As highlighted previously, the Level-0 initial processing, performed to preserve the integrity of the acquired data, occurs in real-time (i.e. in parallel to data reception). By contrast, Level-0 consolidation and downstream processing activities (i.e. to Level-1) are delayed until triggered by the recovery on-ground of the ancillary data. This occurs at the end of every satellite pass. Ancillary data are ingested together with the MSI raw data at the beginning of the Level-0 consolidation processing collates and appends all necessary metadata required for archiving and onward processing to Level-1.

1. Level-0 viewing model initialisation: This processing step computes the viewing model for generation of the Preliminary Quicklook (PQL)
2. PQL processing: This step includes the PQL resampling and the computation of ancillary data for the consolidated Level-0 product
3. Preliminary cloud mask processing: This processing step generates the cloud mask from the PQL based on spectral criteria
4. PQL compression: Performs the compression of the PQL

The position of these initial processes can be seen in the middle of Figure 1, up to the output block named Level-0 (Consolidated).

The Level-0 product is raw image data (on-board compressed) in raw Instrument Source Packet (ISP) format. It is initially processed to safeguard the integrity of the Near-Real-Time-acquired data. As summarised in Figure 1 this operation includes:

• MSI telemetry analysis: Concatenates acquired ISP data into granules and performs data analysis and error detection functions
• Datation: The datation of the individual lines in an image enables the exact capture time of each ISP within a granule to be recorded
• Low resolution image extraction: This processing step extracts the low-resolution images for quicklook generation
• Satellite ancillary telemetry analysis: Compares extracted satellite ancillary data from bit values and checks it relative to pre-defined admissible ranges

Level-0 Processing Workflow