Exploring beneath the PIG Ice Shelf with the Autosub3 AUV

On 31st January 2009, two numbers: ldquorange and bearingrdquo flashing up on a laptop screen, indicated that Autosub3 had returned from its last mission beneath the Pine Island Glacier (PIG) Ice Shelf in the Western Antarctic. The Autosub technical team from NOCS, Southampton, onboard the US ice breaker Nathanial B Palmer breathed a collective sigh of relief. Any significant technical failure would have resulted in total loss of the multi million Euro Autonomous Underwater Vehicle with no hope of recovery from 60 km into the ice shelf cavity. This was the last of six successful missions to investigate the shape the ice shelf, the sea bed bathymetry, the currents and the physical oceanography within the ice cavity. Each are vital to understanding the interaction between the sea water and the ice shelf, and quantifying whether the melting rate is changing. During the cruise, Autosub3 had run beneath the ice for almost 4 days and for 510 km. Autosub3 had been exploring the Pine Island Glacier, a floating extension of the West Antarctic ice sheet, as part of an international team effort lead by Dr Adrian Jenkins of the British Antarctic Survey and Dr Stanley Jacobs of the Lamont-Doherty Earth Observatory, New York. Autosub3 was launched from the Nathaniel B Palmer, an American icebreaker, as part of the two month cruise to investigate the oceanography, biology and glaciology of the Southern Amundsen Sea. This paper will concentrate on the technical aspects of the Autosub3 vehicle and its missions under the PIG, and seek to answer a number of questions: How did the AUV successfully dead reckon navigate for over 24 hours, and return accurately to the rendezvous point? How did we cope with the possibility of ice bergs or sea ice drifting over the recovery position ? How did Autosub3 (almost always) avoid collision with the jagged ice shelf above, or the unknown depths of the seabed? How did we communicate with the vehicle at the start and the end of missions? How did we manage risk, and prior to the cruise, what modifications and testing did we apply to the AUV to improve the overall reliability? What measures did we take during the cruise to further improve our chances of a successful outcome ? The paper will outline the history of the use of AUVs for polar science. Results from the recent cruise will be presented showing the actual mission tracks, with the echo sounder isonified ice draft and seabed. Not all went completely to plan: the paper will also describe the events of Autosub's close scrape on its 4
^th
mission under the PIG.