Updated May 13, 1999
By Tim Dobbyn

WASHINGTON (Reuters) - A congressional watchdog agency Wednesday urged Americans who use Global Positioning System (GPS) navigational receivers to check that they can cope with an internal date change in the system due in August. Some experts see the date change as a bigger problem for users of the GPS than the Year 2000 computer problem.

GPS is a satellite-based navigation system run by the Department of Defense but plays an increasingly important role in civilian airline operations, truck fleet tracking, recreational boating and computer maps in cars.

The General Accounting Office told a congressional panel that precise time signals in the system were also used to synchronize commercial communications networks and ignorance of the August date change and the ability of the GPS receiver being used could cause network failures.

"GPS is not just a thing to have on your bass boat to find the best fishing hole," said GAO communications technical director Keith Rhodes.

The August date rollover for GPS occurs because the system was designed to ignore calendar dates but keep precise time measured in seconds and weeks.

Only 1,024 weeks were allotted from Jan. 6 in 1980 before the system resets to zero. That is due to occur over the weekend of August 21-22.

Older GPS receivers could become confused by the reset, giving either wrong times or positions, or no information at all. Users needed to quickly establish with their suppliers whether their receivers were at risk, Rhodes said.

Although GPS is not the primary means of navigation for passenger aircraft, it is starting to be used on some New England and Caribbean routes to fly more direct paths than current radio navigation beacons allow. It is also use on long oceanic flights as an alternative to inertial guidance.

Air Transport Association Year 2000 coordinator Tom Browne said major carriers had been aware of the August problem for some time, adding: "You are talking about the low-end equipment, not the units installed on regulated passenger jets."

A list of GPS devices tested for both the end-of-week rollover and the Year 2000 computer problem has been compiled by the Air Force and can be found on the Internet at: http://gps.laafb.af.mil/y2000/index.html.

The Defense Department told a joint hearing of the House Science and House Government Reform subcommittees that the GPS system itself would continue working through both dates.

"We do not anticipate any show stoppers to occur resulting in disruption of this crucial navigation and time-keeping capability," said Deputy Assistant Secretary of Defense Marvin Langston.

The Year 2000, or Y2K, problem arises because many computers and their programs allocated just two digits for the year in dates. Unless repaired or replaced, these computers may misread the Year 2000 as 1900 or simply fail to work.

Langston said the satellites themselves were never affected and the ground-based computers that controlled the network had all been upgraded. Some computers used to analyze the network were still being repaired but they were not critical to the system's operation, he said.

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Repeating the obvious: you should never rely upon only one method of navigation.
Capt. Matt

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GPS - Y2K and sooner!