Time References in GNSS

Fundamentals
Title	Time References in GNSS
Author(s)	J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain.
Level	Basic
Year of Publication	2011

Regarding the GNSS, the GPS Time (GPST), GLONASS Time (GLONASST), Galileo System Time (GST) and BeiDou Time (BDT) are the reference times used in GPS, GLONASS and Galileo applications, respectively.

GPS Time (GPST) is a continuous time scale (no leap seconds) defined by the GPS Control segment on the basis of a set of atomic clocks at the Monitor Stations and onboard the satellites. It starts at 0^h UTC (midnight) of January 5th to 6th 1980 (6.^d0). At that epoch, the difference TAI−UTC was 19 seconds, thence GPS−UTC=n − 19^s. GPS time is synchronised with the UTC(USNO) at 1 microsecond level (modulo one second), but actually is kept within 25 ns.

GLONASS Time (GLONASST) is generated by the GLONASS Central Synchroniser and the difference between the UTC(SU) and GLONASST should not exceed 1 millisecond plus three hours (i.e.,[math]\displaystyle{ GLONASST=UTC(SU)+3^h-\tau }[/math], where [math]\displaystyle{ |\tau|\lt 1 milisec. }[/math]), but [math]\displaystyle{ \tau }[/math] is typically better than 1 microsecond. Note: Unlike GPS, Galileo or Compass, GLONASS time scale implements leap seconds, like UTC.

Galileo System Time (GST) is a continuous time scale maintained by the Galileo Central Segment and synchronised with TAI with a nominal offset below 50 ns. The GST start epoch is 0^h UTC on Sunday, 22 August 1999 (midnight between 21 and 22 August).

BeiDou Time (BDT) is a continuous time scale starting at 0^h UTC on January 1st, 2006 and is synchronised with UTC within 100 ns< (modulo one second), [BeiDou-SIS-ICD-Test, 2011].

References

• BeiDou-SIS-ICD-Test, 2011. Technical report. BeiDou Navigation Satellite System Signal In Space. Interface Control Document (Test version), December 2011.