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December 2020 - updated September 2021 to reflect the withdrawal of the receiver upgrade kits.

Important notice about the GPS Week-Number Roll-Over problem for 2021

The previous version of this page covered events which occurred in 2019 and 2020.

Some problems are yet to appear

A number of Güralp GPS receivers are affected by the GPS WNROThe Week Number Roll Over problem is caused by a fault in the original design of the US Navy's NAVSTAR GPS system. For more details, please see the Background information section, below. problem. The symptoms appear on different dates, depending on the hardware and firmware of the receiver. Many receivers have failed already, requiring mitigating actions. All affected receivers which have not already been upgraded or replaced will start announcing incorrect dates after the 29th of May, 2021.

Am I affected?

Check the colour, the serial number and the history of your receiver:

If you have a white receiver which is not excluded by the rules above, it is probably affected. You can check by unscrewing the domed lid and looking at the circuit-boards inside.

unscrewing the dome lid of a GPS receiver

Removing the lid is easiest if you brace the receiver's spike and connector against the top edge of a desk:

In extreme cases, a strap-wrench may be required.

a strap-wrench

Receivers with one PCBPCB stands for Printed Circuit Board, also known as a circuit card. assembly, like those on the left, are not affected. Those with two or three, stacked above each other like those on the right, are affected.

What will happen?

At the end of 29th day of May, 2021, instead of rolling over to the 30th, an affected receiver will start announcing dates which are 1024 weeks early (i.e. beginning with the 14th of October, 2001).

Sometime after that, the attached digitiser will resynchronise to the incorrect date and your seismic data will have incorrect time-stamps.

What can I do?

This information applies to users of:

DM 24 Mk1 digitisers can no longer be supported.

Users of other Güralp digitisers and instruments should contact for advice.

If you have an affected receiver, the following options are available:

Receiver upgrade kits are no longer available for order as of 24th September, 2021.
CD24 digitisers, including those embedded in instruments such as the 6TD, will need to be running the latest firmware before they can use the new receivers. If your CD24 requires a firmware upgrade, please see the WNRO CD24 firmware upgrade page.

In addition to upgrading replacing the hardware, a firmware upgrade for DM24 Mk3 digitisers is available which should be applied in most cases. The upgrade allows the manual setting of a "pivot date". If a connected GPS receiver reports a date earlier than the configured pivot date, the firmware will add 1024 weeks to the reported date - possibly repeatedly - until the resulting date is later than the pivot date. This upgrade allows the DM24 to handle all future GPS WNRO events.

Although this upgrade addresses the problems anticipated over the next few months and years, we still recommend performing the hardware upgrade replacing the hardware as well.

We are not able to provide an equivalent firmware update for CD24 digitisers, DM24 Mk2 digitisers or the corresponding integrated instruments. For these types of systems, a GPS receiver hardware upgrade replacement is the only solution.

The new receivers offer the following advantages:

A list of applicable systems, a detailed description of the firmware upgrade procedure and a link from which to download the new firmware are available at the WNRO firmware update for DM24 Mk3 and the WNRO firmware update for CD24 web pages.

For more information, please contact or .

Background information

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.

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 second roll-over occurred on April the 6th, 2019, and it is the delayed effect of that with which we are now dealing.

The next roll-over will occur on November the 20th, 2038, when the Week Number field will again change from 1023 to zero. All current Güralp GPS receivers are designed to avoid any problems resulting from that.

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.

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 as a result but some of these solutions were time-limited, leading to the current problems.