Methods of Determining Towfish Location for Improvement of Sidescan Sonar Image Positioning
Defence Research Establishment Atlantic Dartmouth, Nova Scotia Canada
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New methods for improving the positioning of post-processed sidescan sonar seabed imagery have been developed and evaluated. During sea-trial Q260, July 2001, Trackpoint determined sonar towfish position and ship position data were recorded in several configurations in the sonar data files and in accompanying log files. The Trackpoint system uses a ship-mounted receiver to determine range and bearing to a transponder on the sonar towfish. In post-processing of the sidescan sonar data to obtain geo-referenced seabed imagery, various methods of using this towfish position information have been developed. Varying degrees of conditioning of the Trackpoint position data were required, as well assume receding of the in-house sonar data mosaicking software. A method for determining towfish position from the cable length entries in the sonar operators logbook was also developed and used with reasonable success. Resulting positioning errors were estimated from the separation between multiple images of targets in seabed mosaics made from several survey swaths, as well as between geological features appearing in the images. It was found that use of the Trackpoint system for towfish location did improve positioning of the resulting sidescan sonar seabed images, as well as helping to resolve some apparent inconsistencies in positioning caused by unpredicted towfish ship-following behaviour.