Galileo runs on Galileo System Time (GST), which is under responsibility of the Galileo Mission Segment (GMS). GST is fixed on the ground at the Galileo Control Centre in Fucino (Italy) by the Precise Timing Facility (PTF), based on the average of different atomic clocks.
The GST start epoch is 00:00 UT on Sunday 22nd August 1999 (midnight between 21st and 22nd August). At the start epoch, GST was ahead of UTC by 13 leap seconds. Since then 3 additional leap seconds have been introduced (31 Dec. 2005 and 2008, and 30 Jun. 2012). Therefore, currently GST is ahead of UTC by 16 seconds.
The GST within the Galileo message is given as a 32 bit binary number composed of two parameters as follows:
- The Week Number (WN) is a 12 bits counter that gives the sequential week number from the origin of the Galileo time. It covers 4096 weeks (about 78 years). Then the counter is reset to zero to cover additional period modulo 4096.
- The Time of Week (TOW) defined as the number of seconds, in 20 bits, that have occurred since the transition from the previous week. The TOW covers an entire week from 0 to 604799 seconds and is reset to zero at the end of each week.