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 on-ground recovery 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: 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: 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: 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

Figure 1: Level-0 Processing Workflow