Water column current profile aided localisation for 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, transiting from the surface, where GPS is available, to the seafloor presents a localisation problem since both GPS and DVL are unavailable in the midwater column. This paper proposes an alternative approach 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. Simulation results show performance with sensor fusion of a low cost IMU and DVL bottom lock at the sea floor can achieve 20 cm per minute (2 σ) position error growth. Results with real data from an Autonomous Benthic Explorer (ABE) dive show that this method is applicable and a promising approach to navigation for untended deepwater autonomous vehicle operations.