GPS week numbering and how it is processed within the receiver
If you have a Septentrio GPS (GNSS) receiver, feel free to also consult our Support Notification document about the upcoming GPS week number rollover.
Questions & Answers (Q&A)
What is the GPS week number and why does it “roll over”?
GPS uses its own timescale: “GPS time”. The starting point (time zero) of GPS time was chosen to be midnight of 5-6 January 1980. The GPS satellites transmit the time in two parts: the week number (the number of weeks since time zero) and the elapsed number of seconds within that week.
In the 1970s, when the GPS system was originally designed, 10 bits were assigned to the week number in the navigation data. As a result, the transmitted week number can have a value from 0 to 1023 (= 210 - 1). After the maximum value is reached, the week number “rolls over” to 0 and starts counting again. So the transmitted week number jumps back to zero every 1024 weeks (about 20 years).
Is the GPS week number rollover an unexpected event?
No. It is the result of a system design choice which is well known and documented. A specific note highlighting the fact that the week number rolls over has been part of the official GPS signal specification since at least 1993. Before both rollovers so far, the U.S. Government has issued an official statement to remind of the upcoming rollover. So, both receiver manufacturers and users are well aware.
When does the rollover happen?
The first rollover happened at midnight of 21-22 August 1999. The second rollover will happen at midnight (GPS time) of 6-7 April 2019. Note that this corresponds to 23:59:42 UTC on 6 April 2019, because GPS time is unaffected by so-called leap seconds. In Central European Summer Time, this corresponds to 01:59:42 CEST on Sunday 7 April 2019.
Couldn’t the GPS system use more than 10 bits for the week number?
Most GPS satellites are already transmitting a 13-bit week number, which will only roll over in the year 2137. Septentrio fully supports the “modernised” GPS signals, called “L5”, “L2C”, and “L1C”. Only the legacy “L1 C/A” GPS signal contains a 10‑bit week number.
What sector would be more likely to get affected if the GPS week number rollover would cause difficulties?
In case of issues related to a week number rollover, the most severe effect would probably be on applications relying on the date and time provided by an affected GPS receiver. In fact, time synchronisation based on GPS (and other Global Navigation Satellite Systems) is used in many critical applications including wireless communication networks, financial transactions and power grid synchronisation. Only old receivers which have never received software updates are affected. Critical applications should not be using obsolete equipment.
How do Septentrio receivers deal with a rollover of the 10-bit GPS week number?
The fact that the legacy 10-bit GPS week number “rolls over” every 1024 weeks introduces an ambiguity of the transmitted time of about 20 years. This is taken into account in the receiver software. The ambiguity can easily be solved by assuming that the current date cannot be before a hard-coded date (for example, the software release date). Using this approach, there is a window of about 20 years during which the rollover has no impact whatsoever. Receivers should have a sufficiently recent software version to ensure that they resolve the 20‑year ambiguity correctly. Because Septentrio regularly issues (free of charge) software updates for its receivers, this approach solves the problem for all practical purposes.
Septentrio receivers also fully support the modernised GPS signals containing a 13-bit week number, as discussed above. In addition, Septentrio receivers not only use the American GPS system but can also make use of all other Global Navigation Satellite Systems. The week number transmitted by the European Galileo system, for example, will only roll over in the year 2078. So, nowadays, a rollover of the “legacy” 10-bit GPS week number is no longer a concern.