A SAR image study of a snow-covered area in the French Alps

Two radar experiments were performed in the French Alps in December 1989 and April 1990. The main objective was to study the signature of snow-covered terrain for different snow conditions. The E-SAR (Band X) from DLR was used, and the data were processed by CNES. The study area was situated at Les Arcs, a ski resort, with elevations ranging from 1600 m to 2600 m. Ground data included snow depth, liquid water content, and snow and air temperature. A SPOT image was available for December and aerial photographs for the following July. In December there was almost no snow, while in April the area was completely snow covered. Because of the highly varying topography, a DEM (digital elevation model) was used to correct the image geometrically and radiometrically. A simple scheme was developed for these corrections. The snow was slightly wet in April and many features appearing on the image are due to the snow itself or to the snow-ground interface. In particular, the ski runs are clearly visible due to the different structure of the packed snow. Only a small area could be compared between December and April due to technical problems in December. It appears that the snow free and snow-covered signatures are very similar for this particular terrain, which is a golf course in summer, that is, smooth wet soil with short grass. These experiments show that geometrical corrections are necessary when studying mountainous area. Furthermore calibrated data are required to compare snow free and snow-covered terrain when the snow is slightly wet and therefore slightly absorbant with respect to the electromagnetic waves.