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# 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

GNSS Systems strongly rely on measuring the time of arrival of radio signals propagation. Thus, each GNSS System has its own time reference from which all elements of the Space, Control and User segments are time synchronized, as well most of the GNSS-based applications. The most relevant GNSS time references are briefly described below.

## GPS Time (GPST)

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 0h UTC (midnight) of January 5th to 6th 1980 (6.d0). At that epoch, the difference TAI−UTC was 19 seconds, thence GPS−UTC=n − 19s. GPS time is synchronised with the UTC(USNO) at 1 microsecond level (modulo one second), but actually is kept within 25 ns.[1]

## GLONASS Time (GLONASST)

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[footnotes 1] (i.e.,$GLONASST=UTC(SU)+3^h-\tau$, where $|\tau|\lt 1 milisec.$), but $\tau$ is typically better than 1 microsecond. Note: Unlike GPS, Galileo or BeiDou, GLONASS time scale implements leap seconds, like UTC.[2]

## Galileo System Time (GST)

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, GST(T0), is defined 13 seconds before 0:00:00 UTC on Sunday, 22 August 1999 (midnight between 21 and 22 August).[3]

## BeiDou Time (BDT)

BeiDou Time (BDT) is a continuous time scale starting at 0h UTC on January 1st, 2006 and is synchronised with UTC within 100 ns< (modulo one second).[4]

## Notes

1. ^ The difference between Moscow Time and Greenwich Mean Time (GMT).

## References

1. ^ GPS Interface Specification, Navstar GPS Space Segment/Navigation User Segment Interfaces (IS-GPS-200G), GPS Directorate, Revision G, 21 September 2011
2. ^ GLONASS Interface Control Document, Navigation radiosignal In bands L1, L2 (ICD L1, L2 GLONASS), Russian Institute of Space Device Engineering, Edition 5.1, 2008
3. ^ Galileo Open Service Signal In Space Interface Control Document (OS SIS ICD), GSA, Issue 1.1, September 2010
4. ^ BeiDou Signal In Space Interface Control Document, Open Service Signal (BDS-SIS-ICD-2.0), China Satellite Navigation Office, Version 2.0, December 2013