Prototyping algorithm for retrieving FAPAR using MSG data in the context of the LSA SAF project

This paper describes the prototyping algorithm developed for retrieving the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) using MSG data in the framework of Satellite Application Facility on Land Surface Analysis (LSA SAF). The prototyping relies on the Roujean and Bréon (1995) method, which is based on simulations of visible and near infrared reflectance values in an optimal geometry. A relationship is found between a vegetation index and daily FAPAR. The algorithm has been applied to one year of MSG BRDF data since August 2005, using a temporal frequency of 5-days, and then validated against a set of operational satellite FAPAR products such as MODIS, MERIS, SeaWiFS and CYCLOPES, as well as with ground-truth information. MSG FAPAR products present a reliable spatial and temporal distribution of retrievals. The inter-comparison of the MSG FAPAR product with different operational products shows the large existing differences among FAPAR products. However, the best consistency is found between MSG and MERIS (only July 2006 was evaluated) or between MSG and CYCLOPES (available only for 2003). At a pixel level, the RMS over selected validation sites is lower than 0.1 between MSG and MERIS or MSG and CYCLOPES products, which demonstrates the competitiveness of the proposed prototype.