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INTRODUCTION TO CAMOUFLAGE AND DECEPTION

INTRODUCTION TO

CAMOUFLAGE AND DECEPTION

JV Ramana Rao Director (Retd) Defence Laboratory Jodhpur

DEFENCE RESEARCH & DEVELOPMENT ORGANISATION MINISTRY OF DEFENCE NEW DELHI 1 1 0 01 1

-

1999

DRDO Monographs/ Special Publications Series

INTRODUCTION TO CAMOUFLAGE AND DECEPTION

JV Ramana Rao

Series Editors Editor-in-Chief Associate Editor-in-Chief M Singh SS Murthy Editor DS Bedi

Asst Editor A Saravanan

Production Printing SB Gupta SK Tyagi

Cover Design SK Saxena

Associate Editor Ashok Kumar

Marketing RK Dua

O 1999, Defence Scientific Information & Documentation Centre (DESIDOC),Defence R&D Organisation, Delhi-110 054.

All rights reserved. Except a s permitted under the Indian Copyright Act 1957, no part of this publication may be reproduced, distributed or transmitted, stored in a database or a retrieval system, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the Publisher.

The views expressed in the book are those of the author only. The Editors or Publisher do not assume responsibility for the statements/ opinions expressed by the author.

ISBN: 81-86514-02-7 Printed a n d published by Director, DESIDOC,Metcalfe House, Delhi-1 10 054.

CONTENTS Preface Acknowledgements

CHAPTER 1 INTRODUCTION CHAPTER 2 MODERN MILITARY TECHNOLOGY AND ITS FUTURE TRENDS 2.1

Introduction

2.2 2.2.1 2.2.2 2.2.3 2 2.4

Land Warfare Main Battle Tank The Infantry The Artillery Role of Air Defence

2.2.5

Nuclear, Biological and Chemical Warfare Surveillance and Target Acquisition Systems Command, Control and Communication (C3) Air Warfare Air Defence Aircraft Survival in the Enemy's Airspace Combat Aircraft and Weapons Future Air Warfare Naval Warfare Submarines Antisubmarine Warfare Future Trends

CHAPTER 3 CAMOUFLAGE IN NATURE 3.1 c3.2 3.2.1 3.2.1.1

Introduction Concealment Colour Matching Variable Colour Resemblance

Studies on Animal Colouration Countershading Disruptive Colouration Shadow Suppression Role of Concealing Colouration Concealment in Offence Studies on Concealing Colouration Advertisement The Warning Colouration Disguise Resemblance to Objects in the Background Diverting Attention to Non-vital Part Mimicry Other Forms of Camouflage Camouflage in Plants Evolution of Camouflage Conclusion

CHAPTER 4 VISUAL CAMOUFLAGE 4.1

Introduction

41

4.2

Visual Camouflage

41

4.3

The Human Eye

41

4.3.1

Visual Acuity

43

4.3.2

Dark and Light Adaptations

43

4.4

Characteristics of Light Relevant to Visual Camouflage

45

4.4.1

Colour

45

4.4.1.1

Geometrical Representations of Surface Colours in Terms of Lightness, Hue and Saturation

46

4.4.1.2

Measurement of Colour

46

4.4.2

Texture

47

4.4.3

Brightness (Contrast)

47

4.5

Sensors in the Visible Region

48

4.5.1

Electrooptical Instruments

4.5.1.1

Image Intensifiers

4.5.1.2

Low Light Level Television

Lasers Rangefinding Target Designation Target Illumination Tracking Photography Platforms Photo-reconnaissance - Aerial Factors Affecting Photographic Reconnaissance Aerial Camera

Advantages/disadvantages of Photographic Reconnaissance T V Cameras Optical Mechanical Scanners Linear Imaging Self-scanning Sensor (LISS) Military Satellites Factors Affecting Recognition in the Visible Region Shape Size Colour Texture Shadow Pattern Site Association Basic Principles of Camouflage in the Visible Region Hiding Arboriculture in Desert Region Screens Obscurants (Smoke Screens) Blending Colour Matching Countershading Disruptive Colouration Shadow Elimination Deception

Camouflaging of Military Objects by Disruptive Pattern Painting Studies on Disruptive Pattern Painting Dual Texture Gradient Pattern Paintings (DTG) Computerised Generation of Disruptive Patterns Camouflaging by Nets Properties of Net Materials Applications of Nets Manufacturers of Nets Psychological Camouflage Neurophysiological Principles of Visual Perception Studies on Target Characteristics and Target Context on Detection Psychological Studies Related to Camouflaging of nfilitary Objects Miscellaneous Camouflage Devices Foams Reflectance Camouflage Antishine Devices Vehicle Track Erasers Computer-based Evaluation of Camouflage New Areas of Visual Camouflage Metarners Spectral Camouflage

CHAPTER 5 INFRARED CAMOUFLAGE Introduction What is Infrared Camouflage? Infrared Radiation Sources of Infrared Radiation Natural Sources Man-made Sources Carbon Arc Tungsten Lamp Xenon Arc Lamp Laser

Nernst Glower Globar Terminology Radiant Energy (U) Radiant Flux or Radiant Power (P) Radiant Emittance (W) Radiant Intensity (J) Radiance (N) Radiant Photon Emittance (Q) Irradiance (H) Spectral Radiant Flux (P, ) Radiant Emissivity (E) Radiant Reflectance (p) Radiant Absorptance (a) Radiant Transmittance (T) Laws Governing Radiation Emitted by Heated Objec Kirchhoff s Law Stefan-Boltzmann's Law Wien's Displacement Law RayleighJeansJ Law Planck's Law Properties of Infrared Radiation Propagation Characteristics Extinction Coefficient Atmospheric Windows Emissivity Measurement of Infrared Emissivity Ernissivity and Temperature Effects on Contrast Relative Effects of Temperature and Emissivity Differences on Radiant Flux Per Unit Area Infrared Sensors Pre- World War I1 Scenario Post- World War I1 Scenario Principle of an Infrared Sensing System Classification of Infrared Sensing Systems Infrared Detectors

Thermal Detectors Quantum Detectors Far Infrared Materials General Discussion on IR Detector Materials Performance Characteristics of a Detector Noise Equivalent Power (NEP) Detectivity (D) Infrared Sensing System Infrared Telescope Vidicon Photothermionic Image Converter Infrared Photography Evaporograph Thermal Imaging System Basic Elements of a Thermal Imaging System Objective Lens System Optomechanical Scanner Detector Bank Electronic Signal Processing and Display Performance Characteristics Applications of Thermal Imaging System Land Applications Air-borne Applications Sea Applications Manufacturers Differences Between Thermal Imaging System and Image Intensifier Future Trends General Considerations Concerning IR Operations with Thermal Imaging Systems Image Processing Single-element Scan Multi-element Scan Parallel-Scan Serial-parallel Scan Focal-plane Processing Arrays (FPAs)

Staring Arrays Schottky Barrier FPAs Charge Transfer Device Focal Planes IR Signatures of Military Objects and Backgrounds IR Signature of Aircraft IR Signature of Ship IR Signature of Tank IR Signature of Personnel IR Signature of Backgrounds Thermal Scenes - Characterisation of Backgrounds Scene Objects Computer Generated Imagery Components of Synthetic Scenes Paradigm for IR Synthetic Image Generation IR Signature Suppression (IRSS) of Warships Spectral Characteristics of IR Signature(s) of Ships

IR Signature Suppression The Dres Ball The Eductor Diffuser IR Signature Suppression of Aircraft Suppression of Plume Signature Suppression of the Signature of Hot Parts Suppression of Signature of Aircraft Body Suppression of Signature of Unresolved Aircraft IR Signature Suppression of Tank Passive Countermeasures Reactive Countermeasures Signature Suppression of Ground Objects Suppression of Signature of Non-hardware Thermal Camouflage Equipment and Materials Disruptive Patterns Camouflage Screens Thermal Blankets or Tarps

CHAPTER 6 MICROWAVE CAMOUFLAGE Introduction What are Microwaves? Properties of Microwaves Microwave Spectrum Radar Frequencies Historical Development of Microwaves Generation of Microwaves Microwave Vacuum Tube Devices Applications of Microwave Tubes Microwave Solid State Devices Microwave Sensors Principle of Radar Historical Development of Radar Radar Equation Typical Radar Types of Radars Continuous Wave (CW) Radar Frequency Modulated Continuous Wave (FM-CW)Radar Pulse Doppler Radar and Moving Target Indicater (MTI) Tracking Radar Side Looking Air-borne Radar (SLAR) Synthetic Aperture Radar (SAR) Millimeter Wave Radar Role of Radar in War Types of Radars Used in War Battlefield Surveillance Radar (BSR) Weapon Locating Radar (WLR) Air Defence Radar (ADR) Other Types of Radar Radar Cross Section (RCS) Expression for RCS Methods for the Prediction of RCS RCS of Flat Plate

RCS of Re-entrant Bodies (Corner Reflectors)

20 1

General Discussion on RCS of Simple Bodies

203

RCS of Military Objects RCS of Aircraft RCS of Ship RCS of Tank

203 203 203 207

Advantages a n d Disadvantages of Prediction Techniques 207 RCS of Targets - Experimental Determination 208 Outdoor Ranges

209

Indoor Ranges Methods for Reduction of RCS

210 210

Shaping B-2 Bomber and F- 117A Fighter

21 1 212

Ship Radar Absorbing Materials (RAMS)

2 12 2 13

Theory Practical Radar Absorbing Materials

2 14 2 14

Types of Radar Absorbing Materials

215

Salisbury Screen

215

McMiIian Absorber

216

Dallenbach Layer Jaumann Absorber and Graded Dielectric Absorber

218 218

Magnetic Absorber Radar Absorbing Structures (RAS)

2 19 22 1

Circuit Analog Absorbers (CAs)

222

R-cards

223

Passive Cancellation Active Cancellation Current Research on Radar Absorbing Materials

224 224 224

CHAPTER 7 DECEPTION 7.1 7.2 7.3 7.4

Introduction What is Deception ? Disinformation Psychological and General Aspects of Deception

229 23 1 231 232

7.5

Deception Equipment

7.5.1

Dummies

7.5.2

Decoys

7.6

Candidates for Dummies and Decoys

7.6.1

Criteria for Selection

7.6.2

General Criteria

7.6.3

Sensor-specific Criteria

7.7

Background for a n Effective Deception Strategy

7.8

Dummies/Decoys of Military Objects Dummies and Decoys of Visible Region

7.8.1 7.8.2

Decoys (IR & Radar)

7.8.2. I

Chaff Decoy

7.8.2.2

Infrared Flares

7.9

Various Decoys (Published in Literature)

CHAPTER 8 MATERIALS FOR CAMOUFLAGE APPLICATIONS Introduction Radar Absorbing Materials (RAMS) Magnetic Materials Dielectric Materials Artificial Dielectrics Conducting Polymers Chiral and Two-dimensional Polymers Schiff Base Salts Infrared Camouflage Materials Physical Principles Attenuat~onof Infrared Signatures Obscuration Surface Treatment Coating Materials for Camouflage in Infrared Region Coating Materials for Camouflage in Visible Region Paints Pigments for Forest and Jungle Areas Pigments for Desert Regions Pigments for Ocean Environment

8.4.2 8.4.4 8.4.5 8.5 8.5.1 8.5.2 8.5.3 8.6 8.7

Antireflective Coatings Aqueous Foam Smoke Nets Materials for Multispectral Camouflage Surface Coatings Composites Multispectral Camouflage Nets Materials for Acoustic Camouflage Futuristic Camouflage Materials

8.7.1 8.7.2 8.7.3

Chromogenic Materials Luminescent Materials Polymers and Composites

8.4.3

CHAPTER 9 STEALTH TECHNOLOGY Introduction What is Stealth? Historical Background of Stealth Technology Military Objects Requiring Stealth Stealth Aircraft LockheedJAirforce F- 1 17A Constructional Details of F-117A NorthropJBoeing B-2 Advanced Technology Bomber (ATB) Stealth Warships Acoustic Signature Radar Cross Section Infrared Signature Magnetic Signature Electric Signature Other Signatures Stealth Tank Stealth Submarine Stealth Helicopter Stealth RPVs

9.11

Stealth Missiles

312

9.12

Airship

312

CHAPTER PO R&D WORK ON CAMOUFLAGE AND DECEPTION IN DRDO 3 1 5 10.1

Introduction

10.2 10.3 10.4 10.5 10.6

Visual Camouflage Infrared Camouflage Microwave Camouflage Multispectral Camouflage Materials Naval Camouflage Force Multipliers

10.7

CHAPTER 11 CONCLUSION Appendix - A Appendix - B Index

PREFACE This introductory book on camouflage and deception is primarily intended for dissemination of knowledge and information in the field. The subject is a military science that has no counterpart in the civil sector, and a s such, no university teaches and gives degrees in the field. Camouflage and deception is an integral part of nature. For self-preservation,which is the central problem of biological evolution, all animals, small and big, both in offence and defence, adopt strategies and counterstrategies. These very principles significantly form the basis of camouflage in war. The means adopted by animals in nature have bewildering diversity and complexity all of which do not seem to have counterparts or could be duplicated even in the modern war of today. One typical example is that of the chameleon which almost instantaneously changes its colours in order to blend with its background. A s yet, there does not seem to be any means in the present day war by which a military vehicle can change its colour automatically in order to blend with the background, as it moves from one background to another. The field of camouflage and deception was existing more as a military art than science, until and during World War 11. Since then, it has developed into a science. The field is inter-disciplinary and draws knowledge from several branches of science and engineering. The stealth technology, of the modern war of today, which greatly enhances the combat survivability of a fighter aircraft or bomber in the enemy's territory is a complex synthesis of several technologies. The rapid advancements that have been taking place in military sensor technologies, in turn, demand more and more sophisticated countermeasures. This is a war between the strategies and counterstrategies. Countermeasures, signature s u p p r e s s i o n / s i g n a t u r e management, stealth, low observables: these are the modern terms being employed in place of the classical terminology - camouflage, concealment and deception. The author, however, has entitled this book in the classical terminology. This book has been written in eleven chapters based on the information available in open literature. Chapter 1, starting with the origin of camouflage, provides an introduction to the field. Chapter 2

provides glimpses of modem military technology and its future trends. This has been introduced in order to have a better appreciation of the importance of countermeasures in war. Chapter 3 deals with camouflage in nature. This provides the basic concepts of camouflage. Chapter 4 covers camouflage in the visible region. Camouflage in war started with ways and means to defy detection by the human eye. Before dealing with the different techniques of visual camouflage, the various sensors that are used in the visible region of the electromagnetic spectrum are briefly described. Chapter 5 starts with the basics of infrared radiation, then discusses the infrared sourcesnatural and man-made, infrared sensors and imaging systems, and infrared signatures of major military targets. Then the various infrared countermeasures are dealt with. Chapter 6 deals with basics of microwaves-generation, properties, microwave sensors, different types of radars, radar cross-section and its prediction and measurement, RCS of major military objects - aircraft, ships and tank, radar absorbing materials and paints, and RCS reduction methods. Chapter 7 briefly touches upon the role of deception in war in general and deception equipment in particular. Chapter 8 deals with camouflage materials for suppression of signatures in visible, infrared and microwave regions, including signatures of non-electromagnetic nature such a s acoustic. Chapter 9 briefly touches upon stealth technology - its history, and its application to major military platforms. Chapter 10 gives a brief account of some aspects of research and development activities in the field carried out in DRDO laboratories. Chapter 11 summarises the various facets of the field and future trends. The technologies associated with target acquisition are rapidly advancing. New tools, such as artificial intelligence, neural networks, pattern recognition and automatic target recognition, may further enhance sensor capabilities. These might lead to counterstealth technologies demanding counter-counterstealth measures. The entire approach towards the field must be viewed from the scenario mentioned above. The field has to counter more challenges in future.

Hyderabad February 1999

J V RaMANA RAO

ACKNOWLEDGEMENTS At the outset I would like to express my deep sense of gratitude to Dr APJ Abdul Kalam, SA to the Minister of Defence and Director General DRDO, Government of India, who has been the driving force behind this task and who has given me this assignment. I would also like to express my grateful thanks to Shri A Nagaratnarn, former Director, Defence Laboratory, Jodhpur (DW), for going through the manuscript of the book and providing several valuable suggestions. I have been greatly helped by Defence Research a n d Development Laboratory (DRDL), Research Centre Imarat (RCI), Defence Metallurgical Research Laboratory (DMRL), Hyderabad; Aeronautical Development Establishment (ADE), Electronics and Radar Development E s t a b l i s h m e n t (LRDE), Aeronautical Development Agency (ADA), and Centre for Artificial Intelligence and Robotics (CAIR), Bangalore; Research and Development Establishment (Engineers) (R&DE Engineers)), Armaments Research and Development Establishment (ARDE), Institute of Armament Technology (IAT), High Energy Materials Research Laboratory (HEMRL),and College of Military Engineering (CME), Pune; Defence Materials and Stores Research and Development Establishment (DMSRDE), Kanpur; Instrument Research and Development Establishment (IRDE), Defence Electronics Applications Laboratory (DEAL),Dehradun; Defence Science Centre (DSC),Solidstate Physics Laboratory (SPL),and Defence Scientific Information and Documentation Centre (DESIDOC), s el hi; and Combat Vehicles Research a n d Development Establishment (CVRDE), Chennai, in extending library facilities, through Xerox copies of articles and papers published in open literature. I express my sincere thanks to the Directors of all these laboratories.

I take this opportunity to thank Prof J Srihari Rao, Regional Engineering College, Warangal, for his support in the preparation of material pertaining to generation of microwaves and radars; Prof Raghavendra Gadagkar, Indian Institute of Science (IISc),Bangalore; Prof J Sobhanadri and Prof VRK Murthy, Indian Institute of Technoloo (IIT), Chennai for their valuable suggestions and discussions.

I gratefully acknowledge the support provided by Dr AR Reddy, former Director, DW for extending every type of facility that I have asked for and to Dr SS Murthy, Director, DESIDOC and Dr Ramesh Kumar, Director of Materials, and their colleagues for their support and valuable suggestions.

It is a great pleasure to acknowledge the excellent support provided by Dr N h m a r , and Dr SR Vadera, DM, in writing the chapter on Materials for Camouflage Applications. But for their support, it would have been difficult for me to do justice to this chapter. I would also like to express my sincere thanks to Shri Anil Das, DW, for his assistance in writing the chapter on Deception. I take this opportunity to thank Shri P Rama Seshu, Dr Krishna Kumar, Shri SN Puspak, Shri Ramesh Chandra Saxena, Shri BL Soni and Shri N Bohra, my ex-colleagues in DW, for the services rendered by them. I gratefully acknowledge the support provided by Dr Kartikeya V Sarabhai, Director, Centre for Environment Education (CEE), Ahmedabad. I would also like to place on record my sincere thanks and appreciation to Smt Meena Raghunathan, Programme Coordinator; Shri Mukesh Barad, Artist; and Shri DM Thumber, Artist of CEE for their excellent work in the preparation of some of the illustrations of the Chapter - Camouflage in Nature.

I would like to place on record my sincere thanks and appreciation to Shri MS Verma, Shri RP Sharma, Shri Virendra Vikram and Shri Mangi La1 for preparing drawings of figures; to Shri Madho Singh and Rajender Vimal for preparing colour transparencies; and to Shri ML Choudhary, Shri BT Mathai, Shri M R Pate1 and Shri Ajay Singh, of DW, f ~ the r excellent assistance provided by them in word processing.

J V Ramana Rao

CHAPTER 1 INTRODUCTION The word 'camouflage7h a s its origin in the French word camoufler which means 'to disguise7'. When the word entered the English dictionary initially, it had a limited meaning, implying concealment or disguise of military objects in order to prevent detection by the enemy. The only sensor available in the early days was the human eye. The means to camouflage a military object were foliage and other locally available material. The concept of camouflage is a s old a s nature, and it has been an integral part of it. All animals, small and big, are found to employ several methods of concealment and disguise for self-preservation, both in defence and offence. Practically no animal is safe, since for every animal there is a predator. Both the predator and the prey have to adopt strategies for their survival. Thus there is a n evolutionary arms race between different species and also within the same species. In the progress of biological evolution, both the predators and the prey have to constantly and equally improve their strategies and then pass them from generation to generation2s3.In the arms race in nature there exists a bewildering diversity in the strategies and counterstrategies adopted by different animals. All these techniques may be termed as camouflage and deception in nature2r3.Although there may not be a counterpart in the present day arms race to each and every strategy adopted by animals in nature, these very principles, by and large, form the basis of camouflage in war. Whether it is concealment or disguise, deception is inherent in all the methods. Human civilization, beginning with primitive man, has been using camouflage, concealment and deception in various forms for different purposes, particularly in wars. The basic philosophy remaining one and the same, the changes that have come are in the methodology of application and the levels of sophistication. Several examples can be cited from ancient wars in which camouflage was extensively utilised with great advantage. The

2

Introduction to Camouflage and Deception

German legend4 "The Nibelungenlied" describes the camouflaging cap, the 'Tarnkappe'. Siegfried wins the cap from the dwarf king Alberich. The cap makes him invisible. It makes him defeat Brunhilde, the Queen of Iceland, in battle. The could not conquer Troy for ten years, not until they employed a ruse-the wooden Trojan horse. The Greeks hid themselves in its belly. The horse was pulled inside the city by the Trojans which led to the conquer of Troy. The use of twigs and leaves on the caps and moving under natural cover by Genghis Khan's mounted mongols, and leaving of camp fires burning by George Washington after departing from the camp, are but a few examples where last minute decisions on camouflage measures had changed failures to successes. Camouflage was employed by the French army during World War I in order to prevent detection of guns and vehicles from the enemy's observation5. Camouflage which was existing more a s a military art became a science during World War 11. At that time a wide range of military objects, such a s individual soldiers, guns, vehicles, tanks, airfields and shipyards, needed protection against aerial observation through naked eye and aerial photographs6. This provided the impetus to develop the field of camouflage and deception on scientific lines. Even during World War 11, the field was essentially confined to the ways and means to disguise military objects against human observation, i.e. camouflaging of military objects against sensors which were available in the visible region. Technological advances in the field of remote sensing covering a wide range of the electromagnetic spectrum have in turn demanded equivalent countermeasures. Prior to World War 11, camouflaging of military objects against sensors employed in the infrared region of the electromagnetic spectrum did not seem to have been employed, a s no such sensors were available. In the subsequent wars, such a s in the Vietnam War, new detectors beyond the visible region of the electromagnetic spectrum came into use. The need for camouflaging military objects beyond the red end of the visible region had arisen with the development of infrared false colour photographic film during World War I1 which provided an impetus for research and development in the field of infrared radiation. Since then, the field has seen rapid growth, in particular in the area of military reconnaissance, surveillance and target acquisition. This in turn has put great stress on countermeasures to defy detection by infrared systems. Thus progress in the field of infrared engineering became synonymous with the development of military infrared7. Much of the work done in the field was classified and not available in open literature. The field of infrared camouflage known under different names - infrared

Introduction

countermeasures, infrared signature suppression, etc - has become vital to the success of any military operations in the various theatres of war - the land, the air and the sea. Radar had played a very important role in World War I1 when several developments took place in radar technology in US, Britain and Germany. This i n t u r n h a d p u t a great s t r e s s on r a d a r countermeasures - a modern name for microwave camouflage. The major military objects which need microwave camouflage are the fighter aircraft, the naval warship and the tank. These objects can be detected by t h e i r d i s t i n c t microwave s i g n a t u r e s . T h e countermeasures involve suppression of these signatures. The microwave signature of a military object is known today as its radar cross-section (RCS). Thus the problem of microwave camouflage is one of reducing RCS of military objects of interest to such a n extent that the object escapes detection by radar. All military objects do not require microwave camouflage. Only those objects which come under the influence of radar threat are the candidates for microwave camouflage. The development of radar is synonymous with the development of microwave e l e c t r o n i c s . T h e r e have b e e n t r e m e n d o u s advancements in radar technology in the post-World War I1 period. Developments such a s digital signal processing and phased array antennas have greatly enhanced radar capabilities. All these developments will continue to demand radar countermeasures. Throughout history, besides the conventional methods of camouflage, deception h a s been employed simultaneously as a force multiplier and to enhance combat survivability. Application of deception techniques in all their subtlety and sophistication peaked during the 1991 Gulf War. The technically developed and heavily resource-backed Allied Forces brandished advanced decoys and deception equipment. The Iraqis effectively displayed deception by relatively simpler techniques8-lo. In many situations, camouflage combined with deception would be more effective. In some cases, it is deception equipment alone that can meet the requirement. Camouflage is concerned with the suppression of signature(s)which the military object may have by which it may be detected. Deception is concerned with simulation of the concerned signature(s).Increase in the signal-to-noise ratio increases detectability of the object by the sensor concerned. Increase in the noise-to-signal ratio increases the degree of camouflage. The objective of various camouflage methods is to increase noise-to-signal ratio. The technological explosion of the 20th century - in the fields of electronics, computer revolution, materials research, a n d sensor

3

4


technology have brought in unimaginable advances in military hardware, weapons, weapon controls, and delivery systems, mobility, reconnaissance, surveillance and target acquisition systems. Simultaneously, along with these developments, the role of countermeasures has become increasingly important, demanding improvements in the existing countermeasures and development of new methods and techniques. The conventional methods of camouflage and deception are no longer adequate in the present-day advanced technology warfare scenario. The field has acquired new dimensions under the names such a s stealth technology, low observable technology, very low observable technology, or signature management. In this context, the conventional methods of camouflage, concealment and deception serve only the preliminary stages. The concept of multispectral/ polyspectral camouflage under the name stealth technology h a s to embody countermeasures to detection by radar, infrared, visible and acoustic sensors and any other sensor that may be employed. Stealth or low observable technology as applied to a weapon platform such as a combat aircraft considers several aspects of the design right from inception with the primary objective of incorporating low observable features without affecting the performance of the aircraft". REFERENCES

1.

2. 3. 4. 5.

6.

7. 8. 9.

Hinkle, C. W. The Encyclopedia Americana. The International Reference Work. Americana Corporation, Washington DC, 1958. p 268-70. Owen, D. Camouflage and mimicry. Oxford University Press, 1980. Cott, H. B. Adaptive coloration in animals. Methuen & Co. Ltd. London, 1966. Jurgen Erbe. Thoughts on camouflage and deception. Milita y Technology. 1987, XI(9),85-87. Now you see me, now you don't - military camouflage. Defence, 1993, XXIV(2), 10-14. Goetz, P. W. (Editor-in-Chief). The New Encyclopedia Britannica. Encyclopedia Britannica Inc., 1988. p 77 1. Hudson, R.D. Infrared system engineering. John Wiley & Sons, New York, 1969. Soviet Electronic National Defence. 1985, 35-42. Soviet military thought. Militay Review. 1982, 6, 25.

Introduction

10. 11.

International Defence Review. 1985, 8, 1235-57. Schmieder, D.E. & Walker, G.W. Camouflage, suppression and screening systems. In Countermeasure Systems, Vol7, Edited by David H. Pollock. The Infrared & Electrooptical Systems Handbook, ERIM, Michigan & SPIE Optical Engineering Press, Washington, 1993.

5

CHAPTER 2 MODERN MILITARY TECHNOLOGY AND ITS FUTURE TRENDS 2.1

INTRODUCTION Modern military technology and its future trends have been discussed at length by Friedman et a1 in their book on Advanced Technology Warfare1.The information given in this chaptkr is based on the above reference. It is not difficult to comprehend for a layman to what levels science and technology have progressed and the impact they have made in the various theatres of war - the land, the air and the sea. The impact of the technological explosion of the 20th century has made space another combat zone through the employment of satellites. Developments in the field of electronics, the computer revolution, and improvements in the performance of existing materials and development of new materials satisfying critical requirements, have significantly added to the armamentarium of military hardware making the weapon platforms, weapon systems and controls more and more sophisticated and complex. This can be very well gauged, for example, from the ballistic and cruise missiles currently available. If the military hardware continues to grow in sophistication at the present rate, it would be difficult to predict the nature and magnitude of future wars. A s weapon platforms and weapon systems grow in their capabilities to zero-on to the target, the availability of effective countermeasures will become a key factor for combat survivability.

2.2

LAND WARFARE Today's war on the ground has become highly complex with the introduction of improved technologies into combat systems which has greatly enhanced the range and lethality of the weapon

8


systems employed by ground forces. To counteract the multitude of highly potent sensors and weapon systems with which the airspace above the ground is filled, there are equally potent air defence systems on the ground providing protection to the battlefield on the ground. 2.2.1 Main Battle Tank The main battle tank (MBT)has a major role in land warfare. Its design has been continuously absorbing the developments in related technologies with special reference to fire power, protection and mobility. As a result, the speed (reaching a maximum of around 70 kmph on level ground), the calibre of the main guns, the firing range, the accuracy of fire, the thickness of the armour, have all increased. Different countries have their own designs with variations in relative importance to fire power, mobility and protection. Besides the large calibre gun, tanks have night vision equipment, thermal imagers, electronic fire control computers and navigational aids which enhance the tank's performance a s a weapon system. The most vulnerable part of the tank is its armour (particularly against top attack) which is receiving maximum attention for protection against anti-tank weapons - missiles, rockets and guns. The American M- 1, German Leopard-2, and the Soviet T-80 utilise composite armours for effective protection. The British CHOBAM a m o u r has layers of metals, ceramics and plastics, which can defeat the High Explosives Squash Head (HESH)round. This armour has been countered by the long rod penetrator projectile which can pierce through the armour. Then came the reactive armour. This utilises an explosive on its outer surface, to prevent the projectile from entering the armour. The fourth generation Russian tanks, such a s T-64, T-72 and T-80 have incorporated some special features. Their gun can fire Armour Piercing, Fin- Stabilized Discarding Sabot (APFSDS)projectile, which can defeat the High Explosive Anti-Tank (HEAT)ammunition as well as High Explosive Squash Head (HESH). India is also manufacturing the Fin- Stabilised Armour Piercing Discarding Sabot (FSAPDS)projectile. In India, DRDO has developed Kanchan a m o u r which is being incorporated in MBT Arjun. Significant advances can be expected to take place in the tanks of the future, mainly in terms of fire power, manoeuvrability and armour strength, to provide greater strength to land warfare and to withstand the anti-tank weapons - mines, missiles, rockets and guns, particularly the anti-tank guided missiles (ATGMs) whose technology has been greatly advancing in terms of their guidance systems, fire power and ability to distinguish between real and false targets.

Modern Military Technology and its Future Trends

Besides MBT, robotic tanks such a s robotic obstacle breaching tank (ROBAT)-which can breach minefields in hostile environment, autonomous land vehicles, and programmable robot observer with logical enemy response (PROWLER)will be new additions from US. This class of vehicles will be equipped with microcomputers with artificial intelligence software and a variety of sensors that will enable them to patrol the battlefield. Various other vehicles such as Armoured Personnel Carriers (APCs)- battle taxis, BMPs (Boyeva Mashina Pekhoti) of Russia, and BFVs (Bradley Fighting Vehicle) of US are being deployed for a number of roles.

2.2.2

The Infantry

Modern technology has brought in many changes in the battlefield environment of the infantryman. These changes have resulted in increased mobility, improved anti-tank capability, better personal equipment, more portable weapons, etc. The infantryman of the future will probably wear a helmet made of kevlar, then a number of sensors such a s image intensifier, thermal imager, and gyro-stabilised laser target designator, as well as a pocket-sized computer with several tens of megabytes of memory.

The Artillery Major technological advances have enhanced the performance of artillery. Revolutionary advances in the field of electronics have brought in improved communications, observation means, survey capabilities and artillery fire. The artillery has a variety of munitions, including nuclear weapons, a t its disposal. New @idance systems, such a s those used in US Copperhead and Precision Guided Munitions (PGMs), have greatly enhanced their capabilities while hitting the targets. The Multiple Launched Rocket System (MLRS),which was under development by US, UK, Federal Republic of Germany and France, had a provision to incorporate a terminal guidance warhead to defeat armour and another warhead for chemical weapons. A development for rocket artillery weapons will be the Cannon Launched Guided Missile (CLGP). Another US concept is the Seek and Destroy Armour (SADARM)which has the capability to seek and destroy individual targets. Yet another development would be STAFF (Smart Target Activated Fire and Forget). This will be fired in the general direction of the target. Radio waves reflected from the target would be picked up by an antenna in the nose of the projectile which is guided by an onboard computer.

2.2.3

9

10


2.2.4 Role of Air Defence The role of air defence is to protect ground installations and forces from aerial attacks. This is accomplished by radar directed Surfac-to-Air Missiles (SAMs).The US designed PATRIOT counters high speed aircraft and missiles a t all altitudes a s well as jammers and other electronic countermeasures (ECMs). 2.2.5 Nuclear, Biological and Chemical Warfare The prospect of nuclear war (although not fortunately resorted to till now) significantly changes military planning. Here it is the dispersal of troops that is to be adopted instead of the normal method of concentrating them. Chemical warfare (CW) has high potential. It involves the use of nerve agents, toxins and psychological agents. These nerve agents are derived from Tabun, Sarin and Soman. In biological warfare (BW),toxins derived from bacteriological organisms such a s botulin would be employed. Psychological agents a r e derived from psychochemicals such as (lysergic acid diethylamide) LSD. These chemical agents can be spread by several means such as aerial bombs, artillery shells or by aerosol sprays. 2.2.6

Surveillance and Target Acquisition Systems For rapidly reacting to any adverse situation in the battlefield, it is essential to find out the concentration and disposition of the opponent. Several advances have taken place in this area which have greatly enhanced battlefield surveillance. The means through which this is accomplished are from ground, air, a s well a s space. The equipment used include optical instruments, electrooptical devices, radars, and thermal imaging systems. These can be hand held, tripod mounted, mounted on ground platforms, helicopters, remotely piloted vehicles, drones, aircraft and satellites. 2.2.7

Command, Control and Communication (C3) The electronic revolution has made a great impact on command, control and communication (C3) systems in the battlefield and elsewhere. Information can flow from FEBA (Forward Edge of the Battle Area) and FLOT (ForwardLine of Own Troops).The operational area will have a matrix of trunk nodes. The principal component of a node is a vehicle containing a computer-controlled electronic switching device which functions a s an automatic exchange. The exchange connects one user to another. The jobs of the widely dispersed and highly sophisticated arms of today have become simpler and rapid, with the advances that have taken place in C3.

Modern Military Technology and its Future Trends

2.3

AIR WARFARE Today war in .air is backed by more advanced technologies than war in the other two theatres - land and sea. Obviously it is more complex and sophisticated, and greatly influences the war in other theatres. Technologies have advanced to such a n extent that today there may be no need for a pilot, or human crews in a combat aircraft, from which targets such as a tank or ship or another craft can be hit and destroyed. What used to be done by the pilot's eyes is now performed by a wide range of sensors, viz., electrooptical, infrared, microwave etc. Further, cruise missiles carrying warheads and satellites have added new dimensions to war in general and to air warfare and space warfare in particular. 2.3.1 Air Defence It is known that for a n aircraft to survive in enemy's air space, it has to fly a t the lowest possible levels to minimise radar detection. This has brought attention on to low level air defence systems. The introduction of Airborne Warning and Control System (AWACS) which utilises a multimode radar a t a height of less than 10,000 m (10km) has extended the range of vision to about 400 km, enabling detection of targets flying close to the ground. Besides, AWACS can provide a lot of information on friendly and hostile forces. But, a t the same time, every defence system is countered by measures incorporated in the combat aircraft. These include: Electrooptical Countermeasures (EOCM),Infrared Countermeasures (IRCM),and Eelectronic Countermeasures (ECM).The various types of missiles that home on to targets can be countered by jammers, decoys and other false sources. The Triple A (anti-aircraft)or SAMs can engage any aircraft or missile a t a range of 3-8 km. Aircraft Survival in the Enemy's Airspace 2.3.2 Aircraft survival in the enemy's airspace depends upon its ability to escape detection by all possible sensors which the enemy might use. The aircraft has to be made stealthy, involving reduction in Radar Cross-Section (RCS), suppression of infrared emissions and all visible signs such as contrails. Stealth technology takes care of these aspects to reduce the chances of detection. Also, to distract detection by infrared sensors, expendable decoys in the form of flares and infrared pulse emitters are employed. Protection of airfields during major wars is not a n easy task. The accepted line of action is to proyide additional support airfield defences employing hardened aircraft shelters and aircraft that can take off between the craters.

11

12


2.3.3 Combat Aircraft a n d Weapons The combat aircraft carries on board a variety of sensors for different roles - ranging from navigation to weapon delivery, including recovery a t the base. These sensors could be active or passive, and are described elsewhere. In order to reduce emissions which can be detected, use of active sensors has to be reduced. The weapons carried by the aircraft are guided missiles which receive guidance from external radar, or lasers, or have self-guidance which rely upon IR radiation emitted by the target. The antiship missile flies extremely low in the sea-skimming mode to escape detection. Most of the antiship missiles use active radar homing. Today there is a wide range of air-to-surface missiles such a s wire-guided antitank missiles having a range of a few hundred meters and cruise missiles which can home on a target over 3,000 km. For air-to-air operations, the weapons are guns and guided missiles. Air-to-Air Missiles (AAMs)of short range depend on IR homing. Fire and forget come under this category. The other category of AAMs is of medium range which relies on radar homing.

2.3.4 Future Air Warfare The present-day stealth technology which is using passive measures may be countered by further developments in sensor technologies. So, for enhancing the chances of survival in the airspace, active measures of stealth will probably be necessary. They involve manipulation of the electromagnetic and nonelectromagnetic signatures associated with the aircraft to create confusion. Another problem which has still not been probably solved is the suppression of IR signature of the aircraft which is being utilised by missiles for homing on. NAVAL WARFARE Naval warfare has incorporated the latest technologies in its surface warships, surface weapons, submarines, submarine weapons, sensors, and command, control and communications. During the last five decades, there have been rapid strides in surface warship technologies. The surface warship h a s to simultaneously perform several tasks, viz., Air Defence (AD),AntiSubmarine War (ASW),besides antisurface roles. In order to perform these roles, it has to equip itself with active and passive sensors s u c h a s Air Defence Radars, Surface Surveillance Radars, Electrooptical Systems, Hull-Mounted Sonars, and Variable Depth Sonars. In terms of warship armament, a surface warship has to equip itself with weaponry to attack and defeat other surface warships, submarines and aircraft. The most important weapon 2.4

Modem Military Technology and its Future Trends

for both offensive and defensive actions is the missile. The French Exocet is a n antiship missile (ASM)which can be ship-launched as well as air-launched. The Soviet antiship missile 'Shaddock' has a nuclear warhead. The US Navy's Harpoon has penetration blast warhead. In general, antiship missile guidance is programmed in such a manner that the missile hits at the central portion of the hull so that the vital services of the ship are damaged. There are however countermeasures to antiship missile, such as chaff decoys which provide a screen around the ship, and infrared flares which provide alternate targets to the incoming surface and air-launched missiles. Besides missiles, other armaments include modern naval guns, such a s the US Navy's Phalanx CIWS (Closein Weapons System). The characteristic feature of this system is that it has extremely fast reaction time and heavy volume of fire. With regard to aircraft carrier scene, the wkstern navies have dominated for many years. The vertical/short takeoff and landing V/STOL aircraft are comparatively inexpensive. The primary role of sea-based aircraft are Antisubmarine Warfare (ASW), strike/ attack, Air Defence (AD), electronic warfare and Airborne Early Warnilng (AEW). 2.4.1 Submarines The submarine is an effective underwater weapon platform. Submarines have both acoustic and non-acoustic signatures. In order to reduce the chances of detection, their signatures have to be suppressed. Research efforts are being directed towards comparatively less noisy submarines with better speeds and deeper diving capabilities. Nuclear propulsion is one of the greatest achievements in the submarine technology of the post World War I1 period. Now the submarine does not have to come to surface for refilling its air requirement and recharging its batteries, and its speed far exceeds that of attacking surface vessels. The torpedo has been the underwater weapon for attack and patrol submarines since long. In comparativeIy recent years, new weapons are being added. Submarine-Launched Cruise Missile (SLCM) is one such weapon which can have a range u p to 1,000 km. One important role being played by the submarine is mine laying. The important sensors of a submerged submarine are sonar and hydrophone.

2.4.2 Antisubmarine Warfare Submarine detection technology is critical to antisubmarine warfare. The characteristics of a moving submarine provide the necessary means of detecting it. Navies of the world a r e

13

f:4


concentrating their R&D efforts towards improving the existing methods, and discovering new methods of detection, localisation and categorisation of submarines. Antisubmarine warfare mainly involves five steps: search, contact, approach, attack, and close combat and disengagement. During the search stage, a surface warship or a submarine or airborne aircraft searches for the submarine in a certain region. In this stage, passive sensors together with inputs from other platforms are employed. Contact stage involves detection and classification. Detection implies the presence of a n object in a given area. Classification tells whether the object detected is a submarine. From the nature of the signature received from the submarine, further information on the type and class of submarine is obtained. The approach stage utilises passive means for localisation of the target. In the attack stage the weapon, usually a torpedo, is launched. In the final stage the submarine comes back to its original position of quiet. Mines also play a major role in antisubmarine warfare. Besides sonar (the active sensor) and hydrophone (the passive sensor) other non-acoustic means, such a s the wake, which sets in temperature disturbances, can be used for detection of submarines. Submarines are also detectable by the electrical and magnetic fields they create. 2.5

FUTURETRENDS Probably the most important contributory factor to the advanced technology warfare of today is the electronic revolution developments i n solid-state electronics, miniaturisation of electronics, very high speed integrated circuits (VHSIC) and digital computers. All these developments are finding application in military hardware and military systems and will continue to do so to enhance the combat effectiveness in the various theatres of war. A wide range of the electromagnetic spectrum will find increasing applications in modern warfare. Millitimeter wave systems may soon find t h e ~way r into weapon systems. Radiometers as passive seekers can also be used in dual mode along with active systems such a s lasers. Millimeter wave seekers will find application in antitank missiles. Developments may take place in sonar, which is the means for locating and tracking submarines. Passive sensors provide only the bearing of the target. Active sonar signals alert the victim. Towedarray sonars which have been developed in comparatively recent years can meet the requirement of long range detection of targets. A s regards electrooptical systems, non-imaging infrared homing systems are used in antiaircraft missiles and antiship missiles.

Modem Military Technology and its Future Trends

Imaging infrared systems are used in air-to-ground weapon systems and night vision systems. Lasers have found several applications in war, such a s missile guidance, ranging and target designation, and laser-guided bombs. The Rockwell AGM-114A Hellfire was one such missile guidance application. Laser h a s entered the field of radar. The British Aerospace Dynamics Laserfire is a n example. This type of radar has the advantage of high angular resolution a s well a s good range resolution. The Hyper-Velocity Missile which was being developed by Vought for USAF is a multifunctional system. It can not only detect but classify multiple moving tagets and transmit the commands to a formation of several missiles. In the area of communications, a combination of advancements in the field of electronics, such a s integrated circuit electronics and microprocessers and computer technology has brought in flexibility and reliability in military command and control. Electronic warfare {EW) consists of countermeasures to enemy's surveillance, target acquisition, tracking and guidance systems. Radar and infrared warning receivers detect signals from hostile radars and tactical missiles respectively. The chaff and flare decoys are the countermeasures against radar and infrared homing systems. The shape, scope and speed of future wars may be estimated mainly from the technological,advancements taking place in the field of electronics, materials and computers. With this background of the present-day military technology, it would not be difficult to assess the role and importance of camouflage and deception in war. All vital equipment need the cover of camouflage and/or deception for sustenance, combat survivability and for successfully completing missions.

REFERENCE 1.

Friedman, R.S.; Miller, D.; Gunston, B.; Richardson, D.; Hobbs, D. & Warmer, M. Advanced technology warfare. Salmander Books Ltd., London, 1985.

15

CHAPTER 3 CAMOUFLAGE IN NATURE INTRODUCTION The origin of the biological world may be traced back to a few thousand million years. It consists of a multitude of organic species. The central problem of biological evolution is self-preservation. The basic requirements of self-preservation are food, habitat, security, procreation, rearing the offspring, and transferring the genetic characteristics to the next generation. The various organisms have to constantly interact with the environment in which they live, with other species, and with their own species. The interactions are not simple. The environment taken a s a whole is a complex web consisting of the organism's surroundings, habitat, predators, enemies and competitors. Adaptability to this complex web is the primary requisite for survival. The probability of sunrival is determined by the degree of adaptability. The foremost requirement for an animal to survive is food. A simple food chain consists of plant-plant feeder-predator1. While some animals depend on plants for their food, a majority of animals are predatory. Although in this food chain, only one predator is given, four or even five predators may be added to the chain. It may be said that the predator at the end of the chain is free from other predators. Practically no animal is safe, and for every animal there is a predator. For survival, both the predators and the prey have to adopt strategies. The prey is constantly on the alert to avoid recognition by the predator. The predator is constantly on the lookout to locate and capture its prey for food. This is a n arms race consisting of strategies and counterstrategies, the former for defence and the latter for offence. Evolutionary arms race takes place between different species as well a s species of the same kind. It is a conflict between the predator and prey, between hunter and hunted, and the aggressor and the victim of aggression.

3.1

18

Introduction to Camouflageand Deception

In the process of biological evolution, both the predator and the prey have to constantly and equally improve their strategies and then pass them from generation togeneration. The strategies adopted by the animals in nature are highly varied and there do not seem to exist counterparts for all of them even in the present-day arms race. In the arms race in nature, there exists a bewildering diversity of strategies and counterstrategies. All these techniques may be termed a s camouflage. A s applied to nature, camouflage may be defined as the means by which animals escape the notice of predators or a s a device or expedient designed to conceal or deceive. Whatever be the strategy adopted, deception i s inherent in all of them since the true appearance of the animal is replaced by a false one. CONCEALMENT Concealment is a widely adopted method of camouflage. In nature, there are a variety of backgrounds characterised by homogeneity and heterogeneity in colour, and structural simplicity and complexity. The predominant colours of various backgrounds are green and brown, besides sea blue and grey. These colours occur either singly or in combination, along with tonal and hue variations. Forests, woodlands, mountainous and rocky regions are complex backgrounds while deserts, seas and snow regions are simple. Cott gives a n excellent account of camouflage in nature in his book2, 'Adaptive Coloration in Animals'. The various principles that are found to operate by which different animals are concealed in their respective backgrounds are: Colour matching Countershading Disruptive colouration and Shadow suppression 3.2

3.2.1 Colour Matching The first and the foremost requirement for a n animal to blend with its background is to have on its body the prevailing colour of the environment. Several varieties of caterpillars, butterflies, grasshoppers, mantids, frogs, and birds are predominantly green in colour, which reduces their probability of detection in their green background. Lizards and several other species living on boughs, tree trunks and barks are usually brown in colour. Animals living in deserts have on their bodies dusty brown coats. In snow-bound areas white colour is predominantly seen on birds and mammals. Fishes which dwell in water have transparent bodies. Those living on sea shore and sea

bottom bearr OPE their bodies appropriate colours h m a ~ s i n with g their backgrounds. Multicolours are found on species which Eve on flowers. Thus concealment is attained in animals in nature, broadly, by bearing colour resemblance to their respective environment. BJariarhle Cobour Resemblance Many anirnals, besides possessing colour resemblance, have the ability to change their colour depending upon the requirement. During their life history, while in the young stages, many geometridae like the oalc beauty in the larval form display colour charactc~risticsof twigs on which they rest2. A s the lava grows and becomes arr adult, it changes its colour to that of the bark on which it Jlves. Some i~lsectssuch as buttedies exhibit variation r have in colour between wet and dry seasons. Some ~ t h eanimals the ability lo rapidly iz?d almost instantaneously change their colour in their effort to attain the colour of the background. Chameleon and cuttlefish are two examples of this kind. There are some other categories wh5.h exhibit polyrnorg~hismin colouration3. In some of these cases it has been proved that by being different from the rest of the crowd animals escape attention of their predators or prey. This is probably because atlixnals develop a search image by which they ignore unfamiliar or different animals. Some animals, besides possessing colour resemblance, have structural resemblances to their respective backgrounds. The yellow-wattled Lapwing2builds its nest on bare ground and lays its eggs in a small depression in the ground (Fig. 3.1). In spite of the heterogeneity of the background in terms of coliaur and structure, the bird, its nest and its eggs blend remarkably weli with surroundings. Similarly, the sandgrousa: which lays pinkish eggs on hare ground among fallen leaves, the wood cock among fallen and broken oak trees, the ringed plover morrg pebbles a d the ptarmigan among Iichen-covered rocks, all are b e a u t i k i examples of camouflage in complex backgrounds. Specid mention may be made of the Eurasion bittern Botunurus stelaris [Fig, 3.2) which nests =amongreeds'. When in danger, it straigl~fcnsits head and erects its neck in a straight line. A s the reeds of the backgro~lntdare blown with the winds, the bird too, ,dong with the vertical stripes on its body and neck, sv~ays,blending well with the background. Other examples of colour and structurd resemblance YO their immediate background, namely, leaves, 3.2.1.1

Camouflage in nature

sticks, pebbles etc, are leaf insects, stick insects, walking sticks, and leaf-like frogs. 3.2.1.2 Studies on Animal Colouration Pioneering work on animal colouration was carried out by Poulton4j5.According to him, the green colour of various caterpillars is due to the presence of chlorophyll which is derived from the food they eat. The green colour of tree frogs is due to selective absorption and reflection of light6. Light falling on the animal through green leaves is predominantly yellow in colour, which is absorbed by the deep-seated melanophores. The guanophores reflect back radiation lying in the green region. Where there is no green foliage, blue radiation is more pronounced. Laboratory experiments showed that exposure to blue light results in brown pigment, which explains the brown colour of animals living in brown backgrounds. By and large, the green and brown pigments of grasshoppers are either genetical or produced in response to stimuli from the environment. Rapid changes of colours in cuttlefish are due to the presence of special cells in the skin which contain black melanin. Regulation of melanin can cause different kinds of colours7.The physiological mechanisms responsible for such rapid changes in colour are complex. Several factors come into play such as reflex activities induced through the sense of sight, controlled by hormones in the blood, or in some cases due to direct action of light on skin. It is unlikely that adaptive colouration is accidental. There is positive evidence in support of the biological value of visual concealment a s a means of protection from predators which hunt by sight. The studies of Poulton and Sanders8,di Cesnolag,Younglo, S ~ r n n e r ~ ~ .Carrick14, '*J~, Iselyls, Collenette16and Cheesman17are in support of this view.

Countershading The principle of countershading is also found to be operative in the camouflage scheme of animals2. An animal possessing colour matching with its background can still be recognised by the unequal illumination of different parts of the body. Figure 3.3 shows a white cock against white background; yet, it is conspicuously seen and recognized. The back of the bird receives more light from the top, its breast receives less light, and its vertical parts have the same illumination a s that of the background. This gives rise to unequal illumination on different parts of the body. The light and shade effects so produced completely offset the colour matching of the bird with the background and renders the animal recognisable. Thus, despite

3.2.2

21

22


Figure 3.3. A white cock against a white background.

colour matching with background, any solid object can be recognised by the effect of light and shade alone. Even if the cock were coloured by a darker shade and seen against the darker background, it can still be recognised. But the bird can be rendered invisible by making its back darker and belly lighter. This destroys the light and shade effects. The solid body of the bird appears to get flattened. Then the colour matching becomes effective, rendering the bird invisible. This is known a s the principle of countershading. This effect is observed in various fishes and many desert animals-mammals, birds, snakes, lizards, etc. Another example is the hare which bears a darker tone on its back and lighter tone on its belly. The theory of concealment by countershading was put forth by Thayer18-21.Figure 3 . 4 illustrates Thayer's principle of countershading. Figure 3.4(a) shows the appearance of light and shade produced in a uniformly coloured body matching with the background and illuminated from above, and figure 3.4(b) the appearance of a counter-shaded body illuminated uniformly from all directions. Figure 3.4(c) gives the appearance of flatness caused by corrntershading and light falling from the top.


Figure 3.4. Thayer's principle of countershading. (a) Self-coloured when illuminated from above (b) Countershaded when uniformly illuminated (c) Top-lighting and countershaded

Figure 3.5. Bush Buck

- An illustration of obliterative shading.

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Another example of this principle can be seen in Bush Buck (Fig. 3.5). In this case the animal has white spots on its flanks. These spots resemble flecks of sunlight falling on its body through vegetation. The animal a t first glance melts into the background2. Effects of countershading can also be produced by employing patterns of alternating black and white stripes or spots or patches. When these patterns are observed from different distances, depending upon the density of the patterns, a point will be reached where, because of failure of resolution, they blend and provide the necessary countershading effects. MottramZ2was the first to discuss the effects of such patterns. A glaring example of this kind from nature is the zebra (Equus burchelligranti,Fig. 3.6).The zebra has black and white bands on its body. The black stripes are dense where it receives more illumination, and less dense where it receives less illumination. This type of pattern produces the necessary countershading effects. Also, the stripes which are perpendicular to the contour, obliterate (discussed subsequently) the form of the animal. The zebra escapes attention from its predators during dawn and dusk when it is vulnerable to attack. There are many other animals which have patterns producing countershading effects a t blending distances at which they are vulnerable to attack.

Figure 3.6. Zebra - Black and white stripes producing countershading.


Although countershading in nature gives a degree of invisibility to the animal, further studies are needed to establish its role and value.

3.2.3 Disruptive Colouration Colour matching combined with countershading provides adequate concealment against a simple background of uniform colour when the animal is not in motion. But this is a n ideal situation. Most of the animals move from place to place and hence are seen by their predators against constantly changing backgrounds. Under these conditions nature provides effective camouflage to the various animals by the application of disruptive colouration2. The two important characteristics by which any object is recognised are'specific surface area and specific contour by which it is bounded. These two characteristics fix the size and shape of the object by which it is recognised. So, if these two characteristics are

Figuse 3.7. Disruptive patterns on some snakes.

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26

Intmduction to Camouflage and Deception

destroyed, recognition is not possible. This is what is accomplished in disruptive colouration. A large variety of animals in nature are camouflaged by disruptive colouration, where animals wear on their bodies patches of irregular shape oriented at random in two or three colours and of different sizes. Figure 3.7 shows some of the disruptive patterns found on some snakes. These patterns divert the attention of the observer away from the real appearance of the animal, and atterltion is drawn towards these patches which bear no relationship to its actual shape and size. The efficacy of the patterns depends upon a number of factors. Firstly, some of the patches should have the same colour as that of the background, while that of the others should strongly differ from that of the background. Secondly, the patches should cut the contour of the body perpendicularly rather than running parallel to the contour. Thirdly, there should be maximum colour contrast between adjacent patches. A single object wearing these patterns is replaced by a large number of dissimilar and small objects. In this way the real appearance of the animal is

(a)

(b)

(c)

(dl

Figure 3.8. Disruptive patterns illustrating the principle of differential blending.


masked. The pattern by itself may be dazzliig but it contradicts the form of the animal. Another feature observed in these patterns is that the patches bear striking resemblances to the various objects of the background in which the animal moves. Figure 3.8 shows how the recognition is rendered more and more difficult. Left hand figures of each series are the real forms of a fish, a butterfly and a bird, seen against a uniform white background. Figures number (b)of each series show these three animals with disruptive patterns, but not matching with their background. Despite mismatch with the background colour, there will be some difficulty experienced in recognising their true form. In figures (c) and fd) of each series, the background matches with one element of the pattern. One of the elements blending with the background, if not totally preventing recognition, at least ddays the recognition of the true form of the animal. This illustrates differential blending. Figure 3.9 illustrates the principle of maximum disruptive contrast between the background and one of the elements

Figure 3.9. Principle of maximum disruptive contrast.

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28


of the background. First figures of each horizontal row are shaded in such a way as to correspond with their respective backgrounds,

Figure 3.10. Rana adspersa.

black in the first, white in the second, and intermediate in the third. The second figure of each horizontal row has a pattern. The white elements in the middle first row sharply differ in contrast from the black background. The black element in the middle second row sharply differ in contrast from the white background. The white element in the middle third row differs in contrast from the grey background. When one looks at these figures from a distance, the elements of the figure having maximum contrast from the background distract attention from the true form of the figure. In the third of each row, the backgrounds are nonuniform, having broken surroundings. In such a situation, the efficacy of the patterns is further enhanced. This principle has wide application is nature. Even the simplest disruptive pattern such as the one found on the body of the East African Rana adspersa (Fig . 3.10) is effective in hindering recognition. The body of the frog bears brown and green colours. A yellow stripe which is conspicuous, starting lfrom the snout and running along its back, divides the body approximately into two parts. The yellow line resembles a blade of grass or twig; further it bisects the frog into two apparently different entities. The conspicuous yellow line catching the observer's eye diverts the attention away ffom the real form of the animal. Such simple patterns are found in nature on many kinds of birds, grasshoppers etc.


Flipre 3.11.1Megalixalusfomasinii.

Besides patterns which produce disruption of form and size, there are patterns which join together separate parts of the body, rendering recognition difficult. When component parts like legs, wings, eyes, mouths and fins are separately seen, recognition is easier. But if by some means these parts are brought together, giving the impression of a single entity, recognition becomes difficult. The tiny frog

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Megalixalusfonasinii has stripes on its back and on legs. While the animal is in resting position it folds the limbs close to the sides of the body. The appearance totally contradicts the real form of the animal (Fig. 3. 1 1). Similar techniques a r e adopted by giraffes, jaguars, grasshoppers, moths, frogs, mantids, etc., to escape recognition by predators or grey. Figure 3.12 shows a woodcock. Here disruption is produced by dazzle. The most difficult of all the organs which are difficult to conceal are the eyes. Whatever be the background, a circular or round object attracts maximum attention, and a s such its concealment is necessary for hindering recognition of the animal. During the day, in the case of tree frogs and snakes, the round eyes get constricted into narrow slits. In the case of the common frog, there is a dark patch of irregular shape around the eye extending into the body which masks the eye.

3.2.4 Shadow Suppression An animal which is otherwise well camouflaged by the methods mentioned above can still be recognised indirectly by its shadow cast on the ground under the action of sunlight. The probability of recognition by the shadow depends on the nature of the surface on which it is cast. If the surface is smooth, the shadow is well defined; if the surface is uneven or irregular, the shadow is distorted or broken, making recognition difficult. Shadow suppression becomes more important in the case of animals such a s lizards, birds, butterflies, moths, etc. whose habitats are relatively flat and even, and exposed to sunlight. Butterflies compress their shadow into unrecognisable form by resting with their wings folded and orient their body with respect to the direction of sunlight in such a way that the shadow gets reduced practically into a line. Some butterflies tilt their bodies or wings in such a way that their shadow is hidden. Moths keep their body and wings depressed to the surface and crouch low. In some categories of animals, the dark elements in their disruptive patterns and the shadow together make recognition difficult. 3.2.5 Role of Concealing Colouration There is no general agreement among biologists a s regards the role of cryptic dress worn by various animals on their bodies in nature. The question that arises is whether the cryptic colours are merely accidental or whether they have been evolved for survival or concealment. This aspect has been discussed a t length by Cott2.


In general, there is a striking resemblance between the colouration put on by frogs, toads, grasshoppers, butterflies, etc. and their surroundings. Such a phenomenon is also observed in several birds such as bustards, larks, night jars etc. In all these cases, the cryptic dress is providing protection from the predators, enemies etc. On the other hand, there are some birds such as eagles, falcons, kites, harriers, etc. which do not possess any concealing colouration and are conspicuous, the reason being that they have no natural predators. Birds such a s macaws, cockattos, and ostriches have strong and sharp beaks, a s a means of protection. There are others which live in a colony in large numbers. There are some other categories of birds which depend upon their speed of motion, evade attack by diving or bolting and are aerial in habits. In other words, cryptic colouration becomes necessary in the case of animals of terrestrial origin of small, moderate size and unarmed. Camouflage is effective only when the animal is stationary. However perfect the cryptic dress may be, while in motion its efficacy abruptly drops down. In the case of birds which are nocturnal in habits and rest during the day, camouflage becomes a vital biological necessity. The bird kiwi spends its daytime in holes and burrows, and so it does not need any camouflage, and it does not put on any cryptic dress. Likewise, starlings, kingfishers, woodpeckers which rest in holes, in trees and underground, have no cryptic dress. In the case of birds like larks and bustards which lay their nests on bare grounds, cryptic colouration is well developed. During periods of incubation there appears to be a correlation between appearance and nesting habits. In the case of eggs which are not otherwise protected, s u c h as those of larks, pipits, sandpipers, and sandgrouses, they are all cryptically coloured in such a way as to blend with their surroundings. On the other hand, the eggs of woodpeckers, hornbills, barbets and parrots are white and hence do not possess any cryptic colouration; but they do not need protection by camouflage as the nests are inside holes in trees and underground. But there are a few exceptions where the eggs which are otherwise not protected do not have any cryptic colouration. In general, there seems to be a close correlation between the cryptic dress of eggs and the environment in which they are incubated. It appears that, in general, cryptic colouration and cryptic instincts have evolved in response to the need for concealment. 3.2.6 Concealment in Offence Camouflage is employed not only in defence but also in offence. Animals can sense events happening a t a distance through sight,

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sound and smell. While capturing prey, animals alter their appearance, suppress noise and obliterate smell. For aggressive as well as protective function, the predatory animals employ the same principles of camouflage. The tiger and the panther are practically invisible to their prey almost till the latter are attacked and caught. Whatever be the degree of camouflage, once the animal is in motion, it attracts attention. Basically, the problem of concealment in offence differs from that in defence. The prey anim,al, while in defence, remains motionless and escapes observation by its cryptic dress. On the other hand, the predator cannot remain still, it has to be active and in motion, and still not attract the attention of its prey. Besides adopting cryptic colouration, the movements of the predator have to be stealthy and skilful while approaching and attacking the prey. Fishes are good examples of stealthy approach. Firstly, their bodies are excessively thin, and secondly their movements are precise and cautious. All these render the fish most inconspicuous to its prey. Until the final assault, the leaf fish is least observable by its prey. During the assault, the approach is so slow and stealthy that its movements do not produce any detectable noise. Other examples of stealthy approach are those of the spider and snake while catching their prey. Owls, while in flight and approaching their prey, depend upon a combination of darkness and silence. The flight of an owl, in spite of high speed, is not accompanied by detectable noise. This is due to structural modifications of its feathersz3.For masking scent, animals make use of the wind direction. They approach the prey against the wind.

Studies on Concealing Colouration Cott2gives an excellent review of the views of different workers on the function of concealing colouration. Deanz4is of the view that cryptic colouration in animals is accidental, in support of which he cites some instances. According to Cott, those instances cannot be used a s a n argument against the theories of cryptic colouration. The combined use of colour matching, obliterative shading and disruptive colouration, all in a single animal, cannot be chance effects without any biological significance. We find a general agreement between the cryptic dress in a wide variety of animals and the diverse surroundings in which the respective animals live. All the observed facts taken together indicate that concealing colouration has been a n important end rather than an incidental by-product. 3.2.7


Some critics put forth the argument that cryptic colouration is the result of a physical process a s opposed to the need for concealment in nature. But the function of concealing colouration in biological evolution is not to be mixed u p with the mechanism of the concealing colouration. Although the physical and chemical mechanisms vary in different cases, the underlying optical principles of concealment are the same in all the cases. Cryptic colouration does not imply total immunity from attack but chances of survival are increased. Some critics argue that it is stillness which is more important than concealing colouration. Whether the animal is a t rest or in motion, a cryptic colour scheme has its significance. Some argue that since many animals do not have colour vision, concealing colouration cannot be adaptive. Many studies have been carried out on birds, insects, fishes and mammals, and it is established that several animals have a wide range of colour vision. Another argument against adaptive colouration is that many protected forms are widely preyed upon in nature. Adaptive colouration cannot afford protection against all animals. The greatest supporters of t h e theories of concealing colouration, warning colouration and mimicry have been without exception naturalists - Darwin, Wallace, Bates, Alcock, Belt, Hale Carpenter, Graham Kerr, Hingston, Hudson, Julian Huxley, Miles Moss, Mortesen, Poulton, and Shelford -whose experience of natural history has convinced them of the adaptive significance of the various phenomena of cryptic colouration2. Literature on this subject has been descriptive. Relatively few experiments have been carried out to test the validity of the theory of concealing colouration. It is of utmost importance that further data based upon experimental observations should be obtained in relation to the theories of concealing colouration. 3.3

ADVERTISEMENT A s anti-thesis to concealment in nature, there are several animals which by their appearance are conspicuous. Although this characteristic is the opposite of concealment, it serves the same vital needs - food, safety and reproduction. By being conspicuous the animal advertises its inedible qualities. Thereby it is avoided by the predator. The colours red, black and yellow in combination are utilised for drawing attention.

The Warning Colouration In contrast to concealment which involves cryptic colouration, stillness and concealing attitude, advertisement involves display of 3.3.1

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34


conspicuous colours, movements, sound, smell etc. Animals which take recourse to advertisement have structures and behaviours which can cause real harm when attacked. The poisonous stings and bites of many insects, spiders and snakes are examples of the above category. These animals combine warning colouration along with occasional use of the sting or bite to keep the danger away. There is hardly any need for a stinging animal like bee, wasp or hornet to hide its appearance and not let the victim know what struck it. So, instead of hiding, it leaves a permanent impression of its appearance and sting in the mind of its victim, so that bee or wasp is never approached subsequently. Salamanders (Salamandra rnaculosa), tree snakes (Dipsado morphus dendrophilus), sea snakes (Pelamydrus platurus), and sawflies (Athalia cordata) use the combination of black and yellow colours for drawing attention. Some collect in large numbers with the same aim. The porcupine, when threatened, employs several means to give warning to its enemy. The anirna.1 opens its thorny spines, it stamps, squeals, rattles and stinks when in danger. The puffer fish (Tetrodontidae)grows in size and resembles a balloon when in danger. The little porcupine fishes gather together creating a n appearance of a large fish when in danger. There are some animals which exhibit bright colours all of a sudden when there is a threat. Snakes employ hissing sounds for drawing attention. Some animals release irritating secretions. There are categories of animals which emit nauseous odour. Butterflies employ odour for conspicuousness. There are some other categories of animals which possess tough bodies. All these animals, with the dangerous characteristics they possess, remain conspicuous so that they are avoided. There are some animals which are palatable and defenceless. But they get protection by borrowing the characteristics of aposematic animals which are inedible. Also some birds build their nests very near the habitat of poisonous insects such as bees, wasps, etc. 3.4

DISGUISE Disguise is employed both in defence and in offence.

