Atomic Time

Fundamentals
Title	Atomic Time
Author(s)	J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain.
Level	Basic
Year of Publication	2011

TAI

TAI stands for International Atomic Time and was established as a reference time by the Bureau International de l'Heure. Its initial epoch was matched to the [math]\displaystyle{ 0^{h}0^{m}0^{s} }[/math] of UT2 scale of January 1[math]\displaystyle{ ^{st} }[/math], 1958, so the difference between TAI and UT2 was [math]\displaystyle{ 0 }[/math] in this epoch.

The TAI second duration was defined in 1967 as the same as the ET second, according to the following sentence: The TAI second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom.

It is realized from several high-precision atomic clocks held at standards institutes in various countries, it is, therefore, a statistical time. There is an elaborate process of continuous comparison, leading to a weighted average of all the clocks involved.

After this definition had been done, the TAI second substituted for ET second as the SI second in 1967. With the next three points in mind, one can obtain the relationship between ET and TAI:

• TAI second = ET second

• The difference between ET time and UT2 time at [math]\displaystyle{ 0^{h}0^{m}0^{s} }[/math] of UT2 scale of January 1[math]\displaystyle{ ^{st} }[/math], 1958 was [math]\displaystyle{ 32.184 }[/math] seconds.

• TAI time = UT2 time at [math]\displaystyle{ 0^{h}0^{m}0^{s} }[/math] of UT2 scale of January 1[math]\displaystyle{ ^{st} }[/math], 1958.

Obviously the relationship between TAI scale and ET scale (or TDT scale afterwards) is a constant offset of [math]\displaystyle{ 32.184 }[/math] seconds.

UTC

UTC stands for Universal Time Coordinated and is the compromise between TAI and UT1. In fact UTC, as an atomic time, is as uniform as the TAI scale can be, but it is always kept closer than [math]\displaystyle{ 0.9 }[/math] seconds with respect to UT1, in order to follow earth's rotation variations. This is accomplished by adding (or subtracting) a certain number of Leap Seconds to TAI. This number, which is refreshed periodically, is provided by the IERS (International Earth Rotation Service) .

This time is very important since all currently used time signals are synchronised with UTC.

Official Time is the one used by all nations of the world. It usually differs by an integer number of hours or half an hours with regard to UTC. This difference is given by time zones and the proper adjustments in summer and winter.

GNSS Time

The GPST, GLONASST and GST are the reference times used in GPS, GLONASS and Galileo systems, 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 [math]\displaystyle{ 00:00 }[/math] UTC (midnight) of January 5th to 6th of 1980 (6[math]\displaystyle{ ^d }[/math].0). At that epoch, the difference TAI[math]\displaystyle{ - }[/math]UTC was [math]\displaystyle{ 19 }[/math] seconds, thence GPS[math]\displaystyle{ - }[/math]UTC=[math]\displaystyle{ n-19^s }[/math] where [math]\displaystyle{ n }[/math] is the number of leap-seconds at the working epoch.

GLONASST is generated by the GLONASS Central Synchroniser and the difference between the UTC (SU) and GLONASST should not exceed [math]\displaystyle{ 1 }[/math] millisecond, plus three hours (i.e., [math]\displaystyle{ t_{GLONASS}=t_{UTC(SU)}+3^h-\tau }[/math], where [math]\displaystyle{ |\tau|\lt 1 milisec. }[/math]).

GST will be maintained by the Galileo Central Segment and synchronised with TAI with a nominal offset below [math]\displaystyle{ 50 }[/math] nanoseconds. The starting time of Galileo system time was not already defined at the time this article was written.