GPS Date Rollover Affects Older GPS Units

Published by PANBO the Marine Electronics Hub and written by Ben Stein

On 6 April 2019 older GPS units may experience a range of problems because of a limitation in how they handle dates. Those problems could include not being able to receive location data.

It turns out the way dates are stored in older GPS units has a rollover event every roughly 20 years and we are about to experience the second one since the GPS constellations came online. The rollover last happened in 1999, but in the last 20 years the use and dependence on GPS has increased dramatically.

The Department of Homeland Security has published a bulletin on the upcoming rollover, as has the Coast Guard. Both bulletins make reference to a new GPS spec (IS-GPS-200) that deals with these rollovers gracefully. Unfortunately, the bulletins also warn that not all implementations of the new specification work properly and some GPS units might be prone to trouble either at the 4/6/19 rollover or later.

The underlying problem stems from the way GPS units store dates. The original specification for GPS had dates stored by week in a 10-bit field (2^10 or 2x2x2x2x2x2x2x2x2x2) which is 1,024 weeks. 1,024 weeks is 19 years, 36 weeks. Dates for the GPS constellation start at midnight on January 5th, 1980, so the first rollover occurred on August 21, 1999. Now, 19 years and 36 weeks later the same thing will happen again on April 6th, 2019.

Furuno has stated their units will experience the rollover on March 17th, 2019, almost three weeks earlier than the published date in the notices. Furuno has a list of affected units as well as information on how to cold start the units to restore location information. It appears the date will remain wrong, but the location services should function correctly. The affected units appear to all have been introduced in the early 2000s and ended sale around a decade ago.

Raymarine has posted to their forum that they are confident most of their recent hardware (Axiom, a/c/e/eS-Series MFDs and Raystar 130 & 150 GPS receivers) are unaffected.

Garmin has published a support article indicating they don’t expect trouble for their units as long as they are running current software.

Most of my research indicates the trouble is likely to be restricted to some possible restarts or even factory defaulting of equipment followed by potentially incorrect dates. I don’t believe it’s likely much equipment will end up fully broken, but do think it’s prudent to check the accuracy of your GPS equipment around the 4/6/2019 date, especially if you have older navigational equipment.


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