Ice-Water Ambiguity

At steep incidence angles, especially under high wind conditions, confusion can often occur between open water and sea ice in imagery acquired by single channel SARs such as RADARSAT-1 or ERS1/2. Imagery from multi-polarization SARs can be used to greatly reduce this confusion in two different ways: Due to the minimal backscatter from the water in the cross-polarization (HV or VH), imagery acquired at these polarizations can be used to improve discrimination between the water/ice/land classes: The land and to some degree the ice both experience volume scattering giving rise to a cross-polarized return Figure 9-15 illustrates this for the same area of the Labrador Sea as shown in Figure 9-14.

Figure 9-15
Figure 9-15. Single channel intensity C-band images from SIR-C of the Labrador Sea illustrating the improved contrast of the first-year ice/water boundary of the HV channel due to a lack of multiple scattering from the water surface (from Scheuchl et al 2001c).

Figure 9-16 demonstrates how a ratio of HH /VV can be used to achieve similar results as the different scattering characteristics of the two targets at HH and VV can be used to improve the contrast compared to any single channel.

Figure 9-16
Figure 9-16. Ratios between images of Labrador Sea acquired by SIR-C show the advantages of the co-polarization ratio for improving the contrast of the first-year ice/water boundary (from Scheuchl et al 2001c).

Polarimetric decomposition techniques can be used to generate polarimetric discriminators that can be used to aid interpretation or to use for classification. Figures 9-17 and 9-18 show examples of the contrast between ice and water that is visible in C- and L-band images with particular improvements visible in the use of anisotropy at C-band and the entropy at L-band.

Figure 9-17
Figure 9-17. C-band entropy (H), anisotropy (A), and alpha-angle (Greek small letter alpha) images for the Labrador Sea demonstrating the improved contrast of ice/water using the anisotropy parameter (from Scheuchl et al 2001a).

Figure 9-18
Figure 9-18. L-band entropy (H), anisotropy (A), and alpha-angle (Greek small letter alpha) images for the Labrador Sea demonstrating the improved contrast of ice/water using the entropy parameter (from Scheuchl et al 2001a).