Synthetic Aperture Radar (SAR) refers to a technique for producing fine resolution images from an intrinsically resolution-limited radar system. The wavelengths, λ, that are used for radar remote sensing of the earth’s surface are typically in the range of a few to tens of centimeters. At these wavelengths, the energy radiated from a radar antenna of dimension D disperses quickly at a rate that is equivalent to the beam width λ/D of the antenna. For a typical spaceborne SAR configuration with wavelengths of ~10 cm and an antenna of 10 m size, this beam width is 1/100 radians, or about 0.6 degrees. If we are in space observing the Earth 1000 km below, the beam size on the ground is then 1000 λ/D = 10 km. This intrinsic resolution of the radar system is insufficient for many applications and practical solutions for improving the resolution needed to be found.
Learn more: http://nisar.jpl.nasa.gov/technology/sar/