Satellite observations are ideal for the synoptic observation of expansive and inaccessible areas, serving as both primary and secondary sources of information. However, although there is significant value in the analysis of data from individual sensors, global observing systems and in particular studies requiring different spatial resolutions and long time bases require accurate knowledge of sensor to sensor biases. Therefore the responsivities of all optical radiometers operated in space need to be intercompared and traceable to a common reference standard. Natural targets in polar regions, such as snow or sea ice, are potential vicarious calibration sites, providing uniform surfaces and short overpass times. However, snow and sea ice are not isotropic and can vary spatially, due to parameters such as mineral aerosol loadings. Therefore, the knowledge of surface BDRF is a pre-requisite for the use of satellite data.