Principles of Modern Radar - Volume 2 1891121537

564 CHAPTER 12 Electronic Protectionadditional time to set-on to the radar waveform and generate more efficient jamming.Two additional factors that may limit the duration of a burnthrough waveform are rangewalkand Doppler-walk. Range-walk occurs when a high range-rate target moves throughmultiple range bins over the duration of the burnthrough dwell. This is more likely for awideband waveform used for high range resolution. Similarly, Doppler-walk occurs whena high-acceleration target moves through multiple Doppler filters over the course of thedwell. The signal processor may be able to employ algorithms to compensate for therange-walk and Doppler-walk issues.12.7.4 Narrow-Pulse Range ResolutionRange resolution accomplished through a narrow pulse, or short pulse, can help countertransponder or repeater SSJs that have relatively slow response times or long throughputdelays [6]. A responsive noise SSJ may rely on detection of the pulse leading edge tosynchronize its transmission with the radar waveform. It may also attempt to sample theradar frequency within the early portion of the pulse in order to set-on with RSN. If thepulse is narrow enough, say less than 300 ns, the jammer may not be able to place itssignal on top of the skin return of the target due to these delays. This is especially truefor a radar with PRI agility and pulse-to-pulse frequency agility, in which case the noisejammer must dilute its energy with broadband noise jamming and may have to broadcastcontinuously rather than in a cover-pulse mode.A coherent jammer may also have difficulty with very narrow pulses. The cumulativeinsertion delay from transmission lines to and from the antennas as well as various componentsin the repeater chain may result in a total throughput delay of 200 ns or more. Againstpulse widths of 1 μs, for example, such delays might be tolerated in the jammer design,as most of the radar pulse is still covered. Against narrow pulses, however, the jammingmay cover only a small fraction of the radar pulse or may miss the pulse completely. Otherexamples of EA techniques with time-delayed returns are TB and TRD [6,9]. Jammerswith coherent RF memory and PRI tracking may be able to generate a coherent coverpulse to compensate for this throughput delay if the radar has a predictable PRI waveform.Some repeater jammers have a minimum pulse constraint that may result from limitationsin pulse parameter measurement, encoding, and processing functions, in which case thePRI tracking may never be accomplished.The main disadvantage of the narrow-pulse mode is the potential sacrifice in averagepower against noise jammers if it is used instead of longer-pulse waveforms that areotherwise available to the radar. The target SNR is inversely proportional to the pulse width,as indicated in Equation 12.5. The narrow pulse does not compromise performance if it canbe combined with a higher PRF to maintain average power or if a reduced average powercan be tolerated due to high-RCS or short-range target conditions. The potential loss ofSNR resulting from a narrow pulse is circumvented through the use of pulse compressionwaveforms, described below.12.7.5 Pulse Compression Range ResolutionPulse compression allows the radar to achieve a desired range resolution without sacrificingSNR [6]. As described in [8], instead of transmitting a narrow pulse that corresponds to thedesired range resolution, the radar transmits a much wider pulse. The wide pulse containsan intrapulse PM or FM that results in a signal of the same spectral bandwidth as the