5.5.3 - LSA SAF fAPAR

The fAPAR is estimated from a NDVI-like vegetation index, the RDVI (Renormalized Difference Vegetation Index).
As you have seen, in chapter 4, NDVI is given by the normalized difference between reflectances in channels 0.6 μm and 0.8 μm. RDVI, is a very similar parameter given by:

RDVI_equation

RDVI has a behaviour close to that of NDVI for large LAI, but tends to be more sensitive to changes in vegetation coverage under low LAI conditions. Since the view-illumination geometry may have a large impact on reflectance observations, RDVI is computed using 0.6 Ám and 0.8 Ám surface reflectances corrected to an optimal geometry, which maximizes the correlation with fAPAR. Such corrected reflectance values are estimated, for each channel, from K0, K1 and K2 parameters (Roujean et al. 1992):

refoptimized_equation

corresponding to solar and view zenith angles of 45║ and 60║, respectively. fAPAR estimation explores its linear relationship to RDVI (obtained for the optimal geometry):

fapar_function_RDVI_equation

Next figure shows an example of fAPAR over the Full SEVIRI disk (figure on the left), and the corresponding Error (figure on the right) for the 15th April 2007.

fAPAR_and_Error