News & Press: Space

GPS week roll-over coming up

30 January 2019   (0 Comments)
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The GPS 10-bit week number will ‘roll-over’ from 1,024 to zero in April - possibly confusing some receivers.

The GPS week number is given in the GPS legacy navigation (LNAV) message of 10 bits. Hence every 1,024 weeks (almost 20 years) it jumps from 1,024 back to 0. The next roll-over will occur 18 seconds before the 0000 UTC boundary  between 6 and 7 April 2019. The 18 s represents the current difference between GPS time and UTC. The last roll-over was on 21/22 August 1999.

It should be noted that the modern civil navigation (CNAV) GPS and other GNSS signals all use a 13-bit week number - their first roll-over will occur on 5/6 January 2137.

During the 1999 roll-over, many receivers became confused and stopped working. It is hoped that such problems will not occur again, but manufacturers and operators should be aware of the possibility.

The USAF Global Positioning System Directorate (GPSD) plans to run a test in February to investigate legacy receiver week roll-over behaviour and analyse any abnormalities exhibited. Details of this testing and an associated questionnaire (due by 4 February) can be found at the US Federal Register