Principles of Modern Radar - Volume 2 1891121537

804 CHAPTER 17 Advanced Processing Methods for Passive Bistatic Radar Systemsthe transmitter antenna [7, 8]. As an alternative, it is possible to properly combine thesignals received at different antennas. This solution requires the availability of multiplereceiving channels, but it might be more effective when the transmitter polarization isunknown or when the direct signal polarization experiences slow changes due to thegeometric and atmospheric conditions (e.g., satellite transmissions).2. Assuming that the adopted cancellation algorithms allow a sufficient direct signalcancellation, the polarization diversity might be exploited to reject other interferingsignals being transmitted in cross-polarization in the same or adjacent frequency band.3. Target echoes show a random polarization due to the reflection of the transmitted signalon the target complex structure; thus, using a fixed polarization for the surveillanceantenna might result in a significant SNR degradation depending on the target characteristics.It can therefore be expected that a proper combination of the signals receivedat differently polarized antennas might yield a detection performance improvement forthe resulting PBR.In the following we report some examples of the potential exploitation of differentantenna polarizations to mitigate the effect of co-/adjacent-channel interferences (i.e.,approach 2) in FM-based PBR [60]. To illustrate the effect of interfering signals on FMbasedPBR performance, we exploit the real data set collected during the acquisitioncampaign carried out on April 30, 2009, at the same site and with the same geometrydepicted in Figure 17-31. In this case a quad-channel experimental PBR prototypewas used that allowed for synchronously sampling the analog signals from up to fourinput channels. Simultaneous downconversion of up to 16 arbitrary signal bands (e.g.,16 FM radio channels) is provided by four Graychip GC4016 quad digital downconverters.The described setup allows data to be collected from up to four different FMradio channels, each one from four different antennas. In the considered acquisition campaign,four linearly polarized directive antennas were used. Specifically, two of them wereemployed as reference antennas, one vertically polarized (Ref-V) and one horizontallypolarized (Ref-H), and steered toward a transmitter located on Monte Argentario (seeFigure 17-31). Similarly, the two cross-polarized surveillance antennas (Surv-V, Surv-H)were pointed at about 180 ◦ with respect to the exploited transmitter. The following resultswere evaluated for two FM radio channels for which the performance has beenrecognized to be highly affected by co-/adjacent-channel interferences (channels SFC2and SFC4 in Table 17-8). This is shown in Table 17-9 that reports the comparison oftheoretical cancellation (CA Theo ) and actual cancellation (CA) obtained with the cancellationalgorithm described in Section 17.3. Different combinations are considered forthe reference and surveillance antenna polarizations for both the considered FM channels.The reported results have been averaged over 681 consecutive acquisitions of 1.15sec each. As is apparent, the actual cancellation values are always significantly lowerthan their theoretical expectations. The obtained CA loss cannot be due only to limitationsof the system hardware or the exploited processing techniques. In fact, at differentFM channels only small CA loss is obtained (e.g., the best-performing FM channels ofTable 17-8, SFC1 and SFC3). Nevertheless, the performed analysis demonstrated thathigh CA loss is mainly due to the presence of interfering sources affecting the PBRperformance.The acquired signals have been demodulated and listened to before and after thecancellation stage: as expected, the reference and surveillance signals before cancellationsound identical, whereas in most cases a different radio program can be listened