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Physical Oceanography from an Arctic AUV: Field Comparison with CTD Casts

An autonomous underwater vehicle (AUV) designed for operation at high latitudes and under ice completed its first Arctic field tests from the USCGC Healy in October 2001. The AUV has been under development since 1998, and is being created to provide: unprecedented endurance, ability to navigate at high latitudes, a depth rating of 1500 to 4500 meters depending on payload, and the capability to relay data through the ice to satellites via data buoys.  The ALTEX (Atlantic Layer Tracking EXperiment) AUV's initial applications are focused on tracking the warm Atlantic Layer inflow - the primary source of seawater to the Arctic Ocean.  Consequently the primary payloads are twin, pumped CTD systems.  Oxygen and nitrate sensors provide the ability to use NO (dissolved oxygen corrected by nitrate to account for biological respiration) as a nearly conservative tracer.  An ice profiling sonar allows the AUV to estimate the ice thickness in real-time and is designed to generate high quality post-processed ice draft data comparable to that collected through the SCICEX program. 

 The cruise generated 24 water column and under-ice data sets. Fifty-two CTD casts were made to the depth of AUV operations and often to the ocean bottom.  Their purpose was to provide AUV-CTD comparisons and to track the Atlantic Layer inflow across 5 transects.  Reasonable agreement in temperature, salinity, and tracer concentrations was obtained between the vertical CTD casts and the along-track AUV measurements when one considers temporal and horizontal variability.  The field comparison suggests some improvements to AUV sensor configuration and some advantages to sensor redundancy.  The post-processed ice draft results show reasonable ice profiles and have the potential, when combined with other science data collected, to shed some additional light on upper water column processes in ice-covered regions.  The AUV proved its worth in making autonomous measurements under sea ice too thick for the icebreaker to penetrate effectively.  Overall cruise results include: operating the AUV from the USCGC Healy in the ice pack, demonstrating inertial navigation system performance, obtaining oceanographic sections with the AUV, obtaining ice draft measurements with an AUV born sonar, and testing the data-buoy system.