# Dual Cancelled Channel STAP for Target Detection and DOA Estimation in Passive Radar
## 1 Introduction  
Recent advances in passive radar research have opened interesting new perspectives and application areas [1, 2, 3]. Among them, in the last few years, the potentialities of passive radar mounted onboard moving platforms have been investigated, aiming towards synthetic aperture radar (SAR) imaging [4, 5, 6, 7] and ground moving target indication (GMTI) [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18] applications. One of the main obstacles to the use of passive radar on moving platforms is the motion-induced Doppler distortion of the received signals. On the one hand, this can complicate the reception and reconstruction of the reference signal [19, 20]. On the other hand, in surveillance applications, the detection and direction of arrival (DOA) estimation of slowly moving target echoes are hindered by the Doppler-spread clutter returns from the stationary scene. This issue is exacerbated by the typical broad antenna beams available at VHF/UHF bands of the most widely used illuminators of opportunity. Therefore, a proper suppression of clutter signal is required. This can be achieved by operating in the space-time domain, exploiting systems featuring multiple channels on receive. However, numerous critical aspects brought in by the passive radar framework must be tackled, while possibly preserving the paradigm of a low-complexity system. This topic has been recently addressed by these and other authors, with reference to passive radar sensors equipped with two or more channels on receive, and different effective solutions have been reported in the technical literature. In [8, 9, 10, 11, 12, 13], an approach based on displaced phase centre antenna (DPCA) was shown to be a particularly attractive and undemanding solution for passive radar applications in the case of dual-channel systems. However, the need to mitigate the potential angle-dependentchannel imbalances requires the implementation of adaptive digital calibration strategies, operating across multiple processing stages, for a convenient suppression of direct signal interference and clutter [11; 12; 13]. Moreover, with two channels on receive, only the detection of target echoes can be sought, whereas the problem of its angular localisation within the broad antenna beam remains unsolved. Such limitations can be overcome by employing multiple channels on receive and applying adaptive space-time processing (STAP) techniques [14; 15; 16; 17; 18]. Specifically, a complete scheme for STAP in mobile passive radar has been recently proposed by the authors in [18]. By resorting to a post-Doppler strategy [21; 22; 23], it easily fits into the typical passive radar processing chain and takes advantage of the long integration times to reduce the size of the adaptive cancellation problem and compensate for potential angle-dependent channel errors. A space-time detection scheme is then employed where a clutter-based calibration of the spatial steering vector allows for prevention of accidental rejection of the target echoes arising from a severe channel imbalance. Finally, the accurate target angular localisation is provided by a space-time maximum likelihood estimator (MLE). The approach in [18] enables effective GMTI capability and gains a larger flexibility compared to the DPCA schemes thanks to the adaptive use of a higher number of spatial degrees of freedom (DOF). In fact, clutter cancellation, target detection and angular localisation, and even the spatial steering vector calibration are performed according to a fullarray strategy, i.e., by jointly exploiting all the \(N\) available channels on receive.