The copolar phase difference (CPD) between VV and HH polarization of X-band SAR acquisitions shows a significant dependence on the depth of fresh snow. Phase differences of 5-15 deg/10 cm fresh snow were determined at a frequency of 9.65 GHz by comparing spatial and temporal variations of snow depth (SD) with the CPD. Spatial correlations were derived from snow transect measurements during January 2012 and TanDEM-X acquisitions. Temporal correlations were derived from weather station data and TerraSAR-X acquisitions between December 2012 and April 2013. All measurements were done at a test field near the city Sodankylae, Finland. To explain the observed CPD, a model derives birefringent properties from the microstructure of snow, which is described as aligned elliptical particles. The microscopic description is based on computer tomography observations. Different incidence angles were analyzed in consistency with the model. The temporal evolution of the CPD was linked to the temperature-gradient-driven recrystallization process. Sudden increases in the CPD indicate fresh snow. Slow decreases indicate the subsequent recrystallization process. The background signal of wet soil was considered and causes a small negative offset to the CPD. A quantitative determination of the depth of fresh snow is possible, because the specific CPD per meter of snow can be estimated. Spatial resolutions below are achievable with sensors such as TerraSAR-X or TanDEM-X. This paper presents a theoretical relationship between the microstructure of snow and the CPD and relates the CPD theoretically and empirically to the depth of fresh snow.

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