Understanding SAR
How the 'synthetic aperture' works
A radar's sharpness depends on its antenna size relative to wavelength, so at a 5 cm wavelength an orbiting radar would need an antenna kilometres long to resolve objects a few metres apart — impossible to launch. SAR sidesteps this limit. As the satellite flies, each target stays inside the radar beam for a second or two while the changing geometry imprints a shifting Doppler signature on its echo. By coherently combining the echoes recorded across that stretch of orbit, the processor 'focuses' them as though captured by one enormous antenna. Remarkably, the best along-track resolution works out at roughly half the physical antenna's length and is independent of altitude — the opposite of an optical camera's ground sample distance, which grows coarser the higher you fly.
Radar bands: X, C and L
The microwave band sets the balance between resolution and penetration. Shorter wavelengths (X-band) scatter off fine surface texture and give the crispest images; longer wavelengths (L-band) pass through vegetation and dry soil to sense the structure and moisture beneath. The imaging mode — not the band — mainly sets the swath width and revisit time.
| Band | Frequency | Wavelength | Strengths & example missions |
|---|---|---|---|
| X-band | 8–12 GHz | ~3 cm | Sharpest detail, sensitive to fine texture — Capella, ICEYE, TerraSAR-X |
| C-band | 4–8 GHz | ~5 cm | Balanced workhorse for open data — Sentinel-1, RADARSAT |
| L-band | 1–2 GHz | ~24 cm | Penetrates vegetation & soil, ideal for InSAR — ALOS-2, NISAR |
InSAR: measuring millimetre ground motion
Because SAR records the phase of each returning wave as well as its brightness, two images taken from almost the same viewpoint can be compared to millimetre precision — a technique called Interferometric SAR (InSAR). The phase difference between passes maps minute changes in distance to the ground, exposing subsidence beneath cities and aquifers, the swelling of volcanoes before an eruption, fault movement after earthquakes, and the slow creep of landslides, dams and glaciers. Stacking dozens of acquisitions over months (persistent-scatterer InSAR) pushes sensitivity to a few millimetres per year. The 2000 Shuttle Radar Topography Mission used single-pass interferometry to map the height of most of Earth's land in just eleven days.