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International Atomic Time (abbreviated TAI, from its French name temps atomique international [1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. [2] TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. [3]
The time from an atomic clock onboard each satellite is encoded into the radio signal; the receiver determines how much later it received the signal than it was sent. To do this, a local clock is corrected to the GPS atomic clock time by solving for three dimensions and time based on four or more satellite signals. [11]
The unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock, [3] but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it. TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC).
Atomic clocks are installed at sites of time signal radio transmitters. [113] They are used at some long-wave and medium-wave broadcasting stations to deliver a very precise carrier frequency. [114] Atomic clocks are used in many scientific disciplines, such as for long-baseline interferometry in radio astronomy. [115]
Increasingly, after the calibration in 1958 of the cesium atomic clock by reference to ephemeris time, [17] cesium atomic clocks running on the basis of ephemeris seconds began to be used and kept in step with ephemeris time. The atomic clocks offered a further secondary realization of ET, on a quasi-real time basis [25] that soon proved to be ...
[6] [7] At the time, local mean time was used to set clocks, meaning that every place used its own local time based on its longitude because the time was measured by locally observing the Sun. Philippine Standard Time was instituted through Batas Pambansa Blg. 8 (that defined the metric system ), approved on December 2, 1978, and implemented on ...
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Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. [1] Older systems use telescope observations.