SAR Scene Size Limits for Spatially Invariant Defocus With Near- and Far-Field Geometries

Existing traditional or phase-only autofocus techniques rely on the assumption that the phase-error function, which causes synthetic aperture radar (SAR) image defocus, is spatially invariant. In many cases this assumption holds, but for larger scene sizes or scenarios where the far-field approximation does not hold, the spatially invariant phase error approximation breaks down. In this paper, an approximate upper bound is derived on the spatially variant phase-error component of the phase-error function. The derived bound applies with near- and far-field bistatic geometries, with monostatic as a special case, and is expressed in terms of either known quantities or quantities which typically have a known upper bound. The derived bound enables SAR scene size limits to be computed, such that the phase-error function is approximately spatially invariant according to a specified phase threshold. Example results identifying SAR scene size limits for spatially invariant defocus are presented for both near- and far-field geometries.