## 1 Introduction
Among these satellites are TerraSAR-X and next generation missions, Cosmo SkyMed, Sentinel-1 and the Radarsat mission (Figure 1) [8]. The Spanish National Earth Observation Program (PNOTS, Programa Nacional de Observacion por Satelite ) has developed the PAZ system, an X-band SAR instrument mounted on a TerraSAR-X-like platform [9], to fulfill the objective of providing a constellation of observation satellites to cover Spanish EO needs and to contribute to the Copernicus and Global Earth Observation System of Systems (GEOSS) programs. PAZ is a dual system: civilian and defense. Its potential applications are those considered in Copernicus, whose main goal is to deliver products and services to manage and protect the environment and natural resources and to ensure civil security. Six main areas are considered: maritime environment, land environment, atmospheric monitoring, emergency management, security and climate change monitoring. PAZ will be operational in 2016. Over the last decade, bistatic SAR (BSAR) techniques have been developed using spaceborne, airborne or even fixed platforms [10; 11; 12; 13]. Space-surface BSAR (SS-BSAR), or hybrid BSAR, combines a spaceborne platform and another platform located on or near the Earth's surface. The spaceborne platform can be a SAR sensor [10; 14; 15; 16; 17]. Some of the advantages associated with the use of bistatic systems are the following:  
Figure 1: Operating SAR EO satellites’ main characteristics [8]. * Alternative geometries. * Different scattering mechanisms. * Ground resolution comparable to that of a monostatic system in fixed-receiver bistatic systems [16]. * Advantageous sensitivity requirements due to the shorter distance from target to receiver. The use of a ground-based receiver allows [18]:  
* Low cost experimentation of single and multichannel techniques (only a passive antenna with multiple sub-apertures with no space qualification). * No data-link/data storage limitations. * Data immediately available at the ground receiver. Although the number of available SAR EO constellations is constantly growing (Figure 1), increasing the chances that an area of interest is illuminated by an EO sensor, the orbit properties (revisit time) and on-demand operating principle limit the availability of an illuminating signal. This is an important constraint, but the high cost associated with the development and launch of an EO SAR and its limited lifetime justify the study of applications based on the signal generated from space. Passive BSAR systems have been the object of an intense research activity [19, 20, 21, 22, 23, 24, 25, 26]. The fact that they exploit OIs instead of a controlled transmitter allows the use of other satellite illuminators (other EO sensors, GNSS or communication systems) to provide a continuous monitoring of areas of interest, instead of being limited by the revisit time of one EO satellite or constellation. In this paper, a different approach is considered. The objective is to study the feasibility of the detection and tracking of moving targets using EO SAR satellites as OIs. Taking into consideration that the objective of passive BSAR systems is to generate radar images of Earth's surface for detecting and classifying point and extended targets, both systems could be used as valuable complements of actual active EO SAR systems. More specifically, PRs based on these OIs could provide additional information during the EO SAR sensor calibration and operation stages. The EO SAR and the PR sensors use the same transmitted signal, so novel complementary calibration techniques can be designed. Furthermore, information retrieval from SAR images can be improved. In the first approach, PR could estimate the speed and trajectory of moving targets or provide new target features due to the bistatic geometry. The availability of commercial receiving stations makes the deployment of PRs based on EO SAR sensors in areas of special interest with a low cost feasible.