RADAR AND SENSOR SYSTEMS Basic principle of operation
Radar measurement of range, or distance, is made possible because of the properties of radiated electromagnetic energy: This energy normally travels through space in a straight line, at a constant speed, and will vary only slightly because of atmospheric and weather conditions. Electromagnetic energy travels through air at approximately the speed of light,
300,000 kilometers per second or
186,000 statute miles per second or
162,000 nautical miles per second. Reflection of electromagnetic waves The electromagnetic waves are reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction.
These principles can basically be implemented in a radar system, and allow the determination of the distance, the direction and the height of the reflecting object The radio-frequency (RF) energy emitted by the radar system is transmitted to and reflected from the reflecting object. A small portion of the reflected energy returns to the radar set. This returned energy is called an ECHO, just as it is in sound terminology. Radar sets use the echo to determine the direction and distance of the reflecting object. The word radar is a contraction of RAdio Detecting And R anging. anging. As implied by this contraction, radars are used to detect the presence of an aim (as object of detection) and to determine its location. The contraction implies that the quantity measured is range. While this is correct, modern radars are also used to measure range and angle. Although radar cannot reorganize the color of the object and resolve the detailed features of the target like the human eye, it can see through darkness, fog and rain, and over a much longer range. It can also measure the range, direction, and velocity of the target. Basic radar consists of a transmitter, a receiver, and a transmitting and receiving antenna. A very small portion of the transmitted energy is intercepted and reflected by the target. A part of the reflection is reradiated back to the radar (this is called back-reradiation), as shown in Fig. 7.1. The back-reradiation is received by the radar, amplified, and processed. The range to the target is found from the time it takes for the transmitted signal to travel to the target and back. The direction or angular position of the target is determined by the arrival angle of the returned signal. A directive antenna with a narrow beamwidth is generally used to find the direction. The relative motion of the target can be determined from the doppler shift in the carrier frequency of the returned signal.
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Draft Lecture Notes-Radar
2014
Although the basic concept is fairly simple, the actual implementation of radar could be complicated in order to obtain the information in a complex environment. A sophisticated radar is required to search, detect, and track multiple targets in a hostile environment; to identify the target from land and sea clutter; and to discern the target from its size and shape. To search and track targets would require mechanical or electronic scanning of the antenna beam. For mechanical scanning, a motor or gimbal can be used, but the speed is slow. Phased arrays can be used for electronic scanning, which has the advantages of fast speed and a stationary antenna
Fig 1 Radar and back-radiation Radar can be classified according to locations of deployment, operating functions, applications, and waveforms. 1. Locations: airborne, ground-based, ship or marine, space-based, missile or smart weapon, etc. 2. Functions: search, track, search and track 3. Applications: traffic control, weather, terrain avoidance, collision avoidance, navigation, air defense, remote sensing, imaging or mapping, surveillance, reconnaissance, missile or weapon guidance, weapon fuses, distance measurement (e.g., altimeter), intruder detection, speed measurement (police radar), etc. 4. Waveforms: pulsed, pulse compression, continuous wave (CW), frequenc y modulated Continuous wave (FMCW) Radar can also be classified as monostatic radar or bistatic radar. Monostatic radar uses a single antenna serving as a transmitting and receiving antenna. The transmitting and receiving signals are separated by a duplexer. Bistatic radar uses a separate transmitting and receiving antenna to improve the isolation between transmitter and receiver. Most radar systems are monostatic t ypes. RADAR EQUATION The radar equation gives the range in terms of the characteristics of the transmitter, receiver, antenna, target, and environment. It is a basic equation for understanding radar operation. Consider a simple system configuration, as shown in Fig 2. The radar consists of a transmitter, a receiver, and an antenna for transmitting and receiving. A duplexer is used to separate the transmitting and receiving signals. A circulator is shown in fig 2 used as a duplexer. A switch
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Draft Lecture Notes-Radar
2014
can also be used, since transmitting and receiving are operating at different times. The target could be an aircraft, missile, satellite, ship, tank, car, person, mountain, iceberg, cloud, wind, raindrop, and so on. Different targets will have different radar cross sections δs. The parameter Pt is the transmitted power and Pr is the received power. For a pulse radar, Pt is the peak pulse power. For a CW radar, it is the average power. Since the same antenna is used for transmitting and receiving, we have G Gt Gr gain of antenna Ae Aet Aer effective area of the antenna
Basic Radar system
Note that, 4 Gt 2 Aet Aet At Where is the free space wavelength,
is
the antenna efficiency and At is the antenna aperture