Resemblance to Object's i n the Background Many animals resemble objects of their background. Some fishes resemble dead leaves, some ,other categories acquire the appearance of thin leaves either by compressing or by depressing their bodies or by a combination of obliterative shading and disruptive patterns. Butterflies have wings resembling thin leaves. 3.4.1


Some insects resemble curled leaves. Some type of moths, beetles etc., resemble bark. Lizards, frogs, and birds also utilize this type of deceptive appearance. Some insects such a s mantids, grasshoppers, caterpillars and tree frogs resemble lichen. Some insects bear striking resemblance to bird droppings. Some moths resemble broken twigs. Marine organisms resemble shingle, sea grass etc2. Diverting Attention to Non-vital Part Some animals divert the attack of their predators to non-vital parts of their bodies. A round object attracts maximum attention. The wings of mantids, moths, butterflies etc possess eye-like round patterns at the extreme ends of their wings. Even if the predator attacks, the prey escapes with a minor injury to the extreme end of its wing. In some lizards, the tail end is brilliantly coloured to attract attention. When attacked, the lizard escapes with a minor injury to the tail end. Some animals create the impression of a head a t a wrong part of its body. Head is a vital organ both for the predator and prey. A predator, while capturing the prey by ambush, conceals its head. Some predators possess characteristics by which they allure the prey to the most dangerous part of the body. Some animals like mantids resemble buds of flowers. When insects visit these buds they become food to these animals2. 3.4.2

Mimicry We have seen that some animals have a n inbuilt mechanism by which they can sting or bite and also have warning colouration in order to warn their victims against future attempts at harming them. There are some animals which by merely mimicking the warning colouration of the above animals escape attacks from their predators and enemies. Mimicry has been extensively studied in the case of butterflies3. The most common type of mimicry is called Batesiari mimicry in which a harmless or a nonpoisonous species mimics a harmful or a poisonous one. This form of mimicry was first demonstrated by Bates over a hundred years ago. The North American monarch butterfly is inedible and also warningly coloured. The viceroy butterfly which is palatable mimics the monarch butterfly. Thus by mimicking the monarch in warning colouration, the viceroy gets protection from predators. There is another type of mimicry known as Mullerian mimicry, after the name of the naturalist Fritz Muller. In this form of mimicry two species of butterflies that are both unpalatable mimic each other. Animals try to mimic only those parts which can actually be seen or observed. Mimicry is employed by several species of insects, birds and snakes. Some moths mimic wasps, some spiders look like ants, and some flies bear resemblance to bees. 3.4.3

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36


3.5

OTHER FORMS OF CAMOUFLAGE Besides visual camouflage which we have discussed so far, there are other forms such as ultraviolet camouflage, auditory camouflage and olfactory camouflage3. Many insects have vision in the ultraviolet region. So the colours and patterns on flowers and insects as perceived by them are quite different from our perception. A s regards auditory camouflage, we may cite the example of small birds which, in their effort to help their fellow birds, produce sound signals a t the approach of a predator. These alarm calls are produced in such a way that the predator cannot locate the source of the signals. MarlerZ5has discussed the physical basis of auditory camouflage. His conclusions have been found to be generally correct2'j. Other examples of auditory camouflage are some species of mynas and bulbuls which imitate the calls of other birdsz7. Of the three senses -- sight, sound, and smell - it is the smell that plays the important role in the case of most insects. In all species of ants, most individuals in the colony are sterile and labour for the colony, helping the queen to produce more offspring. In some species of ants, adequate numbers of workers are not produced. As such, these species capture workers from other species which produce a large number of workers. The captured workers, when they become adults, work for the colony. StudiesZ8showed that the Fomica Sunguinea group sprays large quantities of decyl, dodecyl and tetradecyl acetates on the colony raided. This disturbs the olfactory sense of workers in the colony. In the process, sufficient number of pupae are captured to meet the requirements of slaves whose number in the other colony is less. 3.6

CAMOUFLAGE IN PLANTS Camouflage is also found to occur in plants. But it cannot be as effective as in the case of animals, the reason being that the former are immobile, and the latter have the mobility which helps them to fool their predators continuously. The tips of leaves and tendrils of passion flowers mimic eggs of butterflies. Thereby they prevent the Heliconius butterfly from laying eggs on them. Some types of orchids mimic female bees and wasps. Male bees and wasps rest on these flowers for copulation and in the process pollination takes place3.

3.7

EVOLUTION OF CAMOUFLAGE The phenomenon of industrial melanism is an example to illustrate the evolution of camouflage in nature. According to the


theory of natural selection only a small fraction of the individuals of any species produced in each generation will survive. This is because the relatively inferior and less adapted ones get eliminated. Mutations giving rise to differences in adaptiveness take place a t random, a s a result of which the original variety may totally be replaced. Let u s consider one example. Before the mid-19th century the moth Biston betularia in its peppered form was found in England. This rests on bark covered with lichen and is well camouflaged. At the same time there used to exist in small numbers a dark form known a s carbonaria. The lichen-covered barks used to render the dark form conspicuous and hence unprotected. With the onset of industrialisation, the barks of trees began to turn darker and darker with the deposition of soot. This rendered the dark form of the moth camouflaged, while the peppered form got uncamouflaged, t h u s becoming a victim to its predators. In consequence, the proportion of peppered form started decreasing and the darker form began to increase. In the course of a few decades the darker form came to constitute 98 per cent of the pop~lation~ It~was . further found that with reduction in air pollution there was a corresponding reduction in the proportion of the dark form30.

3.8

CONCLUSION The present chapter h a s provided several examples of camouflage in nature in the form of concealment, advertisement and disguise. But not in all the cases has it proved beyond doubt its protective value against predators, enemies etc. In order to obtain clear proof in every case extensive field and laboratory studies are necessary. However, a few general conclusions about camouflage in nature may be drawn, based upon the existing evidence in support of the protective value ascribed to it3. These are: (a) Camouflage is of widespread occurrence in nature; (b) Camouflage combined with appropriate behaviour characteristics provides adequate protection; (c) A single strategy alone is not always successful; and Whatever be the means adopted for sustenance and security by animals in nature, deception is inherent in all of them. REFERENCES

1.

Owen, D. Camouflage and mimicry. Oxford University Press, 1980.

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38

Ihtroduction to Camouflage and Deception

Cott, H. B. Adaptive coloration in animals. Methuen & Co. Ltd., London, 1966. Gadagkar, R. Lessons in the art of deception. Proceedings of the Seminar on Camouflage. 19-2 1 October, 1989, Defence Laboratory, Jodhpur, India. Poulton, E.B. The colors of animals. Int. Sci. Ser. 1890. LXVIII (XIII), +360. Poulton, E.B. The experimental proof that the colours of certain lepidopterous larvae are largely due to,modifiedplant pigments derived from food. Proc. Royal Soc. London, 1893. LIV, 4 17. Noble, G.K. Biology of the amphibia, New York, 1931. Bozler, E. Ueber die Tatigkiet der ernzelnen glatten muskelfaser bei der Kontraktion, ii Mitteilung: Die Chromatophorenmuskeln der Cephalopoden. Zeits. F. Vergl, Physiol. 1928. 7,379-406. Poulton, E.B. & Sanders, C.B. An experimental enquiry into the struggle for existence in certain common insects. Rept. Brit. Assoc. Adv. Sci. 1898. p 906-09. Cesnola, A.P.D. Preliminary note on protective value of colour in Mantis religiosa. Biometrika. 1904, 3, 58. Young, R.T. Some experiments on protective colouration. J. Exp. 2001. 1916, 20, 457. Sumner, F.B. Does protective colouration protect ? Results of some experiments with fishes and birds. Proc. Nat. Acad. Sci, Washington, 1934, 20, 559. Sumner, F.B. Evidence for the protective value of changeable colouration in fishes. Amer. Nut. 1935, LXIX, 245. Sumner, F.B. Studies of protective color change In Experiments with fishes both as predator and prey. Proc. Nut. Acad. Sci., Washington, 1935,21, 345. Camck, R. Experiments to test the efficiency of protective adaptations in insects. Trans. Royal Ent. Soc., London, 1936. 85, 131. Isely, F.B. Survival value of acridian protective colouration, Ecology, 1938, 19, 370. Collenette, C.L. Notes concerning attacks by British birds on butterflies. Proc. Zool. Soc., London. 1935, 20 1.


Cheesman, R.E. In unknown Arabia. London, 1926, XX,+447. Thayer, A.H. The law which underlies protective colouration. The Auk, 1896, 2 , 124. Thayer, A.H. Further remarks on the law which underlies protective colouration. Ibid, 1896, 2 , 3 18. Thayer, A.H. Protective colouration in its relation to mimicry, common warning colours a n d sexual selection. Trans. Ent. Soc. London, 1903, 553. Thayer, A.H. An arrangenment of the theories of mimicry and warning colours. Popular Science, New York, 1909, 75, 550. Mottram, J . C . Some observations on pattern-blending with reference to obliterative shading and concealment of outline. Proc. 2001. Soc., London, 19 15, 679. Graham, R.R. The silent flight of owls. J. Roy. Aero. Soc., 1934, 38, 837. Dean, B. Accidental resemblances among animals: a chapter in un-natural history. Popular Science, 1908, LXXII, 304. Marler, P. Characteristics of some animal calls. Nature, 1955, 176, 6. Halliday T.R. & S l a t e r , P . J . B . Animal b e h a v i o u r . Communications (Vol 2). Blackwell Scientific Publications, Oxford, 1983. Ali, S. The book of Indian birds, Bombay Natural History Society, Bombay, 1979. Regnier F.E. & Wilson, E.O. Chemical communication and "propaganda" in slave-maker ants. Science, 197 1, 172, 267. Kettlewell, B. The evolution of melanism. Clarendon Press, Oxford, 1973. Brakefield, P.M. Industrial melanism: do we have the answers? Trends Eco., 1987, 2 , 117.

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VISUAL CAMOUFLAGE 4.1

INTRODUCTION Despite all the developments in sensor technology - both electromagnetic and non-electromagnetic, and their potentialities the eye, unaided or aided, still remains a most frequently used sensor. A s such, visual camouflage still occupies a very important place in modern war. This chapter deals with visual camouflage embodying the properties of the visible region of the electromagnetic spectrum relevant to the topic and sensors employed, revealing features of various military objects, and detailing principles and methods employed in visual camouflage to reduce target detectability and thereby to enhance survivability. 4.2

VISUAL CAMOUFLAGE Visual camouflage deals with all possible methods, devices and techniques which prevent or delay the recognition of the true nature of the military object, when the human eye, naked or aided is the sensor employed by any optical device. Visual camouflage is concerned with the wavelength range 0.4 - 0.7 pm of the electromagnetic spectrum. The physiology of human vision, visual acuity, and dark and light adaptations of the eye are discussed below. THE HUMAN EYE Figure 4.1 gives the principal parts of human eye1v2.The essential structural components of the eye are: (a) A crystalline lens, consisting of a gelatinous transparent material which focuses the rays of light from a n object into the interior of the eye; (b) Vitreous humour which is located on the backside of the lens; (c) Aqueous humour located in front of the lens and covered by the cornea;

4.3

42

Introduction to Camouflage a n d Deception RETINA

\ VBTREOUS HUMOUR

LENS

\/F AQUEOUS 'HUMOUR

FOVEA OBPsC NERVES

BLIND SPOT

I

ClLlARY MUSCLES

Figure 4.1. Principal parts of human eye. Source:

(d)

(e)

(f)

Surveillance and target acquisition systems. A.L. Rodgers, I.B.R. Fowler, T.K. Garland-collins, J.A. Gould, D.A James a n d W. Roper (reproduced with perm~ssionfrom Brassey's, London). 0 : 1984, Brassey's (UK)Ltd, London

Ciliary muscles which control the shape of the eye lens for focusing objects located at different distances. Actually, it is the curvature of the cornea which is primarily responsible for effecting the necessary bending of light into the interior of the eye; The retina extends over the posterior of the sphere of the eye and has a complex structure consisting of several layers. It converts light falling on it into electrical signals which are conveyed by the optic nerve to the cortex of the brain where the sensation of vision is produced; The rods and the cones lie in the bottom layer of the retina. Rods are responsible for visual perception at luminance levels as low as Cd/m2.The sensitivity of the rods enables us to see even in moonlight and starlight. Low level luminance perception is known as scotopic or night vision. In scotopic vision, colour perception is not possible, i.e., the rods provide only neutral colour perceptions - white, grey or black. The upper luminance level at which rods cease to function is approximately 10 Cd/m2. The cones cause the sensation of colour. Cones get activated at higher levels of luminance such a s daylight vision. Colour vision is known as photopic vision. The cones become active at about Cd/m2. The region of luminance levels within which both the rods and the cones are active is, 10 Cd/m2.This vision is known a s mesopic.

Visual Camouflage

There are approximately 1 x 108 rods, and 6 x lo6 cones in each eye. There are about lo6nerve fibres, each fibre being connected to a large number of rods. The cones are concentrated in the central portion of the retina while the rods are more uniformly distributed throughout. The upper limit of luminance levels of photopic vision is 3 x lo5 Cd/m2. The important characteristics of the eye are visual acuity, and its variation with contrast, dark adaptation, light adaptation, and sensitivity of rods and cones to light of different wavelengths. 4.3.1 Visual Acuity The term visual acuity means the ability of the eye to perceive fine detail or grain in an object scene. It corresponds to the resolving power of an optical instrument. Quantitatively, visual acuity can be expressed a s the smallest angle which two close-by points in a scene subtend such that they are seen a s two separate points. The smaller this angle, the greater is said to be the resolving power. Alternatively, the reciprocal of this angle also gives a measure of visual acuity. This is expressed in reciprocal minutes of arc. Visual acuity is a function of the wavelength of light, diameter of the pupil of the eye (visual angle), and luminance level of the background. For one and the same person, as the luminance level of the scene increases, the resolving power increases. Visual acuity at night will be very low, as compared to daybme. The graph of visual acuity vs luminance (Fig. 4.2) gives the manner in which the visual acuity increases with luminance. The limit of resolution of the human eye, under normal daylight conditions, may be in the range of 60-90 seconds of arc depending upon the diameter of the pupil of the eye. Visual acuity is also a function of the brightness contrast. The higher the brightness contrast, the better will be the visual acuity. 4.3.2 Dark a n d Eight Adaptations The eye cannot adapt itself all of a sudden to changes in luminance levels. A s a person enters a dark room from outside where the luminance level corresponds to sunlight, or when a person, initially in a dark room, comes out where luminance levels are high, he finds it difficult to see the objects until he acquires dark or light adaptation depending on the situation. How much time the eye takes to acquire dark adaptation depends upon the pre-adapting light level. The higher this level, the greater will be the time taken by the eye to acquire dark adaptation. Dark adaptation is a function of wavelength of the pre-adapting light. The longer this wavelength, the less will be the time required to acquire dark adaptation. Hence, it is advisable for soldiers to wear red goggles2 for quicker dark

43

44


Luminance (CdmJ)

Figure 4.2. Variation of visual acuity with luminance. Source:

Surveillance and target asquisition systems. A.L. Rodgers, I.B.R. Fowler,

T.K Garland-Collics, J.A.Gould, D.A.James and W Roper (reproduced with permission from Brassey's, London).@ ; 1984, Brassey's (UK)Ltd, London.

adaptation. Light adaptation is much faster than dark adaptation, and a s such it does not pose much of a problem.Under normal daylight conditions, the eye exhibits maximum sensitivity a t 0.55 pm wavelength. But to a dark adapted eye, the wavelength of maximum sensitivity shifts to 0.5 pm (Purkinjeeffect). In other words, the luminance level required for vision at short wavelengths such a s blue light, under dim light conditions, is less compared to the luminance levels required a t longer wavelengths such as red light. A s such, red objects start losing their colour a s background illumination becomes less and less. So, a s the objects start losing their colour, recognition becomes more and more difficult. The eye possesses a field of view of approximately 120". It can respond to a wide range of luminance levels (10-6- 10-3Cd/m2).It

Visual Camouflage

has a contrast threshold between 2 and 3 per cent3. The eye has ability to detect a black line as thin a s 1 second of arc, located in a bright background. The science of human vision is quite complex and it has been the subject of several investigation^^-^ for purposes of evaluating visual camouflage. CHARACTERISTICS OF LIGHT RELEVANT TO 4.4 VISUAL CAMOUFLAGE Colour, texture and brightness are the important object characteristics of light which enable u s to distinguish one object from another or any object from its background. 4.4.1 Colour Som' has given a detailed account of colour, polour vision and colour measurement. Colour perception is caused by a physiopsychological response of the eye-brain system. According to the Young-Helmholtz theory of colour vision, there are three basic receptors in the cones having maximum sensitivity in the red, green a n d blue regions of the visible spectrum, respectively. All colour sensations are produced by the variations in the magnitudes of the response of these three receptors to external stimuli. This theory h a s been supported by experimental observations that a given colour can be produced by mixing red, green and blue radiation in certain proportions. Although colour sensation involves a physio-psychological process, we can correlate the sensation of colour to the spectral composition of the light causing the sensation. When the spectral energy distribution gets altered, the corresponding light appears to produce a different colour sensation. Let u s examine the colours produced by the spectral components of the white light spectrum. Spectral components having wavelengths from 630 to 700 nm are red in colour, from 590 to 630 nm are orange, from 570 to 590 nm are yellow, from 550 to 570 nm are yellow-green, from 5 10 to 550 nm are green, from 450 to 480 nm are blue, and from 400 to 450 nm are violet. Hue is the gradual change of colour from one to the next. Hue is determined by the position in the spectrum of those radiations that are stronger than the remainder. The depth or the saturation is determined by the degree to which these radiations predominate over the remainder. The quality and intensity of visual sensations are influenced by the quantity of light. In the case of a self-luminous cbject, the greater the amount of the emitted radiation, the brighter it would appear. In the case of diffusely reflecting objects, as is the case with most of the objects, the lightness or darkness of the object depends upon its reflectivity.

46


4.4.1.1

Geometrical representation of surface colours in tenns of lightness, hue and saturation A three-dimensional structure -a colour solid may be developed, within which any surface colour may be located in terms of its appearance, i.e., its lightness, hue and saturation1 (Fig. 4.3). The central vertical axis represents the neutral colours or the grey levels with black and white at the lower and upper ends. The distance between these two extreme points determines the scale of the solid. Colours of any one hue could be located in a vertical plane starting from this axis. The lightness of a colour sample could be indicated by a certain height above the black level. The saturation of the colour could be indicated by a length in the above plane in a direction perpendicular to the central axis. Besides lightness, h u e and saturation, coIour coordinates are utilized for specification of colour a n d its measurement. The mathematical aspects of colour coordinates are given in Appendix-A'.

RED

Figure 4.3. Three-dimensional colour solid. Source:

Proceedings of the Seminar on Camouflage held a t Defence Laboratory, Jodhpur, India; October 19-2 1, 1989. 'Light and its Measurement' by S.C. Som, Calcutta University, Calcutta, India.

4.4.1.2 Measurement of colour The instruments used for the measurement of colour are: (i) visual trichromatic colorimeters which employ visual colour matching, (ii)photoelectric colorimeters which employ three photocell

Visual Camouflage

systems with spectral sensitivities corresponding to the International Commission on Lighting (CommissionInternationale de'1 Exclairage, CIE) X, Y and Z distribution curves, and (iii) visual or physical spectro-photometers for determining the spectral composition of light coming from the sample1. Colour matching of objects with background is one of the most important aspects of visual camouflage. In this context a n understanding of the concept of colour, its quantification and measurement would be useful. 4.4.2 Texture Optical texture is another important characteristic of an object surface or background. To make two surfaces indistinguishable, it is not only colour matching that is important but texture matching is also necessary. Two surfaces having the same colour, if they have different textures, give rise to different appearances, and, thereby, they can be distinguished. Approximately, the ups and downs of a surface may be qualitatively referred to a s the optical texture. Let us take an example to understand what is qualitatively meant by optical texture. When aerial colour photographs of a rice field and a wheat field are observed, we find that although both have the same colour, we can distinguish and identi@ them a s rice and wheat fields by their textural difference. The light and shade effects of a rice field are different from those of the wheat field. The physical characteristics, such as the leaf size, shape, length, and inclination of the leaves to the direction of sunlight are different for rice and wheat plants. Because of these differences, the light and shade effects are different when sunlight falls upon the two species. These effects alter the overall appearance, although basically the colour is the same. Studies on quantification and measurement of texture have been extensively carried outg.

Brightness (Contrast) Difference between the brightness of a n object a n d its background gives rise to brightness contrast. The unit of Luminous intensity is Candela (Cd). Luminous flux is that part of radiant flux which is capable of producing visual sensation and is expressed in terms of the unit lumen. One lumen is the light energy emitted per second within unit solid angle by a uniform point source of one candela. Illuminance is the luminous flux striking unit area of a surface. It is expressed in lux or lumen m-2 . Luminance and brightness are the same and are expressed a s candela m-2.

4.4.3

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Introduction to Camouflage a n d Deception

If Bois the brightness of the object, and B,the brightness of the background, then the brightness contrast is given by

4.5

SENSORS IN THE VISIBLE REGION There are several sensors in the visible region, ofwhich the human eye is the basic surveillance system in the battlefield. The various systems which aid the human eye are: telescopes, binoculars, periscopes, image intensifiers,starlight viewers, low light level television systems, laser systems and many night observation devices. Besides, target characteristics in the visible region can be characterised by airborne instruments such as cameras, line scanners etc. 4.5.1 Electrooptical Instruments For round-the-clock battlefield operations the ability to see well in the dark is very important. At the luminance levels of moonlight and starlight, h u m a n eye performance i s not adequate for surveillance, reconnaissance and target acquisition at night. Several optical systems that can enhance the performance of the eye during the daytime have already been listed in the previous section. During night, the low level light reflected from a scene comes from a variety of sources. These are the moon, planets, stars and skyglow. Objects cannot be seen clearly under these light conditions, rendering night operations difficult. Table 4.1 gives the light levels during day and night under different conditions. Table 4.1. Illuminance levels during day and night

Source:

Level

Lux (I m/ma)

Clear sunlight Good interior working illumination Twilight (dusk) Moonlight (fullmoon) Clear starlight Overcast starlight

1o5 1o3 10'

Very overcast starlight

10 j

10-I 10-3 1o^'

Surveillance and target acquisition systems. A.L. Rodgers, I.B.R. Fowler, T.K. Garland-collins, J . A . Gould, D.A. James & W. Roper (reproduced with permission from Brassey's, O 1984, Brassey's(UK), London).

From the Table 4.1, it is seen that in order to raise the luminance level from overcast starlight to twilight, the gain required is los. Optical aids such as night binoculars can enhance the vision to some extent, but they are not adequate. In order to enhance the

Visual Camouflage

scene brightness at night, electrooptical devices s u c h as image intensifiers are used. These devices make use of the visual part a s well a s near infrared part of the electromagnetic spectrum. Such a system which improves visibility a t 'low field luminance levels is known a s the image intensifier. 4.5.1.1 Image intensifiers Figure 4.4 shows a n image intensifier system2. The objective lens collects radiation from the scene a n d focuses it on the photocathode. The photocathode, on absorbing radiation, emits electrons. The released electrons are accelerated with the help of an electric field. The accelerated electrons, upon striking a phosphor screen, emit visible radiation. The image is viewed through a n eyepiece. Enhanced visibility is obtained by gathering more light from the scene with the objective lens than with the unaided eye, further by the higher photosensitivity and broader spectral response of the photocathode compared to eye, and thirdly by amplification of photoevents. There are several photocathode materials. The S 1 cathode which was used earlier consisted of silver, oxygen and caesium. Then came S20 and S25 photocathodes having better response than S1. A recently developed photocathode material is caesiated gallium arsenide which h a s a large a n d uniform wavelength response. ,

0 mm

.-.-

Optic Window

Focussing Electrode

7

/\

Electron

>-

0

Target Objective lens

't

Photocathode

-ight Out A

Phosphor

Figure 4.4. Image intensifier system. Source:

Surveillance and target acquisition systems. A.L. Rodgers, I.B.R. Fowler,T.K. Garland-collins, J.A.Gould, D.A. J a m e s & W. Roper (reproduced with permission from Brassey's, O 1984, Brassey's (UK), London).

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Single stage tubes have the disadvantage that the gain is not adequate for recognition of objects under starlight conditions. High speed detection is not possible a s the eye needs dark adaptation. These disadvantages are overcome by employing multistage tubes. By using a three-stage tube, we can enhance the brightness of a scene from overcast starlight to twilight. Fibreoptics couples the individual stages. Figure 4.5 shows a threestage image intensifier tube2. From stage to stage the voltage increases by 15 kV. Further amplication is achieved by making use of secondary emission of electrons. The fast moving electrons, upon colliding with the electrons of outer orbits of atoms, knock out electrons. These secondary electrons get accelerated under the applied high voltage, which, on further collisions, result in further emission of electrons. The secondary emission is induced in the tubes of semiconductor glass. Such tubes are inserted between the photocathode and a phosphor screen. In a single stage, gains of the order of lo5 can be obtained. This type of image intensifier tube is smaller in size, lighter in weight and superior in resolution to the earlier versions, besides producing a high brightness. The performance of the image intensifier can be further improved by illuminating the objects employing search lights or laser illuminators or pyrotechnics wherever possible. 195 rnm

Fibre Optic Face Plate /

Figure 4.5. Three-stage image intensifier tube. Source:

Surveillance and target acquisition systems. A.L. Rodgers, I.B.R. Fowler,T.K. Garland-collins, J.A.Gould, D.A. James & W. Roper (reproduced with permission from Brassey's, Q 1984, Brassey's (UK),London).

Low Light Level Television For low light conditions, television camera tubes can also be used2. In a television tube, photons from the scene are collected and focused on to a target by means of a n optical system. The target 4.6.1.2

Visual Camouflage

ejects electrons. This makes the target positively charged. The positive charges accumulated on various regions of the target are proportional to the light intensity variations of the scene. In other words, an electrostatic image of the scene is formed on the target. An electron beam scans the electrostatic image on the target. The positive charges on the target get neutralised. As more number of positive charges are neutralised, more number of electrons are drawn from the scanning beam. This results in a current known as videocurrent which is proportional to the brightness variations in the scene. Low light level television system has certain advantages such as remote viewing and multiple read-out. Also, improving the picture quality is comparatively easier by controlling the tube brightness and amplifier gain. But one serious disadvantage is that it is heavier and bulkier, and also more expensive. The vidicon and orthicon are two such television tubes which can operate over a wide variation in luminance levels ranging from full daylight conditions to twilight conditions. Further developments in these devices resulted in charge coupled devices (CCD) which do not require electron beam scanning. In these devices, the information is stored compactly as electric charges which can be transferred to the output in the required manner. These cameras are rugged and smaller in size. For short range applications, image intensifiers under low light levels enhance the performance of the eye. In short, these devices are ideally suited for short range weapons and short range viewing sights. 4.5.2 Lasers Lasers with their characteristic properties - high intensity, coherence and monochromaticity find several applications in surveillance and target acquisition in the battlefield. These applications inciude ranging, designating, illuminating and tracking2. 4.5.2.1 Rangefinding A laser rangefinder sends a laser pulse of narrow width and high peak power towards the target, the reflected pulse from the target is received, and from the time taken by the pulse from the instant of its transmission till its return, the range is found. Such rangefinders can have a range of 10 k m with a beam divergence of 0.5 mrad. Laser rangefinders are mounted on tanks and fitted in helicopters. 4.5.2.2 T a r g e t Designation Target designation is done by illuminating the target with a laser beam, and a detector in the nose of the aircraft or artillery

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shell homes on the reflected light from the target. It can work u p to 10 km range. The accuracy of designation depends upon the narrowness of the beam. The designator can be used by a soldier on the ground or it can be used from a n aircraft. 4.5.2.3 Target Illumination Laser illuminators enhance the performance of image intensifiers by illuminating the target when the ambient light is not adequate. 4.5.2.4 Tracking Laser tracking is highly advantageous especially for operations at night and low level light conditions. The requirements for tracking vary with the nature of the target, viz., missile, aircraft, hostile target, and cooperative target. In all the cases, along with a conventional radar, the laser tracker is used. Initially, over a wide field of view, radar is used to acquire the target. Once it is acquired, then it is handed over to a laser tracker. The principle of laser tracking is the same as radar tracking. A laser beam has high directivity compared to microwaves used by radar. The visible and the near infrared waves used by laser are much shorter, and hence the size of the laser tracker systems is smaller. The smaller wavelength and good directionality associated with laser systems make them less susceptible to interference than the radar systems where multipath effects become pronounced.

Further developments in lasers include application of laser holography with the help of which three-dimensional view of a scene can be displayed in front of the pilot in a n aircraft.

Photography Photographylo is widely used in military reconnaissance. Cameras of different types which can be mounted on different platforms are used in photographic reconnaissance. The mission can be strategic or tactical. Photography has several distinct advantages over other sensors.

4.5.3

4.5.3.1 Platforms The platform can be either: (i) ground-based, (ii) airborne, or (iii)space-borne. Ground-based cameras are not different from carnezas used in normal photography. The airborne platforms are aircraft, drones and remotely piloted vehicles (RPVs).The space-borne platforms are satellites.

Visual Camouflage

4.5.3.2 Photo-reconnaissance - Aerial Photographs can be taken from different altitudes. They can be taken vertically, obliquely, side-looking, fonvard-looking, and panoramic, depending on the requirement. The velocity of the platform carrying the camera can also vary between wide limits. The nature of photo-reconnaissance is determined by the nature of the mission. Table 4.2 gives the different types of photoreconnaissance. Table 4.2. Different types of photo-reconnaissance Mission

Performance

Strategic

High altitude, medium or high velocity. Long standoff, medium or high velocity

Tactical

Source:

Photography Vertical Oblique, long range

Low altitude, high velocity

Vertica1,side looking, panoramic, forward-looking

Medium altitude, medium velocity

Oblique

Military Reconnaissance - Methods and Devices. Barracuda Camouflage, 1982, DIAB Barracuda, Solna, Sweden.

Table 4.3 gives some typical cameras used in Photoreconnaissance. Table 4.3. Cameras for photo-reconnaissance -

- --

Type of Camera -

-

Focal length (rnm)

Source:

-

35 mm 600 mm 135 mm

-

Field of view Main use

- - -

Wide angle Long-focus Panoramic

-

60" 4.5" 15"X190"

-

Low altitude High altitude Medium altitude


For taking high resolution photos from a satellite, long focus cameras are used. On tactical reconnaissance, an aircraft can have a variety of cameras. Figure 4.6 shows reconnaissance aircraft with pod. Figure 4.7 shows pod with cameras.

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&'

5'

1. Two High Altitude//Long-Range Cameras

5. Night llluminat~onEquipment (Flash Systems)

2. Four Low-AltitudeCameras

6. ECM Data Registration Units

3. Infra-Red Line Scanner

7. Vertical Sighting System

4. Three Low- Altitude Cameras with Infra-Red Film

Figure 4.6. Reconnaissance aircraft with pod. Source:


Frame Camera 2 Positions

Low Altitude Panoramic Camera

H~ghAltitude Panoram~cCameras

2 TV Cameras 2 Positions

Figure 4.7. Pod with cameras. Source:

Military Reconnaissance - Methods and Devices. Barracuda Camouflage, 1982, DlAB Barracuda, Solna, Sweden.

Factors Affecting Photographic Reconnaissance The various factors l o which influence photographic reconnaissance are: Scene characteristics Atmosphere Camera characteristics Platform characteristics Film characteristics 4.5.3.3

Scene characteristics The important scene characteristics are: illumination, brightness contrast, colour contrast and shadows. Unless the scene has adequate illumination, either from sun or by artificial means,

Visual Camouflage

satisfactory photographs cannot be taken. However, low light levels can be compensated for by using more sensitive films. But a t the same time, at higher sensitivities resolution becomes less. Brightness and colour contrast in the scene help in the detection and identification of targets through photography. The nature of shadows depends upon the position of the sun a t the time the photograph is taken. Shadows are very useful in the evaluation and identification process. Atmosphere As the altitude increases, the atmosphere adversely affects contrast between details in aerial photography. Weather conditions, especially haze, adversely affect the quality of photograph. The camera characteristics and platform characteristics are discussed below. 4.5.3.4 Aerial Camera Resolution is the most important quality of a photographic system. It depends upon several factorsI0, viz., Focal length and quality of the lens Photographic film Movement of the platform Vibrations Airstream outside the platfonn Atmosphere Scene contrast and illumination

The smallest details that can be reproduced on the film depend on all these factors. On the negative of the film, we can theoretically resolve 0.0 1 - 0.02 mm if the camera lens as well a s the film employed are of high quality. A s the platform moves, there will be image motion which blurs the image. Besides the movement of the platform, vibration and angular rotation of the viewing system also affect the quality of the image. These effects can be reduced by employing image motion compensation techniques. Vibration can be reduced by proper mounting of camera. The passage of airstream in front of the camera depends on the type of aircraft, aircraft altitude and speed, and the camera mounting in the fuselage. Resolution is expressed normally in radians or milliradians or degrees. It is more conveniently expressed in terms of linear dimensions of the target on the ground. For imaging sensors it is expressed in line pairs per mm.

55

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Introduction to Camouflage a n d Deception

Figure 4.8 gives the ground resolution in meters a t different target distances from the ground for camera lenses of three 'different focal lengths. Table 4.4 gives details of cameras used in aircraft and drones in terms of focal length, type of mounting and mission. Distance to target (km)

Ground Resolution (m)

Figure 4.8. Ground resolution at different target distances. Source:

Militaly Reconnaissance - Methods and Devices. Barracuda Camouflage, 1982, DIAB Barracuda, Solna, Sweden.

Table 4.4. Focal l e n e h and camera mountine for different missions Focal length (mm) 35 70

Source:

Camera mounting

Mission type

Vertical; oblique Vertical; forward oblique; oblique panoramic Vertical; forward oblique Vertical, panoramic Vertical, panoramic Vertical

Low altitude, high velocity Low/medium altitude, high velocity Medium altitude Medium altitude High altitude High altitude

Military Reconnaissance - Methods and Devices. Barracuda Camouflage, 1982, DIAB Barracuda. Solna, Sweden.

Visual Camouflage

Magazine and framing are two other important factors associated with the camera. An overlap of 60 per cent can produce stereoscopic vision. The camera magazine can have 100 m of 70 mm film. A frame rate of 6 pictures per second is adequate for typical high velocity/low altitude mission. One important type of frame camera is multiband camera. This camera photographs a scene simultaneously in individual spectral bands, rather than in a single broad band. Depending on the number of spectral bands in which photographs of the scene are taken, it can have four lenses or seven lenses or even u p to a maximum of nine lenses. On the same film, photos in all the spectral bands are taken. Alternatively, there c a n be a number of independent cameras, corresponding to each spectral band. Such photographs, when observed simultaneously under a n additive colour viewer, provide tonal enhancement. Objects which otherwise are not distinguished in single broad-band photography can be distinguished in multiband photography. The 12S airborne multiband camera (manufactured by International Imaging System, USA) has four separate lens-filter assemblies. It forms four images, each of size 89 mm x 89 mm on the film of area 240 mrn x 240 mm. The multiband camera flown in the Skylab had six lenses. It gives six photographs of the same scene in six different spectral bands covering the visible and near infrared regions. Latest in the development of photographic cameras is the large format camera placed on board the Space Shuttle 17. It was developed by Itek's optical division under a NASA contract. Each picture frame covers about 165 km x 335 krn from the 220 km orbit and has a spatial resolution of about 1 lm. Film

The commonly used films for military reconnaissance are panchromatic and infrared black and white. Colour and infrared false colour films a r e also used for special purposes. The panchromatic film r e s p o n d s to t h e visible region of t h e electromagnetic spectrum (0.35 to 0.67 pm). It gives the image in grey tones. Black and white infrared film is specially sensitized to register the near infrared region. Figure 4.9 gives the colour sensitivity of different films.

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300

400

500

600

700

800

900

Wavelength (nm) Figure 4.9. Relative colour sensitivity of different films. Source:

Military Reconnaissance - Methods and Devices. Barracuda Camouflage, 1982, DlAB Barracuda, Solna, Sweden.

Adugntages/disadvantages of Photographic Reconnaissance Advantages: (a) Very high resolution. (b) Pictures can be had in 3-dimensions under stereoscopic observation. (c) By comparing the images obtained in different parts of the spectrum, additional information can be obtained. Disadvantages (a) Photography needs illumination on the scene. During day time it is sunlight but during night, light from flash or laser is necessary. (b) Visibility must be good. Visibility will be affected by clouds, fog, haze etc. (c) Information processing takes long time.If the film processing is done only after the aircraft has landed it needs at least two 4.5.3.6

Visual Camouflage

(d)

hours from the time the mission starts till decisions can be made on the basis of the information obtained from the photograph. Although in-flight processing a n d d a t a transmission to the ground are possible, they are not used to a great extent. Since the data are not available directly in the electronic form, they are not immediately amenable to digital processing. For unmanned space flight, the film has to be ejected out for recovery which complicates the system.

4.5.4 TV Cameras TV cameras located in the nose of an RPVIO can be used for aerial reconnaissance. The picture can be relayed to the operator on his monitor. It can also be recorded for later evaluation. Tiros- 111, a meteorological satellite launched in 1960, carried a vidicon camera for routinely viewing the earth for world weather studies. The best example of high resolution TV camera was the return beam vidicon (RBV) used in the LANDSAT series". The spectral bands (LANDSAT 1 and 2) were 0.475 - 0.575 ym, 0.580 0.680 pm and 0.698 - 0.830 ym. 4.5.5

Optical Mechanical Scanners O n a multispectral scanner (MSS), the radiation reflected/ emitted from the scene is intercepted by a scan mirror which diverts the radiation to a collecting telescope from where it enters a spectral dispersing system. The radiation dispersed into different spectral bands falls on the corresponding detectors. In the visible region, both photomultipliers and photodiodes are used as detectors. These cover the range 0.4 to 1.1 pm. Typical examples of airborne MSS include 1P Channel M2S developed by Bendix (USA).The best possible improvement has been probably achieved in Thematic Mapper (TM). It has seven spectral bands in visible, near infrared, middle infrared and thermal infrared. It provides 30 m resolution in the visible, near and middle infrared bands, and 120 m resolution in the thermal infrared from an orbiting altitude of 705 km. Linear Imaging Self-scanning Sensor (LISS) 4.5.6 In this the basic sensor is a linear array of solid-state detectors1'.The array may be made ofphotodiodes, phototransistors or charge-coupled-devices (CCDs). CCDs are discussed in the chapter on infrared camouflage. The French Space Agency launched an observation satellite system (SPOT)which carried a CCD-based camera. This camera has a resolution of 10 m in the visible region and 20 m ia? the near infrared region a t a flight altitude of 805 km.

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4.5.7 Military Satellites Ever since space age began, more than 1600 satellites" have been launched, 60 per cent of which are military satellites. USA collects data from four separate types of reconnaissance satellites. These were area-survey satellite, close-look satellite, Big-Bird, and KH- 11. They all carried an imaging sensor. They had the ability to manoeuvre in space to place themselves in a particular orbit at a particular time.

The area-survey satellites were used during the 1960s and early 1970s to obtain broad photographic coverage of large areas. Each satellite had a short life (around a month) and carried cameras that took photographs with wide angle lens. The film was automatically processed on board the satellite, scanned by a digitiser and the data transmitted to the earth. These satellites burnt upon re-entry and so were not reusable. The 'close-look' satellite employed cameras to obtain high resolution photographs of areas of interest. A t a n altitude of 150 km the photographs were expected to have a resolution between 0.25 m and 0.5 m. Once a roll of film had been exposed, it was ejected in a capsule that was either retrieved in mid-air by an aircraft or was picked u p a t sea. The lifetime of these satellites was only five days in the early 1970s. The satellite, Big-Bird combines the reconnaissance attributes of both the area-survey and close-look satellites a s it carries sensors of both high and low resolution images. The Big-Bird was first launched i n the early 1970s. The sensors were primarily photographic, and, a s with the close-look satellite, the film was returned to earth by capsule. The satellite series KH-11 which was first launched in 1976 was used by the military but operated by the CIA. These satellites fly on a relatively fixed orbit at an altitude of 250 km and cany a multispectral scanner similar to Thematic Mapper. It has a resolution of 0.20 m and sensor is a CCD. FACTORS AFFECTING RECOGNITION IN THE VISIBLE REGION In the visible region, object detection/recognition/identification, to a large extent, is carried out by the sensor - the human eye unaided, or aided by an optical instrument or an electro-optical system, or through a photograph of the object. Visual perception is a three-stage process. The first stage depends on the object, its physical properties and the background in which the object is located, and the properties 4.6

Visual Camouflage

of light. The second stage is a physiological process which depends on the construction of the eye and how it forms an image of the object on the retina. The fhird stage is a psychological process in which the image formed on the retina is interpreted by the brain leading to the recognition of the object. The retinal image, which is physical, is transformed into a mental image as interpreted by the brain based upon previous knowledge. Under this section we shall only deal with stage one - the properties of the object, its background and the light reflected by it which enters the eye. The physical properties of the object which contribute to recognition or identification. These are particularly relevant in the interpretation of aerial photographs are a s follows: 4.6.1

Shape Every object has a characteristic shape, bounded by surfaces and contours with which we are familiar. The observer's previous knowledge about the object and his familiarity with it will greatly help in the identification process, even if the quality of image as perceived by his sight is not good. Shape is a very important characteristic of an object by which identification can be conclusively done. 4.6.2 Size The size of a n object is another useful parameter on which identification may be based. Familiarity or previous knowledge about the size is a very useful clue. 4.6.3 Colour The light reflected by a n object depends on its colour. Different objects reflect different amounts and wavelengths of light energy. The contrast that is produced by differential reflection of light by different objects and its background aids in interpretation. 4.6.4 Texture Two objects may have the same colour, but if their optical textures are different, they can be distinguished a s two separate objects. The optical texture depends on the surface condition - degree of smoothness and roughness. The light and shade effqcts are produced by texture. 4.6.5 Shadow Even if the object is not clearly seen, it can be identified from the shadow of the object. This is especially an useful clue in aerial photo-interpretation.

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Pattern The pattern is a characteristic feature of many man-made objects and of some natural features. Individual objects in a pattern may not be discernible but if the pattern is perceived, the objects constituting the pattern can be inferred.

4.6.6

4.6.7 Site Knowledge of location of the object with respect to terrain features or other objects is helpfubl in object identification. Association All solid objects are presented to the eye a s elements of colour, varying in size, shape, texture and hue and saturation, and lightness and darkness12.These differences enable us to distinguish one object from another. If these differences are made to vanish, one object cannot be distinguished from another. Some of the objects are so commonly associated with one another that the presence of one indicates or confirms the other (e.g., tank with its associated track and gun barrel). All military objects, e.g., a tank, a n aircraft or a ship, have their characteristic shapes, sizes, textures, site and association and shadows from which (if they are not adequately camouflaged), they can be identified by visual means with the naked eye or an eye aided with a n optical instrument or electrooptical system or a photograph. 4.6.$

BASIC PRINCIPLES OF CAMOUFLAGE IN THE VISIBLE REGION The various methods by which military objects are camouflaged in the visible region may be divided into four groups: Hiding, Blending, Deception, and Miscellaneous techniques. 4.7

Any single method may not adequately camouflage the object, and, a s such, in any actual situation, two or three methods are simultaneously used. Hiding In hiding, the objects are physically hidden by the use of natural materials such as vegetation -natural and cut - and artificial materials such a s nets, screens etc. 4.9.1

Visual Camouflage

Camouflaging of military objects by vegetation is by far the best method. A s such, wherever adequate vegetation is available, it should be put to use. It is a countermeasure against a variety of sensors. Besides live vegetation, cut vegetation can also be used to a limited extent. The disadvantage with cut vegetation is that, with time, its chlorophyll content drops, giving rise to colour contrast. Besides, the concealed objects can be detected in infrared photography. 4.7.1.1 Arboriculture (in desert region) In regions where there is plenty of vegetation, it can be advantageously used for concealment of military objects, small and big. But where vegetation is scarce, especially in arid and semi-arid regions, Saxena and Solanki13give a detailed account of desert and semi-desert plant species a n d their possible application for concealment of military objects. In these regions, extremes of temperature, moisture, stress, and nutritional deficiency are the principal limitations for plant growth and development. Under these hostile conditions, only a limited number of species survive. The environmental stresses are responsible for a wide range of morphological and physiological adaptations such as reduction in plant height, leaf size, change in growth form, photosynthetic activities taken over by stem and deep root system etc. But these characteristics cannot meet camouflage requirements. Determining how best the prevailing vegetation of arid regions might be utilised for concealment will be the first step. The use of existing hardy shrub species may be drawn in such a fashion that the absence of a clear bole and the spreading crown cover are overmasked by their multiple-stemmed and dense top &over. Wherever the shrub cover is dense, small size military equipment may be concealed. Before planning of plantation of appropriate species which might provide adequate camouflage, we shall discuss the actual distribution pattern of natural plant communities in arid regions.

Distribution of vegetation in arid and semi-arid regions. While discussing the vegetation distribution in these areas, we may divide them into three zones13: (a) 100-200 mm rainfall zone, (b)200-300mm rainfall zone and (c) 300-450 mm rainfall zone. (a]

100-200mm rainfall zone This zone is characterised by dwarf shrub communities and grassland. On flat, gravelly and pediment plains, an open shrub community of Capparis decidua and Zizyphus nummularia are found. On the sandy tracts, Calliponum polyponoides, Haloxylon

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salicornicum, Calotropisprocera, and Leptadenia pyrotechnica grow easily. In this low rainfall zone there will be high intensity of sand dunes. Some of these sand dunes are bare while others have coverage of shrub species like Caligonum leptadenia and Acacia jacquemontii. These dunes are capable of supporting shrub species with fairly high density plantation of shrubs; besides providing stability to the dune, they also help in camouflaging aspects. Fully developed shrub species of CaligonumA jaquemontii, Calotropis or Leptadenia can be utilised for concealing military movements. The rocky and hilly habitats cannot support even shrub species adequately and therefore remain open. Growing of vegetation on such hilly areas is a difficult task due to complete loss of soil. 200-300 mm rainfall zone In this zone, the sand activity is comparatively less, and one can find a mixture of trees and shrubs; while shrub species dominate on sand dunes, a few widely scattered trees are also met with. Acacia senegal, Tecomella undulata, Prosopis cineraria and Salvadora oleoides grow on the lower flanks and/or leeward side of the stabilised sand dunes. The plains having sandy clay loam soils can sustain Salvadora, Prosopis, Capparis species. Alluvial flats with sandy loam soil have Prosopis having a height of 10-14 m with 15 to 20 plants/ ha.

300-450 mm rainfall zone This zone consists of a flat alluvial plain. It supports good growth of Prosopis cineraria (30-120 plants/ha). In hilly areas, one finds Acacia senegal and Anogeisus pendula. In this region, Prosopis, Tecomella, Maytenus and Acacia senegal grow well with fairly good crown which are adequate for concealment of military objects.

Camouflage plantation for desert region Acacia tortilix, Prosopis chilensis, Acacia raddiana, Acacia. nubica, Colophospernum mopane, Acacia salicina, Dichrostachys nutans Parkinsonia aculeta, Holoptelia integrifolia, Zizyphus and Spinachristii are suitable species for arid tract. For semi-arid tract, Eucalyptus camaldulensis, E. terminalis, Acacia auricutifomia, Cassia siamea, Leucaena leucocephala and Peltoforumferugianum are the suitable species. (b)

For rainfall above 350 mm some of the promising species are:

Azadirachta indica, Ailanthudus excelsa, Albizzia lebbek, Acacia nilotica, Hardwickia binata. All these species have fairly dense crowns and tall trunks. Thus for camouflage by arboriculture in arid/ semiarid areas, the above species, when properly grown, are suitable.

Visual Camouflage

Acacia tortilis and Frosopis julifora species are the best suited for arid zone afforestation. Of the two, Prosopis julifora h a s tremendous regenerating capacity in nature. This species has been found to colonize in all the habitats of desert regions including the saline areas. Ecological observations have indicated that Prosopis julifora density per unit area on the wastelands increases during drought years compared to normal years. With continuous prolonged droughts, Acacia tortilis succumb, with majority of its saplings becoming casualities, but it is comparatively sturdy. A. juliflora is more salt-tolerant. Acacia tortilis has large crown size in arid tract. Majority of Prosopisjulif2om are shrubby, and amongst the population a few plants bear clear bole and distinct spreading crown in umbrella shape. Selection of seed from such species will be ideal for camouflage purposes. Amongst exotic shmb species, Dichrostachys nutans and Colophospermum mopane are promising in desert tract. The former is exceptionally drought-hardy and under stress it throws root suckers in all directions. Colophospermurn mopane is also a droughthardy, disease-resistant and self-regenerating shrub for sandy terrain. In general, a mixture of indigenous and exotic species may fulfil the requirement of camouflage application. Depending upon the size of the military objects to be concealed, species may be selected, and their density and pattern of plantation may be decided. While growing vegetation for camouflage purposes, care must be taken that the vegetation should not become an isolated green patch, which might attract attention and become a target. 4.7.1.2 Screens A variety of screens, such as horizontal, vertical and overhead

types, are used for concealing stationary military objects. These screens act as physical barriers between the target and the sensor. Besides, there are smoke screens which provide temporary concealment in situations such as movement of troops.

(a)

Horizontal screens The horizontal screen is put up over the object parallel to the ground and provides protection against aerial reconnaissance. The screen should merge with the natural surroundings. It should be of irregular geometry and the size must be such that it adequately covers the object. Its height from the ground and with respect to the prevailing vegetation must be such that it should not stand out under stereoscopic examination. The screen may be made out of wire or rope, with proper supports and anchorage. It may be garnished with artificial material which blends with the background.

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Inttoduction to Camouflage and Deception

(b)

Vertical screens Vertical screens provide concealment against observation from ground. They are put up around the entire object, or on the side from which the enemy is likely to observe. Such screens may be made out of locally available material such as branches of trees, bushes or wire mesh. These are temporary measures against enemy observation. The movement of troops and vehicles can be concealed for short duration. Similar to the horizontal screen, the vertical screen should not look man-made but should blend with the background. (c)

Overhead covers This type of screen covers the object and the surrounding ground near the object. The cover should blend adequately with the background. 4.7.1.3 Obscurants (Smoke screens) Passive countermeasures are effective in the case of stationary targets. For a military object such as a vehicle when in motion, passive countermeasures are ineffective because any movement is easy to detect by methods ranging from visual observation to Doppler radar. One of the methods for hiding vehicles in motion for short duration is the deployment of smoke. This is known a s concealment by obscuration. An obscurant is a medium inserted between the target and the observer, which by its interaction with the radiation brings down the signal strength below the sensor threshold. The medium is a man-made aerosol which consists of dry and wet particles dispersed and suspended in air. Obscuration countermeasures are discussed a t length in references 14-16. Figure 4.10 shows the mechanisms that come into play in a n obscurant which reduces the strength of the radiation from the target. A s the radiation from the target passes through the obscurant, it undergoes absorption, scattering, emission and transmission. All these effects alter the spectral characteristics of the target, target contrast with its background, background clutter etc. The degree of alteration in the spectral characteristics of the target depends on the properties of the aerosol, viz., mass concentration, size, distribution of particles, vapour condensation, liquid droplet evaporation, optical scattering and absorption, and chemical potential of the aerosol. Besides the properties of the aerosol, the meteorological parameters, viz., wind, temperature, relative humidity, turbulence and radiation from sun, sky, clouds etc. alter the propagation characteristics of the target radiation

Visual Camouflage

Obscurant

Scattering

b Background

Transmittance (a) Direct Transmission Losses

(b) DiffuseMultiple Scattering and Path Radiance

Figure 4.10. Reduction of strength of radiation by an obscurant. Source:

Reproduced with permission from "The Infrared and Electro-optical Systems Handbook" published by ERlM and SPIE Optical Engineering Press, USA. Countermeasure Systems. Edited by David H Pollock, Vol7, Chapter 6 Obsucuration countermeasures by Donald W Hoock (Jr.)and Robert A Sutherland (1993).

passing through the aerosol. Further, the methods by which the aerosol is generated determine the rate of production of particles, particle settling etc. which also influence t h e propagation characteristics. The two important quantities associated with the obscurant on which the transmittance depends are obscurant concentration and its extinction coefficient. The transmission of electromagnetic radiation depends principally on the concentration and optical properties of the aerosol. The optical properties are influenced by the particle size, refractive index at different wavelengths, shape, mass, orientation of particles etc. Calculations have been made of the amount of obscurant needed to produce total concealment. A known amount of obscurant can be introduced which cuts off the transmission below the sensor threshold. When the area of the obscurant cloud is larger than the

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size of the target, total target obscuration can be accomplished. The available obscurants probably absorb electromagnetic radiations effectively in the visible and infrared, including laser radiations. Smoke may be deployed either by individual vehicles or on a large scale to screen the movement of armoured units. In the former case the smoke is deployed by means of vehicle-mounted grenade launchers. Of the different screening smokes, the conventional white type is effective only against visual observation. Smoke grenades (which can be launched from armoured vehicles) require two seconds to begin to become effective which add to the response time of countermeasures based on them. 4.7.2 Blending In camouflaging by blending, the object is made to blend with the background. The object becomes an integral part of the background and is hence rendered invisible, and thereby unrecognisable. Camouflagingby blending involves optical principles which produce illusory effects. Although the object is very much present, it cannot be seen in recognisable form. These are the very principles which nature has applied to camouflage various animals12. Several examples of this kind have been described in Chapter 3.

Colour Matching The first requirement for an object to blend with its general background is that the colour of the object should be the same as that of the background. How far is this feasible for effective camouflage? Secondly, is colour matching alone enough? Firstly, let us take stationary objects for example, a building uniformly painted with the same colour as that of the background. This object then becomes less conspicuous. Now let u s take an object in motion for example, a tank. As it moves, its background will not be uniform, but generally keep on changing in colour. So a tank painted with a single colour cannot blend with all backgrounds. Further, even in one and the same background, besides colour, there should be matching of texture. Seasonal variations also bring in changes in the colour of the background. All these factors make u s infer that an object painted uniformly with a single colour cannot provide adequate camouflage under all possible conditions, although it makes the object less conspicuous under limited conditions. One possible solution for taking into consideration different backgrounds having different colours is that as the object moves from one background to another, the colou on the object should automatically change to that of the background. Till today there are no such materials available, Materials like chromogenics may meet 4.7.2.1

Visual Camouflage

such a requirement in future.But, examples can be cited from nature where animals rapidly and sometimes instantaneously change their colours to those of background. Chameleons are one such example which exhibit marked and rapid colour adjustments ranging from dark brown to sea green1*-17. Besides colour matching which makes an object less conspicuous, let us consider the various other conditions that are to be satisfied for effective camouflage. 4.7.2.2

Countershading

Even when an object has the same colour as that of the background, due to light and shade effects we can recognise the object as discussed in Chapter 3. The light and shade effects are produced by the upper surface receiving more light than the underparts. These effects lighten the tone of the upper parts and darken the lower parts that are in shade. Thus a solid object of uniform colour matching with the background shows up due to its nonuniform illumination and brightness. Light and shade effects help u s to distinguish between a disc and a sphere or between the side of a cube and the side of a cylinder12. In all these examples, the presence of shade is an important factor which gives the appearance of projection or of depth. A s such, a solid body can be distinguished by means of light and shade even when it matches well with its background in colour and texture. The light and shade effects can be flattened by countershading surfaces receiving more light and counterlightening those surfaces which are in the shade using properly graded tones. This removes the appearance of depth and three-dimensional perception, and thereby the object appears optically flat. Countershading can be produced by blended patternsI2.Figure 4.11 shows such patterns. They may consist of alternate black and white markings, and they can be lines or squares or spots. When these patterns are observed from successively increasing distances, the patterns blend to form a uniform grey half-tone. By altering the relative proportion of black and white in the pattern, we can arrive at any desired value of grey tone in order to destroy the light and shade effects. The countershading effects can be advantageously used in the case of aircraft and guns. When an aircraft is viewed from above, the upper portion looks brighter and the lower portion darker. By proper countershading, the light and shade effects can be destr~yed'~. Thus, besides colour matching, countershading also is necessary for effective camouflage.

69

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Introductiott to Camouflage and Lleception

Figure 4.11. Patterns which produce countershading effects.

4.7.2.3 Disruptive Colouration Obliterative shading is not possible for all directions of incident light. As such, colour matching and simultaneous countershading is not possible under all conditions. One of the most important principles of camouflage is that produced by dazzle12. It is an American term which came into popular use during war in connection with camouflage painting of military objects. When an object is painted with irregular patches of colours of varying contrast and tones, the attention of the observer is diverted away from the actual shape of the object but drawn towards the dazzling patches. The patterns which attract attention do not bear any relationship with the shape of the object with which the observer is familiar. Several examples can be cited from nature (already discussed in Chapter 3 where many animals escape detection by their enemies by wearing disruptive patterns on their bodies. The optical principles behind these patterns create confusion in the mind, which is responsible for preventing or delaying the recognition of the true nature of the object. For effective camouflage, the patterns have to follow definite o p t i d principles. The patterns, the colours and tonal contrasts have

Visual Camouflage

to be accordingly chosen. The effect of a disruptive pattern is greatly enhanced when some of its elements closely match the background while others differ sharply from it. This is known as differential blending which implies blending of some components, and others strongly differing in colour and tone from the background. It is not possible to correlate the portions which stand out conspicuously with the real shape of the object as a whole. The disruptive pattern which creates an optical illusion becomes very effective when strongly contrasted tones are employed. The greater the contrast in tone between adjacent elements in a pattern, the greater will be its disruptive function(princip1eof adjacent contrast). These principles have been already discussed in Chapter 3 under camouflage in nature. Besides disruption of surface, obliteration of contour is also equally important. Every solid object has a characteristic boundaxy the shape of which enables us to identi9 it. In order to camouflage an object, the familiar contour of the object must be broken. How a contour can be broken is illustrated in Figure 4.12.

Figure 4.12. Disruptive patterns for contour disruption.

71

72


In each of these four diagrams, half the area is covered by white and the other half by black colour. As these figures are observed from successively increasing distances, each figure starts blending with the background. The blending distance for each figure is not the same. The important optical principle is that for disruption of contour, adequate number of elements of the pattern should cut across the contour. Another important aspect associated with the patterns is that, especially in the case of permanently stationary objects, the pattern drawn on the objects should closely resemble the surroundings. 4.7.2.4 Shadow Elimination So far we have discussed how camouflage can be accomplished by colour resemblance, obliterative shading and dispruptive colouration. There is another important aspect which must be taken care of for effective camouflage. A military object which is otherwise well-camouflaged by the methods mentioned above can be detected by the shadow it casts on the ground under the action of sunlight. The shadow cast by the object may be both conspicuous in tone and characteristic in form. In aerial surveillance shadows, can be very well seen. The character of the shadow depends upon several factors the position of the sun, i.e., the time of the day and season, or the position of any other source of light which falls on the object, or the nature of surface on which the shadow is formed. If the surface is flat or bare ground or a wall, the shadow will be very clear, retaining the shape of the object. If the shadow is cast on a n uneven surface the shadow will not have continuity of outline. If the shadow falls on numerous irregular surfaces lying a t various angles and a t different levels., viz grass, shrubs, or foliage, there will not be a clear shadow, but it will be diffused. Shadow elimination or distortion becomes very important against aerial observation. The various methods for reducing detection probability with respect to shadow are: (a) For an object like a tank, shadow may be studied for different positions of the sun and for different orientations of the object (0- 360")with respect to the direction of sunlight and detection probability in each case recorded. From the analysis of such data, Defence Laboratory, Jodhpur, has arrived at optimum parking positions for minimum detection probability. These would largely depend on the time of the day and the season. Wherever it is feasible the tank may be accordingly parked. The orientation of the body must be such that the area of the shadow is minimum and inconspicuous. The long barrel of the tank gun, in particular, gives a conspicuous shadow.

(b)

Another way of making a shadow less conspicuous is keeping everything low, as lower structures cast shorter shadows.

(c)

Wherever possible, if a military object is kept under the shadow of another object, the shadow of the former may be eliminated.

(d)

Finally, shadows can be distorted by using distortion masks. Such masks can be used for distorting the shadow of the tanks barrel. Also by keeping military objects in pits and trenches partial concealment can be attained.

Deception Deception, whether it is in nature or in war, is implicit in camouflage. To make any object unrecognisable from its real form is itself deception. But besides the various types of camouflage methods and techniques we have discussed so far, there is another form of camouflage in which dummies and decoys are used to simulate real activity. This form of camouflage is specifically known as deception. Quite often, deception equipment along with normal camouflage is used for effective combat survivability of military equipment. Deception in the form of disinformation is also employed in the battlefield. Deception in all its aspects is discussed in chapter 7. CAMOUFLAGING OF MILITARY OBJECTS BY 4.8 DISRUPTIVE PATTERN PAINTING By and large, all static military objects (vehicles,tanks, aircraft, hangers, ships, etc.) are camouflaged by pattern painting. In visual camouflage the three types of painting used are: (i)protetive painting (ii)disruptive pattern painting, and (iii)imitative painting. Protective painting which uses a single colour is used for camouflaging military objects in areas with one overwhelming colour. Disruptive patterns consist of elements of irregular geometry of two or more shades painted a t random. The predominant colour of the terrain is chosen a s the main colour, and the other colours, some of them lighter and others darker than the background colour. The size of the elements and the distance between adjacent elements are determined by the distance from which the enemy is likely to observe. When a disruptively painted object is viewed from successively increasing distances, initially the object can be clearly seen, but on further increasing the distance, a stage is reached when there will be maximum surface and contour disruption, and thereby the object blends with the background. This is the distance for which the pattern is designed. When the distance of observation is further

4.7.3

increased, because ofloss ofm1ution, the adjamt elenrents cannot be seen as separate elements, disruption fails, colour differences vanish, and dl the colours blend into a single colour. Until recent years disruptive patterns were being dram manually and hence were subject to personal preferences. Now-a-days they can be generated in a great variety by computer techniques (as described later). Figure 4.13 shows a disruptive pattern generated by a computer at the Defence Laboratory, Jodhpur, drawn over a mxtande. In imitative painting, the background surrounding the object is imitated on the object. This is done in the case of permanent installations such as buildings,

4.8. X Studies on Disruptive Pattern Painting Not many studies appear to have been carried out to waluat~

the effect dvariables, viz., pattern colour, pattern contrast, number of coIours used in the pattern painting, on the degree of difficulty in detecting the target.

Visual Camouflage

Humphreys a d Jarvis18reported that pattern painting disrupts signature characteristic, reduces target/background contrast a i d distorts the vehicle's geometric lines and overall configuration. It was concluded that camouflage pattern painting is an effective technique whicR reduces visual and near infrared ground target acquisition from ground or air observation. Marrero-Cmacho and M ~ D e r m o talso t ~ ~ concluded from their studies that pattem painting reduces the probability of detection of tactical military vehicles. BucklinZ0reported from his studies on camouflage of small items that when green objects and brown objects were mixed in the same field, the detection rates dropped to less than one-half the rate for either colour alone of the object tested. When a third colour (straw) was added the detection rate further dropped down for the green and straw items but not for brown. It was also found that when green items and straw items were mixed, detection for straw items dropped by one-third while rate for green items more than doubled. Jarvis2' reported the results of a preliminary army field test of five pattern-painted vehicles and one olive drab vehicle in which detection and identification ranges were the measures of camouflage effectiveness. The studies revealed that the patterned vehicles (Germany, MERDC, US Army Mobility Equipment Research and Development Center, Swedish a n d USAREUR) were more difficult to detect than either an olive drab vehicle or a vehicle painted with a British pattem. No differences were found between German, MERDC, Swedish and USAREUR pattern-painted vehicles or between solid olive drab and the British pattern-painted vehicle. The German patterns have two colours, the MERDC and the Swedish army pattern have three to four *~ two colours and the British two colours. G r o s ~ m a n conducted laboratory experiments on a terrain model to assess the effects of pattern, range, lighting and location on the ability of subjects to visually detect tank targets. In the first experiment, each subject observed one of the four patterns (MERDC, Swedish, German and olive drab) a t two ranges. The MERDC took the longest to detect and there was no difference d u e to range. In t h e second experiment, each subject observed five patterns (British in addition to the above) a t each of eight locations under either diffuse or bright/shadow lighting conditions. No difference due to pattern were observed. O'Neill, and J ~ h n s m e y e suggested r~~ that pattern properties which contribute to concealment are: (i)value contrast of pattern and ground: if the overall relative brightness of the target is significantly different from the background, detection is usually is quite easy, (ii) colour contrast of pattern and ground, (iii) intrapattem value differentiation, and (iv)texture gradient contrast with the ground.

75

I ~ u c t i o tno Camoujhge and LIxeptbn

Dud-Texture Gradient Pattern Painting [DTGr] The dud texture w e n t (DTC) patternarwas developed by the Psycholow Committee, Office of Military Leadership, U.S Military Academy. It is derived from the US Army Pattern System developed by the US Army Mobility Equipment Research and Development Command (MERADCOM) at Fort Belvoir, VA. At longer ranges, the DTG merges into a macropattern ofbroad light and dark areas which matches the texture of the ground at that distance. At closer range, the micropattern reso1ution provides a continuing match with the background texture (Fig. 4.14). The figure shows one typical DTG pattern drawn on a body of rectangular cross-section. A number of studies have beencarried out to compare the &cacy of DTG patterns with others. O'Neill, and J~hnrneyer~~ conducted a laboratory experiment in which three groups of subjects viewed a series of 35 mm slides phased at decreasing rmges of targets. Group A viewed the US Army Pattern (UGAPJ (4 colours; fortst w e n 40 per cent, field drab 40 per cent, sand 15 per cent and black 5 per cent). Group B viewed DTG pattern (same number of colours and relative percentages]. Group C viewed a solid forest-green control target. The dependent measures were the distance from the target on detection and identification of target shape. Group B (DTG] differed 4.8.2

Figure 4.14.

pattern (tad-1W,g e k - 8 0 ? + green 10%) on a dark men ba&gmund(lsft) and,yeUow b a c k p o d

WmI.

