Motion compensation of airborne synthetic aperture radars using autofocus

Conventional motion compensation schemes correct for unwanted SAR platform motions using information from an inertial measurement unit (IMU). Autofocus techniques, which focus SAR images, produce an 'autofocus parameter' which is related to the platform motion. In this paper, strong evidence is presented to support the assumption that the contrast optimization autofocus algorithm behaves as a least-squares quadratic fitting to the SAR platform trajectory. Using this assumption, the relationship between the autofocus parameter and across-track accelerations of the SAR platform is derived. This allows the SAR platform motion to be estimated from the autofocus parameter measurements and incorporated in a motion compensation scheme, instead of IMU measurements. Three implementations of motion compensation using autofocus are compared, and the achievable image quality is quantified.