Space variant filtering of polar format spotlight SAR images for wavefront curvature correction and interferometric processing

Abstract

In repeat pass SAR interferometry the scene coherence can be used to detect subtle scene changes that occur in the interval between image collections. In spotlight SAR mode the azimuth resolution of the baseline and repeat pass imagery may be improved by collecting an extended aperture therefore allowing finer resolution estimates of the scene coherence. The formation of the imagery however requires adequate compensation of the wavefront curvature effects which become increasing significant in finer resolution imagery. In N. E. Doren (1997) it is shown that for spotlight SAR imagery, formed using the Polar Format Algorithm (PFA), wavefront curvature effects give rise to residual phase errors that may be removed using an image domain filtering technique. The analysis however is based on the formation of a slant plane image obtained by projection of the scene reflectivity along circular arcs centered on the flight track. In this paper a full 3D imaging geometry is considered to derive an appropriate post PFA image domain filter to form a ground plane image. This allows multipass images to be formed in a common image plane for subsequent interferometric processing. The technique is used to generate a fine resolution coherence map using data collected in a repeat pass SAR experiment using the DSTO Ingara SAR.