International Atomic Time - History

History

Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL). Early atomic time scales consisted of quartz clocks with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. The "Greenwich Atomic" (GA) scale began in 1955 at the Royal Greenwich Observatory. The United States Naval Observatory began the A.1 scale 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder, Colorado. The International Time Bureau (BIH) began a time scale, T_m or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of VLF radio signals. Both the BIH scale and A.1 was defined by an epoch at the beginning of 1958: it was set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding UT2 instant. The procedures used by the BIH evolved, and the name for the time scale changed: "A3" in 1963 and "TA(BIH)" in 1969. This synchronisation was inevitably imperfect, depending as it did on the astronomical realisation of UT2. At the time, UT2 as published by various observatories differed by several centiseconds.

The SI second was defined in terms of the caesium atom in 1967, and in 1971 it was renamed International Atomic Time (TAI).

Also in 1961, UTC began. UTC is a discontinuous time scale composed from segments that are linear transformations of atomic time, the discontinuities being arranged so that UTC approximated UT2 until the end of 1971, and UT1 thereafter. This was a compromise arrangement for a broadcast time scale: a linear transformation of the BIH's atomic time meant that the time scale was stable and internationally synchronised, while approximating UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by public time broadcasts.

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale therefore corresponded to an average of the altitudes of the various clocks. Starting from Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid). Because the clocks had been on average well above sea level, this meant that TAI slowed down, by about 10−12. The former uncorrected time scale continues to be published, under the name EAL (Echelle Atomique Libre, meaning Free Atomic Scale).

The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT). All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant. (The offset is to provide continuity with the older Ephemeris Time.) TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.

The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."