GPS Rollover May Affect Some Older GPS Units
On April 6th, 2019 older GPS units may experience a range of problems because of a limitation in how they handle dates. Date calculations in older GPS units need a rollover event roughly every 20 years. We are about to experience the second one since the GPS constellations came online. The rollover last happened in 1999.
The UK Admiralty has published guidance, as has the US Department of Homeland Security (bulletin on the upcoming rollover), as has the US Coast Guard. The bulletins make reference to a new GPS spec (IS-GPS-200).
Users of Global Positioning System (GPS) receivers should be aware that on 6 April 2019, the GPS Week Number will rollover into the third epoch. This may affect Coordinated Universal Time (UTC) derived from GPS. Older GPS receivers that have not been kept up to date may be effected and give incorrect UTC following the rollover. Navigation solution should not be affected, but associated time tags could be incorrect thus still corrupting navigation data.
The underlying problem stems from the way GPS units store dates. The week number in a GPS signal is encoded into the data stream by a 10-bit field. A binary 10-bit word can represent a maximum of 1,024 weeks, which is approximately 19.7 years. Each 19.7 year period is known in GPS terms as an “epoch”. The last week number rollover was August 21, 1999. GPS time is currently in the second Epoch and the next week number rollover is 6 April 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 were introduced in the early 2000s and ended sale around a decade ago.
Raymarine posted to their forum. 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 do not expect issues for their units as long as they are running current software.
Be Prepared for the Rollover
UK MCA recommends the following actions:
- Ensure all recent firmware updates are completed;
- Check with your GPS manufacturer for further information on GPS-dependent equipment. Ientify what action, if any is, required by the operator;
- Identify what effect this could have on your current operations, PNT (position, navigation and timing) solutions and potential issues if failure occurred;
- Operators of such equipment should monitor and, wherever possible, verify their equipment for any errors.