RADAR RECEIVERS

Introduction

1. The radar receiver has a central role in any radar system; it is responsible for detecting the extremely weak target echoes and extracting target information. With accurate target information, a target can be tracked and destroyed. An understanding of receiver techniques, processing and tracking is essential in order to counter the use of radar or protect friendly radar against enemy Electronic Countermeasures (ECM).

Radar Receiver Function

2. The function of a radar receiver is to amplify the echoes of the transmitted pulse and to process it in such a manner that will provide the maximum discrimination between desired echoes and undesired interference. Remember, echo signals detected at the antenna may range from 1 mW to 0.000,000,000,000,1 mW (10 trillion times smaller). Filtering removes any out-of-band signals, (such as signals from other radars) and attempts to remove any signals created by the receiver’s own electronic components (internal noise).

The Receive Process

3. The radar receiver process involves amplification, filtering and demodulation of detected signals. The majority of radar receivers use superheterodyne (superhet) techniques. A generic superhet receiver layout is given at Figure 1.

Figure 1: Superheterodyne Receiver

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4. A superhet receiver has the following components:

a. A RF Pre-Amplifier. This amplifies the weak RF signals and may limit very strong signals that could saturate the receiver and processor circuits.

b. A Local Oscillator and Mixer. These components are responsible for mixing the incoming radio frequency (RF) with a stable local oscillator to produce an intermediate frequency (IF). This process is called superheterodyning and depending upon the IF selected may require a number of mixing stages.

c. An IF Amplifier. This amplifies limits and filters the IF signal. Amplification at the IF frequency is more stable and less costly than amplification at RF frequencies.

d. A Detector or Signal Demodulator. This component demodulates the IF signal into a form required by the digital signal processor. This is also identified as the Analogue to Digital Conversion stage (ADC) as the analogue signal is converted to digital form for processing in later stages, two distinctive forms of demodulation are:

(1) Video Demodulation. This recovers the amplitude of the signal, (plus interference) and is often referred to as the video signal

(2) In-Phase/Quadrature (I/Q) Demodulation. This recovers the signal-plus-interference amplitude and full signal phase information.

e. Digital Signal Processor. These module processes detected signals to increase the strength of wanted signals (targets), in comparison to unwanted signals (noise). It also determines the noise and gain levels within the receiver. Following this it determines whether or not a target is present and computes range and Doppler shift information. Data is then forwarded antenna, to enable automatic target tracking and the display processor. In more modern radar Target recognition is undertaken at this stage of the receive process.

f. Data Processor. The data processor stores and processes the location of detected targets. It smoothes target data and may provide a track-while-scan capability. Data that is held in registers within the data processor are then fed to the display.

g. Display. The radar display shows the processed data in an easily assimilated form for users. The display will depend upon the function of the radar for example a scope displays may be used for Analysis and Plan Position Indicator (PPI) for Air Defence whilst tabular displays will provide amplifying data for the user.

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Signal Processing

5. Receiver gain is defined as the ratio of the receiver’s output power to its input. Since detected signals are very weak, the radar receiver may need to amplify signals 10 x 109 times. This amplification must be variable to prevent strong signals from saturating sensitive circuits. This is achieved through a number of control techniques implemented at the Signal Processor stage:

a. Pre-set Gain. The receiver gain is set according to the mode of radar operation, for example long-range detection or air-to-air gun mode.

b. Manual Gain Control. The user controls the level of receiver gain.

c. Automatic Gain Control (AGC). AGC is used to maintain the desired level of noise at the ADC stage. The module uses a circuit to monitor the output of the amplifier and adjust the gain level for optimum detection. The gain may be set after averaging many signals (or pulses) and may not follow rapid changes in signal amplitude.

6. An important term in considering radar’s ability to detect very weak echoes is its sensitivity. A receiver’s sensitivity refers to the minimum detectable signal energy that can produce an output in the receiver (see Figure 2). Receiver sensitivities depend upon the function of the radar and vary with the required maximum range and detection of a specific target size. So great is the range of power of reflected echoes that that they would exceed the dynamic range of any fixed gain receiver

7. Sensitivity Time Control (STC) causes the receiver's sensitivity to vary with time in such a way that the amplified radar echo is independent of range. This technique involves gradually increasing the receiver's sensitivity in order to detect weaker echoes from long-range targets.

Figure 2: Receiver Thresholds

Noise

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8. Noise may be defined as any unwanted RF signal that includes interference and clutter. It comes from a variety of sources and at constantly varying strengths (see Figure 3).

