GPS trouble may start sooner than expected

WASHINGTON, D.C. (Aug. 18, 1999) — The possible disruption in the Global Positioning System (GPS) expected this weekend could begin as early as tomorrow, the U.S. military cautioned.

While warnings have focused on the Aug. 21-22 “End of Week” reset of the GPS internal clock to zero, a regular update of location information to the 27 satellites in the system beginning Aug. 19 also contains new time information that may cause some GPS receivers to misinterpret which satellites they are in touch with.

Several vehicle tracking and mapping systems on the market rely on GPS for positioning reports. Whle most newer GPS receivers are capable of handling both the satellite update and the system’s internal date rollover, some units made in1994 or earlier may have trouble.

The August rollover for GPS occurs because the system was designed to ignore calendar dates, but keep precise time measured in seconds and weeks. Only 1024 weeks were allotted from Jan. 6 in 1980 before the system resets to zero.

Some receivers may not work at all, others will take more time to locate satellites and others may display inaccurate positions, times, or dates.