Interpretation of a RADARSAT-2 fully polarimetric time-series for snow cover studies in an Alpine context - first results
Abstract
The aim of the SOAR #1341 project is to perform temporal analyses of changes in Radarsat-2 full-polarimetry parameters on snow cover in a mountainous area. The objective of the present study was to determine whether there is correlation between changes in statistics and changes in physical snow parameters during winter and spring. This methodological paper presents the first results in full polarimetry mode. Eight RADARSAT-2 Quad-Pol images were acquired between January 2009 and February 2011, seven in different snow conditions and one summer snow-free image used as a reference. The selected acquisition mode was Fine with a medium incidence angle (39°). A combination of Landsat-7/SPOT optical images, field measurements and a snow metamorphism model (Crocus) were used for validation. First, a necessary pre-processing step was performed with Radarsat-2 images due to the influence of high mountain topography on the polarimetric signal: projected incidence angle computation by means of a fine digital elevation model (IGN DEM). Next, DEM and optical dataset were re-projected onto the slant range mode of Radarsat-2, the configuration required to preserve the phase signal and polarimetric statistics. The method described here was mainly managed using PolSARpro software from ESA/IETR. The [T3] coherency matrix and the alpha-entropy-anistropy polarimetric decomposition parameters were calculated for each RADARSAT-2 image. Results showed that temporal changes in dry/wet/no snow characteristics can be identified throughout the winter season by analysis of primary polarimetric decomposition parameters compared with measurements made at 10 field sites. Alpha values decreased from dry snow to wet snow and reached minimum in snow-free conditions. In contrast, entropy values increased in inverse proportion.