## 1 Introduction  
A passive radar (PR) is defined as a sensor whose main objective is to detect targets and to estimate parameters, such as position and speed, using non-cooperative transmitters, known as opportunity illuminators (OIs), rather than a dedicated one [1]. Broadcast, communications, radar or radio-navigation signals can be used as OIs. PRs present many advantages over active radars:  
* Low development, implementation and maintenance costs. * Easy deployment without any particular power requirement, using solar panels and batteries, and without complex civil engineering. * Small size and low weight. * Low probability of intercept (LPI). * Invulnerability against the progressive erosion of communication systems, which are demanding the use of traditional radar frequencies. * Avoidance of electromagnetic compatibility or environment impact problems. In active radars, transmitters are usually a large fraction of the radar system cost and design effort and typically require a major share of system prime power and maintenance. The absence of their own transmitter is the main advantage of passive radars from the cost point of view. Of course, the use of non-controlled transmitters that have not been designed for radar purposes makes detection and tracking really complex. Thanks to the possibility of using commercial off-the-shelf (COTS) devices for signal reception (antenna, RF front-end and acquisition systems), an intense research activity has been carried out by research institutions, companies and universities. Technological advances, such as platforms for field-programmable gate array (FPGAs) programming or graphics processing units (GPUs), allow the reduction of the gap between technological demonstrators and prototypes. Actual active radar designs try to move the analog-to-digital conversion stage as close as possible to the antenna. Under this software-defined radio-receiving architecture, active and passive radars' development costs can be comparable, but the active system will require its own transmitter. Due to the high availability of satellite OIs and their almost total invulnerability to natural disasters or physical attacks, these OIs are of special interest. A deep study of the potential OIs is required, because most of the defining characteristics of the resulting PR system are inherited from the characteristics of the OI, in the same way that the behavior of an active radar is determined by the characteristics of its dedicated transmitter. Geostationary satellites, constellations of medium Earth orbit satellites or low Earth orbit satellites have different features with respect to propagation losses, transmitted powers, central frequencies, bandwidths, waveforms, visibility times and geometry variation with time. Systems based on geostationary communication satellites, Global Navigation Satellite System (GNSS) constellations or Earth Observation (EO) systems have been considered [2; 3; 4; 5]. EO SARs present potential advantages associated with the use of radar signals: frequencies characterized by low atmospheric gas absorption and rain attenuation, high radiated powers, which have been designed for fulfilling radar requirements in a monostatic configuration, and low orbits, which reduce the transmitter-receiver and transmitter-target distances compared to other satellite constellations. Synthetic Aperture Radar (SAR) sensors are key pieces of the Earth observation European Program Copernicus, previously known as GMES (Global Monitoring for Environment and Security) [6]. These sensors produce high-resolution remote-sensing imagery using antennas installed aboard mobile platforms, such as aircraft or spacecrafts. Platform movement is used to improve azimuth resolution, through the generation of a larger synthetic antenna. SAR data are used for mapping terrain and sea surfaces and detecting and classifying point and extended targets [7]. The high interest in these sensors has resulted in the launch of several satellites, and more launches are scheduled for the foreseeable future.