Older GPS receivers could believe it is 1999 again

April 5, 2019

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On Saturday at 7:59:42 p.m. EDT (23:59:42 UTC), the constellation 31 active Global Positioning System (GPS) satellites will roll over the GPS week number, putting older receivers at risk of miscalculating the date back to 1999. This roll-over event is being called Y2K19.

Your GPS in your car or mobile phone should be just fine, especially if you apply system updates as they are available. Manufacturers of older, more specialized receivers, particularly those that have not had a firmware in 10-15 years are at risk, but they have been warning users to update for years. The GPS system works by broadcasting the location and time from each satellite. Each receiver uses trilateration to determine your location. Your phone determines the distance to each satellite based the time broadcast and adjusts for Einstein’s Theories of Special and General Relativity. (Time is passing about 38,700 nanoseconds faster each day on each satellite, but that’s a topic for another day).

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An algorithm in the receiver then draws virtual circles around each satellite based on those distances. Your location is where the circles overlap. Add a third dimension to that algorithm (virtual spheres instead of circles) and that overlap point provides your elevation as well.

But that time code being is not something human readable like 2019-04-06T23:59:42+0000. Satellites are sending a series of ones and zeros representing GPS Time which began Jan. 6, 1980, at midnight UTC. There are 10 bits for the week number, enough for 1024 weeks or nearly 20 years, and 16 bits for the seconds that have passed within that week. The GPS week number last rolled over in 1999 with few problems. But there are lot more GPS receivers out there today, in a variety of environments.

The roll-over risk is greatest in the scientific community. The logistics of keeping measuring stations, particularly portable ones placed by seismologists and atmospheric scientists in remote locations, can be complicated. These instruments are there to autonomously gather data over long periods of time using the GPS system for position and/or timestamp data with high accuracy.

Receiver manufacturers and Homeland Security have been issuing information on the potential problem and how to update older receivers for years.

Systems manufactured since 2010 follow the latest standard which uses a 13-bit week number, pushing the next rollover well past 2100. You’ll probably update your phone before then. Images:

GPS Block III-A satellite (image: US Air Force/Lockheed Martin)

GPS receivers use trilateration to convert time signals from multiple satellites to your longitude and latitude on Earth. (image courtesy: GISGeography.com)


GPS.gov: https://www.gps.gov/