Year 2000 Computing Challenge:

Time Issues Affecting the Global Positioning System

T-AIMD-99-187: Published: May 12, 1999. Publicly Released: May 12, 1999.

Additional Materials:


Joel C. Willemssen
(202) 512-6253


Office of Public Affairs
(202) 512-4800

Pursuant to a congressional request, GAO discussed the year 2000 problem and its impact on the Global Positioning System (GPS).

GAO noted that: (1) GPS is affected by both the year 2000 computing problem and an upcoming end-of-week rollover; (2) the upcoming end-of-week rollover is a problem that will occur for the first time on August 21, 1999; (3) instead of using calendar dates, GPS counts weeks, and seconds within a week, from precise clocks on the satellites; (4) this is based on how the signal codes transmitted by the satellite are generated; (5) GPS started at week zero on January 6, 1980; (6) because of its design, the GPS time counter starts over after counting 1,024 weeks; (7) the end of the 1024th week will occur, for the first time, on August 21, 1999; (8) there are three GPS components that may be affected by the year 2000 problem and the end-of-week rollover--space, control, and user; (9) the Air Force Materiel Command (AFMC) reports that the satellite support systems are end-of-week rollover compliant, but that they are not yet year 2000 compliant; (10) AFMC reports that these systems are in the process of either being replaced or renovated and tested; (11) according to AFMC, the ground support systems and user component systems are both year 2000 and end-of-week rollover compliant; (12) according to the Coast Guard Navigation Center, however, the accuracy of navigation on some older GPS receivers may be severely affected by the end-of-week rollover; (13) several activities are ongoing to raise awareness among owners of older GPS receivers of the upcoming end-of-week rollover problem; (14) the Coast Guard Navigation Center has been assigned with responsibility of being the government liaison to the civil sector for GPS; (15) its Internet website explains the potential rollover problem on older receivers and provides an extensive list of manufacturers and points-of-contact; (16) the Air Force has provided a list, also available on the Internet, of specific receivers that have been tested and found to be compliant by the Department of Defense; (17) furthermore, the President's Council on Year 2000 Conversion's Internet site provides links to sources of GPS Year 2000 and end-of-week rollover information; and (18) these activities are important and should be useful to GPS users seeking to determine whether their receivers will operate correctly at the end-of-week rollover.

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