Communications, Radar & Electronic Warfare (201.. - Index of

Non-Communications Electronic Warfare 329Jamming secondary radar would prevent the ground station from receiving all flightaltitude data in the area.Another method of exploiting secondary radar is to return a false return signal,allowing combatant aircraft to pretend to be normal civilian traffic. This is anotherversion of spoofing.18.2 Radar Jamming Techniques18.2.1 Cover Jamm;'lgCover (noise) jamming works in the same way as for communications jamming. It desensitisesthe receiver and makes it more difficult for it to receive valid returns. Theincrease in noise will be picked up by the radar receiver Automatic Gain Control(AGC). This will cause the AGC to compensate for the increased input and in doing soit will cause low level targets to be rejected. AGCs are essential in radar design to allowthe system to work over a wide dynamic range but retain sensitivity from moment tomoment. Since the lowest signal returns are obtained towards the edge of the radar'scoverage, these are the returns that will be affected first. This means that the range ofthe radar will be reduced. If the jamming noise at the radar receiver is sufficiently high,the whole system may cease functioning due to overload.Figure 18.2 shows two phases of ajamming scenario. In the first, the radar is pointingdirectly at the jammer. The jammer to radar path is shown as it passes into the mainbeam of the radar. The radar transmit and target reflection paths are shown below theradar to differentiate them from the jammer path but of course they would really beoverlapping on the radar main beam in practice. In this scenario, the jammer has theadvantage because the reflection from the jammer aircraft skin would be very small.The jammer can transmit a relatively small signal that is larger than the reflectionpower and still degrade the radar receiver response. However, in the second phase, theradar is no longer pointing at the jammer and the jamming signal is in the sidelobes ofthe radar antenna response. In this case, the jammer advantage is gone due to theattenuation of the sidelobes compared to the main beam.Jammer noise is difficult to reject, especially when it enters the main beam. Thereare potential methods to counter it, but all require trade-offs. The principle problem isdue to the AGC in the receiver. The jamming in energy can be reduced by the use ofpulse compression, which counters narrowband energy but in this case, the jammer canuse barrage jamming to counter the counter-measure, or if the jammer is using highenough narrowband noise, it can still saturate the receiver.One other point of interest in this scenario is bum-through. If the jammer powerremains constant as the jamming aircraft approaches the target, its power increases in asquare law function if we simply assume free space loss. The radar signal has to reachthe aircraft and return, doubling the distance. Thus, burn-through will not be achievedif the jammer is heading directly towards the radar.