The main purpose of the Calculate SST Values and Variances process is to compute the coefficient needed to retrieve the SST products. Two main conditions are taken into account:
The variations in viewing geometry across the satellite swath and the differences in these variations between across track and along track views must be incorporated into the SST retrieval coefficients. In addition, the retrieval coefficients are best chosen to be linear with path length (secant of view zenith angle). As a consequence, a Look-Up Table (LUT) is calculated using the nadir normalised path length computed previously.
Retrieval coefficients should also be defined as a function of water vapour. An LUT is then computed from the total column water vapour values present in the meteorological annotations dataset.
Once adequate coefficients have been found, a linear combination of brightness temperatures, weighted by the interpolated coefficients, is computed to obtain SST product and a linear combination of the squares of the BT errors is computed to obtain SST variance.
This process is performed identically for each SST parameter (N2, N3R, N3, D2 and D3 SST products).