Visual Camouflage

77

significantly from the other two groups in that identification and detection in the latter took place a t a farther distance. A field validation test was conducted by ONeill, et aP5 to compare the standard US Army and the DTG patterns in a field environment using a n appropriate subject group representative of observers in a combat environment. Selected subjects viewed a pattern-painted M 113 target vehicle through the commander's sight of a Soviet T-52 Main Battle Tank. One group observed the target painted with the U S Army pattern and the other the DTG pattern. Observations of the time to detection and correct/incorrect detection of the target type were recorded for each subject. It was concluded that the DTG pattern showed a significant improvement in the concealment effect over the standard US Army pattern under the field condition tested. Braaten24found in a series of small scale trials for ground-to-ground and air-to-ground observations that DTG pattern was more effective. He also reported factors other than camouflage pattern which strongly influence effectiveness of ~~ out a n camouflage. Woodruff, Boyd, and B e ~ h w i t hcarried experimental study for assessment of relative detectability of four different paint schemes on MI 13 vehicles viewed against six backgrounds. They found highly significant effects d u e to backgrounds and due to pattern and orientation.

Computerised Generation of Disruptive Patterns Skinner, in his unclassified reportz7described a computer-aided method for generating disruptive patterns for soldier's uniforms. The elements of the unit pattern were generated by a modified leastsquares-fit of a polynomial to a collection of radii whose lengths are randomly distributed within limits and uniformly distributed in angle. The degree of smoothing was determined by the order of the polynomial which was a random odd integer in the range 7 - 17. The unit pattern containing 99 elements was repeated over the entire fabric. The number of colours that could be used were four. The area of each colour could be varied. The method produced random shapes of closed curves approximately circular in shape. There were no discontinuities a t the edges of the unit pattern. The patterns were found to be effective for uniforms. The calculations described in the report were coded in Fortran IV implemented on the Digital Equipment Corporation PDP- 10 computer. At Defence Laboratory, Jodhpur, Jayson a n d P u ~ p a k ~ ~ developed a mathematical model and software for generating disruptive patterns and for superimposing these patterns on simple three-dimentional geometrical solids. In order to generate open and closed curve patterns, B-spline technique was employed. For drawing a R-spline curve, polynomial in parametric form was used. For 4.8.3

simulation ofgeometrical objects boundary representationt d d q u t has been used. Geomesy of the object is defined in terms ofpoints and edges which together constitute the surface and all the surfaces taken together constitute the object. For example, a cube was represented by six bounding faces. Each face was represented by four edges and each edge was a line drawn between two points. Thus a cube was represented by the following structure. (i) The X,Y,Z coordinate of six vertices; Starting and ending point of all ~e 12 edges; (ii) Figure 4.15 gives a computer-generated disruptive pattern superimposed on a truck.

4.1s. Computer-generateddisruptive pattern superimpwsd on a buck.

4,9

CAMOUFLAQIHQ BY NETS

A camouflage n e P * has certain advantages over the methods discussed above. The net, besides making the object blend with its background, distorts the shadow to a certain extent It can also be effectivein the case of objects which are mobile. Camouflage nets together with pattern painting produce a mixture of texture and colours giving more effective c a m d h g e . A modern camouflage net consists of a two-dimensional structure rectangular or square in shape, made out of synthetic fibre. The structure consists of square meshes about 8 to 10 cm size. They are made into modules with sizes ranging from 3 rn x 3 m

Visual Camouflage

to 10 m x 10 m, or any required size. At the periphery of each module there is an edge cord made of synthetic material. This cord facilitates manipulation of the net. Each module is garnished with a PVC foil to form irregular patches. Any number of modules can be joined to required size depending upon the size of the military object to be covered. 4.9.1

(i)

Properties of Net Materials The optical propertie~~~t of the net material should match with those of the background. In the visible and near infrared regions, the diffuse reflectance of the net material should closely match with that of the background. For visual observation the colours of the garnishing material should correspond to those present in the background. Similar to disruptive patterns the colours may be chosen. The density of garnishing decreases from the centre to the periphery of the overall net set. Besides catering for the visible and near-infrared regions, today there are some manufacturers who claim that the nets they manufacture can cater for thermal infrared and microwave regions of the electromagnetic spectrum. In other words, today there appears to be multispectral camouflage nets covering the electromagnetic spectrum from the visible through infrared to radar wavelengths. Besides, nets are available which cater for ultraviolet reflectance; The nets should be light in weight, and compact such that they are easy to erect and handle;

(iii)

They should be waterproof, non-inflammable and rotproof;

(iv)

They should be resistant to bad weather conditions extremes of temperature, humidity and wind velocity. They should also be resistant to rodents and other pests;

(v)

The nets, if reversible, can be used under two different environmental conditions. A typical net is shown in the Fig. 4.16.

79

80


Figure 4.16. Camouflage nets for vehicles, artillery, etc. Source:

Barracuda Camouflage - Synthetic camouflage nets. Barracudaverken, Sweden & Barracuda, France.

4.9.2 Applications of Nets Nets can be used for camouflaging a wide range of objects, both mobile and fixed (personnel, vehicles, weapons, aircraft, ships etc) in different environments - woodlands, desert, snow regions and different seasons. The helmet as well as body can be covered with camouflage nets which provide excellent blending with the background. Fig. 4.17a shows a n infantryman wearing cotton helmet net and a garment made out of camouflage fabric, while Fig 4.17b shows the same infantryman wearing Barracuda helmet net and the Barracuda personal camouflage net30.The superiority of the Barracuda system can be seen from the Fig 4.18a and 4.18b which respectively show a jeep with a trailer and a tank camouflaged by Barracuda nets. Figure 4.19 shows how a truck is camouflaged with a Barracuda net while Fig 4.20 shows an aircraft camouflaged by a Barracuda net 31.

Visual Camouflage

Figure 4.17a. Infantryman wearing cotton helmet net and a garment made out of camouflaged fabric. Source:

Barracuda Camouflage - Synthetic camouflage nets. Barracudaverken, Sweden & Barracuda, France.

Figure 4.17b. Infantryman wearing Barracuda helmet net and a Barracuda personal camouflage net. Source:

-

Barracuda Camouflage Synthetic camouflage nets. Bamacudaverken, Sweden 85 Barracuda, France.


Figure 4.18a. Jeep with trailer (camouflaged with standard net). Source:

Barracuda Camouflage - Synthetic camouflage nets. Barracudaverken, Sweden & Barracuda (France).

Fig 4.18b. Tank camouflaged with standard net. Source:

-

Barracuda Camouflage Synthetic camouflage nets. Barracudaverken Sweden & Barracuda, France.

Visual Camouflage

Fig 4.19. Camouflaging a truck by net. Source:

Barracuda Camouflage -Camouflage nets. Practical Handbook.

Fig 4.20. Camouflaging an aircraft by net. Source:

Barracuda Camouflage -Camouflage nets. hactical Handbook.

Manufacturers of Nets There are several manufacturers all over the world who develop and manufacture camouflage equipment, in particular camouflage nets. The various manufacturers of camouflage equipments/nets are Barracuda Technologies, Sweden; Beivar Research, Development and Engineering Centre, Fort Belvoir, Virginia; Teledyne Brown Engineering, Cummings Research Park, Alabama; Brunswick Corporation Defense Division, Illinois; Sullivan IndustrieslGeneral Image Engineering Corporation, Utah, USA; and Bridport Aviatian Products, The Court Bridport, Airborne Industries Limited, Essex, U.K.These are the principal manufacturers. Other manufacturers include Habemig Camouflage, Austria; Carntex Camouflage Division, Copenhagen, Denmark; idex Foreign Trading, Contracting and Engineering Company Limited, Budapest, Hungary; and Seyntex

4.9.3

63

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Intrvduction to Camouflage and Deception

NVISA., Seyntex-laan- 1, Industriepark Zuid, Belgium3]. In India, DMSRDE (Defence Material & Stores Research & Development Establishment), Kanpur is responsible for design of camouflage nets which are then bulk produced in ordnance factories. 4.10 PSYCHOLOGICAL CAMOUFLAGE Even in the case of a large number of detectors operating over a wide range of the electromagnetic spectrum, ultimately, it is the visual imagery which is deciphered by interpreters, observers or pilots for immediate situational assessment and action. The role of the human being is therefore all important. Any research work in this area has to deal with target and observer variables which would lead to a disruption in the detection, recognition and identification of military targets in a specified set of conditions. This type of work has to center around visual information processing which involves the analysis of visual stimuli which aid target detection, recognition and identification. Certain basic features and the way in which these features are combined are important in this process. Lowered detection is caused by disruption and incorrect combinations of features which result in incoherent information to the observer. These aspects of information processing can be dealt with and reliably indexed by psycho-physiological parameters like the Event Related Potentials (ERPs),the EEG measures and parameters of autonomous nervous system (ANS)activiV2.

Neurophysiological Principles of Visual Perception According to Sokolovs of visual perception a n object in the visual field produces a n external model based on the physical attributes of the object such a s size, shape, colour, texture etc. The external model is produced a t the sensory level. Perception is an end result of a complex stream of sensory inputs. The brain evokes an internal model by comparing the external model with innumerable internal models which are known a s memory traces or engrams. Thus, by picking u p a n internal model which closely matches with the external model, the brain recognises the object. The theory of recognition by component (RBC)explains that an object is recognised by recognising its component parts rather than by simultaneous processing of constituent sub- part^^^,^^. According to the RBC theory, components may be recognised serially and even if some components are missing or not recognised, the object is readily classified. A s mentioned earlier, recognition is accomplished by matching the external and internal models. This is fast if the internal model component is one which is frequently called upon, while on the other hand it will be slow if the internal model component is a n infrequently used one. Further, recognition of component parts 4.10.1

Visual Camouflage

need not necessarily result in the classification of an object. Also, it is not necessary to recognise all major components to arrive at an accurate classification. Some factors crucial for recognition of an object by components are: (i) Are there one or more components, the recognition of which leads to the inference of the object? (ii) What is the minimum amount of information required to recognise an object, if all the components have more or less equal valency or capacity for classifying the object? (iii) What is the relationship between the complexity of the object and its recognition? (iv) What are the factors that can interfere/confound/delay recognition of the object? (v) What are the perceptual processes that aid recognition of the basic geons when minimal information of a n object is presented? Distractability is important in the context of camouflage. It may be defined as the elicitation of orienting responses by irrelevant stimuli while the individual is engaged in directed or focused attention to a relevant stimulus or object. In camouflage we have to study those external visual factors that can effectively induce distractability which can confound and delay target recognition. These may be studied through visually evoked potentials (VEPs).

Studies on Target Characteristics and Target Context on Detection Basic studies on various target characteristics which have shown to affect the sensory system have been reported: Target shape - Carlock, Rayner, and Bucklin3'; size and luminance - Jenkins36; motion and luminance - Regan, and B e ~ e r l e y size, ~ ~ ; orientation, motion in depth - Regan38-40;target contrast variation, pattern degradation, texture, luminance, brightness and colour contrastFenker et aP1. Applied studies on target characteristics suck a s colour Collins, and W h i t t e n b ~ r gand ~ ~ distance-Pabon, et a143have been n ~ ~ out field studies carried out under field conditions. B a l d ~ i carried using model aircraft to estimate the relationship between aircraft size and recognition range, varying the overall size, colour and aspect angle of the models. All these variables were found to affect the recognition range. Characteristics of target context do also affect detection. Similarity of target to non-target stimuli - Farmer and TayloF5,Drury and Clement46;Number of non-targets - Bloomfield, ~~; level of illumination - Faulkner and Wald and T h ~ r n p s o nGeneral

4.10.2

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Murphy48,Mass, Jayson and KjeibeF9;Target and non-target size Bloomfieldso,Position - Pollacks1, Brown and Monk52;Clutter and texture - Bloomfield, Wald and T h ~ m p s o n Distractor ~~; stimuli Harcum and Shaws3, Wertheims4; Sequential visual patterning Lappin and Distance - Galanter and GalanteF6;Display size and fields - Kaplan, Metlay and Lyonss7, Pollacks1; and Colour Bloomfields8. Banks, et alS9studied the effects of search area size on target acquisition with passive night vision devices. Hoffman60 studied detection range, search rate success and rate of acquiring land vehicles as influenced by different environmental conditions and characteristics of observing instruments. Warnick, Chastain, and Ton6' studied long range target recognition and identification of camouflaged armoured vehicles. Porterfield, et aP2 studied visual detection of ground target sites and identification of specific targets as a function of apparent scene illumination. Basic studies on cognitive aspects of visual processing have been carried out by John Bloomfield and his colleague^^^. One such study tried to determine the relationship between the variations in the properties of disruptors and their effectiveness in concealing military targets when placed in various classes of natural backgrounds. Another study carried out was the optimal placement of military targets in cluttered terrains - Bloomfield, Graf and Graffunders8. Woodruff, Boyd, and BeckwithZ6indicated a method for quantifying the detectability of complex targets in natural terrain. They found that factors like patterns and background orientation of M 113 vehicles all had highly significant effects on detectability.

Psychological Studies Related to Camouflaging of Military Objects Bioelectric activity generated in the brain and recorded from the scalp of the human beings varies from DC levels to a few kilohertz (kHz)in frequency and from a fraction of a microvolt to a hundred microvolts or more in amplitude. It is the DC to 3-5 kHz activity that is of interest when evoked potential (EP)and event-related potential (ERP) recordings are conducted. The DC activity or the slow brain potentials such a s contingent negative variation and Bereitschaft potential are generally associated with specific events, or more often with specific psychological states like anticipation, intention or readiness to act, etc. Evoked potentials are obligatory potentials that are produced by stimulation of various receptor organs and represent: basic psychological response of the brain to external stimulation. Most EPs cannot be seen in routine EEG recording because of their low amplitude (0.1 to 0.0 1pV) and their admixture

4.10.3.

Visual Camouflage

with normal background brain wave activity and various artifacts. Removal of background activity is achieved by a process of averaging of several epochs time-locked to the stimulus6'+. Dawson demarcated two zones - primary and secondary - in visually evoked potential (VEP).The primary components consitute the electro-retinogram and components in the secondary zone constitute the cerebral visual evoked potential. Variability in latency of components in the primary zone is minimal while components in the secondary zone show greater inter-individual variability. The primary components are least affected by the psychological state of the individual unlike the cerebral VEPs. There are four cerebral compenents that occur within 250 msec as obligatory potentials. These are N,,, P,,,, N,,, and P,,,. P and N signify positive and negative polarity and underscript relates to latency at which the peak (component)appears. Areas of the brain suggested to be invoked in the electrogenesis of cerebral potentials are area 17 of the striate cortex and parietal lobes. The potentials may also be influenced by limbic activation and modulated by the frontal cortex. As far as visual information processing is concerned, cerebral VEPs are dependent on the characteristics of the visual stimulating target/object and the context in which it is embedded. If the target signature is degraded to such an extent that it matches background, the corresponding reduction in VEP amplitude and delay in latency components signify effective camouflaging. If the reverse takes place, then it can be said to be accelerating detection and identification. VEPs can also be employed for studying the efficacy of various disruptive patterns for camouflaging military objects32. Psychological research related to camouflaging of military objects against human visual perception may be pursued under the following lines32: (i) Visual evoked potentials in relation to psychophysical attributes of simple and complex stimuli;

(ii)

Standardising reliable indices of ERPs for stimuli which are found effective for camouflage; and

(iii) Applied studies in camouflage relating to the conditions/ factors that would lead to the delay of target processing or a reduction in the amount/aspects of information which are conveyed to the human observer.

87

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4.11

MISCELLANEOUS CAMOUFLAGE DEVICES

4.11.1 Foams There are liquid foams being manufactured by FFV Maintenance Export Division, Arboga, Sweded5.These foams can be effectively used to conceal vehicle tracks, roads, runways etc. They can be produced in colours matching with background - green, sand, snow etc. Besides providing colour matching, foams can also have texture-matching properties. There are foam generators which can be mounted on the rear of tanks and other armoured vehicles. As they generate foam, they spread the foam such that the vehicle tracks can be concealed. Besides providing concealment in the visible region, they can cater for thermal infrared region also. Depending upon the stability of the foam, which can be up to 48 hours, it automatically disappears subsequently. Foam generators which can generate foam on large scale are available which can be carried by standard vehicles. The subject of "foam" is discussed in the chapter on Camouflage Materials. 4.11.2 Reflectance Camouflage. Employment of vertical screens has been already discussed. Such screens, although hiding the object, can themselves be quite often seen. If the screen is also made to blend with the background, neither the object behind the screen, nor the screen can be seen. A screen which has mirror-like properties would serve the purpose. Screens consisting of reflecting mylar can be used. As and when required, such a reflecting sheet is kept in front of the object so that the object is hidden. In a foliated background, consisting of bushes, the mirror forms an image of the vegetation in front of it, rendering the mirror invisible. From a distance neither can we see the object nor the reflecting surface. Figures 4.2 1(a)and 4.2 1(b) show a model of a truck and the model together with reflecting surface. In the second figure we can see neither the model nor the reflecting surface. Such a technique can be used in a foliated background only. Another disadvantage is that it is difficult to camouflage big objects a s large reflecting surfaces are difficult to get. Ordinary mirrors cannot be used. Only reflecting sheets which can be rolled can be used. 4 . 1 1 . 3 Antishine Devices A vehicle such as a jeep or a 3-Tonner which is otherwise very well-camouflaged can be seen from kilometers away from air or ground by the shine produced by the windscreen. There is a need to supress the dazzle effect produced by windscreens. When the vehicle is parked, the windscreen can be physically hidden but when

Visual Camouflage

Figare 4.21(a). Model of a truck in a foliated background.

Figure 4.211b). The same truck camouflaged by reflecting surf8ce.

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it is in motion this is not possible as it would obstruct the vision of the driver. There are anti-reflecting materials, which, when applied to the windscreen, reduce the shine by destructive interference. Such nonreflecting films cannot work for all wavelengths in the visible region. Also, when the vipers are used the film may come out; further, due to environmental factors such a s heat, wind, and humidity, the film may gradually lose its properties. There are anti-reflecting gadgets developed by Defence Laboratory, Jodhpur which when fitted in front of the windscreen can cut off the shine without causing hindrance to the vision of the driver. As yet, however, there does not appear to exist any completely satisfactory method by which the dazzle produced by windscreens is eliminated. 4.11.4 Vehicle Track Erasers The tracks produced by vehicles such a s tanks, trucks and jeeps may be pronounced, depending upon the soil conditions of the ground. From the vehicle tracks, military activity can be inferred in any area. As such there is a need to erase or obliterate vehicle tracks such that they are not seen or photographed by aerial reconnaissance. Mechanical devices fitted to the rear wheels have been successfuly developed at Defence Laboratory, Jodhpur to obliterate vehicles tracks. Besides track erasers, vehicle track generators are also needed, in order to simulate dummy qctivity in an area where dummies are deployed. Automatic blending of objects with background. One disadvantage with pattern painting is that whenever an object such a s a vehicle moves from one background such as woodland to another background, such as desert, the pattern needs change. So the requirement is that as the vehicle moves from one background to another, the colours used in the painting should automatically change and match with the background. There are some organic compounds known as liquid crystals. Cholesteric liquid crystals exhibit selective scattering of light. The wavelength which undergoes maximum scattering depends upon several factors such as temperature, electric field etc. Some work appears to have been carried out to explore the potentialities of these materials for automatic blending of objects with background. Actual applications are not found in literature.

Visual Camouflage

4.12

COMPUTER-BASED EVALUATION OF CAMOUFLAGE Kilian and HepfingeF6 reported the construction of a fieldportable, computer-based system for evaluation of personal camouflage. It is based on image analysis technique. The system can be utilised to evaluate the efficacy of camouflage uniforms against unaided human vision. The system which is known a s the Mobile Army Camouflage Evaluation (MACE)consists of hardware and software for acquiring, reducing and analysing images of camouflage-in-background scenes. T h e method involves transformation from a set of monochromatic images to standard colour description coordinates and subsequent comparison of the first and second order statistical measures for the camouflaged soldier and the local background. The MACE system generates a set of 10 monochromatic images of the object samples at equal intervals across the electromagnetic spectrum from 380 - 740 nm. The set of 10 monochromatic images is then transformed to standard colour description coordinates (CIE). Evaluation of camouflage is based on a set of features for each component in the CIELAB colour space which includes statistical texture measures and first-order descriptive statistics over specified regions in the scene a s functions of orientations and scale. The authors state that the MACE system represents a first step towards a fully objective technique for camouflage evaluation and further work is to be done to increase the fidelity to the response characteristics of the human visual system including colour representation, pre-attentive visual processing as well as attentive processing for recognition. 4.13

NEW AREAS OF VISUAL CAMOUFLAGE

Metamers One of the important ways by which objects are detected/ recognised/identified by human vision is by differences in colour and electronically by spectral radiometric differences. In order to defy detection by both human vision and electronically, the object surface should give rise to different spectra (radiometric) leading to different grey level values and spectral band values having the same colours. This can be accomplished by employing metamers which evoke identical colour sensations but with different radiometric functions. Every colour stimulus (radiometric function) comprises two parts -- a fundamental and a residual (fundamental colour ~ is - ~the * . fundamental alone which stimulus and metameric b l a ~ k ) ~ It is processed by the visual system and evokes colour sensation. The residual is not processed and is without effect on colour sensation. 4.13.1

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Introduction to CamoujZage and Deception

Metamers have the same fundamentals but different residuals. It would be necessary to generate various metamers for painting identical type of objects for camouflage purposes. This new technique gives different radiometric functions for each identical object under camouflage. 4.13.2 Spectral Camouflage Spectral camouflage is the camouflage which blends the spectral characteristics of the object surface with the background, a t the same time causing disruption of the object to reduce chances of its recognition and identification. This is possible if each pixel of the object a s recorded by a spectral scanner has the same spectrum as that of the background in which the object is situated (F'uspak). The area to be covered by each paint for spectral matching can be calculated from

Where RA1,R,2, R; A,, A, and A, IZ1 I1

'

=

overall reflectances in bands 1,2 & 3

=

areas painted by paints 1,2 & 3

RZAl, R~~ etc are reflectances of the paint No-1 in bands 1

Using least square method, more exact values can be found with increased number of bands and paints.

REFERENCES 1. Som, S.C. Light and its measurement. Proceedings of the Seminar on Camouflage, Oct 19-21, 1989, Defence Laboratory, Jodhpur, India. p. 45-85. 2.

Rodgers, A.L., et al. Surveillance and target acquisition systems. Brasseys Defence Publishers - a member of the Pergamon Group, Oxford. 1983. 17-27.

3.

Raisbeck, et al. Design goals for future camouflage systems. Report to US Army Mobility Equipment Research and

Visual Camouflage

Development Command. Jan 1981. Contract No. DAAK-79D-0036, Order No. 0006 P 11-20. Duntley, S.Q. Reduction of apparent contrast by atmosphere. JOpt. Soc. Am. 1948, 38, 179. Duntley, S.Q. Visibility of distant objects. J. Opt. Soc. Am. 1948, 38, 237. Blackwell, H.R. Contrast thresholds of the human eye. J. Opt. Sac. Am. 1945,36,624. Middleton, W.E.K. Vision through atmosphere. University of Toronto Press, 1968. Guide to camouflage for DARCOM equipment developers. Report prepared for MERADCOM under contract DAAG5376-c-0 126, Brunswick Corporation. Harvey, L.O. & Geravais, M.J.International representation of visual texture as the basis for the judgement of similarity. J. of Exptl. Psych.: Human Perception and Performance, 1981, 7(4), Military reconnaissance methods and devices - Barracuda Camouflage. Prepared and Printed by A.B. Teleplan on behalf of DIAB-Barracuda, Solna, Sweden 1982, p. 1-62. Remote sensing in relation to camouflage. Status Report. Defence Laboratory, Jodhpur, 1988. Cott, H.B. Adaptive coloration in animals. Methuen & Co. Ltd., London, 1966. Saxena, S.K. & Solanki, K.R. Development of vegetation in Thar Desert. Proceedings of the Seminar on Camouflage, Oct 19-21, 1989, Defence Laboratory, Jodhpur, India. pp. 19521 1. Hook, D.W & Sutherland, R.A. Obscuration countermeasures; chapter 6, Pollock, D.H(Ed). Countermeasure systems. Infrared and Electro-optical Systems Handbook. Vol7., ERIM, and SPIE Optical Engineering Press, Michigan, USA, 1993. Walters, B. Camouflage and chemical warfare protection. Asian Defence Journal, 1993, 1. Knorpp, F.R.Camouflage and deception -A challenge to army. Armament Militay Technology. 1987, XI(9), 88-90 Parker, G.H. Chromatophores. Biol. Rev, Cambridge, 1930, V, 59-90.

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Humphreys, A.H. & Jarvis, S.V. Camouflage pattern painting. Report of USAMERDCs, Camouflage Support team to MASSTER, Fort Belvoir, VA, USAMERDC, February, 1974. Rep. No. 2090. Marr, D. Vision: A computational investigation into the human representation and processing of visual information. San Francisco: W.H. Freeman, 1982. Bucklin, B.L. History and status of small item camouflage. Picatinny Arsenal, Dover, NJ, US Army, April, 1973. Rep. No. SR1-3-490 1. Jarvis, S.V. Technical memorandum: Fort Knox test of camouflage pattern effectiveness. Fort Belvoir, VA, USAMERDC, August, 1974. Grossman, J.D. Effect of camouflage on visual detection. NWCTP-5745. China Lake, California: Naval Weapons Centre, April, 1975. NTIS NO. AD-BOO4 97118. OWeill, T.R. & Johnsmeyer, W.F. Dual tex: Evaluation of dual texture gradient pattern. West Point NY: Military Academy, Office of Military Leadership, April, 1977. NTIS No. AD-A040 34218. Braaten, J . Dual textured camouflage evaluated. Armor. 1980, Jul-Au~,15- 17. 07Neill,T.R. ; Brusitus, J.M.; Taylor, D.L. & Johnsmeyer, W.F. Evaluation of dual texture gradient camouflage pattern. West Point, NY: Military Academy, 1978, July. NTIS No. AD A056 47 116. Woodruff, C.J.; Boyd, R.J. & Beckwith. Comparative assessment of the detectability of patterned M 113 vehicles in natural terrain backgrounds. Ascot Vale, Australia: Materials Research Labs, Dec 1986. Report No. MRL-R- 1035. NTIS No. AD-A18 1 639/6/XAB/XPS. Skinner, D.R. A pseudo-random pattern generator for camouflage. Department of Defence, Australian Defence Scientific Service Materials Research Laboratories, Maribyrnang Victoria, Australia. 1975. Research Report 599. Jayson, P.P. & Puspak, S.N. Report on computer simulation of disruptive patterns on simple, regular, geometrical solids. Defence Laboratory, Jodhpur, 1988. Synthetic Camouflage Nets. Barracuda camouflage. Barracuda, France.

Visual Camouflage

Barracuda - Camouflage Nets Practical Handbook. Foss, C.F. & Gander, T.J.(Ed]. Jane's Military Vehicles and Logistics. 1994-95, 15, p. 677-84 . George, C.S.; Mudgil, Y.K.; Ramana Rao, J.V. & Sarma, T.V.R. Status report on psychological studies in camouflage. Defence Laboratory, Jodhpur, April, 1993. Sokolov, E.N. Perception and conditioned reflex. Pergamon Press, Oxford, 1963. Garner, W.R. Uncertainity and structure a s psychological concepts. Wiley, New York, 1962. Carlock, J.; Rayner, J.C. & Bucklin,B.L. Visual detection and recognition threshold study of four geometric shapes. Rep. No. PA-TN-1956. N J Dover: Picatinny Arsenal, December, 1970. NTIS No. AD 717 004. S.E. Jenkins. Size and luminance discrimination in peripheral visual field. Ascot Vale, Australia: Materials Research Labs, Oct 1979. Rep. No. MRL-R-760. NTIS No. AD-A081 83913. Regan, D. & Beverley, K.I.Figure-ground segregation by motion contrast and by luminance contrast. J. Opt. Soc. Am. 1984, A2(1), p. 433-41. Regan, D. Visual sensitivities and discriminations and their roles in aviation. Halifax, Nova Scotia: Dalhousie Univ., June, 1985. AFOSR-TR-85-0639. NTIS NO. AD A158 962. Regan, D. Visual sensitivities and discriminations and their roles in aviation. Halifax, Nova Scotia: Dalhousie Univ., March 1986. AFOSR-TR-84-003. NTIS NO. AD A170 418/8/XAB/ XPS. Regan, D. Visual sensitivities and discriminations and their roles in aviation. Halifax, Nova Scotia: Dalhousie Univ., September, 1987. AFOSR-TR-88-0907. NTIS No. AD A198 470/7/XAB/XPS. Fenker, R.M.; Dansereau, D.F.; Evans, S.H. & Ellis, A.M. Annotated bibliography of recent reports on the psychological variables affecting performance with camouflaged objects. Fort Worth TX: Texas Christian Univ. Inst. for Study of Cognitive Systems. August, 1975. DAAD05-73-C-0554. NTIS No. ADA017 067. Collins B.L. & Whittenburg, J.A. Defective color vision filters, films, and the detection of camouflaged targets: Annotated bibliography. March, 1974. NTIS No. AD-778 196.

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Pabon, R.J.; Davison, R.A. & Parks, W.I. Analysis of phase IIA of FE 43.8. Fort Leaven Worth, Kans: Army Combined Arms Combat Developments Activity, Feb 1976. Rep. No. CACDATR-2-76. NTIS NO. AD-A025 82316. Baldwin, R.D. Relationship between recognition range and the size, aspect angle, and color of aircraft. Alexandria, VA: Human Resources Research Organisation, February, 1973. Hum RRO Techn. Rep. Farmer, E.W. & Taylor, R.M. Visual search through color displays: Effects of target background similarity and background uniformity. Perception and Psychophysics, 1980, 27(3),p. 267-272. Drury, C.G. & Clement, M.R. The effect of area, density and number of background characters on visual search. Human Factors, 1978, 20(5), p. 597-602. Bloomfield, J.R.; Wald, J . & Thompson, L.A. Visual search: clutter and proximity effects. Minneapolis, MN: Honeywell Inc., 1979. NTIS NO. AD-A1 15 79919. DAAK 70-79-C-0032. Faulkner T.W. & Murphy, T.J. Lighting for difficult visual tasks. Human Factors, 1973, 15(2), 149-62. Mass, J.B.; Jayson, J.K. & Kjeiber, D.A. Effects of spectral differences in illumination on fatigue. Journal of Applied Psychobgy, 1974,59(4), 524-26. Bloomfield, J.R. Visual search in complex fields: Size differences between target disc and surrounding discs. Human Factors, 1972, 14(2), 139-48 . Pollack, I. Detection of changes in spatial position: IV multiple display fields, display aiding and interference. Human Factors, 1974, 16(2), 93- 116. Brown, B. & Monk, T.H. The effect of local target surround and whole background constraint and visual search times. Human factors, 1975, 17(1),81-88. Harcum, E.R. & Shaw, M.R. Cognitive and sensory lateral maskings of tachistoscopic patterns. Journal of Experimental Psychology, 1974, 103(4),663-67. Wertheim, A.H. Distraction in visual search. Soesterberg, Netherlands: Institute for Perception RVQ-TNO, June, 1981. Rep. No. IZF- 1981-7, TBCK-75522. NTIS No. N82-2483013. Lappin, J.S. & Bell, H.H. Perceptual differentiation of sequential visual patterns. Perception & Psychophysics. 1972,12(2-4), 129-34.

Visual Camouflage

Galanter, E. & Galanter, P. Range estimates of distant visual stimuli. Perception & Psychophysics, 1973, 14(2),301-06. Kaplan, I.T.; Metlay, W. & Lyons, C.T. Display size and distribution of search times. Journal of Experimental Psychology, 1972,95(2),334-36. Bloomfield, J.R.; Graf, C.P. & Graffunder, K. Visual factors in target disruption. DAAK 02-75-C-0055, Minneapolis, MN: Honeywell Inc., Dec, 1975. NTIS No. AD-A035 174/2. Banks, J.H., et al. Effects of search area size on target acquisition with passive night vision devices. Arlington, VA: US Army Behaviour and Systems Research Lab, February, 1971, BSRL Techn. Rep. 1168. Hoffman, H.E. The maximum detection range and discovery of land vehicles. Farnborough, England: Royal Aircraft, January, 1970. NTIS No. AD878 422. Rep. No. RAE-Library Trans. 1484. Warnick, W.L.;Chastain, G.D. & Ton, W.H. Long range target recognition and identification of camouflaged armored vehicles. Alexandria, VA: Human Resources Research Organisation, May, 1979. NTIS No. AD A077 86211. Hum RRO Tech Rep 78-7. Porteriield, J.L., et aL Airborne visual reconnaissance as a function of illumination level. Wright Patterson AFB, OH: Aerospace Medical Research Laboratory, 1971. NTIS No. AD728 629. Rep. No. AMRL-TR-71-9. Bloomfield, J.R.; Wald J . & Smith D.A. Final report on the optimal placement of military targets in cluttered terrains. Minneapolis, MN: Honeywell Inc., July 1979. NTIS No. ADA083 500/9. DMK 70-77-C-0056. Halgren, E. Evoked potentials. Boulton, A.A.; Baker, G.B. & Vander, C. Woref-Neuromethods. Vol. 15: Neurophysiological Technics- Applications to Neural Systems, Clifton, N.J, 1990, p. 147-275 FFV Foam Camouflage. Jane's Year Book, Sweden, 1986, p. 897. Kilian, J.C. & Hepfinger, L. Computer-based evaluation of camouflage. SPIE, 1992, 358, 1687. Jozef, C. & Cohen, J. Colour and colour mixture: scalar and vector fundamentals. J. Colour Research and Applications, 1988, 13(1).

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68.

Worthy, A.J. Calculation of metameric reflections. J. Colour Research and Applications, 1988, 13(2).

69.

Michael, H.B.& Gerhard, W. Chromatic adaption and colour constancy: A possible dichotomy. 3. Colour Research and Applications, 1986, 1l(3).

70. Berms, R.S.;Billmeyer Jr,; Fred, W. & Sacher, R.S. Methods for generating spectral reflectance functions leading to colour constant properties. J. Colour Research and Applications, 1985, lO(2).

INFRARED CAMOUFLAGE 5.1

INTRODUCTION

Prior to World War 11, infrared camouflage did not seem to have been employed. But as mentioned by Cott in his book 'Adaptive Coloration in Animals'' and also referred to in Chapter 3, in nature, there are examples, s u c h as certain green caterpillars like Smerinthus ocellatus and certain tree frogs such as Hyla Coerulea, which have in-built mechanisms in them by which they escape detection in the visible region as well a s in the infrared region. That is, these small animals blend with their green background when observed by the normal eye which is sensitive to the visible region, and also blend with the background even if they had been observed by an infrared-sensitive eye. Initially, by the term "camouflage in war" was meant the means employed to defy detection of military objects by sensors available in the visible region of the electromagnetic spectrum, principally, the human eye - unaided and aided. But with the development of infrared false colour film, also known as camouflage detection film, during World War 11, the need for camouflaging military objects beyond the red end of the visible region arose. The infrared false colour film, which is sensitive u p to 0.9 pm,can detect military objects covered with cut foliage in a foliated background. The advent of infrared false colour film provided impetus for research and development in the field of infrared radiation. Since then, the field has steadily grown, with the main thrust on its application to military reconnaissance, surveillance and target acquisition. This, in turn, has put great stress on infrared counter-measures to reduce detection probability of military targets. It may be said that any development in the field of infrared engineering and technology is synonymous with the development of military infrared. Subsequently these developments have found application in industry, medicine and science. Open literature in the field has been limited because of its application in defence.

100

Introduction to Camouflage& Deception

The field of infrared camouflage-known under different names: infrared counter-measures, infrared signature suppression, etchas become vital to the success of any military operations in the various theatres of war. 5.2 WHAT IS INFRARED CAMOUFLAGE? The term infrared camouflage denotes any device or equipment or technique employed to counter detection by an infrared system. Advances in infrared sensor technology have put great stress on infrared camouflage and demanded counter-measures. 5.3 INFRARED RADIATION Infrared radiation was discovered by Sir William Herschel in the year 1800. While observing the solar spectrum, Herschel noticed that radiation a t and beyond the red end of the visible spectrum produced heat. The radiation responsible for causing this effect is the infrared radiation. Broadly, infrared radiation is that portion of the electromagnetic spectrum lying between the visible region and the microwave region. The unit of wavelength in the infrared spectrum is conveniently chosen a s the micrometer (pm)or micron (p) which is rn. The frequencies of this region of the electromagnetic spectrum are of the order of 1013to loi4Hz. The wavelength limits may be put a s 0.75 pm to 1000 pm which may be further subdivided2 into near infrared (NIR) 0.753pm, middle infrared (MIR)-3-6pm,far-infrared (FIR)-6-15pm and extreme-infrared (XIR) 15-1000pm. There are other c;lassificationsfound in literature3.In one such classification, 0.7-3 pm region is referred to a s shortwavelength infrared (SWIR), 3-6 pm region a s middle wavelength infrared (MWIR), 6-16 pm region a s longwavelength infrared (LWIR), and 16-1000 pm region a s far infrared (FIR). However, the universally accepted designations (1986)are: SWIR upto 3 ym, MWIR-3-6 pm, LWIR-6-16 pm and FIR beyond 16 pm. The upper limit of the infrared spectrum - (1000 pm) overlaps the microwave region4. 814 pm is also referred to as therrnal infrared. 5.4 SOURCES OF INFRARED RADIATION Sources of infrared radiation may be broadly divided into two classes - natural and man-made. 5.4.1 Natural Sources All bodies above absolute zero of temperature emit infrared radiation. The laws governing the radiation will be discussed later. The wavelength characteristics of some of the natural sources are given in Table 5.1. Since all heated bodies are sources of infrared

Infrared Camouflage

radiation, infrared radiation is also referred to as heat or thermal radiation. Table 5.1. Natural sources of infrared radiation and their radiation characteristics -

Source

Temperature K

- --

--

Wavelength Pm

Earth Human body Cosmos - - -

---

The s u n is a powerful source of infrared radiation whose characteristics will be discussed later. Other natural sources are s b , clouds, snow etc. 5.4.2 Man-made Sources Man-made sources may be divided into two groups - laboratory sources and other objects such as military targets. There are several laboratory sources of infrared radiation5-lo. They include tungsten lamp, xenon arc lamp, laser, the Nernst glower, the globar, etc. Before we discuss these we shall introduce the concept of a black body. A black body is a theoretical concept and such a source is not possible to make in practice. A real black body approximates an ideal black body in its radiation emitting characteristics. Such sources are of cavity type having a narrow opening of 1.25 cm or less. They operate in the range of temperature 125°C - 1025°C. 5.4.2.1 Carbon arc The carbon arc can operate at a temperature as high a s 6000 K and has wavelength range 2 - 10 pm. This source can be used a s solar simulator. 5.4.2.2 Tungsten lamp This can operate upto temperature a s high a s 3300 K. It provides high intensity infrared in the wavelength range 2 - 3 ym. 5.4.2.3 Xenon arc tamp It is a good source of infrared radiation in the wavelength around 1.5 pm. 5.4.2.4 Laser Laser a s a source of infrared radiation is of comparatively recent origin. Its special a t t r i b u t e s a r e high intensity, monochromaticity and coherence, which find special applications.

101

102

Introduction to Camouflage & Deception

5.4.2.5 The N e m t glower A typical Nernst glower consists of a cylinder 3 cm long and 0.15 cm diameter made by sintering a mixture of zirconium, yttrium, thorium and a few other oxides. At an operating temperature of 2000 K, it emits infrared radiation in the range 2-15 pm. This source is used in infrared spectrometers for measurement of infrared reflectance, absorptance and transmission of materials. 5.5.2.6 The globar It consists of a cylinder 5-10 cm long and 0.5 cm diameter, made up of silicon carbide2.At an operating temperature of 1500 K it emits infrared radiation in the wavelength range 2-15 pm. ~ h e s eman-made sources are used in the laboratory for calibration of radiometers and also as sources of infrared radiation in spectrophotometers and as target simulators. The infrared radiation emission characteristics of military targets will be discussed under a separate section-Infrared signatures of military targets. 5.5 TERMINOLOGY Radiometry deals with measurement of radiant energy, which may be from any part of the electromagneticspectrum ranging from X-rays, through ultraviolet, visible, infrared to microwaves. A radiometer, while measuring radiant energy, absorbs and converts it into some other form like electrical, thermal or chemical, as there is no method for direct measurement of radiant energy. As applied to infrared radiation, the various quantities of relevance, the symbols to denote these quantities, d the units in which they are expressed are the starting point for iscussing the properties and measurement of the radiation. The te inology and symbols of radiometry employed have variations a n d v s o undergo changes. Those that are given here are based on the rep.o,rt of the Committee on Colorimetry of the Optical Society of America which were also used by Hudson2. 5.6.1 Radiant Energy (U) It denotes the energy carried by electromagnetic waves. This can be the energy radiated by a source or received by a detector in a given time interval. It is denoted by the symbol U and its unit is joule. 5.5.2 Radiant Flux or Radiant Power (P) It is the rate at which radiant energy is radiated by a source or received by a detector. It is denoted by the symbol P and its unit

1

Infrared Camouflage

is watt. Radiant flux is a useful quantity which enters into many calculations. A s applied to a source, the terms - radiant emittance, radiant intensity and radiance are used. These are usually measured a t a specific distance from the source. 5.5.3 Radiant Emittance (W) It is the radiant flux ermtted per unit area of a source. It is denoted by the symbol W and its unit is watt/cm2 or watt/m2. 5.5.4 Radiant Intensity (J) It is the radiant flux emitted per unit solid angle. It is denoted by the symbol J and its unit is watt/steradian. In this context, it would be better to distinguish between point sources and extended sources. These are relative terms. In practice it is not possible to have point sources. A source extending over a short distance may be treated a s a point source at a long distance. For example, an extended source such as the tailpipe of a jet aircraft at a distance of 15 km may be treated as a point source. The term radiant intensity refers to a point source. In the case of an extended source, such as sky, it is difficult to define solid angle subtended by a detector a t the source. In the case of extended sources the term radiance (N) is employed. 5.5.5 Radiance (N) It is the radiant flux emitted per unit solid angle and per unit area of the source (extended source), i.e., the radiant intensity per unit area of the source. It is denoted by the symbol N. Its unit is watt/cm2steradian or watt/m2 steradian. In order to measure this quantity, the radiant flux is to be divided by the area of the source and the angle subtended a t the source by the detector. 5.5.6 Radiant Photon Emittance (Q) The response of some types of detectors depends on the number of photons rather than on the radiant flux. In such situations the quantity - the radiant photon emittance (Q) - is of significance. It is the number of photons emitted per second per unit area of the source. 5.5.7 Irradiance (H) This quantity refers to the radiant flux received by a surface. It is the radiant flux striking unit area of a surface. It is denoted by the symbol H and its unit is W ~ mor-W~m-2. The irradiance a t a distance d from a point source of radiant intensity J, neglecting the effects of the intervening medium, is H = J/d2

103

104


5.5.8 Spectral Radiant Flux (P,) It represents qualitatively the radiant flux emitted in a narrow band of wavelengths. It is the radiant flux per unit wavelength interval evaluated at a particular wavelength and is the limiting value of the expression

In order to evaluate the radiant flux P emitted between the wavelengths k,,and &, the following integration is to be carried out: A2

P = IpAdA

5-2

A1

If the integration is carried out between the limits zero to infinity, the value of P represents the total radiant flux. If integration is carried out over an interval of wavelengths, the value of P represents the effective radiant flux. The latter quantity is useful in the case of detectors whose response is limited to a wavelength interval. 5.5.9 Radiant Emissivity (e) It is the ratio of the radiant emittance of a source to that of a black body at the same temperature. It is denoted by the symbol E. It is a numeric varying between the limits zero and unity. This quantity is of great importance in infrared countermeasures and will be dealt with in detail subsequently. 5.5.10 Radiant Reflectance (p) It is the ratio of the reflected radiant flux to the incident radiant flux,and is denoted by the symbol p. It is a numeric varying between the limits zero and unity. 5.5.11 Radiant Absorptance (a) It is the ratio of the absorbed radiant flux to the incident radiant flux,and is denoted by the symbol a.It is a numeric varying between the limits zero and unity. 5.5.12 Radiant Transmittance (T) It is the ratio of the transmitted radiant flux to the incident radiant flux, and is denoted by the symbol T. It is a numeric varying between the limits zero and unity.

LAWS GOVERNING RADIATION EMITTED BY HEATED OBJECTS In infrared countermeasures, we should first of all know the infrared signatures of the military objects concerned. These

5.6

Infrared Camouflage

in turn depend upon the infrared radiation emitted by the objects. For understanding the radiation characteristics, a knowledge of the laws governing the radiation emitted by heated objects is essential. All matter continuously emits and absorbs electromagnetic radiation. The emission is a consequence of continual motion of elementary charged particles within the substance. One of the fundamental laws of classical electromagnetic theory is that an accelerated charged particle radiates energy. As the temperature of a body is raised, the thermal motion of the charged particles of matter - electrons and protons - increases, which results in the emission of radiation. The basic laws which govern the emission and absorption of law, (ii) Stefan-Boltzmann radiation by matter are (i) Kirchhoffs' law, (iii)Wien's Displacement law, (iv)Rayleigh-Jeans law, and (v) Planck's law". 5.6.1 Kirchhoffs Law Kirchhoft's law states that good absorbers of radiation are good emitters of radiation. Consequently, a black body, which is a good absorber of radiation, is also a good emitter of radiation. It has maximum absorptance (a = 1) and hence it emits maximum radiation. Because of this reason, a black body is used to compare the radiation emitted and absorbed by all other bodies. 5.6.2 Stefan-Boltzmann Law This law states that the total amount of radiation emitted by a black body is proportional to the fourth power of its absolute temperature. W, = aAT4t where T o

5-3

absolute temperature of the body in K 5 . 6 7 ~ W/m2 K4 (Stefan's Constant) A = Surface area of the black body in m2 t = time in seconds W,, = Total amount of radiation emitted by the black body For non-black bodies

where A e

= =

=

the emissivity of the surface of the body (defined in section 5.7.3)

105

106


5.6.3 Wien's Displacement Law This law gives the wavelength at which the radiant emittance from a black body at a given' temperature is maximum Assuming that the sun is a black body with a surface temperature of 6000 K,h, turns out to be 480 nm. Wien's law fits the experimental data only at short wavelengths and low temperatures. 5.6.4 Rayleigh - Jeans"Law This law satisfactorily fits the experimental data at long wavelengths and at high temperatures. But, according to the expression, the energy is found to increase without limit with decrease of wavelength. 5.6.5 Planck's Law This law fits the experimentally observed data of radiation emitted by a black body at short as well as long wavelengths. The law can be stated as2

where

w, -

the spectral radiant emittance in W ~ mpm-I - ~ the wavelength in micrometers Planck's constant (6.6256 f 0.0005) x W sec2 - absolute temperature in K - velocity of light (2.997925 0.000003) x 101° cm sec" - 2.rchcz (first radiation constant) = (3.7415 0.0003) x lo4W ~ mpm4 - ~ - ch/ k (second radiation constant) (1.43879 0.00019) x lo4 pm K - Boltzmann's constant = (1.38054 0.00018) x loz3W sec K-I Figure 5.1 gives the variation of spectral radiant emittance of a black body at various temperatures in the range 500-900 K.

h h

-

* * * *

Infrared Camouflage

Wavelength (microns)

Figute 5.1.

Vatiation of spectral radiant emittance of a black body at various temperatures in the range 500-900 K.

Source:

Reprinted by permission of John Wiley and Sons Inc. from Infrared System Engineering by Richard D Hudson, Jr. Copyright O 1969 by John Wiley 81 Sons Inc.

107

108


By integrating Planck's expression between the wavelength limits zero to infinity, we can arrive a t Stefan-Boltzmann law

where W

radiant emittance (W ~ m - ~ ) Stefah - Boltzmann constant We can arrive at Wien's displacement law by differentiating Planck's law and solving for the wavelength a t which the radiant ernittance is maximum h, T = constant 5-9 where h, = wavelength a t which the spectral radiant emittance is maximum. The value of the constant comes out to be 2897.8 0.4 CL~K A s the temperature increases, the wavelength a t which maximum spectral radiant emittance occurs decreases. 5.7 PROPERTIES OF INFRARED RADIATION 5.7.1 Propagation Characteristics Infrared radiation travels with the speed of light, like any other type of electromagnetic radiation, and, in its transit from the source undergoes reflection, scattering, absorption, transmission, diffraction and polarization. In most of the cases, the intervening medium between the source and the detector is the atmosphere. As the radiation passes through the atmosphere, it gets attenuated by its interaction with the various constituents of the atmosphere. This process is known a s extinction. 5.7.2 Extinction Coefficient If x is the path length and a the extinction coefficient, then the transmittance z of the path through the atmosphere can be expressed as2 2 = e-ax 5- 10 Two factors contribute to extinction. We can write o=a+y 5-11 where a is absorption coefficient which takes into account the entire absorption by various gaseous molecules of the atmosphere; y is scattering coefficient which takes into account the entire scattering by various constituents of the atmosphere (including haze, fog, etc) 90th a and y vary widely with the wavelength of the radiation. 0

= =

*

Infrared Camouflage

5.7.3 Atmospheric Windows Figure 5.2 shows the variation of transmittance of the atmosphere, in percent, with wavelength2. From the curve we can see several regions of high transmittance. These are known a s the atmospheric windows. The two most important windows are in the regions 3-5 pm and 8-14 pm. These two windows are exploited by various infrared sensing systems employed in reconnaissance, tracking, searching and target acquisition. These windows restrict the choice of materials for detectors a n d associated optical components. Of the various constituents12 water vapour is the most important absorbing gas interfering with the transmission of the infrared radiation. When the atmosphere is dry (3.5 g/m3), it is almost completely transparent in the windows. When the atmosphere is wet (19 g/m3) it totally blocks the atmospheric windows. Water vapour absorption occurs in two forms - molecular band absorption and continuum absorption. Very complex spectra characterise molecular absorption. Hundreds of vibration-rotation energy level transitions create the water vapour absorption bands. Local humidity conditions can greatly affect the content of water vapour. Next to water vapour, C02is the important constituent of the atmosphere responsible for infrared absorption. It has a constant weight ratio in standard atmosphere. Local effects, such a s automobile exhaust, dense foliage, and factory exhausts, can alter the standard distribution. Carbon dioxide affects both the atmospheric windows 3-5 pm and 8-14 pm. Nitrous oxide has a fairly constant concentration in the atmosphere. It exhibits strong absorption a t about 4.5 pm and 8 pm. Otherwise its absorption is insignificant. Methane has atmospheric absorption centered around 3-5 vm and 8 ym. Ozone exhibits strong absorption a t 9.6 pm. Carbon monoxide exhibits absorption in a band between 3.6 ym and 3.8 pm. Nitrogen affects the atmospheric window 3-5 pm. Oxygen does not have any significant absorption in the atmospheric windows. Table 5.2 shows the concentrations of the principal absorbing gases in the earth's atmosphere. The compositions are percent by volume. Table 5.2. Composition of the atmosphere -

Constituent

Percent b y volume

Constituent

Percent b y volume

Nitrogen

78.088

Kypton

1.14x104

oxygen Areon

20.949 0.93

Nitrous oxide Carbon monoxide

5x 10-~ 20 x l o 4

109

1 10


Constitue~t

Carbon dioxide Neon Helium Methane Source:

Percent by volume 0.033 1.8 x 10" 5.24 x l o * 1.4 x 10.'

Constituent Xenon Hydrogen Ozone Water vapour

Percent by volume 8.6 x l a 6 5 x 10" Variable Variable

NRL report 8311. Atmospheric Effects on Infrared Systems by Goodeil J. B. and Roberts R.E.

The atmosphere, besides the above constituents, contains suspended particles such a s dust, carbon particles, sand, ashes, water droplets, salt s p r a y a n d t h e like whose sizes a n d concentrations depend on local environment, and can therefore vary not only with locale, but temporariIy within a locale. The main contributors to aerosols are sea spray, fog, haze, dust storms and air pollution. Other contributors include forest fires, sea salt, rocks, soil, volcanoes, meteoric dust and biological materials. LOWTRAN - 3B Computer Programme12 developed by the Air Force Geophysics Laboratory, Cambridge, Mass., can be used fbr computing atmospheric transmission knowing t h e local meteorological conditions (temperature, relative humidity and visual range). It is currently the most widely accepted standard for computing atmospheric propagation. The model does not take into account man-made aerosols such as battlefield smoke, dust, etc. Fog and clouds scatter infrared radiation strongly, and as such infrared systems cannot be considered to have all-weather capability. 5.7.4 Emissivity Qualitatively, by the term infrared emissivity E of a n object is meant its ability to emit infrared radiation relative to that of a black body. The rate of emission of a black body is the highest compared to any other body under identical conditions, and hence the rate of emission of all other bodies is compared with that of a black body. The emissivity of a body is the ratio of the radiant emittance of the body to that of the black body under identical conditions. According to this definition, the emissivity of a black body is unity, that of a perfectly reflecting body is zero, and all other bodies have emissivities ranging from zero to one. Emissivity of a material is more a surface property than that of the bulk material. It depends upon the nature of the material, surface conditions of the material, wavelength and temperature. Besides these, there are other factors which have to be specified. In this context, several emissivities are introduced. They are: i) directional emissivity E, which is the emissivity determined a t an angle 0 to the surface per unit solid angle, ii) normal emissivity &, which is the emissivity corresponding to 0 = 0 and iii) hemisphekal

Infrared Camouflage

11 1

1 12

Introduction to Camouflage &SDeception

emissivity chwhich is the emissivity of a source radiating into

a hemisphere. It is given by2 w

I &(a)wa&

5-12

where ~ ( h ) i sthe spectral emissivity over a narrow band of wavelengths hand X+dh. We may classify bodies into three categories on the basis of the variation of €(A): i) For a black body ~ ( h= )E = 1 ii) For a grey body ~ ( h=) E = constant less than unity. iii) A selective radiator for which ~ ( hvaries ) with wavelength.

radiator Wavelength

8

B 8

.r(

elective radiator

3

g

% I?

rn

Wavelength

Figure 5.3.

(a)Wavelength v s spectral emissivity (b) wavelength vs spectral radiant emittance for black body grey body, and selective radiator.

Sow:

Reprinted by permission of John Wiley & Sons. Inc. from Infrared System Engineering by Richard D. Hudson (JR) copyright Q 1969 by John W i & Sons Inc.

Infrared Camouflage

Figure 5.3 shows the variation of spectral emissivity and spectral radiant emittance of the three classes of radiators. The concept of a grey body is useful because heat sources such as jet tailpipes, terrestrial bodies, personnel, etc. can be taken as grey bodies for engineering calculations. Also, selective radiators can be treated as grey bodies over a limited spectral interval. h a n d E,., assume special importance, as, to a large extent, all infrared detection systems depend for their action on the radiant fluxcontained in a small solid angle in a given direction. Actually, except for polished metallic surfaces, there is no significant variation2 between E ~q,,,and E,. 5.7.4.1 Measumment of € @ i r e dembsivity Methods for the determination of ernissivity may be broadly divided into two classes: i) direct methods and ii) indirect methods. There are two direct methods: a) calorimetric and b) radiometric. The calorimetric method measures the total hemispherical emissivity, i.e., in the wavelengths extending from zero to infinity. This method is not suitable where emissivity is required in a narrow band of wavelengths. Radiometric method uses spectroradiometers or broad band pyrometer (thermometer). In this method the radiant flux emitted by the source or surface is measured directly. It is quite satisfactory for the measurement of spectral emissivity or the total emissivity. Measurement of emissivity in 8 - 14 pm band of opaque or partially transparent/transparent materials was carried out by Mukherjee, et all3 by employing Minolta Land Compac-3 IR thermometer, Lillesaetar14 measured emissivity of a wide variety of materials by a radiometric outdoor method. The accuracy of the method compared favourably with that of the other methpds covering the wavelength band 8 - 14 pm according to the author. The difficulties involved in the direct method can be overcome by indirectly arriving at the emissivity from the measurement of reflectivity. These methods use an infrared spectrophotometer and a suitable reflectance attachment. The physical basis for this measurement is that the spectral ernissivity &(A) is given by15

4 4 = ~ - [ p d & + & ( A ) + r,(a)+ rr(AJ]

5- 13

In the case of opaque materials, the diffuse and regular (h) and T~ (h) vanish, and for a components of transmittance II~ rough surface the regular reflectance p, (h) also vanishes and therefore ~ ( hdepends ) only on the diffuse reflectance pd (A) Hence €(it)= 1 -[pd (Ai)l 5- 14

113

114


Polished metals, such as aluminium, copper and nickel, have emissivity of the order of 0.05, and polished cast iron has a higher emmissivity of about 0.2. Building materials such as sand, brick, cement and wood have emmissivity of the order of 0.9, water has still higher value of the order of 0.95 and human skin a s high as l2 Metals have a low emissivity and non-metals have a high ernissivity. As temperature increases, the ernissivity of metals increases with the formation of oxide layer. In the case of non-metals the emissivity decreases with temperature. The radiation emitted by any material originates from the surface, confined to a few microns. So surfaces coated with paint have the emissivity of paint. 5.7.5 EnrAssivity and Temperature Effects o n Contrast Infrared surveillance systems, in order to detect any military target, make use of contrast between the target and the background. In the near infrared region the contrast is caused by difference in reflectivity between the object and the background. In the thermal infrared (MIR and FIR) it is caused by the difference in the radiant emittance between t h e object a n d background. For countermeasures in the near infrared, the reflectivity has to be appropriately matched, and in the case of thermal infrared the radiant emittance of the object relative to the background has to be brought down. Although it is the radiant emittance of the object with respect to the background which is important, we are more familiar with the quantity 'emissivity', and its value for various materials is available in literature. Further, it gives a better idea as to the detectability of the target and the degree of reduction of radiant emittance required to reduce the detectability. Hence we shall devote a little more attention to this property and its relative importance in relation to the temperature of the body in producing thermal contrast. 5.7.6 Relative Effects of Temperature and Emissivity Differences on the Radiant Flux per Unit Area. Let us examine the two parameters-emissivity and temperature of a body-and their relative importance to radiant emittance which contributes to the contrast with respect to the background. It is not possible for the infrared imager to indicate whether the contrast in the imaged scene is caused by differences in emissivity or in temperature. Let u s consider the emissivities E and E + de and temperatures T and T + dT16.l7 and see the change on radiant emittance, W.

Infrared CamoufIage

We can consider three cases: (i) Entire spectrum of radiation where Stefan-Boltzmann law is applicable; (ii) Quasi-monochromatic radiation; (iii) A finite spectral band. Case i: Variations in the total radiation

(where W = dP) i.e., a relative change of 0.1 in temperature causes a relative change of 0.4 in W which is equivalent to a relative change of 0.4 in emissivity. This would imply that temperature changes are quite influential. But for most ambient measurements where T is about 300 K, emissivity has the stronger influence. Case-2 Variations in quasi-monochromatic radiation Let u s take the case where the sensor spectral band is almost monochromatic (1 pm wide) at 10 p.m. In this case the radiant emittance is given by Planck's law W, = ~ ( hc,X-~ ) {exp(x)-1)-I W C ~pm-I - ~ 5-17 where x = c2/hT and the relative change is given by

At the wavelength that maximises radiant contrast with respect to temperature the relation reduces to

For the case T = 300 K and E = 1 for monochromatic radiation at the maximum difference, the equation is

Case iii: Variationsfor ajinite spectral band In this case also it can be shown that for T = 300 K and E = 1, the relative change is given by

I 15

1 16


Thus it is apparent from the above discussion that for 300 K the effect of emissivity is much greater than that of temperature. 5.8 INFRARED SENSORS A knowledge of infrared sensors, sensing systems and their capabilities is of great importance in the design and development of infrared countermeasures. The history of infrared sensing systems may be divided into two parts: pre-World War-I1 scenario and post-World War-I1 scenario. 5.8.1 Pre-World War II Scenario During wars, before the advent of infrared sensing systems, night remained a moment of quiet except that patrols to a limited extent were carried out in the no-man's land. Infrared sensing systems extended the human vision beyond the visible end of the red region of the electromagnetic spectrum and enabled fighting at night18. The history of night vision appears to have started during the period 1910-20, when several patents were taken in this field.These devices were intended for detection of aircraft, artillery, personnel etc. One such infrared system could detect aircraft a t a distance of 1 mile and people at a distance of 1000 ft2. During 1941-43 the Germans incorporated an image converter in the fire control systems of battle tanks. The device proved to be very effective during night operation2.In 1945, US employed an image converter together with a n illuminator mounted on a rifle, enabling a soldier to fire accurately in total darkness a t targets located a t a distance of 75 yards. This was known a s Sniperscope2.lo. Another device developed by US was Snooper-scope. This made it possible to drive vehicles a t night. It had a n image converter and a n infrared illuminator 2* l . Several other infrared equipment reached limited operational status during World War 11. Some of the American equipments were: i) infrared guided bombs, and ii) scanning systems for detection of object by the thermal radiation emitted by them2. Besides these devices, infrared sensitive photographic films sensitive to the range 0.75-0.9 pm were used to detect military objects covered with cut vegetation. Till World War 11, military infrared did not receive much attention. Military infrared, subsequently, particularly infrared sensing systems, have rapidly progressed, enhancing their capabilities manifold. US, Germany and Japan in particular have visualised the potentialities of the infrared region of the electromagnetic spectrum in the battlefield.

Infrared Camouflage

5.8.2 Post-World War I1 Scenario Vietnam war clearly brought out the limitations of the existing night vision technology and the requirement of new technol~gies'~. During late 1950s information on heat seeking infrared guided missiles-Sidewinder and Falcon-was made available in open literature2. It is basically a system which optically focuses the infrared radiation emitted by the scene, mechanically scans the infrared image across a detector and converts it into an electrical signal which is then transformed into a visual image'K19.20. US Army, in 1952, under the name Thermograph, built the first thermal imager2'. During 1956-62 a number of thermographs were developed for the US army. In 1956 the first real time thermal imager under the name FLIR (Forward-LookingInfra-Red) was built at the University of Chicago with the support of the Air Force. In 1960, Perkin Elmer Corporation developed a thermal imager under the name Prism scanner for the US army. The period 1960-74 saw the development of sixty different FLIRs and production of several hundreds of them. Since then spectacular advances have taken place in thermal imaging technology. All the developments were aimed at meeting the military requirements in the field. The desirable requirements are long range, satisfactory operation over wide range of temperatures, good spatial resolution and large field of view. The important field constraints are size, weight, power consumption, field maintenance and cost. Advances in the field of electro-optics, infrared detector materials and solid state electronics have all immensely contributed to thermal imaging technology. Developments in the area of infrared lasers have enhanced the capabilities of thermal imaging systems. In the field of guided missiles, developments towards application of infrared seekers to autonomous target acquisition munitions are taking place22.Work in the area of digital image processing of thermal infrared images is also being pursued. US, France, Germany and UK are actively involved in R&D in the area of thermal imaging technology which would radically enhance the operational capability of armed forces during night and day23. The capability of thermal imaging systems has been amply demonstrated when U S Forces employed these devices in the Operation "Deseit Stormn during Iraq War. The American Forces had a great advantage in their ability to move forces a t night to a critical position and to attack the enemy who could not seez4.

117

1 18


5.8.3

Data Presentatio

Conversion of

Radiation from the Scene

Principle of an Infrared Sensing System The principle of a n infrared sensing system is diagrammatically shown in Figure 5.4. It essentially consists of three steps: (i) The infrared radiation emanated by the target scene is converted into electrical signals with the help of a transducer which is a n infrared detector;

(ii) The electrical signals are processed suitably; (iii) The processed electrical signals are presented in a form applicable in the intended role. From the time the infrared radiation is collected from the scene, till it is presented or used in a suitable form- viz, visual, digital or some o t h e r form, t h e physical processes involved are many and complex. The final data presentation d e p e n d s o n t h e n a t u r e of t h e application. 5.8.4 Classification of Infrared Sensing Systems Infrared sensing systems which are basically electro-optical in nature may be classified a s active and passive systems, or imaging and non-imaging systems. In a n active system the scene of interest is i l l u m i n a t e d by a n infrared s o u r c e (including laser beams) and the reflected radiation is utilized for sensing the target. In a passive system the thermal radiation emitted by the object is collected by an optical system and further processed to obtain the necessary information. In an imaging system the data are displayed in an image form similar to a TV picture. In a non-imaging system the o u t p u t is utilized in different forms depending upon the nature of the application.

