Radio Navigation Aids for Senior Airport Managers

ZULFIQAR ALI MIRANI

Radio Navigation Aids for Sr. Airport Managers

Info: [email protected]

Radio Navigational Aids For Senior Airport Management Course ZULFIQAR ALI MIIRANI

School Of Electronics Engineering

Civil Aviation Training Institute Hyderabad- Pakistan

CONTENTS PART ONE: BASIC COMMUNICATION CONCEPTS A. Fundamental Principles B. Radio Wave Propagation C. Radio Transmission PART TWO: RADIO NAV-AIDS • • • •

NDB VOR DME ILS

PART THREE: RADAR • • •

Primary Surveillance Radar Secondary Surveillance Radar Radar Display System

PART FOUR: CNS/ATM • • • • • •

Background FANS ATM Future Communication Future Navigation Future Surveillance

By: ZULFIQAR ALI MIIRANI Senior Electronics Engineer Civil Aviation Authority- Pakistan Email: Website:

[email protected] http://geocities.com/zamsnet http://geocities.com/zmiiranii

Contents: Radio Navigational Aids for Sr. APM Course

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Radio Navigational Aids for Sr.APM Course A. FUNDAMENTAL PRINCIPLES Alternating Current: An electric current whose intensity and direction change is called Alternating Current. An a-c wave completes ONE cycle after it has made TWO alterations, one in positive direction and one in negative direction. The maximum amount of current occurs at each 90 degrees and 270 degrees instant of the a-c cycle. It repeats this cycle pattern regularly. (Ref. Fig: 1-1)

Figure 1-1: Alternating Current Frequency: Number of cycles in one second (of a radio wave) is called its frequency. It is expressed in Hertz (Hz) or cycles per second (c/s or cps). The entire frequency coverage is divided into sections called frequency spectrum. The division of frequency spectrum is given in Table 1-1 below. Table 1-1: Division of Radio Frequency spectrum: Description Very Low Frequency Low Frequency Medium Frequency High Frequency Very High Frequency Ultra High Frequency Super High Frequency Extremely High Frequency 1

VLF LF MF HF VHF UHF SHF EHF

Frequency1

Wavelength (meters)

Below 30 KHz 30 – 300 KHz 300 – 3,000 KHz 3,000 – 30,000 KHz 30 – 300 MHz 300 – 3,000 MHz 3 – 30 GHz 30 – 300 GHz

30,000 – 10,000 10,000 – 1,000 1000 – 100 100 – 10 10 – 1 1 - 0.1 0.1 – 0.01 0.01 – 0.001

kHz (Kilo Hetz) = 1000 Hz MHz (Mega Hertz) = 1,000,000 Hz or 1,000 kHz GHz (Giga Hertz) = 1,000,000,000 Hz or 1,000 MHz

PART ONE: Basic Communication Concepts

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Radio Navigational Aids for Sr.APM Course

Wavelength: The distance that a wave travels in the time of one cycle is called its wavelength. It is expressed in meters. Speed: Radio waves travel at the speed as of the light i,e 186,000 miles/sec or 300,000 km/sec or 162,000 NM/sec. Frequency, Wavelength and Speed relationship: Figure 1-2, given below, illustrates the relationship between Frequency, Wavelength and the Speed. From the diagram it can be seen that at a frequency of 1 Hz, the length of one cycle (i,e wavelength) is 3 x 108 meters. In case of frequency of 4 Hz, the length of one cycle is 3 x 108 divided by 4; because in one second four cycles or waves are radiated. Where as, in the first case only, one cycle is radiated in one second.

Figure 1-2 Relationship between Frequency, Wavelength Mathematically, relationship between Frequency, Wavelength and the Speed is given by: V=f xλ Where ‘V’ is the velocity (speed) of the light, ‘f’ is frequency in Hz (or cycles per second) and ‘ λ’ is the wavelength in meters. Frequency-Wavelength inter-relationship for radio waves as driven from the above relationship is: Wavelength = 300,000,000 / frequency (in Hz) PART ONE: Basic Communication Concepts

(I) 1-2

Radio Navigational Aids for Sr.APM Course Wavelength = 300,000/ frequency (in kHz) Wavelength = 300/ frequency (in MHz)

(II) (III)

Frequency (in Hz)= 300,000,000 / Wavelength Frequency (in kHz)= 300,000 / Wavelength Frequency (in MHz) = 300,000,000 / Wavelength

(IV) (V) (VI)

Phase: Any stage in a cycle of an alternating current or radio wave is referred to as its phase. It is described as an angle between 00 and 3600, since one cycle (or plane) is divided into 3600. Thus, at a point on an a-c wave at which maximum positive amplitude occurs when the phase is 900. Similarly when the phase is 2700 the amplitude of the wave is minimum (or maximum negative value). It is possible that the two waves are received simultaneously having difference in phase. For example, at a given reference point, one wave may be at maximum positive value, whilst the other is at zero, as illustrated in the figure 1-3 below. The two waves so arrived will tend to cancel each other resulting in weaker final response. It is also possible that the two waves may be received out of phase (with phase difference of 1800) with equal amplitudes.

Figure 1-3: Phase Relationship Human voice contains signals within frequency range between 400 Hz and 4 KHz; power rapidly drops above 4 KHz. Limit of human hearing: Human ear is responsive to frequencies from about 20 Hz to 20 KHz. in terms of frequency, is 20Hz to 20 KHz.


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Knots and Nautical Miles All navigation uses the Nautical Mile as the unit of distance. Traditionally a nautical mile is 6,080 feet but more precisely 6,076.11549 feet. In metric measurement it is 1,852 meters, which is one minute of arc of a great circle of the Earth. Even under the metric system, the unit of distance for navigation is still called the nautical mile. One knot converted to miles per hour (mph) would be approximately 1.15 mph. One mile per hour would be 0.868 knots.

