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# Notations and conventions

## Block diagram symbols

| Symbol | Meaning |
|---|---|
| **S#** | A reverse-chain processing step (S1–S15), one per ATBD §5 block. |
| rounded box | A processing step (a function in `s2_msi_raw_generator.reverse` / `forward_radiometric_atbd`). |
| cylinder | A data store — input product, ADF, or output product. |
| solid arrow | Data flow between steps (a 2-D `(lines, detector_columns)` array). |
| $X$ | raw focal-plane digital number (L0 domain). |
| $L$ | at-sensor band radiance (L1B input), $\mathrm{W \cdot m^{-2} \cdot sr^{-1} \cdot \mu m^{-1}}$. |
| $A$ | per-band absolute-calibration gain (`Band.cal_gain`). |
| $G$ | per-pixel relative response (PRNU), GIPP R2EQOG. |
| $D$ | per-pixel dark signal, GIPP R2EQOG `COEFF_D`. |
| $\alpha, \beta$ | per-band noise-model coefficients, $\sigma = \sqrt{\alpha^2 + \beta \cdot \mathrm{DN}}$. |

**Units & types.** Radiance in $\mathrm{W \cdot m^{-2} \cdot sr^{-1} \cdot \mu m^{-1}}$; DN dimensionless `uint16` in $[0, 4095]$; wavelengths in
nm; line period in ms. Arrays are `(along-track lines, across-track detector columns)`. Detector index
`01`–`12`; bands `B01…B12, B8A` (no panchromatic). Per-unit data keyed `S2A/S2B/S2C`.

**Determinism.** All stochastic steps (noise, synthetic ADF/calibration) use a seeded
`numpy.random.Generator`; a fixed seed yields a reproducible product (REQ-QUAL-004).