Use zyxwvu of Forward Scan Sonar Images for Positioning and Navigation by an AUV zyx * Joe Cuschieri Shahriar Negahdaripour Department of Ocean Engineering Electrical and Computer Engineering Department Florida Atlantic University Boca Raton, FL 33431, USA. Abstract: zyxwvutsrq A zyxwvutsrqpon Forward Scan (FS) sonar has been developed zyxwvut for use by an Ocean Explorer (OEX) class AUV. The sonar op- erates 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 cen- timeters. 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 A U V is approximately 2 meters zyxwvutsrqpo (7 feet). The FS sonar consists of a separate pro- jector and receiver hydrophone. The receive hydrophone is a 64 element linear acoustic array, where each ele- ment is independently sampled and digitized. The signals are digitally processed to simultaneously beamform 129 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 posftioning by the AUV. For navigation, consecutive forward images are processed to calculate the AUV motion. This is achieved by the ap- plication of 2D and 3D motion estimation methodologies developed in computer vision for optical images. I n par- ticular, 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 typzcal results. Figure 1: Ocean Explorer AUV. 'Support for this work is provided by ONR. University of Miami Coral Gables, FL 33124 USA I. Introduction The Ocean Explorer (OEX) class AUV is a multi- purpose autonomous platform that can carry different payloads in its forward section, fig.1. The AUV can take GPS fix positions when near the surface by deploying a GPS antenna. However, once submerged it primarily relies on dead reckoning for position estimation. When performing at sea missions, an acoustic tracking system is used to locate the vehicle. The tracking system, tracks the position of the AUV relative to the support ship, and the GPS position of the support ship. By combining these two tracking data, the absolute position of the AUV is obtained to within kt10 meters. The absolute tracking is not however communicated back to the AUV during the mission. Thus the AUV has no means of adjusting for drift in its position estimates, for accurate navigation. One of the payloads that the AUV can carry is a For- ward Scan (FS) sonar. This Forward Look (FL) scan sonar generates images which are very similar to side scan sonar type images, with forward looking scan lines. The 119 scan lines, covering a 120 degrees sector of view are simultaneously formed to form a complete image of the forward view of the AUV. The primary application for which this sonar was developed is seafloor and mid- water target localization. It is more than an obstacle avoidance sonar since it has a 40 degree vertical field of view and can generate a scan of the seafloor. Fig.2 shows a photo of the FS sonar payload. As a secondary application of the FS sonar, to aid in improving the accuracy of navigation of the AUV, nav- igation information can be extracted from sequences of the FS sonar images. The FS sonar, in its complete im- plementation, that is with on board real-time data and image processing, can generate four image frames per sec- ond, covering the 120 degree sector of view for a range of 50 m. It is shown in this paper that using a sequence of 50 image frames, navigation information can be ex- tracted, which when compared to the navigation data from the AUV dead reckoning and the acoustic tracking shows good promise in using the FS sonar for accur.zte navigation and positioning. 0-7803-5045-6/98/$10.00 01 998 IEEE 752 Authorized licensed use limited to: CSIC. Downloaded on December 10, 2009 at 11:35 from IEEE Xplore. Restrictions apply.