The Global Positioning System provides accurate timing information to many of our critical systems—including, but not limited to, our nation’s power grid, communications infrastructure, financial markets, emergency services, and industrial control systems, to name a few. It also transmits the correct date and time to a receiver by supplying the current GPS week and the number of seconds within that week. This allows the receiver to translate that into a more typical format: day, month, year, and time of day.
The field that contains the week number is a 10-bit binary number. This limits the range of values to 0 through 1023, or 1024 total weeks.
So, what does that mean?
The GPS Week Number count started at approximately midnight on January 6, 1980. The counter hit 1023 and reset once already—on August 21, 1999. Since then, the count has been increasing by 1 each week and is broadcast as part of the GPS signal. Because that field is limited to 1024 values, we’re approaching the next rollover. At the end of week 1023, the GPS week number will reset to 0 again—this will happen at midnight GPS Time on April 6, 2019.
So what could happen on April 6, 2019?
Some GPS receivers—and the systems they support—might not handle the rollover properly. UTC timing displayed on devices or used in data time-stamps could jump by 19.7 years, leading to potential failures. Any software that relies on accurate month/year conversion could also break. Navigation positioning itself should be OK, since GPS time is internally self-consistent, but the associated time tags might be wrong—possibly corrupting navigation data and sending you in the wrong direction.
This issue isn’t limited to April 6 or 7, 2019. If a system isn’t prepared for the rollover, it could fail at any time after that, depending on how it interprets the week number.
What about newer receivers?
Most modern GPS receivers are likely to be fine—if the ICD-200 standard was followed during development. Older receivers, on the other hand, may be problematic. Bottom line: “Trust but verify.” If you rely on GPS timing, now’s the time to check with your manufacturer.
The good news? The next block of GPS satellites will use a 13-bit week number, which will push the next rollover much further into the future. That makes April 6, 2019, the last GPS “zero week” for systems using the original 10-bit format.
