The International System of Units (SI) defines one second as the time corresponding to 9 192 631 770 periods of the radiation emitted by a Cesium-133 atom at the ground state. To achieve the highest possible level of accuracy, the International Bureau of Weights and Measures (BIPM) combines the output of more than 400 commercial atomic clocks and atomic fountains over 50 national laboratories worldwide to determine TAI, the International Atomic Time.
TAI is constantly compared to the rotation of the Earth (a time scale based on this system is called UT1) and since the Earth is a slow clock, the two time scales slowly diverge. Before the difference between the two scales reaches 0.9 seconds, a leap second is added, on the advice of the International Earth Rotation and Reference Systems Service (IERS). The time scale obtained in such a way is called UTC (Universal Time Coordinated) is maintained by the BIPM and is the time scale that forms the basis for the coordinated dissemination of standard frequencies and time signals. This time scale is considered as a “paper” time scale, due to the fact that it is the result of computations, but it is not represented by real (i.e. electric) signals. In this sense, for the practical activities carried out at national and international level, real approximation of UTC, normally named UTC(k), are required. These approximations, normally realized at National Metrology Institutes (NMIs) “Time and Frequency” laboratories, have to be all kept within 100 ns with respect to UTC, as per BIPM recommendations.
For Italy, the local realization of UTC is called UTC(IT) and is generated at INRIM Time and Frequency Laboratory.