Space-based SAR ground moving target indication

Today's demand for space-borne Synthetic Aperture Radar (SAR) data has grown to the point where significant commercial funding of advanced space-borne radar system development has been being made available. The current generation of commercial space-based SAR imaging satellites, such as RADARSAT-2, Sentinel-1, TerraSAR-X/TanDEM-X (PAZ), COSMO-SkyMED and ALOS-2, operate at a single frequency (L-, Cand X-band) and are based on active phased array antenna technology that offers beam agility and adds polarization diversity. Consequently, these modern satellites are equipped with more than one receive channel (i.e., AD-converter) that can also be utilized to record measurements from multiple apertures in along-track direction. This is the principal prerequisite for a ground moving target indication (GMTI)1 capability. While space-based SAR GMTI offers many advantages like global ground coverage and access to strategic regions, it also faces several obstacles such as high satellite velocity, Earth rotation and oftentimes small target reflection energy caused by the enormous distances of more than 1,000 km among others. This book chapter presents the state-of-the-art of space-based SAR-GMTI science and technology with focus on recent advances and the latest direction of research and development (R&D) activities. Owing to an exponential cost jump, technological advances of space-based radars especially with regard to increased power, increased aperture sizes and additional receiver channels have only been somewhat incremental in the last decades. Spacecraft with more than two parallel receive paths are only expected to materialize two generations down the line. Hence, current R&D put emphasis on innovative new concepts trying to circumvent these technological limitations thereby often pushing the resources on existing SAR payloads to their limits.2 Virtually all of these concepts are accompanied by cutting-edge but complex and resource-hungry signal-processing algorithms that only recently became feasible based on the fast-paced evolution in computing power over the last decade. Many of the presented proof-of-concept studies are considered building blocks of future operational space-based SAR capabilities, for instance, the synergy between high-resolution-wide-swath (HRWS) imaging and motion indication and estimation. This chapter attempts to provide a comprehensive, in-depth overview of the theory and the radar signalprocessing techniques required for space-based SAR-GMTI corroborated by real multichannel data from RADARSAT-2.