Synthetic Aperture Radar

Spaceborne remotely sensed imagery has been routinely used as a reconnaissance tool by geologists since the initial launch of the Landsat series of satellites in 1972. More recently, spaceborne sensors such as Thematic Mapper (TM), Seasat Synthetic Aperture Radar (SAR), Shuttle Imaging Radar (SIR-A and SIR-B), and Système pour l'Observation de la Terre (SPOT) have scanned the Earth's surface with other portions of the electromagnetic spectrum in order to sense different features, particularly surface roughness and relief, and to improve spatial resolution. While TM and SPOT images have proven spectacularly effective at differentiating between various rock types, synthetic aperture radar (SAR) is particularly useful at delineating topographically expressed structures. Spaceborne SAR systems also play a major role in exploration of other bodies in the solar system.

Synthetic aperture radar (SAR) is an active sensor where energy is sent from a satellite (or airplane) to the surface at specific intervals in the ultrahigh frequency range of radar. The radar band refers to the specific wavelength sent by the source, and may typically include X-band (4 cm), K-band (2 cm), P-band (1 meter), L-band (23.5 cm), C-band, or others. SAR allows the user to acquire detailed images at any time of day or night and also in inclement weather. A complicated system, SAR basically works by first obtaining a two-dimensional image and then fine tuning that image with computers and sensors to create a decisively more accurate image. SAR provides detailed resolutions of a particular area for governments, military applications, and scientists, but is expensive to others who may wish to use it. The advancement of technology, however, is making it possible and economical in other applications.

The effectiveness of orbital SAR for geologic, particularly structural, studies depends primarily on three factors: (1) roughness contrasts; (2) local incidence angle variations (i.e., topography); and (3) look azimuth relative to topographic trends. Atmospheric or soil moisture can attenuate the strength of the radar signal, as can the types of atomic bonds in the minerals present in surface materials to some degree. Bodies of water are generally smoother than land, and appear as dark, radar smooth terrain. Structure is delineated on land by variations in local incidence angle, with surface roughness controlling the pre cise backscatter dependence. Different SAR satellites have different radar incidence or look angles, and some, such as RADARSAT, are adjustable and specifiable by the user. The 20° look angle chosen for Seasat was intended to maximize the definition of sea conditions, but had the incidental benefit of producing stronger sensitivity to terrain than would larger angles. Look azimuth has been shown to be an extremely important factor for low relief terrain of uniform roughness, with topography within about 20° of the normal to look azimuth being strongly highlighted.

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