synthetic aperture radar
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Synthetic aperture radar (SAR)
Radar, airborne or satellite-borne, that uses special signal processing to produce high-resolution images of the surface of the Earth (or another object) while traversing a considerable flight path. The technique is somewhat like using an antenna as wide as the flight path traversed, that being the large “synthetic aperture,” which would form a very narrow beam. Synthetic aperture radar is extremely valuable in both military and civil remote-sensing applications, providing surface mapping regardless of darkness or weather conditions that hamper other methods.
Resolution is the quality of separating multiple objects clearly. In radar imaging, fine resolution is desired in both the down-range and cross-range dimensions. In radar using pulses, down-range resolution is achieved by using broad-bandwidth pulses, the equivalent of very narrow pulses, allowing the radar to sense separate echoes from objects very closely spaced in range. This technique is called pulse compression; resolution of a few nanoseconds (for example, 5 ns = 5 × 10-9 s gives about 0.75 m or 2.5 ft resolution) is readily achieved in modern radar.
Cross-range resolution is much more difficult to achieve. Generally, the width of the radar's main beam determines the cross-range, or lateral, resolution. For example, a 3° beam width resolves targets at a range of 185 km (100 nautical miles) only if they are separated laterally by more than 100 m (330 ft), not nearly enough resolution for quality imaging.
However, surface objects produce changing Doppler shifts as an airborne radar flies by. In side-looking radar (see illustration), even distant objects actually go from decreasing in range very slightly to increasing in range, producing a Doppler-time function. If the radar can sustain high-quality Doppler processing for as long as the “footprint” of the beam illuminates the scene, these Doppler histories will reveal the lateral placement of objects. In fact, if such processing can be so sustained, the cross-range resolution possible is one-half the physical width of the actual antenna being used, a few feet perhaps. Furthermore, this resolution is independent of range, quite unlike angle-based lateral resolution in conventional radar.
Many synthetic aperture radars use other than just a fixed side-looking beam. Spotlighting involves steering the beam to sustain illumination for a longer time or to illuminate a designated scene at some other angle. The principles remain unchanged: fine resolution in both down-range and cross-range dimensions (achieved by pulse compression and Doppler processing, respectively) permits imaging with picture cells (pixels) of remarkably fine resolution. Many synthetic aperture radars today achieve pixels of less than 1 m (3 ft) square.