## 7 Conclusions  
This paper tackles the problem of radar detection in a 3D target parameter space using a DVB-T-based PRS, with an NULA in the surveillance channel and spatial filtering in the frequency domain. The proposed solution allows the estimation of target azimuth, Doppler and range, to improve the target localization accuracy and to make traffic monitoring easier. Independent acquisition chains were considered for digitizing the signal acquired by each single element of the array. Taking into consideration the requirement of using COTS components in the demonstrator to the greatest possible extent, the commercial Televes 4G NOVA antenna was selected as the array element for the surveillance channel. This antenna provides a good range of steering, reduced dimensions, high front-to-back ratio and the capability to fulfill the high gain requirement for passive radar detection. The use of non-uniform linear arrays was proposed, and a cost function was designed to determine the inter-element distances, to obtain a compromise solution of sidelobe levels and main beam width. The formulated optimization problem was solved using a genetic algorithm. Arrays with five and eleven elements were designed. \(N=5\) was selected because this is the number of available acquisition chains for the surveillance channel of the passive radar demonstrator. \(N=11\) was selected to prove the potential improvement achievable when more acquisition chains are available, as expected in operative systems. Beamforming has been applied in the frequency domain, using a modified version of the previous two-stage algorithms published by other authors:  
* The first step generates a full-dimensional beam-space based on orthogonal beams in the azimuth coverage area where the CFAR detector is applied. Beamforming weights are calculated for guaranteeing an SLL (dB) lower than a specified value (15 dB in the presented results). In the CFAR detector, the windowing technique defines CUTs related to maxima in the beam-space and 3D reference windows that exclude a set of guard cells to reduce the impact of targets contributions around the CUT, in detection threshold estimation. The use of 3D windows reduces CFAR losses. * The angles of arrival of the detected targets are estimated using the output spectrum of a new denser beam-space generated using weight vectors calculated to maximize the directivity. Performance improvement associated with the use of NULA instead of ULA and the combination of weighting techniques to control SLL and maximizing directivity, instead of uniform conventional beamforming, was proven by simulation. The designed NULA constitutes a good base architecture to implement beamformers under the specified design criteria, outperforming the ULA for the whole system coverage sector, especially when eleven elements are considered. Finally, a semiurban radar scenario, with the PRS located at the rooftop terrace of the Polytechnic School of the University of Alcala, was analyzed to validate the capabilities of the considered solution for monitoring terrestrial targets. The environment is characterized by low-height buildings and several roads. PRS coverage was estimated using WinProp (AWE Communications) software and including geographical data, considering that the desired targets are on the ground. A controlled car with a GPS device was used during the measurement campaign. Results obtained with real data validate the conclusions extracted from the simulations of the proposed array architecture and the two-stage array signal processing. This work has been partially funded by the Spanish \"Ministerio de Economia y Competitividad\", under Project TEC2015-71148-R, and the European FP7 Project FP7-COOPERATION-2013-SEC-SCOUT-607019.