Ultimately, UTC is obtained from TAI by adding or removing a leap second as necessary and maintaining the same ticking of the atomic second. These all need to be processed to compensate for the delay due, for example, to the ionosphere, the gravitational field, or the movement of satellites. Similarly, measurements to compare clocks at distance are based either on global navigation satellite systems (GNSS) or on other techniques, such as two-way satellite time and frequency transfer, or via optical fibres. The algorithm for computing TAI is complex, involving estimation, prediction and validation for each type of clock. How BIPM obtains UTCīIPM first computes a weighted average of all the designated atomic clocks to achieve International Atomic Time (TAI). UTC and the UT1‑UTC difference are transmitted by several time and frequency services regulated by the ITU Radiocommunication Sector ( ITU– R - one of three Sectors of the International Telecommunication Union). Whenever this difference approaches 0.9 seconds, a new leap second is announced and applied in all time laboratories. The International Earth Rotation and Reference Systems Service (IERS) determines and publishes the difference between UTC and the Earth rotation angle indicated by UT1. The scale unit, the second, and the reference time scale UTC are defined and realized under the authority of the General Conference on Weights and Measures ( CGPM), where 64 Member States and 36 Associate States and economies are represented.
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