The GPS Week-Number Roll-Over problem

Note: The use of the term "GPS" in this document refers to the US-operated NavStar/GPS Global Navigation Satellite System (GNSS). Other GNSSs, such as GLONASS, Galileo or BeiDou, are not affected by the problem described in this article.

Background information

Although the GPS system can be used to determine the date and time with extreme accuracy, the GPS satellite constellation does not actually transmit the full date to GPS receivers. Instead, a ten-bit value called "Week Number" is transmitted every thirty seconds, as part of each subframe of the "Navigation Message". It is the responsibility of the receiver to calculate the date from this value. (The time within the week is transmitted as the number of seconds since midnight on Saturday/Sunday.)

GPS week zero started at the beginning of 00:00:00 UTC on January the 6th, 1980. A ten-bit field can only hold 1024 different values so this system was never going to last forever. Indeed, week 1023 was first reached on August the 15th, 1999. The following week, the GPS satellites populated the Week Number field with a value of zero. (Because GPS time does not recognise leap-seconds, the "roll-over" from week 1023 to week zero actually took place at the end of 23:59:47 UTC on August the 21st.)

The next roll-over will occur on April the 6th, 2019, when the Week Number field will again change from 1023 to zero.

Historic problems

Manufacturers of GPS receivers must each choose a way to determine the correct date from the GPS Week Number. If the chosen method fails, the announced date will be 1024 weeks - about 19.7 years - in the past or, possibly, the future. One common method uses the date of the version of the firmware as a hint, which works well if the receiver is new or regularly updated. A significant problem with this method arises when the firmware is not updated: the receiver can start producing incorrect dates at the 1024-week anniversary of the firmware date. This means that problems can actually appear at any time, irrespective of the actual roll-over date.

The receivers use the calculated date, along with the information about which satellites will be visible at any particular time and date (the GPS almanac), to calculate which satellites should be in view so, if the date is miscalculated, the performance of the receiver may also decrease dramatically.

Many significant problems were reported after the 1999 roll-over and this became known as the GPS "week-number roll-over" or "WNRO" problem. Many manufacturers had to update their receiver firmware but, fortunately, this means that there are likely to be far fewer problems at the second roll-over in 2019.

Güralp equipment

Güralp digitsers have always used self-contained GPS receiver modules. This means that week-number processing has always happened in the receiver rather than in the digitiser. For this reason, none of our digitisers will be affected by the problem, provided that they are used with a conforming GPS receiver.

Güralp have used a number of different GPS receiver modules over the last 25 years. The current modules are manufactured by uBlox and there is a very clear statement on their user forum regarding this issue:

"there won't be any effects at the 2nd weeknumber roll over in 2019". (link)

Previous Güralp GPS receiver modules used Trimble's "Lassen" modules: first the SQ, adopted in September 2003, and, later, the IQ. The Lassen manual states that one version of their firmware

"would experience ... several seconds of position outages"
(It goes on to say that this bug was fixed in later firmware.) The DM24 is designed to be sceptical of any sudden time-changes and will wait at least two minutes before acting on any such change. The symptom, as described, should not affect the operation of Güralp digitsers.

As in all such cases, Güralp continually monitor the situation and, if any issues do arise, we will respond with updates for supported products as fast as possible.

Further Information

If you would like any more information about GPS WNRO or if you have any concerns about your deployed equipment, please contact This email address is being protected from spambots. You need JavaScript enabled to view it. .

Satellite image courtesy of USAF - http://www.losangeles.af.mil/shared/media/ggallery/hires/AFG-100825-006.jpg, Public Domain, Link

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