Mode-Independent SAR Image Formation Using A Beamforming Framework

Charles (Jack) Jakowatz
Sandia National Laboratories

This paper shows how a mode-independent SAR image formation algorithm can be derived from a simple array processing methodology known generically as "delay and sum beamforming". It amounts to performing phased array processing on the virtual array created during a phase history collection by a SAR system. Specifically, the methodology can handle monostatic SAR modes of strip-map, spotlight, and scan, as well as any bistatic imaging mode. The only requirement is that the radar is relatively narrowband, which is the case for almost all modern operational SARs. The advantages of this methodology include: 1) it inherently treats wavefront curvature; 2) it can create the SAR image on an arbitrary grid, including one that is draped over a pre-existing digital elevation map so that the formed image is automatically orthorectified; and3) it is extremely simple to implement. An interesting feature of the technique is that is turns out to be equivalent to performing back-projection (without any convolution precursor) along curved wavefront arcs. The disadvantage is that the computation time is relatively slow, but can be sped up with clever rearrangement of the operations. The paper will discuss the theory of this methodology and show results from real-world SAR phase history collections, including traditional spotlight, strip, and bistatic modes.

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