Characterisation of the HDRF (as a proxy for BRDF) of snow surfaces at Dome C, Antarctica, for the inter-calibration and inter-comparison of satellite optical data

Measurements of the Hemispherical Directional Reflectance Factor (HDRF) of snow surfaces were performed at Dome C, Antarctica, during the Australis Summer 2011-2012 to support the inter-comparison and inter-calibration of satellite optical sensors. HDRF data were collected with the Gonio Radiometric Spectrometer System (GRASS) which performs hyper-spectral measurements of radiance from the same target surface with independent collectors at a number of viewing and azimuth angles in the 0-60° and 0-360° angular ranges, respectively. The radiance collectors, installed on a hemispheric frame and connected to a spectrometer through fibre optics, have an 8° full cone of acceptance and a viewing footprint varying from 0.049 m2 at nadir to 0.142 m2 at viewing angles of 60°. These relatively small footprints allow for the characterisation of small-scale heterogeneities in the HDRF of observed surfaces. HDRF measurements representative of the Dome C snow surfaces were made at eight different sites along a transect approximately 100 m long. All the sites exhibit similar HDRF distributions with inter-site differences explained by small-scale inhomogeneities of the surface. The measured HDRF display marked forward scattering with anisotropy increasing with wavelength in the 400-1600 nm spectral region. These data complement those from previous measurements performed in the same area with a different technique. Agreement between the two data sets is shown by differences generally lower than 4% between HDRF distributions derived from a previous study and the spatially averaged HDRF from the various sites along a transect presented in this work.