Synthetic aperture radar (SAR)

Synthetic aperture radar creates a large effective antenna by combining returns collected as the satellite moves along its track, which is what lets a modest physical antenna produce fine spatial detail. Because it supplies its own illumination at microwave wavelengths, it images through cloud, smoke and darkness - conditions under which optical sensing produces nothing at all.

For monitoring physical assets this changes what can be relied on rather than merely adding a source. A construction site in a persistently cloudy region, a port during a storm, or a facility at night are all observable on a predictable schedule instead of whenever the sky happens to clear. Interferometric use of repeated passes also measures ground and structural displacement at centimetre scale, which reads subsidence and settlement directly.

The cost is interpretability. A radar image is a measurement of surface roughness, geometry and moisture rather than a photograph, so metal structures dominate, geometric distortion in terrain is inherent, and reading one requires care that an optical image does not. It is therefore used alongside orbital observation in the optical bands rather than instead of it.