Source packet processing aims to unpack, validate and convert to engineering units, the science and auxiliary data required for processing.

Six main sub-steps are defined:

• Defining time and scan limits: as a result of the continuity requirements (SLSTR Level-1B granules must be continuous and provide the same geographical area for nadir and oblique views), the time interval defined for Level-1B granules is not the same as the time interval required for Level-0 granules. This process aims to extract the needed Level-0 data.
• Source packet quality; basic quality checks are performed on each raw packet to identify major problems such as data corruption, ensuring that only "good" data contribute to the product.
• Unpack housekeeping processes; unpack all required housekeeping data such as temperature of the on-board black bodies or instrument health and status information. These housekeeping data are then validated (out-of-range test) and converted to engineering units.
• Specific tests are performed for scanner and flip mirror error, by comparing the scan position and time errors, as well as the position and stability of the flip mirror with regard to the expected values.
• Science data processing unpacks and validates the science data contained in the earth view and black body pixel counts for all available channels from each packet.
• NAVATT packet processing extracts navigation and attitude data.