Infrared Camouflage

Infrared engineering deals with the detection, signal processing and display of infrared radiation in a manner depending on the nature of the application. The infrared systems engineer should be familiar with physical and geometrical optics, infrared physics, electronics for signal processing and electro-optics. 5.8.5 Infrared Detectors The detector is the most important element of an infrared system. All other components are designed and built around it. Basically, an infrared detector is a transducer of radiant energy that converts the energy into some other suitable form such a s a n electric current or for affecting a photographic film, or change some physical property2. Broadly IR detectors may be classified into: i)Thennal detectors and ii) Quantum detectors. Besides, those that do not fall under these two categories may be discussed under 'miscellane~us'~. The first 1R detector was of thermal type - a thermocouple which was used by Herschel in his initial experiments on infrared radiation during 1830s2t3. In 1880 came the bolometers which are also of the thermal category. In the beginning of the 20th century came the quantum detectors which are being widely used since then. Quantum detectors have been steadily advancing with developments in semiconductor materials a n d solid state electronics, leading to two-dimensional detectors known a s staring arrays. Work has been in progress to develop detectors responding to far infrared region beyond 15 pm. Developments in solid state physics and material science such a s growing of super-lattices by molecular beam epitaxy (MBE) are all leading to more and more types of IR detectors responding to far infrared radiation. 5.8.5.1 Thermal detectors In a thermal detector, the heating effect of the infrared radiation causes a change in some electrical property of the detector material. In a thermocouple, rise in temperature of the hot junction sets in an emf. In a bolometer, which is a metal, there will be a change in electrical resistance. In a thermistor, which is a semiconducting material, there will also be a change in electrical resistance. In pyroelectric materials the heat produced results in a change in electrical polarization of the material. A thermocouple consists of two junctions made out of two dissimilar metals. A typical thermocouple consists of a copper wire and an iron wire joined end to end such that two junctions are formed. The junction which absorbs the incident infrared radiation

119

120


is known a s the hot junction and the other junction which is maintained at constant lower temperature is known as the cold junction. The difference in temperature between the two junctions sets u p a thermo-emf which is proportional to the increase in temperature of the hot junction. A number of thermocouples are available, some of which are copper-constantan, bismuth-silver and bismuth-bismuth-tin alloy. With a good thermocouple temperature differences of the order of 10-6OC can be detected. A number of thermocouples connected in series constitute a thermopile. A bolometer consists of two identical strips of a metal sach as gold which are connected to the opposite arms of a Wheatstone's bridge. Initially, when there is no radiation, the bridge is balanced. When radiation is incident on one of the strips of the bolometer there will be a change in its electrical resistance resulting in an imbalance of the bridge. A typical value of the temperature coefficient of electrical resistance of a bolometer material is 0.5 per cent per OC2. Thermistor infrared bolometers were developed by Bell Telephone Laboratories during World War-I1 . A change in the electrical resistance of the thermistor is converted into an electrical signal with the help of a bias voltage applied across the thermistor. . Thermistor bolometers have temperature coefficient of resistance a s high a s 4.2 per cent per OC2. Characteristics of thermal detectors (i) The response of the thermal detectors is independent of the spectral characteristics of the incident infrared radiation; (ii) They can operate a t normal, ambient temperature such that cooling is not required; (iii) They have comparatively low sensitivity and slow response. Their time constant is of the order of a few milliseconds. As such, they are not applicable in search systems or other online situations where high data rates are required2. Thermistor bolometers are rugged and their high resistance helps their matching to a n amplifier. Various thermistor materials and their characteristics are discussed in refren~es~'-'~. Superconducting bolometers have come into the field in comparatively recent years. During the transition from normal to superconducting stage there is tremendous reduction in the electrical resistance. At this stage the temperature coefficient of resistance is of the order of 5000 per cent per OC.Niobium nitride is one s u c h material which is u s e d i n superconducting bolometers2.

Infrared Camouflage

5.8.5.2

Quantum detectors

In a quantum or photon detector, the photons of the infrared radiation interact directly with the electrons of the detector material resulting in direct electrical conversion of radiation. Quantum detectors may be classified under the following groups3 : (i) Photoconductive type (Intrinsic) (ii) Photoconductive type (Extrinsic) (iii) Photovoltaic type (iv) Photoemissive type or Schottky barrier type (v) Photoelectromagnetic type (vi) Blocked impurity band type In general, an interaction between a photon and a n electron of the atom of the detector material results in the ejection of the electron. If it has sufficient energy, it escapes from the surface of the detector material. This phenomenon is known as photoelectric or photoemissive effect. In contrast to this effect which may be called external photoelectric effect (as the ejected electron is observed outside the boundary of the material) there can be several internal photoelectric effects. This happens especially when the infrared photons are of wavelength greater than 1.2 pm. In a semiconductor the incident photon can lift a n electron from a nonconducting state to a conducting state so that a charge carrier is available for conduction. In photon or quantum detector this is the type of interaction that takes place, resulting in an electrical signal. Photoconductive intrinsic type detector In this category, as a result of interaction between the infrared photon and an atom of the semiconductor, an electron from the valence band jumps into the conduction band crossing the forbidden band. In this process a hole is created in the valence band. For every electron that jumps into the conduction band there will be a corresponding hole in the valence band. The necessary condition for this type of interaction to occur is that the energy carried by the incident photon hv 2 Eg, where E, is the bandwidth of the forbidden band. The cut-off wavelength of the infrared photon h, = hc/E, = 1.24/Eg,where E, is in electron volts2. When the detector is biased by applying an electric field, changes in the number of charge carriers vary the current flowing through the detector. This is the principle of action of the photoconductive intrinsic type of detector. In these type of detectors, the bandwidth of the forbidden band is less than one eV. The conductivity is controlled by electrons as well as holes. The first

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photon detector was developed in 1920. It was photoconducting thalluim sulphide with a thermal response u p to 1.2 pm. Lead sulphide was developed between the two World Wars3. Some of the important intrinsic photoconductive detectors a r e silicon, germanium, lead sulphide, lead selenide, indium arsenide and indium a n t i m ~ n i d e ~ O - ~ ~ . Photoconductive extrinsic type detector The smaller the bandwidth of the forbidden band, the larger will be the cut-off wavelength a t which the detector responds. This is accomplished by adding controlled amounts of impurities into a pure semiconductor. When a pentavalent impurity is added to intrinsic germanium it becomes a n n-type semiconductor. When a trivalent impurity is added it becomes a p-type semiconductor. In the n-type, majority current carriers are electrons, and in the ptype the majority current carriers are holes. Doped germanium is used in a number of infrared detectors. Although in principle both p and n types can be used, only the p-type is being used in IR detectors. The incident infrared photon should satisfy the condition that hv 2 E, where E, is the energy of the acceptor above the valence band. These type of detectors need cooling, a s otherwise a t ambient temperature the energy of thermal agitation is large enough to release charge carriers, and none would be left for photon excitation. Detectors of this category responding to wavelengths in the range 3-8 pm need cooling to 77 K and those responding beyond 8 ym need cooling down to a few degrees from absolute zero. Photovoltaic detector This type of detector consists of a p-n junction formed by introducing trivalent and pentavalent impurity atoms into an intrinsic semiconductor. The infrared photons give rise to electronhole pairs which are separated by the electric field inbuilt in the material a t the junction. It has the advantage that it is a selfgenerating device, as it does not need any bias supply. Some of the important photovoltaic detectors are silicon and indium arsenide2. A photovoltaic detector is also known as a photodiode. The basic difference between a photoconductive detector and a photovoltaic detector i s t h a t i n t h e former there i s a n external bias supply whereas in the latter a photovoltage is formed a t the p-n junction. Photoemissive or Schottky barrier type Photoemission is very much used in the near infrared region. In such devices the released photoelectrons enter into a vacuum region, whereas in a Schottky barrier detector the released photoelectrons are collected in a semiconductor such as silicon.

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There exists a barrier potential I$,, a t the interface between the metal and the semiconductor and the incident photon e n e r a must be such thatZ0$,,
C

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i

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Figure 5.5.

Schottky barrier type detector.

Source:

Reprinted with permission from Wiley-VCH verlag Gmbh, Weinheim.

Photo-electromagnetic type detector When a magnetic field is applied perpendicular to the direction of current to an intrinsic type photoconductive detector the electronhole pairs are separated. These can operate upto 7 pm and at room temperature. Figure 5.6 shows the photo-electromagnetic type detector. Their performance is inferior to that photoconductive or photovoltaic type and they are hence not much used. Blocked impurity band type This is basically a n extrinsic type of photo-conductor. It is made by epitaxy and silicon doped with arsenic. It operates a t 10 K and h = 15 pm3 5.8.5.3 Far infrared materials As the wavelength of the infrared radiation increases, the energy carried by the photon becomes less. At such low energies the thermal noise becomes a serious probiem. Today's practical

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upper limit for detection is 200 pm. But with developments that are taking place in superconductor technology and with the availability of high T, superconductors the longer wavelength limit will further increase3,35-37. Incident Radiation

//

Ohmic contact

I L Magnetic field

Signal voltage or current

Figure 5.6.

Photelectromagnetic type detector.

Source:

Reprinted with permission from Wiey-VCH verlag Gmbh,Weinheim.

General Discussion on IR Detector Materials Lead sulphide (1 - 3 pm) operating in the range 300 - 77 K has been in use for the last five decades. PbSe (1-7 pm) operating in the range 300-77 K has been in use in IR systems for missile guidance and control, aircraft search and tracking systems and IR night vision. Indium antimonide (1-5 pm) is used a s a photodiode p-n junction. Several advances have taken place in the development of mercury-cadmium-telluride during 1977-86. It is used for operation in 8-14 pm region operating a t 77 K. It is a semiconductor detector of the greatest practical importance and performance". 5.8.6

Pb-Sn-Te, Si:In, Si:Gu, Pt-Siare others which are being used to cover 1-15 pm. Schottky diode has been the most recent one and it does not need cooling.

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Materials for pyroelectric detectors Materials for pyroelectric detectors are3: Triglycine sulphate (TGS)-single crystals; Organic films : Polyvinylidine difluoride (CH, - CF,) n (PVDF), Ceramics : LiTaO,, PbTaO,, PbTiO,, Pb,, La, TiO,, PbZrj, Tix03,Pb La, (Zry Ti,J,, Mn203, PbZrO, - PbTiO, - Pb,FeNbO, - UO, (PZFNTU)

,

Performance Characteristics of a Detector 5.8.7 The responsivity2 of a detector gives the performance of a detector. It is the detector output per unit input. The responsivity R = V, / HA, 5-22 where, R is expressed in V W-', V, is the rms value of the fundamental component of the signal voltage, H is the rms value of the fundamental component of the irradiance on the detector expressed in W m-2and A, is the sensitive area of the detector in m2.The response time of a detector is the time taken by the detector for its input to reach 63 percent of its final value after a sudden change in the irradiance. Although responsivity is a convenient parameter it does not give any indication of the minimum radiant flux that can be detected. Noise is always present in the output of the detector which obscures the signal. 5.8.7.1 Noise equivalent power (NEP) Noise Equivalent Power (NEP) takes into account the noise facto?. NEP is the radiant flux required to generate an output signal equal to the detector noise. NEP = HA, / (V,/V,) = HA, V, / V, 5-23 where NEP is expressed in watts, V, is the rms value of the noise voltage a t the output of the detector. 5.8.7.2 Detectivity (D) The best detector is the one with the lowest NEP. Accordingly, detectivity (D) is defined a s the reciprocal of NEP. D = 1/ NEP where D has the units of W-l. The lower the NEP, the higher will be the detectivity. Detectability depends upon several factors2,chief of which are wavelength of the incident radiation, temperature of the detector, area of the detector, bias current applied to the detector, chopping frequency and bandwidth of the circuit used to measure detector noise. Jones38introduced the quantity D*, the detectivity referred to an electrical bandwidth of 1 Hz and a detector area of 1 cm2.

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1

D'

=D ( A ~ A = ~ ) NEP ~

D* has the units of c m ( H ~ ) W-I ' / ~ when A, is measured in cm2 and Af is in Hz. The significance of D* is that it is the signal-tonoise ratio when 1 watt is incident on a detector having a sensitive area of 1 cm2and the noise is measured with an electrical bandwidth of 1Hz. 5.9 INFRARED SENSING SYSTEMS We shall now discuss some of the important infrared sensing systems employed in war starting from World War 11. 5.9.1 Infrared Telescope During World War-I1 a night vision system which is basically an infrared telescope was used for night operations in several applications. The heart of the system is an image conversion tube which is sensitive to 0.8- 1.2 pm. When infrared radiation covering these wavelengths impinges on the system it gets converted into visible radiation. The telescope consists of a n optical system for focusing the radiation, a photocathode consisting of silver oxide cesium layer (known a s S 1-photocathode sensitive to 0.8 - 1.2 pm), a n electrostatic focusing system, a phosphorescent screen and a n eyepiece. Such a n infrared night vision system is shown in Figure 5.739. I Voltage

Objective

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Light input

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Visible output

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Figure 5.7.

Infrared telescope (IR night vision system, early version).

The scene of interest is illuminated by infrared radiation from a tungsten lamp fitted with a n infrared filter. The reflected radiation from the scene is focused by an objective system, either of the

Infrared Camouflage

refractive or reflective type, on to the photocathode such that an image of the scene is formed on the photosensitive material. The photocathode emits electrons under the influence of the radiation. The electrons released in this interaction are of thermal energy but are accelerated under the influence of electrostatic field to sufficiently high energies. The energised electrons a r e electrostatically imaged on P-20 phosphorescent screen. The screen, upon receiving the electrons, releases a large number of visible photons. Such a system forms a n image of the scene on the phosphorescent screen with sufficient level of illumination and resolution which are within the range of human vision. This type of instrument was put to use in the following applications i) Battlefield surveillance (1000 m), ii) Weapon sights, iii) Driving binoculars, and iv) Periscopic instruments for tank or armoured vehicles. This type of infrared telescope used in World War I1 belonged to the generation of Zero Image Intensifier Tube. The system suffers from the disadvantage that it is active and the enemy can detect the source of infrared radiation. Several developments have taken place in night vision technology under the class of passive image intensifiers which utilize the background light at night generated by moon and stars. These systems have superseded the infrared telescope, and the system being passive cannot be picked u p by the enemy2~20~40~41. 5.9.2 Vidicon Vidicon2 is similar to television camera. It consists of a small evacuated tube, one end of which is sealed with a flat and transparent plate. The inner side of the plate has a transparent electrically conducting film. It serves a s signal electrode. Over the film is deposited a photoconductive material. At the other end of the tube is an electron gun. The electron gun releases a stream of electrons which scans the photoconductive layer. This is accomplished by deflecting coils mounted outside the tube. The signal electrode is a t a higher potential than the photocathode. The electron beam, in the absence of incoming radiation, keeps the back of the photocathode at the same potential a s the cathode film. When infrared photons enter the system through the transparent window and the transparent and electrically conducting film, electrical conductivity changes, which reduce the potential difference between the front and back of the photocathode. By the scanning of the electron beam an electrical signal is generated. Depending on the type of detector used, cooling is done. Detectors responding u p to 13 pm are employed. Orthicon is another TV type camera which is of the photoemissive type with a cut-off wavelength of 1.3 pm 2.

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Photothermionic Image Converter This is known a s thermicok. In this, photoemission varies according to thermal radiation absorbed. The heart of the system is a multilayer film of thickness of a fraction of a micron. This is known a s retina. The scene of interest is imaged on the retina so that a thermal image is formed on it. A flying spot scans across the retina, thereby releasing photoelectrons. The n u m b e r of photoelectrons emitted at any point i s proportional to the temperature a t that point. Thermicon has a spectral response up to 12 pm2. 5.9.4 Infrared Photography The normal infrared film has a sensitivity upto 0.9 pm. The sensitivity can be extended upto 1.3 pm by applying certain dyes. Such films respond to reflected infrared radiation from different objects in the scene. The infrared reflectivity varies with the object, especially when objects in the battlefield are covered with cut vegetation. The IR reflectivity of cut vegetation drops with time. This is because the reflectivity of vegetation depends upon the chlorophyl content and with time the chlorophyl content drops down. Objects camouflaged by cut vegetation cannot be detected by photography in the visible region whereas they can be detected by near infrared p h ~ t o g r a p h y ~ , ~ ~ . ~ ~ . 5.9.5 Evaporograph When a n oil film is exposed to thermal radiation, the film undergoes evaporation differentially depending on the temperature profile. The film is supported by a material which h a s high absorption coefficient for infrared radiation. The thermal image, depending on temperature variation, gives rise to corresponding variation in the thickness of the oil film. The film is viewed under reflected light. The interference effects in the reflected light form an image which can be viewed2. 5.10 THERMAL IMAGING SYSTEMS Thermal imager, in contrast to a n infrared telescope which is a reflection-band device, is a radiated band system. The image formed by a thermal imager is similar to black and white TV picture. 5.10.1 Basic Elements of a Thermal Imaging System The salient features of a thermal imaging system44are : (i) An objective lens system (ii) A mechanical scanning system (iii) Detector elements (iv) A cooling system (v) Signal processing section, and (vi) Display unit (computer) Figure 5.8 shows the principal components of a thermal imaging system44. 5.9.3

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m m * ) m L O U ) m

130


5.10.1.1

Objective lens system

The objective lens system picks up thermal radiation from the scene. The principal component of the objective system is a converging lens which is made of germanium. Germanium has a good transmission in the infrared and it can also stand the rigours of the battlefield conditions. 5.10.1.2 Optomechanical scanner The radiation collected by the objective lens system falls on the scanner. The scanner consists of a set of eight mirrors fixed on a rotating drum. Each of these mirrors is fixed to the rotating drum in such a way that they are at different angles from the axis of rotation of the drum. The entire scene may be imagined to consist of a number of horizontal bands. A s the drum rotates, the mirror that faces the scene collects the radiation from one of the bands of the scene. The design of the scanner is such that by the time it makes one complete revolution the entire scene consisting of the eight bands will be covered by the eight mirrors. The radiation reflected by the mirrors will fall on the detector arrays. A s the detectors receive the reflected beam, each band of the scene is further split into lines, i.e., each detector scans different parts of the scene. The detectors can also be arranged horizontally instead of vertically. Detectors arranged vertically provide parallel scanning, and detectors arranged horizontally provide serial scanning. 5.10.1.3 Detector bank The detector elements chosen depend upon the wavelengths at which the system is required to operate. If the system is required to operate in 8-14 pm band, the detector material is Hg-Cd-Te, and if the system is required to operate in 3-5 prn band the detector material is InSb. The number of detector elements to be used depends upon a number of factors: performance in terms of response, spatial resolution, scanning mechanism, cooling system, etc. The Hg-CdTe or InSb detector requires cooling to -196°C (77 K) employing liquid nitrogen. There are several ways by which liquid nitrogen can be supplied to the detector array: (i) The liquified gas can be kept in an open vacuum flask from which it can be fed to the detector array; (ii) By employing high pressure from a container and JouleThomson effect, the gas can be cooled and supplied to the detector; or (iii) By utilizing a closed cycle refrigerator.

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Each method has its own advantages and disadvantages. Of the three, however, the third one, although expensive, is preferable, since it has no logistic problems. 5.10.1.4 Electronic signal processing and display The signals from the various detectors enter a n amplifier section from where, through a multiplexer and signal processing section, they may be displayed on a computer display. 5.10.1.5 Performance characteristics The two important characteristics of a thermal imaging system are: (i) Spatial resolution or angular resolution (ii) Thermal resolution or temperature sensitivity The sharpness of the picture depends on the angular resolution. The angular resolution which is expressed in radians is the effective width of the detector element divided by the focal length of the infrared optics. The angular resolution is typically of the order of one milliradian and can also be a s small as 0.5 milliradian. Angular resolution is also referred to as the instantaneous field ofview (IFOV). Thermal resolution is a measure of the smallest difference in temperature that can be resolved. This primarily depends upon the performance of the detector. Temperature resolution is expressed in two ways:Noise Equivalent Temperature Difference (NETD)which is the temperature difference for which the signal-to-noise ratio at the input to the display is unity. The Minimum Resolvable Temperature Difference (MRTD) is the smallest temperature difference that is distinguished on the display. It is of the order of 0.3 K and can be a s small a s 0.1 K. The temperature resolution depends on several factors, viz., optical system performance, detector performance in terms of its sensitivity, and the signal-to-noise ratio of the signal processing circuitry. 5.10.2 Applications of Thermal Imaging System The infrared thermal imaging systems find application in all the theatres of war-land, air and sea44. 5.10.2.1 Land applications. These include: (i)Surveillance - for gathering intelligence about the deployment or movement of men, vehicles and equipment, (ii) Target acquisition - for detection, identification and location of targets for analysis and engagement, (iii) Night observationobservations can be carried out in darkness at considerable distances. The entire land applications are referred to as STANO. In order to carry out STANO, thermal imaging systems ranging from hand-held devices to tripod - or vehicle-mounted systems are used.

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For fire control, thermal images can be used where direct or indirect fire weapons are deployed. In the case of tanks the thermal imager is incorporated into the gunner's or commander's sight and a laser range finder is also integrated with the system. In the case of artillery fire control, the fall of shot can be observed by the earth or dust thrown u p by the shell. In the case of air defence, against the cold sky background the warmer aircraft and helicopters can be detected a s they provide good contrast. Depending upon the nature of the application, the thermal imager can be simple or sophisticated. For night vision for the antiaircraft gun, a relatively simple design is adequate, whereas for a surface-to-air missile system a multimode thermal imager controlled by a computer might be necessary. 5.10.2.2 Air-borne applications Thermal imagers can be used in fixed wing aircraft, helicopters and remotely piloted vehicles. In the case of fixed wing aircraft, they are used for navigation/attack for bombers, strike aircraft etc. These are referred to a s Forward Looking Infrared (FLIR).These are different from Downward Looking Infrared (DLIR)Line Scanners. These scanners scan the ground in the form of a strip along the aircraft track. The FLIR a s well as DLIR basically work in the same manner. The data can be obtained in digital form or in visual form, recorded on a photographic film, or a s a picture in real time. I n the case of navigation, the FLIR provides an unjammable and clutter-free display. In the case of attack on ground targets, FLIR in conjunction with a laser target designator/range finder is used. FLIR in airborne applications is usually housed in a pod as a retrofit. In the case of helicopters also it can be fitted in a pod. In the case of remotely piloted vehicle where it is under ground control, the thermal imager would need to be stabilized and the pictures are transmitted to a base via a broad band data link for real time viewing. 5.10.2.3 Sea applications The thermal imagers are used principally for defence against air or missile attack for detection and tracking of airborne targets a t short to medium ranges. Long range performance is less reliable owing to the long and frequently moist atmospheric path to the target. In certain circumstances at sea, 3-5 pm band will give better range performance than 8-14 pm band. FLIR finds application in maritime patrol aircraft which undertake passive search, surveillance and investigation tasks at night such a s antisubmarine warfare (ASW) and offshore monitoring.

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5.10.3 Manufacturers Barr and Stroud have Limited designed and manufactured a range of thermal imaging equipment44.One such system about which information is available in open literature is SHORTIE. It is an acronym for Short Range Thermal Imaging Equipment. It covers a range of 10 km. It has the following features :Waveband for operation Objective lens system FOV (horizontal) Spatial resolution Thermal resolution Scanner Detector No. of lines Cooling

Display Frame rate Power

8-14 ym 150 mm diameter germanium lens Over 100 millirad Better than 0.5 millirad 0.3OC Mirror drum parallel scan Hg-C-dTe About 260 Liquid nitrogen (8 hours/change or Joule-Thomson minicooler) CRT (can be remotely viewed) 25/s. 25 V batteries

Another manufacturer is M / s N.V. Optische Industry De Dude Delft, which under the name Infrared Line Scanner has designed and developed thermal sensors40. The system has interchangeable detector units for 3-5 pm and 8- 14 pm. It has angular resolution of 20 millirad and the image can be recorded on 70 mm photographic film. It is cooled by a closed cycle cooler requiring no refill. Differences between Thermal Imaging System and 5.10.4 Image Intensifier The difference between a thermal imager and image intensifielA2 is that in the latter the signal photons are emitted into vacuum and subsequently processed, whereas in the former separate sensing elements are connected to their dedicated amplifiers from where the signals go to a display unit. The intensity of an element in the display unit is proporational to the radiant intensity of the corresponding point in the scene. The display can be observed directly or a t some other location through a video. The resolution of thermal imager is less than that of the image intensifier. The sensistivity and resolution of a thermal imager

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depend upon the size and number of detector elements. Sensitivity increases with the total quantity of the detector material, whereas the resolution is inversely proportional to the size of the detector element. For greater sensitivity and resolution there should be a large number of detector elements of small size. The range performance of thermal imaging system is two to three times more than that of the image intensifier. Further, targets appear brighter in thermal imaging system than in image intensifier. Thermal imagers have superior performance in operation a t night a s weapon sights for armour and antiarmour weapons. The longer range of thermal imagers is a n additional advantage.

5.10.5 Future Trends Future developments in night vision technology will be the application of multiple sensors in conjunction with computers. These developments will lead to automatic target recognition systems. 5.10.6 General Considerations Concerning IR Operations with Thermal Imaging Systems Peter Harrison44has brought out the general considerations concerning IR operations: (i) In the absence of solar heating a t night, the background cools down. This gives rise to better thermal contrast between the target and the background which helps in easier detection of targets a t night; (ii) Engine compartments and exhausts with their characteristic hot spots can be clearly located in a thermal display; (iii) Vehicles, owing to their movements and formation of tracks, can be located and classified; (iv) IR radiation can penetrate through smoke, dust and haze better. In situations where target has hot spots, greater detection ranges can be obtained; Rain and fog adversely affwt the performance of IR systems (v) because of attenuation. Further, water film formed on the target suppresses its thermal profile; (vi) Since thermal imagers are passive their presence cannot be detected; (vii) One important point to be kept in mind is the extinction coefficient of the atmosphere. There is an exponential drop in the strength of the signal as it passes through the atmoshpere. Any enhancement of range can be accomplished only through exponential increase in sensitivity of the s y ~ t e m ~ ~ - ~ ~ .

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5.11 IMAGE PROCESSING There are several methods of image processing some of which are discussed below. 5.11.1 Single-element Scan In the case of single-element scan, line by line scanning of the focussed IR image is carried outlg. This gives rise to a n electrical signal a t the single element detector. The amplified signal is employed for driving a display, the scanning of which in synchronisation with the detector element reproduces a n optical counterpart of the thermal image. In order to get a steady image, the image must be scanned a t least 25 times per sec. A single element detector has a limited performance. 5.11.2 Multi-element scan Multi-element scan eliminates many of the limitations of the single element detectors. A s the number of elements is increased, the sensitivity increases. The sensitivity also increases with the detectivity of the individual detectors. Detector manufacturers throughout the world are seeking to develop materials and fabrication technologies to cope with the r e q ~ i r e r n e n t ~ In~multi. element scan, the arrangement of elements is such that a cumulative output is obtained by adding the output of the individual elements (serial scan) or so that a simpler slower-speed scanning system can be used (parallel scan), or a combination of the two is also practical. Figure 5.9 shows serial scanning of a detector array consisting of a row of 8 elements. The scanning mechanism can be the same as that for a single element. In order to get a n output signal equal to 8 times that of the single element, the 8 outputs are combined together in phase. A light emitting diode (LED) or a cathode ray tube [CRT] can be utilised for displaying the image.

Figure 5.9. Serial-scanning of eight element array.

5.1 1.3 Parallel-scan In this there will be one element for each line. The image is scanned in one direction a s shawn in Figure 5.10. Scanning is done horizontally. The required scanning speed i s therefore slower. Figure 5.1 1 shows a practical arrangement of parallel

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Figure 5.10. parallel-scan.

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136


scan. The oscillating mirror is silvered on both sides, one side of which sweeps the infrared image on the row of detector elements. The amplified signal from each detector element actuates a corresponding LED. The second side of the mirror together with the eyepiece forms a visual image. The image is displayed by a row of LEDs.

I

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Figure 5.11. Parallel-scan with an oscillating mirror.

Serial-parallel scan A matrix of 2-dimensional detector element arrays is shown in Figure 5.12, with a serial-parallel scan of 8 image lines at a time. Bringing out separate leads from each element to the signal processing circuits is a practical barrier. It has the output amplitude 5.11.4

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advantage of a serial scan with some of the speed and bandwidth advantage of the parallel-scan.

Figure 5.12. Serial-parallel scan of eight image lines.

5.11.5 Focal-plane Processing Arrays (FPAs) The serial scan has the drawback in that there is requirement of multiple preamplifiers and delay lines in the signal processsing. This drawback has been overcome in the SPRITE (Signal Processing In The Element) detector invented at the Royal Signals and Radar Establishment, Great Malvern, U.K. and developed a t MullardlY In this device a single strip of Hg-Cd-Te having only three connections serves the same purpose a s the complete conventional serial-scan detectors. It utilises only one preamplifier and requires no external delay lines. The signal accumulates within the element and is integrated within the elements itself. 5.11.6 Staring Arrays The SPRITE still suffers from the disadvantage in that it needs mechanical scanning. This drawback is overcome in staring arrays.

Figure 5.13. Photomicrograph of the sensitive area of a staring array.

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138


The optics associated with staring arrays has a limited role. It is restricted to focussing of the infrared image on to the array of detector elements. Figure 5.13 shows a staring array. Scanning is accomplished electronically by circuits integrated with the array. The function of the associated optics is reduced merely to focusing the infrared image on to the array of sensitive elements. 5. PI.% Schottky Barrier FPAs

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Figure 5.14. Camera based on Schottky diode. Source:

Reprinted with permission from Laser Focus World

A dramatic performance improvement can result45when one can incorporate several tens of thousands of detector elements in a n integrated focal plan array (FPA). One such device is the platinum-silicide Schottky barrier FPA. Schottky barrier sensor works similar to the visible TV ame era^^.^^. The focal plane array consists of metallic Schottky electrodes. When illuminated by IR radiation, these electrodes emit hot carriers. The resulting signal is utilised for giving video output. This type of sensor can have a thermal resolution of 0.2 M. Schottky sensors do not have moving optics. The associated electronics are less complex. They are low in cost and easy to manufacture. A camera based on Schottky diode is shown in Fig. 5.14. Schottky barriet FPAs are followed by FPAs of indium antimonide and mercury cadmium telluride doped with silicon.

Schottky barrier infrared charge coupled device (IRCCD). CCDs basically consist of a large number of photosensitive semiconductor elements-either photodiodes or photo-MOS

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elements, - each of which is associated' a separate MOS capacitor for storing the electric charge it produces, when incident photons create electron-hole pairs. The charge-holding capacity of each MOS capacitor is determined by the potential it is submitted to by an associated network of closely spaced electrodes. Staring imagers are the most mature FPA devices, in particular the Schottky barrier FPA operating in 3-4.5 ym range23.The highest performance FPAs are, however, the hybrid photovoltaic Hg-Cd-Te FPAs in staring or non-staring configuration. The technology of staring arrays will find applications in a wide range of systems: (i) (ii)

Homing heads for precision guided munitions; Terminally guided sub-munitions;

(iii) Light weight imagers; and (iv)

Remote surveillance devices

Charge Transfer Device Focal Planes 5.11.8 Metal-Insulator-Semiconductor (MIS)detectors are especially useful for focal plane applications when made with Hg-Cd-Te. Several charge transfer devices based on Hg-Cd-Te, MIS integrated technology have been developed for various applications. They include charge injection devices (CIDs)and charge imaging matrix arrays (CIMsj. Hg-Cd-Te CID staring arrays sensitive in the 8-10 pm range have been developed. The CIM consists of a 2-dimensional array of MIS detectors that can be row-addressed using on-chip MIS switch. The CIM is developed to overcome the limitations of CCDs and C I D S ~ ~ . IR SIGNATURES OF MILITARY OBJECTS AND BACKGROUNDS Variations in radiant emittance of a military object broadly constitute its IR signature which is utilized by an IR sensing system in performing its intended role. A military target has always associated with it a background which has its own radiant emittance characteristics. The infrared system must be able to discern the target from its background. The most important military targets whose IR signatures are utilized are aircraft, ship and tank. The associated backgrounds a r e t h e sky, s e a a n d land, together with atmosphere. The radiant emittance characteristics of these are of relevance in the design and development of countermeasures.

5.12

139

140


5.12.1 IR Signature of Aircraft Several mechanisms come into play that contribute to the aircraft signature. The engine together with solar insolation from outside are the chief sources of infrared radiation. The contribution from the engine depends upon its type. Detailed information on the radiation characteristics of aircraft engines is not available in open literature. However, from the scant information available, calculations on the thermal radiations from the aircraft engines can be made. The two types of engines that are of interest aretheturbojet and the turbofan. The chief components of a turbojet engine are given in Figure 5.15. Compressor

Turbme

Afterburner

\

\\,

& -

Flow diffuser

\ '

\

Combustion chambers

-i --

'\ EGT I\

Exhaust

Talpipe

thermocouples

-

Figure 5.15. The turbojet engine. Source:

Reprinted by permission of John Wiley & Sons Inc. from Infrared System Engineering by Richard D. Hudson, (JR) Copyright O 1969 by John Wiley & Sons. Inc.

Infrared Camouflage

The principal components of a turbojet engine are the compressor, combustion chambers, turbine, exhaust nozzle and tailpipe. A flow diffuser admits air into the engine from where it enters the compressor. In the compressor, the air undergoes compression from where it enters the combustion chamber. In the combustion chamber, the compressed air gets mixed with fuel and the mixture gets burnt. The products of combustion enter the turbine, where power is extracted to run the compressor. Then the gases undergo expansion to the ambient pressure through a nozzle at the end of the tailpipe. The two chief sources of thermal radiation to be considered are: (i)the tailpipe and (ii)the stream of hot gases known a s plume. In general, the tailpipe is the major contributor to thermal radiation. The source of heat for the tailpipe is exhaust gases. From the time the gases leave the turbine till they reach the exhaust nozzle, the temperature remains constant as shown in Figure 5.15. The temperature of the gases a s they leave the turbine is known a s the Exhaust Gas Temperature (EGT).The temperature

1

2

3

4

5

Wavelength (microns)

Figure 5.16. Infrared emission from Bunsen flame. Source:

Reprinted by permission of John Wiley Sons Inc. from Infrared System Engineering by Richard D. Hudson, [JR) Copyright 01969 by John Wiley & Sons Inc.

141

142


of the tailpipe wall may be taken a s equal to EGT, which may be of the order of 700°C during takeoff over a long flight it can be of the order of 500 - 600 "C, and a t lower speeds it may drop down to 350 - 400 "C. While making calculations on the thermal properties of a turbo engine, it may be considered a s a grey body having an emissivity of 0.9. Next to the tailpipe it is the plume radiation which is important. Carbon dioxide a n d water vapour are the chief constituents of the plume. The spectral characteristics of plume radiation are similar to those of Bunsen flame (Fig. 5.16). The temperature of the plume gases at the nozzle is about 15 per cent less than that of EGT. The exhaust gas temperatures of these engines are in the order of 450-650°C. The exhaust gases leaving the tailpipe have temperatures of the order of 350°C. The overall signature of the aircraft comprises: i) radiation from engine nozzle, ii) radiation from skin heated from the engine from inside and solar insolation from outside, iii) top of aircraft due to reflection of the cold sky, and iv) bottom of the aircraft receiving heat from the ground. The threat types and their corresponding spectral bands are given in table 5.3. Table 5.3. Aircraft threat types and spectral bands Threat t.ype -

-

Spectral bands (pm) -

IR missile s e e k e r s

IR S e a r c h a n d t r a c k

FLIR

Source:

Reproduced from 'The Infrared a n d Electro-optical Systems Handbook, vol 7Countermeasure systems, edited by David. H. Pollock - Chapter 2-'Camouflage suppression and screening systems' by David E. Schmieder with permission from the publishers -ERIM and SPIE optical engineering press USA (1993)and the author.

The most intense spectral region for an aircraft is 3-5 pm. The signature varies with the aspect angle or the direction from which the aircraft is viewed47.Figure 5.17 gives the aircraft signature at different aspect angles.

Infrared Camouflage

Figure 5.17.

Aircraft signatures at different angles.

Reproduced from 'The Infrared and Electro-optical Systems Handbook', vol 7Countermeasure systems, edited by David. H. Pollock - Chapter-'Camouflage suppression and screening systems' by David E. Schmieder with permission from the publishers -EMM and SPIE optical engineering press USA (1993)and the author.

Source:

Signature from the rear of the aircraft is prominent as the thermal radiations are contributed by internal gases or heat. Plume signature can be sensed from all aspect angles. The body of the aircraft gets heated by solar radiation due to friction between air molecules and surface. Hudson2arrived a t a relation for estimating the skin temperature which is given by 5-25 T, = To (1+0.2 r M2) where To r

= = =

M

= = =

T,

the ambient air temperature in K the skin temperature in K recovery factor (1.0 at stagnation point i.e. where air stream comes to a complete rest) 0.87 for turbulent flow 0.82 for laminar flow Mach number

I43

144


5.12.2 IR Signature of a Ship A ship has broadly two s i g n a t u r e ~ ~ ~ - ~in ~ -the o nspectral e band 3-5 pm and the other in the spectral band 8-14 pm. The former is used in missiles for homing on to the ship and the latter is used for identifying the ship. The 3-5 pm band signature is provided by the prime movers of propulsion, generation plants and the exhaust gases of the diesel engines used in warship. These exhaust gases are a t temperatures of the order of 200-300°C. The gas turbines that are being used in warships have higher exhaust temperatures, of the order of 500 "C. In general, the dominant sources of radiation in the 3-5 pm band are the exhaust gases, plume and the funnel which carries the exhaust gases. The low temperature parts of the ship which are nearly a t the ambient temperature provide the signature in the 8-14 pm. The low temperature parts are primarily the hull and the superstructure. A ship is detected against sea background or sea and sky together. The contrast between the background and the ship enables the detector to detect the ship. The background signature consists of emission from sea and sky and reflected radiation from other sources e.g. reflection of solar radiation from the sea surface or clouds, or scattering by atmosphere. The contrast that is provided between the background and the ship depends on the atmospheric effects.

IR Signature of a Tank The IR signature of a tank50 is formed by internal heating together with external heating by sun during daytime. The internal heating depends upon the operational status of the tank a s given below: (i) Vehicle stationary with engine running. The hull gets heated around the engine compartment; (ii) Vehicle on the move

5.12.3

A s the vehicle is on the move, the various parts of the hull get heated more intensely. Also parts of the running gear - in particular all rubber and rubberised components - get heated a s a result of mechanical load caused by the motion. After a long drive the area around the drive sprocket also gets heated. Further, the shock absorbers in the running gear get heated up. Areas around the turret engine compartment, and in particular, the exhaust gas and the air outlets, get heated up. A s the tank fires the barrel of the main gun the ground in front of the muzzle gets heated. Also, dust and sand blown u p by hot muzzle flash provide IR signatures. Further, after firing, the barrel of the machine gun and the main gun provide IR signature. A large calibre tank produces IR signature after 3-5 shots.

Infrared Camouflage

Solar insolation contributes to the IR signature due to external heating. The parts that show u p are: (i)parts of low heat capacity (thin plate, metal plates, etc.), and (ii) parts with low heat conductivity (rubber or plastic parts). The following components also provide the IR signature :- (i) pipe jackets, (ii)textile covering along the gun opening in the mantlet, (iii) external stowage boxes made of thin metal plate, (iv) rubber mudguards and rubber track skirts, and v) rubber parts of the armour (if present). A tank crew can also exert a partial influence on the IR signature. The signature of a tank does not remain constant. The signature keeps on changing a s the tank is in operation. The factors on which the IR signature of a tank depends are many. The nature of the terrain or the degree of ground consolidation also influence the IR signature. 5.12.4 IR Signature of Personnel The temperature of the exposed parts of the human body2 (skin) will be of the order of 32OC when the ambient indoor temperature is 2 l o C. Under these conditions the radiant intensity of a n average nude male assuming him to be a point target is 93.5 Wsr-l. At a distance of 1000 feet (300 m), if the atmospheric absorption is ignored, the irradiance due to human body is W. ~ m -About ~ . 32 per cent of this energy is emitted in 8-13 pm region and 1 percent in the 3.2 - 4.8 pm region. The emissivity of human skin is a s high as 0.99 at wavelengths longer than 4 pm and is independent of the colour of the skin. In the presence of clothes the radiant intensity a s well a s irradiance from human body will be less because of the lower values of emissivity and temperature of clothes. 5.12.5 IR Signature of Backgrounds The backgrounds of interest are the sky, the sea and the earth. Sun approximately radiates a s a black body a t temperature 6000 K. The sky radiance is approximately times that of the sun". This arises due to scattering of solar radiation. The maximum radiance of the sky would occur a t 0.5 pm and would have a value of about 3 ~ 1 0W - ~cm-' pm-l sr-l. The atmospheric emission is primarily from water vapour, carbon dioxide and ozone. If the effective temperature of the atmosphere is taken in the range 200300 K, the maximum possible emission from the source is given by the black body radiance for this temperature. The maximum emission occurs at a wavelength of 10 pm and has a value of - ~sr-'. approximately 10-3W ~ mpm-I Thermal Scenes Characterisation s f 5.12.6 Computer modelss1have been developed to describe a variety of thermal scenes. A few have been experimentally verified. These

-

145

146


models will be useful in the design of thermal imaging system as well as countermeasures against infrared detection of military objects. The scenes consist of backgrounds and targets5'. Thermal imaging systems utilise the two spectral bands 3-5 pm and 8- 14 pm in which signals from the scenes are received. The primary scene signal results from variations in radiant emittance from different parts of the scene. These in turn are due to variations in temperature and emissivity. The scene consisting of background and target (s) receives heat from sun during day and loses heat during night. How much the temperature of the object differs from the air temperature depends upon atmospheric conditions - cloud cover, humidity, wind etc. Objects with low emissivity have a tendency to take on the temperature of the air with a lag determined by the thermal mass and thermal conductivity of the object. In the case of objects with high emissivity their temperatures are strongly influenced by the physical characteristics of the scene objects and the radiation characteristics of the sky and atmosphere, unlike objects with low emissivity. Strong winds reduce substantially the temperature excursions within a scene. 5.12.6.1 Backgrounds Figure 5.18 shows five different background materials - sand, grass, asphalt, trees and wood-and their temperature a s a function of the time of the day. Table 5.451 gives the conditions for the calculation of temperature difference shown in the Fig. 5.18. Table 5.4. -

-

-

- - --

Ambient temperature Pressure Declination Visible Range Mixing Ratio Source:

Conditions for the calculation of temperature difference --

--

--

--

-

298 K

Cloud cover

0.6

1013 mbar 0 deg. 20 km 16

Latitude Wind velocity Range -

20"N 20 kmph 4 km

-

NRL report 831 1, May 1979 Characterisation of Thermal Imaging Scene-The Fundamenta,ls of Thermal Imaging Systems by Rosell F.A.

5.12.6.2 Scene objects Some of the scene objects of interest51 are truck, tank, ship etc. A truck when heated by nature appears very much like a truck on a FLIR display. Metal parts which have high reflectivity appear dark. When idling, the engine and exhaust become very hot, exhibiting localized areas of radiation. If the truck is moving along the road, again there is localized heating.

Infrared Camouflage

-

7

1

Wood

//

6

0

\\

Trees

2

4

10

12

AM

Time of day

6

8

PM

Figure 5.18. Temperature difference between various background materials and air as a function of the time of the day. NRL report 8 3 11, May 1979. Characterisation of Thermal Imaging Scene-The Fundamentals of Thermal Imaging Systems by Rosell F.A..

Source:

/

\

1

Solar

Radiation exchange

\

ii

+ ~

Direct

1

'

\

'_ "\Diffuse

r-+=

4

Convection

--- \

.

.-

Conduction

Figure 5.19. Factors which influence heating of a tank. Source:

NRL report 831 1, May 1979 Characterisation of Thermal Imaging Scene-The Fundamentals of Thermal Imaging Systems by Rosell F.A.

147

148


Tanks, which are massive, generally lag behind the terrain in temperature when parked. The engine and exhaust appear very bright when the tank is running . When driven, the bogie wheels and treads, as well as the rest of the tank get heated up, Figure 5.19 shows the various factors which influence the heating of a tank. These are the insolation, the radiation exchange between the tank and its surroundings, convection due to wind or tank motion, internal heat sources such a s the engine, and conduction to the earth.

I-

West

PM

AM Time of day (hours)

Figure 5.20.

Effective radiant contrast between the tank and the grass background as a function of the time of the day for various viewing directions in the 8-12 pm spectral band.

Source:

NRL report 8311, May 1979 Characterisation of Thermal Imaging Scene-The Fundamentals of Thermal Imaging Systems by Rosell F.A.

The signal obtainable from a scene object is a function of the viewing aspect angle and therefore the signals will be time dependent on even a very short-time basis when the sensor is moving. The radiant contrast between the tank and the grass background is shown in Fig. 5.20. The figure shows effective radiant contrast between the tank and the grass background a s a function of time of the day for various viewing directions in the 8-12 pm spectral band. Figure 5.2 1 shows the thermal contrast of a ship against a sea and a n air background a s a function of the time of the day. RatchesS2 a n d Rodax et a153have discussed in detail the characterisation of thermal scenes.

Infrared Camouflage

Ship

//-\ Air

Ship n . \

Air

/

+

--------_----=-_-

= Z _

\ -..

< . - - - - - __---------______ --

-

:1

-

.

Sea

., ...

_ . _ - - - . _--'-:-=-- --_---_

Time (hours) Figure 5.21. Equivalent temperature of ship, the ambient air and the sea in a semi-tropical and cold northern area vs time of the day for a given set of operating conditions. Source:

NRL report 8311, May 1979 Characterisation of Thermal Imaging Scene-The Fundamentals of Thermal Imaging Systems by Rose11 F.A.

5.12.7 Computer Generated Imagery Sheffer and Catheart54 have given an account of eomputirgenerated imagery. Synthetic Image Generation (SIG)finds several applications in military, viz. i) for training, ii) system performance evaluation, iii) algorithm development, and iv) mission planning. To collect actual imagery is prohibitively costly. The advent of FLIR initiated the IR synthetic image generation during 1960s.

149

150

Introductiofi to Camouflage & Deception

Generation of synthetic images in the visible region is comparatively easier, whereas for the generation of imagery in the infrared, the computational complexity is very high. 5.12.7.1 Components of synthetic scenes Synthetic scenes for models of real scenes are 3-D computer representations. All the components of a real scene need not be incorporated in the synthetic scene. Only a relatively small number of important features will be incorporated in the model. The various components of the model are background, objects, atmosphere, dynamic processes, and astronomical objects. Backgrounds considered are terrestrial surfaces viz. terrain, ocean, sky etc. SIC is of interest in detecting, tracking or recognizing a target in its background. By background is meant all the scene features except target. Objects which are natural such a s trees and rocks, a s well a s man-made objects such a s buildings, vehicles and targets are of interest. Atmosphere includes phenomena such as clouds, fog, haze and turbulence. Atmospheric modeling can be done by LOWTRANS5. For any given set of atmospheric parameters, the model provides transmission and solar scattering for given viewing paths. Fire, smoke, explosions, dust, clouds and atmospheric turbulence come under dynamic processes. Astronomical objects include earth, sun and moon. The model takes into account their effects on the scene. The sun is considered a s a black body a t temperature 6000 K. 5.12.7.2 Paradigm for IR Synthetic Image Generation Sheffer and Cat heart54 consider a s an example the scenario of a n FLIR flying on a platform at a n altitude of 50 m and a range of 3 km, and a rural area consisting of trees, shrubs, rocks, cattle and various military vehicles placed in the terrain a t 1000 hrs. It has rained the previous night. Sky is cloudy. There is light haze. It is required to simulate the imagery generated by the FLIR. The authors proposed a paradigm for IR synthetic image generation consisting of the following major steps: Step - 1 This step incorporates all the features of the scene in terms of temperatures and radiances. This takes into a c c o u n t the various factors t h a t influence the temperature and radiance of each element of the scene. These include self-emissions due to internal heat sources, heat exchange witht the environment, and absorbed and reflected solar radiation, and all the scene interactions - background-to-object, object-tobackground and object-to-object.

Infrared Camouflage

Step - 2

In this step computer graphics algorithms are applied for predicting the out put generated from Step-1. The output of the Step-2 is not viewable image. Step - 3 An ideal image is produced from the output of Step-2. Step - 4 This s t e p t a k e s into a c c o u n t p a t h - d e p e n d a n t atmospheric effects. The atmospheric attenuation on the ideal image is calculated for the path between the scene and the sensor. Step - 5 In this Step, the physics of the electromagnetic scattering and absorption in the atmosphere is considered for computing the degraded image a t the sensor. Step - 6 Next, the effects of the sensor on the image must be accounted for. All IR image sensors introduce noise. In order to accurately simulate these effects a sensor model is needed which is based on the phenomenology of the sensor. This requires precise information on circuitry characteristics such a s Instantaneous Field of View (IFOV), s c a n parameters, detector/preamplifier parameters, spectral band pass and signal processing. The output of the Step-6 is the predicted image which is viewable. The electro~nagneticlaboratory of the Georgia Research Institute under the sponsorship of US Army Missile Command started this work. There are several software modules comprising the total SIG package. The program is called GT VISIT. It is designed to generate synthetic imagery in the spectral region 0.4 - 15 pm of a 3-43 scene54. IR SIGNATURE SUPPRESSION (IRSS) OF 5.13 WARSHIPS Infrared signature supperssion is essential for warships. Antiship missiles equipped with IR detecting systems can search, track and identify warships. IR guidance systems fitted in missiles can home on to the target passively. The ship under attack b e s not receive any advance information. The examples of missiles which are equipped with the above type of IR detecting systems are the Hy-2A version of the SILKWORM, PENGUIN Norwegian version, the AGM-I 19, the NATO's first IR guided missile and the SLAM new ship attack version of the H a r p ~ o n ~ ~ , ~ ~ . In spite of the fact that there are several devices avalable today for suppressing IR signatures of ships, it is not possible to reduce the signature to such a n extent so a s to evade detection altogether, because the IR detection technologies have greatly advanced. Today, to detect an object with modern PR sensors a fraction of a degree Celsius temperature difference is adequate58. However, once the

151

152


signature(s)associated with the ship is suppressed, the attention of the missile can be directed towards an IR decoy which becomes a much stronger source of infrared radiation with the requisite spectral characteristics. This enhances the chances of survivability of the warships7. A ship with a n untreated IR signature presents to a thermal imager or IR seeker a s a n extended body of large area with comparatively less brightness. Besides, there will be conspicuously bright localised hot spots with an overall brightness contrast significantly differing from the background. Now we shall discuss the spectral characteristics of IR sources of the ship in detail which are responsible for providing the thermal contrast to the IR detector. 5.13.1 Spectral Characteristics of IR Signature(s)of Ships A ship is a source of IR radiation emitting broadly in the spectral bands 3-5 pm and 8-14 pm a s discussed earlier. Thesc are also the bands which are transmitted by the atmospheric constituents. The exhaust uptakes, the exhaust plumes and th: hull and associated superstructure are the principal parts of thc ship which contribute to the IR signatures. Now we shall consider the relative importance of these sources in terms of radiance and radiant intensity. We shall take a hypothetical case a s discussed by Birk and DavisJ7.They have taken the areas of hull, plume and visible uptake surfaces a s 1500 m2, 20 m2 and 5 m2 respectively. These areas correspond to a side view of the ship. The background temperature is 15°C and the hull is at a temperature 5°C above the background. They have assumed the temperature of the uptake surfaces and plume a s 400 "C, Table 5.5 summarises the data. Table 5.5.

Source

Different sources of IR radiation on a ship and percentage of total black body radiation from these sources, (assumed background temperature = lS°C) Temperature FC)

Assumed area (m2)

% black body

radiation 3-5 pm

8-12pm 26

Hull

20

1500

1

Plume

400

20

4

Exhaust d u c t

400

5

28

Source:

19

Suppressing the Infrared Signatures of Marine Gas Turbines prcsentcd by Birk A X1 a n d Davis WR a t t h e Gas turbine and Aero-Engine Congress a n d Expositior.. June 5-9 1988. Amsterdam; reproduced with permission from ASME International. New York, USA)

Infrared Camouflage

If these sources are assumed to radiate a s black bodies, a certain percentage of the total radiation emitted will be in the two principal spectral bands mentioned earlier.Table 5.6 gives the values of radiance and radiant intensity of hull, plume and exhaust in the two principal spectral bands. Estimates of radiance and radiant intensity of principal sources of a ship (a) without and (b) with the effects of background (assumed background temperature = 15OC)

Table 5.6.

source

Radiance W/sr/m2 3-5 m

Radiant intensity W/sr 8-12 pm

('4

(a)

(b)

(a)

(b)

(a)

(b)

(a)

Hull

1.3

0.02

33

2.45

1950

30

49500

Plume

74

73

1480

1458

-

Exhaust

985

984

4925

4920

3340

Note :

For the plume radiation the percentage o f black body radiation is based on the 4.3-4.55 pm waveband.

Source:

Suppressing the Infrared Signatures of Marine Gas Turbines presented by Birk A.M. and Davis WR at the Gas turbine and Aero-Engine Congiess and Exposition, J u n e 5-9 1988.Amsterdam; (reproduced with permission from ASME International, New York, USA).

668

638

3675

3188

In the analysis given by the authors57the hull and the uptake metal surfaces are assumed as grey bodies with a n emissivity of 0.95. The plume is assumed to have an emissivity of 0.5 and the predominant wavelengths in the plume radiation are 4.3 ym and 4.55 pm the source of which is the selective radiation of carbon dioxide present in the plume. From the above table it is seen that the plume and the uptakes are the highest radiance sources of a ship. A comparison of the values in the table shows that the uptake has a radiance 760 times that of the hull and 13 times that of the plume in the spectral waveband 3-5 pm. A similar comparison in the 8- 12 pm wavelength band shows that the radiance of the uptake is 20 times more than that of the hull. The hull is the most dominant source in the 8- 12 pm band, but the effects of background have also to be taken into consideration. The large hull area compensates for its low radiance. In the 812 ym waveband, the hull is the most significant source in terms of radiant intensity. But in the 3-5 ym waveband all these sources are more or less equally important. Broadly, the IR signature consists of two parts - an extended source consisting of hull and super-structure with its spectral characteristics lying principally in 8-12 pm waveband, and the

153

154


exhaust duct and plume with their spectral characteristics principally lying in the wavelength band 3-5 ym. The latter are the hot spots. Quantitatively, 99% of the ship's signature is in 3-5 pm band while 46% is in the 8-12 pm band; both of which can be reduced by cooling plume and exhaust The approximate percentages of the thermal radiation characteristics of ship's sources are summarised in Table 5.7. Table 5.7. Percentage contribution of different sources to overall ship signature (background at 15°C) - ----

Source

-

% contribution of sources to

total ship signature 3-5 pm

8-12pm

Hull

1

54

Plume

23

0

Exhaust duct

76

46

Source:

Suppressing the Infrared Signatures of Marine Gas Turbines presented by Birk A.M. and Davis WR a t the Gas turbine and Aero-Engine Congress and Exposition, June 5-9 1988. Amsterdam; with reproduced permission from ASME International. New York, USA).

In a real situation the ship's signature, as mentioned earlier, depends upon viewing angles, background conditions and many other factors. 5.13.2 IR Signature Suppression Infrared signature suppression have received considerable attention a s a result of various conflicts around the world. Tribal class Update and Modernisation Programme {TRUMP)and Canadian Patrol Frigate (CPF) were the programmes of the Canadian Navy for the development of IRSS devices5$.In order to adequately reduce the IR signature of the ship to a safe value it is necessary to cool the metal surfaces of the exhaust uptake to near-ambient temperature, and similarly the temperature of the exhaust plume must be brought down to a level where its radiation characteristics are of the same order a s those of the cooled metal surfaces of the exhaust uptakes. The two IRSS devices are: i) The DRES Ball, ii) The Eductor Diffuser.

5.13.3 The DRES Ball The Defence Research Establishment, Suffield (DRES),Canada developed a device under the name DRES BALL It suppresses the IR signature of a ship a t all angles of view. It consists of an outer duct that is film-cooled. The outer duct surrounds a convectively film-cooled optical block centre body and a film-cooled 57p58.

Infrared Camouflage

diffuser. The role of the centre body is to optically block the view down the exhaust uptake trunking i.e. it cuts off the direct line of sight of the heated metal surface. All the metal surfaces are cooled either convectively or by film cooling. The film-cooling layers get mixed with the primary exhaust stream bringing down the temperature of the plume. Air which is introduced into the centre body through four hollow support struts cools the core of the exhaust plume. Figure 5.22 gives the principal parts of the DRES BALL IRSS device. The Eductor Diffuser 5.13.4 This principally consists of a n ejector pump for entraining air to cool the plume and a film-cooled diffuser to cool the metal surfaces. This device has the drawback that it cannot suppress the signature of the metal surfaces a t all viewing angles. It is effective u p to viewing angles 70" above the horizontal of the metal surfaces. Figure 5.23, shows the principal components of the Eductor Diffuser.

Centrr body

,

1

Exhaust

now

/

Figure 5.22. The Dres Ball. Source :

Reproduced from International Defense Review, Mar. 1991, with permission, O Interavia SA, Switzerland.

Effective IR signature suppression. Broadly, the two devices, DRES BALL and Eductor Diffuser, bring down the IR signatures of ships to a considerable limit.

155

156


1

Mixing length

'

hffuser

v Exhaust gases

Figure 5.23. The Eductor Diffuser. Source :

Reproduced from International Defense Review, Mar. 1991, with permission @ Interavia SA Switzerland.

Besides these two devices, other types of IRSS incIude the fan-assisted forced cooling system of the Japanese Maritime SelfDefence Force and the Pepper Pot and a fan-assisted Petal Nozzle system of the UK Royal Navy. There is another device known a s BLISS which is a Boundary Layer Induced Stack Suppressor. This consists of a 4-nozzle eductor accompanied by a mixing tube and entraining BLISS device for metal surface cooling. All these IRSS systems have significant impact on the ship performance parameters such a s back pressure effects on the engine, weight, centre of gravity effects and noise. 5.14

IR SIGNATURE SUPPRESSION OF AIRCRAFT

IR signature of aircraft has been discussed earlier. Here we shall confine ourselves to the salient aspects of the signature from

Infrared Camouflage

the point of view of signature supression of turbofan engine. The IR signature may be described in terms of the contributions made by Fan

Figure 5.24. Turbofan Engine parts. Source:

Reproduced from 'The Infrared and Electro-optical Systems Handbook', vol 7-Countermeasure systems, edited by David. H. Pollock - Chapter 2: 'Camouflage suppression and screening systems' by David E. Schmieder with permission from the publishers -ERIM and SPIE optical engineering press USA (1993)and the author.

the point of view of signature supression of turbofan engine. The IR signature may be described in terms of the contributions made by the plume and hot parts of the engine and the body of the aircraft. The engine is the most important component of the aircraft which contributes to the IR signature. Smoke and contrail also contribute to the signature. Of the various propulsion engines, the turbofan, the turbo-jet and turbo-prop, the first one is the most widely used in military aircraft. Figure 5.24, shows the turbo-fan engine parts47.There are rotating fan blades in front of the compressor. In order to provide direct thrust the fan air bypasses the compressor, burner and the turbine sections. The bypass air can be expelled or ducted to the rear of the engine. The ratio of the fan air mass bypassing the compressor to the mass flowing through the compressor is a cycle parameter and is called the bypass ratio. In order to accomplish cooling of the hot parts the by pass air can be used. 5.14.1 Suppression of Plume Signature In order to suppress the IR radiation of the plume the following methods may be adopted: i) Engine size reduction; Plume radiation depends on the speed, pay-load requirements and airframe size. In order to reduce the plume radiation these parameters have to be reduced. ii) Cycle tailoring.

157

158


As mentioned earlier, it is the turbo-fan cycle which is used in fighter and bomber aircraft. Let u s discuss here the cooling of the turbo-fan cycle. The various parameters that a r e to be considered are: (i) the internal design features, (ii) by pass ratio, and (iii)equipping the engine with axisymmetric nozzle. By mixing the exhaust gases with fan bypass air and further with the free stream air in the ejector, cooling of the plume can be accomplished. This cooling results in the suppression of the signature in the 3-5 pm wavelength band. By making use of a n airframe deck behind the exhaust nozzle obscuration can be accomplished. 5.14.2 Suppression of the Signature of Hot Parts The methods adopted for signature s u p p r e ~ s i o n of~hot ~ engine parts are: (i) obscuration, (ii) cooling, (iii) emissivity control, and (iv) physical size reduction. These techniques are compatible with the plume cooling methods. Unlike the plume the hot parts of the aircraft are not visible a t all angles of view. Further, their view can be obstructed by using a serpentine duct and also by the use of a plug in exhaust nozzle. Although these measures obstruct the viewing of the hot turbine section, they themselves must be cooled. In order to cool these parts use is again made of turbine fan air, inlet and compressor bleed air. Emissivity control must be carried out keeping in view the location of the hot part. A s the emissivity is reduced the reflectivity correspondingly increases. Ernissivity control will be effective when employed in locations that reflect only emissions from cooler surfaces into viewable regions. An optimum combination of emissivity control and cooling would be appropriate. Cooler parts can have higher emissivities. 5.14.3 Suppression of Signature of Aircraft Body Aircraft body signature47can be observed over all aspect angles and a t great distances. An aircraft body signature exhibits broad intensity variations and spatial patterns. The degree of detail that can be obtained is a function of range. At longer ranges it is not possible to resolve details. At such distances only detection is possible and the details get mixed u p with the background clutter. At long distances what is important in camouflage is matching of the spatially integrated radiance with the background radiance. However, at short distances matching of the spatial characteristics with those of background become important. In this context the resolution of the threat sensor is of great importance. We may discuss the body signature under two different conditions: - (i) resolved aircraft, and (ii) unresolved aircraft. At close ranges, the external features of the aircraft-shape, size, internal pattern, structure and silhouette - are all resolved. These features mostly come under visible part of the

Infrared Camouflage

electromagnetic spectrum. In order to suppress these features, we adopt methods of visual camouflage. In the infrared region, contrast can be reduced by controlling emissivity with appropriate coatings and with active heat transfer techniques. Suppression o f Signature o f Unresolved Aircraft 5.14.4 When the various features of the aircraft are not resolved47, what is important is to match the integrated spectral radiance of the body of the aircraft a s a whole with the background radiance. When the aircraft is operating a t high altitudes and also when the skin of the aircraft body has low emissivity, the aircraft body reflects the earth-shine very effectively. This excessive reflection of the earth's shine which can be picked u p by uplooking sensors can be reduced to a certain extent by shaping. Even then, at certain viewing angles, it is not possible to reduce the sunshine in order to match the IR body emissions to the cold background of a high altitude sky. In such cases some form of cooling is necessary. When a n aircraft is at lower altitude and sensors from above the aircraft are looking down, the situation is different. The sensors see the aircraft against a warm earth background. In this case the parts of the aircraft which have high reflectivity can reflect the cold sky giving rise to negative contrast against the ground. Under these conditions adaptive colour techniques such a s the use of chromogenic surface coatings may be effective. There should be adequate matching of IR self-emissions with solar irradiance and earth and sky reflections. Self-emissions become strong at high subsonic and supersonic speeds. Low surface emissivities can satisfy the requirement. But a t the same time the corresponding high reflectivities create a problem arising out of higher solar reflections. All these conflicting requirements may be met by employing glossy planar surfaces. In order to obtain zero contrast with the background, the skin of the aircraft should have emissivity E, given by the relation47 where

background radiance. radiance specularly reflected by the panel towards the threat sensor. LBB,= Planck black body radiance. IR SIGNATURE SUPPRESSION OF TANK 5.15 IR signature of a tank has been already discussed earlier. A tank which is well-camouflaged has a 50% increase in its survival rate in the battlefield. Effective camouflage can increase fire power and mobilitfjO. Lo

=

L,

=

159

160


The thermal signature of a tank may be minimised to a certain extent at the design stage itself. In the majority of designs the engine is kept a t the rear. The countermeasures for surveillance and detection in the case of tanks may be divided into two classes6': i) passive countermeasures, and ii) reactiv~countermeasures. Passive countermeasures become ineffective when a tank is moving. By reactive mode is meant measures taken in response to threats observed or perceived by the crew of the tank. 5.15.1 Passive Countermeasures In passive countermeasures parts of low emissivity are used. When the adjacent patches in a disruptive pattern have different emissivities, disruption of form takes place and IR silhouette effects get reduced. The temperature difference between the tank and its surroundings should not be more than about 2"-6°C. Although this is not easy to achieve, since the temperature of the surroundings within the scene can vary by more than 10°C,this variation renders the thermal signature of the tank to blend with the background clutter. In the near IR region, the reflectance of cut-foliage markedly differs from the live foliage. When viewed a t IR wavelengths the cut- foliage covered vehicle tends to contrast with the surrounding living trees. NIR camouflage was one of the first nonvisible wavelengths to be addressed. Barracuda (Sweden) in 1957 had introduced the first VisibleINIR camouflage net employing IR reflective materials in the form of patches on the net60. Thermal camouflage is achieved by reducing the apparent temperature, size and emissivity of a heat source6]. In practical terms it is almost impossible to eliminate the thermal signature of a vehicle. But it is possible to reduce the signature to a level at which it will cease to present a conspicuous hot spot in a thermal viewer. Exhausts present another problem, for these tend to be angled outwards or upwards to reduce the amount of dust thrown u p and can present a hot cloud above a vehicle. This can be countered to some extent by mixing in air to cool the exhaust gases. Also, exhaust gases will heat u p near-by vegetation or buildings and these will create useful clues for intelligence purposes, sometimes long after the location is vacated. In theory, shining metallic surfaces emit less heat than matt or dark surfaces. Lighter paints reduce thermal signature by reflecting solar radiation. U.S. tanks in Persian Gulf were sprayed with the new Tan 686, a paint which reflects radiation u p to 85%, reducing external surface temperature by lS°C, a s well a s cooling the interior.

