Water-Current and IMU Aided AUV Localization in Deep Mid-Water

Survey class Autonomous Underwater Vehicles (AUVs) typically rely on Doppler Velocity Logs (DVL) for precise navigation near the seafloor. In cases where the distance to the seafloor is greater than the DVL bottom lock range, localizing between the surface where GPS is available and the seafloor presents a localization problem, since both GPS and DVL are unavailable in the mid-water column. Previous work [3] [6] proposed a solution to navigation in the mid-water column that exploits the stability of the vertical water current profile in space over the minutes scale. With repeated measurements of these currents with the Acoustic Doppler Current Profiler (ADCP) mode of the DVL during vertical descent, along with sensor fusion of other low cost sensors, position error growth is constrained during the dive. Following DVL bottom lock, due to correlations in the joint vehicle and water current velocity estimation, the entire velocity history is further constrained. Previous work in this area includes [4] to generalize the ADCP-aided filter to horizontal motion, including using ADCP beam geometry and a water-volume grid approach for the water velocity state space. Furthermore, a number of extensions are developed in [5] to improve navigation performance during missions characterized by prolonged time-scales. In this paper, the ADCP-aided filter is applied to a 25 hour 5000m deep straight line mission, with the environmental effects considered. Also, the addition of IMU acceleration outputs from a navigation grade IMU for the prediction model as an alternative to the constant velocity (CV) model are implemented and analyzed. The re-acquisition of DVL bottom-lock at the end of the mission, simulating the vehicle lowering altitude to within range of the seafloor, is also investigated.