Contents I. Introduction
One of the most popular methods for Earth surface imaging is the synthetic aperture radar (SAR) technique. It was proposed for the first time in 1951 by Carl Willey from the Goodyear Aircraft Corporation [1], [2]. Since that time, SAR has been intensively developed, and many new concepts of image creation have been proposed [3], [4]. Nowadays, SAR imaging has entered a stage of technological maturity and is widely used in sensors mounted on different kinds of moving platforms such as satellites, aircraft, and unmanned aerial vehicles. Most of the SAR techniques are based on monostatic technology, where both transmitter and receiver are onboard the same air- or spaceborne platform. In the last two decades, intensive studies have been carried out on bistatic and multistatic radar observation, including testing of the bistatic SAR concept [5], [6]. The main feature of the bistatic radar system is the physical separation between the transmitter and the receiver. The main advantage of such a configuration is that a target can be observed from different bistatic angles, providing more information about the target. A further step in bistatic radar development is to use transmitters of opportunity for scene illumination instead of dedicated and easily detected SAR transmitters. This leads directly to the idea of passive SAR imaging.
