The synthetic-aperture trick
A satellite carrying a 10 m antenna at 7 km/s pulses radar at the ground. Over a few seconds it traverses a 10 km path. By coherently combining all the returns along that path, the data is processed as if it came from a single 10 km antenna — giving metre-scale resolution from orbit.
All weather, all night
SAR is radar — clouds, rain, smoke, polar night, none of it matters. The European Sentinel-1 satellites image the entire planet every 6 days at 5 m resolution. Glaciologists use it to measure ice flow centimetre by centimetre. Defence analysts use it to count tanks through cloud cover.
Interferometry (InSAR)
Take two SAR images of the same place weeks apart and compare phase. Differences as small as a few millimetres show up — perfect for measuring earthquake displacement, subsidence over abandoned mines, or a volcano slowly inflating before an eruption.
The new commercial wave
ICEYE, Capella Space and Umbra are launching small SAR satellites at high cadence. A site you care about can now be re-imaged in hours, not weeks. The technology that started with a 1978 NASA Seasat is becoming a commodity.