synthetic aperture radar

An airborne radar that produces an image of the scene that is similar to, but not identical with, an optical photograph. A cross-range resolution obtained from a Doppler frequency, along with range resolution, forms the basis of a synthetic aperture radar. The image seen by radar is not the same as observed by optical devices. Radar and optical images differ because of a large difference between the frequencies involved; optical frequencies are approximately 100,000 times higher than radar frequencies. The SAR can operate from long range and through clouds or other atmospheric effects that limit optical and infrared imaging sensors. The resolution of a SAR can be made independent of range, an advantage over passive optical imaging, where the resolution worsens with increasing range. Range resolution is a function of antenna-beam width, whereas azimuth resolution is a function of pulse width. Synthetic aperture radars that map areas of the earth’s surface with resolutions of a few feet or meters can provide information about the nature of the terrain and what is on the surface. If the radar is stationary and the target is moving, then such a radar is called an inverse synthetic aperture radar (ISAR).

Recording of SAR signal on photographic film with the aid of a CRT (cathode ray tube).