Multispectral scanning geometries — the two big families.

• Across-track (“whiskbroom”) — discrete detectors + a scanning mirror sweep perpendicular to flight. Examples: Landsat MSS, TM, ETM+.
• Along-track (“pushbroom”) — a linear array of detectors builds the image as the platform moves forward. Examples: SPOT HRV/HRVIR, IRS LISS, IKONOS, QuickBird, Landsat 8/9 OLI.

Sensors span roughly 0.4–14 µm (visible through thermal-IR).