An Autofocus Technique for Video-SAR

General Atomics Aeronautical Systems, Inc. (GA-ASI) is designing a real-time, video-SAR (synthetic aperture radar) mode for a test bed radar system. Typically, the flight path for video-SAR is circular with the sensor directed inward and broadside relative to the platform heading, providing continuous surveillance over a region of interest. The SAR frames are processed using the backprojection algorithm onto a Cartesian coordinate system at the nominal ground level and oriented in a fixed direction. Standard autofocus techniques are ineffective since the azimuth dimension will be oblique through the images when the synthetic apertures are not centered at a multiple of pi/2 along the flight path. We have developed an algorithm that estimates the phase gradient from pseudo point-scatterers, regardless of the platform position, and focuses the images before they are compiled into a SAR video. Thus the efficiency and utility of fixed frame video-SAR processing is retained, while image sharpness and quality are not compromised due to antenna motion error measurements and/or severe atmospheric effects during propagation.