Atmospheric correction algorithm for CHRIS images application to CASI

The Compact High Resolution Imaging Spectrometer CHRIS sensor was launched on board of PROBA (PROject for on Board Autonomy) the 22nd of October 2001. CHRIS will acquire sets of images over the Belgian coastal zone near Oostende. Within this context CASI (Compact Airborne Spectrographic Imager) images was used as prototype of CHRIS data. This is to assess the performance of an atmospheric correction algorithm for hyperspectral ocean-color sensors (i.e. CHRIS). This approach couples the atmospheric attenuation processes with the underlying physics of water inherent optical properties. The algorithm employed the 6S code (second simulation of the satellite signal in the solar spectrum) to simulate the atmospheric and surface reflectance and the gaseous transmittances. Relationships between the water leaving reflectances at 860 870 and 880 nm are proposed. This is to estimate the water signals at these near infrared (NIR) bands, hence the aerosol reflectances. These negligible water-leaving signals were found to be very important for a reliable atmospheric-correction-algorithm over turbid waters. The choice of this NIR part of the spectrum was to satisfy a certain condition related to the corresponding water absorption coefficient. A look-up-table of total gaseous transmittance has been generated for 42 values of column water vapour. This table was used with a two-band ratio technique to estimate the contribution of water vapour to the total gaseous transmittance. The aerosol optical thicknesses were estimated by fitting calculated atmospheric reflectances at the water vapour window (860-880 nm) to 20 candidates of maritime aerosol models. The performance of the atmospheric correction is being investigated with other sensors (DASI and ROSIS) and in situ measurements.