3 Basic Radar Systems
3.5 Pulsed Radar Architecture
The basic principles of radar operation have already been outlined. The detailed operation of pulse radar is described in this section. A top-level diagram of a pulsed radar system is shown in Figure 3.15.
3.5.1 Pulsed Radar Components
The diagram shows the major elements which are:
Modulator;
Transmitter;
Antenna;
Receiver;
Video processor.
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3.5.1.1 Modulator
The modulator determines the pulse shape and the nature of the radar modulation. Although pulsed transmission is the most elementary form of radar operation, the modulation in a modern multimode radar may take many forms depending upon the nature of information being sought. The operation of the modulator is controlled by the synchroniser which dictates when a pulse should be initiated. The modulator uses the superheterodyne (‘super-het’) principle of modulation to superimpose the modulating signal upon the high-frequency carrier to provide a composite waveform.
3.5.1.2 Transmitter
The transmitter amplifies the modulated carrier signal and feeds it to the antenna via a duplexer. This serves the function of directing the transmitter energy to the antenna waveguide system to be fed by the antenna elements for transmission into the atmosphere.
It also routes the reflected target energy to the receiver.
3.5.1.3 Antenna
The antenna, as has been described, directs the radar energy towards the target and receives the reflected energy from the target. Along with the target echo, a substantial amount of clutter from ground returns is also received. The antenna beam is focused according to the shape of the antenna and the nature of the beam required. Unwanted radar energy enters through the antenna sidelobes as well as the main beam. The antenna also receives noise from a variety of external sources that can help mask the true target signal.
Modulator Transmitter
Receiver Video
Processor
Servo Synchroniser
Power Supply Display
Controls
Duplexer
Receiver Protective
Device
Figure 3.15 Top-level pulsed radar architecture.
Returning energy is passed through a receiver protective device which blocks the large amounts of transmitted power that would cause severe damage to the receiver, but also at the appropriate time allows the reflected target energy to pass through.
3.5.1.4 Receiver
The receiver amplifies the reflected target signal and performs the demodulation process to extract the target data from the surrounding noise, and the resulting target video data are passed to the video processor.
3.5.1.5 Video Processor
The video processor is also controlled by the synchroniser in order that transmitted pulse and target return pulses are coordinated and that a range measurement may be made. The resulting data are coordinated and displayed on the radar display.
3.5.2 Pulsed Modulation
The nature of the pulse modulation in terms of pulse width and frequency of repetition is highly interactive with a number of important radar characteristics and has a significant impact upon the performance of the radar. The basic parameters of a pulsed radar signal are described in Figure 3.16.
In pulsed radar operation, the carrier frequency is modulated by the envelope of a single rectangular pulse; in this case the pulse embraces a fixed carrier frequency. As will subsequently be discovered, in sophisticated radar operations there are advanced forms of modulation/transmission in which the pulse is not rectangular nor the carrier fixed in frequency. The pulse width is denoted by the symbol and is usually fairly narrow, perhaps
Pulse
Figure 3.16 Pulsed radar transmission.
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1 ms in an air-to-air mode. After a time interval called the pulse period, a second pulse is transmitted and the sequence is repeated. The rate at which the pulses are repeated is called the pulse repetition frequency (PRF), and both the pulse width and the PRF are key radar parameters.
It will be noted on the diagram that the terms ‘time domain’ and ‘frequency domain’ are mentioned. The time domain is familiar in everyday life as it is the domain in which we live;
the frequency domain is more abstract but is of great importance to the radar designer. In fact, the time and frequency domains are interdependent and interwoven, and this has a significant impact upon the operation of radar systems. What happens in the time domain affects the frequency domain, and vice versa.
