<title>Space-variant filtering for correction of wavefront curvature effects in spotlight-mode SAR imagery formed via polar formatting</title>

Abstract

Wavefront curvature defocus effects can occur in spotlight-mode SAR imagery wh constructed via the well-known polar formatting algorithm (PFA) under certain scenarios include imaging at close range, use of very low center frequency, and/or imaging of very large scenes. The range migration algorithm (RMA), also known as seismic migration, was developed to accommodate these wavefront curvature effects. However, the along-track upsampling of the phase history data required of the original version of range migration can in certain instances represent a major computational burden. A more recent version of migration processing, the Frequency Domain Replication and Downsampling (FReD) algorithm, obviates the need to upsample, and is accordingly more efficient. In this paper we demonstrate that the combination of traditional polar formatting with appropriate space-variant post-filtering for refocus can be as efficient or even more efficient than FReD under some imaging conditions, as demonstrated by the computer-simulated results in this paper. The post-filter can be pre-calculated from a theoretical derivation of the curvature effect. The conclusion is that the new polar formatting with post filtering algorithm (PF2) should be considered as a viable candidate for a spotight-mode image formation processor when curvature effects are present.

Space-Variant Post-Filtering for Wavefront Curvature Correction in Polar-Formatted Spotlight-Mode SAR Imagery

“…355-365], and demonstrated in [10]. This extension generalizes the space -variant post-filter and extends its utility to many practical imaging scenarios, which typically are not restricted to broadside.…”

“…Consequently, prior to this dissertation and the preliminary papers by the author and his associates [10,11],…”

“…These defocus effects are troublesome to negate due to their space-variant nature. This dissertation is based on a concept originally proposed by Jakomatz, et al, [10] in which a geometric model of the phase return from a point target is used to derive (and ultimately compensate) the phase errors. That concept was further expanded to include squint-mode imaging, as described by Doren, et al, in [11].…”

Signal Processing for a Multiple-Input, Multiple-Output (MIMO) Video Synthetic Aperture Radar (SAR) with Beat Frequency Division Frequency-Modulated Continuous Wave (FMCW)

“…Therefore, these characteristics should be taken into consideration in the design process. The maximum image size can be increased by using wave front curvature compensation methods, as in [33][34][35]. In PFA, the scene size is limited by range curvature, and residual video phase ( [2], Equation (3.130), [36], Equation (B.23)).…”

Wavefront curvature correction in one stationary bistatic SAR image focused via PFA

“…The wavefront curvature phase error in a monostatic PFA was analysed and corrected in [5][6][7][8]. Based on the same method in [6], the wavefront curvature error in a one stationary bistatic SAR image focused via the bistatic PFA is derived and corrected in this Letter.…”