Landsat is the longest continuous Earth-observation record (1972–present, USGS/NASA).

• Orbit: 705 km, sun-synchronous, 16-day repeat.
• Current sensors (Landsat 8 & 9): OLI-2 (9 bands, incl. coastal/aerosol, blue, green, red, NIR, two SWIR, pan at 15 m, cirrus) + TIRS-2 (2 thermal bands at 100 m). VSWIR at 30 m, pan 15 m.
• Heritage bands (TM/ETM+): B3 = Red, B4 = NIR → NDVI = (B4 − B3)/(B4 + B3). On OLI: B4 = Red, B5 = NIR.