Factorized Geometrical Autofocus for Synthetic Aperture Radar Processing

Abstract

Synthetic Aperture Radar (SAR) imagery is a very useful resource for the civilian remote sensingcommunity and for the military. This however presumes that images are focused. There are severalpossible sources for defocusing effects. For airborne SAR, motion measurement errors is the maincause. A defocused image may be compensated by way of autofocus, estimating and correctingerroneous phase components.Standard autofocus strategies are implemented as a separate stage after the image formation(stand-alone autofocus), neglecting the geometrical aspect. In addition, phase errors are usuallyassumed to be space invariant and confined to one dimension. The call for relaxed requirementson inertial measurement systems contradicts these criteria, as it may introduce space variant phaseerrors in two dimensions, i.e. residual space variant Range Cell Migration (RCM).This has motivated the development of a new autofocus approach. The technique, termed theFactorized Geometrical Autofocus (FGA) algorithm, is in principle a Fast Factorized Back-Projection(FFBP) realization with a number of adjustable (geometry) parameters for each factorization step.By altering the aperture in the time domain, it is possible to correct an arbitrary, inaccurate geometry. This in turn indicates that the FGA algorithm has the capacity to compensate for residualspace variant RCM.In appended papers the performance of the algorithm is demonstrated for geometrically constrained autofocus problems. Results for simulated and real (Coherent All RAdio BAnd System II(CARABAS II)) Ultra WideBand (UWB) data sets are presented. Resolution and Peak to SideLobeRatio (PSLR) values for (point/point-like) targets in FGA and reference images are similar withina few percents and tenths of a dB.As an example: the resolution of a trihedral reflector in a reference image and in an FGA imagerespectively, was measured to approximately 3.36 m/3.44 m in azimuth, and to 2.38 m/2.40 m inslant range; the PSLR was in addition measured to about 6.8 dB/6.6 dB.The advantage of a geometrical autofocus approach is clarified further by comparing the FGAalgorithm to a standard strategy, in this case the Phase Gradient Algorithm (PGA)