Frequency allocations of radio spectrum to all users are made by the International Telecommunication Union (ITU), which is a specialized agency of United Nations. ITU’s prime objective is to standardize techniques and operations in telecommunications to achieve end-toend compatibility of international telecommunication connections. International Telegraph and Telephone Consultative Committee (CCITT) is replaced by ITU-T sector which had essentially the same objectives and charter. Some of the allocations to various users of the spectrum are stated in Table 1-2. Some portions among frequency spectrum are also allocated for use of International Civil Aviation operations. Further allocations of the available spectrum are made by ICAO for various applications, as stated in Table 1-3. Table 1-2: Frequency allocations for communication (in General): Description

Frequency

Radio Bands Trans-ocianic communication Medium Frequency Broadcasting Commercial Broadcasting International Broadcasting Land Mobile Television (Low Band: Ch 2 - 6) F M Broadcasting Land Mobile Television (High Band: Ch 7 - 13 ) Television (UHF Band: Ch 14 - 83)

125 – 525 20 – 550 535 – 1605 550 – 1600 5.95 – 29.1 25.01 – 49.60 54 – 88 88 – 108 151.49 – 173.40 174 – 216 470 – 890


KHz KHz KHz KHz MHz MHz MHz MHz MHz MHz MHz

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Radio Navigational Aids for Sr.APM Course Table 1-3: Frequency allocations for Air Navigation and Aviation Communication NavAids: ILS Localizer VOR ILS Glide Slope NDB

108 – 112 112 – 118 328 – 332 200 – 1750

MHz MHz MHz KHz

118 - 132 132 – 152

MHz MHz

Communication: VHF (Low Band) VHF (High Band)

Analog and Digital Signals: The principal feature of analog signals is that they are continuous. In contrast, digital signals consist of values measured at discrete intervals. B. RADIO WAVE PROPAGATION Radio Waves: High frequency currents when pass through a radiator (antenna) produce magnetic and electric fields which radiates in all directions over a long distance. The waves so produced are called Radio Waves or Electromagnetic waves. The direction of propagation of radio waves is perpendicular to both electric and magnetic fields. The propagation of radio waves is illustrated in figure 1-4

Figure 1-4: Propagation of Radio Waves Radio waves are classified in three types: 1. Surface or Ground waves 2. Space waves 3. Sky waves


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Radio Navigational Aids for Sr.APM Course 1. Surface Wave (or Ground Wave) is produced by energy that travels close to the ground and guided to follow the curvature of the earth. Ground wave provides useful communication up to about 400 miles (640 kms). 2. Space Wave travels on a straight path between transmitter and receiver, both situated above surface of the earth. Space Wave is comprised of Direct wave and Ground reflected wave and is limited to line-of-sight transmission. Reflected Wave can cause reception problem if the phase of the two received components is not the same. A complete cancellation of the signal would occur if the two waves arrived 1800 out of phase with equal amplitudes. Such phenomenon is known as ‘Fading’. 3. Sky Wave is transmitted in an upward direction from the earth and returned/reflected back towards the earth by the atmospheric layer known as ‘Ionosphere’. It provides reliable communication up to a distance of about 4000 mile (6400 kms). Ionosphere is a region of the earth's atmosphere where ionization is caused by incoming solar radiation. Layers of ionosphere: There are FOUR layers of ionosphere identified as D, E, F1 and F2. D layer: Exists between 50 – 90 kms above the earth. It disappears after the sun set. E layer: Exists between 90 – 140 kms above the earth. It disappears rapidly after the sun set. F1 layer: Exists between 140 – 250 kms above the earth. It merges with F2 after sun set. F2 layer: Exists day and night. It appears between 150 – 250 kms above the earth in night and 250 – 300 kms in day time. Density of ions in the upper most ionosphere layer is very high which becomes lower while extending toward the earth. Radio wave return to the earth depends upon Ion Density of in the Ionosphere, Frequency of radio waves and Angle of transmission. The refractive ability of the ionosphere increases with the degree of ionization. Abnormally high densities occur during times of peak sunspot activity. The increased ionization during the day is responsible for several important changes in skywave transmission. It causes the sky wave to be returned to the earth nearer to the point of transmission. The extra ionization increases absorption of energy from the sky wave. It could, at times, cause virtually total loss of energy. Absorption usually reduces the effective day light communications range of low-frequency and medium-frequency transmission to groundwave ranges. High Frequency Communication:

HF propagation is characterized by Ground wave and Sky wave components. HF is preferred over VHF and UHF point-to-point communication because HF band frequencies take advantage of sky wave propagation and permit reliable communication up to 4000 miles (6400 Km).


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Radio Navigational Aids for Sr.APM Course Line Of Sight (LOS) Propagation, of an electromagnetic wave, propagation is that in which the direct transmission path from the transmitter to the receiver is unobstructed. The need for LOS propagation is most critical at very high frequencies (frequencies in VHF band and beyond). VHF Communication: a) b) c) d) e) f) g)

Ground wave range of VHF is limited because of high ground absorption at these frequencies. Transmission in VHF band is radiated in a direct path through the atmosphere to the Receiver antenna and therefore called line-of-sight transmission. Transmitting and Receiving points should be sufficiently high to provide transmission path in VHF band communication. Curvature of earth and intervening terrain must be taken into account while calculating VHF range transmission. Obstructions, such as mountains, forests and tall buildings cause signals to be sharply attenuated or to disappear entirely when they come between Transmitter and Receiver. There is no static, in VHF spectrum to contend with, hence, communication may be carried out on even during severe electric storms. VHF signal, however, are subjected to man-made electrical noises / interferences producing the signals of the same frequencies. This type of interference can be eliminated or greatly reduced by proper shielding or by special noise filters.

C. RADIO TRANSMISSION Transmission system deals with the production, transport and delivery of information (or intelligence) from source to destination. A basic communication system include a source (of information), a transmission medium (for transportation) and destination,as shown in the figure below.

Source converts human intelligence into electrical signals; Transmission medium is responsible to carry this information to destination where it is reproduced into its original form. The information may be voice or data, transmission media may be a single channel or network or multiple networks.

