GPS Week Number Rollover

Mar 24, 2022 by Corey D. Ranslem

An interesting phenomenon happened on Jan. 2, 2022 — something that happens only once every 19.6 years. The GPS week number rollover took place. The rollover occurs because of how the system stores the date, and it happens every 1,024 weeks.

This is how it works: The GPS system broadcasts a time and date, including a week number counter, which ranges from zero to 1,023. Once the week counter reaches 1,023, it rolls back over to zero. GPS software that is not properly coded to anticipate the rollover will show issues with the date and could stop working altogether. Some units roll the actual date back by 20 or 40 years. The last time this phenomenon took place was 12 a.m. on Aug. 22, 1999, but GPS usage wasn’t as widespread then as it is today, so that rollover didn’t cause a lot of problems.

There are several GPS units that have been affected across maritime, aviation, and other industries. Within the maritime industry, Furuno GPS or GMDSS devices with built-in Furuno GPS experienced the rollover phenomenon. According to the Furuno website, conducting a cold start of the unit will not fix the problem. You will be required to do a software update, and in some cases the GPS unit will need to be replaced.

The U.S. Coast Guard and MCA put out advisories late in 2021 warning of the upcoming rollover issues. The MCA advisory specifically mentioned the Furuno units. If you notice the date and time of your GPS is not correct, you should immediately contact the manufacturer to determine the best course of action. Incorrect date and time on the GPS could cause issues with safe navigation.

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