By Liz Kalaugher
It wasn’t until remote-sensing specialist Andreas Kääb of the University of Oslo flew over Canada’s Lawrence River and saw floating ice that he came up with an application for the time delay that occurs when satellites take stereo images to map elevation. This delay, which is otherwise an irritation, could in fact be used to measure the motion of ice over periods of just a minute or so, he realised.
Cryospheric scientists currently monitor sea ice motion over hours or days. Kääb believes his minute-scale methods could be useful for testing models of ice motion and investigating shearing of ice floes but ‘I don’t know the purpose yet, that’s why I’m here’. He presented his new techniques to the sea ice community with a poster at DACA-13 in Davos and was pleased by the interest shown.
Kääb has used the approximately 55 second time delay between fixed satellite stereo images taken by ASTER to track sea ice in the Kennedy channel between Greenland and Canada in July 2007. The ice was moving with a maximum speed of roughly 1 m/s, he found. Steerable satellite stereo images taken by Worldview-2 up to 200 seconds apart, meanwhile, showed ice in Cook Inlet, Alaska, travelling at nearly 3 m/s.
Finally, the near simultaneous orbits of Landsat-7 and EO-1, a test instrument for Landsat-8, provided images taken 6.6 minutes apart of the Amery ice shelf in Antarctica. This revealed upwelling water freezing at the surface and the resulting ice crystals being driven away from the shelf by the wind at up to 0.8 m/s, while big icebergs remained motionless.