Page "International Atomic Time" Paragraph 7

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 < sub > m </ sub > 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.