Use of forward scan sonar images for positioning and navigation by an AUV

Abstract

A Forward Scan (FS) sonar has been developed for use by an Ocean Explorer (OEX) class AUV. The sonar operates at a frequency of 220 kHz, with a bandwidth of 40 kHz. The sonar has a 120 degrees forward sector of view, with an azimuth resolution of approximately 2 degrees and a range resolution of approximately 3 centimeters. The OE AUV is 0.5 meter (21 inch) diameter with an adaptable front payload section. With the FS sonar payload, the length of the AUV is approximately 2 meters (7 feet). The FS sonar consists of a separate projector and receiver hydrophone. The receive hydrophone is a 64 element linear acoustic array, where each element is independently sampled and digitized. The signals are digitally processed to simultaneously beamform 119 beams within the sector of view of the FS sonar. Having developed the FS sonar, images from this FS sonar can be used for navigation and positioning by the AUV. For navigation, consecutive forward images are processed to calculate the AUV motion. This is achieved by the application of 2D and 3D motion estimation methodologies developed in computer vision for optical images. In particular, a number of techniques are investigated and it is demonstrated that inaccurate results may be obtained by the application of ad-hoc methods. The advantages of the approach developed here is demonstrated together with typical results.