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# Time References in GNSS

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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.,$\displaystyle{ GLONASST=UTC(SU)+3^h-\tau }$, where $\displaystyle{ |\tau|\lt 1 milisec. }$), but $\displaystyle{ \tau }$ 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[4]. In order to be as consistent as possible with UTC, BDT may steer to an interposed frequency adjustment after a period of time (more than 30 days) according to the situation, but the quantity of adjustment is not to allowed more than 5x10E-15 [5].

## Notes

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