U.S. tests GPS satellites for Y2K compliance

The U.S. Space Command plans to begin today testing the Year 2000 compliance of the Global Positioning System. The test will involve a single satellite and will not interfere with the day-to-day operations of GPS users.

During the tests, which will occur on four days between today and the middle of next month, the Space Command will move the clock on one GPS satellite forward to ensure that none of the date code could cause glitches in the system of GPS satellites. Those satellites provide highly accurate position information to precision-guided weapons fired by U.S. forces in Kosovo, and they provide less-precise civilian signals that are accessed by everyone from recreational hikers to surveyors.

Before moving the clock forward, the satellite will be temporarily set as "unhealthy" so that its signal cannot be used by GPS receivers. The Space Command said there are three more GPS satellites on-orbit than the 24 required to ensure precise positioning anywhere in the world. With one satellite set as "unhealthy" and unable to coordinate locations, GPS users will continue to receive navigation information from the other 26 satellites.

In addition to checking Year 2000 compliance, the test also will verify GPS readiness for a phenomenon known as the End of Week rollover. The GPS clocks count time in weeks and are set to roll over every 20 years. Because GPS system time started on Jan. 6, 1980, when the first satellite went into orbit, the rollover will occur at midnight between Aug. 21 and Aug. 22, when the GPS clock will jump from 1,023 weeks to 0000.

The event is significant because it is the first end-of-week rollover since the GPS constellation was established, and it could be interpreted as an invalid date in GPS receivers that were not designed to accommodate this phenomenon in accordance with manufacturing specifications.

For more information about the GPS constellation, Year 2000 and the end-of-week rollover, go to the Space Command's World Wide Web site at www.spacecom.af.mil/usspace/news8-99.htm.

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