Transmission Media: There are a number of options from which we can choose in selecting a transmission medium. All make use of some form of electromagnetic energy, which can be in the specific form of electricity, radio or light. There are two broad categories of transmission media: 1. Guided or Conducted Transmission Media 2. Un-guided or Radiated Transmission Media Guided media transmission is guided or directed through a conductor or any medium that conducts, or carries on, the signal to the destination. Some examples of guided media are:


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Radio Navigational Aids for Sr.APM Course 1. Twisted Pair

Unshielded Twisted Pair

Shielded Twisted Pair 2. Coaxial cable

3. Optical Fiber:

Un-guided or Radiated media don't make use of conductors. Rather, the signal simply radiates through space between transmitter and receiver, for example: 1. Microwave 2. Satellite 3. Infrared Sound waves (or low frequency intelligence) can not provide efficient communication for long distance for variety of reasons, such as: PART ONE: Basic Communication Concepts

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-

Tx/Rx system requires a huge antenna in length. There will be Jumbling of signals transmitted in a close range. Multiple channels can not be accommodated for Frequency Band limitation. Chances of addition of Noise (or radio interference) are highest since there may be lot of sources, of low frequency generation, found in surrounding.

Higher frequencies provide efficient communication over lower frequencies in radio transmission. They need smaller antennas and accommodate more channels. Since sound waves (or low frequency intelligence) can not travel for long distance, they require a high frequency ‘carrier’ for transportation. The process of impressing a low-frequency intelligence (or information) onto a high frequency ‘carrier’ is called ‘Modulation’. There are three different characteristics of the carrier which are modified so as to allow it to carry intelligence over long distances. They are: 1. Amplitude 2. Frequency 3. Phase Amplitude modulation: Amplitude modulation (AM) is the modulation process in which the intensity, or amplitude, of the carrier wave varies in accordance with the modulating (intelligence) signal. When the carrier is thus modulated, a fraction of the power is converted to sidebands extending above and below the carrier frequency by an amount equal to the highest modulating frequency. HF SSB radio transmission The High Frequency (HF) band is the ranges of frequencies between 3 to 30 MHz. HF radios usually include a frequency range of 2 to 30 MHz. Single Sideband (SSB) is the term used to describe the method of compressing the transmitted information, voice or data, into a compact signal. This has the benefit of reducing the power required to send a signal over a certain distance. This form of transmission uses only half of the radio bandwidth that AM radios use (double sideband). Since only one sideband signal is transmitted, SSB allows more channels for communication within the HF spectrum. HF SSB radios are primarily used for long-range communications where distances of 3000 km and more are possible. Obstructions such as buildings and mountains have little effect on longrange communications. HF radio can cover such large distances because of the way that the transmitted radio signal propagates. HF radio waves are propagated simultaneously by ground, direct and sky waves. Frequency Modulation: If the frequency of the carrier wave is varied in accordance with the change in modulating signal, the process is called frequency modulation (FM), Its principal application is also in radio communication, where it offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. The FM band has become the


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Radio Navigational Aids for Sr.APM Course choice of music listeners because of its low-noise, wide-bandwidth qualities; it is also used for the audio portion of a television broadcast. Phase modulation: Phase modulation is so called because the angle (phase) of the sinewave carrier is changed by the modulating wave. PM technique is very widely used in carrying digital signals over long distances. Pulse Modulation: Pulse modulation involves modulating a carrier that is a train of regularly recurrent pulses. The modulation might vary the amplitude (PAM or pulse amplitude modulation), the duration (PDM or pulse duration modulation), or the presence of the pulses (PCM or pulse code modulation). PCM can be used to send digital data; audio signals on a compact disc use pulse code modulation. Essential Elements of Radio Transmitter and Receiver: Main components of a Transmitter are: a) b) c) d) e)

Microphone Audio Amplifier Oscillator Modulator Antenna

Main components of a Receiver are: a) b) c) d) e)

Antenna Selector and RF Amplifier Detector Audio Amplifier Loud Speaker

Microphone converts audio or sound waves into electrical signals. Oscillator in a transmitter is used to generate radio or carrier signal. Modulation is a process of superimposing of audio onto a carrier signal. Antenna is to radiate or intercept radio waves. Demodulation is a process of retrieving original information from a carrier signal. Detector in a receiver is used to extract audio/information from a carrier signal. Loud Speaker converts electrical signal into sound or audio waves. Morse code is representation of alphabetical characters into the form of Dashes and Dots.


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PART TWO: RADIO NAVIGATION AIDS Radio Navigation Systems En-route Navigation • • •

NDB (Radio Beacons) VOR DME

Aids to Approach, Landing and Departure •

ILS

Non Directional Beacon (NDB) Purpose: It is used with direction finding equipment in the aircraft to provide bearing information of a location on the air route or of an airport. The NDB equipment is installed en-route areas as well as on the airports to provide navigational guidance to the pilot. Operating Frequency: ICAO has assigned Low and Medium Frequency band of 200 – 1750 KHz for NDB operation; where as most of NDB equipments are found operating within frequency band of 200-525 KHz. Construction: NDB consists of 1. LF/MF Transmitter 2. LF/MF Antenna and 3. Monitor Transmission: It radiates a non-directional pattern permitting reception from any point within service range of the facility (usually 200 NM). Station identification code in the form of two letter Morse Code is also transmitted by the NDB. Working Principle: NDB transmitter radiates an RF signal that is intercepted by ADF (Automatic Direction Finder) on an aircraft when tuned to its frequency. ADF receiver uses a rotatable loop antenna that gives a figure of eight (8) pattern and a fixed sense antenna that gives an omni directional pattern. The two patterns given by the two antennas are to produce a resultant pattern called “cardioid or heart shaped pattern”. The phenomenon is illustrated in Figure 2-1. The loop antenna senses the direction of the station but cannot determine whether the bearing is TO or FROM the station. In other words there remains 1800 phase ambiguity which is solved by the sense antenna. It is because the basic loop antenna has two maximum and minimum positions; it is, therefore, difficult to establish exact direction of the transmitter. Where as direction, in “cardioid pattern”, can easily be established since it has only one maximum and one

Part TWO: Navigation Aids

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Radio Navigational Aids for Sr.APM Course minimum or null points. The airborne direction finder equipment tries to locate the null point since it is easier to find out as compared to the maximum point.

