COSMO-SkyMed di Seconda Generazione Spotlight Focusing Chain Improvements and Interferometric and Change Detection Capabilities Enhancement through Common Band Filtering

“COSMO-SkyMed di Seconda Generazione” (CSG) is the follow-on mission of COSMO-SkyMed, the Italian system for Earth Observation. CSG constellation will be composed of two satellites with a Synthetic Aperture Radar payload. CSG will assure the service continuity with respect to the current fully-deployed four-satellite COSMOSkyMed (CSK) system, increasing its operational performances and featuring a number of new or improved products. Among the enhancements required by the Users and the lesson learnt, there are: - the improved processing of Spotlight products; - the possibility, for the final user, to perform a Common Band Filtering (CBF) between two products prior to run co-registration and interferometric processing. The CSG Spotlight focusing chain will be upgraded through the implementation of an Enhanced Omega-k algorithm and a more precise model of the kinematics of the acquisition in order to cope with high-squinted attitude data takes. Moreover, since small errors on targets height induce strong defocusing effects due to incorrect Doppler rate computation, a Precise Topography- and Aperture-dependent Motion Compensation algorithm will be added to the processing chain, allowing high-quality motion compensation even in case of strong topography changes. A prototype already developed in the frame of the CSG B Phase shows a much better performance with respect to the CSK Spotlight processing chain, based on a less time-consuming but less accurate Fast Omega-k focusing algorithm. One of the lessons learnt from the first generation system is the need to deliver interferometric products with a high coherence to fully support applications such as Coherent Change Detection, which are based on the generation of Coherence Maps. The SAR interferometric coherence value depends mainly on the temporal delay between interferometric acquisitions, on the imaged scene typology and its decorrelation time and on the difference of the satellites pointing between the acquisitions. In order to maximize the coherence value at processing level, a Common Band Filter has been required inside the co-registration processing software in order to remove the non-common parts of the Doppler spectrum. The CBF shall be an option of the co-registration step, in order to let the user decide if applying or not the filtering. As a matter of fact, CBF worsen the azimuth resolution, so that the extent of its use must be evaluated by the user in each specific case. A prototype has been already developed in the frame of COSMO-SkyMed Maintenance in Operational Conditions Programme, and the results of the preliminary tests show its capability to strongly enhance the coherence between the interferometric images.