Analysis of Polarimetric SAR Data of Sea-Ice in the Baltic Sea
Eriksson, Leif E.B.; Berg, Anders
Chalmers University of Technology, Dept. of Earth and Space Sciences, SWEDEN

The Baltic Sea has extensive ship traffic all year around and to assist and guide the winter traffic, daily sea ice maps are produced by the national ice services around the Baltic Sea. These maps have traditionally been based on single polarization synthetic aperture radar (SAR) images from Radarsat, Radarsat-2 and Envisat. As part of a campaign to assess the possibility to improve sea ice monitoring and forecasting capabilities with multipolarisation SAR, 22 polarimetric scenes from ALOS PALSAR and Radarsat-2 were collected over the Gulf of Bothnia in 2007 and 2009. Results from the evaluation of backscatter characteristics for each single polarization has been published (Eriksson et al, 2010), but this paper presents the first results from an evaluation of information that fully polarimetric SAR data can provide about sea ice.

To obtain validation data for the satellite images, optical and thermal infrared photos were taken from helicopter whenever weather conditions and staff situation allowed it. The airborne observations were complemented with in situ measurements of ice, snow and weather conditions when the helicopter landed on the ice. In addition, standard meteorological observations were available from several stations in the area covered by the satellite images.

The example in Figure 1 displays a great variety of sea ice types and their corresponding appearance in a Pauli decomposition. The colourful fast ice along the coast indicate the presence of several different scattering mechanisms. The drifting ice types have more homogeneous signatures, but the differences between the ice types are relatively large.

Figure 1. Sea ice outside the city of Umeå in Northern Sweden. This image is a RGB composite of three of the components of a Pauli decomposition of a polarimetric ALOS PALSAR scene acquired 2007-03-22.

The example in Figure 2 shows the polarimetric scattering alpha parameter, anisotropy and entropy for a Radarsat-2 scene acquired in March 2009. The upper and central part is open water, at the lower left border a couple of islands surrounded by fast ice are visible and to the right various types of drifting ice can be seen. Helicopter photos and a more detailed description of the ice types is given in Eriksson et al 2010.

Figure 2 From left to right, alpha, anisotropy and entropy for a Radarsat-2 scene acquired 2009-03-21.

The evaluation shows that there are clear differences in the polarimetric signatures from different ice types, which could provide a base for automated un-supervised ice type classification. However, it is expected that wet snow and melt conditions will make the separation between ice types more difficult.

This work was funded by the Swedish National Space Board, the Swedish Meteorological and Hydrological Institute and the Swedish Maritime Administration. Data from ALOS and Envisat were provided by the European Space Agency within the framework of the ALOS Data European Node Category-1 Proposal titled 'Improved sea-ice monitoring for the Baltic Sea' (AOALO.3562). Data from Radarsat-2 were granted within the Canadian program for Science and Operational Applications Research for Radarsat-2 (SOAR), project number 3924. The Umeå Marine Science Centre in Norrbyn and Lapplandsflyg AB in Umeå are acknowledged for logistical support during the field campaigns.

Eriksson, L. E. B., Borenäs, K., Dierking, W. et al. (2010). Evaluation of new spaceborne SAR sensors for sea-ice monitoring in the Baltic Sea. Canadian Journal of Remote Sensing. 36 (1) s. S56-S73.