The Global Positioning System provides accurate timing information to many of our critical systems – power grid, communications, financial markets, emergency services, and industrial control to name a few. It also includes the ability to transmit the proper date and time to a receiver by supplying the receiver with the current week and the current number of seconds into the week. This allows the receiver to translate the date and time into a more typical format – day, month, year, and time of day.
However, the field that contains the week number is a 10-bit binary number. This limits the range of the week number to 0 – 1023, or 1024 total weeks.
GPS week zero started January 6, 1980. The 1024 weeks counter ran out and rolled over on August 21, 1999. The week counter then reset to zero, and it has been recounting ever since. The next time the counter will reach week 1023 and rollover to zero is on April 6, 2019.
Receivers must properly interpret that week number as the correct date, not 19.7 years into the past or future. To do this, receivers use various methods to ensure that they are providing the correct date. One common method is to use the firmware date as a reference. This works well if the receiver is new or is receiving firmware updates. It is also possible for the user to modify this reference date in some receivers.
Another way is to shift that 1023 window with reference to some firmware or manufacture date within the receiver. Using this method, the problem could occur, but at a different date and year than the actual GPS rollover date.
Because this it is the second time the GPS week rollover will occur, many receiver manufacturers have prepared for it, and newer receivers will continue to operate without issue.
You should be concerned, though, if either of the following applies:
In these cases, we want to verify that an issue will not occur. At a minimum, we recommend consulting your receiver manufacturer to confirm that the issue has been fully tested and will not occur. Many manufacturers have already issued compliance statements, and we expect them to continue to do so over the next year, up until the event occurs. To be sure that your system will not experience any failures related to this issue, it is possible to test for this event using a GPS/GNSS simulator. The requirements for the simulator are straightforward:
If you are using any Spectracom GSG-5 or GSG-6 simulator, you can download a scenario to test for the rollover event: SCENARIO FILE
The scenario file starts the simulation at on April 6, 2019 23:25 and runs for two hours. These settings can also be configured on the front panel of the unit by modifying a present scenario, such as GPSStatic.
GPS is currently undergoing a modernization program to upgrade the signals with new modulation and data message structures. The newer message types (CNAV and MNAV) use 13-bit binary numbers to represent the GPS week number, so the issue should not occur in the future when more receivers are using the newer GPS signals.