SAR-Based Ice Thickness Charts

Routine ice charts published by national ice services WMO ice symbols are used for showing ice conditions in 2-25 km scale. For users at sea this resolution is too rough for ice navigation. In order to provide information in the scale of the ship automated ice thickness charts are published by the Finnish Meteorological Institute (FMI). The algorithm, which combines SAR data and ground truth, provides ice thickness information in 500 m resolution. The products are provided operationally and available for users shortly after SAR data is available, i.e typically a few hours after the instrument overflight.

The mean level ice thickness estimates given in the traditional ice chart by Finnish Ice Service are compiled by an ice analyst from multiple sources including drilling measurements near coastline, systematic field observations, provided by the staff of icebreakers and other ships on sea, and ice growth estimates yielded by ice models. SAR images with a wide coverage are used to produce spatially more accurate ice thickness charts than the routine ice charts.

An ice thickness chart is operationally produced after a SAR image has been received, using the latest available ice chart as an input. Then the ice field boundaries are refined, and the thinnest and the thickest ice areas inside each ice chart segment are identified based on the SAR signal statistics. The resulting thickness chart is then color coded according to navigation restrictions based on ice thickness.

The spatial accuracy of the resulting ice thickness charts as well as that of the routine ice charts have been analyzed using ice thickness measurements based on electromagnetic induction. The performed analysis has shown more accurate results for the SAR-based ice thickness charts. Currently, the ice thickness chart can use Radarsat-1 ScanSAR Wide Mode images and Envisat ASAR Wide Swath images as its input image.

Radarsat-1 is a Canadian space-borne SAR instrument. Radarsat-1 is operating at C-band, its radar frequency is 5.3 GHz corresponding to a wavelength of 5.6 cm. It was launched in late 1995 and it will be relaced by Radarsat-2 in the near future, For more information e.g. see CSA Radarsat-1 pages.

Envisat ASAR (advanced SAR) is a SAR instrument operated by the European Space Agency (ESA). It is one instrument in the Envisat satellite, and also operates at C-band. Envisat was launched in March 2002. For more information, e.g. see ESA Envisat pages.