European satellite services provider ESSP is warning of the potentially critical consequences of ‘GPS Week Rollover’ and is urging airspace users to check their GPS receivers as a priority.
Industry experts are warning that 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data they output may jump backwards in time, resulting in month and year timestamps that are potentially up to 20 years out of date.
ESSP delivers the EGNOS Signal-In-Space, enabling safer and more accurate user-positioning than GPS-only signals and its real-time monitoring ensures transparent information on GNSS performance.
Among other data, the Global Positioning System – more commonly known as GPS – broadcasts the date and time information in a specific format, consisting of the current week and the current number of seconds in the week.
However, the length of the week number field is 10-bit and consequently it is limited to a range of 0 to 1023, or 1024 total weeks (a 19.7 year epoch). The week number is referenced to the start of the GPS Time Epoch, and at the end of week 1023, the week number restarts from 0 (“rollover”) and a new GPS Time Epoch is defined.
With the GPS starting on the 6 January 1980, the first reset of the 1024 weeks counter happened on 21 August 1999, ending the initial GPS Time Epoch. The next reset of the week counter from 1023 to zero will be on Saturday 6 April 2019, at 23:59:42 UTC – note that there is currently an 18-second gap between UTC Time and GPS Time – ending the second GPS Time Epoch.
On Saturday 6 April 2019 the behaviour of an erroneous GPS receiver may vary: from providing incorrect date/time information (e.g. incorrect UTC Time due to the failure on the GPS Time-to-UTC conversion algorithm) which could lead to the complete failure of the receiver.
Some manufactures implement the week number referenced to a date other than 6 January 1980 (e.g. the firmware date is used as reference) so these devices would not be affected by the GPS Week Rollover on the 6 April 2019, – although the problem may affect them on a different date.
PNT expert Jens Hoxmark explains how some devices’ week number-counter could resets from (hexadecimal) $3FF (decimal 1023) back to $000, and this could result in complications with timing for AIS and ADS-B.
“AIS and ADS-B are brilliant systems in non-challenged environments,” he said, “but should only be used for information, not navigation. The AIS GPS-receiver and antenna is usually very low-end-product, and the update-rate would be too low for most dynamic scenarios as it depends on adapting to local traffic-density and VHF-range on the dedicated VHF-channel for vessel-to-vessel-communication with the self organizing multi-way VHF radio data link.”
“All operators and traffic management centres that depend on NAVSTAR GPS should have a Plan-B, not including NAVSTAR GPS, for the next few days, and extra-ordinary checks should be included to confirm that systems are performing according to intentions, in addition to include specific information on the rollover in NOTAMs globally well in advance of the event,” he adds.
“If there are GPS-slaved clocks in the ATM-infrastructure, one method could be to disconnect the GPS-input to the clocks, and keep running on the internal clock across the rollover and ensure that the GPS-receiver works normally prior to connecting it back in again following the rollover-event.
“Never before have so many been so reliant on NAVSTAR GPS during a rollover-event, there will for sure be many incidents, but hopefully accidents can be prevented by people being fully aware of the event.”
ESSP warns that if this event is handled badly, there may be adverse effects so is suggesting the aviation industry to:
- check that the firmware installed on your GPS receiver(s) is up-to-date.
- contact the GPS receiver manufacturer in order to ensure the readiness for the event.
- test the devices in a GPS Week Rollover simulated environment (e.g. a GPS simulator).
“If you are not able to confirm that your GPS receiver is not impacted by the rollover, you should assume its possible failure and take into consideration the appropriate mitigation measures,” said ESSP. Airspace users are invited to contact EGNOS for more information.
NAVSTAR GPS video explainer https://www.youtube.com/watch?v=Tad2REYvb74