A glitch in a satellite-based navigation system caused many users to lose contact with the system, Coast Guard officials said.

Pilots, boaters, motorists and even hikers use the Global Positioning System to pinpoint their locations with the help of satellite signals. To account for variations in the Earth's orbit and rotation, the system requires an exact measure of time.

Many older units were only designed to count a fixed number of weeks--a maximum that lapsed Saturday. Their time clocks had to be reset, making some users vulnerable to a break in service.

GPS, originally designed for the military, is booming in the commercial market and is often available in luxury cars. Aviators and boaters have used the system for at least a decade, and recently hikers have begun to use hand-held units.

A nationwide, private tow-boat agency, Vessel Assist Association of America, based in Newport Beach, reported scattered GPS outages Sunday.

"The biggest problem is that it takes us a long time to find the mariner in distress if we don't have an accurate position. We had some delay in serving boats today," said dispatcher Bob Cunningham. No boaters were in serious danger, he said.

The Coast Guard in California received at least two calls: One boater hadn't yet left the harbor when his GPS failed, and another was lost and fired a flare to summon help, said Petty Officer Dawn Butler.

The Federal Aviation Administration had no reports of problems, said agency spokesman Les Dorr.