Distortion free SAR imagery and change detection

Sources of defocusing and azimuth distortion in synthetic-aperture radar (SAR) imagery are outlined. It is shown how autofocus measurements can be used to measure the aircraft track, and provide phase corrections to the raw SAR data. When the phase-corrected data is processed, correctly focussed imagery, free of any azimuth distortions, is produced. For low look-angle systems, range distortion due to terrain height variation is generally negligible, and therefore the system described produces distortion-free SAR imagery. The change detection problem is also considered: given two SAR images of the same area, locate changes that have occurred in the time between the images being obtained. Two approaches to this problem are described. The first applies a simple featuring operation to the two images, followed by hard limiting and image differencing. This is found to detect changed targets well. In the second method, the two images are segmented, and the segmentation descriptions are compared for inconsistencies indicating changes. This approach allows one to identify changes in targets having distinctive characteristics, such as shape or size.<>