GPS Week Number Rollover
April 6, 2019

To provide the current date, the Global Positioning System (GPS) keeps an internal count of the number of weeks since January 5, 1980. The main civil GPS signal (C/A code) broadcasts the GPS week number using a 10-bit code with a maximum value of 1,023 weeks. This means every 19.7 years, the GPS week number in the C/A code rolls over to zero. This occurred for the first time in August 1999.

The GPS week number again reset to zero at 23:59:42 UTC on April 6, 2019. Many GPS-enabled devices that were not properly designed to account for the rollover event exhibited problems on that date. Other equipment became faulty several months before or after that date, requiring software or firmware patches to restore their function.

To avoid GPS device problems related to week number rollover, please keep your device's software/firmware up to date and report all equipment problems to the manufacturer.

Please also report any suspected week number issues and unusual GPS device behavior to the U.S. Coast Guard Navigation Center. Go there

GPS devices that use only the C/A code and its legacy navigation message (LNAV) can expect the next rollover on November 20, 2038. Newer devices can avoid that event by using the new civil signals being added to GPS. Those signals use a modernized civil navigation message (CNAV) with a 13-bit week number that won't roll over until 2137. Learn more

More Information

• Presentation on GPS Week Roll Over Issue (PDF)
• Article from DOE Office of Electricity (energy.gov)
• Memo for U.S. Owners and Operators Using GPS to Obtain UTC Time (us-cert.gov)
• GPS Equipment Testing Notice (federalregister.gov)
• Interface Specification IS-GPS-200 - Technical info for device makers

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