Realization
TT is a theoretical ideal, not dependent on a particular realization. For practical purposes, TT must be realized by actual clocks in the Earth system.
The main realization of TT is supplied by TAI. The TAI service, running since 1958, attempts to match the rate of proper time on the geoid, using an ensemble of atomic clocks spread over the surface and low orbital space of the Earth. TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings that particular groups of atomic clocks showed at the time. Estimates of TAI are also provided in real time by the institutions that operate the participating clocks. Because of the historical difference between TAI and ET when TT was introduced, the TAI realization of TT is defined thus:
- TT(TAI) = TAI + 32.184 s
Because TAI is never revised once published, it is possible for errors in it to become known and remain uncorrected. It is thus possible to produce a better realization of TT based on reanalysis of historical TAI data. The BIPM has done this approximately annually since 1992. These realizations of TT are named in the form "TT(BIPM08)", with the digits indicating the year of publication. They are published in the form of table of differences from TT(TAI). The latest as of October 2011 is TT(BIPM10).
The international communities of precision timekeeping, astronomy, and radio broadcasts have considered creating a new precision time scale based on observations of an ensemble of pulsars. This new pulsar time scale will serve as an independent means of computing TT, and it may eventually be useful to identify defects in TAI.
Read more about this topic: Terrestrial Time
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