Y2K testing of GPS is critical in order to ensure this national asset is available to civilian, commercial and military users.

The upcoming test is the second Y2K tests slated for the GPS system. The first test occurred from late April to mid-May and successfully tested GPS satellites Block II and IIAs. This second test will evaluate GPS satellites Block IIRs.

During the tests, the clock on one GPS satellite will be moved forward to specific dates to be tested in a Y2K environment. Before moving the clock forward, the satellite will be temporarily set "unhealthy" so that its signal cannot be used by GPS receivers made in accordance with manufacturing specifications*. There are presently three more GPS satellites on-orbit than required for the full constellation of 24 satellites.

Even with one set "unhealthy," GPS users will continue to receive navigation information that exceeds accuracy the Air Force has committed to provide. While this test will be totally transparent to the GPS user with compliant equipment, as with any maintenance on GPS satellites, users will still be notified of the test via a Notice to Navstar Users. This site routinely provides information on the status of the GPS constellation.

In addition to checking Y2K compliance, the test will also verify GPS readiness for a phenomenon known as the End of Week rollover. The GPS end of week rollover happens approximately every 20 years (1023 weeks) because GPS system time, which is counted in weeks, started counting on Jan. 6, 1980. At midnight between Aug. 21 and 22, 1999 the GPS week will rollover from week 1023 to 0000. This event is significant because it is the first EOW rollover since the GPS constellation was established and could be interpreted as an invalid date in GPS receivers that were not designed to accommodate this phenomenon in accordance with manufacturing specifications*.

The Y2K issue stems from the fact that many computer programs use a two-digit date field and assume that the year is 19XX. When the year 2000 occurs within the program, the two-digit date becomes "00" and could be interpreted as an invalid date.

GPS UPDATE - SpaceDaily Special Report

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