As Roll Over Takes Effect  April 6

The IMO has reminded maritime users of the Global Positioning System Standard Positioning Service (GPS-SPS) to check their systems ahead of the week counter roll over on April 6, 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 Some outdated GPS receiver systems may cease to function properly – with potentially serious impacts on navigation.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 January 6, 1980 and can only count 1,023 weeks. The previous roll over occurred on August 21, 1999 when systems reset and began counting towards week 1,023 again. When the GPS system reaches week 1,024, 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 If the GPS receiver is outdated or has not been properly updated, the receiver will revert on April 6, 2019 to reading the week zero as August 1999.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.