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ADVISORY FOR THE AUGUST 1999 GPS WEEK ROLLOVER

NOTE: This information is provided for historical purposes only. For information on when the next GPS rollover will occur please refer to the U.S. Naval Observatory’s page discussing GPS Week Number Rollover. For assistance on how your equipment will handle a GPS week rollover event we recommend you contact your equipment manufacturer.

GPS System Time will roll over at midnight 21-22 August 1999, 132 days before the Year 2000. On 22 August 1999, unless repaired, many GPS receivers will claim that it is 6 January 1980, 23 August will become 7 January, and so on. Accuracy of navigation may also be severely affected. Although it appears that GPS broadcasts do contain sufficient data to ensure that navigation need not be affected by rollover in 1999, it is not proven that the firmware in all receivers will handle the rollovers in stride; some receivers may claim wrong locations in addition to incorrect dates.

Some manufacturers have already solved the problem, but some have not.

This is how the precise rollover date is computed: The timescale origin (time zero) of GPS System Time, 00:00:00 UTC 6 January 1980, is Julian Day 2,444,244.500. A GPS Cycle is 1,024 weeks, or 7,168 days, so the first GPS rollover will occur at Julian Day (2444244.5+7168)= 2,451,412.5, which is 00:00:00 UTC 22 August 1999 AD, which is the midnight between Saturday night the 21st of August, and Sunday morning the 22nd of August, 1999.

Section 3.3.4(b) (page 33) of the ICD-GPS-200, Revision C* (25 September 1997 issue) states that the "GPS Week" count starts at midnight 5-6 January 1980 UTC, and that the GPS Week field is modulo 1024. This means that the week count will roll over 7168/365.25 = 19.6249 years from then, or in 1980+19.625 = 1999.638 (August 21, 1999), only a few years from now.

In the July 1993 update of ICD-GPS-200* , a note was added (also on page 34) saying that the week number *will* roll over, and that users must account for this, but no way to accomplish this is mentioned. I take this note as further evidence that there is no way to tell, given only the signal-in-space definition as of July 1993.

Section 2.3.5 (pages 18-19) of the GPS SPS Signal Specification, 2nd Edition, issued on 2 June 1995, repeats the words and warnings of ICD-GPS-200. You may download the GPS SPS Signal Specification from (now called the "Global Positioning System Standard Positioning Service Performance Standard") from our GPS Technical References page.

The firmware in all affected (mostly older) receivers will have to be replaced. This will involve replacement of PROMs; some are socketed, some are soldered. As a technical matter, the solution is quite simple. It's the logistics that will take some effort.

Without a GPS Simulator, there is no way for users to test a GPS receiver for this problem. Users are encouraged to contact their receiver manufacturer to determine if their receiver will be affected, in particular if a failure of navigation could put lives or property at risk."