Chapter 4. Appendices
4.1 Background information
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.
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, when the Week Number field changed again from 1023 to zero.
Manufacturers of GPS receiver chip-sets 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.
4.2 Receiver identification
Güralp systems have used several different types of chip-set in their GPS receivers. These fall into three categories:
Garmin and Motorola chip-sets: These receivers are no longer supported.
Trimble Lassen chip-sets: These receivers need to be upgraded as described in this document.
U-blox chip-sets: These receivers are not affected by the WNRO problem.
The photos below shows a Garmin-based receiver on the left. This unit has a ridged cap. The receivers in the middle are Motorola-based. While not as large as the Garmin-based units, they are significantly larger than modern receivers. A CD (120 mm diameter) is shown to illustrate the scale. The receivers shown on the right contain either Trimble Lassen or U-blox chipsets. They are 59 mm in diameter.
If you have one of the more modern, smaller (59 mm Ø) receivers, there are a number of ways to determine which chip-set is used.
4.2.1 Serial numbers
The serial number of the receiver is engraved on a white plate screwed to the base of the receiver.
If your serial number begins G3…, it has a u-blox chip-set and does not need to be upgraded.
If your serial number begins G1… or G2…, it may have a Trimble chip-set (either iQ or SQ) and may need to be replaced. Another identification technique is required to distinguish in this case.
4.2.2 Visual inspection
The top of the GPS receiver can be unscrewed to reveal the antenna, processor and associated electronics. This is easiest if you brace the receiver's spike and connector against the top edge of a desk. A rubber glove may also help.
Compare your unit to the photographs below, paying particular attention to the antenna, which is the uppermost component and is identified by its silver top:
Left: Lassen SQ chip-set. The antenna has a cream-coloured casing
Centre: Lassen iQ chip-set. The antenna has a black casing
Right: u-blox chip-set. The antenna has a brown casing
4.2.3 NMEA analysis
The situation regarding NMEA analysis is changing frequently as we receive new information from our suppliers. Please consult www.guralp.com/howtos/gps-wnro for the latest information.