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.

synthetic aperture radar (SAR)

synthetic aperture radar (SAR)click for a larger image
Recording of SAR signal on photographic film with the aid of a CRT (cathode ray tube).
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).
References in periodicals archive ?
How does synthetic aperture radar achieve its impressive resolution?
Imaging with Synthetic Aperture Radar, CRC Press, ISBN 978-0-8493-8239-0 (CRC Press), USA
A second upgrade underway is the synthetic aperture radar (SAR) upgrade.
This book provides a full representation of Inverse Synthetic Aperture Radar (ISAR) imagery, which is a popular and important radar signal processing tool.
Another satellite under construction these days is the unique TECSAR, which employs a Synthetic Aperture Radar payload, designed to provide images during day, night and under all weather conditions.
of Electronic Science and Technology of China, Chengdu) describes multi-antenna synthetic aperture radar (SAR) in microwave remote sensing applications such as high-resolution imaging, wide-swath remote sensing, and ground moving target indication.
Less than a quarter of a century after first entering service on Nasa's Seasat spacecraft, synthetic aperture radar (sar) technology has made the transition from being a highly specialised radar mode to one which is now widely available on the radars of many modern fighter aircraft.
Zhengkang Shen of IGCEA and Peking University's Department of Geophysics, and collaborators acquired two kinds of satellite radar data: Advanced Synthetic Aperture Radar (ASAR) data in C-band from ESA's Envisat satellite and Phased Array type L-band Synthetic Aperture Radar (PALSAR) data from Japan's ALOS satellite.
Inverse synthetic aperture radar imagery and a vessel's range signature are then used against a stored library of data for classification purposes, with the latter creating a ship contour on the radar screen, "and not only a blip".
When fully operational, TECSAR will provide military users with a significant improvement in advanced imaging capabilities using synthetic aperture radar (SAR) technology.
The second edition adds sections on airborne lidar for land surveys, airborne gamma ray spectroscopy, and interferometric synthetic aperture radar.
The modifications were developed beginning in 2001 when the Global Hawk program, working with the Australian government, developed maritime modes for the Global Hawk's synthetic aperture radar.

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