Figure 2-1: NDB Signal’s Patterns Airborne Indication: An airborne radio direction finding (RDF) equipment once tuned to the signal indicates bearing of the NDB transmitter with respect to aircraft heading. Bearing Indicator displays the bearing of the station relative to the nose (heading) of the aircraft. Relative Bearing is the angle formed by the line drawn through the center line of the aircraft and a line drawn from the aircraft to the radio station. Magnetic Bearing is the angle formed by a line drawn from aircraft to the radio station and a line drawn from the aircraft to magnetic north (Bearing to station). Magnetic Bearing = Magnetic Heading + Relative Bearing.

Figure 2-2: Magnetic and Relative Bearing


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Radio Navigational Aids for Sr.APM Course Airborne equipments that interacts with NDB (ground station) is called Automatic Direction Finder (A.D.F) and indicates bearing on a full 360 degree radial. Figure 2-3 shows pictures of ADF.

Figure 2-3: Pictures of ADF

VHF Omni Range (V.O.R) Purpose: It is a radio aid that provides, with inter action of airborne equipment, information about azimuth, the course and TO-FROM to the pilot. AZIMUTH in VOR is a clockwise angle between magnetic north and the line connecting the VOR and the aircraft. The indication is displayed on an “Omni Bearing Indicator” in the aircraft. The COURSE is the information whether aircraft is flying to the left or right of, or exactly on the pre-selected course line. The course information is displayed on a “Flight Path Deviation Indicator”. TO-FROM indication tells the pilot whether an aircraft is approaching to or moving away from VOR stations. Operating Frequency: VOR is assigned to operate in VHF band range from 112 – 118 MHz. Transmission: It radiates two radio signals modulated at 30 Hz, a reference signal which has constant phase through out 360 degrees and a variable signal whose phase varies with variation in azimuth. Station identification code, consisting two or three letters, in the form of Morse code is also transmitted by the VOR. Working principle: The VOR receiver in the aircraft receives the two signals (reference and variable). Difference of phase of the two signals is compared and electrically translated into the number of degrees from


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Radio Navigational Aids for Sr.APM Course the magnetic north. Variable signal is set such that at magnetic north both the signals are exactly in phase. In other directions, positive maximum of variable signal occurs some time later than the maximum of reference signal. Fraction of the cycle which elapses between the occurrence of the two maxima at any point in azimuth, will identify the azimuth angle of that point. As VHF transmissions are line-of-sight; the ground to air range, thus, depends on the elevation of the transmitter site, the height of the aircraft and the power output. Any obstacles (buildings, mountains or other terrain features, including the curvature of the earth) block VOR signals and restrict the distance over which they are received at a given altitude. The VOR is usually located at airfields but as they serve to define designated air routes [airways] they are also installed away from airfields, on the key air route locations Here is yet another example to explain the working of VOR. Imagine a wheel with 360 spokes, at one degree azimuth spacing, with the VOR beacon being the hub. The spokes are numbered clockwise from one to 360 and each spoke or radial represents a magnetic bearing from the VOR beacon. The airborne navigation circuitry measures the phase angle difference between the directional signal phase received and the reference signal phase and interprets that as the angular, or 'radial', indication currently being received. Radials are identified by magnetic bearing – e.g. the 30° radial – and thus form the basis for VOR, and designated air route, navigation. Essentially the system indicates a line of position, from the selected VOR, on which the aircraft is located at any time. Airborne Indications: Indications of VOR information are given on airborne indicators as follows. Course Selector is used in conjunction with left-right needle and To-From indicator to display VOR information. This type of display is called “ Omni Bearing Indicator”. The course selection can be displayed on a 360 degree azimuth dial or a 3 digit counter. The airborne system utilizing the VOR transmissions usually consists of an antenna, a VHF receiver and the separate VOR navigation indicator or 'Omni Bearing Indicator [OBI].

Figure2-4: Pictures of VOR Airborne Indicator A basic Omni Bearing Indicator, as shown above, has a manually operated radial or 'omni Bearing Selector [OBS] which rotates an azimuth ring marked from 0° to 355°. The OBS selected radial – is indicated by the arrow at the top and the reciprocal bearing is indicated by


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Radio Navigational Aids for Sr.APM Course the bottom arrow. The other features of a basic OBI are the TO-FROM indicators, a deviation bar, a deviation indicator needle and a NAV / OFF alarm flag. The Course Deviation Indicator or CDI: The deviation bar and the deviation indicator needle together form the Course Deviation Indicator. If the needle is over the centre point the aircraft is then located at some position along the selected radial – or its reciprocal. The five division marks or dots either side of the centre point are spaced at two degree intervals, thus if the needle is over the third mark, left or right of centre, the aircraft is positioned at a radial six degrees in azimuth from the selected radial, or its reciprocal. [Actually the aircraft is at the centre mark and the needle indicates the position of the selected radial]. Full travel of the needle from the centre to either side represents 10° – or more – of azimuth.

When the aircraft heading agrees generally with the track selector, the track deviation bar (TB) shows the pilot the position relative to the track selected and indicates whether the radial is to the right or left. The TO - FROM indications on the OBI are dependent on the aircraft's position relative to a ground baseline, formed perpendicular to the selected radial and passing through the beacon site. Unlike the NDB the indication is completely independent of the aircraft's heading. The navigation circuitry compares the difference between the radial being received and the radial selected. If the aircraft is located anywhere within range on the radial side of the baseline the 'FROM' indication will be displayed on the OBI and, if located within range on the reciprocal side, the 'TO' indication will be displayed. For example if 040° radial is selected on the OBI, the ground baseline is established between 310° and 130°, as shown in Figure 2-6. If the radial received indicates the aircraft is anywhere in the FROM region as illustrated in the figure and no matter whether it is headed towards or away from the VOR, or in any direction whatsoever, the OBI will display 'FROM'. Similarly if it is in the TO region the OBI will display 'TO' no matter which direction the aircraft is headed. The aircrafts 1 and 2 in the figure will, thus, indicate ‘FROM’ and aircrafts 3 and 4 will display ‘TO’ on the OBI. (Ref: Figure 2-6) There are two areas of ambiguity – near bearings at right angles to the radial [e.g. shown at 120° and 300°] – where the OBI will give fluctuating indications, or display the 'OFF' flag.


