Improvement of a vegetation index using directional measurements from POLDER/ADEOS

A key originality of the POLDER instrument (POLarization and Directionality of Earth Reflectances), is its ability to measure the bidirectional reflectance distribution function (BRDF) in a few days assuming cloud-free conditions. This ability can be used to produce time series of vegetation indices corrected for directional effects. This paper presents the first results of such vegetation indices (called normalized indices throughout this paper), which show that the vegetation indices derived from the atmospherically corrected directional daily measurements depend significantly on the viewing direction, with variation of indices in the range 0.05-0.2. Normalized vegetation indices values, evaluated at the scale of Africa at two different dates, differ substantially (0.1-0.25), both in space and time, from Maximum Value Composite (MVC) vegetation indices. It is argued that the normalized indices have a clearer physical meaning than MVC indices and should lead to a better quantitative description of the biosphere. Areas of improvements of the derivation of vegetation indices corrected for directional effects are identified and will be the subject of future work.