Processing SAR data of rugged terrain by time-domain back-projection

Processing of SAR images of rugged terrain deserves special care because the topography affects the focused image in a number of ways. In order to obtain geometrically and radiometrically corrected SAR images of mountainous areas additional knowledge about the topography and the sensor’s trajectory and attitude has to be included in the processing or post-processing steps. Various well-known focusing techniques are available to transform SAR raw data into a single look complex image such as the range-Doppler, the chirp scaling or the ω-k algorithm. While these algorithms perform the azimuth focusing step in the frequency domain the time-domain back-projection processing technique focuses the data geometrically, i.e., in the time domain. In contrast to the frequency-domain techniques, time-domain back-projection maintains the entire geometric relationship between the sensor and the illuminated area. This implies a couple of advantages: a stringent, terrain-based correction for the elevation antenna gain pattern may be implemented and topography-induced variation of radar brightness can be eliminated in a single step. Further, the SAR image is focused directly onto an arbitrary reconstruction grid and in the desired geodetic reference frame without requiring any additional processing steps. We discuss the influence of rugged terrain on the radiometric properties of focused SAR data and demonstrate how the time-domain back-projection approach accounts for these effects within one integrated processing framework by incorporating both a correction for terrain slope induced variation of radar brightness and a stringent correction for the elevation antenna gain pattern. The algorithm is evaluated for ENVISAT/ASAR image mode data of a mountainous area.