Correlation of imaging sonar acoustic shadows and bathymetry for ROV terrain-relative localization

This paper demonstrates the use of sonar imagery as an automated pilot aid for localization of a remotely-operated vehicle (ROV) with respect to an a priori bathymetric terrain map. Specifically, a method is presented to correlate acoustic shadows in sonar imagery with expected visibility images generated from a bathymetry map, where acoustic shadows are significant drops in the sonar image intensity. An expected visibility image is generated for a given ROV position estimate with respect to a stored terrain map by extracting a bathymetry profile along the sonar scan plane and then evaluating a metric termed “differential height” to quantify visibility probability in the pixel space of the sonar image. Results from Monterey Bay Aquarium Research Institute (MBARI) ROV field trials using a Kongsberg mechanically-scanned imaging sonar are presented that demonstrate localization using the proposed method.