by Rhonda Moniz
22. November 2011 04:55
Studies using Autonomous Underwater Vehicles to navigate relative position to icebergs, as well as mapping the underside of ice flows is not new, but it does have its challenges. AUVs have been operating under the ice for a number of years now. Because icebergs translate and rotate through inertial space, standard vehicle navigation methods are unable to provide iceberg-relative position estimates. AUVs currently use a combination of measurements including dead-reckoning, and acoustic transponder networks, or velocity measurements from a Doppler velocity logger for navigation and control. Coastal sea-ice conditions can change rapidly, and are complex.
Existing AUV systems have a high level of accuracy under static ice sheets, but cannot provide ice-relative navigation of moving ice sheets and icebergs. Iceberg-relative position estimates can be correlated with incoming sonar measurements that show the range to the iceberg surface with an a-priori three-dimensional iceberg map. Iceberg-relative navigation for AUVs would allow for data collection of free-floating icebergs.
By using multi-beam sonar attached to AUVs, scientists are now beginning to get the first complete view of the underside of sea-ice. An AUV operating off NE Greenland in August 2004, was able to obtain the first successful swath sonar measurements under sea ice, showing in unprecedented detail the three-dimensional nature of the under-ice surface. The vehicle was operated from RRS James Clark Ross. The expedition obtained more than 450 track-km of under-ice multi-beam data. The purpose of the cruise was to obtain ice thickness distributions to validate thickness values against other data collected simultaneously from the surface and air. The expedition also set out to conduct transects across, and within Fram Strait so as to obtain estimates of ice flux. Several other possible applications for this technology include mapping rubble fields near oil platforms, charting oil spread under ice, and mapping iceberg shapes in fast moving ice packs.
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