J. David Grossman
GPS “Week Number Rollover” Explained
March 19, 2019 By: J. David Grossman, Executive Director, GPS Innovation Alliance
Nearly forty years ago, on January 6, 1980, while still in its infancy, the GPS constellation began keeping track of time using 19 bits to count seconds and 10-bits to tally the weeks. This has meant that the field indicating the week in the running timestamp broadcast by GPS satellites resets to 0 every time it reaches 1024 weeks. We crossed this point in August 1999 and it will occur once again on April 6, 2019 at 23:59:42 UTC.
This resetting of weeks gives rise to what is now referred to as the upcoming “GPS Week Number Rollover.” This is not a “bug” in GPS. Rather, it is a basic feature of the GPS signal broadcast by GPS satellites, which GPS receivers must handle in order to continue functioning properly.
The latest generation of GPS satellites launched by the US Government has modified this aspect of the signal to increase from 10 bits to 13 bits, which means that the rollover will occur every 157 years. However, the constellation is still dominated by satellites using the older 10 bit format, and currently available receivers use the 10-bit signal. Depending on the design of GPS receivers and the applications using the time and position output of GPS receivers, this infrequent, but regular rollover, of the week number may affect the time output of a receiver or application and in rarer cases the position output may be affected as well.
This is an important issue requiring attention by users of GPS equipment as well as those who maintain applications and systems that rely on GPS signals. GPS receiver manufacturers, including our own member companies, have actively tested their varying products to ensure they continue to perform as designed, and have provided product advisories about how their products may be affected and what steps may need to be taken with respect to their products.
GPS receivers with current firmware should experience no issues with the rollover. Products such as mobile phones which use GPS but receive regular firmware or software updates over the air should not be affected.
The most important step one can take to prepare for the upcoming rollover week is ensuring that every device firmware or software is up-to-date. Just as you would ensure that your computer or smartphone has the latest security patch, updating the software on a GPS-enabled device means you will benefit from the device manufacturers’ extensive testing and guidance.
Many receiver manufacturers have posted guidance on their websites or through service bulletins sent directly to users. Should you have further questions, we encourage you to contact your device manufacturer. With proper planning, you can help anticipate and attempt to address any issues prior to the April 6th rollover date. GPS receiver manufacturers also stand ready to assist customers who experience any issues.
Additional information may be found at www.gpsalliance.org/gps-rollover-week.