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Washout at the GPS Shootout?

Washout at the GPS Shootout?

In the misty, early morning, a group in the park across fromSan Francisco's Civic Center huddled around two sets of strange-looking objects. The objects were radio wave antennas receiving satellite signals, and the people were scientists participating in the GPS Shootout: a competition among four companies developing receivers for the Global Positioning System (GPS). The GPS is the Department of Defense's multi-billion-dollar navigation system, which for a few million dollars more will enable civilian scientists to measure distances of several kilometers to within less than a centimeter.

Because it measures distances between two ground stationsby comparing the times it takes for radio signals to arrive from a satellite, the GPS is not limited, as are most other surveying and navigation methods, by the curvature of the earth. "GPS has turned the surveying world inside out," says John D. Bossler, director of the Center for Mapping at Ohio State University in Columbus. Scientists studying the movement of the plates and the deformation of the crust say GPS will be less expensive, faster and provide more information about height differences of ground stations than existing methods.

Still, there are problems to be worked out, such as gettingprecise locations for the satellite orbits and taking the distorting effects of atmospheric water vapor into account. At the GPS Shootout, one company's antenna was radiating at frequencies that interfered with the other systems, preventing one group from getting results. Of the remaining three, one system measured the 30.471-meter baseline across the park exactly, one was 1 millimeter off and the other missed the mark by 7 millimeters.

The researchers involved said they learned much from theexperiment, although some don't think the results are conclusive. Joked Lloyd Penland of Wild Heerbrugg Instruments, Inc., in Altanta, "We learned not to put the instruments so close together and that we should have GPS Shootouts at other times of the year [when the satellites can be contacted later in the day] so we don't have to get up so early."
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Copyright 1987, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:competition among companies developing receivers for Global Positioning System
Author:Weisburd, Stefi
Publication:Science News
Date:Jan 3, 1987
Words:337
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