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Defense Radars Try Environmental Monitoring
| Article
# : |
10456 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1993 |
2,670 Words |
| Author
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T.M. Georges
T.M. Georges is a physicist with the National Oceanic and
Atmospheric Administration (NOAA) Environmental Research
Laboratories in Boulder, Colorado. He participated in the
Heard Island Experiment at the Ascension Island receiving
station. |
During the last quarter century of the Cold War, the U.S. Air Force spent billions of dollars developing the "ultimate radar"--one that would warn of Soviet bomber attacks while the planes were still thousands of miles from U.S. airspace. Unlike conventional microwave radars, whose coverage is limited to a line of sight, the new radars would look "over the horizon" by bouncing their lower-frequency radio beams off the ionosphere, an electrically charged layer in the upper atmosphere. The investment finally paid off in 1991, with the completion of six huge radar systems known as OTH-B (over the horizon backscatter) to defend the approaches to the U.S. East and West coasts.
The most conspicuous feature of the radars is their enormous transmitting and receiving antenna arrays. At the low frequencies that reflect from the ionosphere (5-28 MHz), the electronically steerable antennas must be roughly a mile long to form the narrow beams, a fraction of a degree wide, needed to perform their tracking tasks. Each transmitting array is 3,630 feet long and covers 400 acres of land, and each receiving array is 4,980 feet long. The transmitting and receiving arrays are about 100 miles apart. Three radars in Maine scan a 180-degree sector of the North Atlantic Ocean, and three more located on the California Oregon border scan the northeastern Pacific Ocean.
The North American Air Defense Command (NORAD), the final user of the new systems, says it is pleased with their performance. During its testing and acceptance phase, the OTH-B radar in Maine routinely "intercepted" Soviet long-range Bear reconnaissance aircraft. In other tests, the radar showed that it could detect and track much smaller targets at great distance, but its exact range and sensitive are classified. There have also been rumors that the radar easily detected U.S. Stealth aircraft returning from the Gulf War. (Stealth aircraft are designed to be invisible to microwave radars, but not to the longer OTH wavelengths. Which are about as long as typical aircraft.) For practice, the OTH-B radars track commercial air traffic, occasionally notifying NORAD of aircraft that stray from their flight plans at all. It is particularly impressive to sit at a radar screen and watch the commercial air traffic in the North Atlantic corridor to Europe--a feat no other single radar can perform.
Not to be outdone by the Air Force, the U.S. Navy developed a parallel OTH radar program of its own--but with a twist. Instead of fixed radars, the Navy designed a less expensive and less powerful but somewhat more flexible radar that could be transported to support regional conflicts like the Gulf
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