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GPS week roll over April 6th

Map with GPS markers

On April 6th, the GPS week counter rolls over and resets to zero. This change may affect Industrial Control Systems (ICSs) and Critical Infrastructure (CI) owners and operators.

This rollover may affect log time stamp information, loss of communication between devices, inability to authenticate multi-factor authentication, or the ability to log in to computers.

What is GPS time and what’s changing?

GPS time is represented by a week number and a number counting seconds into the week. The week counter is stored as a 10-bit number; meaning the value of the week number field can be between 0 and 1023.

On April 6th, the week’s field number will have reached 1023, and will roll over from 1023 to 0. Depending on the manufacturer of the GPS receiver, or versions of firmware, a GPS receiver may be affected by this roll over.

GPS Clocks may display inaccurate date and time information, such as rolling time back to the previous Epoch (August 21, 1999), and GPS Navigation Systems may be affected.

The GPS week counter started at 0 on January 6th, 1980. Since then, the week counter has reached its maximum and been reset once, last occurring on August 21, 1999. The next occurrence of the GPS week rollover for GPS clocks with 10-bit week numbers following April 6th 2019 will be November 20, 2038.

Modernised GPS clocks, and GPS navigation systems may be released with a 13-bit week field, allowing up to 8192 weeks, instead of 1023 weeks, before rolling over to 0. This will increase the time between GPS week roll overs from approximately 20 years, to around 150 years.

Mitigation actions

The ACSC recommends that ICS and CI owners and operators consider the following mitigation actions:

  • Ensure that the firmware for GPS receivers are up-to-date.
  • Contact the manufacturer of the GPS receiver to ensure the device will not be affected by the GPS week roll over.
  • Consider the impact to ICS and CI devices if the GPS receiver stops operating or reports the wrong GPS or UTC timing information.
February 20th, 2019