The time in local time zones can be expressed as an offset from UTC. The time-of-day expressed by UTC is the time at the prime meridian (0° longitude) located near Greenwich, England. The international contributors to TAI, and their offsets, are published in the BIPM Circular-T, which is a document updated monthly. However, NIST and other laboratories maintain real-time versions of UTC that are often within just a few nanoseconds of the UTC calculations. No clock keeps the "official" version of UTC, because TAI and UTC are "paper" time scales that can only be calculated after all of the data from the international contributors are received. We publish the current difference between UTC and TAI in our Time Scale Data and Bulletin Archive.įor more information, see the section on leap seconds. This correction keeps UTC in conjunction with the apparent position of the Sun and the stars, and it is the standard used for all general timekeeping applications. TAI is computed by the International Bureau of Weights and Measures (BIPM) located near Paris, France.Ĭoordinated Universal Time (UTC) is based on TAI, but it is adjusted by leap seconds to account for the difference between the definition of the second and the rotation of Earth. While the stability of TAI is achieved by this weighted average, the accuracy of TAI is derived from data from primary frequency standards, which are clocks built at several national metrology institutes. Thus, if N = 300, the maximum weight is 0.83 %. The most stable clocks receive the most weight in the calculation, but the maximum weight assigned to any clock is limited to A / N, where A = 2.5 and N is the number of clocks. These clocks are located at more than 60 timing laboratories around the world. International Atomic Time (TAI) is an international time scale that is computed by taking the weighted average of more than 300 atomic clocks. What are International Atomic Time (TAI) and Coordinated Universal Time (UTC)? It is distributed through the NIST time and frequency services and continuously compared to the time and frequency standards located around the world. UTC(NIST) serves as a national standard for frequency, time interval, and time-of-day.
The clocks in the UTC(NIST) time scale also contribute to the International Atomic Time (TAI) and Coordinated Universal Time (UTC). The most stable clocks are assigned the most weight. The outputs of the clocks are combined into a single signal by using a weighted average. The number of clocks in the time scale varies, but is typically around ten.
The UTC(NIST) time scale comprises an ensemble of cesium beam and hydrogen maser atomic clocks, which are regularly calibrated by the NIST primary frequency standard. UTC(NIST) is the coordinated universal time scale maintained at NIST. Why is UTC used as the acronym for Coordinated Universal Time instead of CUT?.What are International Atomic Time (TAI) and Coordinated Universal Time (UTC)?.How is UTC(NIST) related to Coordinated Universal Time (UTC) International Atomic Time (TAI), Greenwich Mean Time (GMT), USNO time, GPS time and Zulu time? Two-Way Satellite Time and Frequency Transfer (TWSTFT).
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