That isn’t quite right. UTC was originally (early 1960s) coordinated between the British and Americans. This was related to their existing collaborations in ti

That isn’t quite right. UTC was originally (early 1960s) coordinated between the British and Americans. This was related to their existing collaborations in time and navigation. The definition of the atomic second was established in 1955-1958 by a collaboration between NPL (London) who had the first good atomic clock, and the USNO who were good at the astronometric work to establish the ephemeris second. The USNO and the Greenwich observatory also prepared the Astronomical Almanac together. Later on, as other countries wanted to use the same atomic timescale, the BIH (bureau international de l’heure, based at the Paris observatory) took over. More recently, the BIH was split. The part responsible for atomic time moved to the BIPM (bureau international des poids et mesures, in Sévres just outside Paris) who are responsible for the definition of the second, so it makes sense for them to be responsible for its realisation as well. They maintain atomic time based on contributions from many atomic clocks in many time labs around the world. The astronomy parts of the BIH became the IERS (international earth rotation service), still based at the Paris Observatory. However, a significant part of their practical scientific work is carried out by the USNO. For instance IERS Bulletin A is sent out each week by the USNO. Earth orientation is needed for the GPS as well as for leap seconds.