Navigation, Control and Recovery of the ENDURANCE Under-ice Hovering AUV (2009)
Kristof Richmond, Shilpa Gulati, Chris Flesher, Bartholomew P. Hogan, and William C. Stone
The AUV ENDURANCE was designed to operate in a unique environment: the waters of Lake Bonney in Taylor Valley, one of the Dry Valleys of Antarctica. This lake represents an extreme environment for AUV deployment and operation. The lake waters are permanently covered with an ice layer 3-4 m thick, and exhibit a relatively striking change in composition from the ice roof to the lake bottom, with salinity varying from that of fresh water in contact with the roof down to a sharp halocline at ~12 m depth, then increasing to 4 times that of seawater at the maximum bottom depth of almost 40 m. ENDURANCE was successfully deployed in the West Lobe of Lake Bonney in the 2008-2009 austral summer, and will return to re-sample the same location in the 2009-2010 season. The ENDURANCE mission goals include traversing the entire ~1 km x ~2 km lobe of the lake to obtain a full 3-D synoptic chemical profile, high-resolution mapping of the lake bottom with a multibeam sonar, and close approach for visual and high-resolution sonar examination of the face of the Taylor Glacier at one end of the lake. Thus, to accomplish the science tasks, the vehicle is required to perform larger scale navigation over several kilometers and find its way back home, as well as being capable of fine positioning and control to deploy instruments at points throughout the lake and near the glacier face. Environmental and logistical constraints at Lake Bonney place demands on the vehicle navigation and control. First, in order to avoid disturbing the deeper water layers, vehicle navigation is confined to the 8 m freshwater lens underneath the ice roof. Second, under-ice operation means that ENDURANCE has to be deployed and recovered through a melt hole only a few tens of centimeters larger in diameter than itself. Operation in this stratified environment, close to the solid ice surface, presents some difficulties for sonar sensors, including the vehicle DVL and iUSBL. In addition, in order to be able to profile the entire water column while remaining near the ice roof, ENDURANCE incorporates a spooling profiler system. Profiling is performed by deploying an instrument sonde from a cable spool inside the vehicle at nodes on a pre-determined sampling grid. Accurate profiling requires even deployment of the sonde, without jerking or swaying the cable. In pre-deployment tests, vehicle stationkeeping was refined to be sufficiently smooth and disturbance-free to allow accurate sonde profiling. In the field, an icepicking technique was developed to maximize the portion of the water column sampled, and to minimize disturbances on the instrument sonde even further. In order to ensure successful recovery through the melt hole, ENDURANCE has a three-stage return-tohole strategy. Normal navigation proceeds using a conventional down-looking DVL and an IMU to obtain dead-reckoned position. For recovery, as the vehicle approaches the melt hole, an on-board iUSBL and a transponder hanging under the hole provide relative position updates which are fused with the dead-reckoned navigation. The final approach and up-hole recovery are performed using a visual homing system. Navigation and control results from vehicle trials in temperate lakes and laboratory tests are presented, along with field results from the 2008 campaign.
In International Symposium on Unmanned Untethered Submersible Technology (UUST) 2009.

Shilpa Gulati Ph.D. Alumni gulati [at] mail utexas edu