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).