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Radar remote sensing transmits microwave pulses (wavelengths 1 cm to 1 m) toward Earth's surface and measures the returned signal (backscatter). Synthetic Aperture Radar (SAR) uses the satellite's motion to synthesize a much larger antenna, achieving fine spatial resolution despite long wavelengths. Because the sensor provides its own illumination and microwaves penetrate clouds, rain, and smoke, SAR operates day and night in all weather. Backscatter intensity depends on surface roughness, moisture content, and dielectric properties. SAR also records signal phase, enabling interferometric SAR (InSAR) to measure surface deformation with millimeter precision.
From the passive-vs-active distinction, you understand that active sensors provide their own illumination. Radar remote sensing is the most important active technique, and Synthetic Aperture Radar was the breakthrough that made high-resolution imaging possible from space.
A real antenna's resolution is proportional to wavelength divided by antenna length. At microwave wavelengths, achieving 10-meter resolution from 700 km would require a kilometers-long antenna. SAR solves this by exploiting satellite motion: as it moves along orbit, it records echoes at many positions, then computationally combines them as if from a single enormous antenna. The synthetic aperture is the distance traveled during data collection.
Backscatter intensity depends on surface roughness (relative to wavelength), moisture content (water increases the dielectric constant), incidence angle, and polarization. SAR can transmit and receive in horizontal (H) and vertical (V) polarizations, producing combinations (HH, VV, HV, VH) that respond differently to surface structure. Fully polarimetric SAR enables decomposition into surface, volume, and double-bounce scattering.
Interferometric SAR exploits phase information. By comparing phase from two acquisitions, InSAR measures topography (generating DEMs) and surface deformation (detecting millimeter-scale ground movement). This capability is transformative for monitoring tectonic strain, volcanic inflation, glacial flow, and urban subsidence at continental scales.