The U.S. Coast Guard is urging boaters and other civilian users of the global positioning system to play it safe over the next few days and make sure alternative navigation methods are available.
Over the 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 was uploaded to the satellites Thursday. That may confuse some older receivers, as the time information will assume the weekend clock rollover.
In a final appeal to private citizens and businesses to check their equipment, the Coast Guard warned that while the weekend was the most likely time units more than five years old could shut down or give wrong information, some initial problems for older GPS receivers could begin with Thursday's update.
"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, Virginia.
Designed for the military, the constellation of 27 satellites orbiting 11,000 miles 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 familiar 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.
Rice and other Coast Guard officials at first said the weekend rollover coming up Aug. 21 at 8.00 p.m. on the U.S. East Coast was the key time to focus on.
But later they agreed it was impossible to predict how the older GPS units would react. "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, said Tuesday that the same receiver software bugs that cause problems coping with 20-year clock rollover could trip up the unit on the five-year almanac changes.