Figure 3: Noises, Interference and Clutter

9. Noise may be divided into 2 general groups:

a. Natural sources of noise include:

(1) Galactic or Cosmic Noise depends on solar radiation from our Sun and the stars, as well as meteors and space debris entering the Earth’s atmosphere.

(2) Lightning causes atmospheric Noise.

(3) Thermal Noise is created by all objects above absolute zero as a result of the conversion of thermal energy to electrical energy.

b. Man-made sources of noise include:

(1) Other Radar Transmissions. Often termed interference.

(2) All Electrical Equipment produces unwanted signals, which are random in amplitude and frequency.

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(3) Ignition Noise caused by the starting and continuous operation of electro-mechanical motors.

(4) Radar Jamming will create a higher noise signal than the target echo, in the radar receiver.

10. The noise level within the receiver affects the sensitivity or threshold of the receiver (see Figure 4). As the first stage of the receiver process is to amplify the detected signal it will include the real target echo and the external and internal noise. The combination of external and internal noise detected by a receiver is called the average noise level.

Figure 4: False Alarms

11. If the sensitivity is set too high weak echoes but also real targets, may go undetected. Therefore it is important to reduce internal noise as low as possible but to select an appropriate threshold to ensure the lowest probability of the production of false targets. This is called the Constant False Alarm Rate (CFAR) function.

12. Unfortunately the introduction of CFAR reduces the probability of detection. It also results in a loss of signal to noise ratio and degrades range resolution. CFAR is necessary when automatic tracking cannot handle large numbers of echo returns but for the reasons stated above its use should be avoided if possible.

13. Interference is generally used to describe the interaction of 2 or more radar signals on similar frequencies.

14. Clutter is the term used to describe any unwanted echoes that may appear on a radar display. Different types of radar and different frequencies suffer from different types of clutter. These may include:

a. Weather clutter (cloud and rain).

b. Sea clutter (from waves).

c. Ground clutter.

d. Flocks of birds.

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e. Buildings or Trees.

15. The ease with which a radar receiver can detect a signal depends on the signal-to-noise ratio, (SNR or S/N). For a signal to be detected above the average noise level, the SNR must be greater than one. SNR is a critical parameter in determining the probability of detection for a radar system.

Signal Processing

16. Good receiver design is based on maximising the SNR. The aim of signal processing is therefore to improve the SNR, (and therefore the probability of detection). It also extracts target information for range and velocity calculation. Signal processors may be analogue or digital. The more modern the radar the more likely it is to have digital processing. In general, signal processing will involve Automatic Target detection and tracking this comprises a number of different functions:

a. Automatic Target Detection is achieved by filtering a sequence of returned signals into histograms. Each histogram has a number of small discrete bins for range or Doppler frequency, (for velocity calculation). Range bins are discrete quantities of time. Doppler bins are discrete bandwidths. Filtering may lower the noise level at a particular range or Doppler frequency.

b. Signal integration improves radar’s probability of detection by basing detection upon a number of pulses that are added over time, rather than a single pulse. The pulses can be added in amplitude (non-coherent integration), or in phase (coherent integration). Coherent integration produces less noise and therefore a better SNR.

c. Target Tracking is achieved by the implementation of sophisticated algorithms that take a sequence of target positions smooth them and predict future track positions. The algorithms must take into account target fade, blind speeds and propagation effects as well as platform velocities.

17. Particular types of radars, such as moving target indicator (MTI) and synthetic aperture radar (SAR) carry out more specialised signal processing.

Target Range

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18. The range of a target is calculated by measuring how long the radio waves take to reach the target and return. The time starts when the master timer sends a synchronising pulse to the modulator and ends when the echo is detected in the receiver/signal processor. In pulse radar, the time reference is usually the centroid of the transmitted pulse, although the leading and trailing edges of the pulse can also be used. The synchronising pulse is also sent from the master timer to the receiver or display. The later is to initiate the display time-base. The target range is calculated in the receiver/signal processor or passed directly to the display.

19. The Doppler effect is the change in frequency due to movement. A common example is found in the roar of a racing car, which deepens as it passes by. The velocity of the racing car compressing the sound waves ahead causes this, whilst the sound waves spread out behind. This change in frequency is known as the Doppler shift or Doppler frequency, and is proportional to the object’s (or target’s) relative velocity and the transmitted frequency. (Relative velocity refers to the velocity towards or away from the radar).

