Save the date: GPS Week Number Rollover Event – April 6th 2019
If your networks or systems use Coordinated Universal Time (UTC), April 6th 2019 is probably a date worth marking in advance.
On, or possibly after, this date, some GPS receivers may start to behave strangely. The data they output may jump backwards in time, resulting in month and year timestamps that are potentially up to 20 years out of date.
This is a known issue; in April 2018 the Department of Homeland Security in the United States issued a memo to make GPS users aware of the situation. Any changes, adjustments, or other actions are ultimately the responsibility of the user, so DHS strongly recommends owners and operators of critical infrastructure to prepare for the rollover.
This refers to the GPS Week Rollover on Coordinated Universal Time (UTC) derived from GPS devices.
The detail …
The potential problem revolves around the way that GPS handles the week element of the data that forms part of the navigation signal; specifically the CNAV and MNAV message types.
The week number is encoded into the data stream by a 10-bit field. A binary 10-bit word can represent a maximum of 1,024 weeks, which is approximately 19.7 years. Each 19.7 year period is known in GPS terms as an “epoch”.
At the end of each epoch the receiver resets the week number to zero and starts counting again – a new epoch begins.
The first epoch started when GPS was launched in January 1980; hence the first epoch of GPS time came to an end on 21st August 1999.
As we approach the end of the second epoch, which will fall on 6th April 2019, we may well see problems caused by the rollover. Some GPS receivers, or other systems that utilise the date and time function, may not be able to cope.
Who will be affected?
The list is long and varied; some industries come to mind immediately as they are known to use the accurate timing information provided by the GPS constellation. Financial markets, power generating companies, emergency services and industrial control systems may be affected, as well as fixed-line and cellular communications networks.
GPS tracking devices installed in a fleet management system to schedule and monitor deliveries could cause system errors if they start to provide location data that is potentially up to 20 years out of date.
Since this is the second time a GPS week rollover will occur, many manufacturers will have been aware of it in advance and newer receivers will continue through and beyond the rollover date without issue.
Issues could occur if:
- Receivers have been in operation for more than 10-15 years without firmware updates
- Receivers are part of a critical timing system, i.e. what would be the impact to the system if the GPS receiver starts to output incorrect UTC data?
What can be done?
There is an ongoing multi-billion-dollar program of upgrades to improve the overall performance of the Global Position System.
Included within this program is a move towards a 13-bit date field to represent the week number. Thirteen binary bits represents around 157 years per epoch. Newer GPS receivers won’t be affected by the current end-of-epoch situation.
Notwithstanding that, if unsure the best advice is to check with the manufacturer of the devices you use.
This situation won’t affect a receiver’s ability to navigate and/or calculate precise time, but it has the potential to create week, month and year timestamps that are wildly wrong. Applications which rely on GPS data at that level may be seriously affected.
Other GNSSs, such as the Russian GLONASS, European Galileo or Chinese BeiDou, are not affected by this problem.
Using receivers indoors via a GPS repeater
If your receivers are operated indoors by means of a GPS repeater system, the contents of the GPS signal is not affected at all by the repeater.
Anything that is transmitted by the GPS satellites is passed transparently through to the interior.
Any checks carried out to ascertain the potential effect of the GPS week rollover need not involve the GPS repeater if installed.
If we can help you to overcome the issues caused by being inside a signal-denied space, please do get in touch.
You can email us through the contact page or if you would prefer a good old-fashioned chat we still have a telephone in the office – please call 01326 336 444 in the UK or +44 1326 336 444 from outside the UK.