Integrating a Sonardyne Ranger 2 USBL System in the Autonomous Underwater Vehicle Eagle Ray: Operation and Post-Processing

The Eagle Ray Autonomous Underwater Vehicle (AUV) has been tasked with exploring deep-sea environments along the Northern Gulf Coast and throughout the East Coast of the United States Exclusive Economic Zone. One of the essential products returned from these missions are bathymetry and backscatter produced by a multibeam echosounder; however, the data quality depends on the accuracy of the vehicle navigation. Because satellite-based navigation is unavailable to the vehicle during a mission, the descent relies on inertial navigation, which accumulates errors exponentially over time. A Doppler velocity log is used to aid the inertial navigation system, constraining further error accumulation to an uncertainty typically referenced to the distance traveled during the survey, on the order of a meter of error every kilometer of survey. Employing an ultrashort baseline (USBL) instrument, allows the surface support vessel to acoustically track the position of the AUV. The integration of the Sonardyne Ranger2 USBL into Eagle Ray has provided the opportunity to overhaul not only the acoustic tracking subsystem, but also the acoustic communication capability. Most significant to this work, the tracked position is transmitted to the vehicle and is available as an absolute position reference to further improve the vehicle navigation solution. In this way, USBL corrections are employed to maintain the vehicle track. Because this new capability is still being tested a traditional means of logging the USBL track and merging with the vehicle navigation by post-processing is also being discussed. Initial shallow water field trials of the newly upgraded vehicle suggest that error standard deviation of the vehicle navigation while actively integrating USBL corrections shows no more than 2 meters, and processing of the multibeam soundings reveal no significant artifacts are present in the bathymetry, suggesting that the navigation track is not perturbed by the frequent updates.