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The 'eye' that sees through cloud

The kind of radar that enabled scientists to "see" the surface of cloudshrouded Venus has also helped archaeologists "peer" into the cloud-covered Guatemalan jungle to discover how the ancient Maya managed to feed to 2 to 3 milion people.

This had been a major archaeological puzzle until mapping by airborne radar revealed the grid pattern of a vst agricultural canal system. Follow-up studies on the ground have confirmed that the Maya had a sophisticated system of intensive farming covering many thousands of square kilometers. Archaeologist Richard E. W. Adams of the University of Cambridge, England, estimates there may be as much as 14,000 square km of canal systems under Guatemala's rain forests alone.

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Together with the mapping of 93 percent of the surface of Venus, this discovery shows the power of a type of radar that has an enhanced abiliity to resolve detail. Engineers called it synthetic apeture radar (SAR). It employs a technique that radio astronomers also have used extensively to sharpen the resolution of some radiotelescopes.

This technique enables an antenna of a given size to simulate the resolving power of a much larger dish. The radio or radar system "remembers" what it has scanned as it moves in relation to the target. It can then integrate the data coherently to yield the kind of "picture" that a much larger antenna would provide. In effect, the small antanna moved so as to fill in the surface of a larger antenna -- thus it "sympathesizes" the larger antenna. The Pioneer Venus- mapping radar has the resolution of a system with an antenna some three times the szie of its own dish, which is about 30 centimeters in diameter.

An early version of an SAR radar developed at the Jet Propulsion Laboratory of the US National Aeronautics and Space Administration (NASA) was tested from aircraft over Guatemala and Belize in 1977-78. STudy of some of the images taken then has subsequently revealed the canal patters, according to a recent NASA annoucement. This torpical-forest area is so frequently veiled by cloud that ordinary aerial photography has not been too helpful to archaeologists. But the radar, operating at frequencies taht penetrate the clouds, also had enough resolving power to allow scientists to produce imgaes of varying layers of foliage. Then, from the variations of height of these layers, they could estimate the underlying land forms.

This sharp-eyed kind of radar, which has long been used for military purposes , is beginning to find a number of civil uses. Among other things, it could help environmental satellites monitor ocean wave patterns and the winds that cause them and to trace ocean currents, as demonstrated on recent years by teh SEASAT experimental satellite.

Thus SAR promises to become a widely used "eye" in the sky" for which cloud cover is not obstacle. It is an example of the powerful remote-sensing techniques that have opened all parts of our planet to study and surveillance, as well as giving detailed surveys of planets where not human has yet trod.

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