SAS image reconstruction using Fast Polar Back Projection: comparisons with Fast Factored Back Projection and Fourier-domain imaging

Fast Polar Back-Projection (FPBP) is a variant of the Fast-Factored Back-Projection (FFBP) algorithm, originally developed for ultra-wideband airborne Synthetic Aperture Radar (SAR), but since applied with success to Synthetic Aperture Sonar (SAS). The paper outlines the FPBP and FFBP algorithms, comparing computation time and memory requirements for the two methods. Processing time comparisons with a standard FFT-based method are also given. Since FFBP and FPBP are both approximation methods, computation time also depends on the preset approximation error, which particularly affects azimuth sidelobe level. The paper provides an opportunity to review speed and accuracy estimates made in previous literature. However reduction in computation time is not the decisive advantage of these time-domain methods. The difference from the FFT-based methods lies in the flexibility with which nonlinear platform trajectories, wide swaths, wide bandwidths, and multielement sonar arrays can be handled. It is also straightforward to obtain a set of intermediate physical aperture images for autopositioning purposes.