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The development of atomic clocks has led to many scientific and technological advances such as precise global and regional navigation satellite systems, and applications in the Internet, which depend critically on frequency and time standards. Atomic clocks are installed at sites of time signal radio transmitters. [113]
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]
Since the atomic clocks on board the GPS satellites are precisely tuned, it makes the system a practical engineering application of the scientific theory of relativity in a real-world environment. [16] Placing atomic clocks on artificial satellites to test Einstein's general theory was proposed by Friedwardt Winterberg in 1955. [17]
A modern LF radio-controlled clock. A radio clock or radio-controlled clock (RCC), and often colloquially (and incorrectly [1]) referred to as an "atomic clock", is a type of quartz clock or watch that is automatically synchronized to a time code transmitted by a radio transmitter connected to a time standard such as an atomic clock.
These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.
Satellite clocks are slowed by their orbital speed, but accelerated by their distance out of Earth's gravitational well. Gravitational time dilation has been experimentally measured using atomic clocks on airplanes, such as the Hafele–Keating experiment. The clocks aboard the airplanes were slightly faster than clocks on the ground.
Commercial rubidium clocks are less accurate than caesium atomic clocks, which serve as primary frequency standards, so a rubidium clock is usually used as a secondary frequency standard. Commercial rubidium frequency standards operate by disciplining a crystal oscillator to the rubidium hyperfine transition of 6.8 GHz (6 834 682 610.904 Hz).
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).