BE AWARE – GPS-SPS COUNTER ROLL FROM 6 APRIL

April 5, 2019

This counter roll over could potentially cause some GPS receivers to show the wrong time and position of the ship or stop functioning.  Ships are advised to check with their GPS manufacturer for further details.  They are also advised to check the IMO website for further information and to read the IMO circular SN.1/Circ.182/Add.1 .

Why this happens: the GPS system is designed with a 10 bits format for counting time and weeks. This gives a counting limit of 1023 weeks. The GPS-SPS system was launched 6 January 1980 and the previous week counter roll over was on the 21 August 1999.

When the GPS system reaches week 1024, the system will start counting again from week zero which will be on 6 April 2019. However, experience from 1999 shows that not all GPS receivers are able to make this transition and therefore will read week zero as August 1999 instead of April 2019. Some GPS receivers can be repaired with an update, but others might potentially remain unusable.

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