Navigation Systems

4/12/2018: Upcoming 2019 Global Positioning System week number rollover event

The Department of Homeland Security released a bulletin to assist federal, state, local, and private sector organizations understand the effects of and make preparations for the April 6, 2019 GPS Week Number Rollover event.

Critical Infrastructure owners and operators and other users who obtain Coordinated Universal Time (UTC) from Global Positioning System devices should be aware of the GPS Week Number rollover events and the possible effect a GPS Week Number rollover event may have on the reliability of the reported UTC. The legacy GPS navigation message has a 10 bit parameter that represents Week Number. Thus, the Week Number parameter in the GPS navigation message “rolls over” to zero every 1024 weeks starting from 0000Z January 6, 1980.

The next Week Number rollover will occur April 6, 2019. The IS-GPS-200H interface specification identifies both the 10 bit Week Number parameter and the Week Number rollover events. A GPS device that conforms to the latest IS-GPS-200 and provides UTC should not be adversely affected. However, tests of some GPS devices revealed that not all manufacturer implementations correctly handle the April 6, 2019 Week Number rollover. Additionally, some manufacturer implementations interpret the Week Number parameter relative to a date other than January 5, 1980. These devices should not be affected by the Week Number rollover on April 6, 2019 but may experience a similar rollover event at a future date.


Critical Infrastructure and other owners and operators are strongly encouraged to:

1. Investigate and understand their possible dependencies on GPS for obtaining UTC.
2. Contact the GPS manufacturers of devices they use to obtain UTC in order to understand the manufacturers’ preparedness for the April 6, 2019 WN rollover and actions required by Critical Infrastructure and other owners and operators to ensure proper operation through the April 6, 2019 Week Number rollover.
3. Ensure that the firmware of such devices is up-to-date.

For full details and background information on this issue, view DHS’s bulletin.

This blog is not a replacement or substitute for the formal posting of regulations and updates or existing processes for receiving formal feedback of the same. Links provided on this blog will direct the reader to official source documents, such as the Federal Register, Homeport and the Code of Federal Regulations. These documents remain the official source for regulatory information published by the Coast Guard.

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