Anisotropy-corrected vegetation indexes derived from POLDER ADEOS

A key originality of the polarization and directionality of earth reflectan ces (POLDER) instrument is its ability to measure the bidirectional reflect ance distribution function (BRDF) in a few days assuming cloud-free conditi ons, This ability can be used to produce time series of vegetation indexes corrected for directional effects, This paper presents the first results of such vegetation indexes during a period of four months over the continent of Africa, and an assessment of the noise level of their spatial and tempor al variations. The results show that the vegetation indexes derived from th e atmospherically corrected directional daily measurements depend significa ntly on the viewing direction, with variations of indexes in the range 0.05 -0.2. Composited vegetation indexes corrected for directional effects (call ed anisotropy-corrected indexes) are produced at ten-day intervals as norma lized differences of spectral hemispherical reflectances at 865 and 670 nm, The hemispherical reflectances are calculated by directional integration o f a three-parameter semi-empirical BRDF model adjusted against time series of daily reflectance data, The values of anisotropy-corrected indexes, eval uated 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) veget ation indexes calculated every ten days using a selection of POLDER data ac quired in the vertical across-track acquisition plane, It is argued that th e anisotropy-corrected indexes have a clearer physical meaning than MVC ind exes and should lead to a better quantitative description of the biosphere. A comparative characterization of space and time behavior of anisotropy-co rrected and MVC vegetation indexes shows that anisotropy-corrected indexes have more noisy single date images than the MVC indexes on vegetated areas, but also contain less noiselike oscillations in their temporal profiles, A reas of improvements of the derivation of vegetation indexes corrected for directional effects are identified and will be the subject of future work.