Infrared Camouflage

To date most modern multispectral camouflage has been aimed at a combination of visible, near infrared and radar wavelengths. The missing bands are covered by FIR. Combination of thermal and radar camouflage is difficult to achieve since low emissivity surfaces tend to provide good radar reflections, although Flectalon metal foil thermal blanket provides a degree of attenuation through scatter6'. 5.15.2 Reactive Countermeasures When the vehicle is in motion it is easy to detect it by several methods ranging from visual observation to Doppler radar. Gulf War demonstrated6' the vulnerability of columns of moving vehicles to surveillance by long range radar. We have to think of other methods to reduce the chances of detection, such as decoys or smoke. Smoke is one of the oidest methods61which can be deployed either by individual vehicles when fired a t or on a large scale to screen the movement of armoured units. Smoke can be deployed61 by means of a vehicle-mounted grenade launcher. Smoke on a large scale is now produced by the traditional method of firing shells or rockets, and also by injecting diesel fuel into exhausts of tank engines. The white smoke is effective only against visual observation. This has led to the development of multispectral screening smokes. Such systems can be based either on a combination of absorption, scattering and reflection produced by clouds, containing relatively large particles, or a cloud of IR radiating material, the emission characteristics which provide effective sceening in the IR a s well a s visible regions of the spectrum. This can only be used in a reactive mode a s the number of such smoke grenades that can be carried on a vehicle is limited. Smoke grenades require a t least two seconds to begin to become effective, which adds to the response time of the counter-measures based on them. Neverthless, smoke grenade launchers remain a useful item of armoured vehicle equipment and can be used not only to deploy smoke but also some type of decoys. One type of decoy, namely IR flare, is already offered a s a part of the Galix Combat Vehicle Defence System produced by Etiemne, Lacroire, and can be fired from the same type of launcher a s the smoke and other munitions available with the system. The Galix IR flare has been introduced to disturb the tracking of early types of SACLOS antitank guided missiles with simple beacons. Flares of this type could also be used to decoy housing missiles or sub-munitions with hot spot sensors. Flare decoys are discussed in the chapter on deception. Smokes have been upgraded to increase their thermal signature and opacity, thus degrading the ability of IR sensors.

161

162


5.16 SIGNATURE SUPPRESSION OF GROUND OBJECTS In the case of ground equipment, in order to accomplish reduction of contrast with the background in the infrared region, we must consider the thermal profile of the object concerned and its variation. This depends upon several properties of the object viz. material, color, shade, and angle to the direction of insolation. The profile also depends upon the nature of the background, viz. soil, vegetation, other objects in the background, etc. The thermal profile also varies from season to season, between day and night, and during the day itself. The US camouflage community during 1 9 8 0 arrived ~ ~ ~ a t the conclusion that if the temperature variations in the object are within 4OC of its background, thermal matching can be accomplished from the camouflage point of view. 5.16.1 Suppression o f Signature o f Non-hardware By hiding objects inside shadows of other objects, the detection probability can be reduced in all the bands47-visible, near infrared a n d thermal infrared. This is because of reduction of the corresponding contrast. Although this method cannot diminish the thermal signature due to internal sources of heat, it reduces the contribution from solar insolation. However, the signal strength d u e to internal sources, s u c h a s h e a t exchanger, i n the direction of threat sensor can be reduced by a proper orientation of the object. Natural materials such a s live foliage and cut foliage can also be used a s a cover in order to suppress the thermal signature of ground objects. 5.16.2 Thermal Camouflage Equipment and Materials All ground equipment to a large extent are camouflaged by disruptive pattern painting and screens such a s nets. Disruptive patterns with appropriate reflectance characteristics will be effective in the visible and near infrared regions. But these methods cannot reduce the thermal signatures due to internal sources. The various factors that contribute to the thermal signature due to internal sources are the type of the engine driving the equipment, its location and orientation in the equipment, the size of the equipment, heat transfer properties of the component parts etc. The designer of the equipment must keep all these factors in mind while arriving a t a n overall low thermal signature. Also, when the vehicle is on the move, the tracks formed by the wheels can give rise to thermal signature. Ways and means to reduce such signatures must be developed. As the tank's main

*

Infrared Camouflage

gun is fired, it gives rise to a characteristic signature. By proper cooling of the barrel, this effect may reduced. Wherever possible it would be better to incorporate signature suppression measures a t the design stage itself. This has distinct advantage in that the troops do not need any training7. 5.16.2.1 Disruptive patterns When disruptive pattens are drawn with paints with their reflectance properties matching with those of the background in the visible and near infrared, they are effective against sensors used in those regions. 5.16.2.2 Camouflage screens The camouflage screens known a s camouflage nets are employed all over the world for camouflaging all ground equipment. The US Light-weight Camouflage Screen (LCSS) is one such net. The nets are made in modules consisting of hexagonal-shaped or diamond-shaped garnished screens. Depending on the size of the object to be camouflaged, one or more modules can be used. Any number of modules can be mated to form a big screen. A single module can have a size varying from 10-100 m2 . Here we shall discuss only thermal screening effect of these nets. When a n object is covered with a screen, it partly cuts off solar heating, so that the thermal signature due to solar heating is partly reduced. A s far a s reducing the thermal signature due to internal heating is concerned, it is not that effective because of its significant open space. An object covered with a screen will present a much larger area to a sensor, and, a s such, it becomes a much bigger target when covered. While camouflaging heat-producing equipment with a screen, great care must be taken. The net must be erected in such a way that the exhaust gases do not produce hot spots or raise the overall temperature of the screen. On the whole, with the help of the screens described above, besides reducing detectability by sensors in the visible and near IR region, solar loading and secondary heating effects due to exhaust gases can be minimised. Barracuda, Sweden has been working on the development of camouflage nets with low thermal emissivity. By coating the nets with paints of low thermal emissivity their temperature can brought down. Such nets can merge into the clutter of the natural background.

163

164

Introduction to Camouflage & ~ec&tion

5.1 6.2.3 Thermal blankets or tarps The thermal signature of a vehicle having internal sources of heat can either be suppressed or distorted by spreading the vehicle with a cover known a s thermal blanket or tarp. Such a blanket acts a s a thermal insulator which retards the transfer of heat from internal heat sources to outside. US during 1970s worked on such a blanket47. The thermal blanket developed by Diab-Ban-acuda61(Sweden) carr effectively screen the thermal signature of a tank when it is covered with the blanket. It consists of a metallic film with a plastic coating and a fabric backing and has a pattern of vent holes to allow controlled escape of warm air. Over the thermal blanket a thermal net is put to make the object blend with the background. It consists of a metal foil between layers composed of a t least two polymeric materials of different emissivities. In combination with paints, CAMTEX texture mats developed in Denmark enhance the efficacy. A Swedish company FFV has developed a device similar to tliermal blanket. This consists of a paper bladder filled with a foam material. When a n object with internal heat source is covered with such a system it suppresses the thermal signature due to internal heat sources. Another device developed by a British company is a thermal tarp. It has emissivity of 0.5 on either of its sides. When a n object whose thermal signature is to be suppressed is covered with the, trap the outer temperature will be brought down because of the comparatively lower emissivity.

REFERENCES 1. 2. 3.

4.

5. 6.

Cott, H.B. Adaptive coloration in animals. Methuen & Co, London 1966. Hudson, R.D. Infrared system engineering. John Wiley & Sons, New York, 1969. Gopel, W.; Hesse, J . & Zemel J.N. (Ed)., Sensorscomprehensive survey-Optical Sensor. Vol 6, edited by Wagner, E, Dandilkar, R. Spenner, K. V.C.H., Weinhkim, New York, Bassel, Cambridge, 1992. Spiro, I . J . & Schlessinger, M. Infrared technology fundamentals. Marcel Dekker, Inc. New York and Basel. 1989. Rocca, A. La & Zissis G.J. Field sources of black body radiation. Rev. Sci. Instr. 1955 30, 200. Simmons, F.S.; De Bell, A.G. & Anderson, Q.S., A 2000°C slitaperture black body source. Rev. Sci. Instr. 1961, 32, 1265.

Infrared Camouflage

Quinn, T.J., A practical black body cavity for the calibration of radiation pyrometers. J. Sci. Instr. 1967, 44, 22 1. Yamada, H.Y. A high-temperature black body radiation source Appl. Opt. 1967, 6, 357. Benarie, M.M. Optical pyrometry below red heat. J. Opt. Soc. Am. 1957, 47, 1005. Lalos, G.T.; Corruccini, R.J. & Broida H.P. Design and construction of a black body and its use in the calibration of a grating spectroradiometer. Rev. Sci. Instr. 1958, 29,505. Jamieson, J.A. et al. Infrared physics and engineering. Mc Graw Hill Book Co. Inc., New York, 1963. Goodell, J.B. & Roberts, R.E. Atmospheric effects on infrared systems. NRL Report, 83 11. Mukherjee, R.N. Measurements of thermal infrared emissivity using infrared thermometer. Technical Report, DMSRDE (DRDO),Kanpur, India. Lillesaeter, O. Simple radiometric method for measuring thermal broadband emissivity of material samples; Appl. Opt. 1991, 30 (34), 5086-5089. Clarke, F.J.J. & Larkin, J.A. Measurement of total reflectance, transmittance and emissivity over the thermal IR spectrum. Infrared Phys. 1985, 67 (1121, p. 359-67. Wolfe, W.L. J. Opt. Soc. Am. 1974, 64, 546. Wolfe, W.L., Appl. Opt., 1975, 14, 1937. Accasto, M. Night vision systems. Defence Today/4-5 OPTRONIC. Chiari, J.A. & Morton, F.D. Detectors for thermal imaging, electronic components and applications. 1982 4, Technical Publication, M-82-0099. Dupuy T.N. (Ed-in-chief).Night vision technology application. International Military and Defence Encyclopedia, 1993 4, Brassey's (US),Inc. Washington, New York, p. 1953. Lloyd, J.M. Thermal imaging systems. Plenum Press, New York 1975. Hame, T.G. (Director) Precision guided munition seeker technology. Technology Programme, General DynzEics Pomona Division, Bomona, California, USA. Smith, D.A. 8t Dennis, P.N.G. Future thermal imaging technology. Defence Systems International, 1988, 1, 26 1-65. Acchione , L.A. Electro-optics provide vital military capacity. Signal, 1991, 92-94.

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Wormser, E.M. Properties of thermistor infrared detectors. J. Opt. Soc. Am., 1953 43, p. 15. Melman, I.M. & Meltzer, I.M. Status report on infrared thermistor detectors. Proc. Natl. Electronics Conf, 1962, p. 556. Becker, J.A. Bolometric thermistor. 1947, US Patent No. 2, 414, 792. Wormser E.M. & De Waard R.D. Construction for thermistor bolometers. U.S. Patent No. 2,963, 1960, p. 674. Wormser, E.M. Bolometer, 1961, US Patent No. 2, 983, 888. Cooperstein, R. Method of forming a photosensitive layer of lead sulphide crystals on a glass plate. January 8, 1962 US Patent No. 3,O 18,236. Johnson, T.H.; Cozine H.T. & McLean, B.N. Lead Selenide detectors for ambient temperature operation. Appl. Opt., 1965, 4, 693. McLean, B.N. Method of production of lead selenide photodetector cells. August 22, 1961. US Patent No. 2,997,409. Spencer, H.E. Chemically deposited lead selenide photoconductive cells. February 1964, US Patent No 3,12 1, 022. Morton, F.D. & King R.E.J. Photoconductive indium antimonide detectors. Appl. Opt. 1965. 44, 659. Van Duzer, T., IEEE J. Quantum Electron. 1989,25 (1I), 2365. Rose, K. IEEE Trans. E.D., 1980, 2 7 (I), 118. Bednorz, J. G. & Muller, K.A. 2. Phs., 1986, B-64, p- 189 Jones, R.C. Phenomenological description of response and detecting ability of radiation detectors. Roc. Inst. Radio Engrs. 1959,47, 1495. Baratte H.F. & King D. Night vision Tubes and solid state device. Special Electronics, 1984, 3, 38-40. Jagat Bhushan & Mehta, K.R. Night vision electro-optical instruments. J. Instr. Society of India, 1973. 3 (2),p. 10-190. Csorba, I.P., Current status and performance characteristics of night vision aids. Proceedings of the International Symposium on Optoelectronic Imaging, New Delhi, Dec 2-5, 1985, Chief Editor, Jugal, D.P., IRDE, Dehradun, Tata McGraw Hill Publishing Co Ltd., New Delhi, 1985, p. 34-63. Clarke, W. Photography by Infrared (2nd Ed.) Wiley, New York 1946.

Infrared Camouflage

Infrared and ultraviolet photography. Publication No. M3, Eastman Kodak Co, Rohester, New York, 1961. Harrison, P. Modern, weapon technology-thermal imaging and its military applications. Barr & Stroud Ltd. Mueffelmann, W. & Sato, I. Second generation technology enhances military imaging. Laser Focus World, 1989, p. 109. Ravich L.E. Thermal imaging rev. Laser Focus/Electro-optics, 1986, p. 99. Schmieder, D.E. & Walker, G.W. Camouflage, suppression and screening systems countermeasure systems. Edited by David H. Pollock, Vol7, IR/EO Systems Handbook, ERIM and SPIE Optical Engineering Press, Bellingham, Washington, USA, 1993. lnfrared emission from ships. Technology, Naval Forces, 1991, 1 2 (4),p. 24-29. Gates P.J . Infrared signature of warships. JNaval Engineering, 1990, 30 (I), p. 153-172. Hilmes, R. A.F.V1sand thermal imagers. Military Technology, 1994, 10, p. 77-80. Rosell, F.A. Characterisation of the thermal imaging scene the fundamentals of thermal imaging systems. NRL Report 831 1, May 197'9, p. 7-19. Ratches, J.A. Performance model for thermal imaging systems Optical Engineering, 1976, 15 (6). Rodak, S.; Zege, F. & Stump, W., Thermal signature measurements of four US Army field portable power generators N.V. & E.O.L. Report, Aug 1975. Sheffer, A.D. & Catheart, J . M. Computer generated IR imagery; A first principle modelling approach. SPIC Vol 933, Multispectral Image Processing and Enhancement, 1988, p. 199-206. Kneizys, F.X. et al. Atmospheric transmittance/ra$iance computer code LOWTRAN-6, Airforce Geophysics Laboratory 1983, Report AFGL-TR-83-0 187. Foxwell, D. Soviet navy reduces warship IR signatures. International Defence Review, 1990, 23 (1I), p. 1213-17. Birk, A.M. & Davis, W.R., Suppressingthe infrared signatures of marine gas turbines. Presented at the Gas Turbine and Aero-Engine Congress and Exposition, J u n e 5-9, 1988, Amsterdam, The American Society of Mechanical Engineers The Netherlands p. 1-10.

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58. Foxwell, D. Keeping cool at sea; canada leads the way in IR suppression. International Defence Review, 1991, 3, p. 2505 1. 59. Lynch, T.G. Eliminating "Hot Spotsn, Canadian Naval IR Suppression Systems. Navy International, 1992, 97 (5), p. 119-23. 60. Hammick, M. The Invisible art of camouflage, International Defence Review, 1992, 8, p. 749-54. 6 1. Ogorkiewiez, R.M.Countermeasures for tanks. International Defence Review, 1989, 1,p. 53-57.

MICROWAVE CAMOUFLAGE 6.1

INTRODUCTION Microwave camouflage deals with the various means that are employed to defy detection of military objects by sensors responding to the microwave region of the electromagnetic spectrum. As radar is the chief sensor of microwave region, microwave camouflage is actually referred to a s radar camouflage or radar countermeasures. Microwave camouflage is the principal component of stealth technology or low observable technology. The role of radar in reconnaissance, surveillance and target acquisition has greatly increased in the modern technology warfare scenario. This, i n t u r n , h a s p u t a great s t r e s s on r a d a r countermeasures. The major military objects which need microwave camouflage cover are the fighter aircraft, the naval warship and the tank, which are detected by their distinct microwave signatures. The camouflage measures involve suppression of these signatures. The microwave signature of a military object is known today as its Radar Cross Section (RCS). Thus the problem of microwave camouflage is one of reducing RCS of military objects such that the object escapes detection by radar.Only those military objects which come under the influence of radar threat are the candidates for microwave camouflage. This chapter deals with the basics of microwaves and their generation, microwave sensors, broad categories of radars, role of radar in war, RCS of military objects, its theoretical prediction for objects of simple geometry, and methods for reducing RCS of military objects. 6.2 WHAT ARE MICROWAVES 3 Microwaves are electromagnetic waves of frequencies ranging from a few hundred Mega Hertz (MHz)to a few hundred Giga Hertz (GHz). The prefix "micro" reflects neither the size of the wavelength of these waves nor does it bear any relationship with the unit "micrometer"; it merely indicates the smallness of these waves in comparison with the wavelength of the radio waves'. However,

170


microwaves are discussed separately from radio waves because sf differences in their properties. Although the limits of frequency of the microwave region are open for debate, the microwave spectrum may be said to span the frequency range 300 MHz - 300 GHz. Table 6.1 shows the approximate ranges of wavelength, frequency, time period and photon energy of the microwaves. Table 6.1. Physical characteristics of microwaves Wavelength Frequency Time period

3 ns - 3 ps

Energy (photon)

1.2 x 10-6eV - 1.2 x

1 m - 1 mm

300 MHz - 300 GHz eV

Majority of microwave equipment available today utilize the frequency range 1- 100 GHz.

6.2.1 Properties of Microwaves Microwaves can be propagated through free space or through waveguides, which are different from conventional transmission lines. Unlike radio waves, which are either totally or partially reflected, microwaves pass through the ionosphere. This property is utilized in satellite communication and space transmission. Their wide bandwidth is a distinct advantage in their application in telecommunication a s information carrier. Unlike radiations in the X-ray and ultraviolet regions, these radiations are incapable of causing ionisation, as their photon energy is less than the energy required to break molecular bonds. Microwaves are strongly absorbed by water. This property finds application in microwave cooking. The frequencies of stable atomic oscillators viz, those of hydrogen, cesium and rubidium, lie in the microwave region, and as such atomic clocks make use of these waves1. Absorption of microwave radiation by matter a t resonant frequencies finds application in physical and chemical analyses. Microwaves bear some resemblance to acoustic waves in that their wavelength range (1 m - P mm) corresponds to that of sound in air in the frequency range 300 Hz - 300 kHz2. The microwave spectrum is broadly divided into three bands. These are Ultra-High Frequency; 300 MHz to 3 GHz (UHF),SupraHigh Frequency; 3-30 GHz (SHF), and Extra-High Frequency 30-300 GHz (EHF). The microwave spectrum is sandwiched between the radio frequency spectrum and the optical spectrum. Figure 6.1 gives the physical characteristics of the microwave spectrum.

Microwave Camouflage

6.2.2 Microwave Spectrum 300 MHz

ULTRA HIGH FREQUENCY

CC

5

I

I

3 GHz &

CC

SUPRA HIGH FREQUENCY U

EXTRA HIGH FREQUENCY 300 GHz

Figure 6.1.

The microwave frequency spectrum.

Source:

Reprinted with permission from Introduction to microwaves by Frede Gardiol Artech House, Inc., Norwood, MA, USA http : //WWW.artechhouse.com.

All commercial microwave equipment, by and large, operate in the frequency range 0.5-40 GHz. For example, microwave oven operates a t a b o u t 2.2 GHz, microwave relay (telephone) a t about 4.0 GHz, satellite television a t 4 GHz (downlink) and 6 GHz (uplink) 3.

171

I72


6.2.3 Radar Frequencies Radar frequencies have no fixed limits. Any device that detects and locates targets by transmitting electromagnetic radiation and receiving the reflected radiation from the targets can be termed as radar. In practice, most of the radars operate in the microwave region of the electromagnetic spectrum. Table 6.2 gives the letter designations old and new (NATO's) commonly used for radar frequencies4. Table 6.2. Letter designations for radar frequencies Nomenclature (old) P L S C X

J(Ku) K

Frequency range (GHz) 0.225 1 2 4 8 12 18

Q (Ka)

27

V

40 60 20 60

O(E)

K M

Source:

6.3

-

1 2 4 8 - 12 - 18 - 27 - 40 - 60 - 90 - 40 -100 -

Nomenclature Frequency (new) range (GHz) A u p to 0.25

B C D E

F G H I J L

0.25 0.5 1 2 3 4 6 8 10 40

-

-

-

-

0.5 1 2 3 4 6 18 10 20 60

With permission from RADAR by P.S. Hall, T. K. Garland-collins, R. S. Picton and R. G. Lee. Brassey's, London, O 1991, Brassey's (UK) Ltd. London.

HISTORICAL DEVELOPMENT OF MICROWAVES

The origin of the development1 of the microwave region of the electromagnetic spectrum may be traced back to the pioneering work carried out by Heinrich Hertz (1888) and Marconi (1890) on spark generators whose frequencies entered the microwave region. Hertz extended the frequency to 500 MHz in 1894 before his death. Sir Olive Lodge in 1894 continued the work on spark generators. A metal tube surrounding the spark generator was found to radiate a signal having directional properties. George Southworth (1920-30) a t the Bell Telephone Laboratories in U S was the first to use microwaves for the transmission of information. Sir Robert WattsonWatt in 1930 demonstrated the principle of radar by producing an electromagnetic echo from a n object. World War I1 saw the development and fabrication of radar, employing microwaves, mostly in US. The branch of microwave electronics, in particular, generation of microwaves, started growing first with the development of the magnetron in 1920, the klystron in 1935, subsequently the Backward Wave Oscillator (BWO) and the Travelling Wave Tube (TWT).Then


appeared the solid state device - the Gunn Diode in 1962. During 1970s transistors were utilised and then microwave printed circuits. The development of radar is synonymous with developments in microwave electronics. 6.4 GENERATION OF MICROWAVES In contrast with the conventional electronics a t the lower radio frequencies, microwave electronics features special characteristics, in particular the associated short wavelengths, which are of the same order of magnitude a s those of the circuit components and devices employed. As the frequency is raised to a point where the wavelength becomes comparable to circuit dimensions, transmission delays cannot be ignored. There will be relative increase in the impedance of the connecting leads, terminals, etc, and the effects of distributed inductance and capacitance between leads cannot be neglected. Further, currents circulating in unshielded circuits which are comparable in size with a wavelength start radiating. Thus the conventional low frequency circuit elements do not work a t these frequencies. The open wire transmission lines used for energy transfer a t lower frequencies become lossy due to radiation of energy at the microwave frequencies. Even the coaxial line cannot be effective due to increased dielectric and conductor losses. Thus in the microwave region hollow metallic pipes known as "waveguides" are employed for energy transmission in place of conventional transmission lines. The vacuum tube and solid state electron devices used a t lower frequencies also do not work in the microwave region. Their performance is mainly limited by the electron transit time, interelectrodeljunction capacitances and lead inductancer;. The generation and handling of microwaves acquired great importance during World War-I1 and afterwards because of the necessity for narrow beam antennas required by the high resolution radar. Since then rapid developments have taken place in the field of microwaves-generation, transmission and detection. 6.4.1 Microwave Vacuum Tube Devices The basic difference between the conventional vacuum tube devices and the microwave vacuum tube devices is that the resonator system (tank circuit) in the latter is a n integral part of the electrode structure. Further, the electron transit time (time taken by a n electron to travel from one electrode to another), the main factor which limits the performance of conventional devices a t high frequencies, is made use of in the working of microwave tubes.

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174


Though the mechanism of operation of various microwave oscillators and amplifiers differs in detail, the common factor in all the tubes is the transfer of power from a source of direct voltage to a source of alternating voltage by means of a density modulated stream of electrons. In fact, velocity modulation of electrons is converted into density modulation of electron beam, using electron transit times which are not negligible compared to periods of oscillation. The density modulation of the electron beam results in a net delivery of energy from the electrons to the electric field during the interaction between the two. If the interaction between electrons and R.F. electric field takes place in the relatively short gap between two electrodes, it is referred to a s "localised interaction". On the other hand, if the interaction takes place between the electrons and electric fields moving approximately with the same velocity over a relatively large distance, it is referred to as "extended interaction". Microwave tubes like Two Cavity Klystron and Reflex Klystron employ "localised interaction", whereas it is of "extended" type in Travelling Wave Tube (TWT) amplifier, Backward Wave Oscillator (BWO), Magnetron and Crossed Field Amplifier (CFA).

A comparison of these tubes (typical values) is given in Table 6.3. Table 6.3. Comparison of microwave vacuum tube devices D e w

Frequency Power range, (Gw output

Multicavity Klystron Amplifier

upto 70

500 KW (CW) 30MW (Peak) at 10 GHz

Reflex Klystron Oscillator

upto 25

10-500 MW (CW)

upto 20 Travelling Wave Tube Amplifier (TWT)

lOkw (CW)

Backward upto 100 Wave Oscillator IBWOI

100 W at 300 MHz A few mw at 100 GHz

Magnetron Oscillator

upto 70

800 kw (CW) 40 MW beak) at 10 GHz

Crossed Field Amplifier (CFA)

upto 10

6 MW (Peak)

Gain

Bandwidth

Effiency

1-8%

- 40%

dB 30-50

-

30% 20-30% (Mechanical)

30-40

Octave

20-40%

-

Octave

10-20%

Narrow

> 50%

25%

- 50%

30-40


6.4.2 Applications of Microwave Tubes Klystron amplijier It is used in UHF TV transmitters (CW), long range radar (pulsed),troposcatter links (CW),and earth station transmitters (CW). Reflex Klystron Oscillator It is used a s signal source in microwave generators, local oscillator in microwave receivers, FM oscillator in portable microwave links and pump oscillator for parametric amplifiers. TWTAmpliJier It is used in microwave receivers, wide band ground and satellite communication systems, anti-jamming and countermeasures. BWO It is used a s a signal source in sweep oscillators and as a wide band noise source for countermeasures. Magnetron It is used a s radar transmitter (pulsed),in microwave cooking, telemetry and missile applications. Crossed Field AmpliJier It is used in radar and electronic countermeasures. 6.4.3 Microwave Solid State Devices Research work during the past three decades has resulted in the use of semiconductor devices for applications like microwave generation, amplification, detection, switching and frequency modulation. Microwave solid state devices may be classified a s follows: (i) Based on thejunction properties Microwave bipolar transistors Microwave field effect transistors Microwave tunnel diodes (ii) Based on the bulk effectof semiconductor Transferred electron devices - TEDs Gunn diode Limited space charge accumulation diode ILSA) Indium phosphide diode (InP) Cadmium telluride diode (CdTe) (iii) Based on avalanche effect of the semiconductor Impact ionisation avalanche transit time (IMPATT)diode Trapped plasma avalanche triggered transit time

175

176


(TRAPATT)diode Barrier injected transit time (BARITT)Diode (iv) Based on quantum electronics (Stimulated emission of radiation) Ruby lasers Semiconductor lasers In general, solid state sources produce much smaller powers (tens of milliwatts to a few watts) a s compared to vacuum tube devices. However, with the solid state devices it is possible to reach frequencies exceeding 100 GHz, entering into rnillimetre wave region. Further, amplifiers with low noise figures like GaAs FET amplifiers can be advantageously used in the receivers. Also, these devices require low operating voltages and are suitable for portable groundbased radars requiring short ranges. These are also suitable for integrating the source with radiating elements in phased array radars. 6.5 MICROWAVE SENSORS Microwave sensors may be broadly discussed under two classes, active and passive, radar belonging to the former and radiometry to the latter. Radar is the primary long range sensor for surveillance and target acquisition in the air, on land, on the sea and in space. An active sensor illuminates the target and utilises the characteristics of the reflected signal to get the required information. A passive sensor such a s a microwave radiometer depends for its action on the natural radiation emitted by the target by virtue of its temperature. Microwave radiometry, unlike infrared radiometry, is yet to find application in defence. The principal limitation of microwave radiometry is the low intensity of radiation emitted by terrestrial objects. Advances in microwave radiometry depend primarily upon the improvement of the sensitivity of the radiometer. 6.5.1 Principle of Radar The word radar is a n acronym for Radio Detection And Ranging. Broadly, it deals with the process of detection of radio objects and finding their distances (ranging). The basic principle behind the operation is the fact that electromagnetic waves get reflected whenever there is a change in the properties of the medium. The properties involved are the conductivity, the permittivity and the permeability. Radar sends a burst of electromagnetic energy and records the reflected signal (the echo) from the targeW. If T is the time interval between the instant of transmission of the electromagnetic pulse and the instant of the reception of the


echo and c is the velocity of propagation of electromagnetic waves, then the distance or the range of the target, R is given by : R = cT/2 6-1 R(km)=O.lST 6-2 where T is in microseconds For 1 ps the distance is 150 m. The principle shown in figure 6.27 is almost as old7 as the subject of electromagnetic radiation itself, but the development of conventional radar in its present use took place during World War 11. Radar

+.

Tareet

Transmitted S~gnal

Echo Signal

Figure 6.2. Badc principle of radar.

6.5.2 Historical development of radar Radar development has a long Maxwell, in 1873, theoretically showed that electromagneticwaves could be propagated through the atmosphere. Hertz in 1887 practically demonstrated it. Then Tesla in 1900 put forth the concept of the reflection of electromagnetic waves from a n object. Hulsmeyer, in 1904, made a practical radar. He generated electromagnetic waves with the help of a spark gap and focused them with a parabolic reflector antenna. An echo was picked u p by another parabolic reflector. This simple radar was used on two ships for avoiding collision between them. Even range could be calculated from the angle of the parabolic antenna and its height above water. During the period 1904 to 1935 several countries in the world were engaged in the development of radar. A need for extending the performance of the human eye for military application for target detection was felt for the first time in 1917 when cities in England were attacked by German aircraft. Thus, air defence became the primary impetus for subsequent developments in the field of radar technology. Taylor and Young in the United States, and Alder in England, contributed to the technology. Hyland working at the Naval Research Laboratory in United States was the first to show that an aircraft could be detected by a radar. A pulsed radar was built by 1935 which had a range of 40 km. The experiment conducted at Daventry in 1935 by Wattson-Watt and Wilkins with a transmitter operating at 6 MHz and a separate receiver (located a little away from the transmitter) detected a flying aircraft. In the Battle of Britain

177

178


in 1940, a radar having a range of about 65 km was used against bomber aircraft. Radars were simultaneously developed in Germany and Britain. The invention of the magnetron in 1940 with pulse powers of the order of many kilowatts greatly increased the radar range and capability. During World War I1 several developments took place in radar technology. Post-war developments such as digital signal processing, and phased array antenna have revolutionised radar capabilities. Further, in the years to come, developments in modern electronic technology such as Very High Speed Integrated Circuits (VHSIC)will find their application in digital signal processing in radar. 6.5.3 Radar Equation The radar equation gives the power reflected back by the target in terms of the transmitted power and the distance of the target from the transmitter. From the radar equation the maximum range of the radar can also be deduced. The mathematical treatment of the radar equation a s given by Cheung and Levien3is dealt with in Appendix B'. 6.5.4 A Typical Radar

amplifier I

A

DISPLAY

-1

Transmitter

1 <--I

1

Pulse Modulator

Figure 6.3.

Block diagram of a typical radar system.

Source:

Reprinted with permission from microwaves made simple : principles and applications (ed) W. Stephen Cheung and Frederic H. Levien, Chapter 14, Radar Systems by frederic H Levien. Artech House Inc., Norwood, MA. USA http : //www.artech-house.com.

The block diagram of a typical radar is given in Fig 6.3. It consists of a transmitter which generates and amplifies the microwave signal a t radar frequencies, a duplexer which sends the radar power from the transmitter to the antenna, and a pulse modulator which acts a s on and off switch for the transmitter, causing pulses of radar power to radiate from the antenna3. On the receiver side it has a low noise RF amplifier which amplifies the


weak echo signal from the target, a mixer (a local oscillator which converts the microwave signal to a signal of a more convenient lower frequency), a n IF amplifier which amplifies the converted signal, second detector which eliminates the intermediate frequency (convertedvalue) leaving the base band information, a video amplifier which amplifies the base band signal and a display which displays the received radar echo signal in visual form for interpretation by the operator. 6.5.5 Types of Radars The following types of radars will be briefly d e s ~ r i b e d ~ . ~ . i) Continuous wave (CW)radar ii) Frequency modulated continuous wave (FM-CW)radar iii) Pulse doppler radar and moving target indicator (MTI) iv) Tracking radar v) Side looking air-borne radar (SLAR)and vi) Synthetic aperture radar (SAR)

6.5.6 Continuous Wave (CW)Radar A radar transmitter in its usual concept sends pulses of electromagnetic energy, but it can also transmit waves continuously towards a moving target and measure the Doppler shift for determining the relative velocity between the target and the radar. This is the principle of the CW radar6. Iff, is the frequency of the transmitted signal, then the frequency of the signal reflected back from the moving target (toward or away) is Cf, *fJ where fd is known as the Doppler shift. fd = 0.666 vf0, HZ 6-3 where vr is in km and fo in MHz. The relative velocity vr may be written as v,=v cose where v is the speed of the target and 8 is the angle between the line joining the radar and the target and the trajectory of the target. If the target is moving about the radar in a circular trajectory, f, = 0 . As the operating frequency of the radar transmitter increases, the Doppler shift increases. Figure 6.4 (a) shows the block diagram of CW radar and 6.4 (b)gives the frequency response of the beat frequency amplifier. The signal fo +fd is mixed with f, to produce a Doppler beat note of frequency f,. The sign of therDoppler shift is lost in the process? The amplifier eliminates echoes from stationary targets and amplifies the echo signal sufficiently to operate the display (indicator).The indicator may be an earphone or a frequency meter.

179

180


fo

<---

----

<

fo

CW Transmitter

..---fo

*

fo P

fd

1 .-

Frequency

Figure 6.4.

(a)Block diagram of a CW radar. (b)Response characteristics of a beat frequency amplifier.

Source:

Reproduced with permission from McGraw-Hill Companies Ltd. Introduction to Radar Systems by M.I.Skolnik.

The major drawback of CW radar is that it cannot measure range. But there are many applications such as traffic police radar where only vehicle speed is required, or intrusion alarm to detect movement of intruders. It can be applied for the measurement of velocitgr of missiles. 6.5.7 Frequency Modulated Continuous Wave (FM-CW)Radar In order to measure range some sort of timing mark has to be applied to a CW radar (carrier). One way is to frequency-modulate


the carrier6. Let the transmitter frequency be changed a s a function of time linearly. Figure 6.5(a) shows linear frequency modulation. In the presence of a target at a distance R the echo signal comes back after a time T=2R/c.Dashed line corresponds to the echo signal. When the echo signal is heterodyned with a portion of the transmitter signal in a non-linear element, such a s a diode, a beat note& will be produced. In the absence of a Doppler frequency shift the beat note &represents a measure of target's range and fb= f, where fr is the beat frequency due to-the target's range. If the carrier frequency has a rate of change f, the beat frequency

In any practical situation, it is not possible to change the frequency continuously in one direction only. There is a need for periodicity in modulation. The type of modulation can be triangular or sawtooth or sinusoidal or some other form. If we use the triangular frequency modulation (Fig 6.5(b))a t a rate off, over a range of Af the beat frequency a

The resulting beat frequency vs time is shown in figure 6.5(c). Thus, from the measurement of beat frequency, the range R can be determined. Figure 6.6 shows the block diagram of a fi-equency modulated-continuous wave radar6. In order to produce beat frequency a portion of the transmitter signal is used as reference. The beat frequency is amplified and measured with a counter calibrated in distance. If the target is in motion, a Doppler shift comes into play superimposed on the FM range beat note. The Doppler shift changes the frequency-time plot of the echo signal to be shifted u p or down. The beat frequency is increased by the Doppler shift in one portion of cycle and decreased in the other portion (Fig. 6.7) If we consider the case when the target is approaching, the corresponding beat frequency will be the difference between the beat frequency due to the range frequency f,and the Doppler frequency shift fd.

fb (UP)=fr -fd

6-6

likewise fb

(down) =f, + f d

6-7

18 1

182


Figure 6.5.

Frequency time relationships in FM-CWradar. Solid curve gives the transmitted signal : dashed curve the echo (a) linear frequency mo.dulation (b) triangular frequency modulation (c) beat frequency of (b).

Source:

Reproduced with permission from McGraw-Hill. Introduction to Radar Systems by M.I. Skolnik.


The range frequency f, may be obtained by measuring the average beat frequency. 1

=-

6-8

[fb(~+ ~ f)d d o w n ) ]

2 =f,

Iff, (up) and f, (down) are measured individually, one half the difference between the two frequencies will give the Doppler frequency (V fr=-fd)

Transmithng antenna

Receiving antenna

Reference signal

,

counter

,

Figure 6.6.

Block diagram of a FM-CWradar.

Source:


I f f , < f, the roles of the averaging and difference frequency measurements are reversed. When there is more than one target, the mixer output will have more than one difference frequency. If the system is linear corresponding to each target there will be a frequency component. By employing narrow band filters the different frequency components can be separated and in principle the range to each target can be determined. If the frequency modulated waveform is non-linear, the situation is more complicated. The main application of FM-CW radar is aircraft altimeter. 6.5.8 Pulse Doppler Radar and Moving Target Indicator (MTI) A pulse Doppler radar or MTI is a system that utilises Doppler frequency shift-for discriminating moving targets from fixed targets. Although there are some differences between the two, the basic principle is the same. MTI radar h a s unambiguous range measurement with ambiguous Doppler measurements, whereas the situation is vice versa with pulse Doppler radar. A continuous wave radar may in principle be converted into a pulse radar by

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184


Ii--------_-.___.- Transmitted signal

Time

Figure 6.7.

Source:

----->

Frequency time relationships in FM-CWradar-increase in Doppler shift in one portion of cycle and decrease in the other portion. (a) Solid curve transmitted signal and dashed curve the received signal and (b) Beat frequency. Reproduced with permission from McGraw-Hill. Introduction to Radar Systems by M.I. Skolnik.

incorporating a power amplifier and a modulator to turn the amplifier on and off for generating pulses. In order to take the place of the local oscillator, a small portion of the CW oscillator power is diverted to the receiver. This functions as the coherent reference in order to detect the Doppler frequency shift. By coherent it is meant that the phase of the transmitted signal is maintained in the reference signal. This reference signal is the characteristic feature of the coherent MTI radar. Figure 6.8 shows pulse Doppler radar.


1

\

/ji /

1

Pulse modulator

I

- -

-CW oscillator

Power amplfier

4I

f,

Reference signal -

-

Figure 6.8.

Pulse radar using Doppler information.

Source:

Reproduced with permission from McGraw-Hill Introduction to Radar Systems by M.I. Skolnik.

Let the voltage of CW oscillator be A, sin 2 4 t

and that of the reference signal be Ve = A2sin2zf;t The Doppler-shifted echo signal voltage is V,,

i

= A3 sin 2n-(f,

I

+ f d )t - -

4rift C R" where A, is the amplitude of the signal received from a target at range Roand f,the Doppler frequency shift. The reference signal and the Doppler-shifted echo signal are heterodyned in the mixer stage of the receiver. The low frequency component of the mixer is of concern and the voltage difference is given by

These equations represent sine wave carriers. Pulse modulation is imposed on these wave carriers. In the case of stationary objects

185

286


the Doppler frequency shift& will be zero. Hence Vd8does not vary with time and takes a constant value lying anywhere between +A, to -A, including zero. In the case of a moving target f, ;t 0 and the voltage corresponding to the difference frequency from the mixer will be a function of time (Fig 6.9a). (i) When f d > ;,1 the Doppler signal can be readily identified from the information contained in a single pulse. (Figure 6.9(b)) 1

(ii) When fd < ,; the pulses will be modulated with an amplitude given by the equation above and many pulses will be needed to 1

extract the Doppler information. Situation of the kind fd < ,; is encountered in the case of aircraft detection radar, whereas f, > ', is applicable to the situation where the primary function of the radar is the detection of extra-terrestrial targets such as ballistic missiles and satellites. (Fig 6.9(c))

(4 Figure 6.9.

a) FW echo pulse train.

bj Video pulse train for Doppler frequency

fd

>

1 -. r 1

c) Video pulse train for Doppler frequency fd < 7. Source:


From an A-scope display of video output, we can distinguish moving targets from stationary targets (Fig 6.10). Figure 6.10(a) shows the appearance of a single sweep. The two arrows point out two moving targets among several stationary targets. It is, however, not possible to distinguish moving targets from stationary targets with the help of a single sweep. As shown in Figures 6.1O(a)to (e) depicting successive sweeps in A-scope display, the amplitude of


moving targets varies from sweep to sweep, whereas it is not so in the case of' stationary objects. Targets in motion give rise to what is known as "butterfly effect" as shown in figure 6.10(f). This effect can be utilised for recognition of moving targets on a n A-scope display.

Figure 6.10. Succeessive sweeps of an MTI radar A-Scope display; echo amplitude as a function of time; arrows indicate position gf moving targets. Source:

Reproduced with permission from McGraw-Hill Introduction to Radar Systems by M.I. Skolnik.

6.5.9 Tracking Radar A tracking radar can measure the coordinates of a target and give data which can be used to determine target path and predict its subsequent positions6. The information it provides includes data on

187

188


range, elevation angle, azimuth angle and Doppler shift, or any combination of these. Although it may appear like any other radar, it differs in the method by which tracking is accomplished. A continuous tracking radar provides continuous tracking data on a particular target. The track-while-scan radar provides sampled data on one or more targets while maintaining search for other targets. The continuous tracking radar employs a servo-mechanism actuated by an error signal in order to position antenna beam in angle. The methods used for generating the error signal are: (i)sequential lobing, (ii) conical scan, an,d (iii) simultaneous lobing or monopulse. The information the radar provides may be used by an operator or can be fed to a computer. Before the radar can track a target, it must first of all find it, so the radar may first operate in a search or acquisition mode. Normally, many radar tracking systems employ a separate search radar known a s acquisition radar which designates targets to the tracking radar by providing information about the location of the targets. The tracking radar then performs a limited search in the area in order to acquire the target. If there is not much change from scan to scan, it is possible to reconstruct the track of the target from the data gathered. When the traffic is dense, the d a t a become unmanageable to the operator a n d is fed automatically to a digital computer. Then the process is called Automatic Detection and Track (ADT).When data from more than one radar are combined to provide target tracks, the process is called Automatic Detection and Integrated Track (ADIT) 6.5.10 Side Looking Air-Borne Radar (SLAR) There are several methods by which discrimination between wanted and unwanted targets or between separate targets can be accomplished. Also, by forming an image, discrimination can be improved. In this method, the antenna scans across the screen in a raster fashion or by linear motion. In side-looking air-borne radar, a long antenna is mounted on the side of the aircraft. It has a narrow horizontal beamwidth (0.l oor less). Its vertical beam which is wide provides a wide swath parallel to the aircraft track. The pulse length and the horizontal antenna beamwidth determine the resolution on the ground (20m x 20 m at 16 km range). But as the range increases, the azimuth discrimination deteriorates. This is overcome in the synthetic aperture radar4. 6.5.11 Synthetic Aperture Radar (SAR) In the SLAR described above, the resolution is limited by the size of the antenna. But there is a limit to the size of the antenna. In the synthetic aperture radar this difficulty is overcome by employing additional signal processing. Even at long ranges it has very high


resolution. The principle is given in Fig 6.11. Let u s say that from time t = t, to t = t2, the aircraft has covered a distance L. At t = t, the object P on the ground is just on the edge of the radar antenna beam. The beam has a width 6, corresponding to this situation. At t = t2the object is leaving the other side of the beam. During the time interval t2 - t, for a range R a resolution Re, is achieved. This corresponds to a n apparent beam width of 0,. Although this beamwidth is narrow, it is equivalent to that available if the antenna had actually had an aperture size L. Hence the system gets the name synthetic aperture radar. At t = t, the target return has a small positive Doppler shift. Figure 6.12(a)gives the SAR processing.

Figure 6.11. Synthetic aperture radar (principle). Source: '

With permission from "Radar" by P.S. Hall, T.K. Garland-collins, R.S. Picton and R.G. Lee, Brassey's, London O 1991 Brassey's (UK) Ltd., London.

189

190


Figure 6.12. SAR processing. Source:

With permission from "Radar" by P.S. Hall, T.K.Garland-collins, R.S. Picton and R.G. Lec, Brassey's, London O 1991, Brassey's (UK)Ltd., London.

Between t = t, and t = t, it has no Doppler shift and at t = t, it has a small negative Doppler shift, a s shown in Fig 6.12 (a).Let us take another target Q at the same range. Which is shown by the dotted line. Now the SAR processing involves subtraction of a characteristic of Pgenerated inside the radar from both the Doppler characteristics corresponding to Pand Q. This is shown in the third figure of 6.12(b).The signals from Pand Q have different frequencies but remain constant with time. In order to see only the object P, the combined signal is passed through a filter of narrow bandwidth but centred around& = 0. This results in a high resolution at the point P. By combining range gating with this method, a horizontal row of points can be produced (Fig 6.13). When this process is repeated for a slightly later period (t, + At) - (t, +At),this will correspond to the return around Q. Thus, pixel by pixel high resolution picture is built parallel to the path of the aircraft. SAR generates high resolution pictures across the radar look direction. The resolution obtained with SAR is a function of the synthetic aperture L which in turn depends on actual aperture size of the radar antenna La and the radar range R. The , is determined by La.Resolution on the ground parallel beamwidth 8 to the aircraft track is given bf


In order to approach resolution of this order, a very stable radar oscillator and a very steady aircraft are necessary. Radar Picture

Aircraft tract

Pixel

Good resolution by SAR processing

Good resolution by range gating

Figure 6.13. Creation of SAR image. Source:

With permission from "Radar" by P.S. Hall,T.K.Garland-collins, R.S. Picton and R.G. Lec, Brassey's, London O 1991, Brassey's (UK) Ltd., London.

6.5.12 Millimeter Wave Radar Bhartia and Bah18have discussed the potentialities of millimeter waves and their applications. The wavelength range 10 mm to 1 mm (30-300 GHz) is generally referred to a s the millimetre region of the electromagneticspectrum. The characteristics of this region make them more suitable in certain applications and offer distinct advantages over microwave, infrared and-electro-optical systems. But technological developments in the region have been rather slow in the past because of certain difficulties, chiefly associated with generation and detection of the waves in the region.

1 91

192


The chief characteristics are their short wavelength, wide bandwidth and environmental interaction. Short wavelengths and associated narrow beamwidth result in better resolution, precision in target tracking and discrimination, low angle tracking capability, good resolution for closely spaced objects and high angular resolution for a r e a mapping. The principal atmospheric windows in the millimetre wave region eiist at 8.6,3.2,2.1 and 1.4 mm wavelengths. These correspond to the frequencies 3 5 , 9 4 , 140 and 220 GHz. The bandwidth available a t each of these windows is extremely large, roughly 16, 23, 2 6 a n d 7 0 GHz respectively. This helps i n accommodating all the lower frequencies including microwave frequencies. Likewise, maxima of absorption bands are a t 5, 2.5 and 1.6 mm wavelengths. These correspond to the frequencies 60, 119, a n d 1 8 3 GHz. These have large bandwidths. This wide bandwidth characteristic provides high information rate capability for obtaining fine structure details of target signature, a s well a s high range resolution and increased immunity to interference and jamming. Their attenuation in aerosols, dust, smoke and battlefield contaminants is less than a t IR and optical frequencies. Thus this characteristic also scores over IR/optical systems. Millimeter wave radars have several advantages - viz., small size, small weight, high resolution in both azimuth and range. Also, they can operate in adverse environmental conditions s u c h a s poor visibility. Several research and development programmes for groundg-15, seal6 and air s ~ r v e i l l a n c 22 e ~a~n d even space object i d e n t i f i ~ a t i o n24 ~ ~were . undertaken keeping the potentialities of millimeter waves in mind. US h a s been engaged in developing sophisticated surveillance and target acquisition millimeter wave r a d a r s for t a n k location a n d engagement (STARTLE). Target acquisition millimeter wave radars can carry out volume search, detection, and discrimination of targets from clutter. Also, target classification and identification into categories, e.g., moving tank, types of aircraft (rotary or fixed wing) would be possible. These systems are suitable for Low Level Air Defence (LLAD)applications. They can be used effectively against low flying aircraft. Millimeter wave radars can be used to distinguish between targets in motion and stationary ones, a s even small vibrations of targets can result in a large change in phase. A number of millimeter wave tracking radars a r e being designed. In seekers and missile guidance applications millimeter wave radars complement or replace electro-optical systems because of their characteristics s u c h a s small size, high accuracy and reasonable performance in adverse weather conditions. Several studies on millimeter wave sensors and guidance systems viz., active,


passive, semiactive and dual mode-have been carried out. The primary role of terminal guidance seeker includes range and attitude control, target search and detection, and target tracking and homing. Two types of systems are common, viz., Lock-On-Before-Launch (LOBL) and Lock-On-After-Launch (LOAL). Dual mode operations are possible, with microwave tracking being used for long range search and target acquisition and millimeter wave system for precision tracking, once the target is within the range. Millimeter wave technology, when fully developed, although it cannot completely replace microwaves and IR/EO systems, will be far superior in certain applications and will also find new applications. ROLE OF RADAR IN WAR 6.6 Probably no other sensor finds more applications than radar in war. It is used in all the theatres of war both in defence and in offence. Broadly, its tasks include battlefield surveillance, air defence and weapon tracking and locating. Its physical dimensions can vary widely ranging from enormous permanent structures to pocket size or hand-held meters4. Depending upon the nature of the task it is intended to perform, it can be static having permanent installation, or can be mounted on a mobile platform such a s aircraft, ship, space vehicle, or man-portable, or can be mounted on a weapon system as big as a missile. In the case of objects of large dimensions with RCS greater than 100 m2, detection as well as localisation are possible, besides some degree of target identification. In the case of small objects, the information the radar can give is in bits and snatches. However, the extent of information that can be obtained from the radar output or display depends upon the knowledge and experience of the radar operator or interpretor, besides the ground truth and collateral information that is available. With the rapid advances that are taking place in radar technology and in allied fields, the threat from radar will become greater and greater. The performance of radars will greatly enhance detectability, recognition and identification of important targets and discrimination of targets-stationary or moving and background. 6.6.1 Types of Radars Used in War Radars used in war, based upon their applications, may be discussed under three groups: (i) Battlefield surveillance radar (BSR) (ii) Weapon locating radar (WLR) (iii) Air defence radar (ADR)

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194


Battlefield Surveillance Radar (BSR) This radar category may be divided into two groups - short range operating u p to 30-40 km and long range operating beyond 100 km. The role of these radars in general is to provide timely information about the eneniy's depolyment in the battlefield4. The short range radars, which can be vehicle-mounted or man-portable, can detect objects ranging from helicopters, tracked vehicles, light vehicles, to group of men or even a single man. Ground surveillance MTI radars which are truck-mounted provide coverage of forward area close to forward edge of battle area (FEBA),within a range of 15 km. These radars can do both search and track when a large number of them are used along the front. They can provide target disposition and location information to the artillery units. Mortar and artillery locating radars are reported to detect and determine the source of single mortar rounds and missiles. The long range radars are mounted on platforms such as helicopters and aircraft, both manned and unmanned, and also satellites. When the range is beyond 100 km, the requirement of an elevated platform arises owing to the earth's curvature. SLAR and SAR, which have been discussed already, are the important systems coming under long range airborne radars. SLAR can produce high quality imagery of the surveyed area which can be utilised for strip mapping. SARs are in general employed on high flying reconnaissance aircraft. They can provide high resolution imagery of the target area for creating a precise database to the radar interpreter. With air-to-ground data link the processing time can be a s low as half an hour. A modern SAR can have along flighttrack resolution of 3 m at altitudes of 12 km and a range of 15 km. SAR at a tactical level can give information over target areas of size 10 x 10 km. It is likely that, with high speed digital processing, SAR data can be processed in real time in the aircraft itself. SLAR and SAR appear to pose the greatest threat to targets on the ground because of their high angle and range resolution. 6.6.3 Weapon Locating Radar (WLR) The role of a weapon locating radar, as its name implies, is to detect the launch of an enemy projectile or missile, and also to determine the path of the weapon in flight to the extent required for a computer to compute the location of the launchep. Weapon locating radars are highly complex and very expensive. 6.6.4 Air Defence Radar (ADR) Air defence is one of the greatest priorities for ground forces, as enemy's aircraft and missiles can rapidly approach ground targets

6.6.2


with very little warning. A radar can be very effective against such threats. The role of air defence radar is to detect and destroy the airborne system before it hits the ground object. Air defence radars can be discussed under four groups - strategic, long range, medium range and short range4. The strategic radar must be able to detect intercontinental ballistic missiles or submarine-launched ballistic missiles or even orbital vehicles. Obviously, these radars should have ranges of several thousands of kilometers. The long range radars have ranges beyond 370 km and face threat from aircraft including missiles. The medium range radars have ranges over 200 km. The short range radars have a detection range of 55 km. 6.6.5 Other Types of Radar Other radars4which are used in war include: (i)active homing guidance radar which is incorported in the missile in its homing head. The surveillance radar provides the target parameters to the missile and the tracking radar locks on to the target to be destroyed. Simultaneously, a ground radar continuously monitors the target and guides the missile to the target; (ii)a semi-active homing radar on the ground illuminates the target such that a strong echo is obtained from the target. A tracker on the missile uses the echo for homing on to the target; (iii) a track via missile guidance radar employs a combination of command and surveillance guidance, and (iv)a n identification friend-or-foe (IFF)radar sends a signal that is picked u p by a receiver on board of a friendly aircraft. The receiver triggers a response which is picked u p by a surveillance radar on the ground. This response contains the necessary information in order to find out whether the aircraft is a friend or foe. Other miscellaneous radars4 which may be used in battlefield are: (i)unmanned aircraft radar which employs low power millimeter waves; (ii)remotely piloted vehicle tracking radar which is of short range, used by a pilot or the operator to control the vehicle; (iii)an antitank homing missile radar which utilises millimeter waves for homing missiles to attack MBT; (iv)a millimeter wave radar homing head which will be incorporated in guided weapons for homing on to a target; (v)a passive radar homing head which utilises the passive microwave radiation emitted by the target, and (vi)tank automatic defence radar which automatically activates a gun in the tank upon locating a missile approaching a tank. 6.7 RADAR CROSS SECTION (RCS) Radar Cross Section (RCS)is a n important physical quantity associated with targets where the s e n s o r employed for

195

196


sensing is the radar. The concept of RCS, its prediction, measurement and methods for its reduction have been dealt with in detail by Knott, et aP5. Whenever electromagnetic radiation is intercepted by an obstacle, the energy gets dispersed in all directions. The angular distribution of energy depends upon the characteristics of the obstacle and the quality of the radiation. This energy distribution is in general known a s scattering. In a practical situation, such a s application of radar, the obstacle is the target, a transmitter provides radiation, and a receiver receives a part of the radiation scattered by the target. Quite often, the radar transmitter acts as receiver also and the radiation scattered backward is picked up. This is known as monostatic scattering, as different from bistatic scattering, where a separate receiver located a t a different point is used. When the angle subtended at the target by the transmitter and the receiver is small, the scattering may be considered a s monostatic and when this angle is 180°,it represents forward scattering a s different from back-scattering. The target is attributed a fictitious area known a s cross-section which is a measure of the energy/power scattered by in the direction of the receiver. The antenna of a radar transmitterlreceiver is ascribed to possess an aperture having an effective area which transmits/extracts electromagnetic energy. The product of the effective area of the receiving antenna and the incident power density is the power available to the radar receiver. Similarly, the product of the fictitious area of the target and the incident power density is the power reflected/scattered by the target. The fictitious area is, in general, known as, the scattering cross-section of the target. The term radar cross-section (RCS)is used in the special case when the scattering is monostatic. The symbol used for RCS is o. 6.7.1 Expression for RCS The general expression for RCS of a target can be derived in terms of the strength of the incident electric field and scattered electric field at a large distance. Mathematical treatment of RCS of targets (spherical and cylindrical) has been dealt with by Knott, et a1 and 0 t h e 1 - s ~ ~ ~ ~ ~ . The unit of RCS is mZ. o may be considered as the projected area of an equivalent reflector which has uniform properties in all directions. Such a reflector is a sphere which will reflect the same power per unit solid angle. A sphere of 1.12 m diameter has an echoing area of 1 m2. Except for sphere, for all other geometries the value of RCS varies with the aspect angle relative to the receiver.


able 6.4 gives the typical values of RCS of some common (including military) objects.

Figure 6.14. Radar cross-section of a conducting sphere. Source:

Reproduced with permission from McGraw-Hill Companies Ltd. Radar Handbook by Skolnik, M.I.1970.

Because of the wide variation in its value it is more convenient to express o in a logarithmic scale rather than in a linear scale. Accordingly, o is expressed in decibels dBsm = 1 0 log,,o where dBsm are decibels above a square meter. Figure 6.14 shows the monostatic radar cross-section of a perfectly conducting sphere. The diagram actually shows the variation of o normalised by ra2and the ratio of the circumference offhe sphere to the wavelength 2na5s6.The graph is characterised by three distinct regions: (i) Rayleigh region (low frequency). In this region where the circumference is less than one wavelength, the radar cross section varies inversely as the fourth power of the wavelength; o is independent of the viewing angle.

1 97

198


Table 6.4

Source:

.

Typical values of RCS of usually encountered1 objects including (military)objects

Object

RCS in m2

Jumbo jet

100

B- 17 Flying fortress

80

B-47 Bomber

40

B-52 Bomber

10

B-1 Bomber

1.O

Large fighters

5-6

Small fighters

2-3

Small single engine plane

1.0

Man

1.O

Small bird

0.01

Insect

0.00001

F- 117A Stealth fighter

0.6

B-2 Stealth bomber

0.01

"Reprinted by permission of the society for the Advancement of Material and process Engineering" Stealth Aircraft and Technology from World War I1 to Gulf War History and background by Roger A. Stonier.

(ii)MIE or resonance region. A s the normalised sphere circumference (2ndh) increases (1 < 2 n a l h < 10) or t h e wavelength i s between 1.0 to 10 circumferences, the normalised RCS exhibits oscillatory behaviour. This phenomenon is due to interference between the specularly reflected component of the scattered radiation and the waves which creep around the back of the sphere and then return in the direction of the radar receiver 34. This is also known as resonance region. As the waves creep around the back of the sphere they get damped, and a s such the RCS shows damped oscillations; (iii)Optical region (high frequency) A s the wavelength becomes still smaller (2naIh > 10) the cross section reaches the specular value o = na2(the projected area of the sphere). Whatever be the method adopted for predicting RCS of a sphere which is perfectly conducting, at high frequencies, it attains the value na2. Instead of being perfectly conducting, if the sphere is made of a dielectric material, the variation of RCS with wavelength is shown in the Fig 6.15.

Microwave Camoujlage

The refractive index of the material of the sphere is taken as n= 2.5 k i 0.01. The curve shows the normalised RCS = na2 in dB as a function of the radius of the sphere in wavelength a / h 35. In the case of a dielectric sphere the internal reflection of the fields penetrating into the sphere plays a significant role.

Figure 6.16. RCS of a lossy dielectric sphere. Source:

Reprinted by pennission of IEEE,New York, O 19XX,lEEE Back scatter from a sphere - A short pulse view. J. Rheinstein. IEEE Trans vol AP-16, Jan.

-

1968.

Methods for the Prediction of RCS Methods for prediction of RCS have been dealt with qualitatively ~~ a1, in detail. by Skolnik5and K n o t t ~ et Theoretical study of RCS of basic shapes enables u s to understand the mechanisms involved in the scattering by complex shapes. The methods are either based on exact theories wherever possible, or approximate theories in most of the situations. The values so arrived a t for RCS are subject to verification by comparing with experimentally determined values. The theoretical prediction of RCS involves the treatment of scattering phenomenon in terms of electromagnetic theory. The interaction of the incident electromagnetic field with the scattering body is dealt with by solving Maxwell's equations with appropriate boundary conditions depending on the geometry of the body. The two important mathematical techniques employed to axrive at exact solutions are;

6.7.2

199

200


(i)The separation of variables, and (ii)The integral equation formulation Solutions through the former technique are possible only in a few cases in which the wave equation is separable in a coordinate system in which the body surface coincides with one of the coordinates2'. Examples of such cases are sphere, spheroid, torus, paraboloid etc. The second approach describes the scattering phenomenon in terms of a n integral of various vector products involving the surface electric and magnetic fields and unit vector ~~ is one locally normal to the body surface. C h u - S t r a t t ~ nintegral type of integral employed in the technique. When exact integral equations of Chu-Stratton obtained by using Green's theorem in conjunction with Maxwell's equations are solved by modern computers, they provide the scattered fields from which RCS is c a l ~ u l a t e dThe ~ ~ .method is applicable in situations where the body dimensions do not exceed a few wavelengths of the incident electromagnetic radiation. In reality exact solutions are rare. A s such, in many practical problems approximate theories are adopted. The various approximate theories are: (i) Geometric optics approach A s the name implies, it treats bundles of rays by laws of reflection and refraction. But the method fails to account for interference and polarization characteristics of the radiation30.

(ii) Physical optics approach This approach utilises electromagnetic theory a n d simplified approximations to arrive a t the surface current distributions of the Chu-Stratton integral. But this method fails for accurate specifications of polarisation31. (iii) Geometrical theory of diffraction This utilises geometrical optics for taking into account diffraction effects32.This is a combination of ray approach with wavelength and phase characteristics. But this method fails at caustics. Besides the above, there is the method of equivalent current and physical theory of diffraction for treating edges. 6.7.3 RCS of Flat Plate For predicting the RCS of a flat plate, both physical -opticsand geometrical theory of diffraction have been employed. Figure 6.16 shows the theoretically predicted RCS of a flat plate of size approximately5 h on a side36and comparison with experimentalvalues.


10 1.

Vertical polarization

1 ;

Horizontal polarization

0

Qeometric diffraction

-5 -10 -15

-20

-25 -30 -35 -*0

20

40

60

80

100

Azimuth aspect angle a (degrees)

Figure 6.16. RCS of a flat plate. Source:

6.7.4

Reprinted by permission of IEEE, New York, O 19XX IEEE RCS of a 6.5 by 6.5 inch flat plate' by R.A. Ross, IEEE Trans vol. AP-14,May 1966

RCS of Re-entrant Bodies (corner reflectors)

Corner reflectors are formed by a combination of two or three mutually perpendicular flat plates. The dihedral corner reflector is formed by two flat plates with their planes perpendicular to each other. The trihedral corner reflectors have three faces which are mutually perpendicular. These bodies have large radar cross sections. These are used in order to enhance the radar echoing efficiency in the direction of the receiver. In the case of dihedral corner reflector, double reflection contributed by the two faces provides passive enhancement of the RCS. The physical optics method has been applied to predict the RCS of dihedral comer reflector. But the method is not satisfactory while providing detailed aspect, polarization and biostatic dependence. Geometric theory of diffraction provides more information. Figure 6.17 shows RCS of a dihedral comer reflector with square faces of side 17.9 cm, experimentally determined at a frequency of 9.4 G H z ~The ~ . central part is broad. This is due to the interaction between the two faces with the incident wave. It gets reflected twice, once from each face. The peaks on the left and right side of the central portion are due to direct reflections from each face. The ripples in the middle are due to side lobes of the individual face patterns.

201

Source:

'Reprinted with permission from Radar cross section by E.F.Knott, J. F. Shacffer and M. T.Tuley, Artech House, Inc. Norwood, MS, USA, http://www. artech-house.com

Figure 6.17. RCS pattern of a 100°dihedral comer with square face 17.9 cm along a side measured at 9.4 GHz.

Aspect angel (degrees)


6.7.5 General Discussion on RCS of Simple Bodies Of the various scatterers which we have considered so far, the corner reflectors have the highest RCS. Then, in decreasing order come the flat plate, the cylinder and the sphere. In terms of frequency, the flat plate, the cylinder and the sphere are proportional to F2 ,F1, F0 respectively (where F stands for frequency). In terms of size dependence they are proportional to L4,L3 and L2 respectively (where L stands for the linear dimension) for these objects. 6.7.6 RCS of Military Objects RCS is a measure of the signature of military object concerned in the microwave region. Hence its determination is very imporant before any means to suppress RCS is thought of. Among the various military objects to be considered, aircraft, ship and tank are of the greatest importance. These objects are complex, consisting of a large number of significant scattering centres, besides equally large numbers of less significant ones. 6.7.6.1 RCS of aircraft Figure 6.18 shows the RCS pattern of a scale model of a commercial aircraft. The pattern over 0" to 360' aspect angle is zigzag. Such a pattern arises because of the mutual inference of the scattered waves which are in phase and out of phase. There is a wide variation between the maximum and minimum values of the RCS over 360". Broadly, we may consider the nose-on, broad-side and tail-on aspects for discussion. In each of these views there are a large number of major scattering centres. In the nose-on aspect, the major scattering centres are jet intake ducts, radar antenna a t the nose and leading edges of the wings. They provide corner reflectors, doubly curved and singly curved surfaces. Likewise, for the broadside incidence, upper wing surfaces, fuselage, vertical fins and the horizontal stabilizer are the more significant scattering centres. In the tail-on aspect, engine exhaust ducts are the major scattering centres. More or less, all the mechanisms of scattering come into play in the macroscopic scale. Fig 6.19 gives RCS of an aircraft in mete9 vs azimuth angle. 6.7.6.2 RCS of ship RCS of a ship is probably the largest of all the military objects. Fig 6.20 gives the RCS pattern of a large ship. The values of RCS are shown a t two different frequencies from 0° to 360". The three traces correspond to 20,50 and 80 percentile levels. The ship's hull a t the broadside incidence, the superstxucture the masts and the large assortment of fixtures on board such a s the vents, hoists, lockers, pipes, canduits etc are the significant scattering centres.

203

-120

-90

-60

-30

Source:

0

Pulse Gated Compact RCS Range

30

60

90

compact RCS range' N.A. Howell 1970G-APInternational programme and digest =RE.

Reprinted with permission of IEEE, New York O 19XX IEEE. 'Designof pulse-gated

Figure 6.18. RCS of a model of aircraft Boeing 737.

-180 -150

120

150

180


Fuselage I I

!

Leading edge of wing

I

1

Trailing edge of wing

Azimuth angle go0

n

Figure 6.19. RCS of an ahcraft. Source:

Reprinted with permission from Microwaves made simple : principles and applications (ed)W. Stephen Cheungand Frederic H. Levien, chapter 14 Radar Systems by Frederic W. Levien-Artech Houe Inc., Norwood MA USA. http://www. artech-house.com

Similar to an aircraft, the RCS variation from different angles shows a somewhat zigzag pattern but the amplitude of fluctuation is not as large as it is in the case of aircraft.

205

206


Port

board

board

Figure 6.20. RCS of a ship (a) Frequency = 2.8 GHz, (b)Frequency = 9.225 GHz. Sourae:

Reproduced from Introductionto Radar systems by M.I. Skolnik with permission of the McGraw-Hill Companies O 1980,1962 by McGraw-Hill Inc.