The rectangular pulse results in a response in the frequency domain that has a sin x=x response, the same generic response that determines the pattern of the antenna main beam and sidelobes. However, in this case the response is occurring on an axis relating to frequency rather than angle off-boresight as is the case in the antenna pattern. When the incoming pulse is received, it results in a frequency response of received power portrayed by the sin x=x response and centred upon the radiated frequency f0. The practical limits of the main sin x=x response are f1 centred on fo, that is, f0þ 1=; f0 1= ( f0 being the carrier frequency and the pulse width), and this determines the bandwidth required of the receiver in order to be able to pass all the components of the target return. Therefore, for a 1 ms pulse the receiver bandwidth would need to be 2=¼ 2=ð1 106Þ ¼ 2 106or 2 MHz (see the lower part of Figure 3.16). The narrower the transmitted pulse, the wider will be the bandwidth; the converse also applies.
In basic pulsed radar operation the pulse width also determines the range resolution of which the radar is capable. The radar can only resolve to half the pulse width, as at a lower interval than this part of the pulse has been reflected while part has not yet reached the target.
A pulse of 1 ms duration will be approximately 1000 ft long (as light travels at 3:3 3 108 or 109ft=s and the duration of the pulse is 1 106; distance¼ velocity time). There-fore, a 1 ms pulse will be able to resolve the target range to no less than 500 ft. In fact, by using more complex modulation and demodulation methods called pulse compression, it is possible to achieve much better resolution than this; pulse compression is discussed in Chapter 4.
The pulse period – and therefore PRF – also has an impact upon the radar design that affects target ambiguity, as shown in Figure 3.17. The figure shows an aircraft illuminating two targets and compares the effect of the returns from these targets for two different pulse periods. In pulse period T1 (PRF1¼ 1/T1), the returns from both targets are received before the successive pulse is transmitted, and the range is unambiguous. In the case of the shorter pulse period T2 (PRF2¼ 1/T2), the return from the most distant target occurs after the transmission of the successive pulse and to the radar appears as a relatively close target within the second period. In this case the target range is ambiguous and misleading. The selection of PRF is one of the most difficult choices the radar designer has to make, and some of the effects of range ambiguity are discussed in more detail in Chapter 4.
The fact that the radar only transmits for a portion of the time means that the average power is relative low. The average power is given by the following expression:
Pav¼Ppeak T
where Ppeakis the peak power, is the pulse width and T is the pulse period.
For a peak power of 10 kW, a pulse width of 1 ms and a pulse period of 250 ms, we have
Pav¼104 1 106
106 250 ¼ 40 W
3.5.3 Receiver Characteristics
In order to be able to detect the target, the radar receiver has to able to discriminate from unwanted effects. The main adverse affects are as follows:
1. Noise is either internally generated or radar transmitter induced or externally sourced.
Noise is random and can only be minimised by good design.
2. Clutter due to unwanted returns from the ground and other sources is usually more systematic and can be countered by filtering and processing techniques.
3.5.3.1 Noise
The sources of noise that can affect the ability of the radar receiver to detect a target signal are shown in Figure 3.18. The total system noise includes noise from the following sources:
1. Antenna noise Ta. The antenna noise includes all those sources of noise that are external to the radar, including radiation from the sun, terrain, emissions from man-made objects
Transmitted Pulse
Most Distant Target
Time T1
T2
PRF (kHz) Period, T
(microsec)
0 500 1000 2000
1 2 3 4 5
Range∝
Figure 3.17 Effect of pulse period on target ambiguity.
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and the weather. Noise from jamming may also be included in this category. The radome and the antenna itself may also generate noise. While external noise will be most troublesome when it enters the system via the antenna main beam, it should also be remembered that noise can also enter via the antenna sidelobes.
2. Transmission line noise Tr. This includes noise originating within the waveguide couplers, duplexer and the receiver protection device.
3. Equipment noise Te. The equipment noise is generated within the receiver itself and is the most difficult to counter.
The total system noise, Ts, is the sum of these individual components:
Ts¼ Taþ Trþ Te
The problem with the noise in a receiver is that, once present, it is there to stay. Signal amplification in subsequent stages will only amplify the noise as well as the signal and accentuate the problem of target detection. One technique commonly used is to insert a low-noise amplifier (LNA) at the front of the receiver to amplify the signal proportionately more than the noise. LNAs are also commonly used where antennas (or apertures) are mounted remotely throughout the airframe and where transmission losses might be relatively high.
