A SAR IMAGE STUDY OF A SNOW-COVERED AREA IN THE FRENCH ALPS

Two radar experiments were performed in the French Alps in December 19 89 and April 1990. The main objective was to study the signature of sn ow-covered terrain for different snow conditions. The E-SAR (Band X) f rom 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 1 600 m to 2600 m. Ground data included snow depth, liquid water content , and snow and air temperature. A SPOT image was available for Decembe r 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 mod el) was used to correct the image geometrically and radiometrically. A simple scheme was developed for these corrections. The snow was sligh tly wet in April and many features appearing on the image are due to t he 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 a nd snow-covered signatures are very similar for this particular terrai n, which is a golf course in summer; that is, smooth wet soil with sho rt grass. These experiments show that are necessary when studying moun tainous area. Furthermore calibrated data are required to compare snow -free and snow-covered terrain when the snow is slightly wet and there fore slightly absorbant with respect to the electromagnetic waves.