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GPS week rollover

Satellite orbiting the earth with illuminated cities at night – iStock.com/tifonimages

The Global Positioning System (GPS) is widely used as a reliable and accurate source of time as well as for positioning and navigation. It does, however, need to be employed with care, and one event that may cause problems for some users is the GPS week number roll-over, or reset to zero, that will happen at the end of 6 April 2019.If the event is not handled correctly by the firmware inside a GPS receiver, its date and time outputs may jump to incorrect values when the roll-over occurs.

GPS week number roll-over on 6 April 2019

What is it and what is the risk? What is the GPS week number?

The Global Positioning System (GPS) keeps track of the date and time by counting weeks and elapsed time from the start of the week.

The week number is encoded into the navigation message broadcast by the GPS satellites using 10 bits of information, which results in the week number only having allowed values from 0 to 1023.

 

Why is there a problem?

Because the GPS week number can take only 1024 different values, it resets to zero every 1024 weeks, or approximately 19.6 years. This event is known as the week number roll-over. Any GPS receivers that have not been programmed to handle the reset correctly may start to display an incorrect date and time, and in general will need an upgrade from the manufacturer to correct the problem.

When does the week number roll-over occur?

The origin of GPS time – the start of the first week number 0 – was chosen to be 6 January 1980. The first GPS week number roll-over therefore occurred at 00:00 UTC on 22 August 1999, and some models of GPS receiver experienced problems as a result. The second roll-over will happen at 00:00 UTC on 7 April 2019.

Will all GPS receivers be affected?

The majority of GPS receivers should not display any abnormal behaviour as a result of the roll-over. A properly-designed receiver should be unaffected by the event, and receiver manufacturers would have gained experience of potential problems and solutions following the first roll-over in 1999. However, most GPS equipment in use today has been designed since 1999 and there is a significant risk that the firmware in some GPS devices might not handle the roll-over correctly.

What problems might occur?

The week number roll-over should have no effect on GPS receivers used for positioning and navigation only, and receivers using signals from the other global navigation satellite systems, Galileo, GLONASS and BeiDou, will be unaffected. However, some GPS timing receivers may begin to display an incorrect date, and potentially also an incorrect UTC time, following the roll-over. The problem would persist until the firmware in the receiver is updated.

What can be done to reduce the risk?

Operators of GPS receivers used for timing purposes should contact the equipment supplier for advice, and ensure that the device firmware is updated to the latest version. It may also be appropriate in critical applications to monitor an example of each model of GPS timing receiver in use around the time of the roll-over, or as early as possible on 7 April 2019, to check for any anomalous behaviour.

Will a third GPS week number roll-over occur in another 19 years?

The latest GPS satellites transmit additional civil signals that include a modernized navigation message. This message uses 13 bits to encode the week number rather than 10, which will avoid any further roll-overs occurring in the foreseeable future. By the time the third roll-over will be due in 2038, it is expected that virtually all GPS receivers will be using these new signals.

Further information

EURAMET projects relating to navigation, position and timing include:

-       International timescales with optical clocks (SIB55, ITOC)

-       Compact and high-performing microwave clocks for industrial applications (IND55, Mclocks)

-       Metrology for long distance surveying (SIB60, Surveying)


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