20. The Doppler effect applies to all waveforms. Radar that detects a moving target will notice a change in frequency of the echo, related to the target’s relative velocity. The same principle applies if the radar is moving, as in an air intercept (AI) radar, and the target is not, such as the ground. The higher the transmitted frequency or relative velocity, the higher the Doppler frequency or Doppler shift.21. Doppler frequency is determined by the relative movement of the target as follows:

a. Relative movement towards, (range decreasing), causes the Doppler shift to increase.

b. Relative movement away, (range increasing), causes the Doppler shift to decrease.

c. No relative movement, (constant range), causes no change in frequency.

22. The Doppler effect gives a radar system the target's relative velocity. If the range, angle and movement of a radar system are also fed to the signal processor, an indication of the exact velocity of the target may be calculated. Radar that can use the Doppler effect include:

a. Continuous wave (CW) radar.

b. Frequency modulated continuous wave (FMCW) radar.

c. Pulse Doppler (PD) radar.

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23. By sensing Doppler frequencies, a radar system can substantially reduce or even eliminate clutter. The signal processor to enhance radar’s probability of detecting targets can filter out weather clutter and ground clutter. Radar that uses the Doppler effect include:

a. Moving Target Indication. This involves filtering out ground clutter by its Doppler frequency, and can be used by airborne radar to spot moving ground targets.

b. Look-Down-Shoot-Down. Again this involves filtering out ground clutter, but this time to highlight low-flying aircraft. It is a technique used by all modern fighter radar.

c. Doppler Navigation. This involves using the Doppler effect to measure an aircraft’s own velocity. For this, the antenna beam is generally pointed ahead and down at a shallow angle. The ground echoes are measured in sequence at different azimuth and elevation angles. From this, the aircraft’s ground speed and drift can be calculated and passed to the aircraft’s navigation computer.

d. Doppler Mapping. The Doppler effect can be used by airborne radar to improve their resolution for ground mapping by removing clutter. When the beam is pointing directly ahead the radial velocity vectors of the clutter are close to the Ground speed of the aircraft. However when the beam is pointing to one side the radial velocities of the clutter decrease but the difference across the beam increases. Synthetic aperture radar (SAR) and Doppler beam sharpening (DBS) use this principle to provide high-resolution mapping.

Radar Tracking

24. Tracking is defined as the precise and continuous measurement of a target’s range, angle and/or velocity to determine a target’s flight path and predict its future position. The range resolution of the early warning, search and height finding radar is not usually accurate enough to guide a missile or allow an anti-aircraft artillery (AAA) system to shoot the target down. A high-resolution tracking radar is required to refine the target position, so that it can be engaged and shot-down. The acquisition phase of tracking radar begins when it searches for the target in the approximate position passed by the low-resolution search and height finding radar.

25. Types of Tracking include:

a. Single Target Track occurs when radar follows a single target and ignores all others. The majority of modern radar use single target track.

b. Spotlight Track occurs when radar follows one target for a period of time and then switches to a second target and further targets, before returning to the first target at the end of the cycle.

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c. Track-While-Scan involves sampling the position of several targets once per scan and uses sophisticated smoothing calculations to estimate the position of the targets between scan samples. Typical target updates are between once per second and once per 15 seconds.

d. Multiple Target Tracking involves sampling the position of several targets many times per second. This requires very fast beam steering and scans rates, which are normally possible only with electronically scanned antennas.

26. Having detected the target, the tracking radar must have excellent range resolution (short PW) and angle resolution (narrow beam widths) refine the target's position. With pulse radar, a very high PRF is used to ensure a large number of echoes are received from the target providing a high data rate to the tracking computer. The high data rate increases the accuracy of the interception point for the surface-to-air missile (SAM) or AAA system, particularly against a manoeuvring target.

27. Target data from the receiver tracking circuits are fed to the antenna servomotor this turns the antenna through the required angle so that the target is constantly within its beam. Any error in the target position, generates angle error signals to move the antenna in closed loop system. Range Tracking

28. The main purpose of range tracking is to provide continuous range information on a target. If there is more than one target appearing in the radar beam, range tracking ensures that only the selected target is tracked.

