GPS Time is a continuous counting time scale beginning at the January 5, 1980 to January 6, 1980 midnight. It is split into two parts: a time of week measured in seconds from midnight Sat/Sun and a week number. The time of week is transmitted in an unambiguous manner by the satellites, but only the bottom 10 bits of the week number are transmitted. This means that a receiver will see a week number count that goes up steadily until it reaches 1023 after which it will “roll over” back to zero, before steadily going up again. Such a week rollover will occur approx. every 20 years. The last week rollover occurred in 1999 and the next one will be in 2019.
The Thunderbolt manual states…
The ThunderBolt adjusts for this week rollover by adding 1024 to any week number reported by GPS which is less than week number 936 which began on December 14, 1997. With this technique, the ThunderBolt will provide an accurate translation of GPS week number and TOW to time and date until July 30, 2017.
The Thunderbolt can now not be trusted to give the accurate date and time and the responsibility of the time correction has now been forced onto the hardware/software which is reading data from the Thunderbolt. I have written a fix and released v1.0.4 which attempts to correct the time. Please visit https://github.com/m1dst/Trimble-Thunderbolt-Monitor and download the latest version. Give it a try and report back any issues.
It is very likely I will have to patch again when the GPS system rolls its week over on April 6th 2019.