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5. ORGANIZACIÓN DEL PROGRAMA
5.2 SEGUIMIENTO DEL DOCTORANDO
The operation of a pulsed Doppler radar opens totally new areas that need to be considered.
The derivation of target velocity using the Doppler shift has significant advantages and also permits low-level targets to be detected and tracked when flying in ground clutter regions.
However, the radar sidelobes also collect a significant amount of ground clutter over a range of Doppler frequencies, and the characteristics of this clutter need to be fully understood for the best performance to be achieved.
Figure 4.6 shows the principal clutter returns for a pulse Doppler radar. These are as follows:
1. Mainlobe clutter (MLC) in the direction of the main beam. This is related to the velocity of the aircraft/radar, as has already been described in the Doppler navigator in Chapter 3.
The velocity component of the mainlobe clutter reduces as the antenna boresight is moved to the left or right of the aircraft track. This happens as the component of forward velocity in the Doppler shift is reduced by the cosine of the angle on the antenna boresight with respect to aircraft track. Similarly, the size of the MLC Doppler shift is modified according to the cosine of the antenna look angle (depression angle).
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2. An altitude or ground return resulting from stray energy being reflected from the terrain directly underneath the aircraft. Since the terrain producing this return is directly below the aircraft, this return includes no Doppler component when the aircraft is flying straight and level over flat terrain. It therefore usually represents the zero Doppler shift position of the spectrum. However, there will be a Doppler bias if the aircraft is ascending or descending or the terrain below the aircraft is not level.
3. Returns are experienced across a whole area extending from ahead of the main beam clutter to well behind the aircraft ground return. This return is due to energy entering the system via the antenna sidelobes and is therefore known as sidelobe clutter. The sidelobe clutter region extends over a region that approximates to2 Vr=.
The typical Doppler spectrum resulting from an aircraft flying at velocity Vr, with pulsed Doppler antenna transmitting over terrain, is shown in Figure 4.7. On the diagram, Doppler frequency is increasing from left to right, starting with the extreme negative frequencies on the left, through the altitude line with zero Doppler shift to the extreme positive frequencies on the right. On the example shown there are the three main components of clutter already described.
The beginning of the sidelobe occurs at a point called the opening rate, below which targets flying much slower than the radar may become disengaged from the most negative aspect of the sidelobe clutter (SLC). At this point the Doppler shift from the receding terrain is negative and is approximately equal to the velocity of the radarð¼ 2 Vr=Þ.
The other extent of the SLC is called the closing rate, which is a positive Doppler shift ð¼ þ2 Vr=Þ. The altitude line approximates to zero Doppler shift, while the main lobe clutter (MLC) has a positive shift but less than the closing rate (depending upon the antenna look angle).
The diagram shows a total of five targets from left to right (with the magnitude of the target echoes greatly exaggerated):
Ground Return
Main Lobe Clutter Look Angle
Left of Track
Right of Track
Side Lobe Clutter Figure 4.6 Principal returns for a pulse Doppler radar.
1. Target 1 has a high opening rate (Doppler shift >2 Vr=) and appears to the left of the negative SLC.
2. Target 2 has a low opening rate and appears through the negative SLC.
3. Target 3 has a low closing rate and appears through the positive SLC.
4. Target 4 is flying at the same velocity or tangentially to the radar and is masked by the MLC.
5. Target 5 is approaching at high speed with a high closing rate and appears in the clutter-free zone to the right of the SLC (Doppler shift >þ2 Vr=).
It should be noted that target echoes appearing outside the SLC, altitude line and MLC regions will still have to be detected among the receiver noise, as for standard pulsed radar.
It can be seen how many variables affect where clutter and the target appear with respect to each other: radar velocity and radar and target relative velocities, antenna position, target geometry, etc. As many of these variables alter, as they rapidly will in a dynamic combat situation, the shape and relative positions of the target echoes and clutter regions will change quickly with respect to one another.
In certain situations, especially when operating at medium PRF, unwanted returns can be received in the sidelobes from very large ground targets. Industrial plants such as refineries and chemical processing facilities can produce significant returns that can be captured in this way, even if the sidelobe gain is 30 dB below the main beam. A solution to this problem is to use a guard horn and guard receiver channel, (Figure 4.8).