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Figure 2-6: VOR TO-FROM Indication If the aircraft is out of station range and cannot receive a reliable, usable signal the TOFROM/OFF indicator displays OFF. Also, the OFF flag is displayed when the aircraft is directly over the station, when abeam of the station in the area of ambiguity (i.e., directly on either side of the station) or when beyond the reception range of the station selected.

Distance Measuring Equipment (DME) The DME system is to provide continuous and accurate indication of the slant range distance (expressed in nautical miles) of an equipped from an equipped ground reference point (i,e ground DME facility). System Components: The system consists of two basic components, one fitted into the aircraft and the other installed on the ground. The aircraft equipment is referred to as INTRROGATOR and the ground component as TRANSPONDER. Operating Frequencies: The system operates within frequency band of 960 MHz to 1215 MHz. The interrogation and Reply frequencies are assigned frequencies with, at least, 1 MHz separation. The DME operates in the ultra-high frequency (UHF) band and therefore is restricted to line-ofsight transmission. Principle of Operation: The interrogator, from the aircraft, transmits interrogation pulses to the pre-selected ground station from where transponder returns the signal synchronized with interrogations. The time elapsed between interrogation and the arrival of response is measured and transformed into distance. The transmission is composed of pulse group with a pre-arranged spacing (i,e 12 microseconds) between the pulses of the two groups.


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Figure 2-7: DME System Diagram showing Airborne and Ground Elements Both receivers, the ground and airborne, employ pulse decoders which are adjusted to pass only pulse pairs of prescribed spacing. The main purpose of twin pulse technique is to discriminate against pulse interference as might be produced by the other extraneous sources. The ground station receives the transmission (interrogation) from the aircraft, retain that transmission for 50 microseconds and then transmit a reply to the query of the aircraft. The purpose of the delay of 50 microseconds is to eliminate the error in range calculation, as various systems may take different signal processing time. The distance measurement is a function of both ground and airborne equipments, where: Airborne Equipment a) b) c) d) e) f)

Produce and transmit a properly coded interrogation signal. Receive and process the replies transmitted by a ground transponder. Examine the coding and reject the improper signals. Sort out the correct replies. Measure the elapsed time between an interrogation and its reply and converts it into a distance information. Recognize and reproduce the identification signal transmitted by the transponder.


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Radio Navigational Aids for Sr.APM Course Whereas, the main functions of a transponder are: a) Receive interrogation from the aircraft interrogator. b) Examine the coding of interrogation pulses and initiate replies only to those which possess the proper spacing. c) Generate properly coded reply pulses pairs on the assigned reply frequency. d) Maintain 50 microseconds reply delay for system accuracy. e) Transmit identification signal when working independently. Range determination in DME: Distance measurement in DME utilizes two way travel time of the pulses and is called range time. As used in DME range time is the interval of time between transmission of an interrogation to the reception of a reply to that interrogation (exclusive of system delay and pulse pair spacing) Distance is a product of velocity and time and is given by s = c x t where ‘s’ is distance, ‘c’ is velocity of radio waves (300,000 kms/s) and ‘t’ is time. Travel Time t for One NM will be 1852 divided by 300 microseconds. Notice that range time is the time required for a signal to travel a given distance twice, therefore range time for One NM is the time it takes a signal to travel actual distance of Two NM. Range Time

= =

2 x 1852 / 300 microseconds 12.36 microseconds

Therefore Range Time for One NM is 12.36 microseconds; which is the time the signal takes to travel for two nautical miles.

Conversion of time into distance: In Figure 2-8, the system timing, are; t1 = One way travel time from aircraft to ground station t2 = System delay (50 microseconds) t3 = One way travel time from ground station to aircraft. t4 = Pulse spacing at aircraft (12 microseconds) Therefore; Range time = Total elapsed time – (t4 +t2) The distance (in NM) = Range Time divided by 12.36 microseconds Example: If total lapse time between aircraft interrogation and reply is 185.6 microsec, the distance in NM will be: Range time = 185.5 – 62 [where 62 is equal to t2 + t4] = 123.6 Part TWO: Navigation Aids

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Radio Navigational Aids for Sr.APM Course Distance

= Range time / 12.36 = 123.6 / 12.36 = 10 NM

Figure 2-8: DME ange determination principle Pulse Repetition Frequency of Interrogation: The PRF of interrogator should not exceed 30 pairs of pulses per second. Aircraft Handling Capability: The transponder equipment should be capable of handling 100 aircrafts or peak traffic which ever is less. Transmission of identification signal : All transponders are to transmit an identification signal in one of the following forms. a) An “independent identification” in Morse coed form b) An “associated” signal for transponders associated with a VHF facility (VOR or ILS localizer frequencies) which itself transmits an identity signal. The airborne receiving equipment provides automatic DME selection through a coupled VOR/lLS receiver or, in other words, selection of the appropriate VOR or ILS (LOC) frequency will automatically tune the DME.


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Radio Navigational Aids for Sr.APM Course Coverage: DME facility provides coverage up to 200 NM.

Instrument Landing System (I.L.S) ILS is a radio aid to the final approach and is used only within a short distance from the airport. Its purpose is to help the pilot land the airplane. It is very helpful when visibility is limited and the pilot cannot see the airport and runway. ILS facilities are a highly accurate and dependable means of navigating to the runway in IFR conditions. The landing path in ILS is determined by intersection of two planes a vertical plane and a horizontal plane. Horizontal plane contains information of the Central Line of a runway and Vertical plane provides Glide Path angle. When using the ILS, the pilot determines aircraft position primarily by reference to instruments. The ILS provides the lateral and vertical guidance necessary to fly a precision approach. A Precision Approach is an approved descent procedure using a navigation facility aligned with a runway where glide slope information is given. When all components of the ILS system are available, including the approved approach procedure, the pilot may execute a precision approach. ILS was developed in 1946 and was finally deemed completely developed in 1973, when the solid state systems were deployed.

