Synthetic Aperture Radar Images

A radar system illuminates an area with microwaves, and records the strength and travel-time of the returned signals. This allows the distance (or range) of the reflecting objects to be determined. As in an optical instrument, the resolution of such a system is affected by the size of the aperture: a larger aperture gives a finer resolution. However, scaling up to radar wavelengths has problems. For example, to achieve an (azimuth) resolution of $100$m with a $5$cm radar from a range of $800$km requires an aperture some $400$m long.

If the radar is attached to a moving platform, either a satellite or an aircraft, then it is possible to combine reflected signals from along the flight path to synthesise a very long antenna. The aperture, or area used to receive signals, is created artificially during the signal processing. This is a Synthetic Aperture Radar ( SAR).