The Global Positioning System provides accurate timing information to many of our critical systems – AIS, GPS, Speedlog and SATCOM systems 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.
The next time the counter will reach week 1023 and rollover to zero is on April 6, 2019
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.
Should I Be Concerned?
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:
- Receiver has been fielded for more than 10-15 years without firmware updates
- Receiver is a core component of a critical timing system – ask yourself – what is the impact to my system if the GPS receiver stopped operating or put out wrong GPS or UTC information
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.
Below are statements from some of the major suppliers - click on the supplier name and read their statement:
Should your equipment be on one of these lists we will be happy to assist you in either arranging upgrade of software or installing new equipment.
If you have any questions, please do not hesitate to contact us!