Figure 1 presents the CAF generation for a passive radar with a single radiating element surveillance antenna. Figure 2 shows a general view of the radar scenario. The IoO illuminates the area of interest (sea) and all the potential clutter sources (urban areas, motorways, train tracks, wind farms, harbour, airports, etc.). Although main ground clutter echoes are located at the back of the surveillance antenna, from bearings of very low sensitivity, their intensities are significantly higher than those generated by the vessels to be sought, and will correlate with the reference signal, generating strong contributions compared to target ones. The bistatic range is defined as the difference between the paths IoO-target-PR and the direct path IoO-PR: \(R_{B}=R_{IoO-target}+R_{target-PR}-R_{IoO-PR}\). The bistatic velocity is defined in terms of Doppler shift and operation frequency: \(v_{B}=f_{D}\cdot\lambda\). The resolution of the system is defined by the acquisition bandwidth and the integration time for CAF processing: the bistatic range resolution depends on the acquisition bandwidth (\(\Delta R_{B}=c/BW_{acq}\)), while Doppler resolution depends on the integration time (\(\Delta f_{D}=1/T_{int}\)). The bistatic velocity is calculated as the addition of the target velocity components along the target-IoO and target-PR directions, denoted as \(v_{IoO}\) and \(v_{PR}\), respectively, which is \(v_{B}=v_{IoO}+v_{PR}\). One of the main PR interferences, besides clutter, is the Direct Path Interference (DPI) or direct IoO signal acquired by the surveillance channel. It correlates with the reference signal and produces a high level peak at the CAF origin, and raises the pedestal throughout the bistatic range-Doppler domain. On the other hand, CAF contributions of echoes generated by non-desired stationary targets (surface clutter, for example) present maximum values along the zero Doppler line and increase the pedestal throughout the CAF domain. Different techniques for DPI and zero Doppler interferences cancellation have been studied in the literature, such as Extensive CancellationAlgorithm (ECA) [11] and block Normalised Mean Least Squares (NMLS) [11; 12; 13; 14]. DPI can also be reduced through the  
Figure 1: PR geometry scheme and operation principle. Reference signal (blue), surveillance signal components (red): targets’ echoes (continuous), sea clutter (dashed) and Direct Path Interference or DPI (dash and dotted). Figure 2: General view of the radar scenario with its main components. antenna system by designing an antenna with low gain in the direction where IoO is located, or introducing a null via beamforming techniques if an array is being employed [14]. In most applications, radar operation covers detection and localisation tasks. The CAF maps targets and interference contributions in the range Doppler plane. Bearings can be estimated from Direction of Arrival Techniques (DoA) based on the use of surveillance array antennas in bistatic configurations and/or multi-static configurations involving a set of IoOs and/or a set of PRs. The DoA can be estimated using monopulse techniques or by employing arrays of antennas. The advantages of employing digital phased arrays in combination with array processing techniques are numerous, from beam shape flexibility and simultaneous beam generation to jamming and clutter suppression without forgetting the angle estimation accuracy [15]. Multiple beams can be generated and a CAF can be calculated for each beam, but to exploit the mapping generated by the CAF, a CAF can be generated for each single radiating element (using the acquired signal and the reference one), and beamforming coefficients can be calculated in the transformed CAF domain for different (range, Doppler) pairs. Adaptive Beamforming (ABF) is commonly employed in active radars to suppress jamming signals enabling target detection [16]. Deterministic and adaptive beamformers have also been designed to improve target detection and localisation performances [4; 5; 17; 18].