International time and frequency coordination

The significant advances in the development of international coordination in time determination and dissemination are briefly reviewed. Before 1955, time was determined exclusively by astronomical observations and international cooperation was primarily the concern of the International Astronomical Union (IAU). In the past 15 years, an independent time scale, based on atomic phenomena, has been realized. Astronomical and atomic time each have their field of application, and both require international cooperation in order to achieve the standards of accuracy now demanded. Responsibility for coordination has been assigned to the Bureau International de l´Heure (BIH). The adoption by the General Conference of Weights and Measures (CGPM) of an atomic definition of the second as the basic time interval in the international system of units has led to the establishment of an international scale of atomic time (IAT). In the dissemination of time by radio, a compromise system designed to meet the needs for both astronomical and atomic time was introduced experimentally and later recommended for general adoption by the International Radio Consultative Committee (CCIR). Following the recommendations of the international working party, an improved compromise system was inaugurated on January 1, 1972 and is now being followed by all major time services. Developments of new observational techniques now supplement conventional astronomical methods and offer a further useful field for international cooperation.