By Bradley Graham
Washington Post Staff Writer
Saturday, March 13, 1999; Page A12

In a case of clashing military and civilian interests, the U.S. Coast Guard has told the Pentagon not to expect approval of any more training exercises in U.S. waters that involve the muddling of satellite navigational signals.

Coast Guard authorities say they are worried that the military's periodic jamming of the Global Positioning System (GPS) could interfere with the Coast Guard's requirement to provide reliable navigational assistance to civilian ships that have come to rely solely on GPS.

The U.S. military also depends heavily on GPS for navigation and weapons guidance. The military-run system, introduced in the early 1990s, provides precise longitudinal and latitudinal coordinates to anything on land, at sea or in the air that has a GPS receiver.

To protect against possible enemy sabotage, the Pentagon has been running a series of jamming exercises. The tests help develop anti-jamming technology and also train military crews to identify GPS interference and switch to alternative means of navigation. But these efforts to disrupt GPS service by interrupting the signals also can complicate travel by civilian ships and aircraft that use the 24-satellite network to help guide them across the ocean or through the sky.

A Coast Guard system for improving GPS is scheduled to become fully operational on Monday. With it, the Coast Guard has assured mariners that GPS signals will be available 99.7 percent of the time.

"That allows for only a couple of hours a month of down time for maintenance, and no time for military training," said Cmdr. Peter Keane, a spokesman for the Coast Guard's navigation center. "After Monday, we're going to say we will most likely disapprove any request for military testing that touches navigable waters of the United States."

Following a number of GPS tests over land, the Defense Department picked an area off North Carolina to stage its first jamming exercise over water last month. Ahead of the week-long exercise, the Coast Guard and Federal Aviation Administration issued notices cautioning pilots and ship operators against relying on GPS signals in the affected areas during specified four-hour periods.

In the tests, and during previous ones held over North Carolina and Nevada in the past two years, there were no reports of civilian ships or planes veering off course or encountering safety problems attributable to faulty GPS readings. But military officials have acknowledged there is little way of ensuring that all those likely to be affected by the jamming will read and observe the warnings given in advance.

Asked yesterday about the Coast Guard action, Pentagon spokesmen said the matter is under review. In view of the conflict, Langhorne Bond, a former head of the FAA, wrote Defense Secretary William S. Cohen and Transportation Secretary Rodney E. Slater this week urging a reassessment of government plans to phase out ground-based navigational aids and allow civilian planes and ships to navigate solely by satellite systems.

"Both the Coast Guard and the Pentagon have reasonable concerns," Bond said yesterday. "It's the policy that allows civilian users to switch to GPS-only that's at fault and needs to be reconsidered."