Context overview

        flowchart LR
    BKT[("public S3 bucket dpr-common:<br/>S2 L1A · ESA PSD L0 SAFE · GIPP")]
    L1["S2B L1B<br/>(EOPF Zarr, radiance / counts)<br/>— reverse chain input"]
    GIPP["operational S2A GIPP<br/>R2EQOG / R2DEPI / BLINDP /<br/>R2PARA / R2CRCO (XML)"]
    ADF["ADF<br/>SRF (spectral characterisation)"]
    E2ES["s2_msi_raw_generator reverse chain<br/>(invert L0→L1B corrections;<br/>CCSDS-122 + packetize)"]
    L0["Synthetic L0 RAW EOProduct<br/>(Zarr, compressed-ISP streams + STAC,<br/>PSFD names — ICD-IF-NAME)"]
    CAL["calibration sub-set:<br/>synth diffuser + dark →<br/>derived ADF (inverse-crime)"]
    MSI["msi-processor l0_decode<br/>→ L1A′"]
    REP["validation & report:<br/>Synthetic-Synthetic L0 vs reference ESA L0 'img'<br/>(10/20 m ≤~4 DN) · L0plus codec<br/>round-trip decode==L1A bit-exact"]
    BKT -->|"s3fetch"| L1
    BKT -.->|"ESA L0 (structural ref)"| REP
    L1 --> E2ES
    GIPP --> E2ES
    ADF --> E2ES
    E2ES --> Synthetic L0
    E2ES -.-> CAL
    L0 --> MSI --> REP
    L1 -.-> REP
    

Inputs. The primary input is a S2B L1B (radiance / counts) EOPF Zarr granule; the operational S2A GIPP (per-pixel dark + relative response/PRNU, defects, offsets, crosstalk, on-board-eq); and the SRF for spectral characterisation. L1A is not an input — it is an intermediate the reverse chain produces on the way down to Synthetic L0.

Processing. The reverse chain runs the S2 L1B backwards through the exact inverse of the operational L0→L1B radiometric correction chain — invert offset, relative-response/PRNU, dark, un-bin, SWIR re-stage, defective, crosstalk, on-board-eq — to reconstruct L1A → L0plus → Synthetic L0. MTF-deconvolution is OFF, so PSF and noise are not re-applied. A separate calibration sub-set synthesises sun-diffuser + dark acquisitions and derives the calibration coefficients back — the coefficients a downstream processor would actually use (inverse-crime cure).

Output. The reconstructed Synthetic L0 RAW EOProduct (the ICD-IF-L0 Zarr: 156 detector/band frames, quality masks, optional CCSDS ISP telemetry, STAC + sensor-configuration metadata); the reverse chain also emits the L1A and L0plus (CCSDS-122 ISP) intermediates en route.

Verification context. The reconstructed L0 is compared against the reference ESA L0 ‘img’: the 10/20 m bands agree to ≤~4 DN. As a supporting check, the L0plus codec round-trip is bit-exact — decode(L0plus) == L1A.

Calibration database (ADF output)

Besides the Synthetic L0 RAW product, the generator also derives the radiometric calibration coefficients and writes them as a versioned set of EOPF Auxiliary Data Files — the calibration database — that the downstream processor (the L1PP blocks of msi-processor) consumes directly. This is the single shared sensor-model ADF of the E2ES ⇄ processor coupling: the generator produces the ADF; the processor keeps calibration internal. Coefficients are derived (synthetic diffuser + dark), not the truth ADF, so the round-trip is non-tautological.

        flowchart LR
    subgraph SRC["derived coefficients (per band)"]
        PR["NUC gain g_d"]
        OF["NUC offset o_d"]
        DK["dark k"]
        AA["abs gain (~1/A)"]
        ES["ESUN (solar irradiance)"]
    end
    subgraph DB["cal-DB — EOPF ADFs (zarr v2)"]
        NUC["nuc.zarr<br/>/gain, /offset"]
        DARK["dark.zarr<br/>/dark_offset"]
        RADO["radiometric.zarr<br/>/gain, /offset"]
        SPECT["spectral.zarr<br/>/esun"]
    end
    PR --> NUC
    OF --> NUC
    DK --> DARK
    AA --> RADO
    ES --> SPECT
    

The NUC gain/offset follow the processor’s two-point convention (estimate_nuc); the absolute radiometric.gain is diffuser-derived (\(\approx 1/\mathrm{cal\_gain}\)); spectral.zarr carries the per-band ESUN (Thuillier 2003, S2A — ATBD §A.3) the processor’s toa unit needs for TOA reflectance. Written by s2_msi_raw_generator.adf_writer (s2_msi_raw_generator.caldb, pipeline phase build-caldb).