Construction: ILS comprises of the following three components:


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Radio Navigational Aids for Sr.APM Course 1. Localizer 2. Glide Slope 3. Marker Beacons

LOCALIZER: Localizer is installed at the STOP END of a runway. It provides central line information to the pilot approaching the aircraft for landing. The localizer signal is transmitted at the far end of the runway. Operating Frequency: It is assigned specific transmitting frequency in the VHF band ranging between 108 MHz to 112 MHz. Localizer frequencies, however, are only on odd-tenths, with 50 kHz spacing between each frequency. Transmission: A composite field pattern is transmitted by localizer antenna which is modulated by 90 Hz and 150 Hz tones. The radiation pattern produces course sector such that one tone is predominated on one side and the other tone predominating on the opposite side. 150 Hz tone is identified as BLUE SECTOR and is always on the RIGHT of the pilot of the approaching aircraft, while 90 Hz identified as YELLOW SECTOR and is on the LEFT. The overlap between the two areas provides the on-track signal. Each localizer is identified by two or three letter Morse code designator preceded by the letter “I”, which is modulated at 1020 Hz signal and transmitted six times per minute. For example: IKC for ILS of Karachi Airport.


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Radio Navigational Aids for Sr.APM Course A voice transmission feature is also provided at each localizer as an additional means of communication with the aircraft. Airborne Localizer Receiver: The localizer signal is received in the aircraft by a localizer receiver. The localizer signal activates the vertical needle called the Track Bar (TB). Assuming a final approach track aligned north and south (see ILS Localizer Signal Pattern, figure below), an aircraft right of the extended centerline of the runway (position 1) is in the area modulated at 150 Hz. The TB is deflected to the left. Conversely, if the aircraft is in the area left of the runway centerline, the 90 Hz signal causes the TB to deflect to the right (position 2). In the overlap area, both signals apply a force to the needle, causing a partial deflection in the direction of the strongest signal. Thus, if an aircraft is approximately on the approach track bur slightly to the right, the TB is deflected slightly to the left. This indicates that a correction to the left is necessary to place the aircraft in precise alignment.

At the point where the 90 Hz and 150 Hz signals are of equal intensity, the TB is centered, indicating that the aircraft is located precisely on the approach track (position 3). When an OFF flag appears in front of the vertical needle, it indicates that the signal is too weak, and, therefore, the needle indications arc unreliable. A momentary OFF flag, or brief TB needle deflections, or both, may occur when obstructions or other aircraft pass between the transmitting antenna and the receiving aircraft.


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Radio Navigational Aids for Sr.APM Course The localizer receiver, in some airborne installations, is combined with the VOR receiver in a single unit. The two receivers share some electronic circuits and also the same frequency selector, volume control, and ON-OFF control. Coverage: Localizer coverage sector extends from the centre localizer antenna system to distance of 25 NM or 10 NM depending upon the category of the ILS used. The localizer back course is used on some, but not all ILS systems. Where the back course is approved for landing purposes, it is generally provided with a 75 MHz back marker facility or NDB located 3 to 5 NM from touchdown. The course is checked periodically to ensure that it is positioned within specified tolerances. GLIDE SLOPE: Glide slope is installed at the APPROACH END of a runway. The transmitter is located 750 to 1,250 feet (ft) down the runway from the threshold, offset 400 to 600 ft from the runway centerline. The Glide path is adjusted to project an angle of 20 (degrees) above the horizon. This angle may vary between 20 and 4.50 degrees depending upon obstructions along an approach angle. Operating Frequency: It is to operate in UHF band range between 328 to 332 MHz with a 50 kHz spacing between each channel. Each Glide Slope frequency is paired with a specific Localizer frequency thereby permitting a pilot to automatically select the current Glide Slope frequency by selecting a desired Localizer frequency Transmission: The course projected by the Glide Slope is same as of the Localizer course operating on its side, with the upper side of the course modulated at 90 Hz and the lower side at 150 Hz. The ILS glide path sector is located in the vertical plane and is divided by the radiated glide path in two parts called upper sector and lower sector, referring respectively to the sectors above and below the glide path. The sectors of Glide Slope are NOT identified as Blue and Yellow as of Localizer. AIRBORNE GLIDE PATH RECEIVER: The glide slope signal is received by a UHF receiver in the aircraft The glide slope signal activates the glide slope needle, located in conjunction with the TB (see Glide Slope Signal Pattern figure, below). There is a separate OFF flag in the navigation indicator for the glide slope needle. This flag appears when the glide slope signal is too weak. As happens with the localizer, the glide slope needle shows full deflection until the aircraft reaches the point of signal overlap. At this time, the needle shows a partial deflection in the direction of the strongest signal. When both signals are equal, the needle centers horizontally, indicating that the aircraft is precisely on the glide path. The pilot may determine precise location with respect to the approach path by referring to a single instrument because the navigation indicator provides both vertical and lateral guidance.


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Radio Navigational Aids for Sr.APM Course In the Glide Slope Signal Pattern, figure below, position 1 shows both needles centered; indicating that the aircraft is located in the center of the approach path. The indication at position-2 tells the pilot to fly down and left to correct the approach path. Position 3 shows the requirement to fly up and right to reach the proper path. The apparent sensitivity of the instrument increases as the aircraft nears the runway. The pilot must monitor it carefully to keep the needle centered. As said before, a full deflection of the needle indicates that the aircraft is either high or low but there is no indication of how high or low.