29. Range tracking is achieved through the use of range gates. The process is as follows:

a. After initial detection, a range gate is placed over the selected target. This range gate is split into 2 halves; the first half called the early gate is placed slightly short of the estimated position of the target. The other half, called the late gate, is placed slightly beyond the estimated range.

b. Both gates output a signal that is passed to a subtraction circuit. If the target is not positioned exactly under the range gates, the amplitudes from the gates will be different and an error signal will be generated.

c. The error signal automatically moves the split gate until the amplitude of the echo is equal in both halves. It also drives the servomotor to move the antenna. This technique is known as a tracking loop.

d. The rate at which the range changes (range rate) is required by the computer to predict the correct position for the range gate when the next echo is due to arrive. Range rate is mathematically calculated.

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Figure 5: Ranges Gate (Lock On)

30. Many modern radar systems use additional range gates known as guard gates. Guard gates assist in track initiation, sensing separations from targets being tracked (such as missile launches), and to counter certain types of deception ECM.

Velocity Tracking

31. If a target is moving relative to radar, the frequency in the reflected echoes will be modified by the Doppler shift. Some tracking radar track targets in velocity by using Doppler information. This can be achieved using a velocity gate. The method of operation is as follows.

a. A velocity gate is a narrowband band pass filter. The radar adjusts the tuning of the filter automatically in response to changes in target speed.

b. This may be done by a split-gate technique, very similar to that of range tracking.

c. Two parallel band pass filters cover successive ranges of Doppler shifts.

d. If the signals received in them are of unequal strengths, an error signal will retune the filters until the signal strengths are equal.

e. The radar has now locked-on.

32. SAM radars that use semi-active guidance and air intercept (AI) radars use velocity tracking.

Angle Tracking

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33. To accurately aim AAA or to guide a missile to its target, radar must measure target angle both in azimuth and elevation. Angle errors must come from only one target; otherwise the system will not accurately track the required target. When many targets are present only the required target must be selected and all others rejected. This is achieved through the process of gating. Targets can be selected within a gate, based on time of arrival (range gating) or the Doppler shift of a target (velocity gating).

34. To obtain accurate angle information, a pencil beam could be used. Unfortunately this would illuminate only a small area and it would be difficult to locate a target and then keep it in the tracking beam. To overcome this problem different types of radar have been developed that maintain accurate angle tracking whilst covering a relatively large area. 3 main types of angle tracking have been developed: Sequential lobing, Conical scan, and Monopulse, these techniques are discussed below.

Sequential Lobing

35. This method of angle tracking was developed during World War II and is also known as lobe switching or lobe angle tracking. It is still used by some older fire control radar, modern multi-mode radars and for satellite tracking.

36. In sequential lobing radar, a single pencil beam is switched between 2 positions that generate overlapping coverage to measure target angle in azimuth. The displacement off of the central axis or boresight is known as the squint angle. The signal strengths are compared, over time, from the different beam positions. If the target is not on the boresight, the signal strengths will not be equal and an error signal is generated. This moves the antenna radar boresight towards the stronger echo. When the 2 echo strengths are equal, the target is on the boresight in azimuth. By switching the pencil beam between 2 positions, one above the other, it is also possible to track the target in elevation. By switching the beam between all 4 different positions, the target is tracked in both azimuth and elevation. Lobing rates can be as high as 50 Hz. Sequential lobing is an old method of angle tracking and is gradually being replaced by monopulse tracking.

37. Sequential lobing radar requires a train of pulses to achieve target tracking. Additionally, power is wasted due to the squint angle (the target is not in the centre of the beam). The measurement of signal strength in each beam position at slightly different times means that the system is relatively easy to jam by transmitting noise into one or more of the beam positions.

38. The Lobe-On-Receive-Only (LORO) technique overcomes the inherent weakness in sequential lobing. LORO uses a non-scanning or floodlight transmitted beam. Separate receive-only antennas are scanned to give the sequential lobing comparison. Electronically scanned radars also use LORO. The AN/APG-66 in the F-16 is an example.

Conical Scan

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39. A conical scanning (conscan) radar uses a single squinted pencil beam that is rapidly rotated around the boresight of the radar. Typically, the radar may use a 2 beam, rotated at 50 to 100 Hz, with a squint angle of 2. The higher rotation rate gives a higher data rate than sequential lobing radars.

40. There are 3 methods of achieving conical scan radar: Mechanically rotating an offset waveguide feed to a parabolic reflector, mechanically wobbling the parabolic dish. Changing the phase of the signal fed to the antenna.