6.7.6.3 RCS of Tank Figure 6.21 gives RCS of a tank. Over an angle 0° to 360° aspect angle RCS exhibits significant variation. o0

Figure 6.21.

RCS of a tank.

Source:

Reproduced from Radar by P.S. Hall, T. K. Garland-collins, R. S. Picton and R. G. Lee with permission of Brasseys (UK) O 1991 Brassey's (UK) Ltd.. London.

6.7.7 Advantages asd disadvantages of prediction techniques Advcmtage~~~*~~.~~

(i) Quite often, in many situations, prediction techniques are more economical than actual measurements indoor or outdoor; (ii) Even before a prototype or model of the target is available, prediction techniques help u s in giving the RCS. This is quite often useful in the design of the target for reducing the RCS; (iii)With the predicted values we can analyze the data with respect to various shapes and also materials used for reducing the RCS; (iv) Once the target is modelled, calculation and analysis and their variation with frequency and polarization are a question of computer time.

208


Disadvantages (i) Predicted values cannot take all factors into consideration and, as such, these values are always underestimates; (ii) The method cannot be used for complex bodies for which electromagnetic modelling is difficult. On the whole, the prediction method based on mathematical modelling and subsequent confirmation with high speed computers would be of great help both for designing a controlled RCS target and radar equipment design. 6.7.8 RCS of Targets Experimental Determination Mathematical modelling methods are applicable only to simple geometrical structures. An actual target has to be broken down into a number of simple structures before the prediction techniques can be applied. Although experimental determination is more expensive, it is comparatively easier, once a model of the target, either full scale or small scale, can be made. Broadly, there are two methods, viz., (i)The outdoor method, and (ii) the indoor method. Knott, et aZZ5have discussed a t length the various aspects of RCS measurements employing outdoor and indoor ranges. The objectives of carrying out RCS measurements are broadly a s follows: Measurement Objectives These measurements enable u s to understand the basic scattering processes and are useful for checking the validity and the limitations of the theory of RCS. Secondly, RCS measurements provide data for identification of 'flare spots'. Thirdly, for verifying the RCS performance of a new system, although computer programmes are capable of predicting the performance, experimental values are extremely useful. Also, they would be useful for building a database. An RCS measurement facility, whether it is outdoor or indoor, should have the following: (i) An instrumentation radar for transmitting and receiving the signals; (ii) Recording instruments-analog or digital for storing the information;

-

(iii) A target mount which can be rotated; (iv) An anechoic environment o r low background signal environment; and (v) A test target of full size or scaled-down model.


For carrying out measurements on large targets, outdoor ranges are used. On small targets and scaled-down models measurements are carried out indoor. 6.7.8.1 Outdoor Ranges There are two reasons for employing outdoor ranges. Firstly, far field range requirement can be achieved, and, secondly, measurements can be carried out only outdoor on large targets. Instrumentation for outdoor ranges is relatively simple. Skolnik's Radar Handbook5 gives a detailed account of radar instrumentation needed and different conditions and requirements. A simple instrumentation radar for measurement of RCS consists of an RF source of a few hundred watts of power. The RF power is amplified by a Travelling Wave Tube Amplifier (TWTA). A trigger pulse from a pulse generator turns on the TWTA for a short time. The pulse widths employed are in the range 0.1 to 0.5 ps. Either a single antenna for transmitting and receiving can be used, or two separate antennas can be employed. An RF attenuator adjusts the signal level into the receiver. The target signal together with that of a local oscillator is fed to a mixer preamplifier from which the signal is delivered to an IF amplifier. The IF amplifier is usually of the dynamic range of 60-70 dB. The output is recorded on an analog chart recorder or on a digital recording system. The salient features of an outdoor measurement range are: (i) A target support system; (ii) Range geometry keeping in view the height of the antenna, target, range etc; (iii) instrumentation radar; (iv) Measurement of background return most of which is due to the target support system; and (v) System calibration with the help of a known return from a primary calibration standard. The target is mounted and observations are taken for different angular positions of the target by rotating the target mount. Ground plane effect which arises due to the proximity of the ground to the antenna can be advantageously utilised for increasing the overall sensitivity of the system by concentrating more energy on the target. Since the targets for which RCS is required are from defence, usually these ranges are owned by defence departments. However, large aerospace companies such as McDonnell Douglas and Boeing have outdoor ranges. Information on radar ranges existing in various countries is not available in open literature.

209

210


6.7.8.2 Indoor RCS Ranges

One of the most important advantages of carrying out measurements indoor is that they are free from environmental factors such a s wind, rain etc. Outdoor measurements cannot be carried out in rain, a s rain drops cause clutter. High velocity winds create the problem of instability to the target mount. Another disadvantage of the outdoor measurements is that they can be observed by satellites from above. Indoor measurements have certain limitations. Measurements can be carried out only on small size targets. One very important problem faced by indoor measurement range is the elimination or reduction of the effects of reflection from walls. The chamber in which measurements are carried out has to be of appropriate geometry and the walls of the chamber should be covered with anechoic material with respect to the frequencies being used in the measurement. The essential features of a n indoor range are: (i) The geometry of the chamber The chamber is of tapered design. This geometry removes specular reflections from side walls. The walls are covered by pyramidal absorbers whose thickness is determined by the lowest frequency that would be used. The lower the frequency, the thicker will be the pyramid. The material used for the pyramid is polyurethane foamed rubber; (ii) The radar used is FM-CW type; (iii) RCS measurements can be carried out at frequencies as low as 120 MHz and as high as 93 GHz. When measurements are carried out on scaled-down models of real targets, the radar frequencies have to be correspondingly raised. 6.8 METHODS FOR REDUCTION OF RCS Reduction of RCS of military objects is the same as suppression of radar signature of the object concerned. In classical terms it may be known as microwave camouflage, or in modern terminology it constitutes a major component of Stealth or Low Observable Technology (LOT).Methods of reduction of RCS have been dealt with by several authors 25,26,38,39.Knotts, et al 25 have discussed a t length the various methods for the reduction of RCS of various weapon platforms. Many of the weapon platforms such as combat aircraft, warship, armoured vehicles, missiles and weapons are detected by radar over long ranges. In order to reduce their detectability so that they successfully cany out their mission, the RCS of these military objects


must be reduced below the detection probability. Basic techniques for reducing RCS of targets fall into four categories. They are: (i) (ii) (iii) (iv)

Shaping; Use ofRAMs; Passive cancellation; and Active cancellation

6.8.1 Shaping

Today, computer-based RCS modelling, numerical methods of predicting RCS, and measurements on small scale and full scale models and actual objects have made it possible to arrive a t a tentative design of a n antiradar target before it is actually fabricated. These methodologies give a n insight into the various shapes and sizes that affect RCS a t different frequencies and aspect angles and identification of hot spot catering centres and resonant scatterers. The concept of reducing RCS by shaping is simple. The designer of the antiradar military target has to orient or alter the shape of the surface which is responsible for the RCS in such a way that the incident radar signal is deflected away from the receiving radar. This involves reshaping without altering the other design parameters which might affect the performance of the system. While shaping, the two important parameters that must be considered are the aspect angle, i.e., the direction from which the transmitted signal is incident on the target surface, and the frequency. These two broadly depend on the nature of the threat. Accordingly, reshaping and other measures have to be taken to reduce the overall RCS. The entire military target may be decomposed into its constituent components and their geometrical shapes and sizes studied. The basic geometrical surfaces which we know are to be avoided are flat, cylindrical surfaces, sharp edges, abrupt angles, re-entrant structures, corner reflectors and cavities. These have to be changed or oriented in such a way that the return signal is either reduced to the extent required or deviated into a different direction. Sleek, smooth and continuously varying contours and rounded shapes are to be used. These changes in the military object, if it is an aircraft, should be aerodynamically acceptable. A shape which has low RCS a t all aspect angles is an ideal situation not possible to achieve in practice, and compromises a n d trade-offs occur throughout the design with concomitant loss of aerodynamic efficiency. Let u s take two specific examples with reference to aircraft.

21 1

212


6.8.1.1 8-2 Bomber and F-117Afighter

Stonier has discussed the salient aspects of RCS of B-2 bomber ~ " ~shape . of the B-2 bomber is such and F- 117A fighter a i r ~ r a f t ~The that it deflects a large percentage of radar energy away from the direction of the transmitterjreceiver antenna. There is no conventional fuselage and vertical tail. The engine inlets have an unusual design consisting of irregular edges, smoothly rounded in some areas and with skip angles and flat faces in other areas in order to effectively deflect the radar echoes. F- 117A has broad flat exteriors which are inclined in such a way that the radar signal is deflected away from the direction of the receiver. It has compact, smooth external geometry and overall reduced size. It has no external protuberances and the tips of the wings are rounded. The flat faces are angled in such a way as to deny signal returns to the searching radar. The body has thin, swept wings, butterfly tail, finely pointed nose, bubble canopy, triangular shaped wind shield and air intakes which are carefully designed and shielded from ground-based surveillance. For aircraft, various geometrical shapes have been studied keeping in view the aerodynamic aspects. The complex contoured surfaces must be manufactured to precision dimensions. Such precision can be achieved better with composites than metals. It seems unlikely that any aerodynamically viable shape can avoid having a large RCS when viewed from some angles. B-2bomber shows u p a very large and flat surface from above and below. Presumably it can easily be detected by radar from these aspect angles. 6.8.1.2 Ship F o ~ w e lhas l ~ ~discussed the salignt aspects of RCS of a ship. Both the macro- and micro-geometry of the ship must be taken into consideration. The frequencies which are used for detection of ships lie in the range of 10 MHz to 300 GHz. Naval targets are sufficiently large for frequencies from 1 GHz and higher. Both the entire target and super-structure must be taken into consideration. The entire structure can reflect radar energy. Also, sections of super, structure such as hand rails, bollards, wire ropes, deck lockers and weapon system such as radars, guns, missile launchers etc. will reflect radar energy. Such objects are approximately of the same size as the wavelength of the incident radar and are termed resonant scatterers. Resonant scatterers may be eliminated to some extent by housing them within the super-structure. The superstructure is designed in such a way that the reflected radar energy is deflected away from the direction of the enemy's radars. This has led to the box-like


box-like superstructure, but with sloping sides. Hot spots are treated with radar absorbing materials. New ships are being designed with reduced RCS with the help of computer design. The vertical section of the hull and the reflecting water constitute corner reflector geometry which can give rise to large RCS. Low RCS hull design would entail substantial costs. Any low observability depends on the angle the hull sides make with the sea. In comparison to the aircraft, the RCS of a ship cannot be reduced to the same extent. Two important approaches may be adopted. Firstly, the RCS may be reduced to an extent a t which it is indistinguishable from a decoy such as a floating corner reflector or a chaff cloud, or the signal received from an off-board decoy. At that point, a properly warned ship ought to be able to evade radar guided missiles, even from those which it cannot see. Decoy technology might become much more important if the stealth technology makes incoming weapons almost undetectable. Air-dropped ship simulators already exist in the U S Navy. They are sufficiently powerful to imitate RCS of ships. Shaping cannot altogether reduce RCS, whereas radar absorbing materials can drastically reduce residual RCS. Careful designing can substantially reduce RCS by using shaping smoothing surfaces and minimising the number of openings and re-entrant structures and also keeping weapons and sensors inside the superstructure when not in use. 6.8.2 Radar Absorbimg Materials ( M s ) Knott, et a125have given a detailed treatment of the mechanism of absorption of various radar absorbing materials. Salient aspects ~ ~the , ~design ~. stage, of the RAMS have been discussed by S t ~ n i e r At reduction of RCS by shaping of the composite part responsible for contributing towards radar echo is possible, in case the change of shape does not adversely affect the performance of the intended role of the component or the system a s a whole. Also, once the design is completed and the system is in the final form, reshaping of any composite part may not be possible. In all these situations reduction of RCS can be done by the application of radar absorbing materials (RAMS). Since World War I1 the science and technology of radar absorbing materials have greatly advanced, and today the field k3s become an important part of stealth technology. Initial experimects on these materials commenced during 1940s. In general, it was known that less dense materials like wood, textiles and plastic are more transparentlless reflecting to microwaves than more dense materials.

21 3

21 4


6.8.2.1 Theory Basically, when a radar wave is incident upon a radar absorbing material (RAM) it does not allow the wave to undergo reflection; instead, it transmits the wave and then dissipates the energy, either by absorption, or by destructive interference. In order to avoid reflection the material should match the impedance of free space to that of the surface being shielded. Absorption of energy is accomplished by "lossy" dielectric and/or magnetic properties of the material. Special materials have to be made in order to incorporate the "lossy" property to the extent required. The design and development of a RAM involves: (i) Type of the material - dielectric or magnetic; (ii) Impedance; (iii) Loss factor; (iv) Thickness; (v) Optical design. By an appropriate combination of the various factors radar absorbing materials responding to a narrow band or broad band of frequencies can be developed. Knott, et a1 have derived the expressions for: (i) reflection coefficient, (ii) scattering from flat dielectric multilyered material for normal incidence, and (iii) scattering for oblique incidence.

6.8.2.2 Practical Radar Absorbing Materials The requisites of an ideal radar absorbing material25are: (i) Available in thin sheets or as a paint which can be sprayed or coated on to the surface of the target; (ii) Light; (iii) Cater for a broad band of frequencies; (iv) Flexible; (v) Water repellent; (vi) Environment resistant; and (vii) Economical. A radar absorbing material can be attached to the target permanently in the form of a sheet, or it can be in the form of a panel which can be fixed as and when required, or a s a paint which can be sprayed on to otherwise inaccessible parts, or it can be a structural material. The method of application to be adopted depends on the situation. A permanently attached panel adds weight and is exposed to the enemy who may measure the RCS value of the target surface. Panels which can be attached when required do not have this disadvantage. Each method of application h a s its own advantages and disadvantages.


6.8.2.3. Q p e s of radar absorbing materials Radar absorbing materials can be classified in different ways. In terms of frequency response they may be classified as narrow (resonant)or broad band. A s per the physical mechanism they may be divided a s dielectric and magnetic or circuit analog absorbers. They can be classified a s single layer or multilayer. Salisbury screens and Dallenbach layers are single layer dielectric absorbers; when made as multiple layer absorbers they are known as Jaumann absorbers. There are graded dielectric absorbers where the gradient of the dielectric constant varies in the layers. Then there are magnetic radar absorbing materials, circuit analog absorbers, hybrid materials and Radar Absorbing Structure (RAS).There are specular and nonspecular and radar absorbing materials. The type of absorbers used depends broadly upon the nature of application. The requirement can be broad band or narrow band. Additional weight can be detrimental. 6.8.2.4 Salkbury screen It is one of the earliest types of absorbers invented during 1940s. When used in single layer it responds to a narrow band of frequencies and hence is termed a s a resonant absorber.

Resistive sheet

_-:

1 &-.-I

I

'_-

Metal Backing

Incident plane wave Plastic foam or honeycomb spacer

Figure 6.22. Sdlsburg screen. Source:

Reprinted with permission from Radar Cross section by E.F. Knott, J. F. Shaeffer and M. T. Tuley, Artech house, Inc., Nonvood, MA, USA. http : //www. artechhouse.com.

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Salisbury screen consists of a thin layer of a lossy material placed in front of the target surface at a distance of a quarter wave length or odd multiple of Al4. The space between the two is filled by a low dielectric constant material, usually foam or honeycomb. A s shown in Fig 6.22, d is the distance between the metallic ground sheet and dielectric RAM. The sheet should have 377 ohms per square element, or

The dielectric constant of the spacer will be in the range 1.03 to 1.1. Let u s consider the theoretical performance of a Salisbury screen with d = 0.5 inches (1.2 mm). Figure (6.23)(a)shows the variation of reflected power with frequency for different impedances (377 ohms, 300 ohms and 200 ohms) of the Salisbury screen. It can be seen that the best performance is obtained for a resistivity of 377 ohms. The reflection coefficient exhibits a minimum a t a frequency of 5.9 GHz and A = 5 cm. A t 300 ohms resistivity, the reflectivity is 18 dB and a t 200 ohms it is -10 dB. By increasing the spacer thickness similar performance can be obtained a t lower frequencies. Salisbury screen cannot be used for broadband applications. A s the spacer thickness is increased there will be rapid oscillations in its performance. For better mechanical rigidity higher density foams or plastics may be used a s spacers. The thickness of the spacer can be reduced by using a spacer with higher dielectric constant. But by doing so the bandwidth is reduced. The Salisbury screen can also be considered for oblique angle incidence. Figure 6.23 (b)shows variation of reflection coefficient of a Salisbury screen2' a s a function of the angle of incidence. A s the angle of incidence changes from 0 - 90" the reflection coefficient increases in the manner shown by the curve. An absorption of the order of 30 dB (99.9%)can be accomplished over a narrow band. But these absorbers suffer from some disadvantages such a s poor environmental resistance, poor mechanical stability, substantial thickness, particularly a t low frequencies, and also high ~ o s t ~ ~ , ~ ~ . 6.8.2.5 McMillcm absorber McMillan absorber is similar to Salisbury screen and was patented in 1959 26' 39. It makes use of destructive interference for rendering itself non-reflecting. Between the front resistive skin and the ground metallic surface, there is flexible foam rubber. When a wave is incident upon it, it partly undergoes transmission and partly reflection. The transmitted portion, upon striking the ground plane, undergoes reflection which in turn undergoes a series of multiple reflections. Each reflection gives rise to a transmitted portion above the front skin since the foam rubber is of A/4 thickness or odd multiple of A/4. The condition for destructive interference is satisfied


at the front surface and hence no reflection takes place. Thus McMillan absorber is a narrow band absorber.

0

15

30

45

60

75

90

Incidence angle (degrees)

Figure 6.23. (a) Variation of reflected power with frequency for different impedances (377 ohms, 300 ohms and 200 ohms); (b)variation of reflection coefficient of a Salibury screen as a function of the angle of incidence. Source:

Reprinted with permission from Radar cross section by E.F. Knott, J. F. Shaeffer and M. T. Tuley, Artech House, Inc., Norwood, MA, USA .http://www.artechhouse.com.

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6.8.2.6 Dallenbach layer Dallenbach absorber has been dealt with by Knott, et a1 25 and StonieP, 39 and others. It is a single layer absorber consisting of a homogeneous slab that can have either dielectric or magnetic absorbing material. The impedance of the lossy slab is matched with that of free space. Theoretically, if p, = &,forthe material there will be no reflection. The actual absorption depends upon the loss properties of the material E ; , p ; . By increasing the permeability and permittivity of the layer the refractive index 6can be increased. Consequently, there will be a decrease in the thickness for the layer. Compared to Salisbury screen, its thickness will be less by 75%. The transmitted energy into the layer undergoes partial absorption before it is reflected from the ground metal surface. Further, the portion that undergoes reflection a t the ground surface gets absorbed in the material. If there is any energy transmitted out of the layer and if it is out of phase by 180" from the wave reflected from the front surface, there will be destructive interference and hence no reflection. 6.8.2.7 Jaumann absorber and Graded dielectric absorber These are multilayer absorbers which extend the bandwidth of the a b ~ o r b e rand ~ ~ may , ~ ~be considered a s extensions of Salisbury and Dallenbach absorbers. In the Jaumann absorber, the impedance tapers off from layer to layer starting from the front sheet to a low value for the ground plane. The electrical properties are controlled by varying the amount of fillers in each layer in order to achieve the required amount of absorption characteristics. The bandwidth depends upon the number of sheets. For a sheet thickness 0.76 cm the increase in fractional bandwidth is 0.27. For 4 sheets of total thickness 3.05 cm the corresponding increase should be 4 times more, i.e., 1.2. Figure (6.24) shows the performance of multilayer Jaumann absorber and the variation of reflected power with frequency. If the concentration of the filler is too high the conductivity will be high and the energy reflected correspondingly increases rather than being absorbed within the material. If the filler concentration is too low there will not be enough absorption within the material and a large amount of the energy of the wave will be reflected from the internal surfaces or ground plane. The Jaumann absorber with multiple plastic sheets replaces the multiple layer Dallenbach absorber. The absorber can be tuned to different frequencies by varying the impedances of the various layers. The impedance sheets are separated by honeycomb spacers of h/4 thickness.


-

Figure 6.24. Performance of multilayer Jaumann absorber variation of reflected power with frequency at different thicknesses. Source:

Reprinted with permission from Radar cross section by E.F. Knott, J.F. Shaeffer and M.T.Tuley, Artech House, Inc, Nonvood, MA, USA. http://www. artechhouse.com.

Instead of tapering t h e s h e e t resistance, a graded dielectric can be used. The mathematical approach for its solution i s to analytically arrive a t the values of p a n d E a s a function of distance from the ground plane over a given frequency range. Graded dielectric absorbers are commercially available manufactured by Emerson and C ~ r n i n gFig ~ ~6.25 . shows measured reflectivity data of a commercial three layer graded dielectric absorber of thickness of about 1 cm. 6.8.2.8 Magnetic absorber

The basic difference between a magnetic absorber and a dielectric absorber is that in the former dissipation of energy takes place principally by magnetic means, although simultaneous dielectric loss also occurs.

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Frequency (GHz)

Figure 6.25. Frequency Vs reflectivity of a three layer graded dielectric absorber. Source:

Reprinted with permission from Radar cross section by E.F. Knott, J.F. Shaeffer and M.T.Tuley, Artech House, Inc, Norwood, MA, USA. http://www. artechhouse.com

Magnetic absorbers consist chiefly of ferrites and carbonyl iron

- a form of pure iron powder. These materials are loaded as fillers in

a flexible matrix consisting of elastomeric polymers such as polyisoprene, neoprene, nitrile rubber, silicone, urethane and fluoroe l a s t ~ r n e r s In ~ ~order ~ ~ ~ .to achieve the required permeability characteristics, thickness and magnetic properties of these materials are controued. This enables "tuning" to achieve the desired loss factor and impedance. Magnetic absorbers are generally thinner by a factor of about ten than dielectric absorbers for the same frequency range. Reduction in reflectivity can be of the order of 20-25 dB (99-99.7%) absorption for material tuned to one or two frequencies. Broadband attenuation can be of the order of 12 dB (95%a b s o r p t i ~ n ) ~ ~ ~ ~ ~ .


Magnetic absorbers can be obtained by spraying or brushing. Required thickness can be obtained by depositing several thin layers. Such materials are used for reducing RCS due to travelling and creeping waves. Such magnetic paints can be used to cover irregular surfaces. Above the Curie temperature (usually 500 - 1000°C)the magnetic properties deteriorate. Ferrite materials are sintered in the form of small rigid tiles. Proper bonding techniques must be employed to fix them on the surface of targets. These materials, although heavier, give good performance a t lower frequencies a t reasonably small thicknees. Analogous to dielectric Salisbury screens, magnetic Salisbury screens can be designed, but adjusting the values of permeability is rather difficult. Sintered ferrite magnetic absorbers can be used even above 1000°C. In general, ferro-magnetic materials can be constructed with Curie temperatures in the range 500- 1000°C but they pose serious problems of chemical stability. Multilayer magnetic absorbers which can work in the frequency range 1- 15 GHz can be constructed in four layers with a maximum thickness of 7.5 mm. 6.8.3 Radar Absorbing Structures (RAS) So far, we have discussed materials which are required to be applied to the surface of the target in order to impart radar absorbing properties to it. That is, we make the assumption that the structural material of which the target is made has no radar absorbing properties. Mow we shall discuss materials which have radar absorbing properties and which can be used a s structural materials, so that there is no additional weight for the target. In general, the less dense the material employed, the lower the radar reflectivity. Radar absorbing properties can be imparted to the target through mechanical design of the composite structure. These are known a s Radar Absorbing Structures (RAS). One such structure has a surface made of quartz glass, fibre glass or aramid fabric reinforced plastic composites. The matrix resin can be loaded with a lossy material or with fillers to control its electrical properties and/or surface impedance in conjunction with a separate dielectric or magnetic absorber, the thickness of which is integrated into the structure. Carbon fibre is used a s the backing structural material. These composites are available a s prepregs which are then moulded to the required shape/contour. The structures made out of fibre reinforced plastic composite materials are light in weight besides possessing high ~ t r e n g t h Also, ~ ~ , ~single ~ . skin absorbers with metallic mesh materials can be made. In these a specific thickness of a

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dielectric layer separates each metal fabric layer. Nippen Electric Company (NEC)manufactures a product which absorbs radar energy. It consists of multiple layers of a stainless steel wire and polyethylene fibre woven f a b r i ~ ~ ~ , ~ ~ . Carbon fibre reinforced plastic material h a s been used extensively a s a structural material in B-2 B0mbe9~.It provides high tensile and compression strength and stiffness resulting in reduction in weight. It can take any complex shape which satisfies aerodynamic charateristics as well as RCS reduction charateristics. Required complex shapes can be obtained by moulding the resin matrix composite prepregs. Further, composite structures possess the advantage that they are cost-competitive with metallic parts. 6.8.4 Circuit Analog Absorbers (CAs)

Circuit analog absorbers are discussed a t length by Knott, et

aP5.Unlike the continuous resistive sheets of Salisbury screen and Jaumann absorbers, conducting material is deposited in geometrical patterns such a s dipoles, crosses, triangles etc, as shown in Fig 6.2625.For such absorbers the term Circuit Analog Absorber (CA)is used, a s the properties of these patterns are expressed in terms of resistance, reactive capacitance and reactive inductance. Further, the design and analysis of these absorbers involve equivalent circuit techniques. The design of circuit analog absorbers is related to that of band-pass or band-stop surfaces. The latter are frequency-selective surfaces (FSS)which do not absorb RF energy. The design of CA involves: (i) Calculation of admittance values for each layer of CA as a function of frequency;

(ii) Aniving a t a geometry and conductance combination which matches with admittance value of each layer. Depending on the size and spacing of the elements the geometry is specified; (iii) Calculation of the performance of the design at the admittance values arrived at; and (iv) Repetition of the process till all the parameters are optimised; Compared to Salisbury screens or Jaumann absorbers, the tailoring of the admittance properties of CAs can be better done and better performance can be achieved. But optirnisation of the variables controlling the admittance properties is rather complicated and involves use of sophisticated computer programmes.


(a) Strips~Wires

(b) Intersecting Wires

(d) Crossed Dipoles

(e) Dual Period Strips

(c) Dipoles

(f) Jerusalem Cross

Figure 6.26. Circuit analog element geometries. Source:

Reprinted with permission from Radar Cross Section by E.F. Knott, J.F. Shaeffer and M.T.Tuley, Artech House, Inc, Norwood, MA, USA. http://www. artechhouse.com.

The substrate material normally consists of polyimide (Kapton] film39.The geometric shapes have complex patterns. The electric properties (dielectricconstant, loss factor) of the various layers cater for different frequencies of the band. The multiple layers catering for different frequencies are spaced at h / 4 to obtain broadband properties. The electrical properties vary progressively through the entire thickness of the absorber (as in the case of broadband applications). 6.8.5 R-Cards R-Cards 25,39 are basically resistive thin films and fabrics. These films are made of gold, copper or nichrome with a dielectric substrate. The thickness of the metal film is less than a micron. When the surface resistance of these absorbers varies along the length of the film it is known a s linear R-card. If the resistance varies radially, it is known a s radial R-card i.e. the R cards have gradient resistivity coatings which are either deposited in varying thicknesses and/or varying composition such that the resistance can vary from 0.2 to 3000 ohms per square depending upon the specific design. Depending upon the contour of the surface (say, an

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aeroplane surface) the surface resistance can be made to vary in several ways. The variation of resistance can be made quadratic, cubic, exponential, or logarithmic. Supposing that the alsorber is applied on the leading edge surface of a n aeroplane wing, the minimum surface resistance will be a t the tip of the leading edge, and then the surface resistance increases systematicallyas it extends from the edge. This RAM design caters for different angles of incidence of the incoming radar signals striking a t various points of the aircraft surface. 6.8.6 Passive Cancellation Passive cancellation, in principle, involves cancelling a n echo source in the object with the help of another echo source whose amplitude and phase have to be adjusted accordingly. In practice this is very difficult. Reduction can be accomplished at one frequency only. Actual targets which are very big in size may have several echo sources running into dozens. A small change in viewing angle or frequency may satisfy the condition for reinforcement instead of cancellation. Hence this method is not being adopted in practice generallyz5. 6.8.7 Active Cancellation Active cancellation, in principle, involves emission of a signal by the target whose amplitude and phase cancel the reflected energy. The target must be able to follow carefully the arrival of the incident signal with respect to its angle of incidence, intensity, frequency and wave-form. It is a very difficult task and it is not known whether it is being used anywhere25. 6.8.8 Current Research on Radar Absorbing Materials It has been reported that conducting polymers have high potential a s radar absorbing materials. Retynl Schiff base salt is a recently developed RAM coating. It is reported that it can reduce radar reflectivity by 80 percent. These materials are discussed in the chapter on camouflage materials. There is a requirement for thin broad-band absorbers. Materials which are based on purely dielectric loss are thick for low frequency applications. Materials which are based on magnetic losses alone are quite effective a t low frequencies. A hybrid RAM might make use of a suitable combination of dielectric, magnetic and circuit analog absorbers. REFERENCES 1. Gardiol, F. Introduction to microwaves. Artech House Inc., Dedham, 1984.


Cook, N.P. Microwave principles and systems. Prentice-Hall, New Jersey, 1986. Microwaves made simple: principles and applications. Edited by Cheung W.S. & Levien F.H.. Artech House Inc., Dedham, 1985. Hall, P.S.; Garland-Collins T.K.; Picton R.S. & Lee R.G. Radar. Brassey's, UK, 1991. Skolnik M.I. Radar handbook. (Editor-in-Chief) McGraw Hill. Inc., USA 1970. Skolnik M.I. Introduction to radar systems (Second Ed.) McGraw Hill Book Co. 1981. Rodgers, A.L; Fowler I.B.R. ; Garland-Collins T.K.; Gould, J.A, James G.A. & Roper, W. Surveillance and target acquisition systems. Brassey's Defence Publishers, 1983. Bhartia P. & Bahl I.J. Millimeter wave engineering and applications. A. Wiley Inter-Science Publication, John Wiley and Sons, 1984. Winderman J.B. & Hulderman G.N. Solid state millimeter wave pulse compression radar sensor. Microwave J.,20, 1977 (1I), 45. Long M.W.; Rivers W.K. & Butterworth J.C. Combat surveillance radar AN/MPS-29 (%E-1).Sixth Tri-Service Radar Symposium Rec. DARPA Publications, 1960, p. 230-44. Dyer, F.B. & Goodman(Jr) R.M. Vehicle mounted millimeter radar. Eighteenth Tri-Service Radar Symposium Rec., DARPA Publications, 1972, p. 473-95. Long M.W. et al. Combat surveillance radar. Final report on contract DA SC-74870. Georgia Institute of Technology, 1960. Holm W.A.; Foster W.S & Loefer G.R. Combined electro-optical/ millimeter wave radar sensor system. DARPA Tri-Service Millimeter Wave Conference, 1977, p 94-103. Hair T. Battlefield surveillance. Proceedings of the military microwave conference. microwave Exhibitors and Publishers Ltd., Kent, UK, 1980, p 281-291. Barret (Jr)C.R. & Ryan D.A. Surveillance and target acquisition radar for tank location and engagement (STARTLE).Proceedings of the Military Microwave Conference. Microwave Exhibitors & Publishers Ltd., Kent, UK, 1980, p 1-8. Layman G.E. SEATRACKS - A millimeter wave radar fire control system. IEEE LASCON - 78 Res. 1978, p. 2 11-16.

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Holmes T.R. & Flick E.A. A millimeter wave radar for the US army helicopters in the 80's. IEEE Proc, NAECON-80. 1980, 2, p. 712-716 Kosowsky, L. et al. A millimeter wave radar for the mini-RPV. AIAA/DARPA Conference on Smart Sensors. November 1978. Wagner R.H. et al, Millimeter wave radar for RPV applications. Presented a t the twenty third annual Tri-Service Radar Symposium, July 1977. Pearce R.H. et al. 3.2 millimeter surveillance radar for the US army mini RPV. Presented a t the AGARD AVPjGCP Joint Symposium Avionics/Guidance and Control for RPV's Symposium. October 1976, Florence, Italy, 16, p. 16-20. Lynn, V.L., HOWLS radar development.AGARD AVP/GCPJoint Symposium Avionics/Guidance a n d Control for RPV's Symposium. October 1976, Florence, Italy, Paper no. 31. Novak, L.M. & Vote F.W. Millimeter airborne radar target detection and selection techniques. IEEE Proc. NAECON-79. 1979, 2, p. 807-817 Hofhnan L.A. et al. A 94GHz radar for space object identification. IEEE Trans. Microwave Theory Tech. Dec 1979, MIT-17 (12), p. 1145-49. Dybdal R.B. Millimeter radar application to SOI. IEEE EASCON77 Rec., 1977, p. 16-4A/ 16-41. Knott, E.F.; Shaeffer J.F. & Tubey, M.T.Radar cross section its prediction, measurement and reduction. Artech House, Inc., Nonvood, Book M a r t Press, Inc. North Bergen, N J 1985. Stonier R.A. Stealth aircraft and technology - World War I1 to the Gulf War SAMPE Journal, 1991 , 2 7 (4) p. 9-16. Mentzer J.R. Scattering and diffractionof radiowaves. Pergamon Press, New York, 1955. Stratton J.A. Electromagnetic theory. McGraw Hill Co, New York, 1941. Richmond J.H. Digital computer solutions of the rigorous equations for scattering problems. Proc. IEEE, Aug 1965, 53, p 796-804. Spencer R.C. Back scattering from conducting. Air force Cambridge Research Lab Report, E5070 April 1951. Crispin, (Jr)J.W. & Siege1 K.M.Methods of radar cross section analysis. Academic Press, New York, 1968. Keller J.B., Geometrical theory of diffraction J. Opt. Soc. Am. 1962, 52, 116-30.


33. Electromagnetic and acoustic scattering by simple shapes. Brown J .J,Senior T.B.A. & Uslenghi P.L. (Eds) North Holland, Amsterdam, 1969. 34. Senior, T.B.A., A survey of analytical techniques for radar cross section estimates. Proc. IEEE, -pug 1965, 53, 822-33. 35. Rheinstein J. Backscatter from spheres: A short pulse view. IEEE Trans, Vol AP-16 1968, Jan. 89-97. 36. Ross, R.A., Radar cross section of rectangular flat plates. IEEE Trans., May 1966, Vol AP-14,329-35. 37. Jones J. Stealth technology: the art of black magic. Edited by Thurber, M. AERO, A Division of TAB Books Inc., Blue Ridge Summit, 1989. 38. Bessi, F. & Zacca, F. Introduction to stealth. Military Technology, MIL TECH 5/ 1989. 39. Stonier R.A. Stealth aircraft and technology - World War I1 to the Gulf-Part 11. Applications and Design, SAMPE Journal, 1991, 27 (5). 40. Foxwell, D. Stealth-the essence of modern frigate design. International Defence Review, 1990,9, p. 984-94. 41. Amin, M.B. & James, J. R.. Techniques : for utilization of hexagonal Ferrites in Radar absorbers (Part I) Broadband planar coatings. The Radio and electronics engineer. 1981, 51(5),209-18.

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DECEPTION 7.1

INTRODUCTION Deception, similar to camouflage and concealment, is new neither in nature nor in war. For self-preservation, whatever means that the prey and predator adopt, in defence and offence, let it be camouflage or concealment, or any other means such as mimicry, deception is inherent. In the progress of biological evolution, there is an evolutionary arms race, in which deception is invariably employed as a means in strategies and counterstrategies-the predator while on the lookout to locate and capture its prey, and the prey while it is constantly on the alert to avoid recognition and capture1. Human civilization, too, beginning with the primitive man, has been using deception in various forms for different purposes. Particularly in wars, deception has been in vogue from ancient times. In the words of Sun Tzu (500 BC) - The Art of War, "All warfare is based upon deception; when able to attack, we must seem unable, when using our forces, we must seem inactive, when we are near, we must make the enemy believe that we are away, when far away, we must make him believe that we are near. Hold out baits to entice the enemy. Feign disorder and crush him2s3t4. The basic philosophy remaining one and the same, the changes that have come from olden times to these modem times are in the methodology of application and the levels of sophistication of the means of deception- from the torch light decoys to the TacticalAir-Launched Decoys (TALDs); from the Trojan Horse to the Tomahawk. During the American War of Independence, in late 18th century5, the British troops who used to dress in bright red coats, because of which they were nicknamed "Red Coats", were conspicuously visible in any background, thus becoming easy targets to American riflemen. But on realising this, they changed their dress to buck-skin which was not spotted in the background.

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In the first World War, khaki and grey had become standard colours of clothing outfits. This ruse was picked up from the Afghan war of 1880, where the Afghan soldiers wore khaki uniforms which blended well with the barren muddy countryside. Khaki is the Urdu word for dust-coloured3. The Britishers during the North Africa Campaign5 had learnt the benefits of deception for better survival. They had developed the technique of blending into the background of foliage by attaching pieces of hessian strips and artificial foliage on their helmets and weapons. These techniques were incorporated into the World War I1 operations3. The Americans, on the other hand, because of their better mobility and superior fire-power and material resources, considered deception an unneccessary subtlety. But the deception operations were too successful to be ignored for long and the reluctance gave way to full acceptance, and the US Army became quite proficient in deceptive arts by the end of World War 11. Their techniques have evolved into a highly effective combat multiplie9. In the Falkland campaign in 19825, the British commanders were misled into believing that the Argentinian airfield at Portstanley was damaged and rendered unservicable by bombardment. Reconnaissance pictures showed bomb craters which supported this view. But it was discovered only much later that the entire operation was a deliberately planned, and effectively executed, deception exercise. Early every morning, the otherwise normal and undamaged runways were covered with heaps of mud in bombcrater like appearance, by a gang of labourers. At dusk they would clear all this mud and make the runways operational for supply aircraft to use them through the night cover, without being detected by the enemys. In another example from the Falkland campaign, the British naval aircraft carrier HMS Hennes survived a missile attack by an Argetinian missile-carrying aircraft Super Etendard. The Exocet missile fired at Hermes was successfully diverted when the missile radar seeker got confused by the chaff decoys deployed by the warship. On the other hand, another warship, Atlantic Conveyor, which had not deployed the chaff decoys and was in the near vicinity of Hermes, got struck by the missile6. The Soviet Union had made deception a formal part of their war doctrines, even before World War 11. Maskirovka is a well documented Soviet deception doctrine. The man behind its development was General Ogark~v'*~*~. Applications of deception techniques in all their subtlety and sophistication peaked during the 1991 Gulf War. The technically developed and heavily resource-backed Allied Forces brandished advanced decoys and deception equipment. The Iraqis effectively displayed deception with relatively simpler techniques.

Deception

Winston Churchill during World War I1 had stated: "In War, truth is so precious, that it should always be attended by a body of lies"lO.Immediately following World War I1 Gen Dwight D Eisenhower admonished the American Army to remember the important role that deception played during the war ".... no major operation should be undertaken without planning and executing appropriate deceptive measuresu3. One of Britain's most experienced deception planners, Brig. Gen Dudley Clarke, who was head of the 'A' Force - a deception planning team-when planning with Gen Wave11 an attack against the Italians, had expressed a creed that they always followed: "what do you want the enemy to do; not what do you want him to think."2 7.2 WHAT IS DECEPTION? A s applied to war, deception is a deliberate and rational effort to mislead the adversary by manipulation, distortion, or falsification of evidence to induce him to react in a manner prejudicial to his interest and/or to the benefit of the deceiverl1J2. Deception may also be defined a s a technique that leads to surprise which enables the deceiver to take advantage of the enemy's weakness and overcome his strength. Throughout history deception has been employed not only to deceive enemy battle-field commanders, but also the highest ranking enemy policy makers. Deception has several facets. These are: Camouflage and concealment; Deception equipment in the form of dummies, decoys feints, and ruses; Disinformation. Before proceeding further, the last facet 'disinformation' may be elaborated upon a little. 7.3 DISINFORMATION Disinformation is spreading false information carried out openly or secretly. The intention of the disinformer is to alter the perceptions or the behaviour of the persons on the receiving side. The ultimate function of disinformation a s applied to defence is to affect the decision making in a manner favourable to the disinformer. Diversion is another term employed by the military which involves diverting the enemy's attention away from the location of main operation. A s applied to movement of troops, by diversion is meant the act of altering the precribed route to reach the destination.

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Research has been carried out on psychological aspects of disinformation with the objective of neutralising or minimising the adverse effects12. The Soviets considered 'Maskirovka' their doctrine of deception and disinformation3. It is an active continuous process of encompassing demonstrations, feints, camouflage imitation and false information. Its main objective is to convince the enemy about events and activities which in reality are not there. Interestingly, the Soviets considered 'Maskirovka' important not only to warp the enemy's perception of Soviet positions, intentions and missions, but also, because of Soviet penchant for secrecy, they wanted to alter the perceptions of their own units and clients as well. The Soviets considered successful deception a s contingent upon surprise. 7.4 PSYCHOLOGICAL AND GENERAL ASPECTS OF DECEPTION Deception is false presentation and false propaganda. A psychological coloration to a deception effort can very positively help in toning it up to influence the adversary, mind, to induce him to a reaction which puts him into a disadvantage1&12. In his 'Art of War' treatise referred to earlier, Sun Tsu stressed the importance of manipulating the human mind and behaviour in times of crisis and war. In the 20th century, deceptive psychology has been used systematically as an important new discipline in war. Some key factors from the psychological perceptive that can affect the accomplishment of the overall deception mission are mentioned below. The mind to a very great extent relies upon preconceived ideas. The brain accumulates a database of hypotheses. The mind is gratified by evidence that confirms its preconceptions and thinking attitudes. In other words, the mind believes what it wishes to believe. The mind tends to dislike confusing and ambiguous situations. The effort and the resulting stress is to resolve uncertainities, and, especially under battle pressures, as quickly as possible. This can lead to jumping to conclusions. The tendency is to allocate priority to anything new or different or interesting, and shift to the rear anything of routine nature. Surprise, when thrown in, can catch the attention with more certainity than anything else. Routine dulls the senses and one may not spot marginal or gradual changes.

Deception

Credibility of a particular line of thought is enhanced when the illusion or ruse is seen from more than one source.The greater the sources or opportunities the mind has to 'see' the proposal, the more convinced it will become. In a reduced fidelity simulation, the more prominent or pronounced ploy registers better on the mind, making a greater influence on it. Thus by knowing what attitude the mind can take, under differentcircumstances, the necessary stimuli can be tailored and toned to influence the mind in the required direction. Deception has been exhibited in all battles from World War I1 such as Operation Fortitude, Barbarossa, Battle of Alamin and the Tobruk Offensive of 194112. In situations where camouflage and concealment by any possible means are not effective, or not practicable, deception has to be employed. The fundamental characteristics2 of deception are: It is low in cost; It confuses the enemy resulting in prolonging his indecisiveness; It tends not to destroy, but it tends to save both assets and lives. It is therefore significantly nonlethal warfare; It enhances your ability to survive; In combination with secrecy and integrated effort,it becomes a force multiplier. Although initially deception in the battlefield was existing more as a military art than science, today in the modern war, with the advent of high technology multispectral sensor systems deployed in a variety of platforms including satellites, deception has become a technology in its own right. It deals with development of dummies and decoys. Principally the role of deception equipment is that they become force multipliers, thereby enhancing the force's ability to survive. Deployment of deception equipment weakens the strength of the enemy, by offering more targets than those that actually exist, distracts the attention of the enemy and fire away from essential installations, and deceives the enemy a s to the identity, strength, intention or degree of activity in an area. 7.5 DECEPTION EQUIPMENT The most important facet of deception is deception equipment. A plethora of equipments are being developed for use as deception devices. These devices based upon their different characteristics viz, physical dimensions, emissions, deployment etc-may be classified into different types or groups.(Table 7.1)

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Broadly they may be classified into two types: Dummies and decoys. Table 7.1. Classification of deception equipment Main types Subtypes Activity classification Signature-wise classification With respect to

Dummies 3-dimensional Z-dimensional Passive Mp~k-ups

Non-expendable

Decoys One dimensional Active Monospectral Bispectral Multispectral Polyspectral Expendable

7.5.1 Dummies A dummy displays all the external visual features of a military object with regard to shape, solidity, and size. But it cannot perform the various functions of the object it depicts. A dummy is generally ground-based. It is usually stationary or static. Any movable type is mobile, not so much from the point of depicting motion of the actual object, but from the point of facilitating change of deployment position or location over short distance. A dummy of this kind possesses all the optical (visual) characteristics of the original object, and, therefore, when viewed through sensors in the visible region, it can be mistaken for the original object. This type of deception equipment may therefore be classified as 3-dimensional (3-D) dummies with monospectral simulation (visible region only). However, in the present day war scenario of multispectral surveillance, sensors in the infrared and microwave regions can detect the monospectral (visual)3-D dummies a s false. It therefore may become necessary to simulate in the 3-D dummies, besides the signature in visible region, the infrared and/or microwave s i g n a t u r e s also. S u c h d u m m i e s may be designated a s multispectral. Besides the various types of electromagnetic signatures the military object may have other types of signatures such a s acoustic, seismic, electric and magnetic. In specific situations, some of these additonal signatures may also have to be incorporated in the deception equipment. Such types of devices may be classified as polyspectral. Thus the larger the simulation bands in a deception device, the more difficult it becomes to discriminate it Erom the original

Deception

object even by multispeotral sensor systems. The larger number of cues combine to support as well as strengthen one another to enhance credibility of the device. Dummies are deployed generally in numbers to depict a tactical manoeuvre to deceive surveillance efforts of the adversary. They are designed to be used a number of times and are therefore categorised as non-expendables. Wherever IR signature simulation is effected through conversion of some energy into heat, e.g. fuel burning or electric heating, the deception device becomes a n active system. Dummies are mostly land-based and a big object such a s a ship may not have 3-dimensional dummies deployed on the sea. 7.5.2 Decoys In general, in a decoy, only the most prominent signature of the object by which it is detected, depending upon a particular war situation, is simulated. A decoy of a military object does not possess the visual external appearance of the original object. Thus it oan be detected by the sensors of the regions other than that which it is simulating. An obvious consequence of this detection is a countermeasure reaction directed against the decoy. Therefore, decoys are, by and large, not static but are mobile, and move in a manner to depict the motions of the original target object, thus offering themselves as a more realistic bait and a t the same time executing evasive countermeasure tactics. Again, decoys are carried in suitable numbers on the parent object which is meant to be either protected or is the launch platform. Because of this reason, as well a s the preceding one, a decoy is of a fractional size of the original object. An infrared decoy may merely consist of a strong source of heat having the IR spectral characteristics of the original object. In a microwave decoy, only the microwave signature of the object is simulated by very much smaller sized corner- reflectors giving intense reflected returns to match the original radar cross-section of the object. Another example is that of a noise-generator decoy. Its physical size is totally insignificant as compared to the bulk of the object it protects, i.e. a ship. Such types of decoys that have practically insignificant dimensions, a s compared to the (parent) object they protect, can be categorised as one-dimensional (1-D) decoys. Also, since each of the above examples of decoys is having signature of only one particular region of the signal spectrum, they can be grouped as monospectral.

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The IR decoy produces its own IR signature by burning its pyrotechnic material. The noise generator produces its own acoustic signature. Such types of decoys that generate their own signals are designated a s active decoys. Most decoys are active a n d consume their resources of signature simulation energy. They have only one-time deployment capability and they are therefore categorised as expendable. In naval defence, mostly against antiship missiles that may be air-launched or sea-skimming, certain off-board decoys are used. These are categorised a s soft-kill decoys13- resorting to active jammers, generating noise or signals to deceive or distract the incoming missile sensors and inducing them away from the ship target so that the missiles go waste. The noise generators may be in the form of IR flares or microwave chaff dispensers. Hard-kill decoys are designed to engage the incoming missiles and destroy them; they may be in the form of antimissile missiles, or radar controlled rapid fire guns. The former systems have greater advantage over the latter, of being very low cost and engaging much less ship installation area. Though IR flares a n d chaff dispensers are individually monospectral decoys, sometimes, they are used together in colocated efforts to cope effectively with multispectral sensors in missile-seeker heads. 7.6 CANDIDATES FOR DUMMIES AND DECOYS In any battle, the most frequent and susceptible targets for the enemy are objects that are most exposed or vulnerable to attack by the adversary. This number for such targets becomes fairly large, when we consider the various objects deployed in different theatres of war-land, air and sea. These objects may be directly involved on the battlefronts or indirectly involved a s battle support effort on the rear. This prohibitive number can be short-listed by considering only those objects which, under the prevalent war scenarios, are critical or sensitive. The former, of course, takes priority over the latter. In order to gain full advantage from deception under any tactical situation, the defence forces must be involved with it from the earliest stages of both planning and preparation-in other words possess "deception preparedness". Candidates for dummies and decoys must be identified and devices made ready and stocked, and training and deployment expertise imparted. With regard to dummies, considering the three forces, from the point of view of their different functions, their different war

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approaches and battle scenarios, they are used to the maximum by land forces. In the case of the navy, because of its formidablesized systems like ships and submarines, the use of l ~ o k - ~ k e full-sized deception equipment is neither economical nor practically feasibleldesirable. When we consider decoys, it is found that all the forces have abundant use of these devices, with the navy taking the lead in variety a s well a s applicaton. 7.6.1 Criteria for Selection In picking out candidates for dummies and decoys certain general criteria14 may be used, However, specific situational considerations may need to be applied for different scenarios/ circumstances. The criteria can be categorised under two main groups i.e. general criteria which consider operational situations, and sensorspecific criteria which deal with behaviour of the target in the electromagnetic regions. Each of these categories is further subdivided - the general criteria on the basis of the target situational parameters, and the sensor-specific criteria on the basis of different regions of the electromagnetic spectrum a s well a s acoustic and seismic regions. Again, each of these sub-divisions can be further split into three groups, each depending upon the state or intensity of each parameter. These are high, medium and low. The first-high-deals with signature strength or the quantity of details enough to identify the target; the second medium-is lower in details and can indicate target recognition and the third low-is still lower and may indicate only u p to detection level. The above details are shown in the from of a chart below : Salient points of t h e operational parameters a n d the vulnerability to detection by surveillance or acquisition by a weapon sensor will be discussed in brief. 7.6.2 General Criteria Size and number : Physical aspects i.e. dimensions of length, width and height; Operational aspects: Number deployed on battlefield, spread and orientation; This determines the vulnerability of the target Location to ground or airborne surveillance, e.g., the front line locations are vulnerable to both ground and air sensors;

I-

1

I

,

I

7 >

,/.'

I

High

Location Mobility Detection

I -

Size

artillcry, short range sensors

Vulnerability & Detection value

Activity Operation"

1 Visual

.L

aircraft, long range sensors

Acoustic seismic

- -

/"

Detection

1

Radar

Sensor-spccific -

Laser Infrared

4 Surveillance

Recognition

Medium

Association

Criteria for dummies and decoys

t

1 I

!

,

t

c. 0

Go

@ mb

%

0

3

0 CI

0"

Deception

Frequency of movement during normal field operations; high - e.g. tank company medium - e.g. medium guns battery low - e.g. tactical headquarters : Degree of accuracy by which targets are Detection value acquired or engaged; Targetting: When within artillery range and short range missile system; : Through aircraft and in the range of long Surveillance range missile; : These signatures refer to the signatures due Associative signatures to general activity or situation. These are different from electromagnetic and nonelectromagnetic signatures of the targets. Examples a r e g u n flashes a n d track markings. They are classified a s high; if they a r e total giveaway, giving reliable cues medium which need to be confirmed and; verified, and low if only suggestive of certain equipment/deployment. 7.6.3 Sensor-Specific Criteria These relate to signatures of the electromagnetic spectrum and can be categorised into five different kinds. Visual vulnerability The detection prbbability, aided or unaided, is dependent on shape, size, hue, colour contrast, mobility: High -. locations near the forward edge of battlefield area i) (FEBA); here visual surveillance is maximum; movement is also frequent.; ii) Medium - direct ground observation stand-off aerial sensors; iii) Low - subject to only long range airborne reccee; blending with background is high. Infrared detection vulnerabiliity Very-short wave infrared (VSWIR)- detection is by imaging systems : High - operation zone near FEBA, within short and medium line of sight, contrast from background is sufficient to afford detection; Medium - within line of sight range, signature images are not very distinct; Long - outside the range of VSWIR sendors. Mobility

:

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Medium wave (IRj detection is by uncooled or moderately cooled IR detectors of very hot sources and hot spots, target self-emissions, applicable in seeking and homing devices. The high, medium and low groupings are similar to the' ones mentioned above. Long wave (IR) - detection is usually by cooled detectors (liquid nitrogen cooling), of surface emittance, thermal lag and inertia. The resulting imagery of thermal signature gives sufficient details to detect and identify targets. Laser detection vulnerability - degree to which surface characteristics s u c h as skin or coatings composition and roughness, aspect angle and contrast with background provide laser reflectivity of useful quality capable of being received and analysed by sensors High - significant amount of energy is reflected and detected; Medium - reduction of reflectivity a t particular wavelength; Low - minimum reflectivity from the target. Radar detection vulnerability - magnitude of target scattering cross section a n d the signature relative to background, based on combination of size, configuration and metallic components: High value - ideal reflectivity characteristics e.g. tactical bridges; Medium value - reflectivity sufficient to require further investigation e.g. a medium gun battery; Low value - reflectivity low enough to blend with normal background, e.g. Infantry Brigade. Acoustic and seismic vulnerability - detection by acoustic and seismic detectors, transmitted vibrations to air or groundmagnitude, characteristics, shape, duty cycle: High - large impulses delivered to air or ground near forward edge of battle area (FEBA); ~kdium - smaller vibrations or vibrations emitted at longer range; Low - unidentifiable characteristics, or out of range of sensor systems. All the above criteria form the basis of analysis for classifying targets. One has to use discretion in the selection of the important targets so as to keep the numbers reasonable and manageable. Also, mission-associated conditions and characteristics e.g. location on battlefield, type of material employed, mobility, resources available for the extra effort needed, including logistic support with respect to place and operational constraints, have all to be given proper weightage.

Deception

7.7

BACKGROUND FOR AN EFFECTIVE DECEPTION

STRATEGY The enemy, through his strategic reconnaissance facilities, collects information on his adversary in good time and builds u p his data bank. Through his tactical reccee systems, he acquires information needed towards a n imminent battle action. The basic purpose of both types of reconnaissance is to gather information regarding the following - time, location, kind, number and activity. With regard to this information gathering effort by the enemy, if we can find answers to some pertinent questions, listed below, we shall be in a position to plan out an effective deception scheme or strategy to deny the adversary the correct information or feed him with disinformation. Some pertinent questions as given by Savoie in his pape? are: What are my high signature items from enemy's point of view?. What methods should I use to deny these signatures to the enemy e.g. camouflage or deception techniques? What signals and when should I send to the enemy to enhance the deception ruse? What is the enemy's decision making time and what is his reaction time required to respond to my signals?. Has he received the signals a s per my plans and strategy? How and a t what levels are deception plans to be linked with other u n i t s or the exercise integrated in a joint a n d coordinated plan ? Our effort and accent should be to gather information on the enemy's likely course of action and adjust the deception plan, if possible, to the enemy's momentum of planning, direction and tactics, thereby convincing him that his predetermined course of action is the proper one. We can thus with greater certainty and speed line him into our deception trap. 7.8 DUMMIES/DECOYS OF MILITARY OBJECTS Of the various military objects, the major items that need deception equipment are tanks, aircraft, ships and submarines. Majority of the deception equipment reported in literature are decoys of expendable type. Non-expendable types such a s 3-D dummies are intended for military targets in a static position.

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7.8.1 Dummies and Decoys of Visible Region

Deception devices in this region, because of the high discriminating and resolving power of the optical instruments being used a s sensors and surveillance devices, must imitate the original objects not only in correct size and shape, but also in the display of necessary smaller details, like surface clutter fitments, contours of prominently exposed surfaces, shade and shadow effects, texture of the skin surface, and visual reflectances from acute sighting angles. With all these details to be considered, the dummies tend to become operationally and logistically somewhat cumbersome. But the need for surface details clutter can be considerably reduced by providing camouflage cover through artificial means such a s nets or masks, or natural means such a s foliage or dunes. Means of erecting and dismantling m u s t be simple a n d not timeconsuming. The dummies must be easily transportable from place to place and the individual parts must be stackable and must not occupy large volume or space. They should be light and strong and stable against high velocity winds. Their economics is an important criterion. 7.8.2 Decoys (IR & Radar) The major expendable decoys are chaff and IR flares. The former mimics the RCS of the military object concerned and becomes the virtual target to a radar-guided weapon, the latter mimics the hot spots of the military object concerned and offers itself a s the target to a heat-seeking missile. Chaff and IR decoys reported in literature are briefly discussed in the following sections. 7.8.2.1 Chaff Decoy Chaff is one of the commonest forms of decoy designed to counter hostile radar system. The chaff decoy presents tracking radars with a n alternative target with respect to that which they are really seeking utilising homing radars which are mounted in the nose of antiship and antiaircraft missiles. ChaffIs consists of thousands of thin rods of conductive material such a s aluminium, or metallised glass fibre rods, or silver nylon monofilaments whose length is equal to half the wavelength of the frequency at which the hostile radar is anticipated to will operate. Each of the rods constitutes a dipole which reflects the hostile radar pulse. These fibre rods may have diameter in the range 0.5 - 2 -0mil and RCS varying between 0 and 0.866 m ZThe RCS depends upon the view angle. The expected RCS averaged

Deception

over all viewing angles is 0.15 to 0.20, depending on the length-todiameter ratio of the dipole. The chaff must cover a broad band of frequencies to provide effective cover. The alignment of the dipole rods should be able to cater to counter horizontally polarised, vertically polarised, as well a s circularly polarised transmissions of the radar. The maximum cross-section occurs a t chaff lengths that are approximately integral multiples of one-half the illuminating wavelength. Chaff may also be characterised by assigning an average cross-section to it. The average is around the frequency range a t which the peak cross-section occurs. The bandwidth is normally 10-15% of the mid-frequency and the chaff is assigned a cross-section of zero outside the bandwidth. Several millions of individual elements are required to generate a chaff cloud capable of producing a false target with a radar cross-section equivalent to the target (if the target is a warship). Chaff is dispensed in millions, in many methods ranging from pyrotechnique to mechanical. Self-protective chaff is ejected from cartidges containing a n explosive to effect rapid blooming. The chaff expands to form a chaff cloud. It must expand rapidly so that it presents a cross-section larger than the target. Blooming in a time of the order of 50 ms may be required. As viewed by a radar, the RCS of the chaff cloud will be nomally less than the physical area viewed by it. The RCS presented by the chaff cloud depends upon the number N of the chaff dipoles visible to the radar. It is claimed that 1 lb (approx. 0.5 kg) of chaff will yield a 60 m2 target at all frequencies over 1- 10 GHzL5. If N represents the total number of dipoles in a chaff cloud, and A, the physical area viewed by the radar, then the number of dipoles per unit of the projected area (n)15is equal to The RCS is given by where u, = 0.15A2 (dipole cross section) when n is large, noebecomes large, i.e. a becomes equal to A,. . When n is small, nocis small, and 1-nu,so that The efficacy of chaff depends on the environmental factors, essentially wind. The slightest m o u n t of wind in the wrong direction will begin to dissipate chaff cloud and nullify its effect. Taking wind factor into consideration, chaff must be continually deployed.

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Chaff continues to be deployed in naval units16 and aircraft with comparatievly low cost and simplicity. Chaff is a uniquely versatile countermeasure; however its performance against sophisticated electronic counter-countermeasures (ECCM) is questionable13. There are several modes by which chaff may be deployed. Before the identification of target group occurs, i.e. in the initial stages of engagement, confusion chaff clouds can be deployed from a long range. Such chaff clouds present a multitude of decoys and create confusion denying range and bearing information13. Distraction chaff are deployed to distract the missile seeker away from the true target when the missile is inbound. Seduction chaff is deployed when a missile seeker has locked on to its target. Accurate modelling of chaff behaviour is not a particularly simple exercise. It is in this area that probably future efforts will be concentrated 17. The US Navy is researching and developing several new materials for chaff, but the information is classified1'. Cheming Ltd., Portsmouth, U.K. is studying the aerodynamic behaviour and radar characteristics of existing and novel chaff materials1'. ML Aviation UK has developed SUPER BARRICADE which uses the latest chaff dispersion technology to create large, rapid blooming chaff cloudslg. Loral Hycor Mk36 SRBOC (US)is a rapid blooming decoy system. The MK36 is a family of mortar launchers deploying chaff cartridgeslg. There are numerous systems available for deploying chaff ranging from highly sophisticated systems such as those developed by CSEE in France and Plessey Aerospace in UK to very basic sys&ms such as those developed by wallop in the UK20. 7.8.2.2 Infrared Flares The increasing effectiveness of missiles equipped with infrared seeker heads in the Falklands and the Middle East provided impetus to intense development in infrared decoy^^'^^^. Infrared decoys in the form of infrared flares are widely employed to protect aircraft and ships against heat-seeking missiles. The starting point in the design of an infrared flare decoy is to have an assessment of the detection range of the threat missile a n d t h e environmental characteristics. The important characteristics of a flare decoy are: peak intensity, rise time, spectral characteristics, function time, ejection velocity and aerodynamic characteristics. All these have been discussed in detail by BruneZ2. The cost of the decoy depends upon the number of characteristics reproduced. An infrared decoy is basically an intense source of

Deception

heat and small in size, possessing radiation properties matching with those of the military system to be protected. The most important characteristic is the peak intensity22which should be higher than that of the target to be protected in all the threat bands. One typical example of a n IR flare of a conventional aircraft consumes 0.5 to 1 MW of power for each 1000 W/sr of inband radiant intensity a t flight condition^^^. Rise time is the time in which the radiant intensity reaches effective level. This should happen before the decoy leaves the seeker's field of view. The is of the order of a fraction of a second for a n aircraft decoy. The heat required for most IR decoys comes from chemical sources22.Appropriate spectral matching corresponding to threat wavelengths must be accomplished. The flare will have the maximum intensity a t the critical wavelength band. The function time during which the decoy persists should be long enough to ensure that there is no possibility of target reacquisiton. motechnic flares are used in aircraft IR decoys. These are released whenever there is a cue from a missile warning system. The main parts of a pyrotechnique flare22are a cartridge case which is simply a cover for the flare, a n ejector charge which is a n electrically initiated gun powder charge, a mechanism for initiating the flare and a n end cap which serves a s a seal. Atomised magnesium powder and poiytetrafluoroethylene (FTFE) resin are used for accomplishing pyrotechnique reaction. In the case of a flare for fighter aircraft, the rate of consumption of the fuel is more than 100 g/s23.The performance of a flare is affected by wind speed and altitude. The radiation intensity decreases a s the wind speed increases. The burn rate becomes more and more complicated a s the altitude increases. The flare decoys are launched precisely a t a time and location in such a way a s to maximise the threat miss distance. The flare cartridges loaded into a magazine are dispensed, for example, by a countermeasures dispenser. The dispensing rate is controlled electronically and by computational algorithms. Similar to IR decoys for aircraft, IR decoys are designed for shipboard use. Most of the threat IR seekers employ mid-lnng wavelength bands. The two bands of interest are 8-14 prn and 3-5 pm wavelength ranges. The contrast signature (with respect to the surrounding sea) of a combat ship is less than 1000 W/sr in the 8-14 pm band. The signature level in this band, compared to that of 3-5 pm band, is several times higher.

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The important parameters of the ship to be considered in the design of a n IR decoy are area, temperature and emissivity cha,.: 2teristics. Similar to the modes of deployment of decoys for aircraft, seduction, distraction and dilution can be employed in the case of ship also. Other IR decoys employed in the case of ships are the liquid fuelled decoys and floating solid fuel decoys. The fuel can be a simple hydrocarbon which is expelled through a nozzle and then ignited. The liquid fuel decoy is supported on the ocean surface by a floatation assembly. The US Navy MK 186 Torch is a typical example of liquid fuelled decoy. The floating solid fuel decoy floats in the ocean a n d projects a radiant plume of combustion products of magnesium flare-carbon fuel. In the case of shipboard decoys, two types of launchers are used - the mortar launched system and the rocket laurfched systemz2. VARIOUS DECOYS (Published in Literature) 7.9 Data on various decoys published in literature are briefly described here. Tactical Air-launched decoy (TAED)23,on engagement, keeps the enemy radars operating, thereby increasing the chances of their destruction by anti-radiation missiles. It has a length of about 92" (230 cm) a n d width of about 10" (about 2 5 cm). It weighs approximately 400 pounds (app.180 kg). The decoy can be used in active a s well passive modes. In the active mode, a n antenna processes the SAM radar signature and then sends back the enhanced signature of a full scale aircraft. In the passive mode the decoy uses a reflective lens located in its nose. The efficacy of TALD has become evident in the operation 'Desert Storm'. It was launched from A-6, A-7, F/A-18 and S-3 aircraft. TALD forced Iraqi i-adars to stay on the air, easing their destruction by missiles. TALD was produced by Brunswick Corporation's Defence Division, CaIifornia, US. Encouraging results fiom the 137 TALDs deployed by the US Navy during the Gulf War were one of the factors which led to the contract for the Improved Tactical Air Launched Decoy (ITALD),which is expected to provide higher speeds, greater range and more realistic flight profiles. Antiradar missile decoy (ARMD)24was being produced for the U S Army and Airforce by ITT Gilfillan USA. It is a miniature transmitter which protects radar in the field. Each decoy will radiate signals emulating transmission characteristics of the radar station it is set to protect, thus confusing any missile tracking and targetting. The earliest operational American example of Towed/Free Flight Decoy17 is Sanders POET - Primed Oscillator Expendable

Deception

Transponder. It is a n active deception jammer. It can be deployed in such a way that it provides the launch vehicle maximum protection against active and semiactive radar guided threats. POET h a s been superseded by t h e Texas I n s t r u m e n t s Generic Expendable (GENE-X)round which is the first decoy to incorporate18 the monolithic microwave integrated circuit technology (MMIC). The latest generation US device is the STRAP (Straight Through Repeater Antenna Performance). The Big Boy is another retrievable towed decoy jointly developed by Tracor Aerospace Austin TX and Boeing Military Airplanes Seattle WA. Big Boy also operates by propagating a larger RCS than that of the aircraft it is protecting17. Mention may be made of some of t h e off-board countermeasure^'^. The demand on naval air defences has been greatly increasing. The types of antiship missiles range from large transonic cruise missiles such as STYX and the Silkworm to the latest generation of highly intelligent sea-skimmers such a s Exocet Block-2 and Harpoon Block C. The countermeasures consist of hard-kill and soft-kill systems. The hard-kill systern is expensive and its size often renders it unsuitable for installation in smaller platforms. The off-board decoy is the most viable soft-kill countermeasure system to antiship missile. Chaff decoy (which has been already discussed) will remain the primary soft-kill medium against radar guided threats. Active off-board decoys will continue to have a major impact on antiship missile defence. In order to counter laser-guided weapons, a n experimental free flying decoy, containing a laser jammer and power supply, was being developed by Tracor Aerospace and Messerschmitt Boelkow-Blohm GMbH. It employs a fibre optic based laser warning system to interrogate the incoming laser beam and determine its angle of arrivalz5. Weapon decoys countering the torpedo will be briefly describedz6.The torpedo threat is more deadly to the surface ship than the antiship missile. Ships and submarines are highly vulnerable to underwater explosions, in particular to torpedo hits. There is therefore a definite need to provide a fully integrated antitorpedo defensive system. The method employed is to deploy torpedo decoys such a s US SLQ-25 Nixie for the surface ships. The torpedo decoy is essentially a noise maker. Such a system is a softkill measure. It can be in two forms - (i)a noise maker stream astern of a surface warship, or (ii) an expendable decoy launched from a submerged submarine. The U S Nixie manufactured by Frequency Engineering Laboratories comprises two small light towed bodies which transmit acoustic signals intended to complement the ship's acoustic signature and decoy the torpedo away from the ship.

