Sub-pixel correction in comparison between in situ and satellite derived spectral reflectance

In comparison between in situ and satellite-derived spectral reflectance so as to validate a newly launched satellite sensor, the difficulty is that the ground measurement is generally carried out at a higher resolution than the satellite sensor, especially for moderate or high resolution sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) or the Advanced Very High Resolution Radiometer (AVHRR) etc. To overcome this difficulty, we adopt sub-pixel correction method in a comparison study between in situ and MODIS-derived spectral reflectances of snow and sea ice in the Amundsen Sea, Antarctica. The position of the in situ measurement is accurately located within the imagery granule using the combination of small angle approximation and direct matching algorithms. Apparent reflectance retrieved from MODIS granule for the station pixel is atmospherically corrected. The corresponding spectral albedo and directional reflectance at the same viewing geometry as MODIS are derived from ground-based spectroradiometer measurements. The averaged spectral reflectance and albedo in the vicinity of each ice station are simulated for the corresponding MODIS pixel from ground spectral measurements by weighing over different surface types (various ice types and open water) so that reflectance difference at a sub-pixel level is appropriately corrected. Values of ground-based MODIS spectral simulation are compared with satellite-derived values so that the discrepancy is estimated.