Coverage: The Glide Slope is to provide signals sufficient to allow satisfactory operation in the sector of 8 degrees on each side of the Glide path to a distance of 10 NM. MARKER BEACONS: Marker beacons associated with ILS are designated as Outer Marker (OM), Middle Marker (MM) and Inner Marker (IM) and are located along a localizer front course at specific distances from the approach end of the runway. Operating Frequency: Marker Beacons operate at a frequency of 75 MHz. Outer Marker


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Radio Navigational Aids for Sr.APM Course It is located at 4 to 7 miles from the approach end of a runway and identified by transmission of continuous dashes. The OM is modulated at 400 Hz that activates PURPLE light on Pilots instrument panel. Middle Marker MM is located at approximately 3,500 feet (1050 meters) from the approach end of runway and identified by transmission of alternating dots and dashes. The MM is modulated at 1,300 Hz that activates AMBER light on Pilots instrument panel. Inner Marker IM is identified by transmission of continuous dots transmitted at a rate of 6 dots per second. It is installed between 250 feet and 1500 feet (450 meters) from the runway threshold. The signal is modulated at 3000 Hz and activates WHITE light. Coverage: The markers are adjusted to provide coverage as specified below. IM: MM: OM:

100 – 200 meters 200 – 400 meters 400 – 400 meters

Airborne Equipment: 1. Cross Point Indicator 2. ILS Marker Receiver Cross Point Indicator: CPI is prominently located in front panel of the pilot and used to indicate ILS signals. The vertical needle in CPI indicates position of the Localizer course and tells the pilot whether aircraft is right on the central line of the runway or deviating on left or right from the central line. Horizontal needle in CPI indicates the position of the aircraft with respect to glide angle. ILS Marker Receiver: It consists of three-light indicator mounted on the instrument panel in the aircraft. Activation of these lights is controlled by the modulating frequencies of OM, MM and IM as described earlier. The use of the light indicator with aural marker receiver enables pilot to have a double check when an aircraft passes over the markers. ILS Facility Performance Categories: ILS is classified by category in accordance with the capabilities of the ground equipment. Facility Performance Category-I ILS: An ILS which provides guidance information from the coverage limit of the ILS to the point at which the localizer course line intersects the ILS glide path at a height of 60 m (200 ft) or less above the horizontal plane containing the threshold. Category I ILS provides guidance information down to a decision height (DH) of not less than 200 ft.


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Radio Navigational Aids for Sr.APM Course Facility Performance Category-II ILS: An ILS which provides guidance information from the coverage limit of the ILS to the point at which the localizer course line intersects the ILS glide path at a height of 15 m (50 ft) or less above the shorizontal plane containing the threshold. This way a DH of not less than 100 ft. on the radar altimeter is authorized for Category II ILS approaches. Facility Performance Category-III ILS: An ILS which, with the aid of ancillary equipment where necessary, provides guidance information from the coverage limit of the facility to, and along, the surface of the runway. Two ILS at opposite ends of a single runway: At a location where two separate ILS facilities serve at opposite ends of a single runway, an interlock should ensure that only the ILS serving the approach direction shall radiate.


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PART THREE: RADAR PRINCIPLES RADAR The term “RADAR” is derived from “RADIO DETECTION AND RANGING”. Radar is a method whereby radio waves are transmitted into the air in a specific direction and are received when they are reflected by an object in the path of the beam. RANGE in RADAR is determined by measuring the time, radio wave takes, from the radiation to return of its echo; whereas DIRECTION is determined from the position of antenna at the time of reception of signal. Range determination in Radar: The distance of an object from a Radar station is called “slant range” or simply “range”. Range in Radar is determined by an expression given below. Range =

cxt 2

Where ‘c’ is speed of radio waves and ‘t’ is the time elapsed from transmission of radio waves to the reception of echo. Example: If total time elapsed, from transmission of radio waves to the reception of echo, is 1000 microseconds. Velocity of radio waves is constant and given as 161,800 NM per second. Then Range =

161,800 x 1000 µ 2

Range = 80.9 or 81 NM Time elapsed from Tx to Rx of radio waves to travel for One NM is Time

= =

2 x 1 NM / 161,800 NM per sec 12.36 microseconds

Another method of range determination for the elapsed time is given below: Range = =

1000 microseconds / 12.36 microseconds 80.9 NM or 81 NM

FUNCTIONAL CLASSIFICATION OF ATC RADAR SYSTEMS: 1. 2. 3. 4.

En-route Radar Terminal Approach Radar Precision Approach Radar Ground Movement Radar

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TYPES OF RADAR 1. Primary Radar It provides “Range and Bearing” information to the Air Traffic Control Center. It does not need cooperation of the aircraft for providing information as above for it depends upon reflection of the radio waves transmitted by the system itself. 2. Secondary Radar It provides “identification and altitude” information to ground ATC. It works with cooperation of the aircraft. The information produced by the Secondary Radar is therefore function of both ground equipment and airborne equipment.

PRIMARY RADAR Basic Primary Radar System: A basic radar system is illustrated in Figure 3-1 below:

Figure 3-1 Basic Radar System Process: Pulses of electromagnetic energy are transmitted in a particular direction by radar transmitter. A portion of this energy is reflected by the objects, which comes into the path of the radar radiation, and collected by the radar receiver. The range information is then extracted from the received signal which is displayed on the Radar Scope along with bearing of the object, which is determined from the direction of the antenna at the time of reception of the signal. PRIMARY RADAR CONSTRUCTION: Practical Primary Radar system is composed of following essential components. a) Timer or Synchronizer

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Radio Navigational Aids for Sr.APM Course b) c) d) e) f) g)

Modulator Transmitter Antenna Duplexer or TR switch Receiver and Indicator or Radar Scope

Figure 3-2: Block Diagram of Primary Radar System SYSTEM OPERATION: Timer or Synchronizer generates small triggering pulses for start and control of cycle of operation. These pulses are supplied to the Modulator and Indicator units. Modulator produces larger pulses for excitation of Transmitter (Oscillator: Magnetron or Klystron). The transmitter then sends a burst of RF energy to the Duplexer unit. For transmission of RF signal Duplexer will be switched to provide passage to RF energy from transmitter to antenna, which in turn radiates the energy in the specific direction.

