Feasibility of sea ice typing with synthetic aperture radar (SAR): Merging of Landsat thematic mapper and ERS 1 SAR satellite imagery

Abstract

ERS 1 synthetic aperture radar (SAR) and Landsat thematic mapper (TM) images were acquired for the same area in the Beaufort Sea, April 16 and 18, 1992. The two image pairs were colocated to the same grid (25‐m resolution), and a supervised ice type classification was performed on the TM images in order to classify ice free, nilas, gray ice, gray‐white ice, thin first‐year ice, medium and thick first‐year ice, and old ice. Comparison of the collocated SAR pixels showed that ice‐free areas can only be classified under calm wind conditions (<3 m s−1) and for surface winds >10 m s−1 based on the backscattering coefficient alone. This is true for pack ice regions during the cold months of the year where ice‐free areas are spatially limited and where the capillary waves that cause SAR backscatter are dampened by entrained ice crystals. For nilas, two distinct backscatter classes were found at −17 dB and at −10 dB. The higher backscattering coefficient is attributed to the presence of frost flowers on light nilas. Gray and gray‐white ice have a backscatter signature similar to first‐year ice and therefore can not be distinguished by SAR alone. First‐year and old ice can be clearly separated based on their backscattering coefficient. The performance of the Geophysical Processor System ice classifier was tested against the Landsat derived ice products. It was found that smooth first‐year ice and rough first‐year ice were not significantly different in the backscatter domain. Ice concentration estimates based on ERS 1 C band SAR showed an error range of 5 to 8% for high ice concentration regions, mainly due to misclassified ice‐free and smooth first‐year ice areas. This error is expected to increase for areas of lower ice concentration. The combination of C band SAR and TM channels 2, 4, and 6 resulted in ice typing performance with an estimated accuracy of 90% for all seven ice classes.