size Let us first assume that there is no misalignment (which means the maximum of the antenna beam is aimed at the target), no polarization mismatch, no loss in the atmosphere, and no impedance mismatch at the antenna feed. Later, a loss term will be incorporated to account for the above losses. The target is assumed to be located in the far-field region of the antenna The power density (in watts per square meter) at the target location from an isotropic antenna is given by pt power density 4 R2
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Draft Lecture Notes-Radar
2014
For a radar using a directive antenna with a gain of Gt , the power density at the target location should be increased by Gt times. We have pt Gt Power density at target location from a directive antenn a = 4 R 2 The measure of the amount of incident power intercepted by the target and reradiated back in the direction of the radar is denoted by the radar cross section , where is in square meters and is defined as power backscaterred at theradar powerdesnity at the t arg et Therefore, the backscattered power at the target location is p G Power backscattered to radar (W) = t t 2 4 R p G Power density backscattered by target and returned to radar location= t t 2 4 R 4 R2 The radar receiving antenna captures only a small portion of this backscattered power. The captured receiving power is given by; p G Aef Pr = returned power captured by radar(w)= t t 2 4 R 4 R 2 2 Gr Where Aef 4 There fore pt Gt Gr 2 pr 4 R 2 4 R2 4 But for monostatic Radar, Gr Gt pr
pt
G 2 2
4 R 2 4 R2 4 This is the Radar Equation If the minimum allowable signal power is Smin, then we have the maximum allowable range when the received signal is Smin. Let Pr= Smin 1
p G 4 t 3 4 S min 2
Then R Rmax
2
The maximum radar range Rmax is the distance beyond which the required signal is too small for the required system Radar Cross section The RCS of a target is the effective area defined as the ratio of backscattered power to the incident power density. The larger the RCS, the higher the power backscattered to the radar. The RCS depends on the actual size of the target, the shape of the target, the materials of the target, the frequency and polarization of the incident wave, and the incident and reflected angles relative to the target. The RCS can be considered as the effective area of the target. It does not necessarily have a simple relationship to the
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physical area, but the larger the target size, the larger the cross section is likely to be.
PULSE RADAR A pulse radar transmits a train of rectangular pulses, each pulse consisting of a short burst of microwave signals, as shown in Fig below
. The pulse has a width t and a pulse repetition period Tp =1/fp , where fp is the pulse repetition frequency (PRF) or pulse repetition rate. The transmitting pulse hits the target and returns to the radar at some time tR later depending on the distance, where tR is the round-trip time of a pulsed microwave signal. The target range can be determined by
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where c is the speed of light. To avoid range ambiguities, the maximum tR should be less than Tp. The maximum range without ambiguity requires
The average power is related to the peak power by
where Pt is the peak pulse power. A matched filter is normally designed to maximize the output peak signal to average noise power ratio. The ideal matched-filter receiver cannot always be exactly realized in practice but can be approximated with practical receiver circuits. For optimal performance, the pulse width is designed such that B 1 Where B is the bandwidth 7.6 CONTINUOUS-WAVE OR DOPPLER RADAR Continuous-wave or doppler radar is a simple type of radar. It can be used to detect a moving target and determine the velocity of the target. It is well known in acoustics and optics that if there is a relative movement between the source (oscillator) and the observer, an apparent shift in frequency will result. The phenomenon is called the doppler effect, and the frequency shift is the doppler shift. Doppler shift is the basis of CW or doppler radar. Consider that a radar transmitter has a frequency f0 and the relative target velocity is vr. If R is the distance from the radar to the target, the total number of wavelengths contained in the twoway round trip between the target and radar is 2 R / 0 The total angular excursion or phase made by the electromagnetic wave during its transit to and from the target is 2R 2 0 If the target is in relative motion with the radar, R and are continuously changing. The change in with respect to time gives a frequency shift d . The doppler angular frequency shift d is given by
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Draft Lecture Notes-Radar
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d 2 f d
d dt
4 dR
4
0 dt 0
vr
There fore
where f0 is the transmitting signal frequency, c is the speed of light, and vr is the relative velocity of the target. Since vr is normally much smaller than c, fd is very small unless f0 is at a high (microwave) frequency. The received signal frequency is f 0 f d . The plus sign is for an approaching target and the minus sign for a receding target. For a target that is not directly moving toward or away from a radar as shown in Fig below, the relative velocity vr may be written as vr v cos where v is the target speed and is the angle between the target trajectory and the line joining the target and radar. It can be seen that
Therefore, the Doppler shift is zero when the trajectory is perpendicular to the radar line o f sight.
