Principles of Modern Radar - Volume 2 1891121537

12.8 Receiver-Based EP 567RSN bandwidth. The jammer power is thus diluted by 23 dB (a factor of 1000/5 = 200),whereas the radar power per frequency is diluted by only 10 dB, corresponding to the 10distinct frequencies: a 13 dB reduction of JSR. A more efficient jammer may be able togenerate multiple spot noise waveforms, however, thereby incurring only a 10-dB dilutionloss and resulting in no net advantage to the radar.Another possible result is that the jammer lacks sufficient bandwidth to detect andrespond to all frequencies simultaneously and may therefore miss one or more frequenciesentirely. If the radar is able to receive and process these frequencies, it may be able to getan unjammed look at the target. A jammer may overcome this by time-sharing betweenthe multiple frequencies if the radar uses multiple simultaneous frequency radiation inconjunction with a multiple-pulse waveform.The multiple simultaneous frequency radiation capability is a potentially costly feature.For sidelobe operation, it may involve auxiliary transmitters and antennas. For mainlobe operation, it may involve multiple exciters and transmitters or it may force a sharedtransmitter to operate in its linear region in order to avoid intermodulation products, therebyreducing radiated power.12.7.9 Pulse Repetition Frequency VariationThe variability of the radar PRF drives up the complexity of a transponder or repeaterthat is required to identify and possibly temporally track radars in the environment. Theradar PRF can be varied pulse to pulse or, in the case of a pulse-Doppler radar, burstto burst (sometimes referred to as dwell to dwell). Pulse-to-pulse jitter introduces small,random-like variations in the radar PRI; pulse-to-pulse stagger introduces regular switchingbetween a several specific PRIs. Deceptive jammers attempting to project false targetsin front of their true range will be unable to do so if the radar PRI is not predictable frompulse to pulse. For burst-to-burst variations, the jammer must first measure the new PRIand adapt its EA transmission accordingly. It may also have to correlate the previous andcurrent PRIs to the same emitter being countered before resuming the EA technique. Theseeffects may potentially interrupt and delay the jammer response with each PRF change.12.8 RECEIVER-BASED EPThe receiver encompasses the signal path that accepts the RF input from the antenna, translatesthe signal to one or more IF stages, and finally converts the signal to digital wordsthrough an ADC. Many of the receiver design practices used to optimize radar performanceunder natural environments, such as those related to dynamic range and spectral filtering,also provide inherent EP benefits. Receiver EP capability may be further enhanced to handlespecific types of jamming through specialized gain and filtering methods or additionaldetection channels. A description of radar receiver design is provided in [8].12.8.1 RF PreselectionThe RF preselector is a front-end RF filter that passes the frequency spectrum of the radarwaveform but rejects out-of-band interference. The radar receiver–signal processor chaintypically employs band-limiting IF filters in its down-conversion mixing stages as well asmatched filter processing in the detection stage. Although the RF preselector is much wider