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# Introduction

## Purpose
This Software Design Document (SDD) describes the architecture and detailed design of the **Sentinel-2 MSI Synthetic Raw Data Generator** (`s2_msi_raw_generator`). It is the design counterpart of the Software Requirements Specification
(`docs/srs.md`) and the Algorithm Theoretical Basis Document (`docs/atbd/atbd.md`), authored per
**ECSS-E-ST-40C Rev.1** (SDD DRD), tailored for a single Configuration Software Component (CSC).

## Objective
Define how the software **inverts** the operational Sentinel-2 L0→L1B radiometric correction chain,
running a Sentinel-2B **L1B** product backwards through the exact inverse of each operational step
— undoing offset, relative-response/PRNU, dark, un-binning, SWIR re-staging, defective-pixel, crosstalk
and on-board equalisation — to reconstruct **L1A → L0plus → Synthetic L0**. MTF-deconvolution is OFF, so PSF and
noise are **not** re-applied. Success is validated against the ESA **L0 'img'** (Synthetic L0
DN agrees within ≤ ~4 DN on the 10/20 m bands), so that the requirements in the SRS are met and
verifiable. The design also supports the in-flight **calibration sub-set** (inverse-crime cure). The
inversion stages correspond to the reverse chain steps of ATBD §5 (exact numbering per the ATBD §5 stage
listing).

## Scope
Covers the static architecture (the `s2_msi_raw_generator` Python package and its modules), the dynamic data flow of
the reverse chain, the key design decisions, and the internal interfaces between components. Geometry
inversion (L1C entry / de-orthorectification) is **out of scope** — cancelled per Issue #17, because
L1A/L1B are already in per-detector sensor geometry. External-processor integration is out of scope by
design (the deliverable is an original, self-contained implementation from public references).

## Content
- **Overview** (`overview.md`) — static architecture (component view, data flow, dependency graph),
  dynamic architecture, behaviour, interface context, and resource budget.
- **Design** (`design.md`) — general design, overall architecture and key design decisions, per-module
  detailed design, and internal interface design.
- **Traceability** (`traceability.md`) — the requirements-to-design-to-code-to-test matrix.