Sonar-based iceberg-relative navigation for autonomous underwater vehicles

Iceberg-relative navigation for autonomous underwater vehicles (AUVs) will enable a new mode of data collection for studies of free-floating icebergs. Compared to current data collection methods, autonomous underwater vehicles offer substantially expanded coverage area and continuous sampling. However, because icebergs translate and rotate through inertial space, standard vehicle navigation methods which rely on inertial sensors are unable to provide iceberg-relative position estimates.Presented here is a new iceberg-relative vehicle navigation technique which is an extension of existing work in terrain-relative navigation. The technique comprises a mapping step and localization step, each of which is modified here to account for the translation and the rotation of free-floating icebergs.In the mapping step, the AUV circumnavigates the iceberg at a sequence of constant depths, collecting multibeam sonar imagery of the iceberg's submerged surface. A map is then generated in post-processing by projecting these sonar data from their corresponding vehicle positions (accounting for iceberg motion) in a frame that is fixed to the iceberg. Overlapping sonar data from the beginning and end of a circumnavigation provide the information necessary to enforce self-consistency of the iceberg map.In the localization step, the AUV uses the previously generated map to determine its position and orientation with respect to the iceberg by correlating incoming sonar ranges with the map. The estimator works by maintaining explicit estimates not only of the vehicle position and orientation, but also of the iceberg translation and rotation rates through inertial space.Results from a proof-of-concept field demonstration of this new iceberg-relative AUV navigation technique prove the feasibility of both generating a self-consistent three-dimensional map of a moving iceberg and localizing a vehicle's position with respect to that iceberg. The data for the experiment were collected using a sideways looking multibeam sonar mounted on the side of the RVIB Nathaniel B Palmer during a circumnavigation of a small iceberg in the Scotia Sea.