## 1 Introduction  
A Passive Radar (PR) is a system that has the ability of detect and track targets without transmitting any electromagnetic signal, by taking advantage of the signals from other communication systems that are available in the scenario where the PR operates. The transmitter that emits that signal is called Illuminator of Opportunity (IoO). The IoO can be either from terrestrial communications systems or from satellite ones. In PRs, the lack of devoted electromagnetic emission provides some useful advantages with respect to active radars, such as reduced cost and low power supply, which means portability potential, null electromagnetic pollution and low probability of interception. On the contrary, the exploitation of signals not designed for detection purposes and the null control of transmission parameters makes complex signal processing techniques necessary to detect the low energy scattered by the targets. The fact that a PR does not use a dedicated transmitter usually makes necessary the use of two acquisition channels: the reference one that acquires the direct signal from the IoO and the surveillance one that points towards the interest scenario and acquires the signal scattered by the targets (Figure 1). PR processing is based on the correlation of the surveillance signal with Doppler shifted copies of the reference one, which is called Cross-Ambiguity Function (CAF). The output of the CAF is a Range-Doppler (RD) map that usually defines the observation space for the detector design. Satellite IoOs are being considered for PR operation in the last years due to their high availability and nearby global coverage [1]. DVB-S satellites are geostationary, which simplifies PR scenario geometry, their electromagnetic characterization and guarantees the availability of stable illuminating signals. DVB-S operates at frequencies in X and Ku bands (from 10.7 GHz to 12.75 GHz), which implies high propagation losses. Besides that, the transmitters are installed in satellites located at thousands of kilometres away from the Earth surface, so the power scattered from the targets is very low. In general, high gain surveillance antennas are required, especially if targets of interest are ground vehicles. The relation between the antenna gain and the radiation pattern defines a complex design problem: the generation of wide enough radiation beams for coverage sectors of interest in practical situations, providing the required gain to fulfil sensitivity requirements. Characteristics of the signal and the PR geometry and desired performance define the main requirements of the antenna system: gain, beamwidth, frequency and bandwidth. Common solutions for DVB-S reception are based on commercial parabolic dish antennas, which provide high efficiency and gain but a narrow pencil beam [2; 3; 4]. Those antennas can fulfil coverage requirements for communications applications, but in radar the pencil beam radiation pattern limits the surveillance area. An alternative to commercial parabolic antennas, specially for these kind of applications, are reflectarray antennas. A reflectarray is a planar surface composed of hundreds or thousands of printed elements (microstrip patches), each of which are pre-designed with a phase adjustment, in order to reflect the collimated electromagnetic field in a given direction, when it is illuminated by a feed [5]. These kind of antennas combine advantages from reflector antennas with the performance versatility of arrays, but avoiding the complex feeding networks [6]. Today, these antennas have become a really interesting solution due to their easy and low cost manufacturing employing the printed microstrip technology. They have been studied for different applications, mainly satellite communications and radar [7; 8; 9; 10; 11].