Published in the December, 1996, GOFC Newsletter
Some GPS receivers may no longer operate correctly after 21 August, 1999 unless returned to the factory for an upgrade. Affected units will think that 22 August 1999 is actually 6 January, 1980. Since GPS receivers use the current date and time to calculate their position, accuracy of the affected units will be destroyed.
This has nothing to do with the infamous "year 2000 problem" looming over the computer industry, although the situation is similar. Many business-related computer programs, especially those written years ago for big mainframe computers, cannot deal with dates later than 31 December, 1999. These programs think that the next day is 1 January, 1900 because the leading "19" in the year is understood to be present, and the computer only stores the last two digits of the year. As a result, a mortgage payment received and posted on 31 December, 1999, one day ahead of a due date of 1 January, 2000, is viewed by affected programs as arriving 100 years too late -- the computer thinks the payment was due on 1 January, 1900!
The GPS problem occurs because, among other things, GPS satellites broadcast the current date and time to your GPS receiver. Your receiver then uses this information to calculate its position from the location of the satellites it can "see." Unfortunately, the date information broadcasted by a GPS satellite is given as the number of weeks elapsed since 6 January, 1980 -- the official "birthday" of the GPS system -- and the amount of computer space allocated in the GPS system for storing the week number only can hold 1023 weeks. (It's a 10-bit value for all you computer nerds that may be interested.)
On 22 August, 1999, the GPS week number will reset to zero, since the previous week was GPS week number 1023. Some older GPS units were not programmed to handle rollover of the GPS week back to zero. An affected unit will become totally lost on 22 August, 1999. It will not "see" the location of GPS satellites correctly, because it will look for them where they were supposed to be on 6 January, 1980, instead of 22 August, 1999.
Garmin, manufacturer of the best selling marine GPS units, has stated that all its units will handle the rollover correctly. Most likely recent units from other reputable manufacturers will do so as well. However, those of you with older, non-Garmin units should check with the manufacturer. Also, be sure to check for this problem before buying a used GPS unit or a bargain-priced new unit from a no-name manufacturer.
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This page last updated 3 November, 1996
Charles H. Courtney (chc@dkeep.com)