The GPS 2019 Week Rollover - What You Need to Know

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What’s the Key Issue?

The Global Positioning System provides accurate timing information to many of our critical systems – power grid, communications, financial markets, emergency services, and industrial control 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.

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.

Party Like It’s 1999? Not

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. Many manufacturers have already issued compliance statements, and we expect them to continue to do so over the next year, up until the event occurs. To be sure that your system will not experience any failures related to this issue, it is possible to test for this event using a GPS/GNSS simulator. The requirements for the simulator are straightforward:

  • Minimum of 4 GPS Satellites simulated
  • GPS L1 C/A signal type
  • Possibility to program custom date and time

If you are using any Spectracom GSG-5 or GSG-6 simulator, you can download a scenario to test for the rollover event: Scenario File

The scenario file starts the simulation at on April 6, 2019 23:25 and runs for two hours. These settings can also be configured on the front panel of the unit by modifying a present scenario, such as GPSStatic.

How to Test For the Rollover

  1. Set GSG transmit power to a suitable level for your receiver (typically -115 to -125dBm)
    1. In the Options – Transmit Power menu
  2. Start the Rollover scenario on the GSG
    1. Select scenario from the scenario list
    2. Press Start
  3. Cold reset the receiver
  4. Allow the receiver to get a fix, and report the correct date and time
    1. As observed on the GSG front panel
  5. Observe the rollover from April 6 to April 7, 2019
    1. Verify the receiver still outputs the correct date and time
  6. Cold reset the receiver
    1. Keep the simulation running
  7. Verify that the receiver can obtain the correct date and time
    1. This verifies the receiver’s capability to obtain the correct date and time in the next GPS epoch without having any previous information from GPS.

Will It Happen Again?

GPS is currently undergoing a modernization program to upgrade the signals with new modulation and data message structures. The newer message types (CNAV and MNAV) use 13-bit binary numbers to represent the GPS week number, so the issue should not occur in the future when more receivers are using the newer GPS signals.

Need additional info? Get help from an Orolia technical expert.


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Lisa Perdue
ABOUT THE AUTHOR
Lisa Perdue

Lisa Perdue is a world-leading expert in testing critical GPS and GNSS systems. She has trained hundreds of engineers and technicians who are responsible for high-reliability positioning, navigation and timing (PNT) applications. She took a lead role in the development of the first GNSS Vulnerability Test System and speaks widely on the topic at many industry conferences. Lisa Perdue is currently the Simulation Product Line Director at Orolia, directing the organization's GNSS simulation activities and contributing to its entire portfolio of resilient PNT solutions. She has more than 15 years of navigation and RF systems experience, which includes 10 years of service with the U.S. Navy, where she was a certified master training specialist.