IMO has issued a warning to maritime users of the Global Positioning System Standard Positioning Service (GPS-SPS) urging them to check their systems ahead of the week counter roll over, which took place on 6 April.
The roll over happens because the GPS system transmits time to GPS receivers using a format of time and weeks as a 10-bit value, which began on 6 January 1980, and can only count 1023 weeks. When the GPS system reaches week 1024, the system reverts back to week zero.
However, some GPS receivers cannot properly make the transition from week 1023 to week 1024. If the receiver is outdated, or hasn’t been updated as required, it will have reverted on 6 April to reading week zero as August 1999 – the date of the previous roll over.
The internal clocks of these faulty GPS receivers will experience a lack of absolute reference and may give the wrong time and position, or may lock up altogether.
While some receivers are repairable with upgrades, others will have become unusable. IMO has advised maritime GPS users to check with the device’s manufacturer regarding the status of their receiver.
The GPS-SPS has been recognized by IMO as a component of the world-wide radionavigation system for 23 years.