Principles of Modern Radar - Volume 2 1891121537

756 CHAPTER 17 Advanced Processing Methods for Passive Bistatic Radar Systemsfor transmission of information, the waveforms of opportunity have an intrinsic randombehavior. This implies that very often their AFs have time-varying sidelobe structures alongboth bistatic range and Doppler. Moreover, these sidelobes exist at a level not greatly lowerthan the peak [3]. This can lead to the following:1. Strong clutter echoes masking targets with high Doppler frequencies2. A small fraction of the direct signal being received via the sidelobe/backlobe of thesurveillance antenna (still significantly larger than the clutter echo) that masks targetecho signals3. Strong target echoes masking other echoes from other targets of a lower level, even inthe presence of large range-Doppler separationsIn addition to using carefully designed waveforms, in active radar systems theseproblems are typically addressed either by applying tapering to the received signal tolower the sidelobes or by using MTI canceller filters to remove the strong stationaryclutter echoes. Unfortunately, neither of these can be directly applied to passive radar.The standard taper functions are typically effective in controlling sidelobes due to thesignal characteristics at the leading and trailing edges of the conventional radar pulsesthat have a well-known shape and can be modified by appropriate filtering. In contrast,the typical sidelobes of the waveforms of opportunity are due to fast amplitude and phasemodulations of the waveform spectrum related to the transmitted information content thatis obviously unknown and time varying. This makes it impossible to design filters for PBRthat are able to control the sidelobes to very low levels.Also, the masking effect due to the low peak sidelobe level of the AF cannot be removedusing conventional MTI techniques. A standard canceller is based on subtractingtwo or more consecutive echoes (with appropriate weights), with the idea that the echofrom stationary targets in two consecutive echoes would be exactly the same and the differencewill cancel the undesired contributions from the stationary background. In passiveradar, two successive batches of the received signals are different even if they have beenreflected by the same stationary background, since the transmitted waveform changescontinuously.Hence, proper adaptive cancellation filters have been designed for this purpose [4, 29–34]. The goal of the cancellation stage is to remove as much as possible the undesiredinterference to extract the desired target signal from the surveillance signal observation.The complex envelope of the signal received at the surveillance channel can be written aswhereN T∑s surv (t) = A surv d(t) + a m d(t − τ Tm )e j2π f DmtN Sm=1∑+ c i d (t − τ ci ) + n surv (t) 0 ≤ t < T 0 (17.12)i=1T 0 = observation timed(t) = the complex envelope of the direct signal (a delayed replica of the transmittedsignal)A surv = the complex amplitude of the direct signal received via the sidelobe/backlobeof the surveillance antenna