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Title: General formulation for wavefront curvature correction in polar-formatted spotlight-mode SAR images using space-variant post-filtering

Abstract

In this paper, the authors introduce a general formulation for wavefront curvature correction in spotlight-mode SAR images formed using the polar-formatting algorithm (PFA). This correction is achieved through the use of an efficient, image domain space-variant filter which is applied as a post-processing step to PFA. Wavefront curvature defocus effects occur in certain SAR collection modes that include imaging at close range, using low center frequency, and/or imaging very large scenes. The formulation is general in that it corrects for wavefront curvature in roadside as well as squinted collection modes, with no computational penalty for correcting squint-mode images. Algorithms such as the range migration technique (also known as seismic migration), and a recent enhancement known as frequency domain replication, FReD, have been developed to accommodate these wavefront curvature effects. However, they exhibit no clear computational advantage over space-variant post-filtering in conjunction with polar formatting (PF2). This paper will present the basic concepts of the formulation, and will provide computer results demonstrating the capabilities of space-variant post-filtering.

Authors:
; ; ;
Publication Date:
Research Org.:
Sandia National Labs., Albuquerque, NM (United States)
Sponsoring Org.:
USDOE, Washington, DC (United States)
OSTI Identifier:
532556
Report Number(s):
SAND-97-0233C; CONF-971070-1
ON: DE97007950; BR: GC40400000; TRN: AHC29721%%106
DOE Contract Number:
AC04-94AL85000
Resource Type:
Conference
Resource Relation:
Conference: IEEE Signal Processing Society international conference on image processing, Santa Barbara, CA (United States), 26 Oct 1997; Other Information: PBD: [1997]
Country of Publication:
United States
Language:
English
Subject:
44 INSTRUMENTATION, INCLUDING NUCLEAR AND PARTICLE DETECTORS; 99 MATHEMATICS, COMPUTERS, INFORMATION SCIENCE, MANAGEMENT, LAW, MISCELLANEOUS; RADAR; IMAGE PROCESSING; CORRECTIONS; DIGITAL FILTERS; ALGORITHMS; COMPUTERIZED SIMULATION; FORTRAN

Citation Formats

Doren, N.E., Jakowatz, C.V. Jr., Wahl, D.E., and Thompson, P.A. General formulation for wavefront curvature correction in polar-formatted spotlight-mode SAR images using space-variant post-filtering. United States: N. p., 1997. Web.
Doren, N.E., Jakowatz, C.V. Jr., Wahl, D.E., & Thompson, P.A. General formulation for wavefront curvature correction in polar-formatted spotlight-mode SAR images using space-variant post-filtering. United States.
Doren, N.E., Jakowatz, C.V. Jr., Wahl, D.E., and Thompson, P.A. Wed . "General formulation for wavefront curvature correction in polar-formatted spotlight-mode SAR images using space-variant post-filtering". United States. doi:. https://www.osti.gov/servlets/purl/532556.
@article{osti_532556,
title = {General formulation for wavefront curvature correction in polar-formatted spotlight-mode SAR images using space-variant post-filtering},
author = {Doren, N.E. and Jakowatz, C.V. Jr. and Wahl, D.E. and Thompson, P.A.},
abstractNote = {In this paper, the authors introduce a general formulation for wavefront curvature correction in spotlight-mode SAR images formed using the polar-formatting algorithm (PFA). This correction is achieved through the use of an efficient, image domain space-variant filter which is applied as a post-processing step to PFA. Wavefront curvature defocus effects occur in certain SAR collection modes that include imaging at close range, using low center frequency, and/or imaging very large scenes. The formulation is general in that it corrects for wavefront curvature in roadside as well as squinted collection modes, with no computational penalty for correcting squint-mode images. Algorithms such as the range migration technique (also known as seismic migration), and a recent enhancement known as frequency domain replication, FReD, have been developed to accommodate these wavefront curvature effects. However, they exhibit no clear computational advantage over space-variant post-filtering in conjunction with polar formatting (PF2). This paper will present the basic concepts of the formulation, and will provide computer results demonstrating the capabilities of space-variant post-filtering.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Oct 01 00:00:00 EDT 1997},
month = {Wed Oct 01 00:00:00 EDT 1997}
}

Conference:
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  • Wavefront curvature defocus effects occur in spotlight-mode SAR imagery when reconstructed via the well-known polar-formatting algorithm (PFA) under certain imaging scenarios. These include imaging at close range, using a very low radar center frequency, utilizing high resolution, and/or imaging very large scenes. Wavefront curvature effects arise from the unrealistic assumption of strictly planar wavefronts illuminating the imaged scene. This dissertation presents a method for the correction of wavefront curvature defocus effects under these scenarios, concentrating on the generalized: squint-mode imaging scenario and its computational aspects. This correction is accomplished through an efficient one-dimensional, image domain filter applied as a post-processingmore » step to PF.4. This post-filter, referred to as SVPF, is precalculated from a theoretical derivation of the wavefront curvature effect and varies as a function of scene location. Prior to SVPF, severe restrictions were placed on the imaged scene size in order to avoid defocus effects under these scenarios when using PFA. The SVPF algorithm eliminates the need for scene size restrictions when wavefront curvature effects are present, correcting for wavefront curvature in broadside as well as squinted collection modes while imposing little additional computational penalty for squinted images. This dissertation covers the theoretical development, implementation and analysis of the generalized, squint-mode SVPF algorithm (of which broadside-mode is a special case) and provides examples of its capabilities and limitations as well as offering guidelines for maximizing its computational efficiency. Tradeoffs between the PFA/SVPF combination and other spotlight-mode SAR image formation techniques are discussed with regard to computational burden, image quality, and imaging geometry constraints. It is demonstrated that other methods fail to exhibit a clear computational advantage over polar-formatting in conjunction with SVPF. This research concludes that PFA in conjunction with SVPF provides a computationally efficient spotlight-mode image formation solution that solves the wavefront curvature problem for most standoff distances and patch sizes, regardless of squint, resolution or radar center frequency. Additional advantages are that SVPF is not iterative and has no dependence on the visual contents of the scene: resulting in a deterministic computational complexity which typically adds only thirty percent to the overall image formation time.« less
  • Wavefront curvature defocus effects can occur in spotlight-mode SAR imagery when reconstructed via the well-known polar formatting algorithm (PFA) under certain scenarios that 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, obviatesmore » the need to upsample, and is accordingly more efficient. In this paper the authors 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 spotlight-mode image formation processor when curvature effects are present.« less
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