Processing issues for very low incident angle synthetic aperture interferometric space-borne radar

In preparation for the Surface Water and Ocean Topography (SWOT) mission, scheduled for launch around October 2020, ONERA and CNES evaluated SAR processors intended for airborne SAR processing in configurations close to that of the eventual space platform. Already during preliminary studies from aircraft (with ONERA operated touring moto-glider carrying the BuSARd radar, and later on the JPL operated airSWOT radar), difficulties due to both extremely low incident angle and high frequency band emerged. The most significant issues are: first the strong topographic effects on the aperture, which makes the spectral support strongly varying across swath even for small separation between the reference trajectory and the effective one; second, the large distortion across-track induced by even small altitude deviation. These two effects combine in making interferometric pairs of SAR images difficult to co-generate and to motion compensate to an accurate prior altitude reference. Mitigation issues are proposed that allow to use faster frequency domain processing and simpler surface water altitude retrieval.