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From the noise radiated by submarines they are sensed and hit by torpedoes. The transient noises arising from a rapid last minute manoeuvre of a submarine could be very revealing to an alert enemy. In this situation it becomes necessary for the submarine to deploy decoys instantly. The Submerged Signal Ejector (SSE), which was originally used for the purpose of discharging pyrotechnics to locate the position of a submarine in d i s t r e s s , h a d been identified a s suitable for deploying countermeasures and for gathering intelligence from submarines. For this application, these devices were renamed a s the Submarine Signal and Decoy Ejector (SSDE).This provides instant deployment of decoys the moment a n incoming weapon is detected. The SSDE becomes a part of the tactical weapon system. The BANDFISH developed by Dowty Maritime Sonar and Communications Systems Limited is an example of such a decoy. The Bandfish is a jammer which transmits a high intensity acoustic signal covering a wide range of frequencies adequate to counter the perceived threat. The decoy may be launched from all submarine operating depths27. The French Navy's SLAT28(System de Lutte Anti-Torpille) softkill torpedo countermeasure h a s two principal componentsantitorpedo warning system and countermeasure system. The wake produced by a surface ship is large, leading to a persistant signature which can be targetted by a torpedo equipped with a high frequency upward-looking A wake is caused by several effects such as modification of thermal structures within the sea, turbulence, stemwaves etc. But the main source of the wake detectable by the high frequency sonar in a wake homer is the accumulation of air bubbles due to the motion of the vessel through the water. The acoustic wake is defined a s the volume of ocean which has acquired an increased capacity for absorbing and scattering sonar energy. It may not be difficult to produce a false wake behind a ship employing a bubble generator which would mimic the acoustic back-scattering effects of a real wake. A false wake of right dimensions so produced a t the right time might be sufficient to counteract a torpedo's re-attack logic. Alternatively, a device that is capable of creating a false wake could be towed well astern of the ship so that a wake homer acquires the device before it is able to acquire the ship's wake. The various camouflage measures, unless supplemented with dummies and decoys, are not likely to be effective. In a stress situation, a n enemy pilot is likely to attack the first target he sees1'. He can be misdirected by the use of dummy targets (3-D/2-D).

Deception

They must also be camouflaged inadequately. US, Swedish and British companies manufacture solid and inflatable dummies and decoys simulating aircraft, tanks, APCs, artillery pieces and air defence system. The best known manufacturer in the, field is the Swedish firm of Barracuda. Bridges can be of crucial importance. A large number of dummy bridges may well divert and waste enemy interdiction effort.

REFERENCES Cott, H.B. adaptive coloration in animals. Methuen & Co Ltd. London, 1966. Le Hockey, J . D. Are we deceiving any one? Naval Proceedings, 2. Sept 1989, p. 53 . 3. Savoie, A.T. Are we deceiving ourselves, Military Review, March 1987, LXVII(3) p. 37-45. 4. Uhle-Wettler, F. I n t e r n a t i o n a l military a n d defense encyclopedia. Trevor Dupuy N. (Editor-in-Chief), 1993, 2,(C-F), p.708. 5. How is it done ? Readers Digest. Readers Digest Association, London, New York, Sydney, 3rd Reprint, 1992, 76. 6. Electronic warfare supplement. International Defence Review. 1985, 12, p. 55 7. Soviet electronic. National Defence, April 1985, p. 35-42 . 8. Soviet military thought, Military Review, 1982, 6, p. 25. 9. G U S M. International Defence Review, 1985, 8, p. 1235-57 10. Col. O'M Dewar; M.K. Camouflage: advances in defence technology. Defence Systems, International, 1990, p. 227-32. 11. Godson R. I n t e r n a t i o n a l m i l i t a r y a n d defence encyclopedia. Col Trevor N. Dupuy, (Editor in Chief) 1993, 2(C-F) p. 768-71. 12. Maj.Shivane A.B., Deception: an operational imperative. The Cavalier, p . P - 11. 13. Scott, R. Off board countermeasures technology (Part-I) : soft kill payloads get smarter. Naval Technology, 1994, 15 (4), p. 16-22. 14. Arthur D' Little (Inc.) Design goals for future camouflage systems. Report to US Army Mobility Equipment Research and Development Command Order No. 0006, J a n 1981. 15. Golden, J.R.A. Radar electronic warfare. AIAA Education Series, J .S. P r ~ e m i e r n i e (Series s~ Editor-in-Chief),p. 106-119. 1.

249

MATERIALS FOR CAMOUFLAGE APPLICATIONS INTRODUCTIQN The important role played by materials technology in modern warfare cannot be overemphasised. But for the results achieved in materials science, some of the technological advancements that have taken place i n military hardware, weapons, sensors a n d countermeasures would not have been possible. Quite often, the criticality of a material, in terms of meeting exacting specifications, becomes the deciding factor in the proper functioning of the equipment. In this scenario, with the ever increasing advancements in military technologies, the need for improved materials a s well a s new materials with specific characteristics will continuously increase. The rapidly advancing technologies associated with military reconnaissance, surveillance and target acquisition systems are calling for equivalent developments i n camouflage devices, techniques, equipment and materials. To a large extent, any countermeasure to a target acquisition system directly or indirectly depends on materials. New and improved materials are enhancing the performance of sensor systems. This in turn is placing great demand on improved and new materials to defeat the improved performance of the sensor systems. Initially, the need for advanced camouflage materials was hardly felt. To cater for the human eye, the countermeasures required did not involve any man-made materials. Objects were concealed by' locally available vegetation or other materials. Gradually the demand for man-made materials arose, as the range and performance of human vision was extended by the use of optical and electro-optical instruments. These materials include paints, as surface appliques and nets, in order to make the objects blend with the background such that the role of the aided h u m a n eye is defeated. The introduction of the near infrared sensitive photographic film placed a demand on paints which can defeat the infrared film besides the

8.1

252


human eye. Simultaneously, new sensing systems, viz. night vision infrared systems, thermal imaging systems, infrared seekers and homing systems, besides the long range, round-the-clock, allweather r a d a r systems, have been added to t h e list of reconnaissance, surveillance and target acquisition systems. Further, sensors of nonelectromagnetic type, such a s acoustic, seismic, magnetic and electric, are being used for target acquisition. Each of these sensors performs its role by picking u p the signature which the object may have. With the developments in sensor technologies the requirement to simultaneously suppress a large number of signatures has arisen. This complex scenario has put a great stress on camouflage materials. No single or simple material can simultaneously suppress all possible signatures below the threshold value of the sensors concerned. Broadly, camouflage materials may be divided into the following classes: those for visible, infrared and microwave (radar) regions. Besides these materials which cater for electromagnetic signatures only, materials for suppression of signatures of nonelectromagnetic nature such as acoustic are required. The presentday approach is one of signature management and multispectral camouflage. It i s a n integrated a p p r o a c h to arrive a t countermeasures to defeat all possible sensors by which the military object may be detected. In this chapter, the various camouflage materials which are used for signature suppression based upon the information available in open literature are discussed. 8.2. RADAR ABSORBING MATERIALS ( A s discussed in Chapter 6, radar dominates the surveillance network; not only does it detect airborne objects a t much greater distances than infrared, acoustic or ultraviolet sensors, but it can also be used in all kinds of weather. Since the first use of radar, engineers and scientists have sought ways to mask weaponry from electronic surveillance with the ultimate goal to keep the enemy radar screen dark, as if the surveillance beam had passed through empty space. A s discussed earlier, this is done by various means, including surface shapes designed to deflect waves away from the incident electromagneticbeam, the use of radar absorbent structures and modified surfaces, and employment of onboard avionics. These measures, by no means fool-proof, known a s stealth techniques (whichare described in a subsequent chapter) are applied in varying degrees to airplanes, ships and tanks. Some radars, especially those using long wavelengths, are hard to delude. In aircraft designs that give stealthiness, stealth measures necessarily compromise performance, range and payload. However, aeroplanes

Materials for Camouflage Applications

can be much more effective the longer they can hide, and weaker echo means a diminished target for missiles and better ability to employ deceptive countermeasures. Radar absorbing materials (RAMs) in the form of surface coatings or structural materials play a n important role by complementing or augmenting reduction in RCS of both military and civilian targets where the reduction of radar echo from metallic structures can be a vital requirement. Radar echo includes specular (or direct) reflections, edge diffractions, multiple reflections and creeping waves (which propagate along the body surface and emerge at the opposite edge). The tangle of variables makes the design of radar absorbing materials very tricky. The task is further complicated by the facts that i) different radars affect the target in different ways, and ii) there are continuous improvements in radar system performances. Fundamental to the design process of radar absorbing material is comprehensive knowledge of the electrical properties of the material over the frequency range of interest. A s discussed in Chapter 6, these properties are described by the complex permeability (p) and dielectric permittivity (E) p = p, ( I -tan 6J E = E, ( I -tan 6,) (tan 6 is loss tangent) z=JP/E

z = impedence (377 2 ! for air) 8-3 Many materials exhibit only dielectric properties, but some display both dielectric and magnetic properties and are attractive a s radar absorbing materials because they can provide good absorption performance a t lower thickness. In most RAMs, the first step is to make the total pathway (of energy within the RAM) equal to halfa wavelength so that the residual reflection from the back face is exactly out of phase with front face reflection. However, RAM can be much thinner than the nominal wavelength of the radar and still achieve cancellation because the wavelength inside the material is much shorter than in free space. In addition, refraction within the RAM keeps the internal path length close to constant over a wide range of incidence angles. RAMs are tailored so that the energy that travels through them bounces off the substructure and escapes. Additionally, the RAM coatings applied on surfaces of aircraft for stealthy applications must be thin, weigh a s little a s possible, withstand stressing temperatures, pressures and erosive environments, generally be covered by materials to keep things together structurally, and must not disturb the smooth contours of the airframe.

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Though most of the R&D work on RAMs is classified, various types of known RAMs are discussed in the following section. 8.2.1 Magnetic Materials Fenites Fenrites are ferrimagnetic materials of general chemical formula MO(Fe,O,), where MO is a metal oxide of a divalent metal such as Fe, Mn, Co, Ni, Mg, Zn, Cd. They are prepared by thoroughly mixing the iron oxide and the metallic oxide MO. The mixture is then heated a t a temperature which is less than the melting point of either of the oxides, where the oxides sinter together into a spinel crystal structure1.

Figure 8.1.

Two octants of the spinel structure. The large spheres represent the oxygen ions. The small black and white spheres represent the metal ions on tetrahedral and octahedral sites respectively.

Source:

Reproduced from the book 'Fenites'by J. Smit and H.P. J. Wijn (1959) (figure 31-2) chapter VIIIJ, with permission from Philips Research, Library and Documentation Eindhoven, The Netherlands.


In ferrites, usually the iron ion is surrounded by six oxygen ions, in octahedral configuration, and metal ion (M) in a tetrahedral configuration, (Fig 8.1). The distribution of ions between the two types of sites is determined by a delicate balance of contributions, s u c h as the magnitude, of the ionic radii, their electronic configurations, and electrostatic energy of the lattice. The important atomic property that contributes to the observed magnetic properties is electronic spin quantum number. The unpaired spins of the metal atom which contribute to the magnetic moment and vector sum of the resulting magnetic moments give rise to the observed magnetic moment of a given atom. The magnetic moment of a given ion is affected by the crystal field exerted by oxygen atoms on that ion, and the net magnetic moment of a unit cell is the sum of magnetic moments in the unit cell. Iron atoms may be in tetrahedral or octahedral field whereas metal atoms are in the tetrahedral field. In this type of structure, iron atoms have a tendency to reverse their spins in the A and B sites, leaving only the metal ions to contribute to the magnetization of the ferrite. Conversely, by adjusting the ratio of Fe/metal atoms in the ferrite, it is possible to design a material with any desired saturation magnetization. Saturation magnetization is important because the oscillating magnetic vector of propagating electromagnetic radiation in the microwave region can couple to spinning electrons in ferrites. The degree of coupling depends upon the magnitude and direction of the electron spin. Saturation magnetization is the net magnetization due to all electrons, and the degree of coupling depends upon saturation magnetization. Essentially, each unit cell of ferrite acts a s a tiny magnet with finite magnetic moment and all these magnetic moments add up, provided their directions are the same. The region of the crystal where this condition is satisfied is called a domain. The vector sum of all these domain magnetic moments is the observed saturation magnetization of a given material. Although these randomly oriented domains may be directed in the field direction under applied field, they revert to their equilibrium positions once the applied field is removed. The mechanism of interaction or coupling of microwave radiation with ferrite materials in the applied magnetic field is a s follows: When a solid ferrite material is placed in a magnetic field, it will have a resonance frequency given by the equation OI = YHi 8-4 where o is the midband resonance frequency, y is the gyromagnetic ratio, and Hi is the internal magnetic field in presence

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of applied magnetic field Ha.In other words, electrons not only spin about their own axes, but also precess about the stationary magnetic field H, a t a frequency given by the equation 8-4, if damping losses are neglected. If microwave radiation corresponding to the resonance frequency and polarization is incident upon these spins, the oscillating magnetic vector of incident radiation couples with spins. The energy exchange manifests by changing the angle between the spin axis of the electrons and the internal magnetic field H,. The coupling is expressed as loss of energy with unit of decibel (dB). Usually the loss is not sharp and has a range of frequencies called linewidth. The smaller the linewidth, the sharper the peak, or viceversa. Further, H, is a function of the geometry of the monolithic ferrite material. If there is no magnetic field and the only field is that derived from the microwaves, then the mechanism of interaction between ferrimagnetic materials and suitable frequency in the microwave region may be different and is not discussed clearly in the literature. A brief review of the available information is given here. Although it is possible to couple electromagnetic radiation with electronic energy levels in an atom, ferrite materials exhibit high insulating and passive dielectric properties, and as such, the only alternative for propagating electromagnetic waves through them would be a function of their magnetic properties. A s explained earlier, ferrimagnetic materials exhibit magnetic property by spatial ordering of electron spin orientation of electrons from magnetic ions in the crystal. Radiation in the microwave region, under proper conditions, may spontaneously realign electron spins during magnetization. In a macroscopically demagnetized sample, the domains are arranged in haphazard orientations. Low frequency microwave radiation is typically attenuated by domain wall movement and high frequency microwave radiation by rotational resonance effects. Another mechanism of microwave absorption by ferrimagnetic material is nonlinear absorption a s a function of radiation power level. At milliwatt power level the absorption loss can be small, but a t higher power level sudden increase in absorption is observed beyond a certain value which continues until saturation is achieved2s3.The generally accepted mechanism for this observation is the excitation of spin waves when the microwave field exceeds a critical value. The critical field is dependent upon many factors such a s geometry, magnetization, R.F. power level, main linewidths, gyromagnetic ratio and operating frequency. Another related


mechanism is that there exists a spectrum of spin waves in any material with varying energy levels excitable by different critical field levels. In such a situation, the absorption will increase as microwave power increases a s more s p i n s a r e excited. However, i n polycrystalline powders the grain boundaries provide sufficient discontinuity to break u p spin waves. To summarize, there are several mechanisms at play in ferrite materials to absorb energy from microwave radiations. However, no database has been established on the magnetic properties of these materials. In several cases, their composition itself is not defined, nor their electrical and magnetic properties. Even in the cases where these are defined, the properties are not easy to tabulate because there is no standard method of measurements. ~ - the ~ formula Ni,.,Zn,Fe,O, Paints using Ni-Zn f e r r i t e ~ of (x I 0.5) in polyurethane or epoxy resins are commercially available as radar absorbing materials. Carbonyliron Fine powder of iron dispersed in epoxy resin is being sold, under the trade name Eccosorb, a s radar absorbing paint. Eccosorb coatings are capable of attenuating surface current from 50 MHz through microwave frequencies. These coatings are useful in reducing cavities, acting as attenuator in transmission lines, modifying antenna radiation pattern by being applied to radiating elements, and reducing radar cross section of complex objects. The core material of these paints, i.e., fine powder of iron, also known a s carbonyl iron, is obtained by heating iron to 100-200°C under 50-200 atmospheres of carbon monoxide to form iron pentacarbonyl, Fe(CO),. The latter is then vaporised and decomposed at around 250°C to give iron as a fine powder free from most impurities. Traces of still remaining carbon and oxygen are removed by sintering the material in vacuum when they are removed as carbon monoxide. 8.2.2 Dielectric Materials Dielectrics or graded absorbers operate by effectively altering the dielectric properties of a material at different depths. The material surface impedance is designed to closely match that of free space, (Z =m= 370 ohms), which encourages radar wave absorption and produces little reflection from the front face a s the wave progresses through the material (through the changing dielectric constant), loses and dissipates the wave's electromagnetic energy. These losses can be explained in terms of phonon-photon conversion phenomenon in polar dielectric insulators.

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In direct phonon-photon conversion, the polarization induced in crystal lattices by incoming electromagnetic fields relaxes into nmmal oscillation modes which eventually redistribute the electromagnetic energy into thermal phonons. This type of loss mechanism depends on the number of available normal modes of ions in the lattice, which is a strong function of temperature and of frequency. This mechanism is particularly ineffective a t low temperatures, where, for instance, crystalline solids exhibit very low attenuation constants. This process becomes more effective at high temperatures, up to a point where, together with increasing semiconduction losses, i t c a n lead to thermal runway phenomenonl0.l1. In this category of microwave absorbers, materials are available in the following forms: Honeycomb -A light-weight, broad-band absorber effective over 2-18 GHz range. This material can be used in load-bearing structures. The core, resin system, cell size and thickness can be adjusted to meet the requirements of a particular application. Depending on the thickness, honeycomb-based absorbers can provide 10-15 dB attenuation over a specific frequency range; Open-structured netting material, either left open or filled with urethane foam, is rugged, light in weight (2 oz/ft2 or 0.6 kg/ m2),low cost and typically provides 15 dB attenuation from 8100 GHz; Salisbury screen - A quarter wave tuned frequency absorber formed a s a light-weight laminate with reflective backing. 8.2.3 Artificial Dielectrics A composite of conducting particles dispersed into a dielectric insulating material (ceramicor polymer matrix)is known as artificial dielectric. These composites make use of metallic behaviour of conductive particles to get effective and controlled microwave absorption. These materials and their properties have been known for a long time and have been the subject of extensive theoretical and experimental s t ~ d i e s ' ~ - ' ~ . The basic loss mechanism here is scattering of electrons within each elementary grain due to current induced in each grain by the incoming electromagneticwave. Depending on the size and resistivity of the grains, the losses in each grain can be electric or magnetic, and a particular frequency can be selectively attenuated16. Typically, radiation is best absorbed if the grain diameter is of the order of the skin depth in the grain material. By choosing a proper conductivity material in powder form, and by knowing its


resistivity, grain size (or size distribution), its concentration and the dielectric properties of the host material, it is in principle possible to obtain specific combination of dielectric constant and losses. However, the actual choice of materials that will yield specific electrical and magnetic properties is difficult in practice, because of the poor control of the powder properties, and it is even more difficult if the artificial dielectric is to be manufactured in the ceramic form. Artificial dielectrics present properties which are much milder functions of the constituents' properties than other types of lossy materials. 8.2.4 Conducting Polymers Synthetic polymers, whose long molecules contain hundreds of identical structural units, have been insulating electrical equipments since the turn of the century. The discovery, in the 7OYs,of polymers with high electrical conductivity stirred intense interest in the research and development community. The first conducting polymer was obtained in 1977 by a group of American scientists17 from acetylene - a common gas used for welding purposes. This report led to the synthesis of a large number of conducting polymers obtained from a variety of organic monomers and ~ ~ 'aromatic^.^^-^^ ~ The most important such a s h e t e r o c y ~ l i c s ~ criterion for selection of monomers to give conducting polymers is the formation of conjugate bonding in the backbone of the polymer obtained. Such polymers on interaction with electron acceptors (oxidizing agents) or electron donors (reducing agents) create charge carriers and get doped with these species. The former withdraws electrons while the latter adds them. Among the common dopants are halogens (like iodine or bromine), transition element cations (like ferric or ceric cations), organic oxidising agents (like chloronil o r dichlorodicyanoquinone), and inorganic reducing agents (like sodium, potassium metals). Though there is no single method for synthesising conducting polymers derived from different types of monomers, they can be obtained by both chemical and electrochemical methods of polymerizationz4. In the chemical procedure, the common polymerization methods a r e adopted, e.g., polyacetylene, by polyparaphenylene and polyphenylene sulphide can be o b t ~ n e d Ziggler-Natta17, Friedel Craft25 and nucleophilic displacement reaction26, respectively. Electrochemical procedures have mainly been used in the synthesis of polymers from heterocyclic rnonomer~~~-~'.

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in organic solvent

Figure 8.2. Electrochemical method of polymerisation.

Electrochemical method (Fig 8.2) offers the advantages of simultaneous and homogeneous incorporation of dopants during polymerization and close control over the polymerization parameters. The method is limited, however, by the fact that the yield of polymer is restricted to the area of working electrode. For this reason (among others), chemical procedures are used when large quantities of materials are required. Several conducting polymers are now available commercially. Polymers, in general, are relativeIy light and easy to process. The combination of metal-like or semiconductor-like conductivities together with the availability of other properties such as low environmental contamination, redox behaviour, optical changes (linear and nonlinear) and, above all, modulating/tuning of many of the properties via organic chemistry, have generated great interest in these materials. The earlier academic interest has overflowed into developmental efforts in national laboratories, defence, and commercial companies worldwide. Several products, based on conducting polymers, such a s capacitors, light weight rechargeable batteries, high capacity (4 MB) magnetic disks for computers, coatings for antistatic and anticorrosion applications, thermoplastic conducting polymer blends, etc. are available commercially now in the international market.


Like in the other sectors, in defence also, the response of conducting polymers to a very wide frequency r a n g e of electromagnetic spectrum (fromDC through optical frequencies) has given the possibility of applications such a s static charge dissipation, lightning strike protection, embedded antennas, sensors, antiradar aids - technologies generally known a s 'smart-skins'. In the application of conducting polymers a s antiradar (stealth) material their response to the frequency range 107-10" Hz is of special interest. For efficient use of absorber, without any significant reflectance, it is important to grade the microwave absorption with thickness. The absorption is related to the quantity

tan 6 = PEE^) = E" / E' where tan 6 is the loss tangent, o and

E conductivity and dielectric constant respectively a t frequency o.Here q,is the dielectric constant of vacuum and E" and E' are the real and imaginary parts of the dielectric constant. Microwave studies on polyaniline reveal that loss tangent increases monotonically3* with protonation and for the most conducting form (50 percent protonation) cs = 1n-'cm-' E = 100 and tan 6 = 2.5. The value of tan 6 varies with the protonation level/ dopant concentration. Although their conductivity is intermediate among conducting polymers, polyaniline emeraldine salts (Figure 8.3)have a large dielectric constant. Table 8.1 compares the values of tan 6 and dielectric constants E,for microwave materids available in the market, together with a few polymers and ceramics.

Table 8.1. Dielectric Properties Material

tan 6

GHz

Er

Paper Snow (-20°C) Nylon Water1 Polyethylene Teflon Fused silica Fused Quartz Bakelite2 Eccosorb VF3 Polyaniline

0.04 0.0004 0.01 0.26 0.0001 0.0003 0.00008 0.00003 0.01 1.2 2.6k0.6

10 10 10 10 35 50 35

2.62 1.26 2.73 34.0 2.3 2.05 2.01 3.80

Source:

3.5:: 6.5

41.0 105k15

Reprinted with permission from Microwave Journal, page 162, Feb. 1989. 1 conducting water; 2 - used as attenuator or loading material; 3 - a trademark of Emerson and Cuming

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Figure 8.3.

The PAN1 Polymer (a) reduced form; (b) The emeraldine base; and (c) 50 percent protonated emeraldine salt.

Source:

Reprinted with permission from Microwave Journal, 1989,February, p.162.

The microwave response of several conducting polymers has been studied by several workers using a variety of techniques. Scattering data, obtained from network a n a l y ~ e r s ~show ~ - ~ that ~, several processes influence the dielectric properties of these materials. The chemical composition, conjugation length, nature and concentration of dopants or ionic impurities, interchain distance and morphological structures are some of the dominating factors that influence the dielectric constant and loss tangent of the polymer. The ability of conducting polymers to absorb microwave and other electromagnetic radiations can also result in another interesting application40, i.e., welding of plastic joints through remote heating a s a spin-off benefit. For instance, conducting polyaniline may be placed between two pieces of plastics (e.g. high density polyethylene whose surfaces need to be melted in order to fuse them together). Exposing the components of insulating polymer with a layer of polyaniline between them to microwave radiation can lead to the formation of a strong bond. A related application of remote heating is to form part of a polymer vessel of a low melting polymer which has the conducting polyaniline applied to it. The exposure of this configuration to microwave radiation will lead to melting of the polymer adjacent to polyaniline and hence forming a remote break seal. 8.2.5 Chiral and Two Dimensional Polymers Besides conducting polymers, two other types of polymers viz. chira141and two dimensional polymeF2, the latter also having an


element of chirality, have been reported a s potential radar absorbing materials. Chirality in a molecule or crystal arises when the structure has sufficiently low symmetry that it is not superimposable on its mirror image and is responsible for optical activity in that substance (Fig 8.4). A large number of liquid crystalline polymers are known to show chiraliW3.

Figure 8.4.

Simple model for chiral molecufes: (a) Schematic representationof helical macromolecule @) "Twistedbelt" as a simple model for cellulose chain (c)Primitive model for a molecule of the chiral nematic with a substitution group.

Source:

Reproduced from "Liquid crystalline and Mesomorphic polymers" editors-V. P. Shipaev and Lui Lam, Figure 101, page-4 chapter-1 (1994)with permission of Springer-vetlag GmbH & Co. KG Heidelberg, Germany.

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The first attempt to study the effect of relatively large scale He measured chiral structure was made by Lindman in 191444945. the plane of polarization of a linearly polarised plane wave after its passage through an ensemble of copper helices of one handedness embedded in a host medium; an analogous study was done by Tinoc and Freeman in 1957. Their experiment was of a qualitative nature, and considered the phenomenon of optical activity at a much lower frequency range than the optical studies. For the millimeter and microwave frequencies the name 'electromagneticactivity has been suggested by Lakhtakia, et aP6instead of optical activity'. At these frequency ranges the medium parameters such as E, p and chirality denoted by K rather than refractive index for left and, right handcircularly polarised light, play the major role. In the past few years chiral media have gained considerable interest due to their potential applications and feasibility in microwave engineering. Interesting applications are in the area of low reflection coatings a n d corrections of polarisation in inhomogeneous lens antennas. Two Dimensional Polymers Polymers are usually defined to be chains of thousands of small monomers linked together. Biologically important arnylopectin (a multiply branched molecule and the principal component of starches) a t 9x107daltons, or DNA (basicallyladder polymer of base pairs and backbone chains) a t 6x10' daltons are amongst the largest known polymer molecules. Most of the synthetic polymer molecules of commercial importance are linear chains weighing between 5x 1O4 and 5x106 daltons. There are two general backbone types : rigid rods and flexible coils. The linear chain and its close cousins (branched chains, rings, network and so on) are all basically one dimensional in that they may be described by reference to unit line (which represents location along a simple linear chain) with the appropriate added description of the branching nodes and their interconnections. There are two-dimensional polymer-like assemblies that occur naturally such as in cell membranes, where large molecules line up parallel to each other and are held together in a layer by electrostatic attraction. But these structures are unstable because electrostatic forces are weak compared to the covalent bonds formed between atoms that share electrons. ~ ~ reported the synthesis Recently, Stupp and c o - w o r k e r ~have of a homogenous two-dimensional polymer which can be 10 nm thick and several square millimeters in area with molecular weight in billions of daltons.


Acrylate end

Benzoic acid ester

Cyanide group

Phenol ester

Biphenyl

Hydrocarbon tail

Figure 8.5.

Molecular structure of two-dimensional polymer,

Source:

Reproduced from Science,6 Feburary 1993, p. 14 - Molecular structure of two dimensional polymer by John Emsley with permission of American Association for the Advancement of Science.

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This molecule was synthesised from three components. The central section is a short chain of seven carbon atoms with a benzoic acid group a t one end and phenol at the other. Along this chain is a cyanide group. The benzoic acid end was joined, via an ester link, to a shorter chain of four carbon atoms which has an acrylic group at the far end. To the phenol end a biphenyl derivative with a hydrocarbon trail was attached, again by means of an ester bond (Fig 8.5). Applications that have been suggested for these polymers include lubricants, in semiconductors, optical materials and selective membranes. In addition to these, other very important potential defence applications are outlined below: (i)

Due to the presence of the chiral part the material can act as second harmonic generator, and the wavelength entering in the media can be halved. This property would allow, for example, applications enabling IR transmitters on the battlefield to be detected;

(ii] Since the sheet thickness is double the length of the monomer, extremely thin, yet very uniform and flat, sheets of such polymers with a large dielectric constant could be formed. These sheets which are flexible can curl under certain conditions of temperature and chemical environments. Further, if the outer surface (formed by the circular ends) is modified in such a way a s to bond on one side to a metal surface, such a s the skin of a n airplane, and the other side to ferrite compounds, then a radar absorbing material of extremely light weight and uniformity would be the result. Since the molecules forming part of the bonded structure would be very large, the absorbed energy would be readily transmitted away via the sheet structure and an even better absorber provided than by a chemical layer applied to the skin. 8.2.6 Schiff Base Salts In 1987, a group of researchers led by Robert B. Birge at Carnegie Mellon University in USA identified a group of non-ferrite based materials capable of absorbing radio frequency transmission that could reduce aircraft radar reflectance by 80%48. These materials, known as retinal Schiff base salts, are polymers containing a double bonded carbon-nitrogen (-CH=N-)structure linking divalent groups in the linear backbone of the molecule chains, (Pig. 8.6). These salts are obtained by the reaction of retinal, a n aldehyde, a precursor of vitamin A, with a n amine. These salts are highly polar, black in colour, and phydcally resemble graphite. Their importance is that they absorb RF energy

Materialsfor CamouflageApplications

H3C

"\ CH3

R = Mkyl or aryl group

Figure 8.6. Molecular structure of retinal schiff-basesalt.

a s well a s or better than ferrite-based paints for about one tenth of the weight; and specific salts seem to absorb specific R F energies. Salts, however, require modification so that a n ensemble of them could absorb over the entire radar RF range. The radar energy absorbed by these salts is dissipated a s heat. However, the heat rejected by the molecules is so small that the temperature of a coated surface may increase by only l ~ l O OC - ~which should not be a concern about increased infrared signature. 8.3 INFRARED CAMOUFLAGE MATERIALS The various sensors that are used in the infrared region of the electromagnetic spectrum have already been discussed in Chapter 5. The threat perceptions thus posed by IR detectors give rise to the need for thin coatings of materials a t object surfaces to control absorption and reflection of IR radiation with an optimum balance of these properties in the near infrared (NIR) and thermal infrared (TIR) regions. For example, low NIR reflecting paints strongly absorb the NIR component of solar radiation and as a result increase thermal infrared signatures. 8.3.1 Physical Principles For camouflage in the infrared, the regions of special interest are: NIR (0.75 - 3.0 pm), MIR (3 - 6 pm) and FIR (6-15 pm). The selfemissions and reflection by objects can be understood from the principles of black body radiation and solar radiations. Detailed discussions have already been made in Chapter 5. 8.3.2 Attenuation of Infrared Signatures In principle, emitted infrared radiation from an object and causing its signature in the infrared can be curbed by three mechanisms, viz., scattering, absorption and reflection, adopting one or a combination of the following methods49:

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268


Obscuration Shape tailoring Surface treatment Active cooling Wake control Among these methods, obscuration and surface treatment are the most important ones and are discussed here in detail. Shape tailoring is often useful in combination with surface treatment and is closely related to that followed in signature control in the microwave region. Active cooling is common in designs for suppression of heat generated by an engine. Wake control should get more attention as success is achieved in controlling the more direct signature contributors. These methods are not discussed further. 8.3.2.1 Obscuration Obscuration has been already referred to in Chapter 4. It is often easier to hide a signature source than to eliminate it. Accordingly, obscuration is one of the more common suppression techniques. Obscuration can take many forms. It can range from simple baffle, designed to obstruct the line of sight to a hot part, to a camouflage net, to a smoke. Obscuration is done on the assumption that the object accomplishing the obscuration will be easier to control than the object it hides49. Obscuration works on the principle of scattering. Scattering of visible and infrared radiation is a single stage process and is commonly referred to as being either elastic or inelastic. Elastic scattering, also referred as Rayleigh scattering, is the one in which radiation retains the same quantity of energy and momentum and hence keeps its frequency unchanged. In inelastic scattering, known a s Raman scattering, the energy is exchanged with the scattering object and shifted by a n amount equal to the change in vibrational energy of the material through which the radiation is passing. Both kinds of scattering processes result from electromagnetic radiation perturbing the electronic cloud surrounding the molecules of t h e irradiated material. Rayleigh scattering occurs in the presence of small independent particles commonly present in colloidal a n d other s u s p e n s i o n s containing particles with sizes comparable to or larger than the wavelength involved. For a fixed value of the scattering power m, the wavelength A. most efficiently scattered by a particle of diameter d is given by


where

In this equation n is the refractive index of the medium which in case of paint is the same as for the resin. Taking advantage of the scattering phenomenon in smoke, suitable sizes of suspended particles are generated to obscure signature in the visible and infrared regions0. 8.3.2.2 Surface Treatment Surface treatment by way of putting coatings of different materials on the surface alters surface characteristics by modifying their reflection, self-emission and directional properties, which act a s a major tool for the design of camouflage in infrared region a s well as in the visible and microwave regions. In the infrared region the main emphasis of putting such coatings on a surface, however, is to modify its reflection and self-emission characteristics. The important relationship between emissivity and reflectivity is given by the following equations : Diffuse E, = l -p 8-7 Specular E,

=

1-pd (8, $)

8-8

where p = total reflection and 8, @ = viewing angles A major implication of this equation in signature control is that a low emissivity surface must have a high reflectivity, and, conversely, low surface reflectivity results in high emissivity. A practical implication is that, when placing a high temperature object in its natural environment, low emissivity coatings may reduce selfemission but still result in significant overall apparent emissions because of reflections. A logical consequence is that situations can arise where low emissivity, or surface treatment, can, by itself, provide the desired signature control level. Figures 8.7a and 8.7b and Figures 8.8a and 8.8b show representative emissivities required for infrared camouflage by surface treatment whenever there is such possibility. In these figures, target minus background AT'S were computed from the relation : AT= T (target) - T (background).

269

270


0.6 ;-

I

Sky Background

Ground level 1962 U S Std. atmospher planar panel. Specular reflectance night 3-5 micron band

PANEL AT PC) / -

-- - 0

Figure 8.7a. Emlssivity required for zero contrast as against horizon sky at 3- 5-pm band. Source:

Reproduced from "The infrared and electro optical systems hand book. Vol. 7 Countermeasuresystems, edited by David H. Pollock - chapter-2. Camouflage, suppression and screening systems by David E. Schmieder (section 2.1 to 2.5) with permission from the publishers-ERIM & SPIE Optical Engineering Press USA (1993)and the author.


8- 12 Micron band

PANEL

Fig. 8.7b. Source:

T(C)

Emissivity required for zero contrast as against horizon sky at 8-12 pm. Reproduced from "The infrared and electro optical systems hand book.Vol.7Countermeasure systems, edited by David H. Pollock - Chapter-2 Camouflage, suppression and Screening systems by David E. Schmieder (section 2.1 to 2.5) with permission from the publishers-ERIM & SPIE Optical Engineering Press USA (1993)and the author.

271

272

Introduction to Camouflage Ck Deception

Terrain Background

Figure 8.8a.

Source:

3-5 Micron band

Emissivity required for zero contrast as a function of panel tilt angle when viewed against terrain at 3-5 pm band. Reproduced from The infrared and electro optical systems hand book. Vol. 7 Countermeasure systems, edited by David H. Pollock - chapter-2 .Camouflage, suppression and screening systems" by David E. Schmieder [section 2.1 to 2.5) with permission from the publishers-ERlM & SPIE Optical Engineering Press USA (1993)and the author.

Materials for CamouflageApplications

-

Figure 8.8b.

Source:

8-12 Micron Band

Emissivity required for zero contrast as a function of band tilt angle when viewed against terrain at 8-12 pm band. Reproduced from The infrared and electro optical systems hand book. Vol. 7 Countermeasure systems, edited by David H. Pollock - chapter-2 "Camouflage, suppression and screening systems" by David E. Schmieder (section 2.1 to 2.5) with permission from the publishers-ERIM & SPIE Optical Engineering Press USA (1993)and the author.

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Both target and background temperatures are physical temperatures. For terrain, because unit emissivity was assumed, the background temperature was assumed a s the apparent black body radiant temperature. However, for the sky background, apparent radiant temperature was lower than its physical temperature. Examination of these emissivity plots reveals that a warmer terrain background allows higher emissivities for a given target AT'S than does a horizon sky background because the latter is less dense than the former. In general, spectral reflectivity requirements are scenario-and application-dependent. However, nominal requirements can be stated from signature generation principles and typical vehicle usage. In near infrared (NIR),reflectivities need to be dramatically higher against live foliage background. In the mid-and long wave (thermal)infrared, reflectivities need to be high on high temperature parts so that the resulting low emissivities can reduce self-emission50. Required emissivity values can easily range from 0.05 to 0.6. These low emissivities can result in high daytime solar reflections as well as high day and night ground reflections. So, in some cases, as with high altitude aircraft, reflectivities must be kept low, and other means, such a s active cooling techniques, must be used to control the mid-infrared and long wave infrared emissions. 8.3.2.3 Coating Materials For Camouflage i n Infrared Region Coatings offer the potential to reduce heat-induced selfemissions by reducing surface emissivity, and such coatings are obtained in the form of paints. Before beginning the formulation of any coating with specified infrared properties, it is important to define the illuminant and conditions of illumination, the spectral response of the detector or viewer, the desired visual colour of the coating, and the spectral distribution of the radiation to be reflected or absorbed. The next step is to select a vehicle (binder)with good heat resistance and minimal absorption in the infrared bands of interest. Pigments are then selected for their emissivity (reflectivity) properties. Binder Resim There are two primary requirements for resins in tailored coatings. First, the resin must protect the pigment and preserve its infrared properties throughout the service life of the coating, and, secondly, it should exhibit very weak interaction with the electromagnetic radiation of interest. Though most of the organic resins are free from significant absorption in the NIR region, they show strong absorption in the thermal IR regions51.

Materialsfor CamouflageApplications

Strong absorption in the thermal IR regions can be avoided by choosing resins which do not contain common functional groups. For example, poly(viny1idenefluoride) resins are almost transparent to, and unaffected by solar radiation. They have only very weak absorption in the thermal IR region52in addition to having excellent weather stability. Dimethyl silicone resins have emittance values lower than those of fully organic resins and have been used for low emittance coatings53. Further, absorption by resins can also be reduced by the selection of pigments which because of their refractive indices and particle sizes scatter light effectively in the band where resins absorb. This technique can be used to great advantage in the thermal IR region. In addition, leafing metallic pigments which form a practically continuous film reduce the penetration of incident radiation and absorption by the underlying resins. Pigments In conventional paints the main role of the pigment is to provide opacity and colour through control of reflectivity of the paint with its right index of refraction as compared to the binder resins. However, pigments exert their principal influence on the optical and NIR properties of coatingP. Pigments used in the infrared region are selected based on their emissivities, scattering and reflectance properties in this region. Plgments and Coatings for Near Infrared A wide variety of useful NIR properties are obtained when pigments are dispersed in film. Titanium dioxide pigments reflect NIR radiation very well as indicated by their high value of scattering power. Titanium dioxide with a particle size of 10 pm reflects well between 0.8 and 2.3 pm with very little or no effect on the visual colour of the coating. Pigments which absorb in the visible region (required for visual colouration) but are transparent in the NIR region can be considered to be extenders so far a s NIR radiation is concerned, that is, they can be used to modify the visual appearance of the paint without affecting the infrared properties. Organic pigments such as perylene black, phthalocyanine blues and greens and carbazole dioxazine violet are useful for the purpose. Conventional extender pigments such a s barytes, amorphous and diatomacious silica, and talc are transparerit and non-reflective throughout the visible and NIR regions. These pigments are valuable where transparency in the NIR is needed, and they do not interfere with the performance of other pigments. Carbon black absorbs strongly throughout in the NIR and thermal IR region55;when used in small amounts it is effective in decreasing the infrared reflectance

275

276


of the paint in near and thermal infrar-ect regions without greatly affecting the colour. In painting military objects 'to protect against systems which use the NIR wave bands and operate in low light conditions, it should be understood that the term 'low reflecting infrared paint' generally means a paint that is highly absorbing in NIR (with consequently higher signature in TIR on exposure to sunlight). Likewise, the term 'infrared reflecting paint' refers to highly reflecting NIR paint (with a consequently lower thermal signature on exposure to sunlight). Pigment and Coating for Thermal infrared Region Coatings using conventional pigments and working on scattering principles are ruled out for this region because of the difficulty in getting a balance between the size of the pigment particles required to scatter in this region and overall desired s m o o t h n e ~ s ~ ~ . Inorganic pigments show strong and broad absorption bands. For example, carbonate absorbs at about 7pm, silicate at about 9 ym and oxides between 9 and 30 ym. Organic pigments such as the perylene blacks, phthalocyanine blue and green, and carbazole dioxazine violet show strong sharp absorption bands throughout the thermal infrared, but principally between 6 and 11 pm. The properties of coatings based on these pigments are likely to be influenced by the pigments used and are wavelength dependent. Grey body approximation for these coatings is likely to lead to considerable error. For these reasons metallic pigments in the form of flakes are generally used to achieve both emissivity and reflectance coatings in the thermal infrared region. Metals show low emissivity because they contain free electrons that provide for high refractive indices over broad bands57. Figure 8.9 shows an example58of the broad band control that is achieved with aluminium having various surface conditions. Furthermore, metals absorb poorly but scatter and reflect strongly in the thermal infrared region due to high density of their electrons59.A coating filled with metals can be considered as a grey body. Less reflectivity and more spectral shaping can be accomplished by choosing lower conductivity materials. Semiconductors show less conductivity than metals. Their lower electron density causes semiconductors to have a high refractive index, with corresponding high reflectivity a t longer wavelengths and transparency at lower wavelengths, with reduced refractive index a n d extinction coefficients. This effect can be controlled by varying the population of charge carriers through changing the composition or temperature.


.2

-

.1

-

0

>... ............--.-.. '.-..............:.,.---. Sanded ........................ ... Polished ............................. ...... %--

I

----*----

.............................'"1-

1.o

10.0

Wavelength (Microns)

Figure 8.9.

Effects of sufrace finish on emissivity.

Source:

Reproduced from "The infrared and electro optical systems hand book. Vol. 7 Countermeasuresystems,edited by David H. Pollock- Chapter-2 Camouflage, suppression and screening systems by David E. Schrnieder (section 2.1 to 2.5) with permission from the publishers-ERIM % SPlE Optical Engineering Press USA (1993)and the author.

Still more spectral reflectivity tailoring can be achieved by going to multiple layer approach. The technique is comparable to the approach used in interference filters and has some of the same tailorability advantages as well as angular dependency limitations. Figure 8.10 shows the results of efforts to create a coating with low visible reflectance and high infrared reflectance with a cut-on wavelength near 3.0 pm60.This result was achieved with a pigment consisting of a 17-layer, 3 pm thick structure of silicon, silicon oxynitride and silicon dioxide. An alternative method of hiding the thermal signature of a vehicle is to place a thermal barrier directly between the heatproducing object and the sensor, as described in Chapter 5.

277

278

Introduction to Camouflage8a Deception

loo

r

Single pigment flake immerdes in binder at normal incidence

Wavelength (pn)

Figure 8.10. Theoreticalspectral pezfonnance of pigmnet design with emiwivity transition at 3 pm. Sowce:

Reproduced from "Theinfrared and electro optical systems hand book. Vol. 7 Countermeasuresystems, edited by David H. Pollock chapter-2 Camouflage, suppression w d screening systems by David E. Schmieder (section 2.1 to 2.5) &th permission from the publishers-ENM 8s SPIE Optical Engineering Ress USA (1993)and the author.

-

Fabrics from both inorganic and organic fibres such a s polyamides, polybenzimidazole, p ~ l y a m i d e i r n i d e ~and ~ d~minosilicate~~ have been reported for this application. The thermal protective performance of these fabrics is related to their thickness, density, air permeability and moisture content. 8.4 COATING MATERIALS FOR CAMOUFLAGE IN THE VISIBLE REGION Basically, the concept of camouflage in the visible region is to confuse the human eye, and it can be accomplished by three methods, viz., hiding, blending and deception, as already discussed in Chapter 4 earlier. In all of these methods materials play the central role. For example, hiding is achieved by naturaI/artificial vegetation, special coated nets, foam and smoke, blending by using paints of different colours in disruptive patterns, and deception by the use of


decoys or dummies of the real objects. The materials used for camouflage in the visible region in different forms are discussed below. 8.4.1 Paints A paint, a pigmented liquid composition which is converted to an opaque solid film after application a s a thin layer, is essential for the protection and decoration of the majority of manufactured metallic goods and architectural and industrial structures which characterize our complex material civilization. The foremost requirements of a paint for camouflage application are: (i) It should match in its optical properties with the background; (ii) It should be stable; (iii)It should be corrosion-resistant; and (iv) It should be non-glossy. Many of the conventional paints might satisfy the above requirements and may therefore be used for camouflagein the visible region because of their matching reflectivity in the region to that of the background, i.e., vegetation or sandy terrains.

..i ,fl

,'

\

Undercoat

\ \

Topcoat

I

"8

'

:' "

\

/

/,/,<,/;

8

-..

1

Primer

Substrote

Figure 8.11. Paint composition. Source:

Reproduced from 'The infrared and electro optical systems hand book."vol. 7-Countermeasure systems, edited by David H. Pollock - chapter-2 "Camouflage,suppression and Screening systems" by David E. Schmieder (section2.1 to 2.5) with permission from the publishers-ERIM & SPIE Optical Engineering Ress USA (1993) and the author.

279

280

lntroduction to Camouflage 86 Deception

These paints may have one to three separate layer types and be in the forrn of a primer, undercoat and topcoat. (Fig. 8.11). The main purpose of the primer is to provide a mechanism for each successivelayer to adhere to the underlying surface and to a certain extent to serve against corrosion of the metal. The undercoat (which is also the topcoat if there is no additional layer)contains the primary pigment designed to control spectral reflectivity. A transparent topcoat may be used to control surface roughness, provide abrasion resistance and contamination protection. The important coating for camouflage application is the undercoat and Table 8.2 describes the major constituents and their functions. Key ingredients of undercoat are the pigments and binders. Main optical properties of binders are their transparency and index of refraction; likewise, the main optical properties of pigments are their index of refraction and opacity. Pigment controls reflectivity by offering a high index of refraction relative to the binder medium. In general, reflectivity p for homogenous medium at normal incidence is given by the following equation

where n, is the refractive index of the binder and n, is that of the pigment.

Table 8.2. Paint constituents Pigment (filler)

Primary, material used to impart colour; remains, insoluble, provides, protection, hardness, weatherability, roughness

Dyestuff

Secondarymaterial used to impart colour; solublein solvents and/or binder, transparent in their coats; limited uses

Binder (polymer)

Holds pigment particles together, to substrate; transparent to visible light

Solvents

Provide application mobility, evaporate

Additives

Driers, wetting, antisag, flattening and similar agents

Source:

Reproduced from "The infrared and electro optical systems hand book." vol. 7-Countermeasure systems, edited by David H. Pollock - chapter-2 "Camouflage,suppression and Screening systems" by David E. Schrnieder (section2.1 to 2.5) with permission from the publishers-ERIMB SPlE Optical Engineering Press USA (1993)and the author.


Camouflage coatings adjust the normal reflectance of a n object so that it resembles its immediate environment, thus making it indistinguishable from its surroundings under natural as well a s artificial illumination. For terrestrial applications the most important environments when blending concealment is desired are foliage and the desert regions. A s discussed in Chapter 4, pattern painting with various camouflage colours can also break u p the sharp boundaries of an object and make it less conspicuous. For maximum effectiveness, the various coatings used in the pattern must be significantly different in both optical and infrared r e f l e ~ t a n c e ~ ~ . One of the most common pigments in general use64is titanium dioxide (TiO,), which has a refractive index of approximately 2.8. Most binders are made from oils or polymers that offer a refractive index near 1.5. In addition to influencing the overall optical properties of paints, binders also strongly determine other importmt general properties of paint such a s durability, maintainability and adherability. Some of the common examples of polymers used a s binders in paints are alkydes (polyester-based), polyesters epoxy resins, polyurethanes, silicone resins and acrylics. On the other hand, vegetable oils obtained from oilseeds such as linseed, cottonseed, etc. have constituted the largest category of binder resins used in the paint industry. In general, all of the binders can have uses in camouflage paint formulations. However, some of these binders have unique properties worth noting. For instance, epoxy resins and polyurethane resins offer a high degree of flexibility, toughness, abrasion resistance, and resistance to chemicals. In addition, they offer good weather resistance and adhesion to metals. Silicone resins, on the other hand, also provide high levels of heat resistance. The choice of pigment in camouflage paint is made based on the type of the terrain as discussed below. 8.4.1.1 Pigments for Forest and Jungle Areas Pigments used to provide green coatings for these areas are selected for their ability to duplicate the reflectance properties of chlorophyll, the green pigment responsible for the characteristic colour. The reflectance spectrum of green foilage, i.e., leaves, grass, etc, a s shown in Fig 8.12, exhibits a peak new 550 nm, low values in the visible red region, a dip in the far visible red a t 650-680 nm, a sharp rise between 680 and 7 10 nm, and uniformly high values above 720 nm. The goal for camouflage coating in forest area is to match this spectrum.

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282


A B C D

GRASS YOUNG SYCAMORE LEAF BEECH LEAF OLD SYCAMORE LEAF

PHOTOGRAPHIC INFUA-RED 500 550

600

650

700

750

BOO

850

900

WAVELENGTH IN MILLIMICRONS

FIGURE 8.12. Spectrophotometric curves for green vegetation. Source:

-

Reproduced from Journal of Colour Chemists Association Opticd properties of pigments in Near Infra-Red, Figure 7, page T 23, Vol. 41, 1958with permission of the Journal of Oil & Colour Chemists ' Association.'

Many green pigments given in Table 8.3 resemble the colour of chlorophyll, but, unlike chlorophyll, show strong absorption in the near infrared. Chromium trioxide is green in colour and possesses a reflectance curve similar to that of chlorophyll and was the principal pigment in earlier visible and near infrared camouflage paint. For some specifications which do not call for reflectance to rise rapidly in the near infrared, chromium trioxide tinted to the desired visual shade can still be used as the basic pigment. Various green shades can also be obtained'j6using formulations based on lead chromate (chrome yellow), basic lead chromate (chrome orange) and molybdated lead chromate (moly orange) and tinted with red and blue pigments. However, their use has been curtailed in United States because of extremely high lead content.

Materialsfor CamoufIage Applications

Table 8.3. Pigments for different types of terrains Terrain Light green, dark green Forest green, Olive drab green 383 Field drab, Earth yellow Aircraft gren, olive drab 34087 Sand Interior aircraft black 37038 Aircraft white 37875 Aircraft red 3436 Aircraft grey Interior aircraft grey 3623 1 Aircraft insignia blue 35044

Pigment Acid insoluble green pigments predominantly composed of cobalt, zinc and chromium oxides with other oxides. Permitted carbazole dioxazine violet, yellow iron oxide, red iron oxide, chromium oxide, permanent maroon, light stable organic yellow. Yellow iron oxide, red iron oxide, chromium oxide, titanium dioxide carbon black. Yellow iron oxide, red iron oxide, carbon black, iron xoide Yellow iron oxide, red iron oxide, chmmium oxide, titanium dioxide, carbazole dioxazine violet Carbon black, black iron oxide Titanium dioxide Titanium dioxide, light stable organic red Titanium dioxide, carbon black, yellow iron oxide Copper phthalocyanine blue, carbon or lamp black, titanium dioxide

Source :US military specification MILC-53039(NE),16 April 1984

8.4.1.2 Pigments for Desert Regions For these regions, natural sand and earth colours in a range of grey, red, orange, yellow and brown colours must be reproduced. For desert colours and woodland browns and black, iron oxides ranging from red to yellow to black, chromium trioxide, titanium dioxide and carbon black are used. 8.4.1.3 Pigments for Ocean Environments Paints now used on the hull and superstructure of warships often exhibit relatively high solar absorption because of the grey colours required for visual camouflage. These colours may vary from bluish to greenish grey depending on the waters in which the vessel is likely to operate and have reflectance near 30%. Historically, naval coatings have contained significant amounts of carbon b l a ~ k ~ 'Effective , ~ ~ . solar heat reflectivecoatings have now been made in the grey shades established for warships by replacing carbon black by an organic perylene black which absorbs strongly in the visible region but is transparent throughout infrared. In general, most paints are composites of several matenial types. Therefore, the simple formulae that predict reflectivity for homogenous materials do not apply to the paints. The reflectivity of such paints can be predicted from the properties of the medium and the suspended particulate materials. 8.4.2 Antireflective Coatings As discussed in Chapter 4, a vehicle, which otherwise is well camouflaged, may be detected from a long distance while moving in

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daylight due to the glare caused by the reflection of the visible portion of the sunlight. The glare a t the windscreen can be controlled either by fixing a mechanical gadget in front of the screen or by applying a thin and transparent coating of a metal oxide or a polymeric substance, on to the wind screen surfaces. The underlying principle69for a material to be antireflective on a substrate, for example, glass, is the following :

(i) n2 = where

,/F n,

8-10

refractive index of glass (1.5) = refractive index of the material n, = refractive index of air (1) n, (ii) thickness (d) of the coating should follow the relation =

where h is the wavelength of light Materials showing antireflective characteristic on glass are discussed by Ritter70. A few of s u c h material are thorium tetrafluoride, cerium trifluoride, magnesium oxide and titanium Oxide. Antireflective coatings from these materials are obtained by depositing single layer of a substance or multiple layers of more than one substance of thickness d. The coating of metal oxides on glass substrates is done by using one of the following methods: Plasma vapour deposition Vacuum deposition Chemical vapour deposition Spray pyrolytic, and Dip coating/sol gel Spray pyrolytic and dip coating/sol gel techniques are commonly used for coating large-sized substrates. Organometdlic compounds containing metal-carbon or metal-oxygen (alkoxides) are employed as the starting materials. Thin film coating procedure by using dip coating technique is shown in Fig 8.13.71 8.4.3 Aqueous Foam Foam which is a system consisting of dispersion of gaseous particles in liquid or solid medium is a multicomponent material. Solid foam, i.e., dispersion of gas in solid medium, is known for its thermal insulation and cushion applications. Liquid foam, i.e.,

Materials for CamouflageApplications

dispersion of gaseouslair particles in liquid (aqueous) medium, is used for household/industrial cleaning, laundry and fire fighting applications.

Oxide film Reaction products

(0.07pm)

>

H20

<.-- (Vapor)

----- -.Oven --- - --

.

-

Figure 8.13.

Solvent

- -.. (400-500OC) -*--.

>

Antireflective coating by dip coating technique.

Though liquid foam, (Fig 8.14), is made mainly of a gas (air) and a liquid with dissolved/dispersed surfactant molecules, its physical properties w e far removed from those of its constituents. For example, foam behaves both like a solid and a liquid. Because of the presence of thin liquid films, it offers large interfacial area for transfer processes to occur. These special characteristics of foam have indeed been used to develop a number of industrial applications. The first use of foam was perhaps made by the insect froghopper. The nymph of this species secretes foam a s soon a s it hatches out from the egg and

285

286


covers itself with it to get protection not only from predator but also from rain and the sun. The rigid structure, its large surface area, colour and entrapped air, are responsible for these special properties of liquid foam. The same properties of foam are utilized for crop protection from frost and prevention of heat losses in greenhouses.

Figure 8.14. Structure of foam (dispersionof gas in liquid media). Source :

Reproduced from Organic Coatings Science and Technology by O.D.Parfitt"Dispersion of Gas in Liquid media' with permission of Marcel Dekker Inc. New York.

Because of the special structure and low liquid content, foam is ideal for containing material and is used to reduce coal dust explosions in coal mines. Further, the addition& property of foam to distribute the contained material uniformly throughout the medium has been used widely in the fabric industry to distribute finishing agents such as dyes or resins on fabrics. During the recovery of petroleum from oil wells foam is used in many ways, e.g., to remove water, as a drilling fluid, or a diverting agent. In addition to the numerous industrial applications as mentioned above, foam is now seriously considered for camouflage in the visible region of large static objects such as runways, connecting roads, combat vehicles, minefields, and headquarters' buildings, by producing them in different colours to match with the background. The advantages of using liquid foam for camouflage applications are many, and some of them are given below: A free liquid foam can easily be produced right on the site and also removed easily with water or simple mechanical device after its utility;


287

It can be spread over a large area to cover tracks and roads in minutes. Foam is time-saving; It is adaptive to the surroundings. Pigmentation can be created in a dye mixer, allowing colour variations to s u i t the surroundings. Besides, texturally it matches with the background; Tanks and trucks leave highly visible tracks. These tracks reveal the target even after the vehicles themselves have been successfully camouflaged. The use of foam in covering these tracks minimizes the threat of detection even in the infrared region. The materials used in generating these foams are air, water, c h the alkali metal salts, alkyl and foaming a g e n t ~ ~ ~ - w h iare sulphates, alkyl aryl sulphonates, quaternary ammonium salts, metal soaps, alkyloamides, etc, while stabilizers include long chain fatty alcohols and alkyloamides. By suitable combination of the foaming agents, colouring agents and stabilizers in water, foam in any colour and varying stability can be generated by passing air through the foaming composition. 8.4.4 Smoke Smoke has already been referred to in Chapter 4. Suspended solid particles in gaseous (air) medium were used for the first time during World War I for the protection of infantry reserves and river crossing from enemy's observations. Early World War I smokes which were black, proved unreliable and unstable, and were subsequently replaced by white phosphorous and smoke generated from heating a mixture of carbon tetrachloride, zinc metal and he~achloroethane~~. After World War I1 it became apparent that i) smoke, deployment depends upon knowledge of terrain, circumstances, geography, meterological conditions, ii) smoke can be very effective in denying enemy observation, thereby degrading the enemy's direct and indirect combat power, iii) even the most effectively generated smoke becomes useless if either misplaced (insufficient time to deploy, wrong wind direction, etc.) or mishandled (insufficient planning, inexperienced operators, poor training, etc.), and iv) smoke from white phosphorous, fog oil, or hexachloroethane can seriously degrade optical sighting systems including ATGMS, visual sighting system and the neodymium laser, a type used extensively in range finders and target designators. Smoke tactics a s well a s the smoke itself are now considered as combat multiplierss0 as they can be used to deny the enemy information about one's own measures and emplacements;

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reduce the efficiency of enemy's target detection; obscure terrain features from enemy's airforce; interrupt enemy operations on the battlefield; surprise the enemy; and deceive the enemy. Further, computerised war gaming74has shown that the use of smoke can reduce friendly losses by 25 percent and slow the enemy rate of advance by 50 percent. The primary attenuation mechanism in smoke, or, for that matter, in aerosol or dust particles, is the scattering of light. The scattering efficiency is determined by the relationship between the distribution of particle sizes and the wavelength of the detector's operation. The highest scattering efficiency occurs when the particle's radius and the wavelength of the radiation are nearly equal [assuming spherical particles). Some of the commonly used materials for generating smoke are white phosphorous, hexachloroethane, fog oil, and diesel fuel fog. Smoke is generated from pyrotechnics, grenades or vehicle engine smoke systems, and can be deployed in four different ways in the battlefield, viz., obscuring, screening, decoy and masking. In order to reduce the threat of detection from increasing use of sensors working in the mid- and far infrared bands programmes are under way to device new munitions that provide protection in these bands. 8.4.5 Nets A s discussed earlier in the Chapter on Visual Camouflage, nets are used extensively to give coverage to the strategic military targets against reconnaissance. This is accomplished by providing coloured patterns to the fish net (garnish) similar to the surroundings, while maintaining the consistent continuation of surroundings contours. This aspect is incorporated while using camouflage net with the help of a superstructure. These nets are incised to produce visual texture and movements (i.e., similar to leaf movement].The visual camouflage nets are usually manufactured in three colour combinations (woodland, desert and snow) and two basic types (radar scattering and radar transparent). In addition to garnished net, open cell nets of strong plastics such as polypropylene are also available. These nets, because of their low density, high strength and large area, could be covered without actual net draping or "umbrella points"49. 8.5 MATERIALS FOR MULTISPECTRAL CAMOPJFLAGE In the modern era of reconnaissance systems that operate in more than one region of the electromagnetic spectrum, both the


design and material development goal concepts expand to multispectral camouflage, i.e., to cause protected items to become indistinguishable from the background in the view of a designated set of sensors at some set of designated ranges. Since about 1970, camouflage systems, mostly screens or nets, have incorporated protection in visual, near infrared, and radar spectral ranges, and are thus multispectral to a degree. The main element missing todate is some treatment of the 3 to 5 pm and 8 to 12 pm infrared spectral regions. The international camouflage community has acknowledged the existence of this gap and a great deal of work is being carried out to solve this problem. For material development the main concern is to achieve surface modification, through application of coatings, of objects and structural material of objects in order to give multispectral camouflage effects. Some of the measures are discussed below. 8.5.1 Surface Coatings As on today there is no single material which can be used in the form of coatings to give signature suppression in multispectral wavelength regions. However, as has been discussed in previous sections, materials are available for the individual regions. Designed successive coatings of these paints thus can meet the objective of multispectral camouflage provided they are compatible. Design of some of the materials like conducting polymers may probably provide visual camouflage during the day and absorb radar enerw, for example, a t night. However, it appears that these coatings cannot be an effective RAM and camouflage a t the same time75. 8.5.2 Composites The development of advanced composites has certainly been important, and may be even more so in the near future, for the radar absorbing structures. Thermoplastic matrix composites offer high structural strength, low cost, and good absorbance. Reinforced carbon-carbon (RCC) - produced by laying u p carbon fibres in a special matrix, which is then baked and carbonized - is strongly radar absorbent and heat resistant, and, with additional loading with suitable materials, is likely to play a role in multispectral camouflaged structures. Poly-p-phenylene-benzobis-thiazole (PBT) doped with iodine by ion implantation method76 shows better conductivity than that of carbon-impregnated composites ahd is considered as a better radar absorbing structural material. At present no suitable surface coating or structural multispectral camouflage material seems to be readily available;

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however, concentrated efforts are on. One of the approaches being pursued in this regard is to make a combination of pigmentation and texture with radar absorbing properties to combine visual and near infrared camouflage with substantial reduction in radar cross section. Further, a design approach is likely to extend these characteristics to the infrared wavelengths. 8.5.3 Multispectral Camouflage N e t s Multispectral camouflage nets have recently been developed by Ball Corpn. in USA. These nets make use of light weight materials assembled a s a series of 1-inch (2.5 cm) bands - separated by open, yet tightly woven, small fibres which weigh less than 1.5 kg/m2. Two persons can unpack and assemble the net system over a n F15 or F- 16 fighter, without touching the vehicle, in about 15 minutes without any special support49. Tests of these nets had shown that they will withstand wi~lds upto 60 mph (100 kmph). The nets' open weave also permits snow and rain to pass through, avoiding snow and water accumulation that could collapse the light structures. From the outside, sunilluminated nets appear to be opaque. Looking from inside through their relatively open weave allows enough natural light to pass through so that maintenance personnel can easily work on sheltered aircraft inside. These nets are available a s double-sided units with patterns suitable for different environments, i.e., snow and partial snow, concrete and asphalt, forest green, and sand. Further, the net materials have 1R shielding and capabilities for absorption of radar energy. Exactly how the net material accomplishes visual, IR and radar camouflage is a closely guarded secret. 8.6 MATERIALS FOR ACOUSTIC CAMOUFLAGE Submarine tiles cast from rubber or neoprene compounds containing macro or micro air-filled cells have been used for several years to reduce the acoustic signature of submarines. These tiles are designed specifically to meet the most stringent acoustic requirements and a t the same time to provide the lowest global volume compressibility vs depth. The following four main types of materials which are usually glued in place on a submarine are available for the control of underwater acoustic signature: Materials for anechoic treatments which absorb active sonar through viscoelastic loss and local strain deformation; Transmission loss decoupling materials which act as reflectors, reflecting incident acoustic energy away from a source, in order


to reduce a submarine's own signatures, and decoupling internally generated noise from the surrounding water; Vibration damping materials which act a s mechanical absorbers; and Flow noise reduction acoustic coatings designed for protection of acoustic transmitters on a submarine. The choice of adhesives is of key importance for their successful installation. The adhesive must be compatible with both the acoustic material and the substrate (or with any anticorrosion primer applied to the substrate). Epoxy is found a s a suitable adhesive. Recently, spray-on acoustic coatings based on polyurethane have been reported77 to replace acoustic tiles. Other polymeric candidates for acoustic camouflage are interpenetrating network which are intimate mixtures of two polymers with polymers (IPNS)~~ high and low glass transition temperatures cross-linked together. Further, in many active control systems, solid piezo-ceramics are used a s the actuating device^^^.^^ since they offer high electromechanical coupling, linear transmission responses and flexibility that lend to coatingss1. Two s u c h piezocomposite candidates include piezorubber and piezoceramic rods in polymer. 8.7 FUTURISTIC CAMOUFLAGE MATERIALS The materials described thus far produce fixed properties such a s colour, reflectivity, emissivity and absorption, in different segments of the electromagnetic spectrum. It means that the camouflage is a n average or "best guessn design based on the most likely background conditions. The ideal expectation from a material for camouflage, however, is its ability for adaptive signature control of the object in different segments, i.e., visible, infrared and microwave, of the electromagnetic spectrum, and its operation under different background conditions. Some of the futuristic materials for camouflage are discussed below. 8.7.1 Chromogenic Materials Some existing and proven technologies in other areas offer opportunities for adaptive signature control, For instance, passive and active technologies available for visible and infrared applications in solar energy and display device areas could be used in camouflage to better match background conditions over the long term. Though the practical application of these technologies to camouflage requirements is in its infancy, their general principles are discussed here.