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Draft Lecture Notes-Radar
2014
Doppler or CW radar block diagram. Exercise A police radar operating at 10.5 GHz is used to track a car’s speed. If a car is moving at a speed of 100 km/h and is directly aproaching the police radar, what is the doppler shift frequency in hertz? Continuous-wave radar is relatively simple as compared to pulse radar, since no pulse modulation is needed
Extra content The following figure shows the operating principle of primary radar. The radar antenna illuminates the target with a microwave signal, which is then reflected and picked up by a receiving device. The electrical signal picked up by the receiving antenna is called echo or return. The radar signal is generated by a powerful transmitter and received by a highly sensitive receiver.
Block diagram of a primary radar with the signal flow Signal Routing
The radar transmitter produces short duration high-power RF- pulses of energ y.
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The duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver. The antenna transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception. The transmitted pulses are radiated into space by the antenna as an electromagnetic wave. This wave travels in a straight line with a con stant velocity and will be reflected by an aim.
The antenna receives the back scattered echo signals.
During reception the duplexer lead the weakly echo signals to the receiver.
The hypersensitive receiver amplifies and demodulates the received RF-signals. The receiver provides video signals on the output. The indicator should present to the observer a continuous, easily understandable, graphic picture of the relative position of radar targets.
All targets produce a diffuse reflection i.e. it is reflected in a wide number of directions. The reflected signal is also called scattering. Backscatter is the term given to reflections in the opposite direction to the incident rays. Radar signals can be displayed on the traditional plan position indicator (PPI) or other more advanced radar display systems. A PPI has a rotating vector with the radar at the origin, which indicates the pointing direction of the antenna and hence the bearing of targets. It shows a map-like picture of the area covered by the radar beam. Signal Timing Most functions of a radar set are time-dependent. Time synchronization between the transmitter and receiver of a radar set is required for range measurement. Radar systems radiate each pulse during transmit time (or Pulse Width τ ), wait for returning echoes during listening or rest time, and then radiate the next pulse, as shown in figure below A so called synchronizer coordinates the timing for range determination and supplies the synchronizing signals for the radar. It sent simultaneously signals to the transmitter, which sends a new pulse, and to the indicator, and other associated circuits.
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The time between the beginning of one pulse and the start of the next pulse is called pulserepetition time (PRT) and is equal to the reciprocal of PRF as follows:
The Pulse Repetition Frequency (PRF) of the radar system is the number of pulses that are transmitted per second. The frequency of pulse transmission affects the maximum range that can be displayed . Ranging The distance of the aim is determined from the running time of the high-frequency transmitted signal and the propagation speed c0. The actual range of a target from the radar is known as slant range. Slant range is the line of sight distance between the radar and the object illuminated. While ground range is the horizontal distance between the emitter and its target and its calculation requires knowledge of the target's elevation. Since the waves travel to a target and back, the round trip time is divided by two in order to obtain the time the wave took to reach the target. Therefore the following formula arises for the slant range
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Draft Lecture Notes-Radar
2014
If the respective running time tdelay is known, then the distance R between a target and the radar set can be calculated by using this equation Maximum Unambiguous Range A problem with pulsed radars and range measurement is how to unambiguously determine the range to the target if the target returns a strong echo. This problem arises because of the fact that pulsed radars typically transmit a sequence of pulses. The radar receiver measures the time between the leading edges of the last transmitting pulse and the echo pulse. It is possible that an echo will be received from a long range target after the transmission of a second transmitting pulse.
In this case, the radar will determine the wrong time interval and therefore the wrong range. The measurement process assumes that the pulse is associated with the second transmitted pulse and declares a much reduced range for the target. This is called range ambiguity and occurs where there are strong targets at a range in excess of the pulse repetition time. The pulse repetition time defines a maximum unambiguous range. To increase the value of the unambiguous range, it is necessary to increase the PRT, this means: to reduce the PRF. Echo signals arriving after the reception time are placed either into the
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Draft Lecture Notes-Radar
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o
o
transmit time where they remain unconsidered since the radar equipment isn't ready to receive during this time, or into the following reception time where they lead to measuring failures (ambiguous returns).
