## 2 Problem Formulation  
Currently, IDEPAR consists of eight acquisition channels with a maximum bandwidth of 100 MHz and operating frequencies ranging from 40 MHz to 6 GHz. Within the possible opportunity signals available in this range, DVB-T has been selected for the research carried out in this article. One channel is dedicated to reference signal acquisition, and the other seven are used to acquire the signals captured by the seven single radiating elements of the surveillance array. The reference channel employs a commercial antenna _Televes DAT 75_ which provides high directivity. The surveillance antenna is the Non-Uniform Linear Array (NULA) described in [5; 23] and shown in Figure 3. The beam-width of these antennas at DVB-T frequencies is \(\pm 42^{\circ}\) in the azimuth plane, so the total field of view of the system employing the array is \(84^{\circ}\). The main parameters of the array were obtained for two steering angles: \(Pti_{sa})=0^{\circ}\) and \(Pti_{sa})=\pm 30^{\circ}\). Table 1 summarises the array gain (\(G_{\Phi_{sa}=0^{\circ}}\) and \(G_{\Phi_{sa}=\pm 30^{\circ}}\)), the Half Power BeamWidth, HPBW, (\(BW_{3 dB_{\Phi_{sa}=0^{\circ}}}\) and \(BW_{3 dB_{\Phi_{sa}=\pm 30^{\circ}}}\)), and the SideLobe Level (\(SLL_{\Phi_{sa}=0^{\circ}}\) and \(SLL_{\Phi_{sa}=\pm 30^{\circ}}\)). Array signal processing techniques are applied in the CAF transformed domain [4]. The reference signal is used to generate seven CAFs, one for each surveillance single radiating element. Figure 4 depicts the processing architecture. The CAF generation is included in the Processing Stage. For each [m,p] pair, a snap shot is obtained that will be applied to DoA and beamforming techniques. Digital beamforming is applied, applying the corresponding set of weights **w** to each to each snapshot to form a beam in the desired steering direction, fulfilling maximum directivity. Next, five-orthogonal beams within the single element HPBW are generated. Those beams are steered to azimuth angles \(\phi_{sa}=[-30.5^{\circ},-14.5^{\circ},0^{\circ},14.5^{\circ},30.5^{\circ}]\). Due to the sign convention applied, negative steering beams are at the right of bore-sight in the surveillance area seeking direction. The output of each beam is obtained following Equation (2), where \(\textbf{X}[m,p]\) is a matrix of \(N\)-elements rows times the number of total samples of each RD map (RD cells) (Equation (3), \begin{table}
\begin{tabular}{c c} \hline \(G_{\Phi_{sa}=0^{\circ}}\) & 15.2 dBi \\ \(BW_{3 dB_{\Phi_{sa}=0^{\circ}}}\) & 13.6\({}^{\circ}\) \\ \(SLL_{\Phi_{sa}=0^{\circ}}\) & 14.9 dB \\ \(G_{\Phi_{sa}=\pm 30^{\circ}}\) & 14.4 dBi \\ \(BW_{3 dB_{\Phi_{sa}=\pm 30^{\circ}}}\) & 15.5\({}^{\circ}\) \\ \(SLL_{\Phi_{sa}=\pm 30^{\circ}}\) & 13.6 dB \\ \hline \end{tabular}
\end{table} 
Table 1: Main parameters of the surveillance array (Figure 3) [5]. \(G_{\Phi_{sa}=0^{\circ}}\) and \(G_{\Phi_{sa}=\pm 30^{\circ}}\) are the array gain at steering angles (sa) \(0^{\circ}\) and \(\pm 30^{\circ}\), respectively, \(BW_{3 dB_{\Phi_{sa}=sa^{\circ}}}\) is the half power beamwidth (HPBW) for both steerings, and \(SLL_{\Phi_{sa}=sa^{\circ}}\) is the SideLobe Level. Figure 3: Prototype of single radiating element and back view of Non-Uniform Linear Array employed for the surveillance channel in the trials scenario. where the subscript indicates the element or antenna that is used in each CAF) and \(\mathbf{Y}[m,p]\) is the output RD map for each desired beam. \[\mathbf{Y}[m,p]=\mathbf{w}^{H}\mathbf{X}[m,p] \tag{2}\]  
\[\mathbf{X}=\left[s^{1}_{CAF}[m,p];s^{2}_{CAF}[m,p];\cdots;s^{N}_{CAF}[m,p]\right] \tag{3}\]  
The measurements considered in this article were carried out during trials in a coastal scenario in the south of Spain (Cadiz). The main elements of the scenario that can be seen in Figure 5 are:  
* The Area of Interest (AoI) or surveillance area defined by the surveillance antenna broadside direction (270\({}^{\circ}\)N) and the single radiating element HPBW.