GPS Week Number Roll Over (WNRO)

An accurate accounting of time is of critical importance to nearly every modern organization. Because of its low cost and universal availability, The Global Positioning System (GPS) has become the standard time keeper for many commercial users across the globe.

GPS Week Number Rollover. Image of a satellite.  Image of numbers 1, 0 and 9.

What is the GPS Week Number Rollover?

GPS uses a week counter that enables receivers to calculate the appropriate date.  That week counter uses 10 bits and needs to be reset every 1,024 weeks—roughly every 20 years. On April 6, 2019—the first Saturday in April—the GPS week counter will reset to zero. The good news is that with the modernization of GPS the week number counter will be increased to 13 bits and we will not have another event like this for approximately 157 years.

How do I Prepare for the GPS Week Number Rollover?

According to the GPS Innovation Alliance[1], GPS-enabled equipment with current firmware should experience no issues due to the rollover. The most important step that individuals and organizations can take to prepare for the April 6th rollover is to ensure that all devices using GPS have up-to-date firmware or software.

  1. The GPS Week Number Roll Over (WNRO) is a standard design feature of GPS.  It is not an unknown “bug”.  Devices that conform to the Interface Control Document (current is J) should not be affected.
  2. DHS issued the initial advisory in April 2018. Manufacturers have been testing their equipment and issuing service bulletins.  If you have questions about a specific receiver make and model particularly older than 10 years, DHS recommends that you should contact your manufacturer for further guidance.
  3. Since this occurs nearly every 20 years, CISA recommend that you take these steps to be operationally resilient to any effects of the NWRO.
    1. Take an Inventory of where GPS receivers are found in your operations and systems.
    2. Determine the manufacturer, make, model and if any were built before 2010.
    3. Contact manufacturer (or review its website) for recommendations to update firmware, software or HARDWARE, as needed.
    4. Review and update standard operating procedures and business continuity plans for how to react and report any disruption of GPS equipment or alarms in your systems that indicate a GPS anomaly or loss of signal.
      1. Ensure all end-users and operators of GPS-enabled equipment found in your operations are aware and prepared for the event.
      2. Understand the impacts to your system and operations, if receivers are off-line.
    5. Take this opportunity to update or develop a timing and synchronization plan for your systems (See DHS papers on "Managing Time").

DHS issued the initial notice in April of 2018.   
 
DHS also encourages you to review how time is used in your organizations and develop plans for manage your timing requirements.  There are two documents to assist in this effort.  
 
The first, Corporate Leadership Resilient Timing, is designed for business leaders to explain why managing timing requirements is important. 
 
The second document, Technical Level Resilient Timing, provides an overview for businesses to follow to identify timing requirements and selecting appropriate timing source(s): 

What Resources are Available?

For additional information on the 2019 GPS Week Number Rollover, please visit:

 

[1] Grossman, J. David. “GPS Innovation Alliance Blog.” GPS Innovation Alliance (GPSIA), GPS Innovation Alliance, 19 Mar. 2019, www.gpsalliance.org/blog/gps-week-number-rollover-explained.

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