Distortion and break-up of sidescan images - criteria and reconstruction by geocoding

Image distortion and degradation caused by platform motion are familiar to all operators of side-scan sonars. Modest motions can distort unprocessed images, that is, the images normally generated on the ship. Image processing can remove distortions by resampling the image onto a geographic reference frame based on logged platform motion. This process is often called geocoding. To allow processing, though, the platform attitude must not change so rapidly that coverage gaps result, or the overlap between transmitted and received beams is much reduced. The criteria which apply to both unprocessed and geocoded images, for a high-resolution single-beam-perside sidescan sonar, are presented. The criteria are too complex for simple summaries such as an upper limit on yaw rate, but the relative importance of the towfish degrees of freedom can be seen. Several examples, acquired with a towfish in tidal turbulence, are shown of uncorrected images which are changed significantly by geocoding, including images which appear as two separate objects in the raw image but which are united by geocoding. Platform stability indicators are calculated and correlated with distortion and break-up