Similar to OLCI "retrieval" processing, SLSTR data are retrieved and prepared for use in Level-1 main processing. The nadir parts of the SLSTR swaths are loaded and transformed to the original acquisition geometry. Annotations are, if necessary, interpolated on the original grid.
The ortho-geolocation function for the reference channel (Su) is retrieved and de-registration to the other channels of the instrument is estimated in accordance with an instrument characterisation file. The radiometric units of SLSTR measurements are converted to TOA reflectances.