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West Europe, US propose rival satellite navigation systems

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Western Europe is proposing an all-civilian system of satellite navigation that would rival United States plans for a similar network under the auspices of the Department of Defense.

According to the 11-nation European Space Agency, which is based in Paris, the civilian system would better suit the needs of users such as ships and aircraft. This is because the satellites would be less complex and thus less expensive than those in the military network.

And the European spacecraft would be run by a civilian agency that is committed to providing a service. Under current plans for the US satellites, they would be switched off or their frequencies altered in times of international emergency or war.

The US plans are much further ahead than those of the Europeans. But there is still a lot of controversy over exactly how the American system, called Navstar, will be implemented.

In both systems, a set of satellites encircles the world at an altitude of some 20,000 kilometers (12,500 miles). The West Europeans want 24 space vehicles; the US is counting on 18.

The European system is estimated to cost about $2.5 billion; the US network, more than twice that much.

The vehicles relay signals from earth stations to small computerized terminals on ships, aircraft, or land vehicles. The terminals receive ''fixes'' from several spacecraft simultaneously and can work out exactly where they are.

The information would pinpoint position to an accuracy of 100 meters in three dimensions. A satellite system would represent an advance on current methods of navigation, which are inaccurate or do not give continuous coverage.

The Department of Defense already has six experimental Navstar vehicles in orbit. A prototype satellite navigation system called Transit provides an intermittent service.

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