Synthetic Aperture Radar, an active microwave imaging technology that can observe the Earth's surface through clouds and at night. SAR is critical for flood mapping, soil moisture retrieval, and ground deformation monitoring in water resource applications.
Synthetic Aperture Radar (SAR) is an active remote sensing technology that transmits microwave pulses (typically in C-band, L-band, or X-band) and records the backscattered signal to create high-resolution images of the Earth's surface. Unlike optical sensors, SAR operates independently of solar illumination and cloud cover, making it invaluable for monitoring hydrological events that often occur during cloudy or nighttime conditions. In flood mapping, SAR exploits the specular reflection of smooth water surfaces, which appear dark in SAR imagery, enabling rapid delineation of flood extent during disaster response. Interferometric SAR (InSAR) measures ground surface deformation with millimeter precision, used to detect land subsidence caused by groundwater overdraft in areas like California's Central Valley and Mexico City. SAR backscatter is sensitive to soil moisture content, and algorithms such as those used in the Sentinel-1-based soil moisture products retrieve near-surface moisture at field scales. Polarimetric SAR data provide additional information about surface roughness and vegetation structure relevant to wetland mapping and crop water stress assessment. The Copernicus Sentinel-1 constellation provides free, systematic SAR coverage globally with a 6-12 day revisit cycle.