Multichannel passive radar systems : signal processing techniques and design strategies

Passive Coherent Location (PCL) system is one of the most rapidly developing technology in the radar field and significant progress has been made in recent years. However, some aspects that are not under the control of the radar designer still prevent PCL technology from reaching its point of maturity. The goal of this thesis is to address this issue by resorting to the exploitation of information diversity conveyed by multiple receiving channels. The main novelties that this research has led to can be identified in two main areas, one concerning the exploitation of polarization diversity for target detection, the other regarding the exploitation of spatial and frequency diversity for target localization purposes. Although the ideas underlying the main achievements reported in this work arise from the need to overcome PCL systems limitations, the research performed to reach this goal has led to achievements that have a broad scope of application, not limited to passive radar systems, which increases the scientific value of this work. In the first part of this thesis, we deal with the problem of target detection in coherent radar systems exploiting polarimetric diversity. We build upon the demonstrated benefits that a suitable use of signals collected by differently polarized antennas can lead to, and we present a new polarimetric adaptive target detection scheme. To this aim, we resort to a parametric approach and we model the disturbance affecting the data as a multi-channel AR process. First, we show the effectiveness of the proposed strategy via an extensive simulated analysis, then we carry out a performance assessment under spectral model mismatch conditions, and finally we demonstrate its validity against real data, collected by both active and passive radar systems. In the second part of this thesis, we address the problem of target direction of arrival (DoA) estimation in systems that jointly exploit spatial and frequency diversity. We derive a reliable performance characterization of a multi-carrier maximum likelihood DoA estimator in the threshold region. The capability of predicting the performance of the considered estimator in low signal-to-noise ratio scenarios is a powerful tool that can also be used to select the best performing receiving system layout, given a number of constraints. The experimental results obtained using configurations selected according to this criterion show that a suitable exploitation of spatial and frequency diversity allows to both extend the angular sector where the target DoA can be unambiguously estimated and improve the estimation accuracy.