## 2 Problem Formulation
It covers the main naval routes from and to Cadiz harbour and includes the approaching route of aerial targets to Rota Military Base airport, with a huge number of communication and radar systems. * The IoO is located next to Jerez de la Frontera, at a distance of 21 km from PR location. The central acquisition frequency is tuned to 774 MHz, comprising two DVB-T channels of around 8 MHz each. * Wind farms areas which are harmful interference sources due to the high Radar Cross Section (RCS) of wind turbines, up to 60 dBsm [24; 25; 26]. * A set of roads just at the back of the PR system, with ground targets that are considered to interfere, as they are not expected in the defined area of interest. The radiation pattern of the two beams under study in this article is also superimposed in the scenario: beam 1 steered to \(-30.5^{\circ}\) and beam 3 steered to \(0^{\circ}\). The side lobes due to array configuration can be appreciated. Side lobes, even providing considerably less gain than the main lobe, can cause the detection of targets in more than one beam, i.e., not only in the beam whose HPBW contains the target's direction. Side lobes are also responsible for interferences coming from different directions than the main one; for example the wind turbines in the considered scenario, which worsen the detection capability. Taking into consideration targets' dynamics, CPIs of 500 ms were generated with a Pulse Repetition Interval (PRI) of 250 ms, and processed for Zero-Doppler interference cancellation using an ECA filter, and CAF generation. Seven CAFs (one for every single radiating element of the surveillance array) were obtained, and the set of orthogonal beams was generated in this transformed RD domain. Figure 6 shows the maxima of the CAFs for the rightmost beam (\(-30.5^{\circ}\) with respect to the array boresight, \(300.5^{\circ}\)N) and the central one (at the boresight, \(270^{\circ}\)N). These two beams under study are represented in Figure 5 overlayed in the scenario. Maxima were calculated along a block of 160 consecutive CPIs. The first beam contains the area closest to the harbour where more targets are present. There are a lot of high-power scatterers that group during the acquisition time, forming trajectories in this beam, whereas no targets are appreciated in beam 3. Figure 4: Scheme of processing stages employed showing beamforming processing in range-Doppler domain. The main problems observed in these RD maps are the high-level lines located at different ranges that extend along the Doppler dimension crossing the zero-Doppler. These structures not only can mask low-Doppler targets at the same ranges, but can also increase the CAF pedestal level, reducing the SIR of targets far from the position of the lines in the CAF. Figure 7 shows a more detailed analysis of beam 3. This beam is interesting due to the absence of targets of interest. In Figure 7a, four main clutter contributions are observed. One is around range bin 156. The second one, which is more dispersed, extends from range bin 296 to bin 353; another one appears at range bin 605, and the last one appears at range bin 935. Figure 7b depicts isorange contours for these range bins and allows the identification of wind farms as potential interference sources. This cannot prove that those interferences are due to wind farms and not due to sea clutter, as they can not be decoupled, but the coincidence of iso-range curves with the areas of wind turbines indicates that these may be the sources. Figure 5: Sketch of the trials scenario located in Cádiz. It shows the IoO and PR locations, the Area of Interest and wind turbines interference areas. The pattern of the two beams under study are also superimposed: beam 1 steered to \(-30.5^{\circ}\) and beam 3 steered to \(0^{\circ}\). Figure 6: Cumulative of maxima of RD during the acquisition time for two orthogonal steerings. In addition to the increase in the RD map background and possible targets undercover, another problem is the non-stationary character of interference sources over time.