Use of a remote sensing data repository for in-flight calibration of optical sensors over terrestrial targets.

The present study is parr of an investigation aimed at optimizing the use of desertic sites for absolute or relative calibration of satellite visible sensors. This effort includes characterization of the surface, gathering of climatology or atmospheric data sets, ground- and air-based measurements as well as results of calibration of various sensors over these sites. All these measurements and estimates are stored in a repository and made available to various methods for calibration. Post-launch degradation and relative sensitivity of various sensors have been estimated using north african desertic sites as radiometrically stable targets. The selected areas have first been characterize in terms of bidirectional and spectral reflectances by making use of POLDER capabilities, then to cross-calibrate SeaWifs, VEGETATION onboard SPOT4 and AVHRR onboard NOAA-14 by reference to POLDER. Results are compared with absolute and relative calibration issued from other sources. Extensive period of time are spanned to assess the ability of this method to monitor long term trends in sensor evolutions. Results of this cross calibration will be presented. The method developed for this study will be presented as well, in order to make it applicable to other sensors (e.g. optical sensors onboard EOS-AM1, ENVISAT). A sensitivity study has also been realized, considering synthetic data, allowing to evaluate the main contributions to the error budget. The need for aerosol optical thickness is then evidenced, and will lead to the set up of a sun photometer on one of the selected sites in 1999.