Metode avansate de imagistică radar cu apertură sintetică pentru configurații monostatice și bistatice / Advanced synthetic aperture radar computational imaging methods for monostatic and bistatic configurations

As an emerging tool in Earth Observation (EO), synthetic aperture radar (SAR) imaging ensures the continuity of the Earth surface monitorization in optical challenging conditions. The thesis proposes solutions for both SAR image focusing and SAR image enhancement. For the first solution, the classic Back-Projection (BP) is combined with compressive sensing (CS) techniques assuming the spatial sparsity of various scenes. Precisely, the BP linear operator was derived for a user-defined imaging grid and employed as a sparsifying dictionary in the CS framework in conjunction with a modified greedy solver. The CS-BP SAR processor is able to deal with both monostatic and bistatic in the two-dimensional image space. In the second solution, the intrinsic bistatic low resolution is boosted by exploiting the data recorded over multiple apertures. The variability of the receiver looking angles is treated by a Keystone transform whereas the main achievement (resolution enhancement and sidelobe suppression) is obtained through the CS techniques used for azimuth profile reconstruction. Another asset of the proposed bistatic imaging workflow is the CS recovery using chirp dictionaries. The third SAR processor is constructed on the sub-aperture-based BP core which operates on special imaging grids. Two sub-problems were approached. A geometric/vectorial method for polar/elliptical grid generation (unambiguously) on arbitrary plane was derived. The generated grid was integrated in a sped-up BP processor which accommodates beam-sweeping scanning modes by using an angle constrained back-projection step. Finally, a solution that harnesses the co-polarization images for denoising the crosspolarization SAR product (Sentinel-1 GRDM) recorded over low backscattering areas (calm waters, deserts, polar regions) on TOPSAR mode was proposed.