Water Vapour Retrieval
The "maximum likelihood" water vapour column over a pixel is estimated by comparing Radiative Transfer (RT) based simulations of the ratio 900/885 with the corresponding OLCI measurement. The RT-simulations are approximated by a product of the precise atmospheric transmission (using exponential sums of uncorrelated k-distribution terms) and an estimation of the scattering – absorption - interaction – factor, stored in pre-calculated LUTs. The optimization is done by the gradient descent ("newton-secant") method. It usually requires 1 to 5 iterations.
A priori Land/Ocean classification is used to select among different LUTs for the computation of correction factor for the scattering – absorption interaction.