The receiver noise is defined as noise per unit of receiver gain:
Receiver noise¼noise at output of receiver receiver gain
RF Amplifier IF Amplifier
Mixer
Ta Antenna Noise Tr Transmission
Line Noise Te Equipment Noise
Ts System Noise
Figure 3.18 Sources of noise affecting radar signal.
The receiver gain can be easily measured using laboratory techniques.
The receiver noise may be characterised by a figure of merit or noise figure Fn. This is defined as the ratio of the noise figure of the actual (imperfect) receiver to the hypothetical ideal receiver providing equal gain. Therefore:
Fn¼noise output of actual receiver noise output of ideal receiver
An ideal receiver would produce no noise; the only noise that would exist would be that from external sources. This external noise can be represented as though resulting from thermal agitation in a conductor (resistor) since the two have similar spectral characteristics.
Therefore, in the derivation of Fn, for both ideal and actual receivers, the thermal noise can be portrayed as the voltage across a resistor. Thermal noise is governed by the random motion of the free electrons within the conductor and is uniformly spread across the entire spectrum. This motion is determined by the absolute temperature of the notional resistor, denoted by T0. Also, the noise depends upon the receiver bandwidth B. Thus, to derive the mean noise power for an ideal receiver, the expression
mean noise power¼ k T0 B ðWÞ
may be used for an ideal receiver, where k is Boltzmann’s constant¼ 1.38 1023W s/K, T0is the absolute temperature of the resistor representing the external noise (K) and B is the receiver bandwidth (Hz).
The external noise is the same for both receivers, and by convention T0 is taken to be 290 K which is close to room temperature. Where the external noise is small by comparison with that generated by the receiver, as is usually the case, the mean noise figure for an actual receiver may be determined by the following:
Mean noise power¼ Fn k T0 B ðWÞ
As was shown earlier, the total noise may represented by Ts where the mean noise power (all sources)¼ k Ts B.
The nature of the modulation used also has an impact upon receiver noise. This is shown in Figure 3.19. The figure shows the simple comparison of narrow and broad rectangular pulse modulation. It was shown earlier in Figure 3.16 that the bandwidth needed to accommodate all the frequency components of a rectangular pulse was governed by the sin x=x waveform, and that the theoretical bandwidth was 2= . The narrow (sharper) pulse 1
needs a greater bandwidth than the broader pulse 2. The narrow pulse gives an improved range resolution and, for a given pulse period (PRF), a reduced mean power, so it can be seen that there are performance trade-offs to consider that affect bandwidth and hence receiver noise.
In practical systems a compromise is allowed and generally a bandwidth of 1= is regarded as sufficient. Therefore, it is common practice to narrow the IF bandpass filter until it is 1= wide, just wide enough to pass the bulk of the target-related energy but reject the unwanted noise. This design is called a matched filter, and the mean noise energy per pulse is kT0= .
In the Doppler radars addressed in Chapter 4 the Doppler filters downstream of the IF filter are much finer, and greater noise and clutter rejection result.
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The detection and extraction of a target echo from a background of noise depends upon the four factors outlined below:
The average power radiated in the direction of the target;
The proportion of the radiated energy reflected back in the direction of the radar;
The proportion of power recaptured by the radar antenna;
The length of time the antenna beam is trained upon the target.
Average power is determined by the relationship of the peak power, Ppeak, transmitted by the radar and the modulation characteristics of pulse width, , and pulse period, T, as shown in the previous section. The antenna gain, GD, also increases the power density related to the beamwidth(s) and beam geometry.
As the radiated signal is directed towards the target, it spreads out an increasing area, proportional to R2, where R is the range from the radar. This means that the power density reduces by a factor of 1=R2 as the energy is propagated in the direction of the target.
A fraction of the energy incident upon the target will be reflected back in the direction of the radar. In the simplest form the target may be considered to be a simple sphere with a specific cross-sectional area, denoted by the symbol and specified in square metres. The reality is much more complicated than that, and other factors such as reflectivity and directivity play a great part, as will be seen in the discussion on low observability or stealth in Chapter 4.
