DEFINITION OF SYNTHETIC APERTURE RADAR

Space-borne remotely sensed

imagery has been routinely acquired and used as a reconnaissance

tool by geologists since the initial launch of the Landsat

series of satellites in 1972. More recently, space-borne sensors

such as Thematic Mapper (TM), Seasat Synthetic Aperture

Radar (SAR), Shuttle Imaging Radar (SIR-A and SIR-B),

and SPOT (Système pour l’Observation de la Terre) 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 proved 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. Synthetic aperture

radar is very complicated, but it 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. It is useful in military, science, and mapping

as it provides detailed resolutions of a particular area. Synthetic

aperture radar is widely used by governments and militaries

but is expensive for 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 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. The strength of

the radar signal may also be attenuated by atmospheric or

soil moisture and is affected to some degree by the types of

atomic bonds in the minerals present in surface materials.

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, the precise backscatter

dependence being controlled by surface roughness. 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.

See also REMOTE SENSING.

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