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Adaptive materials (coatings) can be grouped into categories named after the mechanism responsible for the adaptation process. Materials whose optical properties are subject to change by the application of external stimuli are called chromogenic. Some of the major chromogenic materials are : Thennochromic: Optical properties are reversible under the influence of temperature. Example : cholesteric liquid crystals; Electrochrornic: Optical properties are reversible under the influence of an electric field. Examples: viologenes, conducting polymers: Photochromic: Optical properties are reversible under the influence of incident radiation flux. Examples: Inorganic complexes, organic dyes and conducting polymers. There are many other materials reported in the l i t e r a t ~ r e ~ ~ - ~ ' showing these properties. Such materials, are now being considered for giving adaptive camouflage in the visible region just like a chameleon does. 8.7.2 Luminescent Materials One of the conducting polymers, viz., polyphenylene vinylene (PPV), showing very good luminescent properties associated with its processing advantage of being a polymer, looks like a good candidate to replace liquid crystals (LCs) in many applicationsa8. There are further interesting possibilities of using this material in both active and passive camouflage application in the optical regionsg. In passive camouflage PPV would be used in a similar way as is camouflage paint today. It could, however, also be tailored to form part of an active system: a sheet of PPV film could be modified to emit in specific colours a s required for a particular application, and attached to an intelligent processing and environmentalsensing system. Possible applications for this active approach to camouflage therefore include: (i) Camouflage sheet to cover vehicles or facilities ; (ii) Optical stealth coatings or foils for vehicles applied to outer hull of land vehicles, aircraft and even submarines and ships. For aircraft applications, the coatings on the underside would reproduce what the platform's sensors see above the aircraft. This active deception ploy, plus the speed at which the aircraft travels, would make it almost impossible for an enemy gunner to track his target optically. In submarine applications, this material promises the means to defeat a new mode of detection in antisubmarine warfare - the blue-green laser.


8.7.3 Polymers and Compssites

The prime focus of materials research in the polymer compounds and composites field to-date h a s been "passive" applications, many of which involve substitution of metals in existing applications. The driving force for advances in this area has been the composite paradigm (anisotropic design) and computer-aided design, manufacture and modelling, and the underpinning detailed molecular level understanding from advanced characterization and measurement facilities. What is required for multispectral camouflage and other applications? The future lies in the ability of these materials to integrate many functions in order to act a s "activenor "perceptive" or "intelligentn materials. Since this emerging area will be new to many, brief definitions of the terminologies are given below: An intelligent rnaterial/system is one which is capable of understanding and of obtaining information; A perceptive material/system is one which is capable of intuitive recognition or action by which response to external signals/stimuli may be interpreted and acted on. Polymer composites which comprise high performance polymeric matrix have replaced metals and metal alloys in many structural applications in aerospace, transport a n d marine industries since these materials offer the advantages of lower specific gravity, higher specific tensile modulus and tensile strength a s compared to those of metals. Two high modulus fibres in use for making composites for structural applications are Kevlar developed by Du Pont and Twaroon developed by Akzo, a Dutch company. Although still under development, many of the newer composite structures will combine excellent absorption characteristics with physical attributes such as high tensile and compression strengths, resistance to impact, fuel and hydraulic fluid tolerance, and light weight. Several materials scientists and aircraft manufacturers are working on the design and development of these composites to meet the multispectrd requirements, and some of their products probably are being employed in the next generation aircraft such a s B-2 bomber and advanced tactical fighter. Two classes of new polymers, i.e., Piquid crystalline polymer and conducting p o l p e r , possess unique combination of rnechsenical, electrical, optical, etc. properties, and a n ample scope for tailoring these properties through 'molecular design'. This provides a n excellent opportunity to development into "perceptive"multispectral camouflage materials, both a s surface applique and structural materials in the form of composites for strategic military objects.

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REFERENCES Clark. D.J. Magnetic oxides (Part 2). J o h n Wiley & Sons, New York, 1975. 2. Bboemargen, N. & Wang, S. Phys. Rev., 1954 9 3 , 72. 3. Suhl, M. J. Phys. Solid., 1975, 1, 209. 4. McCauly, J.W. Malpin (Jr), B.M.; Hynes, T., & Eite1men.D. Ceram, Eng. Sci. Proc., 1980, 1 (7-8), 356. 5. Mitushi Toatsu Chemicals Inc., J a p a n Kokai Tokyo Koho 09 NOV 1983, 3P,58,192-201. 6. Vardan, V.K.; Vasundra Vardan, V.; Ma, Y. & Hall W.F. IEEE Trans. Microwave Theory Tech. 1986, 34(2), 257. 7. Ozaki T. & State, T. J a p a n Kokai Tokyo Koho. 17 May 1979, 7961,231. 8. Kishimito A.; Yoshino, H.; Watanbe, T. & Hashimoto, Y. Japan, 1 6 Sept 1977, 08,110,500. 9. Nicolas, J. Rev.; Phys. Appl, 1974, 9 , 847. 10. Newnham, R.E.; Jang, S.J.; Ming, Xu, & Joe, F. Ceramics Transactions, 1991, 2 1,5 1. 1 1. Kenkre, V.K. Ceramic Transaction, 199 1, 2 1 , 69. 12. Kharadly, M.M.Z. & Jackson W. Proc Institute El. Engg. 1953, 9100, 199. 13. Collin, R.E. Field theory of guided waves (2nd Ed.). New York, 1991. 14. Doyle, W.T. & Jacobs, I.S., Phys. Rev., 1990, 4 2 (15), 9319. 15. Russel, N.E.; Garland, J.C. & Tanner D.B. Phys. Rev. 1981, B 2 3 , 632. 16. Jacobs, I.S. Advanced artificial dielectric materials for millimeter wave length application. Schenectady. 1990, Report 90-SRD0 0 1, GE CRD. 17. Shirakawa, H.; Louis, E.J; Mac Diarrnid, A.G. & Heeger, A. J. J. Chem. Soc. Chem. Commun. 1997, 578 18. Kanazawa, K.K. ; Diaz, A.F. Geiss, R.H. ; Gill, W.D. ;Kwak J.F. & Street, G.B., J. Chem. Soc. Chem. Commun., 1979, 854. 19. Lin, J.W.P. & Dudek L.P., J Polym. Sci. Polym. Lett. (Ed.) 1980 18,348 20. Rabolt, J.F.; Clark, T. C. ; Kanazawa, K.K. ;Reynold, J .R. & Street G.B. J. Chem. Soc. Chem. Cornmun., 1980,347. 2 1. Oshaka, T.; Ohnuki Y.; Oyama, N; Katagin, G. & Kapisaka K. J. Electroanal. Chem., 1984, 161, 399. 1.


22. Chance, R.R.; Shacklette, L.W.; Mitler, G.G.; Ivory, D.M.; Sowa J.M., Elsenbaumer R.L. & Baughman, R.H. J. Chem. Soc. Chem. Commun., 1980, 348. 23. Shacklette, L.W.; Chance, R.R.; Ivory, D.M.; Miller, G.G. & Baughman, R.H. Synth. Met., 1979, 1,307. 24. Malhotra, B.D.; Kumar, N. & Chandra, S. Prog. Polym. Sci, 1986, 12, 179. 25. Ito, T.; Shirakawa, H. & Ikeda, S. J. Polym. Sci. Polym. Chem. (Ed)., 1974, 12, 11. 26. Gau, S.C.; Milliken, J ; Pron A.; MacDiarmid, A.G. & Heeger, A.J. J. Chem. Soc. Chem. Commun. 1979, 662. 27. Tourillon, G. & Garnier, F. J. Electroanal. Chem., 1982, 135, 173. 28. DosSanto, M.C. & de Melo, C.P. Solid State Commun., 1984, 50, 389. 29. Kaufrnan, J.H.; Colaneri, N.; Scott, J.C., & Street, G.B. Phys. Rev. Lett., 1984, 5 3 , 11000. 30. Hayes, W.; Contemp. Phys., 1986, 26, 421. 31. DosSantos, M.C. de Melo, C.P. & Brandi, H.S. Solid State Commun, 1 9 8 4 , 5 2 , 9 9 . 32. Javadi, H.H.S.; Cromack, K.R.; Mac Diarmid, A.G. & Epstein A.J. Phys. Rev., 1989, B39, 3579. 33. Javedi, H.H.S. Microwave. J., 1989, 162. 34. Colaneri, N. & Shacklette L., IEEE Trans. Instrum. Meas., 1992, 41, 2891. 35. Dudeck, K. & Buckley, L. IEEE Trans. Instrum. Meas., 1992, 41, 5. 36. Joo,'J.; Moss, B. & Burford, R. Phys. Rev., 1994, B49, 2977. 37. Joo J . & Epstein A.J. Appl. Phys. Lett., 1994, 6 5 , 18 & 2278. 38. Kohlman, R.S. et al, Phys. Rev. Lett., 1995, 74, 5 & 773. 39. Buckley, L.J. & Eashoo, M. Synth. Met., 1996, 78, 1-6. 40. Epstein, A.J., & MacDiarmid A.G. Science and application of conducting polymers. Adam Hilger, New York, 199 1, 141. 41. Vardan, V.K.; Vardan, V.V., Lakhatia, A. J. Waver-Muter, Interact., 1987, 2, 71. 42. Knoth, A. International Defence Review. 1994, 6 , 93. 43. Shibaev, V.P. & Lui Lam. Liquid crystalline and mesomorphic polymers. Springer-Verlag, New York, 1993. 44. Lindman, K.F. Ann. Phys., 1920, 6 3 , 621.

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45. Landman, K.F., Ann. Phys., 1922, 69, 270. 46. Lakhatia, A; Vardan, V.K., & Vardan, V.V. Time-harmonic electromagnetic fields in chiral media. Springer Verlag, New York. 1989. 47. Stupp, S.I.; Son, S; Lin, H.C. & Li, L.S. Science, 1993,259,59. 48. Aviation Week and Space Technology. Washington, 1987, 22. 49. Schmieder, D.E. & Walkar G.W. Infrared and Electrooptical Handbook, 1993, 7 , 157. 50. Holst, G.C., Armor, 1984, 20. 51. An infrared spectroscopy atlas for coating technology. Federation of Societies for Coating Technology, Blue Bell PA. 1990. 52. McCann, J.E. Surface Coatings. Australia, 1990, 27, 8. 53. Mar, H.Y.B & Zimmer, P.B. US Patent, 4, 131,593, Dec. 1978. 54. Wake, L.V. J. Oil & Colour Chem. Assoc., 1990, 73(2), 78. 55. Taylor, D. J. Oil and Colour Chem. Assoc., 1958, 41, 707. 56. Sward, G.G. &Jacobson, A.E. Paint testing manual (13th Ed.), Special Technical Publication, 500, Am. Soc. Test Mater., Philadelphia PA, 218, 1972. 57. Hecht, E. & Zazac A., OPTICS, Addison-Wesley, Reading, MA, 1979. 58. Edward, D.K. & Catton, I. Radiation characteristics of rough and oxidized metals In: Advances in therrnophysical properties of extreme temperatures. S. Cratch, (Ed.).American Society of Mechanical Engineers, 1965. 59. Hagen, E. & Rubens, H. Annalen. de. chimique Physique, 1904, 2, 441. 60. Palik, (Ed.) Handbook of optical constants of solids. Academic Press, Orlando, FL, 1985. 61. Carlesson, D.J.&Wiles, D.M. Cand. Text. J., 1973, 90(6), 107. 62. Pvidal, J.; Sauman, Z.; Kashnik, F.; Franc, V. & Hampel, M. Czech. CS 192.137.37, J a n 1982. 63. Pusch, G.; British patent, 1 605, 131, 1981. 64. Surface Coatings (vol. 1) 2nd Ed. Tafe Educational Book. Oil and Colour Chemists Association, Kensington, Australia, 1983. 65. US Military Specifications MIL-C-53039 (ME) 16 April 1984. 66. US Military Specification MIL-E-46117A Enamel, Alkyd, Lusterless, Solar heat reflecting, Olive drab, Oct 6, 1970.


67. US Federal Specification TT-P-490E, Enamel, Silicone Alkyd Copolymer, Semigloss for Exterior and Interior Non-Residential use, September 25, 1975. 68. US Military Specification DOD-E-24635, Enamel, Gray, Silicone Alkyd Copolymer, Semigloss (for Exterior use) September 13, 1984. 69. Coss J.T. & Hass, G. Physics of thin films. Academic Press, New York 1965, 2, 223. 70. Ritter E. Physics of thin films. Academic Press, New York, 1975, 8 , p. 42. 7 1. Dislich, H. & Hussman, E., Thin solid film. 1981, 77, 129. 72. Noguchi, S. Kogakuo to Seibutsu. 1972, 10 (31, 199-203. 73. Piette, P. St. Artificial aerosols. Naval Research Lab Memorandum Report, 4197, April 1983. 74. Washer, R.J. & Flack, L.T.G. Smoke obscuration symposium IV, April 1982. 75. Aviation Week and Space Technology. 1993, 136. 76. Chen, C. & Naishadham, K., IEEE Proceedings South Eastern 1990, p.38. 77. Marine Defence, March 1994, p.26 78. Capps, R.N. J.Acoustical Society of America, 1983, 73, 2000. 79. Bailey, T. & Hubbard, J.E. (Jr.).J. Guid. Control, 1985, 8, 605. 80. Tzou, H.S. & Gadre, M. J . Sound. Vibration, 1990, 136, 477. 81. Newnham, R.E. Chem. Tech., 1987, 38. 82. Granqvist C.G. Spectrally selective surfaces for heating and cooling application. (Vol.TTO I), SPIE Optical Engineering Press, Bellingham, WA, 1989. 83. Lampert, C.M. & Granqvist, C. G. Large-area chromogenic materials and devices for transmittance control. (Vol IS041 SPIE Optical Engineering Press, Bellingham, WA, 1988. 84. Berglund, C.N. IEEE Transaction on Electron Devices, 1969, 16(5),432. 85. Morin, F.J. Phy. Rev. Lett., 1959, 3, 34. 86. Goodenough, J.B., Phys. Rev., 1960, 117 (6), 1442. 87. Soward, G. et al, Proc. SPIE., 1987, 823, 90. 88. Kumar N.; Vadera S.R.; Singh, Jeevan Dass G.; Negi S. C, Aparna P. & Tuli A., Def: Sci. J., 1996, 46, 9 1. 89. International Defence Review, 1993, 9 15.

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STEALTH TECHNOLOGY 9.1

INTRODUCTION

The concept of stealth is not new to warfare. From ancient times, stealth has been in use in various forms. Modern stealth technology has its roots somewhere prior to World War I1. Probably the need for stealth has arisen ever since the aeroplane became an instrument of war. Stealth, as it stands today in modem war, has become a means of survival for any modem weapon platfom/weapon delivery system/ weapon, in all the theatres of war - air, sea and land. Broadly speaking, by stealth is meant the act of avoiding detection of any weapon platform employed in offence, by all possible sensors which the enemy may possess and put into operation. However, the term 'stealth' is more often associated with the act of defying detection by radar which has become a highly potent threat sensor with a long range capability. Initially the thrust in the development of stealth technology was against air defence systems. Consequently, R&D efforts were directed towards making jet fighters and bombers undetectable by enemy's radars. Today stealth technology has extended to naval ships on sea, torpedoes inside sea, and even to tanks and other vehicles on land. Stealth technology is inter-disciplinary and a complex synthesis of several technologies which are diverse in naturei. It has to embody countermeasures to detection by radar, infrared, visible and acoustic and other types of sensors. The technologies developed or being developed towards achieving this objective are also known as low observables or low observable technologies. The aim of stealth techno lo^ is to reduce the probability of detection and thereby accomplish the mission successfully. A dynamic and integrated approach towards accomplishing a balanced multi-spectral camouflage against the threat sensor system of today is essential to meet the requirement in the continuously changing war scenario.

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9.2

WHAT IS STEALTH?

In the layman's language, the meaning of the word 'stealth' is the act of doing a thing so slowly, quietly and secretively that it cannot be noticed by others. A s has been explained in chapter 3, stealth is present in nature too2. From time immemorial, man as a hunter while approaching his victim has been adopting stealth. In short, stealth comprises various means employed in offence to avoid detection by the other party. One typical example where stealth plays a very important role is the fighter aircraft. The aircraft in the air is seen conspicuously against a uniform background if it is not made stealthy. Similarly, a ship on a uniform background of sea and/or sky can be easily detected unless made stealthy. Ofcourse, on land, there is a lot of heterogeneity for a military object such as a tank. Tanks of the future may also have to be made stealthy. Stealth technology or low observable technology deals with the design of weapon platforms from the begining stage itself to include low observable features a s a major design goal, rather than a s a retrofit capabiliw. 9.3

HISTORICAL BACKGROUND OF STEALTH TECHNOLOGY

The history of modern stealth technology has its beginning in the early 1900s' with its application to aircraft. Initially, in Germany, transparent materials for aircraft wings, fuselage and empennage were used in an attempt to render the aircraft 'invisible'. In 1912' Lt Eduard Nittner, a n Austro-Hungarian airforce officer, flew a monoplane with its external parts covered with a transparent material, known as emaillit, which was derived from celluloid. When the plane was flying at a n altitude of 900 feet (300 m], it was invisible to observers on the ground. At a flying altitude of 700 feet (200 m), the internal framework of the outer frame was faintly visible. Germans during the World War I used cellon - another transparent material - to cover outer parts of aircraft, but without much success. In 1935 the Soviets1employed another transparent material called rodoid for covering the airframe and applied white paint to the internal parts to render them difficult to be seen through the transparent outer frame. Although the aeroplane YakovPev Air-4 could not be spotted from the ground, its inner white frame was visible from a distance of about a few hundred feet. The advent of radar, during World W a r 11, necessitated the development of a new stealth technology for hiding a military aircraft from radar. Stealth projects were undertaken by United States and

Stealth Technology

Germany during that period1. Attempts to employ radar absorbing materials were made for the first time. In 1945, Northrop in US developed a radar absorbing material known as MX-410'. This material, although somewhat effective in reducing the radar return, when applied with several coats, increased the weight of the aeroplane considerably, which adversely affected the flying capacity. Subsequent developments and specific details of the stealth technology are not known in detail a s the subject is highly classified. On August 22, 1980 the United States government a t a press conference1 held at the Pentagon officially disclosed that they have a Stealth Technology Programme. The development of ground-launched and air-launched missiles, with radar fire control systems, subsequent to World War 11, had substantially enhanced the effectiveness of air defence systems. Thus the capabilities of radar as a primary means of detecting aircraft and directing fire against them had become abundantly clear. This necessitated the development of techniques to frustrate radarcontrolled air defence systems. Initially, Electronic countermeasures (ECM) were employed to reduce the effect of radars1. United States and Russia simultaneously initiated R&D work leading to technology to degrade the effectiveness of radars. Although air defence systems were employing other types of sensors in addition to radar, the thrust of R&D activity was primarily directed towards a technology to defy detection by radar. The early versions of the technology were initially applied to reconnaissance aircraft1, and subsequently extended to various weapon platforms. However, in future, the technology has to take care of all possible sensors by which the weapon platforms may be detected. 9.4 MILITARY OBJECTS REQUIRING STEALTH In general, any military vehicle in flight, in air, sea surface or sub-surface and land, which cannot be camouflaged by conventiond methods need the cover of stealth. J e t fighters and bombers are two important classes of aircraft requiring stealth as they are under constant exposure to fighter planes and ground-to-air missiles. F- 117A Stealth Fighter Aircraft developed by Lockheed, and B-2 Stealth Bomber developed by Northrop for US Air Force, are two such examples. Besides the aircraft proper, antennas employed in these aircraft for their own radar systems, such a s Slotted Planner Array and Conformal Phased Array type, a s well a s radomes which cover radar and microwave antennas, need stealth protection. Besides manned aircraft, unmanned aircraft

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such a s Remotely Piloted Vehicles (RPVs) need the application of stealth technology. All major warships which are constantly under surveillance of the enemy's radar, infrared and other sensors and targets of missiles and anti-ship missiles do need the application of stealth technology. In order to avoid detection of ships by cruise missiles which fly very low, Air-borne-Early-Warning Airships provided with stealth are employed. In order to avoid detection by ships and other submarines and Anti-submarine Warfare (ASW) aircraft, submarines employ a variety of countermeasures. The torpedo, the underwater weapon which is deployed either from submarine or from aircraft or helicopter, should have a highly sophisticated degree of stealth technology in order to successfully complete its mission. Missiles also need the capacity to remain undetected just as much a s their targets. Likewise helicopters have to be stealthy. In general, stealth can be applied to any military object. The relative importance of the object and its vulnerability to the various sensors and the effectiveness of stealth and cost are the factors which must be taken into consideration1. Radar threat has been constantly increasing day by day. Developments in terms of range, resolving power and response time are taking place rapidly. Further developments in digital filtering and signal processing will make radar more and more effective. The potential of SLAR and SAR provide the greatest threat to ground targets. Also, substantial developments in millimeter wave radar are taking place. These extend the application of stealth technology to other objects of military importance in addition to demanding improvements in the existing low observable technologies. 9.5

STEALTH AIRCRAFT The advent of radar guided missiles in 1950s4necessitated antidetection methods for aircraft. Initially, electronic countermeasures - such a s jamming, muffling or distorting countermeasures - were in use. These measures were found to be inadequate with the development of a new generation of Surface-to-Air Missile (SAMs). Then R&D work on manned stealth fighter aircraft began around mid-1970s. Although the techniques of stealth were not new to aircraft designers, the means to apply the techniques were not available. The application of stealth technology to fighter aircraft has made quantum jumps with the advent of supercomputers and availability of composite materials. The former enhanced the speed of

Stealth Technology

aircraft design and the latter provided the right type of materials for airframe and structure1. Of the various stealth aircraft, the F-1I7A Fighter Aircraft and the Boeing B-2 Advanced Technology Bomber of the U S Air Force will be described here as given in the literature1. 9.5.1 LockheedJAir Force F-117A The F-117A was developed under US Air Force programme designated as CSIRS (Covert SurvivableIn-Weather Reconnaissance/ Strike).This is part of a highly classified American "black" programme under the code name 'Have Blue'. A series of studies carried out in early 1973 led to the development of F-117A. Encouraged by the results obtained in these studies, US Air Force invited proposals from the aerospace industry for the construction of the prototype for demonstration. A new name, 'Experimental Stealth Tactical (XST)' was given to the programme. Several companies had responded. The production contract was given to Lockheed in 1976. The requirements for the aircraft were that it should have : i) very low RCS in all aspect angles, in particular'head on', ii) reduced infrared signature, iii) reduced acoustic signature, iv) reduced visual signature, and v) ability to cany ECM/ECCM/ESM equipment, so that it may be fielded a s an effective stealth aircraft. Lockheed entrusted the project to its Skunkworks. The prototype flew in November 1977 a t the Tonopah Test Range located in Nellis AFB, near Las Vegas. It was proved to be effective against radar, infrared, optical, acoustic and electronic detection systems. The US government awarded Lockheed a contract in 1981 for producing 59 numbers of full size version of XST under the name F117A. The F-117A aircraft first flew in June 1981 from Tonopah Base and became operational in 19835.In 1988 US Air Force released a photograph of this aircraft. According to eye witnesses of XST prototype which could occasionally be seen from public roads near Tonopah, the aircraft had bat-like appearance and good manoeuvrability. It had a singleseat cockpit. The canopy had flat radar scattering panels. The wind shield was V-shaped. The aircraft had a rounded appearance from all angles. The forward fuselage and wings had the appearance of a wide inverted V. Also it had folding wings. Its engine consisted of a modified non-after burning General Electric CJ6 10 turbojet. 9.5.1.1 Constructional details of F-117A Radar cross section reduction was accomplished by employing Fibaloy for structural frame parts, for skin panels, spars, ribs and longerons. Fibaloy, a product of Dow Chemical Co, is produced by

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embedding glass fibres in plastic. It is an excellent radar absorbing structural material. Another structural material used was reinforced carbon fibre developed by Air Force Materials Laboratory at Wright Patterson AFB in Dayton Ohio. This material, besides having good RAM properties, reduces the infrared signature. It is particularly useful in high temperature parts such a s outer skin panels near the engines. Besides the RAM and RAS employed, the unique shape of F117A is another contributory factor towards RCS reduction. All edges were rounded. Skins were made of fibaloy. The skin consisted of multiple layers filled with tiny fibers oriented in a specific alignment, spacing a n d density for maximum reduction in RCS. The construction of multiple layers for maximum RCS reduction was probably the most secret element of stealth technology. Various parts were made out of super-plastics developed by US Air Force Materials Laboratory and commercial firms. These are thermoplastics that are stronger and lighter than steel and titanium, and do not reflect radar energy. Another important aspect contributing to the reduction of RCS of F-1 17A aircraft was its internal structural architecture. A design known as 'cut-diamond' was reportedly used. It employed several thousands of flat surfaces, which were so angled that when a radar beam is incident on them, only a few surfaces reflect the radar energy in the direction of the radar receiver. All external surfaces and some of the internal metal parts were coated with ironball RAM. In order to suppress visual signature, both active and passive camouflage techniques that change colour to match with the background were probably employed. Flat-black colour for night and dull grey colour for day were reported to have been used. An active camouflage technique, referred to a s "background clutter signal-to-aircraft RCS matching*, protects the aircraft flying a t low level from look-down interceptor radars by matching its overall RCS with that of the terrain below. ECMIESM equipment were housed in such a way that the use of antennae on the outside of the airframe, which significantly contribute to RCS, was dispensed with. Reduction in IR signature was accomplished by mixing fanbypass air and air from cooling baffles with exhaust gases. This also helps in reducing acoustic signature. The use of special coatings and cooling baffles further reduces the IR signature.

Stealth Technology

The noise of the turbofan engines is reduced by surrounding them with a matrix sandwich of polymers and pyramidic noiseabsorbing structures. It is reported that the aircraft produces a medium level humming noise a t a distance of 100 feet (30 m) and a slight whine was heard while taking off. During night operations, all lights were out. The guidance systems employed were passive from which no signals could be detected. In order to defeat terrain-masking of hostile radar installations, the F- 117Ausually flies close to the ground below the speed of sound. This also protects the aircraft from infrared guided weapons or infrared detector systems. The aircraft is employed for covert reconnaissance and covert strikes on preselected targets. The F- l17A stealth fighter is the world's first operational aircraft designed to employ low observable technolo@. In the Gulf War the efficacy of F- 1178 was amply demonstrated7. The operation 'Desert Storm' which began on January 16, 1991, started with a wellcoordinated assault by US AF F-117 stealth fighters7, which were referred to as 'Shabba' or ghost by America's Saudi allies. The aircraft flewover one thousand sorties against Iraq's heavily defended targets7. No stealth fighters were touched by enemy's air defence. In most cases the F-1 17's destroyed critical facilities using a single bomb. While F- 117s comprised only 2.5 percent of the Air Force's combat aircraft in operation 'Desert Storm', they struck 31 percent of the targets bombed in the first 24 hours of the air campaign7. Stealth fighters struck the heavily defended targets in down-town Bagdad with unprecedented accuracy. The Iraqi air defender could not detect the F- 117. Stealth characteristics meant that fewer aircraft are needed to complete a mission. Just eight F-117s could do the same job a s a standard mission of 32 non-stealthy planes dropping unguided bombs with 16 fighter escorts, eight Wild Weasel defence suppression aircraft, four radar jammers and 15 tankers for refuelling the aircraft. Thus Desert Storm has shown the world the value of stealth7. 9.5.2 Northrop/Boeing B-2Advanced Technology Bomber (ATB] A decision to launch a full scale low observable penetrating bomber programme was taken by the Carter Administration in 1980. A contract was placed with Northrop/Boeing for constmction of prototype preproduction aircraft. The basic design of the aircraft was flight tested in 1982. A full-scale mockup of the B-2 was made ready by the end of 1985'.

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The aircraft was powered by four 19,000 lb GE-F 118-GE-100 turbofan engines, was triangular in shape, and had a flying wing design without vertical tail surfaces5.The exact RCS of B-2 is not known. Quoted values1 range from 0.01 m2 to millionth of a m2. The contributory factors towards its low RCS were its internal structures which were made of titanium coated with epoxy graphite composites. Its sensor-defeating architecture was accomplished by computer-aided design techniques. An anechoic filtering structure involving combinations of plastics, epoxy graphite carbon fibres, and ceramics contributed towards reducing infrared a s well as radar signature. Engine heat was absorbed by the use of ultradense carbon foams. Also active and passive infrared countermeasures were employed. B-2 first flew in 1989. Other new stealth airplanes include the US Navy A- 12 'Avenger' with more advanced stealth characteristics than B-2, and the F-22 Advanced Tactical Fighter (HTF). 9.6 STEALTH WARSHIPS Attention is also being paid to incorporating stealth features in major warships8.Stealth in naval environment poses a greater challenge to designers than stealth of aircraft. Warships are generally larger than aircraft and exhibit a more complex regimeg . Warships and combat aircraft have some characteristics which are common a s they both present hot targets against a fairly uniform background1'. They however differ in other aspects. Surface ships have two media - air and water that have very different transmission characteristics and pose problems such a s radar multipath effects1'. The advent of highly sensitive radar and IR-guided antiship missile, besides the new generation of acoustic homing torpedoes and modern acoustic/magnetic mines, have all put a great stress on countermeasures demanding reduction in radar cross section, acoustic and infrared signatures created by ships and submarines. Stealth technology for warships involves management of the various signatures taking into consideration their relative importance and arriving a t a right balance of signatures. For example, the reduction of IR radiation usually has an adverse effect on the radar cross section. The object of signature management is to achieve optimum combination of various factors starting from design, choice of shapes, materials, internal and external treatments with appliques that will give a n overall result a t a n acceptable cost1.

Stealth Technology

The various signatures that are to be considered in a stealth warship are:

Radar Infrared Acoustic Magnetic Electric Hydrodynamic wake Extra low frequency Miscellaneous such as contaminants, bioluminescence etc. Methods for reducing RCS and infrared signature have been already discussed in the chapters on microwave camouflage and infrared camouflage. Measures for reduction of RCS include shaping and profiling of the ship and use of RAM, RAP and RAS. Reduction of IR signature is accomplished by the use of DRES BALL, and EDUCTOR DIFFUSER and covering material such as FLETACAMl1. 9.6.1 Acoustic Signature Surface warships are strong sources of underwater noise which can be easily detected by passive sonars from very long distances u p to hundreds of kilometres away. Further, active sonars can detect ships which reflect incident sound waves. Also, with the help of underwater noise signature, detection and classification of surface ships can be done. Propellers and hull are sources of cavitation and broad-band noise. Generators, diesel engines and various internal pumps are sources of distinctive sounds a t specific frequencies and amplitudes that can both aid and hamper detection. The modem detection methods enable classification of the target specifying the class of ship . Progress in sonar sensors and signal processing technology is bringing down the minimum acceptable noise levels. This in turn places demand on noise control measures. Minimum noise levels are accomplished by the design of high performance machinery isolation systems and methods for reducing cavitation around propellers and the hull". Noise isolation systems use a wide range of techniques such a s double elastic mounting systems. In such applications attention is also to be paid to the secondary transmission paths. Active control techniques were developed to further enhance the high degree of isolation which can be accomplished by wellbalanced, passive isolation treatments. In rotating machinery, moving parts can be dynamically balanced to minimise the noise

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from shafts and connections to other machinery". Also, equipment may be mounted in acoustically insulated boxes in order to isolate them from the hull. Propeller noise is another important source which must be taken care of. It may be divided into two types : i) low frequency noise generated by the rotation of the blade, where the frequency depends on the thickness of the blade and the load, and ii) cavitation noise caused by the implosion of the cavitation bubbles. The latter consists of a broad band of high frequencies. An air-breathing system fitted to the propeller blade prevents the onset of cavitation. It exudes bubbles from holes around the edge of the propeller blade1'. An airbreathing system known a s Masker fitted to the ship's hull also delays onset of cavitation. But this system aggravates the situation in the event of a threat from wake-homing torpedoes. These problems associated with the Masker system have been overcome by the use of a n alternative screening system. Dowty Signature Management in the US" made a system employing decoupling tiles which are bonded to the outer hull around the machinery area using a contact adhesive. This provides an impedance mismatch between the ship's hull and the water to resist the passage of sound between the two. Coatings are also applied to hulls in the form of decoupling or damping layers. This is intended to reduce the level of machineryinduced noise radiated into water. Anechoic coatings are designed to absorb the incident energy generated by active sonar equipment. Other materials which can be applied include continuous coatings in which anechoic material is applied to large flat or curved surfaces. The decoupling tiles and anechoic coatings reduce the detection range and classification capabilities of the enemy sensors. They also improve the performance of the ship's own sensors. 9.6.2 Radar Cross Section As mentioned in the chapter on microwave camouflage, the basic methods of reducing RCS are the same, with some variations depending on the nature of the object. Methods for reduction of RCS include appropriate design techniques, use of radar absorbing materials, surface coatings etc. Dowty Signature Management developed a RAM panel known of FLEXIRAM". This material was used on ships involved in the operation 'Desert Storm'. The employment of these panels reduced the ship's RCS making it more difficult for attacking aircraft and missiles to find and attack the ship. A 30 dB reduction results in 75% reduction in detection range.

Stealth Technology

9.6.3 Infrared Signature

Means for suppressing infrared signature have been already discussed in the chapter on infrared camouflage. They aim a t reducing the temperature differences between different areas in a ship. Another method is to produce specific hot spots, thereby creating a false target. This diverts the sensor from the real target to the false image. A quilt known a s FEECTACAM has been developed in relatively recent years". This quilt, because of air trapped in it, has low conductivity, and hence reduces thermal contrast and target signature. In addition, it reduces RCS. The FLECTACANI blanket comprises FLECTALON R insulating fibres knitted between standard camouflage material, liquid repellant, and is coloured to match with the background. FLECTALON R is made from strips of 12 micron metallised polyester film which is stable upto 265°C and which can reflect u p to 95% of radiant heat. 9.6.4 Magnetic Signature Magnetic signature management has been a subject of study by naval engineers and scientists since long. Reduction of magnetic signature is accomplished1' either by magnetically treating the ship in special facilities - deperming - or by fitting the ship with special active degaussing coils through which electric current is passed. The strength of the electric current is controlled from within the ship in such a way that a t any time and in any geographic position, the ship's magnetic signature plus that caused by its movement through the earth's magnetic field is silenced. A combination of deperming and active degaussing processes is also employed. Deperming involves putting the ship inside a n arrangement of coils or placing a n arrangement of coils around the ship and then passing a powerful electric current through the coils to create a magnetic field in a drection opposite to that of the magnetic field of the ship. This cancels the ship's magnetic signature . Alternatively, depenning is used to create a permanent magnetic field on the ship which is matched to the area in which it will operate. Degaussing coils are built into the ship during consfmction to provide magnetic field correction facilities. The coils are fed with electric current provided from special computer-controlledgenerators to create an opposing magnetic field which is continuously matched to the ship's changing magnetic field as it crosses the ocean.

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9.6.5 Electric Signature

Surface ships produce a n extremely low frequency electric signature (ELFE) t h a t can trigger mines or be detected by comparatively simple and inexpensive surveillance devices. The signature results when electric currents - which may result from the electrochemical reaction between the steel hull and the bronze propeller or from an active cathodic protection system - return to the hull through the shaft. The widely varying impedances of the shaft bearings and seals, as a function of shaft angle, cause a modulation of this return current which generates the ELFE wave lo. Davis Engineering has developed an Active Shaft Grounding (ASG)system that uses a n electronic transconductance amplifier in a closed loop arrangement to maintain the propeller shaft below 1 mV, thereby minimising the ELFE signatures. 9.6.6 Other Signatures There are other types of signatures associated with a ship by which it may be detected. One of the most common for which probably nothing can be done is the pressure signature". Another signature known as hydrodynamic wake can be detected u p to 40 krn behind the ship by remote sensing techniques. Synthetic Aperture Radar (SAR)on aircraft and satellites can provide sufficient evidence of presence of ships. The proper interpretation of such data can lead to actual ship identification. Proper hull design, decreased draft, contra-rotating propellers and non-conventional propeller systems like pump jet can reduce hydrodynamic wake. Chemical traces left in the discharges from cooling water or engine exhausts, bioluminescence caused by the disturbance of minute organisms, hydrodynamic pressure, and surface wave patterns can now be detected by satellites. 9.7 STEALTH TANK The success of stealth technology in the Gulf War provided impetus to the Department of Defence and US combat vehicle makers to make future battle tanks far less visible to the enemy radars and other sensors. Composite armour, exhaust reduction measures, sensor jamming equipment and deception are some of the new technologies, US companies were contemplating to "give fighting vehicles the signature of a light bulb" a s one top industry executive said. Under pressure from US Congress, the US Army combat vehicle makers and Pentagon's Defence Advanced Research Projects Agency (DARPA)have been secretly working towards a stealth tank since the mid-1980s12.

Stealth Technology

For decades, the tank designers have been increasing the thickness of the armour plates. A s the armour weight increases, the demand for more powerful engines increases. More powerful engines give rise to stronger heat and acoustic signatures. So there is a need to find improved methods of reducing all these signatures12. It is said that the use of composite materials provides greater protection and gives vehicles a low signature. Composites can be formed into any desired shape. A composite surface does not reflect like a metal. 9.8 STEALTH SUBMARINE Detection of submarines is based on the noise generated by the various sources of noise including the hydrodynamic noise which is due to the water passing around the hull, the noise of the movement of propellers, gears, pumps and other machinery, noise produced by cavitation etc4. Efforts were directed towards refinements in mounting of machinery, and employing smaller parts in order to reduce noise. Further, with the advent of nuclear power, the hull has undergone changes in its design which has resulted in the reduction of hydrodynamic flow noise. Attention is now being paid towards using new materials for reducing weight and detectability and using active submarine stealth measures such a s acoustic jamming, interference and deception. 9.9 STEALTH HELICOPTER Helicopters possessing stealth characteristics have been under development by US. These include Phalanx Dragon being designed by Phalanx Organisation Inc based in Long Beach California, and Bell/McDonnell Douglas LHX having low infrared and acoustic signatures and a low - RCS fuselage made of RAM1. McDonnell Douglas MH-6 is an operational stealth helicopter. American forces used the MH-6 during the invasion of Grenada in 1980. Its stealth features include modifications to the engine and main rotor to suppress infrared and acoustic signatures. It is reportedly one of the quietest helicopters in the world, capable of hovering a few hundred feet from a person without being heard. STEALTH RPVS Much stealth research has been carried out on unmanned aircraft. U S has extensively used stealth technology on remotely piloted vehicles, some of which are operational and many under development1. 9.10

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The Aquila of US Army built by Lockheed can perform a variety of missions in a hostile environment and survive. The main factor responsible for its survivability is its use of stealth technology. The aircraft has practically no visual, acoustic, infrared and radar signatures. It is of small size and the airframe is made of Kevlar. Its RCS is so low that, during tests, it could be tracked only when metal was added. It is shaped like a flying wing with a forward fuselage that is curved. Other stealth unmanned aircraft of US include US Army CM30, US Army CM-44 Boeing/DARPA Teal Cameo and Leading Systems and Amber which were under development. 9.11 STEALTH MISSILES Like an aircraft, a missile also should have the capability of remaining undetected. US has several stealth missiles both in operation and under development l . The AGM-86b is one such missile which is in operation. It is very small in size and made of metal alloys and radar transparent materials. It has practically negligible infrared signature. Its fuselage and nose design are such that their contribution to RCS is low. It can fly a t low levels so that its chance of detection is minimum. Another such missile is BGM/AGM- 109 Tomahawk cruise missile. It can be launched from aircraft, ships, submarines and groundbased 1aunchers.The stealth missiles under development are DARPA PROJECT LORAINE, Lockheed cruise missiles and Northrop AGM136 Tacit Rainbow.

9.12 AIRSHIP Airship is a new concept to counter threats to ships a t sea by the cruise missiles which fly very low, virtually intermingled with radar clutter1. These airships can look down and instantly detect and counter an incoming cruise missile. The airship incorporates the latest in the design of the engine, airborne electronics and search radar technology. Although reducing the airship's radar, infrared and acoustic signatures may not be a problem, making it blend with the ever changing background of sea, sky and clouds will pose difficulties. REFERENCES 1. Jones, J. Stealth technology the art of black magic (Ed). Matt Thurber AERO, A Division of TAB Books Inc., Blue Ridge summit, PA 17214, 1989.

Stealth Technology

Cott, H.B. Adaptive coloration in animals. Methuen & Co Ltd, London, 1966. Schmieder, D.E. & Walker G.W. Camouflage, suppression and screening systems. In countermeasure systems, (Vol 7). The Infrared and Electro-optical Systems Hand Book (Ed). Pollock, D.H, Copublished by Environmental Research Institute, Michigan and SPIE Optical Engineering Press, Washington, 1993. Goldberg, J.H. Stealth a n d counterstealth technology. International Military and Defence Encyclopedia, (5)Col. Trevor N. Dupuy, USA(Retd.) Ed.-in-chief, Brassey's (US), Inc. Washington, New York, pp 2544-255 1, 1993. Stonier, R.A. Stealth aircraft and technology. World War I1 to the Gulf - Part 11. Applications and Design, SAMPE Journal. 1991, 2 7 (5) . Stealth technology of the future .... which is Now, VAW, V 1991. United States Air Force in Operation Desert Storm, VAYU, V 1991. Foxwell, D, Stealth the essence of modern frigate design. International Defence Review, 1990.9 p. 984-94. Linder, B.R, (US Navy), US Naval Institute Proceedings. 1993, 119/7/ 1, p. 85-88. Hewish, M. Stealth at sea signature management enhances attack and defence. International Defence Review, 1994, 27, 45-49. Brown D.K. & Tupper, E.C. The naval architecture of surface warships royal instituition of naval architects paper 1988, Signature Management, Navy International, 199 1, 96(5), 152-58. Baker, C. & Polsky, D. Gulf success prods effects to develop stealth tank. Defence News, May 1991,16.

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CHAPTER 18 R&D WORK ON CAMOUFLAGE AND DECEPTION IN DRDO 10.1

INTRODUCTION

Indian armed forces are deployed in a variety of trying environments ranging from the hot and dry heat of Rajasthan desert to the rain-lashed tropics of the northeast, from the sub-zero conditions of the Himalayan heights in the north to the hot humid archipelago of Andaman and Car Nicobar, a s well a s in special situations such a s in submarines, space missions and Antarctic expeditions. They operate a bewildering variety of military hardwareweapon platforms, weapons, weapon delivery systems, command, control and communications systems. It is therefore apparent that continuous indigenous R&D in camouflage and countermeasures is vital for survival and success, particularly in view of the fact that the entire subject is shrouded in secrecy, and no country would be willing to pass on know-how nor would material in open literatuxe be of great relevance, unless adapted to specific situations. In view of the above considerations, the Defence Research and Development Organisation (DRDO)has been giving special emphasis in recent years to research and development in the field of camouflage and deception. The laboratories which are mainly involved in this activity are; Defence Materials Stores Research and Development Establishment (DMSRDE), Kanpur; Defence Laboratory, Jodhpur (DW); Naval Materials Research Laboratory (NMRL) Mumbai; and Naval Physical and Oceanographic Laboratory (NPOL),Kochi.

10.2 VISUAL CAMOUFLAGE DMSRDE has been working on the development of camouflage paints and synthetic nets. The laboratory has developed paints and pre-garnished light weight synthetic camouflage nets for visible and NIR regions, conforming to colour schemes/patterns suitable for green belt areas, desert terrains and coastal areas. It has also

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developed polystyrene emulsions, polystyyene solvents and silicatebased camouflage paints for effectively camouflaging runways and taxi tracks. DW has been canying out basic, applied and development work in the field to cater to the visible a n d other regions of t$e electromagnetic spectrum. The laboratory has generated computeraided disruptive patterns of different types with different colour schemes suitable for various geographical regions. It has achieved significant results in the development of camouflage by arboriculture. DW is also engaged in studies on psychological aspects of visual camouflage and development of suitable devices and equipment for effective camouflage in the visible region. 10.3 INFRARED CAMOUFLAGE DMSRDE has developed paints in various shades, e.g. olive green, deep brunswick green, light green biege, and dark brown, having suitable NIR reflectance values. The paints are solvent-based, impart matt finish and have outdoor life of 1.5 to 2 years. These can be applied over mild steel, tarpaulins, PVC sheets, wooden structures, rubber sheets and cement walls. These paints are effective for both NIR and visible regions. The laboratory has an ongoing programme on the development of thermal camouflage materials. DW has been pursuing work on infrared camouflage mainly to study (i) thermal signatures of various objects, (ii) suppression of thermal signatures by thermal insulation and paints, (iii)simulation of thermal signatures, and (iv) development of methods of imaging thermal objects. 10.4 MICROWAVE CAMOUFLAGE DMSRDE has obtained encouraging results in the development of light weight microwave absorbers using Retinyl Schiff base salts. These were synthesised in the laboratory and characterised by IR spectroscopy and elemental analysis. Absorber samples of finite thickness were prepared and tested for radar absorption at X-band frequencies. The laboratory has also conducted studies on Epoxy/SiC, silicone rubberlcarbon and epoxy/carbonyl iron systems for reflection/transmission characteristics and dielectric properties at X-band frequencies. These materials can be applied on static targets for RCS reduction. It has also developed a camouflage net which caters for visible, NIR and radar bands. The net is a synthetic, pregarnished camouflage system which consists of a well-designed garnish structure developed from PVC-coated nylon fabric.

R & D work in camouflage and deception, in DRDO

DW has been carrying out work on theoretical designing of broadband electrical RAM coatings using multilayers. 10.5 MULTISPECTRAL CAMOUFLAGE MATERIALS DMSRDE and DW have been carrying out R&D on development of multi-spectral camouflage materials. DW has synthesised a large number of materials such as conducting polymers, liquid crystals, liquid foam and materials for anti-reflective coatings. DMSRDE has initiated a programme for developing a multi-spectral camouflage system which should be able to cater for visible, NIR, thermal IR and centimeter and millimeter wave radar regions. 10.6 NAVAL CAMOUFLAGE NPOL and NMRL have been carrying out R&D leading to naval camouflage, to cater for a variety of signatures, the chief of which is the acoustic signatures of naval objects. NMRL has been working on viscoelastic materials including polyurethanes which produce reduction of acoustic energy transmission and hence reduced possibilities of detection. The laboratory has also been working on shaping of naval platforms for RCS reduction. In addition, NMRL is working on the development of RAM sheathing and paints based on dielectric absorbing materials. It has also been working on the development of composite-based RAMS. The laboratory, in collaboration with NPL and NPOL, is trying to develop more effective underwater sensing (passive)materids using PZT polymer composites. 10.7 FORCE MULTIPLIERS DW has made considerable progress in the development of deception equipment for employing a s force multipliers for tactical advantage.

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CHAPTER 11 CONCLUSION The technological explosion of the 20th century has made a tremendous impact on military hardware - weapon platforms, weapons, weapon delivery systems, mobility, command, control and communications, reconnaissance, surveillance and target acquisition systems, and countermeasures. In consequence, wars have become highly technologically oriented a n d sophisticated. These developments are not merely confined to the principal theatres of war - the land, the air and the sea - but are also entering space through military satellites1. Rapid strides in the fields of electronics, computer science and engineering and material science have brought in unimaginable improvements in military sensor technology. These developments have greatly enhanced the capabilities of reconnaissance, surveillance and target acquisition systems employed in the various theatres of war1. Radar technology has seen tremendous advances since World War I1 in terms of range, resolution, detection, tracking, and ability to engage targets. A s these developments continue, it will be possible to employ multimode, multi-frequency and multipolarisation radars, besides solid state transmitters/receivers which can be mounted on aircraft's skin. All these developments pose great threat to major targets. Infrared sensing systems also have progressed significantly. The FLIR can give images of targets day and night, though a t short range. Although IR systems are not a s powerFul and a s reliable a s radars, they cannot be detected, as they are passive. IR guided missiles which use IRST have a range u p to 160 km. They are used a s widely as radar-based systems. IRST systems will probably play a greater role in future wars with the developments that are taking place in the medium and long wave IR monolithic focal plane arrays, which enhance sensitivity and resolution.The use of IR thermal

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imaging systems will further increase with developments in pyroelectric detectors. In the technology of electro-optical systems, rapid advancements are taking place. Laser systems for target ranging and designation are being widely used. Now, with the advent of multiple sensor fusion and correlation, together with the use of Signature Intercept Sensors (SIGINT)and Communications Intercept Reports (COMINT), detection, classification and tracking become easier and information can be obtained in real time. Such a multiple sensor system will have the ability to detect a variety of targets - stationary, moving, emitting and communicating. Multiple sensor systems can have the ability to detect targets which are hidden. The multiple sensors cover a wide range of the electromagnetic spectrum and are complementary to each other. Besides the threat posed by these sensors individually and collectively, new areas such as pattern recognition, artificial intelligence, automatic target recognition and neural network applications are advancing very fast. These advances further enhance the capabilities of target acquisition systems2-'. The developments required for countermeasures are to be viewed in this scenario. Stealth or low observable technology which has come u p is trying to face these challenges. Although primarily the technology is intended to defy detection of fighter aircraft and bombers by radar, it has to counter detection by visible, near IR, thermal infrared, acoustic, electrical, magnetic, seismic signals etc depending upon the actual situation. Conventional methods of camouflage and deception cater for only the preliminary stages of the stealth technology. Starting from the design stage of the military object, stealth technology has to take into consideration all aspects - shape, structure, surface materials and surface appliques - to reduce all possible signatures to levels from which they cannot be detecteda. The efficacy of s t e a l t h technology of F-117 aircraft and B-2 bomber was amply demonstrated in the Gulf W a r in 1991. Stealth technology will in future probably be extended to many other military objects depending upon developments in sensor systems such as millimeter wave radars. Besides stealth, deception equipment in the form of chaff decoys and IF? flares are being employed to divert the attention of the weapon from its trajectory.

Conclusion

However, today, there appears to be systems which can distinguish between a decoy and real object. A s stealth technology advances further, it is imperative that technologies to counter stealth should also be developed. These developments involve more powerful and versatile sensors. In turn, these countermeasures to stealth may spur research work on second generation of stealth, which may probably employ active measures, unlike the passive measures such a s shaping, masking etc of the first generation. The new stealth technology may create false signatures to confuse the sensors. Further, the weapon platforms, weapons and weapon delivery systems may have characteristics which may change depending upon the requirements. These variations in the characteristics may make the task of the sensors difficultg. On the whole, the field of camouflage and deception, or in its modern terminology, 'countermeasures7or 'low observables', has to face greater challenges in future. It demands a judicious combination of research in a variety of basic sciences, their application to specific situations, development of newer materials to cater to a wide variety of exacting specifications, innovative design techniques, and costeffective production technologies. It is a never-ending battle of wits between advances in sensor technology a n d corresponding countermeasures.

REFERENCES 1.

Friedman, S. & Miller, Gunston, P; Richardson, Doug Hobbs D. Warmer M.; Advanced technology warfare. Salmander Book, Published by Salmander Books Ltd., London, 1985.

2.

Lupo J.C. Defence applications of neural networks, IEEE Communications Magazine, Nov 1990.

3.

Geisenheyner, S. Special report: possible applications of neuorocomputing in defence. Annada International, 1990,l.

4.

Boyle, D., AI-Computer may starting to Think. International Defence Review, 1990,2.

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Roth, M .W. Survey of neural network technology for automatic target recognition. IEEE Transactions on Neural Network March 1990, 1.

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Boyle, D. Fusing the data in the search of identity. International Defence Review, 1989,8.

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Moore, R. Sensor technology. International Military and Defence Encyclopdia, N. Dupuy T.N., (Ed.-in-chief), 5 , Brassey's (US), Inc. Washington(l993).pp. 2393.

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JO&S, J . Stealth technology-The art of black magic, (Ed).Matt Thurber. AERO, A Division of TAB Books Inc., Blue Ridge summit PA 17214,1989.

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Goldberg, J.H. Stealth technology and counterstealth technology. International Military and Defence Encyclopedia, Dupuy T.N., (Ed.-in-chief),4 , Brassey's (US) Inc. Washington, New York, 1993. pp. 1953.

A. 1

Grassman's Laws Colour coordinates are arrived a t according to a well-defined experimental procedure which is based upon Grassman's laws. Of the three laws, the two which are of relevance in the present context are a s follows: (i) A colour can be matched by a mixture of three selected radiations; (ii) If two colours are matched in turn by mixtures of three radiations, the two colours, when mixed by suitable means, will be matched by the sum of the two mixtures. A.2 Trichromatic Colorimetry In trichromatic colorimetry, saturated red, green and blue radiations are mixed additively, in appropriate proportions. White colour is matched by a correct mixture of red, green and blue. Intermediate hues can be obtained by combining these three stimuli in appropriate proportions. Law 1 stated above can be verified experimentally with a trichromatic colorimeter. Mathematical additivity of Law 2 stated above may be expressed a s follows : Let (C,) and (C2)be two colours which are to be matched. The colorimeter indicates the amounts of the stimuli (R),(G),and (B)in some arbitrary linear scale. Let u, v and w represent the amounts of the stimuli (R), (G) and (B). Algebraically the two colours (C,) and (C2)may be represented as A- B C I (C,) = u1 (R) + v1 (GI + wl (B) where cl and c2 are the amounts of (C1)and (C2)expressed in arbitrary scale. If the two colours (C1)and (C2)are mixed together .and the mixture is matched on the colorimeter, we would have 8-3 cl (Cl) + c2 (C2) = U3 (R) + V3 (GI + W3 (B) If the additivity law of Grassman's holds good, ther,

324


The work of Ives (1975), Guild (1931) Wright (1928), Stiles (19551, and CIE (Commission Internationale de 17Eclairage,1931, 1964)put the foundation of the science and the technique of colour measurement on a firm basis.

Trichomatic Equation and Chromaticity Coordinates A colour is matched in a trichomatic colorimeter as follows. A-7 q (C) E u (R) + v (G) + w (B) where u, v, w are the amounts of (R), (G), and (B) in some arbitrary scales of the instrument and q is a measure of the colour (C).The above equation is not an algebraic equation but a colour matching equation. The units of (R), (G)and (B) are defined on the basis of a match on a standard white. On this basis, any colour can be measured quantitatively in terms of red, green and blue stimuli. The colour equation for may be written a s A- 8 C (C) = R (R) + G (G) + B (B) where R, G and B are called tristimulus values which are numerical quantities. The equation is referred tg as trichomatic equation. Now we write C=R+G+B A-9 which is an algebraic equation. In order to separate the quantity of (C) from its colour quality, the trichromatic equation is divided on both sides by C. Then the equation reduces to A- 10 1.0 (C) ,r ,(R)+ g ,(G)+ b ,(B) where r = R / R+G+B, g = G / R+G+B, b=B / R+G+B andr+g+b = l Here, r,g and b are called the chromaticity coordinates of C. A s r + g + b = 1, C is dependent only on two independent coordinates. Therefore, C may be represented graphically on a two dimensional diagram known a s the chromaticity diagram or chromaticity chart. In the chromaticity diagram, r is taken on the X axis, and g on the Y axis, and the colour C is located on the diagram by its coordinates (r,g),and the third coordinate b b=1-(r+g) A-11 A.3

.On this diagram, the coordinates of matching stimuli (R) (G) and (B) are (1.O, 0.0), (0.0, 1.O) and (0.0, 0.0) respectively. A.4 Colour Coordinates from Spectral Composition Let r; g;, bAbe the spectral tristimulus values or the distribution coefficients which would match each wavelength in terms of the instrument stimuli (R), (G) and (B). Ex is the spectral energy distribution of C, then the tristimulus values needed to match C can be arrived a t as follows:

m

B=~E,~,cu 0

The CIE (ROB)System of Colour Measurement The C.1.E in 1931 introduced the standardisation of the values of 6 , Sr and 5 in terms of a C.I.E.standard observer. This was done by identifying the matching stimuli (R), (G) and (B) with the monochromatic wavelength - 700 nm (R), 546.1 nm (G) and 435.8 nm (B)respectively. - n I 3st of 1 e applications, the integrals given under equations A- 12, A- 13 and A- 14 may be replaced by discrete summation k5

*

The uncertainties associated with visual colorimetry were removed by the specification of the CIE standard observer. By the use of tabulated tristhulus values of and q,the chromaticity coordinates of the stimulus in the C.I.E. (RGB) system can be straightaway calculated. This provides the physical basis of colorimetry. The only physical measurement involved in this technique is the determination of E va- les w'5 the help of a spectroradiometer.

c,

326


A.6

The CIE (XYZ) System

The CIE [RGB]system has one major drawback. As the matching stimuli [R], [GI [B] are monochromatic spectral colours, the trichromatic equations for other spectral radiations involve negative quantities. The negative signs occur not only in the spectral tristimulus values but also in the chromaticity coordinates. As such, it was thought advisable to avoid these negative quantities in the specifications of colours. Therefore CIE in 1931 recommended the adoption of an allpositive system a s the standard reference system for colour specification. Thus the CIE adopted a system of non-physical and [Z] in the [RGB] system in such a way reference stimuli [XI that all real physical stimuli could be expressed entirely in terms of positive stimulus values and chromaticity coordinates. As the (Y) and (2)are known, i.e. [XI, M chromaticity coordinates of (X), and [Z],in the [RGB] system, the spectral tristimulus values r:, , 9; ,

[v

and 5; for the equal-energy spectra may be transformed to the corresponding values in the CIE (XYZ) system. The spectral tristimulus values in the CIE (XYZ)are usually denoted by 4, &, 6 . The so called chromaticity coordinates x y z of the spectral colours may be calculated as

The chromaticity coordinates of any other colour in terms of its spectral composition E may be computed in a direct manner as

IE,Z.&

O

Z=

'

-

0

and x = X/X+Y+Z,....etc

B.1

Radar Equation The equation relating the transmitted power, received power and the distance to the target known as the radar (range)equation can be derived as follows: Let P,be the power transmitted (peak)by the radar transmitting antenna, assumed to be isotropic. The power density (PD) at a distance R from the transmitter is given by

Normally, the antenna will be directive. If the power of the antenna along the direction of the target is G, the power density at the target (power per unit area) is given by

The transmitted signal which is received by the target undergoes reflection at the target and comes back to the transmitter as an echo. The power density of the signal back at the radar is given by

where a is known as the radar cross section (RCS)of the target. The echo is picked up by the radar's antenna. The power received by the radar is given by

where A, is the effective receiving antenna area. The radar equation in its simplest form is given by

Solving for R,

328


The maximum range of the radar k , corresponds to the value of R at the minimum detectable (received)power so that

where

Smhis the minimum detectable received signal. From the above equation we can get expressions for various other characteristics.

can also be shown a s equal to

where his the wavelength of the radar electromagnetic energy related by

In order to double the range the power of the transmitter has to be raised 16 times. As the size of the antenna is increased the transmitted power is concentrated to a narrow beam and simultaneously there will be a larger capture area and there will be increase in the range.

A Absorption 211,213,219 materials 252 solar 283 Adaptation 43 Advanced technology warfare 2-1 Advertisement 33 Aerial observation photographs 1, 2 Air defence systems 9 warfare 12 Antireflective coatings 284 Antishine devices 88 Antisubmarine warfare 1 3 Animal colouration 21 Arboriculture 63 Armoured personnel camer 22 Armour piercing fin stabilised discarding sabot projectile 8 Artificial intelligence 8 Artillery 9

B

auditory 35 chromogenic materials 291 evaluation 91 infrared 100 radiation 101 visual 35,41 psychological 84 reflectance 88 screens 163 Coating materials 274,278 antireflective 283 composites 289 surface 289 Colour, colouration 45 disruptive 25 matching 18,68 measurement 46 resemblances 19 warning 33 Concealment 18,22 colouration 30,32 cryptic 30 offence 31 Contour obliteration 71 Contrast 47 temperature effects 114 Countershading 21-24,69

Blending 68 Brightness 47

C Camera 55 Camouflage 1, 18,30 acoustic 290

Deception 3,73,229 background 241 equipment 233-237 psychological aspects 232

330 Introduction to Carnoufloge & Deception

Decoys 235 chaff 242 IR & radar 242 tactical (TALD) 246 Dielectric materials 257 artificial 258 Disguise 34 Disinforrnation 23 1 Disruptive colouration 70 conlast 27 pattern 28 Diverting attention 34 Dual texture gradient 76 Dummies 234 Eductor diffuser 155 Electronic warfare 15 Elimination of shadow 72 Evaluation, see camouflage Ferrites 254 Fiballoy 303 Foams 88 aqueous284 structure 286 Flares 244 Generation of disruptive patterns 77 Globar 102 Graded absorbers 257 Gyromagnetic ratio 255

Heterocyclics 259 Hexachloroethane 287

Infrared sensors 1 16 camouflage materials 267 sensing systems 1 18 photography 128 signatures 140 suppression 151 attenuation 267 telescopes 126 Instruments 48 Image intensifiers 49, 133 amplification 50 rangefinding 51 Irradiance 103 Jungle area pigments see pigments Kevlar 293 Kirchofls law 105 Laser 15 Loss tangent 253,261 Luminescent materials 292

Metamers 91 Microwaves 170 generation 173 devices 174 sensors 176 camouflaging 73 Mimicry 35 Multispectral camouflage 288

Navalwasfare '12 Nets 78,288 manufacturers 83 properties 79 Nernst glower 102 Noise equivalent power 125 Nuclear propulsion 13 Object resemblance 34 Obscuration 268 Obscurants 66 Optical detector 130 principles 71 texture 47 Paints 279 Passive countermeasures 160 Pattern painting 73 Phased array radar 175 Platforms 52 Photography 52 Photo-reconnaissance 53 factors 54 Pigments 275,281 jungle area 282 desert 283 ocean 283 Planck's law 106 Plantation 64 Plume signature, see suppression Polymers, conducting 259 chiral & two dimensional 262, 264 materials 293

RCS I98 aircraft 203 flat plate 200 reduction 2I0 ship 203 tank 207 Radar 3, 186 absorption 2 13 materials 252 air defence 194 cross-section 170, 195 method of prediction 199 frequencies 172 instrumen tation 209 measurements 2 10 tracking 187-191 weapon locating 194 Radiance 103 Radiant absorbtence 104 emnittance 103 Intensity 103

photon emittance 103 reflectance 103 Rainfall zone 63 Reactive countermeasures 161 SateUtes 60 Scanners 59 Screens 64 dallanbach 2 18 jaurnann 2.18 magnetic 219 mcmillan 217 salisbury 2 15 screens 67 Signature electric 310

332

Introduction to camouflage 86 Deception

infrared 309 magnetic 309 Smoke 287 Sensors 48, 59 Spectral reflectivity 277 Stefan-Boltzmann law 105 Stealth 300-308 aircraft 302 airship 312 helicopter 31 1 missiles 312 submarine 31 1 tank 310 warships 306 Submarine 13 Suppression hot parts 158 plume signature, 157 shadow 30 tank 159 Surveillance 234,239 Target characteristics 85 Target aquisition systems 10 illumination 52 Tarps 164 Thermal imagers 132 Tracking 52 Transferred electronic devices

175 Tungsten l.amp 101 Ultraviolet region 170 Urethane linkage 85 Vaporization of fuel 84 Vidicon 127 Visible region 60 Visual perception 84 Visually evoked potential 87 Vulnerability acoustic 241 laser detectiog 240 radar detection 240 Wake 14 Warfare biological 10 chemical 10 nuclear 10 Waveguides 170,173 X-ray 101 Xenon 110 Xenon arc lamp 101

ISBN: 81-86514-02-7

Defena Scknti& Information& IkumenFaaiwr Defence RweaKh & Dsrrelopment OqpksPlom Mintsty of Defence Metcalfe H-, Oelhi I I0 054

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