As the energy is reflected back to the target, the 1=R2 effect applies in terms of the reduction in received power density. The impact of this effect means that the energy received at the radar has been reduced by a total factor of 1=R4in its outward and return path to and from the target. This has an impact upon the ability of the target signal to be detected above the noise, as shown in Figure 3.20. The figure shows how the returning signal (not to scale)
Narrow Pulse
Broad Pulse
Matched Receiver Bandwidth Time Domain
Frequency Domain
Noise
τ1 τ2
1
τ1 1τ2
Figure 3.19 Effect of different pulses on the receiver bandwidth.
decreases with increasing range to the point where the signal is not detectable against the noise background.
As will be seen, the equation governing the strength of the return signal is a fourth-power law, and this means that the receiver has to accommodate a very large dynamic excursion in terms of variation in target signal strength as the range varies. In certain modes this is addressed by a technique called sensitivity time control (STC) in which the receiver gain is reduced at very short ranges and increased progressively during the range sweep. This technique is sometimes referred to as swept gain and to some extent mitigates the problem of extremely high signal returns at short range.
Another technique is often used to counter this effect and prevent the receiver amplifiers from saturating: if the receivers saturate, then both signal and noise will merge as the amplifiers clip both noise and target signal returns. In this case, automatic gain control (AGC), as the name suggests, automatically reduces amplifier gain to prevent saturation occurring.
The actual detection of the target signal is determined by the setting of a target detection threshold as shown in Figure 3.21. This shows two targets, A and B, against a background of noise on a time axis: A and B are obviously at different ranges from the radar. The figure shows the importance of setting the target threshold correctly with respect to the mean noise level. If the threshold is set low, then it may be anticipated that more targets may be detected.
However, as the diagram shows, setting a low target threshold has the accompanying risk of detecting a spurious target – called a false alarm. For the low threshold setting shown, the radar would detect three targets: genuine targets A and B and the false alarm.
Conversely, there are problems with setting the threshold too high to avoid false alarms. In this case the return from genuine target A is lost and only target B is detected.
One of the major factors affecting target detection was antenna time on the target. So far, only the detection of a target using a single pulse has been considered. In fact, as the radar beam sweeps through the target, a number of successive pulses will illuminate the target in a short period of time. Most radars have the capability of integrating the detected output over a number of pulses, and this has significant advantages, as can be seen from Figure 3.22.
Noise
Target Range Energy
Signal
Figure 3.20 Effect of range upon the target echo.
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Noise is generally random in terms of amplitude and phase. The target return is more systematic and repetitive in nature, at least over a range of successive pulses in an antenna scan. The effect of integrating noise over a series of pulses is to end up with noise at more or less the mean noise level before integration. The converse is true for a real target return. The target return is aggregated during the integration process and the result is a much stronger target return. The figure shows that as an example – integration over 12 pulses produces an
Mean Noise Level Low Threshold High Threshold
Noise
Target A
Target B
False Alarm
Time
Figure 3.21 Receiver threshold setting.
Noise
Successive Pulses
Signal
Integrated Signal
Integrated Noise Time on Target
(Integration Period)
Mean Noise Level Target Detection Threshold
Figure 3.22 Effect of integration over several pulses.
integrated signal that comfortably exceeds the target detection threshold, whereas the integrated noise does not. This occurs in spite of the fact that each of the individual target signals are well below the target detection threshold and without pulse integration would each be subsumed by noise. This shows the powerful capability of ‘extracting’ a signal from noise using integration techniques.
The actual antenna time on target depends upon a combination of three factors:
The antenna scan or slew rate;
The antenna beamwidth;
The PRF.
Taking some simple figures by way of illustration, if the antenna scan rate is 60 deg/s and the 3 dB beamwidth is 3 , then the antenna will dwell upon a target for 1/20th of a second. If a medium PRF of 1000 Hz is assumed, then the antenna will theoretically have a total of 50
Taking some simple figures by way of illustration, if the antenna scan rate is 60 deg/s and the 3 dB beamwidth is 3 , then the antenna will dwell upon a target for 1/20th of a second. If a medium PRF of 1000 Hz is assumed, then the antenna will theoretically have a total of 50