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 29^th of May, 2021.

Am I affected?

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

• If your receiver is black, you are not affected.
• If your serial number begins G3…, you are not affected.
• If you have applied a WNRO hardware upgrade to your receiver, you are not affected.

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.

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.

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

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

• The entire receiver can be replaced with a new model which is unaffected by the WNRO problem.

• Güralp can upgrade your receiver with a new PCB assembly. This will involve returning your receiver to the factory.

• Your local distributor may be able to upgrade your receiver with a new PCB assembly. This will involve returning your receiver to your distributor. Please contact your distributor to see whether they offer this service.

• You can purchase a "field upgrade kit" for your receiver, which will allow you to replace its existing PCB assembly with a new one which is immune to the WNRO problem. The procedure is straightforward and requires only a few standard tools. You can browse the GPS receiver upgrade procedure in an on-line HTML version or download a PDF.

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.

The new receivers offer the following advantages:

• Improved leap-second support. With older receivers, the digitiser should be manually notified in advance of each leap-second. The new receivers do this automatically using a custom NMEA sentence.

• Field upgradeable. The operation of the GPS system and the receiver chip-set are beyond our control and there have already been unpredicted problems, such as the GPS WNRO issue and the Trimble leap-second bug. The supervisory processor can be reprogrammed in the field to adapt the receiver should any new problems appear, offering a high degree of future-proofing.

• Lower power consumption: reduced by >50% for ten-pin units with a 15 V supply and by >30% for six-pin units from a 5 V supply (as provided by a CD24).

• Interchangeability. Unlike earlier receivers, the CD24 and DM24 models of the new receivers are identical other than the connector, allowing inventories to be simplified by the use of adapter cables.

• No lithium cell. Earlier receivers relied on built-in lithium cells to maintain settings when powered down. Degraded cells could corrupt the configuration or leak and damage the receiver.

• More channels. The new receivers support 56 channels, compared to 12 for the Trimble iQ or 8 for the Trimble SQ modules.

• Faster start-ups. The new receivers can obtain a fix in a fraction of the time taken by the older models.

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

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 6^th, 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 15^th, 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 21^st.)

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

The next roll-over will occur on November the 20^th, 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.