The maximum unambiguous range for given radar system can be determined by using the formula:
The pulse repetition time ( PRT ) of the radar is important when determining the maximum range because target return-times that exceed the PRT of the radar system appear at incorrect locations (ranges) on the radar screen. Returns that appear at these incorrect ranges are referred as ambiguous returns or second time around (second-sweep) echoes. The pulse width τ in this equation indicates that the complete echo impulse must be received.
Radar Parameters Elevation Angle The elevation angle is the angle between the horizontal plane and the line of sight, measured in the vertical plane. Height The height of a target over the earth's surface is called height or altitude. Accuracy Accuracy is the degree of conformance between the estimated or measured position and/or the velocity of a platform at a given time and its true position or velocity Radar Resolution The target resolution of radar is its ability to distinguish between targets that are very close in either range or bearing. Weapons-control radar, which requires great precision, should be able to distinguish between targets that are only yards apart. Search radar is usually less precise and only distinguishes between targets that are hundreds of yards or even miles apart. Radar resolution is usually divided into two categories; range resolution and an gular (bearing) resolution.
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Draft Lecture Notes-Radar
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Angular Resolution Angular resolution is the minimum angular separation at which two equal targets at the same range can be separated. Range Resolution Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing but at different ranges. The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator. Therefore, the theoretical range resolution of a radar system can be calculated from the following formula:
Theoretical Maximum Radar Range equation
The radar equation represents the physical dependences of the transmit power, that is the wave propagation up to the receiving of the echo-signals. Furthermore one can assess the performance of the radar with the radar equation. The received energy is an extremely small part of the transmitted energy
The radar equation relates the important parameters affecting the received signal of radar. Now we assess what kinds of factors are expressed in this radar equation. is the peak power transmitted by the radar. This is a known value of the radar. It is important to kno Ptx directly related to the transmitted power. is the power returned to the radar from a target. This is an unknown value of the radar, Prx but it is one that is directly calculated. To detect a target, this power must be greater than the minimum detectable signal of the receiver.
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Draft Lecture Notes-Radar
2014
Antenna Gain
The antenna gain of the radar is a known value. This is a measure of the antenna's ability to focus outgoing energy into the directed beam. G
Maximum radiation int ensity Average radiation int ensity
Antenna gain describes the degree to which an antenna concentrates electromagnetic energy in a narrow angular beam. The two parameters associated with the gain of an antenna are the directive gain and directivity. The gain of an antenna serves as a figure of merit relative to an isotropic source with the directivity of an isotropic antenna being equal to 1. The power received from a given target is directly related to the square of the antenna gain, while the antenna is used both for transmitting and receiving.
Antenna Aperture Remember: the same antenna is used during transmission and reception. In case of transmission the whole energy will be processed by the antenna. In case of receiving, the antenna has got the same gain, but the antenna receives a part of the incoming energy only. But as a second effect is that of the antenna's aperture, which describes how well an antenna can pick up power from an incoming electromagnetic wave. As a receiver, antenna aperture can be visualized as the area of a circle constructed broadside to incoming radiation where all radiation passing within the circle is delivered by the antenna to a matched load. Thus incoming power density (watts per square meter) • aperture (square meters) = available power from antenna (watts). Antenna gain is directly proportional to aperture. An isotropic antenna has an aperture of λ² / 4π . An antenna with a gain of G has an aperture of G • λ² / 4π . Radar Cross Section The size and ability of a target to reflect radar energy can be summarized into a single term, σt , known as the radar cross-section RCS, which has units of m². If absolutely all of the incident
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Draft Lecture Notes-Radar
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radar energy on the target were reflected equally in all directions, then the radar cross section would be equal to the target's cross-sectional area as seen by the transmitter. In practice, some energy is absorbed and the reflected energy is not distributed equally in all directions. Therefore, the radar cross-section is quite difficult to estimate and is normally determined by measurement. The target radar cross sectional area depends of: o
the airplane’s physical geometry and exterior features,
o
the direction of the illuminating radar,
o
the radar transmitters frequency,
o
used material types of the reflecting surface.
Free-space Path Loss R is the target range of the term in the equation. This value can be calculated by measuring the time it takes the signal to return. The range is important since the power obtaining a reflecting object is inversely related to the square of its range from the radar. Free-space path loss is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space, with no obstacles nearby to cause reflection or diffraction. The power loss is proportional to the square of the distance between the radars transmitter and the reflecting obstacle.
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Draft Lecture Notes-Radar
2014
Kyu/ET422
Draft Lecture Notes-Radar
2014
