IMO Urges Users Of GPS-SPS To Check Their Systems Ahead Of GPS Roll-Over

April 9, 2019 IMO

IMO Urges Users Of GPS-SPS To Check Their Systems Ahead Of GPS Roll-Over


Maritime users of the Global Positioning System Standard Positioning Service (GPS-SPS) are urged to check their systems ahead of the week counter roll over on 6 April 2019. Some outdated GPS receiver systems may cease to function properly – with potentially serious impacts on navigation.


The roll over occurs because the GPS system transmits time to GPS receivers using a format of time and weeks as a 10-bit value, which started from 6 January 1980, and can only count 1023 weeks. The previous roll over was on 21 August 1999, when systems reset and began counting towards week 1023 again. When the GPS system reaches week 1024, the system will revert back to week zero.


Some GPS receivers are known to be unable to make the transition from week 1023 to 1024. If the GPS receiver is outdated or has not been properly updated, the receiver will revert on 6 April 2019 to reading the week zero as August 1999. The internal clocks of these GPS receivers will experience a lack of absolute reference and may give the wrong time and position or may lock up permanently. Some of these GPS receivers are repairable with upgrades and others will become unusable.

Maritime users are advised to check the status of their receiver with their GPS manufacturer. IMO has issued a safety of navigation circular SN.1/Circ.182/Add.1 warning maritime users to take action for the roll over.

The GPS-SPS has been recognized by IMO as a component of the world-wide radionavigation system since 1996.




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