Although attention has been focused on the Aug. 21-22 reset of the GPS internal clock to zero, some users of the satellite-based navigation system may start to experience problems as early as Thursday, both the U.S. Air Force and the Coast Guard warned.
The reason is that a regular update of location information to the 27 satellites in the system beginning Aug. 19 also contains new time information that could cause some older GPS receivers to misinterpret which satellites they are ``viewing.''
GPS plays an increasingly important role in civilian life, including airline operations, truck fleet tracking, recreational boating and computer maps in cars.
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. The event has been compared to GPS' own version of the Year 2000 computer glitch that arises because too little memory was allocated to the year in dates.
Most recently manufactured GPS receivers are capable of handling both the satellite update and the system's internal date rollover but some units made 1994 or earlier are in doubt.
Some receivers may not work at all, others will take more time to locate satellites and others may display inaccurate positions, times or dates.
``Users need to check their equipment regularly over the next few days,'' said Ronea Alger, chief spokeswoman at Los Angeles Air Force Base, that runs the GPS programme.
The Coast Guard is urging boaters and other users that include private pilots to play safe and always have more than one navigation tool available.
Boat/U.S., a boatowners' organisation, said it was concerned by the recent emergence of Thursday as a problem after it had spent months sounding warnings about the Aug. 21-22 clock reset.
Spokeswoman Becky Squires said people on voyages expecting to get a few extra days out of their GPS equipment could be at risk. ``I think that's a little disturbing,'' she said.
Boat/U.S. has advised its members that most GPS units made after 1994 are ready for the GPS date rollover.
Coast Guard Master Chief Petty Officer Rick Hamilton said the Aug. 19 problem could have the biggest impact on the increasing number of users who rely on the precise time information generated by GPS to coordinated communications networks.
``We're talking about cellular phone systems, fax and page systems and some power grids,'' said Hamilton. ``We call it the invisible GPS community.''
The Aircraft Owners and Pilots Association (AOPA) played down the risk to aviation from the GPS changes.
AOPA spokesman Drew Steketee said the control panel units used in instrument flight conditions were subject to government orders to be end-of-week compliant while hand-held GPS units sometimes used under visual flight rules were meant as backups only.
``People in the aviation industry have foreseen this date for some time, the press is just waking up to it this month,'' Steketee said.
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