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**Active vs. passive remote sensing** (the slide's diagram didn't survive PDF conversion — here's the concept in full). - **Passive** — the sensor only *receives* natural energy. The source is the Sun (reflected solar) or the target's own thermal emission. Most optical/thermal imagers are passive. Examples: Landsat OLI/TIRS, MODIS, SPOT, IKONOS. - **Active** — the sensor emits its own energy pulse and measures what bounces back. Not Sun-dependent, works day or night, and usually penetrates clouds. Examples: **RADAR**, **LiDAR**, scatterometers, SRTM. - **Short-answer mnemonic:** *Passive listens; active shouts and listens.*

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