GPS / GNSS Week Number Rollover

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 5-6 April 2019. Note that this corresponds to 23:59:42 UTC on 5 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 Saturday 6 April 2019.

In Australia this corresponds to 11:59:42 AEDT on Saturday 6 April 2019.