An efficient wavefront curvature correction algorithm for turntable spotlight ISAR systems using stepped frequency waveforms

Abstract

Wavefront curvature phenomena have a noticeably negative impact on the final formed image, removing the distortion and defocusing effects become an important step in a turntable ISAR imaging system. This paper outlines an efficient wavefront curvature correction algorithm that corrects the distortion effects by implementing a set of 1-D operations in spatial frequency before the final image is formed. By implementing the algorithm in spatial frequency space, several advantages over post-image formation wavefront curvature correction techniques can be realized. The first advantage is that this algorithm can be broken down into a set of 1-D operations that can be applied across each row or column of the 2-D spatial frequency data grid, while post-image formation techniques can require a 2-D spatially varying filter, or a pixel-by-pixel remapping, to "unwarp" and focus the image. The second advantage of this algorithm stems from the fact that the number of data samples in the spatial frequency domain is often much smaller than the number of samples in the final image, because relatively large discrete Fourier transforms (DFTs) are used in image formation to create finely-detailed imagery. Performing the correction in the frequency domain thus requires operation on fewer samples than does correcting the warping in the more densely-sampled image domain.