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.