WASHINGTON ? The Global Positioning Satellite System, vital to commercial aviation and marine shipping, could be shut down during a national crisis to prevent terrorists from using it during an attack, the White House said.

A Bush Administration official who spoke to a small group of reporters on condition of anonymity, said any shutdown of the network inside the United States would come under only the most remarkable circumstances.

Bush also told the Defense Department to develop plans to disable, in certain areas, an enemy?s access to the U.S. navigational satellites and to similar systems like the Galileo GPS system under development by the European Union.

The military increasingly uses GPS technology to move troops across large areas and direct bombs and missiles. But the auto industry also uses GPS for onboard navigation systems. Both General Motors? Onstar and an after market system called Guidepoint, developed in Pontiac, use GPS.

?We?re glad the Bush Administration is paying so much attention to GPS technology,? said Guidepoint spokesman Brian Edwards. ?The President has pledged to make the GPS system better, more stable and that it will continue to be free. That?s going to be a benefit to businesses and consumers.

If there were a shutdown – and it sounds like that would only happen in the most extreme situation – we?re prepared to manage through it,? Edwards said. ?In fact, one of the things that makes Guidepoint different is that we have a 24/7/365 response center staffed with trained professionals who can help in an emergency. So even if there were a temporary shutdown of the GPS system by the government, Guidepoint would be in better position than most others in our industry to deliver needed services.?

The directives to the Defense Department and the Homeland Security Department were part of a space policy that Bush signed this month. It designates the GPS network as a critical infrastructure for the U.S. government. Part of the new policy is classified; other parts were disclosed Wednesday.

The U.S. network is comprised of more than two dozen satellites that act as beacons, sending location-specific radio signals that are recognized by devices popular with motorists, hikers, pilots and sailors.

Bush also said the government will make the network signals more resistant to deliberate or inadvertent jamming.