Time References in GNSS

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	==GPS Time (GPST)==		==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 0<sup>h</sup> UTC (midnight) of January 5th to 6th 1980 (6.<sup>d</sup>0). At that epoch, the difference TAI−UTC was 19 seconds, thence GPS−UTC=n − 19<sup>s</sup>. GPS time is synchronised with the UTC(USNO) at 1 microsecond level (modulo one second), but actually is kept within 25 ns.<ref>[~~http~~://~~www~~.gps.gov/technical/icwg/IS-GPS-~~200G~~.pdf GPS ~~Interface Specification~~, Navstar GPS Space Segment/Navigation User Segment Interfaces ~~(IS-GPS-200G)~~]~~, GPS Directorate, Revision G, 21 September 2011~~</ref>		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<sup>h</sup> UTC (midnight) of January 5th to 6th 1980 (6.<sup>d</sup>0). At that epoch, the difference TAI−UTC was 19 seconds, thence GPS−UTC=n − 19<sup>s</sup>. GPS time is synchronised with the UTC(USNO) at 1 microsecond level (modulo one second), but actually is kept within 25 ns.<ref name="GPS ICD 200">[https://archive.gps.gov/technical/icwg/IS-GPS-200N.pdf GPS ICD-200 Revision N, "Navstar GPS Space Segment/Navigation User Segment Interfaces"]</ref>

	==GLONASS Time (GLONASST)==		==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<ref group="footnotes">The difference between Moscow Time and Greenwich Mean Time (GMT).</ref> (i.e.,<math>GLONASST=UTC(SU)+3^h-\tau</math>, where <math>\|\tau\|< 1		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<ref group="footnotes">The difference between Moscow Time and Greenwich Mean Time (GMT).</ref> (i.e.,<math>GLONASST=UTC(SU)+3^h-\tau</math>, where <math>\|\tau\|< 1
	milisec.</math>), but <math>\tau</math> is typically better than 1 microsecond. Note: Unlike GPS, Galileo or BeiDou, GLONASS time scale implements leap seconds, like UTC.<ref>~~[http://www.spacecorp.ru/upload/iblock/1c4/cgs-aaixymyt%205.1%20ENG%20v%202014.02.18w.pdf~~ GLONASS Interface Control Document~~, Navigation radiosignal In bands L1, L2 (ICD L1, L2 GLONASS)]~~, Russian Institute of Space Device Engineering, Edition 5.1, 2008</ref>		milisec.</math>), but <math>\tau</math> is typically better than 1 microsecond. Note: Unlike GPS, Galileo or BeiDou, GLONASS time scale implements leap seconds, like UTC.<ref name="GLONASS ICD">GLONASS Interface Control Document, Russian Institute of Space Device Engineering, Edition 5.1, 2008</ref>

	==Galileo System Time (GST)==		==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).<ref>[~~http~~://ec.europa.eu/~~DocsRoom~~/~~documents~~/~~11870~~/~~attachments~~/1/~~translations~~/~~en/renditions/native~~ Galileo Open Service Signal In Space Interface Control Document (OS SIS ICD)], ~~GSA~~, Issue 1.1, ~~September 2010~~</ref>		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).<ref name="Galileo OS ICD">[https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo_OS_SIS_ICD_v2.2.pdf Galileo Open Service Signal In Space Interface Control Document (OS SIS ICD), EUSPA, Issue 2.2, November 2025]</ref>

	==BeiDou Time (BDT)==		==BeiDou Time (BDT)==
	BeiDou Time (BDT) is a continuous time scale starting at 0<sup>h</sup> UTC on January 1st, 2006<ref> BeiDou Signal In Space Interface Control Document, Open Service Signal ~~(BDS-SIS-ICD-2.0)~~, China Satellite Navigation Office, Version 2.0, ~~December 2013~~</ref>. 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 <ref>[https://nap.nationalacademies.org/read/13292/chapter/10#87 National Academies of Sciences, Engineering, and Medicine. 2012. Global Navigation Satellite Systems: Report of a Joint Workshop of the National Academy of Engineering and the Chinese Academy of Engineering. Washington, DC: The National Academies Press]</ref>.		BeiDou Time (BDT) is a continuous time scale starting at 0<sup>h</sup> UTC on January 1st, 2006<ref name="BDS B2b ICD">[http://en.beidou.gov.cn/SYSTEMS/ICD/202008/P020231201537880833625.pdf BeiDou Navigation Satellite System Signal In Space Interface Control Document Open Service Signal B2b, China Satellite Navigation Office, Version 1.0, July 2020]</ref>. 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 <ref>[https://nap.nationalacademies.org/read/13292/chapter/10#87 National Academies of Sciences, Engineering, and Medicine. 2012. Global Navigation Satellite Systems: Report of a Joint Workshop of the National Academy of Engineering and the Chinese Academy of Engineering. Washington, DC: The National Academies Press]</ref>.

	==Notes==		==Notes==

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Time References in GNSS