Observations and Analysis of Sea Ice Motion with the Ice Buoy DRIVA during the 2010 Spring Field Campaign in the Bay of Bothnia

Rapport, 2011

The ice drift buoy DRIVA is tested for the first time in the environment for which it was designed. The communication with the buoy via satellite phone is working well. Temperatures could be read during the entire test period, and except from an unexplained loss of GPS data during 20 days, the GPS position was transmitted as expected. The rotary encoders did not function properly during the campaign. A sea ice drift algorithm is validated against the buoy data using ten image pairs acquired by the C‐band SAR satellite ENVISAT. The accuracy of the algorithm decreases with time and highest accuracy is achieved with the first three image pairs. For the first image pair, the computed translation differs by only 160 m from the translation measured by the buoy, which is a good result considering the pixel resolution of approximately 150 m. The root mean square error of the algorithm for the first three image pairs is 5% for the ice speed and 4 degrees for the direction of the ice. The decreasing accuracy can be related to an increase of melting during the campaign. The sea ice is very dynamic during the period, making the images very different. Large ice floes break into smaller pieces after a short time. As the ice floes melt and get smaller, rotation increases, and the shapes of the ice floes are not preserved. It appears that the buoy floats within a streak of slush ice during the last days of the time period. Tracking of the ice is more difficult under such circumstances, because the contours of an area of slush ice are constantly changing.