A new autofocus technique for wide-band wide-angle synthetic aperture radar

A small difference between the true range to the scatterer and that estimated from the radar time delay results in defocusing of Synthetic Aperture Radar (SAR) imagery in both cross-range and range directions. The amount of defocusing is a function of the integration angle and range resolution. SARs with large integration angles and high range resolution are particularly sensitivive to the range error. In the UltraWide Band (UWB) P-3 SAR, for example, there is an error in the knowledge of the absolute range on the order of 100m, which results in very poorly focused images. The current processing procedure is to form multiple SAR images using a different range estimate to form each image, where the range estimates used span the expected variation in the range error. The SAR image with the best image quality is used. This is clearly a very time consuming approach and not suitable for a real-time system. This paper describes a new technique called Autofocus Estimation of Range Error (AERE) which employs a conventional autofocus algorithm to estimate the phase error in a poorly focused image and then converts this into an estimate of the absolute range. The image is then reprocessed with the improved absolute range estimate. This paper discusses how this technique is implemented and demonstrates the improvement in image quality that can be achieved with this technique as opposed to using conventional autofocus techniques.