41. Method of Operation. When the target is off boresight, the returning echoes vary in amplitude at the conscan rate. The phase and depth of this amplitude modulation depends on the angular difference between the target line-of-sight and the boresight. By comparing the conical scan modulation with 2 reference signals (azimuth and elevation), tracking error signals can be applied to an electro-mechanical servo to position the boresight on the target. On boresight, the conical scan gives a constant amplitude signal. The radar system is then locked-on.

42. Like sequential lobing, conscan radars do not achieve instantaneous tracking and suffer from crossover losses. Additionally, conical scan radars are relatively easy to jam by transmitting noise into one or more of the beam positions. Conical-Scan-On-Receive-Only (COSRO) overcomes the problem by using 2 antennas; one to transmit and the other to receive. The conical scan pattern is produced purely by the receive antenna. This hides the conscan modulation rate from a jammer in the same manner as LORO.

43. The Scan-With-Compensation technique is similar to conscan except that 2 pencil beams are rotated around a boresight instead of one. One beam is active (that is transmits and receives); the other is passive and connected to a separate receiver. The 2 beams are arranged such that they scan opposite sides of the circle. The outputs of the 2 receivers, which are in anti-phase, are fed into a subtracting circuit. This results in an enhanced error signal, which cannot be deceived by jamming.

Figure 6: Monopulse Beams

Monopulse Tracking

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44. Unlike sequential and conical tracking, where several scans are required to establish a target track, a monopulse tracking radar can theoretically obtain angle on the basis of transmitting and receiving a single pulse. Monopulse tracking is also known as simultaneous lobing.

45. With a monopulse tracking radar, a single beam is used to illuminate the target. On receive, the echo is split into a number of separate overlapping receive paths. These correspond to relative positions around the target, equivalent to squinted, simultaneous lobes. This can be done by mechanical means, such as having 4 feed horns within a single reflector antenna or, at a very basic level, having 4 separate antennas and receivers. Alternatively, a phased array antenna can be used.

46. The received signals are passed to a waveguide hybrid junction, known as a comparator, which separates the signal into Range, elevation and azimuth channels (see Figure 7). The channels are compared and the sum () and difference () computed. The sum is used for detection and ranging. The elevation and traverse differences are used for angle tracking.

47. If the target is not on radar boresight, an error signal is generated and the antenna moved. When no error signal is generated, the target is exactly on the radar boresight and no movement is required. Theoretically monopulse tracking can achieve angle tracking from a single pulse. In reality, several pulses are used.

48. Most modern tracking radars use monopulse techniques. Pulse, continuous wave and pulse Doppler waveforms can use monopulse tracking, as can phased array radars.

49. Monopulse tracking is achieved in one of two ways:

a. Amplitude comparison monopulse radar compares the signal strength, or amplitude of the target echo in 4 positions. If, typically, 4 feed horns are used, the distance between the horns needs to be very small (a few wavelengths across), so that the phases of signal fed to the 4 receive quadrants are within a few degrees of one another. The sum and differences are calculated as described above. Variations in target amplitude caused by multipathing and other circuit measurement inaccuracies give rise to errors. Consequently this represents the older method of monopulse tracking and suffers from slightly more errors than phase comparison monopulse.

b. Phase comparison monopulse radar compares the difference in phase of the target echo at each position, using typically 4 antennas, such as in a phased array. The phase difference can be converted into an angle off-boresight and this error signal used to move the boresight onto the target. Phase comparison monopulse is more accurate than amplitude comparison monopulse. All modern phased array tracking and multi-function Radars use phase comparison monopulse.

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Figure 7: Monopulse Calculations

50. Monopulse tracking generally offers the following advantages over sequential lobing and conical scan radar:

a. It is difficult for the target to know that it is being tracked because there is no variations in scan modulation.

b. Monopulse radar suffers less wear from mechanical vibration.

c. Higher data rates are generated, which can mean tracking is more accurate.

d. Less susceptible to basic jamming.

51. Monopulse Tracking has the following major disadvantages:

a. The complexity of the monopulse system. Multiple receivers are required, with accurate, balanced outputs. Gain and phase shifts in the 3 error channels must be tightly controlled.

b. They are more expensive than sequential lobing and conscan radar.

Summary

52. The radar receiver plays a major role in the detection of targets and presenting usable information to the operator. An understanding of the above basic receiver processing techniques will enable a radar operator to negate the effects of false targets and clutter.

53. Knowledge of how a radar receiver works is also very important for EW, as it provides a basis for exploiting the weaknesses of a radar system.

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