Water column current aided localisation for significant horizontal trajectories with Autonomous Underwater Vehicles

Survey class Autonomous Underwater Vehicles (AUVs) rely on Doppler Velocity Logs (DVL) for precise navigation near the seafloor. In cases where the seafloor depth is greater than the DVL bottom lock range, localising between the surface, where GPS is available, and the seafloor presents a localisation problem since both GPS and DVL are unavailable in the mid-water column. Previous work proposes a solution to navigation in the mid-water column that exploits the fact that current profiles of water columns are stable over time. With reobservation of these currents with the ADCP (Acoustic Doppler Current Profiler) mode of the DVL during descent, along with sensor fusion of other low cost sensors, position error growth can be constrained to near the initial velocity uncertainty of the vehicle at the sea surface during the dive, and following DVL bottom lock, the entire velocity history is constrained to an error similar to the DVL velocity uncertainty, and coupled with a tactical grade IMU, 12m (2σ) per hour position error growth is possible. Application on real data coupled with view-based SLAM, without the use of an IMU, has also been demonstrated on the Sirius AUV. The limitation of this method is that it does not accommodate significant horizontal transit such that the water current velocity within the horizontal layer changes spatially, instead focusing on vertical motion through the water column. This paper addresses this limitation by extending the method through a number of modifications, with the result allowing accurate localisation during significant horizontal transits. Preliminary results from the Sirius AUV are outlined showing the applicability of this method.