Synthetic Aperture Radar (SAR) satellite imagery is a form of Earth observation data captured using active radar sensors mounted on satellites. SAR satellites send microwave radar signals toward the Earth's surface and capture the reflected echoes, in contrast to optical satellites that depend on sunlight. This makes it possible for SAR systems to picture the Earth through clouds, smoke, rain, and haze, day or night.

Working principle of SAR Imaging

A side-looking radar antenna on an SAR spacecraft sends short and coherent microwave pulses in the direction of the Earth's surface. SAR runs consistently day and night because it is an active system that is not dependent on sunlight. The radar pulses interact with surface elements like buildings and infrastructure, vegetation and topography, water bodies, and soil when they hit the ground. Roughness, shape, moisture content, and material characteristics all affect how each feature reflects (backscatters) the signal. After receiving the reflected radar echoes, the satellite logs information such as phase (the wave's relative time) and amplitude (the strength of the signal). For advanced SAR methods like deformation mapping, phase information is essential. 

The transmitted radar signal's bandwidth and pulse length define the range resolution. The time delay of the returning signal distinguishes whether the objects are closer or farther away from the satellite. As the satellite advances, the same ground target is illuminated multiple times from slightly different positions. SAR processing coherently combines these multiple echoes to form a synthetic antenna aperture much larger than the physical antenna. This process enhances azimuth (along-track) resolution, independent of satellite altitude.

Due to satellite motion, returned signals experience Doppler frequency shifts, such as targets ahead of the satellite having higher Doppler frequencies or targets behind having lower Doppler frequencies. By analyzing these Doppler variations, the SAR processor precisely locates targets along the flight path.

SAR imaging is used for monitoring forests, urban areas, agriculture, wetlands, and near-real-time flood extent mapping during storms and cyclones. They are used to identify fault slip, subsidence, and slope instability along with maritime security and illegal fishing detection. This imaging is used to monitor cities, mining zones, and groundwater extraction areas. It is also used for forest carbon stock estimation, tracking deforestation-linked emissions, and groundwater monitoring via surface deformation.

Click here to learn more about different techniques of SAR imaging.