Starting with Level-1B TOA radiances, this process derives reflectances corrected for gaseous absorption. It also allows for consolidation of the pixel classifications from Level-1B. Water vapour retrieval is a by-product of gas correction.

This process is divided into five main steps:

1. Extraction of data from the input Level-1B packaged product including quality checks, conversion from radiance to reflectance (of both measurements and error estimates) and interpolation tie-points data onto a pixel grid.
2. Pixel classification.
3. Correction of the TOA reflectance for the absorption by atmospheric gases, for clear sky pixels.
4. Water vapour retrieval: retrieval of the Integrated Column Water Vapour
5. Surface reclassification of clear-sky pixels where necessary.

Note that the order of the steps is important.