An Autofocus Approach for Residual Motion Errors with Application to Airborne Repeat-Pass SAR Interferometry

Abstract

Airborne repeat-pass SAR systems are very sensible to sub-wavelength deviations from the reference track. To enable repeat-pass interferometry a high-precision navigation system is needed. Due to the limit of accuracy of such systems, deviations in the order of centimeters remain between the real track and the processed one causing mainly undesirable phase undulations and misregistration in the interferograms, referred as residual motion errors. Up to now only interferometric approaches, as multisquint, are used to compensate for such residual errors. In this paper we present for the first time the use of the autofocus technique for residual motion errors in the repeat-pass interferometric context. A very robust autofocus technique has to be used to cope with the demands of the repeatpass applications. We propose a new robust autofocus algorithm based on the WLS (Weighted Least-Squares) phase estimation and the Phase Curvature Autofocus (PCA) extended to the range-dependent case. We call this new algorithm WPCA (Weighted PCA). Differently from multisquint, the autofocus approach has the advantage of being able to estimate motion deviations independently leading to better focused data and correct impulse response positioning. As a consequence better coherence and interferometric phase accuracy are achieved. Repeat-pass interferometry based only on image processing gains in robustness and reliability since its performance does not deteriorate with time decorrelation and no assumptions need to be made on the interferometric phase. Repeat-pass data of the E-SAR system of the German Aerospace Center (DLR) are used to demostrate the performance of the proposed approach.