The guard horn has a broad low gain response and the gain is selected such that it is positioned below the antenna main beam response but just above the response of the first
Side Lobe Clutter (SLC)
Target 1 Target 2 Target 3 Target 4 Target 5
= - 2 x Vr
Figure 4.7 Typical Doppler spectrum.
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sidelobes. The receiver architecture features two parallel channels for the main antenna and the guard horn. Normally, the main returns from targets will be detected via the main beam and usual signal processing will occur. Target returns will be received in the guard channel but, owing to the low gain of the guard horn, will not be detected. When a target is detected simultaneously through the guard horn and the main antenna channels, decision logic will cause the main receiver output to be inhibited. Therefore, any targets that would have been detected from the sidelobes may be suppressed from the display. This technique may also be used as an electronics countermeasure tool to negate unwanted jamming entering via the antenna sidelobes.
4.2.1 Range Ambiguities
The effect of range ambiguities in a basic pulsed radar was discussed in Chapter 3 and shown in Figure 3.17. However, that diagram portrayed an air-to-air engagement where no ground clutter was present. The situation becomes more complex in pulsed Doppler when significant ground clutter has to be taken into account. The problem is simply stated in Figure 4.9.
This diagram shows a situation where a pulsed Doppler radar is looking down at three targets: two aircraft and a moving ground vehicle. The range return comprises three main elements:
The altitude return on the left;
The first target, clear of the MLC;
The second and third targets obscured by the MLC;
If a high PRF is being used, then the range sweep may be less than the total range shown in the diagram.
Figure 4.8 Use of a guard channel to reduce sidelobe clutter.
It may be assumed that, owing to the use of the high PRF, the range is split into effectively three unambiguous sectors or zones. The overall effect is to superimpose the range zones on top of each other as far as the receiver is concerned. This situation is more complex than the example given in Chapter 3 as there is also the altitude return and MLC to consider. The overall effect is shown in Figure 4.10.
The effect of superimposing the range zones leads to the composite return at the lower right; this is far more difficult to unscramble than for the simple air-to-air non-clutter case.
The one target that was detectable outside clutter has now been totally subsumed. This extremely simple example shows how altering the PRF – in this case increasing it, probably for good reason – has had the effect of losing the target in a combination of the altitude return and MLC.
4.2.2 Effect of the PRF on the Frequency Spectrum – Doppler Ambiguities The effect of the PRF on the frequency spectrum also needs to be considered. In an earlier description the frequency spectrum of a short coherent pulse train was shown in Figure 3.4 to Figure 4.9 Representative flight profile.
Zone 1 Zone 2 Zone 3
Zone 1
Zone 2
Zone 3
Composite
Figure 4.10 Effect of superimposing range zones.
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be a series of sin x=x frequency responses repeated at an interval determined by the PRFð f2Þ.
This was an ideal case and represented the ideal pulse response. As we have seen in Figure 4.7, the true Doppler frequency response is in reality far more complex. Figure 4.11 shows the real situation for a high PRF.
The spectrum comprises replica sets of the true Doppler frequency response repeated at intervals determined by the PRF and modulated by a sin x=x envelope of width 2= , where is the width of the pulse train. In this case the full Doppler passband, containing all the components of the Doppler spectrum, is clearly seen, as the high PRF spaces out the Doppler spectra so that there is no mutual interference with the sidebands (which are also replicas of the Doppler frequency response).
If the PRF is reduced, then the situation portrayed in Figure 4.12 can occur. The sin x=x envelope is unaltered since the length of the pulse train is unchanged. However, the true Doppler return is now overlapping with upper and lower sidebands, giving the very confusing and ambiguous composite Doppler profile shown at the bottom of the figure.
It is clear that, when considering the operation, of pulsed Doppler in a look-down mode of operation, the system design characteristics need to be chosen with care. In particular, the selection of PRF is particularly crucial.
