Principles of Modern Radar - Volume 2 1891121537

792 CHAPTER 17 Advanced Processing Methods for Passive Bistatic Radar SystemsHowever, in many PBR applications, we might assume that the exploited transmitter ofopportunity is simultaneously broadcasting different signals at different carrier frequencieswithin the assigned band. The availability of N ch different frequency channels providesa large diversity, and it is likely that the joint exploitation of the corresponding signalsyields a significant performance improvement for the resulting multiple-frequency (MF)PBR. Specifically, the signals received at different frequency channels might be jointlyexploited to:1. Make the detection scheme robust with respect to both the content of the broadcastchannel and the propagation channel conditions, thus improving the detection performanceof the PBR and its reliability.2. Improve the localization capability of the system by properly combining the measurementsobtained using the signals transmitted at different carrier frequencies.The first issue is mainly addressed in the following. PBR systems exploiting FM radiosignals are discussed since they are very well suited for the proposed MF approach. In fact,an FM radio transmitter usually broadcasts different radio channels in the 88–108 MHzband that can be simultaneously acquired by a single receiving channel with reasonablebandwidth. Moreover, the FM-based PBR represents a worse case since it has been recognizedto be highly affected by the time-varying characteristics of both the transmittedwaveform and the propagation channel conditions [47, 55–58], as briefly described in thefollowing.The standard assigns a nominal frequency bandwidth of 200 kHz to each FM radiobroadcast channel. However, the PSD of a FM signal might experience a considerablevariability with dependence on the program content and the time of transmission. Forexample, an FM speech channel shows a high degree of temporal variability in PSDwidth and in turn in range resolution; in addition, the presence of long periods of silenceor speech pauses can dramatically degrade the achievable range resolution. In contrast,music programs usually lead to wider PSD and exhibit less variation, thus resulting in abetter range resolution, [3, 47]. Therefore, the actual range resolution is highly varying withthe instantaneous transmitted waveform and thus with the considered FM radio channel.Figure 17-28 reports the spectrograms of different data files (of about 1.1 sec) collectedat different times and FM radio channels. It clearly shows the fast temporal variability ofthe FM signal instantaneous frequency bandwidth and the highly different behavior of thewaveforms contemporaneously transmitted at different carriers.A second highly varying characteristic of the AF concerns the level of its sidelobes.Figure 17-29 shows the PSLR evaluated for 32 different real data tracks with differentprogram contents. With the exception of the silence leading to a periodic AF caused bythe stereo pilot tone (totally ambiguous, thus yielding a PSLR of 0 dB), a PSLR of about20 dB is typical for music tracks. The low values confirm that significant masking effectscan be present for weak targets. It is likely that a voice track contains silence pauses, so ityields usually slightly worse PSLR. The three categories of content (music, voice, silence)are typically all present in turn on each FM channel, with a mixing largely depending onthe type of radio channel with a large variability inside the FM band.Recall that the global power transmission level of the considered FM radio channelsstrongly affects the PBR detection performance. The transmitting stations broadcast manyradio programs on different FM channels, with different power levels due to differentcoverage requirements. An FM radio channel can experience an abrupt power loss due