Look Up Table (LUT) of bidirectional reflectance factors (BRF) representing the AVHRR
NOAA like data are created using the physically-based semi-discrete model of Gobron
et al. (1997) to represent the spectral and directional reflectance of horizontally homogeneous plant canopies, as well as to compute the values of FAPAR in each of them.
The sampling of the vegetation parameters and angular values were chosen to cover a
wide range of environmental conditions. These simulations constitute the basic information used to optimise the formulae. The sampling selected to generate the LUT has
been chosen so as to generate a robust global FAPAR algorithm.
Once this LUT was created, the design of the algorithm consisted in defining the mathematical combination of spectral bands which will best account for the variations of
the variable of interest (here, FAPAR) on the basis of (simulated) measurements, while
minimising the effect of perturbing factors such as angular effects.
In the case of bare soil simulations, the Hapke modified soil model of Pinty et al. (1989)
is used with a fixed hot-spot parameter equal to 0.2 and an asymmetry factor equal to
-0.1. The soil data required to specify the lower boundary condition in this model were
taken from Price (1995). The value of single albedo, for each spectral bands, have been
inverted to obtain the same albedo value as the lambertian assumption is made.