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Time References in GNSS
Regarding the GNSS, the GPS Time (GPST), GLONASS Time (GLONASST) and Galileo System Time (GST) are the reference times used in GPS, GLONASS and Galileo applications, respectively.
GPST is defined by the GPS Control segment on the basis of a set of atomic clocks aboard the satellites and in the Monitor Stations. It is synchronised with the UTC(USNO) at nanosecond level. The origin epoch of GPS time is 0h UTC (midnight) of January 5th to 6th of 1980 (6d· 0). At that epoch, the difference TAI−UTC was 19 seconds, thence GPS−UTC=n − 19s.
GLONASST is generated by the GLONASS Central Synchroniser and the differ- ence between the UTC(SU) and GLONASST should not exceed 1 millisecond plus three hours1 (i.e., tGLONASS = tUTC(SU)+3h−τ, where |τ| < 1milisec.).
GST will be maintained by the Galileo Central Segment and synchronised with TAI with a nominal offset below 50 ns. The starting time of Galileo system is not already defined.