A synthetic-aperture radar (SAR) builds up an image by illuminating the target area as the aircraft moves along a flight path. As the radar moves, a pulse is transmitted at each position and the return echoes pass through the receiver and are stored. The returned echoes are Doppler-shifted because the radar is moving relative to the ground. Comparing the Doppler-shifted frequencies (positive as the radar moves away from a target and negative as it moves towards the target) to a reference frequency allows returned signals to be focused on a single point, or target. This effectively increases the length of the antenna that is imaging that particular point. As the longer the antenna, the finer the resolution in azimuth (along track), this method therefore allows the SAR to generate high-resolution imagery. The radar wavelengths are also much longer than those in the visible or infrared spectrum, so SARs can therefore image through clouds, vegetation, sand and other forms of "clutter". NASA/JPL's AirSAR can operate in L-, C- and P-band frequencies, or when operating as an interferometer in C-band can simultaneously generate topographic height data.
Source: Flight International