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GPS Week Rollover
March 19, 2019

Take action: GPS Week Rollover is coming!

If you haven’t heard of the GPS Week Rollover and its potentially critical consequences, or haven’t taken any action, now it is the time to do so, and you can do it by reading this article. Due to this GPS event and its effects, explained hereinafter, EGNOS encourages all its users to check their GPS receivers.

The GPS Week Number Rollover Event

Among other data, GPS broadcast 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.

Critical aftereffects and what you should do

On Saturday 6 April 2019 the behavior of an erroneous GPS receiver may vary: from providing incorrect date/time information (e.g. incorrect UTC Time due to failure on the GPS Time-to-UTC conversion algorithm) to leading up to the complete failure of the receiver. Also be aware that some manufactures implement the week number referenced to a date different than 6 January 1980 (e.g. the firmware date is used as reference). These devices would not be affected by the GPS Week Rollover on the 6 April 2019, but rather the problem would occur on a different date.

In summary, a wrong handling of this event may cause adverse effects, and therefore we suggest following these actions1 as a proactive measure:

  • To check that the firmware installed on your GPS receiver(s) is up-to-date.
  • To contact the GPS receiver manufacturer in order to ensure the readiness for the event.
  • To 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.

Check your GPS receiver now, and please, do not hesitate to contact us in case you have any question!

 1 The United States has issued a Memorandum related to the upcoming rollover event, providing recommendations to be taken by GPS users.