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Satellite secrecy doesn't sink scientists.

Satellite secrecy doesn't sink scientists

The U.S. Department of Defense last March began "degrading" information coming down from its Global Positioning System (GPS), a network of precise navigational satellites. The long-expected change -- which involves adding bogus data to prevent exploitation by unauthorized users such as enemy forces -- worried many scientists who rely on the satellite signals for hints of impending earthquakes and volcanic eruptions, amont the other types of information.

Now, after months of working with the degraded data, scientists say they can largely sidestep the problem, although some worry about Defense Department plans for tinkering further with GPS signals.

At this week's meeting of the American Geophysical Union in San Francisco, geoscientists debated the research ramifications of the recent change. Thomas P. Yunck of NASA's Jet Propulsion Laboratory in Pasadena, Calif., maintains that "the effect on precise scientific users ranges from zero to a minor inconvenience." But other researchers contend that GPS signals have become more difficult to interpret.

The U.S. military created the GPS network in the late 1970s to provide instantaneous and accurate navigational information for its troops, ships and missiles. Scientists soon realized they could use the signals to gauge distances on Earth with extraordinarily high precision. Because the GPS can measure the distance between two points 1,000 kilometers apart with an accuracy of better than 1 centimeter, it has enabled geoscientists to monitor subtle warpings of the crust along the San Andreas fault. The military satellite have also provided navigational guidance for commercial ships, and in the future could allow aviation engineers to outfit commercial planes with automated landing systems.

After several brief experimental periods, the Defense Department activated its data-degrading system, called selective availability (SA), on a full-time basis in March. SA inserts false information about the orbits and the precise timing systems used by each GPS satellite. Ironically, officials had to turn SA off after the Iraqi invasion of Kuwait in August because U.S. troops in Saudi Arabia do not carry receivers capable of correcting the altered signals.

Unlike navigators, who use the GPS's to fix their location, geophysicists circumvent many of the problems associated with SA because they instead measure distances between two points, each with its own receiver. But some geophysicists say they have had difficulty using combinations of old-style and new receivers, which do not detect the GPS signals at exactly the same time. Kurt L. Feigl of the Massachusetts Institute of Technology reports that SA reduced measurement precision by 10 to 50 percent in experiments using a combination of newer and older receivers.

Even with synchronized receivers, some investigators have found they must spend two to three times as long analyzing the satellite information in order to weed out the bogus data. "It's made life more difficult for us," says Yehuda bock of the Scripps Institution of Oceanography in La Jolla, Calif.

Yunck, on the other hand, asserts that "with the techniques we use [at the Jet Propulsion Laboratory], it doesn't take any longer to edit the data [with SA on]."

Many geophysicists voice concern over the Defense Department's plan to degrade other parts of the GPS signal through a process known as anti-spoofing. Air Force Lt. Col. Jules G. McNeff told scientists at this week's meeting that the Pentagon will activate anti-spoofing by 1993. Yunck acknowledges that the program will make it more difficult to process GPS data, but he contends that researchers with sophisticated receivers and editing programs can nonetheless maintain their current precision levels.
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Title Annotation:U.S. Defense Department and the Global Positioning System satellites
Author:Monastersky, Richard
Publication:Science News
Date:Dec 8, 1990
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