WASHINGTON, Sept. 10 � The Global Positioning System � which airlines plan to use to land flights in zero visibility, railroads want for avoiding train collisions and ships use to navigate shoals � is vulnerable to interference and even "spoofing" by enemies, the Transportation Department said today.

A report ordered by President Bill Clinton two years ago suggested that older technologies, which are more expensive and less precise but also more robust, needed to be maintained as a backups.

"There is a growing awareness within the transportation community that the safety and economic risks associated with loss or degradation of the G.P.S. signal have been underestimated," the report said.

The receivers are also used by motorists, hikers and even surveyors, although for them failures would presumably not be potentially catastrophic. Banks and electric utilities also use the signals for precise time measurements.

The report did not rank the risks or say exactly what the government should do; instead the transportation secretary gave the Federal Aviation Administration, the Federal Railroad Administration and other agencies 60 days to report on how they would maintain safety.

The G.P.S. system has tremendous potential for increasing operating efficiencies in aviation, helping planes fly more direct routes and land in low visibility in places that lack conventional instrument landing systems. By letting vehicles determine their positions and radio that information back to a central control point, it can help controllers get more use out resources like railroad tracks and runways.

But the radio signals, which come from satellites, have a strength of less than one quadrillionth of a watt when they reach the receiver; a transmitter of only several watts can blank out large areas. Today's report said an adversary could broadcast a counterfeit signal that would confuse the receivers.

And in case of war, the satellites themselves might become targets, the report said.

The aviation agency's existing system costs about $100 million a year to maintain. That includes about 1,000 radio beacons, which pilots use to triangulate their positions; 1,200 instrument landing systems that guide planes to runways; and scores of radar stations, which controllers use to determine the position of planes. If the planes themselves knew with precision where they were, they could radio that information to the ground and make the radar obsolete.

At the agency, Steven Zaidman, the associate administrator for research and acquisitions, said that he had not yet read the report but that "it's clear that we're not going to get as much savings" as initially hoped.

The aviation administration will not face the decision about what to retire until 2010, Mr. Zaidman said, to give commercial and private planes time to equip themselves with G.P.S. receivers and to let the agency install equipment near airports that would augment the satellite signals, making the signal accurate enough for low-visibility landings.