GNSS-based passive radar

This chapter is an introduction to passive synthetic aperture radar (SAR) using global navigation satellite systems (GNSS), as illuminators of opportunity. Such systems include the global positioning system (GPS), the Russian GLONASS system, or the forthcoming Galileo constellation. Apart from the traditional benefits of a passive radar system, which include cost efficiency, license-free and covert operation, GNSS-based systems have a number of relative merits compared to passive systems based on terrestrial illuminating sources. One of its most prominent features is the potential for persistent monitoring anywhere in the world due to the global GNSS coverage. In addition, as GNSS guarantee a number of satellites illuminating the same point on Earth from multiple aspect angles simultaneously, this provides an opportunity to enhance radar information space. This can be done either by comparing individual passive SAR images obtained from multiple satellite perspectives, or combining them using multistatic SAR techniques. On the other hand, the relatively low power flux density near the Earth’s surface restricts the field of view of applications for such a system to monitoring local areas. At the same time, and partially due to its rather broad scope, this technology has not yet reached the maturity of terrestrial-based systems. As such, the purpose of this chapter is to present methods and results on the fundamental science and technology behind GNSS-based SAR, as a stepping stone to realizing its full potential. This includes an introduction to the system concept and its fundamental parameters (power budget/resolution), signal processing algorithms for signal synchronization and image formation, as well as proof-of-concept results for advanced techniques such as change detection, multi-perspective and multistatic imaging.