Autofocus of stripmap SAS data using the range-variant SPGA algorithm

This paper presents an extension of the Stripmap Phase Gradient Autofocus (SPGA) algorithm to allow operation in environments where the grazing angle varies with range. SPGA is designed for stripmap autofocus in a low-grazing angle scenario. Autofocus performance is degraded when imagery that has varying grazing angles is autofocused using SPGA. The SPGA extension presented here models the combined effect of slant-plane towfish sway and heave on the data. This enhanced modeling allows the algorithm to operate in situations where the grazing angle to the scene varies with range. Results from the application of the new algorithm to field-collected and simulated data sets are demonstrated. The range-variance SPGA extensions improve autofocus performance on simulation data that has varying grazing-angle. The field-collected dataset shown has little range variance and shows only minor improvement. We also present a short summary of simulation approximations giving consideration to quantitative autofocus testing.