4.2.3 Range and Doppler Ambiguities
The operation of a pulse Doppler radar can be affected by both range and Doppler ambiguities. The range ambiguity is determined by the 1=x relationship already described in Chapter 3 (Figure 3.17) and is a fairly straightforward relationship. The determination of Doppler ambiguity is more involved since it depends on the PRF, the velocity of the radar and the carrier frequency (wavelength) being used (Figure 4.13). The gap between two adjacent Doppler spectra is determined by the value of the radar PRF such that the altitude
fr fr fffrrr fffrrr fffrrr fffrrr
sin x
x envelope
High PRF
fr fr
Doppler Passband
-fo 0 +fo
Figure 4.11 Effect of a high PRF on the frequency spectrum.
fr fr fffrrr fffrrr fffrrr fffrrr sin x
x envelope
Low
PRF fr fr fr fr
-fo
-2fo 0 +fo +2fo
True Doppler Profile
+ 1st Lower Sideband
+ 1st Upper Sideband
+ 2nd Lower Sideband
+ 2nd Upper Sideband
Composite Doppler Profile
⇒
⇒
⇐
⇐
⇐
Figure 4.12 Effect of a low PRF on the frequency spectrum.
0
0
0
0
fr
fr
fr
fr Maximum Unambiguous
Positive Doppler
fr
2Vr
l T
T
T
T
Figure 4.13 Definition of maximum unambiguous Doppler.
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line of the second spectrum will occur at frequency fr after the altitude line for the first spectra (the altitude line also represents zero Doppler shift). However, the negative SLC extends back from the second altitude line. The maximum unambiguous positive Doppler is defined as fr 2 Vr=, as shown in the figure. The reason can be seen from the diagram. It shows four closing targets with increasing closing velocity from top to bottom. As the target velocity increases, so the target Doppler shift increases in frequency and moves further to the right until finally, in the last example, the target return has been subsumed by the negative SLC of the previous pulse.
The effect of ambiguous range and ambiguous Doppler is shown in Figure 4.14. In the top left of the diagram the effect of range ambiguity with respect to range and PRF is shown. The range area in which no ambiguity occurs is represented by the shaded portion shown at the bottom left part of the plot, adjoining the axis. Everything to the right of the curve represents areas where the range is ambiguous. This simple 1=x relationship is unaffected by changes in Vr or .
The areas affected by Doppler ambiguity are shown in the top right picture. The unambiguous Doppler areas are shown as the shaded portion at the bottom right. Every point on the diagram to the left of the diagram represents ambiguous Doppler.
At the bottom of Figure 4.14, both diagrams are combined to give a total picture of range and Doppler ambiguous and unambiguous areas. The shape of this diagram depends upon the carrier frequency and therefore wavelength and radar velocity. For the example shown, is 3 cm, which is equivalent to a carrier frequency of 10 Ghz, typical of a fighter radar; Vr is assumed to be 1000 knots. Therefore, the diagram is based upon realistic figures relating to a supersonic engagement.
Range (nm) Range Rate (knots)
PRF (kHz)
Figure 4.14 Areas prone to range and Doppler ambiguity.
Both range and Doppler ambiguities may be resolved by changing the PRF at which the radar is operating, and the use of staggered or multiple PRFs is often used for this purpose.
Increasing increases the size of the unambiguous Doppler envelope, while decreasing has the reverse effect. Therefore, an AWACS radar operating at3 GHz/10 cm will have a much bigger unambiguous Doppler envelope than that shown in Figure 4.14. Decreasing radar velocity has the reverse effect, and therefore a fighter radar closing at only 500 knots will have a much smaller unambiguous Doppler envelope than the one shown in Figure 4.14.
Figure 4.15, shows the difference between the unambiguous Doppler zones for an AWACS and a fighter aircraft. The choice of PRF is crucial to obtaining the optimum performance of the radar. Normally, the three PRF bands shown at the top of the figure are considered:
Low PRF 250–4000 Hz;
Medium PRF 10–20 kHz;
High PRF 100–300 kHz.
These figures are indicative; precise figures may vary from radar to radar, depending upon the design drivers and the precise performance being sought.
The advantages and disadvantages of each of the PRF bands depend in large measure upon the type of radar mode being used and the nature of the target engagement. For an extensive Figure 4.15 Factors affecting the unambiguous Doppler zone.
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review of the benefits and drawbacks of each PRF type, see Stimson (1998) and Skolnik (2001).