Extensions to polar formatting with spatially variant post-filtering
2011
The polar format algorithm (PFA) is computationally faster than back projection for producing spotlight mode synthetic
aperture radar (SAR). This is very important in applications such as video SAR for persistent surveillance, as images
may need to be produced in real time. PFA's speed is largely due to making a planar wavefront assumption and forming
the image onto a regular grid of pixels lying in a plane. Unfortunately, both assumptions cause loss of focus in airborne
persistent surveillance applications. The planar wavefront assumption causes a loss of focus in the scene for pixels that
are far from scene center. The planar grid of image pixels causes loss of the depth of focus for conic flight geometries.
In this paper, we present a method to compensate for the loss of depth of focus while warping the image onto a terrain
map to produce orthorectified imagery. This technique applies a spatially variant post-filter and resampling to correct
the defocus while dewarping the image. This work builds on spatially variant post-filtering techniques previously
developed at Sandia National Laboratories in that it incorporates corrections for terrain height and circular flight paths.
This approach produces high quality SAR images many times faster than back projection.
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