# Introduction This Data Processing Model (DPM) describes the processing chain of the Sentinel-2 MSI Synthetic Raw Data Generator (`s2_msi_raw_generator`) — the **reverse chain** that runs a Sentinel-2B **L1B** product backwards through the **exact inverse of the operational L0→L1B radiometric chain** (invert offset, relative-response/PRNU, dark, un-bin, SWIR re-stage, defective, crosstalk, on-board equalization) to reconstruct **L1A → L0plus (CCSDS-122 ISP) → Synthetic L0**. MTF-deconvolution is **OFF**, so PSF and noise are **not** re-applied. Success is the Synthetic L0 compared against the reference ESA L0 `img` (10/20 m bands ≤ ~4 DN). It complements the ATBD (`docs/atbd/atbd.md`, the per-step physics) and the SDD (`docs/sdd/`, the software structure). DRD: ECSS-E-ST-40C Rev.1, tailored for EOPF processor. ## Processing chain The chain **inverts the operational L0→L1B radiometric chain** of the `msi-processor`, step by step, to reconstruct focal-plane counts from a S2 L1B. It is **radiometric-only** (input is already in per-detector sensor geometry). Because MTF-deconvolution is off, **no PSF re-blur and no noise are re-applied**; the reverse chain runs the ordered inversion steps that reconstruct **L1A → L0plus → Synthetic L0** (ATBD §5): ```mermaid flowchart TD IN["S2 L1B at-sensor radiance
(per-detector geometry)"] S1["S1 · radiance → equalized signal DN (X = A·G·L + D, A = cal_gain)"] S3["S3 · undo scene framing / round-clamp"] S4["S4 · invert radiometric offset (−100, GIPP R2PARA)"] S5["S5 · un-bin 60 m bands (B01/B09/B10)"] S7["S7 · invert relative response / PRNU (GIPP R2EQOG, cubic VNIR / bilinear SWIR)"] S8["S8 · SWIR re-arrangement, reverse (B10/B11/B12)"] S9["S9 · invert inter-band crosstalk (GIPP R2CRCO ≈ 0 for S2A)"] S10["S10 · re-insert blind/defective pixels (GIPP R2DEPI / BLINDP)"] S11["S11 · invert per-pixel dark signal (GIPP R2EQOG COEFF_D)"] S12["S12 · invert onboard equalization"] S14["S14 · quantize to 12-bit uint16 [0, 4095] → Synthetic L1A"] S15a["S15a · assemble L0plus · CCSDS-122 lossless compress (onboard step; ccsds122)"] S15b["S15b · L0 · ISP packetize (SEQ flags + CUC) + SAD telemetry + STAC/PSFD naming"] GD["L0plus codec round-trip:
decode(L0plus) == L1A bit-exact (read_l0_isp_dn)"] VAL["validate: Synthetic L0 vs reference ESA L0 `img`
(10/20 m bands ≤ ~4 DN)"] IN --> S1 --> S3 --> S4 --> S5 --> S7 --> S8 --> S9 --> S10 --> S11 --> S12 --> S14 --> S15a --> S15b S15a -.-> GD S15b -.-> VAL ``` **Realized execution order.** `reverse.reverse_mvp` runs `S1 → S7 → S11 → S12 → S14` — the DN-domain inversion steps that reconstruct the L1A focal-plane counts; **no PSF re-blur and no noise are re-applied** (MTF-deconvolution off). `reverse.reverse_full` additionally inserts S8 (SWIR re-arrangement, reverse) and S10 (defects) as genuine reverse chain steps. Verification is the **L0plus codec round-trip** — `decode(L0plus)` reproduces the Synthetic L1A **bit-exactly** (`read_l0_isp_dn`) — and the comparison of the reconstructed L0 against the reference ESA L0 `img` (10/20 m bands ≤ ~4 DN).