Principles of Modern Radar - Volume 2 1891121537

17.6 Multichannel Processing for Detection Performance Improvement 803channels when available. In this regard, it is potentially of great interest to explore thecapability of automatically selecting the frequency channels to be integrated. Such automaticselection would allow the desired detection performance to be achieved withoutsignificantly increasing the number of channels and hence the computational load.The MF operation was introduced as a way to counteract the time-varying detectionperformance of the PBR. To this purpose, noncoherent integration strategies have beenadopted. However, as previously mentioned, the joint exploitation of different frequencychannels might also improve the target localization capability of the PBR system.In the first instance, a reasonable improvement in the localization accuracy can beobtained by properly combining (averaging) the measurements obtained using the signalstransmitted at different carrier frequencies. This would yield a remarkable advantage,especially for the DOA estimation in low-frequency PBR when this is obtained using asimple interferometric approach [59].Moreover a coherent combination of the 2D-CCFs obtained with the multiple receivedsignals can be exploited aiming at increasing the bistatic range estimation accuracy. Theavailability of multiple channels would result in a global wider bandwidth signal to beexploited by the PBR system. However, due to the frequency separation among the differentchannels, this signal presents a discontinuous PSD that yields high ambiguities inthe corresponding ACF. Thus, this approach cannot be used to improve the radar rangeresolution, namely the capability to distinguish two targets but can improve the singlechannel estimation of the bistatic range of a given detected target largely affected by theexploited signal ACF.17.6.2 Exploitation of Multiple PolarimetricPassive Radar ChannelsThe transmitter and receiver antenna polarization play an important role in designing aPBR system. The exploited transmitter of opportunity might use vertical, horizontal, orcircular polarization. Usually broadcast transmitters use linear polarization; most DABtransmissions are vertically polarized, whereas the horizontal (or mixed) polarization isalso used for FM radio and DVB-T signals. Circular polarization is most often used forsatellite communications. In contrast different solutions are available for the transmitterantenna used by a base station in wireless and cellular networking; in these cases, polarizationdiversity is usually adopted to minimize the effects of selective fading of thehorizontal and vertical components of a signal. Accordingly, proper polarizations mightbe exploited for the reference and the surveillance antennas used by a PBR system aimingat improving its performance. Assuming the availability of at least two receiving antennaswith different (orthogonal) polarizations, many approaches might be envisaged to use theconveyed diversity aiming at specific goals.Typically, at the reference channel, the main objective is to recover a good copy of thetransmitted signal; to this purpose a switching or a selection strategy might be employedto pick out the signal from the preferred antenna (e.g., the one yielding the highest DNR).Alternatively, the signals collected at the two orthogonally polarized antennas might becombined to reject the multipath contributions.With reference to the surveillance channel, different objectives might be pursued:1. The polarization diversity has been exploited to increase the direct signal rejectionthat can be obtained by selecting the antenna with cross-polarization with respect to