Radar

Radar is an active microwave sensing technique that transmits coherent pulses toward Earth's surface and records the backscattered signal to produce amplitude and phase imagery. Because radar generates its own illumination at centimetre wavelengths, it operates in all weather conditions, day and night, independent of solar geometry.

The Synthetic Aperture Radar (SAR) principle exploits platform motion to synthesise a large virtual antenna aperture: a system at satellite altitude would require approximately 4,250 m of physical antenna to achieve 10 m resolution at X-band (around 3 cm wavelength), but SAR achieves comparable resolution with a roughly 10 m antenna by coherently processing returns across the flight path. Backscatter intensity encodes surface roughness and dielectric constant (a proxy for soil moisture), while phase-difference between repeat passes (InSAR) encodes line-of-sight surface displacement with centimetre-level precision. Polarimetric SAR decomposition separates surface, volume, and double-bounce scattering to characterise vegetation structure and urban fabric.

Operational frequency bands span X-band (8-12 GHz, high-resolution urban and infrastructure mapping), C-band (4-8 GHz, global operational mapping and agriculture), L-band (1-2 GHz, deep vegetation penetration and InSAR coherence), and P-band (0.3-1 GHz, experimental biomass estimation). Representative missions include Sentinel-1 A/B (ESA, C-band, 6-day revisit), ALOS-2 PALSAR-2 (JAXA, L-band), TerraSAR-X and TanDEM-X (DLR, X-band), and NISAR (NASA/ISRO, L+S dual-band, open data). Commercial X-band constellations including Capella, ICEYE, and Umbra provide sub-day revisit at high resolution.

Headline applications include surface deformation mapping for earthquakes, volcanic inflation, subsidence, and landslides via InSAR; ice sheet velocity and calving front dynamics; flood extent and surface water mapping; forest biomass and canopy structure estimation at L- and P-band; maritime vessel detection and ocean wind field retrieval; agricultural crop monitoring and soil moisture; and digital elevation model generation (TanDEM-X global 12 m DEM).

Key limitations are geometric distortions including foreshortening, layover, and shadow in steep terrain, InSAR temporal decorrelation over vegetated or rapidly changing surfaces, and the additional processing complexity relative to optical imagery.