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Radio Navigational Aids for Sr.APM Course A small portion of this energy is reflected back by the object(s) in the path of the radar beam. This energy is collected back by the Receiver and fed to the Indicator unit. The Radar Display unit, also called Radar Scope, has a dual function: a) It measures the elapsed time between transmission and reception of radar signal and converts it into the range information, and b) Displays information into a useable form for ATC purpose. Primary Radar Indicator: Information made available by the primary radar is displayed on an indicator called Plan Position Indicator (PPI). Cathode ray Tube (CRT) is found suitable to be used as PPI to display radar information as close as the real situation. It makes interpretation of radar easier than other types of indicator. In PPI the scanning (sweep) starts from the center of the screen and moves outward. The distance between the center and the circumference of the screen represents the maximum range at which the radar is required to provide coverage. When the spot reaches the edge of the screen, it returns to the center extremely fast to start the next scan. This action is known as ‘Flyback’. To display the range of an object, the spot starts its sweep as the pulse is transmitted (by the antenna) and a ‘blip’ is shown at the time when ‘echo’ of the transmitted signal is received. The sweep is arranged to rotate in steps with the rotation of the radar antenna, to show the bearing of the objects appearing in the path of the radar beam.

Figure 3-3: Primary Radar Display (or Radar Scope) Common Terms used in Radar System:

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Radar Echo: is the signal received (reflected) from an object that appears in the path of the radar beam. Radar Blip: is a visual indication on a display of a signal reflected from an object. Range Marks: appears as concentric rings with their center at the beginning of the time base. Each range mark corresponds to a specified distance from the center of the scope say 5, 10, 15, 20, 25 and so on. The range mark generator produces a series of regularly spaced pulses at intervals corresponding to the range marks. Video map: is presentation of useful information (such as airways, reporting points, boundaries etc) on a radar scope. FACTORS AFFECTING RADAR PERFORMANCE: 1. Noise generated within the receiver 2. External Noise caused by natural phenomenon 3. Signals reflected by natural phenomenon 4. Signals reflected by land masses 5. The curvature of the earth 6. The size and shape of the object and the material of which it is made 7. The power of transmitter 8. The frequency of the transmitter 9. The sensitivity of the receiver 10. The shape and dimensions of the radar beam 11. The time interval between pulses and pulse width.

SECONDARY RADAR Secondary Radar, or Secondary Surveillance Radar (SSR) as generally called today, was originally named as IFF “Identification Friend or Foe” system. It is composed of two main equipments; one installed at Ground called ‘INTERROGATOR’ and other fitted in the aircraft called as ‘TRANSPONDER’. The system is illustrated in Figure 3-4. Operating Frequencies: 1030 MHz is used as the carrier frequency of the interrogation and 1090 MHz is used as the carrier frequency of the reply transmission. PRINCIPLE OF OPERATION: The interrogator transmits a series of pulses with specific time intervals, as standardized by ICAO, over a directional antenna. The pulses are received by the Transponder, which after fixed time delay responds with a series of pulses which are coded with information about identity and altitude of the aircraft. Interrogation Transmission: The interrogation consists of two transmitted pulses designated as P1 and P3. A control pulse P2 is transmitted following the first interrogation pulse P1. The interval between P1 and P3 determines the mode of interrogation and shall be as follows: Mode A 8 ±0.2 microseconds

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Radio Navigational Aids for Sr.APM Course Mode C 21 ±0.2 microseconds. The interval between P1 and P2 shall be 2.0 microseconds. The duration of pulses P1, P2 and P3 shall be 0.8 plus or minus 0.1microsecond.

Figure 3-4: Secondary Surveillance Radar System

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Radio Navigational Aids for Sr.APM Course Transmission Modes in SSR: Six different combinations of interrogation pulses are standardized, each having a specific meaning. These combinations are termed as MODES in SSR system. The Figure 3-5 shows the position of P1, P2 and P3 pulses for each mode of interrogation.

Figure 3-5: Modes of Interrogation in SSR Interrogation modes (ground-to-air) 1) Mode A: to elicit transponder replies for identity and surveillance. 2) Mode C: to elicit transponder replies for automatic pressure-altitude transmission and surveillance. 3) Inter-mode: a) Mode A/C/S all-cal l: to elicit replies for surveillance of Mode A/C transponders and for the acquisition of Mode S transponder. b) Mode A/C-only all-call: to elicit replies for surveillance of Mode A/C transponders; Mode S transponder does not reply. 4) Mode S: a) Mode S-only all-call: to elicit replies for acquisition of Mode S transponders.

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Radio Navigational Aids for Sr.APM Course b) Broadcast: to transmit information to all Mode S transponders. No replies are elicited. c) Selective: for surveillance of, and communication with, individual Mode S transponders. For each interrogation, a reply is elicited only from the transponder uniquely addressed by the interrogation. Reply Transmission: In Mode-A and Mode-B information of identity, which is set by the pilot, is sent to the ground interrogator. On an interrogation in Mode-C, the coded information from altimeter (pressure-altitude) is transmitted to the ground station without involvement of an action of the pilot. The reply function employ a signal comprising two ‘framing pulses’ spaced 20.3 microseconds as the most elementary code. Transponder reply format is shown in Figure 3-6. The reply of transponder contains two types of pulses: (a) Frame pulses F1 and F2 (b) Combination of Information pulses Information pulses. Information pulses are spaced in increments of 1.45 microseconds from the first framing pulse. The designation and position of these information pulses is illustrated in the following figure. Note The position of the “X” pulse is specified only as a technical standard to safeguard possible future use.

Figure 3-6: Transponder Reply Format Code nomenclature: The combination of A, B, C and D pulses, as shown in figure above, allows 4096 codes. The range of codes, in ABCD format, is from 0000 to 7777. It may be noticed that there are NO 8’s and 9’s used in the code system because each ‘ABCD” pulse group contains only three pulses which allow transmission of only three binary digits (bits) in each group. There are maximum eight (decimal) counts possible with three (binary) digits or bits which are represented from 0 through 7 (in decimal system). Reserved Codes: The following Mode A codes are reserved for special purposes:

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Code 7700: to provide recognition of an aircraft in an emergency. Code 7600: to provide recognition of an aircraft with radio communication failure. Code 7500: to provide recognition of an aircraft which is being subjected to unlawful interference. Mode A code 2000 is reserved to provide recognition of an aircraft which has not received any instructions from air traffic control units to operate the transponder. Mode A code 0000 should be reserved for allocation subject to regional agreement, as a general purpose code.

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Radio Navigation Aids for Senior Airport Managers

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