"Don't rely totally on GPS. You should have some alternative means of navigation," said Capt. Tom Rice, Commander of the U.S. Coast Guard Navigation Center, in Alexandria, Va.
This coming weekend, the 1,024-week time clock in the satellite-based GPS system turns over to zero after nearly 20 years, creating difficulties for some receivers.
Complicating the matter is a major update of time and position data that starts being uploaded to the satellites Thursday. That may confuse some older receivers as the time information will assume the weekend clock rollover.
Designed for the military, the constellation of 27 satellites orbiting 11,000 miles (17,700 kms) above the earth plays an increasingly important role in civilian life.
GPS applications include airline operations, truck fleet tracking and computer maps in cars in addition to allowing boaters to find favorite fishing holes in an otherwise featureless sea.
A less well-known but extremely important use of GPS involves using the precise time signals from the system to coordinate telecommunications networks.
Coast Guard officials told reporters that users needed to be aware of possible problems but not be alarmed. The majority of users were expected to have no problems but the owners of older units were urged to check with manufacturers.
Many makers have developed software patches or new chips to allow older receivers to cope. A list of manufacturers and contacts is available on the Internet at www.navcen.uscg.mil/gps/geninfo/y2k/default.htm.
Rice and other Coast Guard officials at first said the weekend rollover coming up on Aug. 21 at 8.00 p.m. on the U.S. east coast (midnight GMT) was the key time to focus on.
But later they agreed it was impossible to predict how the older GPS units would react to the "almanac" updates to the satellites that begins Thursday at 6.00 p.m. EDT (2200 GMT).
"If it sees something that it just doesn't understand, we are going into an area where anything can happen," said Commander Gary Shank, chief of GPS planning for the Coast Guard.
The U.S. Air Force, which manages the GPS system, told Reuters on Tuesday that the same receiver software bugs that cause problems coping with the 20-year clock rollover could trip up a unit on almanac changes that include their own five-year clock reset.
John Lovell, director of quality at Sunnyvale, Calif.-based Trimble Navigation Ltd., said his company had been testing its products for two years.
Some Trimble GPS systems described as non-compliant would still operate after a delay or a resetting of the memory. "In many products it will roll through without a hitch," he said.
The Air Transport Association, that represents the major airlines, said the GPS changes presented no safety or operational problems for commercial carriers.
"It's been well known this was going to happen, the fixes have been done and we think there's going to be no problem at all," said ATA spokesman David Fuscus.
In an ironic twist Wednesday, while the Coast Guard was holding its news conference, government scientists were using GPS to measure the height of the Washington Monument. Answer: 555 feet (169 meters), five and nine-tenths inches above the ground.
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