Exploiting polarimetric diversity in passive radar

Radar polarimetry has been widely used in radar systems aiming at the detection, discrimination, and recognition of targets of interest among other interfering sources. It becomes an essential tool in challenging radar scenarios where the achievable performance is not under control of the radar designer. This is certainly the case of passive radar whose performance largely varies with the radiative properties of the transmitter of opportunity as well as with the severity of the electromagnetic (EM) scenario that typically includes many interfering sources. Passive radar, also known as passive coherent location (PCL), technology has been attracting significant research interest over the past two decades. The wide interest received by PCL sensors allowed them to increasingly reach a point of maturity. Nevertheless, by relying on signals emitted by illuminators of opportunity (IOs) to detect and localize targets, the performance of PCL sensors might be largely time-varying and leading to an unreliable surveillance system. The main limitations stem from the lack of control over the exploited waveform structure as well as from the strong direct signal and multipath contributions. In addition, significant interference can be experienced due to co-channel or adjacent-channel transmissions, especially when broadcast emitters are exploited as IOs. Recently, among the advanced processing strategies devised to overcome these limitations, the exploitation of polarization diversity has been considered as a mean to increase the reliability of passive radar with particular reference to the target detection task. Specifically, the availability of multi-polarimetric channels on receive has been demonstrated to provide robustness against the target echo fading that results from the induced variable polarization. Moreover, a proper combination of signals received via differently polarized antennas has been shown to improve the target identification capability against the interfering sources, thanks to the wave polarization diversity. In this chapter, we take this perspective and illustrate practical multi-polarimetric passive radar architectures together with the corresponding signal processing techniques. Different approaches are compared both in terms of achievable performance and based on considerations regarding the resulting system complexity. The aim is to offer the interested reader with an overview of existing solutions and to provide hints for the identification of the most suitable architecture for the application at hand. The discussion is supported by results from real-world tests performed by means of experimental passive radar receivers operating at different frequency bands.