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**The four elements in every passive optical remote-sensing chain** — memorize the letters. - **A** — the **energy source / illumination** (usually the Sun for optical; the sensor itself for active systems like radar). - **B** — the **Earth's atmosphere** (traversed twice for reflected solar: down then back up). - **C** — the **Earth's surface** (the target — reflects/absorbs/transmits). - **D** — the **sensor** (records the radiance that reaches it).

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