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# Software reuse file

ECSS-E-ST-40C Rev.1 / ECSS-Q-ST-80C. This SRF records reused software and reused data, and states the
originality of the reverse-chain Synthetic L0 reconstruction tool: it runs a Sentinel-2B **L1B** backwards
through the exact inverse of the operational Synthetic L0->L1B radiometric chain to reconstruct **L1A -> L0plus -> Synthetic L0**,
validated against the reference ESA L0 `img`.
<!-- NAMING CASCADE: repo s2-msi-raw-generator / package s2_msi_raw_generator is a project-wide rename
     across all ECSS docs; not applied unilaterally here. -->

## Introduction

The deliverable reuses only general-purpose, permissively-licensed libraries and **public** ESA/Copernicus
calibration **data**. No mission-processor source code is reused or vendored. All algorithms are original
implementations from public specifications.

## Reused software

| Component | Role | License |
|-----------|------|---------|
| `numpy` | numerical arrays, the entire processing core | BSD-3-Clause |
| `zarr` | EOPF L1A/L1B read + Synthetic L0 RAW write (optional `read` extra) | MIT |
| Python stdlib `json` + `xml.etree.ElementTree` | GIPP JSON parser (+ XML fixtures for tests) | PSF |
| `pytest` | test runner (dev only) | MIT |


No EOPF CPM, no geometry library, and no credentialed package is required for the referenceized path.

## Reused data

All reused data is **public** ESA/Copernicus reference data; only the data values are used (no code):

| Data | Use | Source / provenance |
|------|-----|---------------------|
| Spectral Response Functions | per-unit centre/bandwidth/equivalent wavelength | SRF doc COPE-GSEG-EOPG-TN-15-0007 v4.0 |
| Operational GIPP | per-pixel dark + relative response, defects, offsets | R2EQOG / R2DEPI / BLINDP / R2PARA / R2CRCO (read as data) |
| Sentinel-2 L1 ATBD | forward radiometric model $X = A\cdot G\cdot L + D$ (the algorithm we invert) | S2-PDGS-MPC-ATBD-L1 §4.1.1 (public specification) |

## Originality statement

The reverse-chain inverse radiometric chain (invert offset, relative-response/PRNU, dark, un-bin, SWIR
re-stage, defective, crosstalk, on-board-eq; MTF-deconvolution OFF), the GIPP reader, the calibration
sub-set and the L1A/L0plus/Synthetic L0 product assembly are **original implementations** written from the public
Sentinel-2 L1 ATBD and the GIPP data layout. **No external mission-processor source code is copied,
vendored, imported or referenced by name** in this deliverable. This satisfies REQ-QUAL-003 (originality),
verified by inspection (source review + name grep) in the V&V report. The reverse chain's own V&V uses only this
project's code: the L0plus codec round-trip (decode(L0plus)==L1A, bit-exact) and validation of the
Synthetic L0 against the reference ESA L0 `img` (10/20 m bands <=~4 DN).