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Atomic clock

Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on more advanced physics involving cesium beams and fountains. National standards agencies maintain an accuracy of 10-9 seconds per day, and a precision equal to the frequency of the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized with the passing of day and night based on astronomical observations.

The most accurate atomic clocks are moderated by precise astronomical measurements and the insertion and removal of leap seconds at the very end of June and December.

The first atomic clock was built in 1949 at the U.S. National Bureau of Standards.

Table of contents
1 External links

How they work

Frequency reference masers use glowing chambers of ionized gas, most often caesium, because that is now how the standard second is defined.

Since 1967, the International System of Units (SI) has defined the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Caesium-133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see Cesium standard). Other physical quantities, like the volt and meter, rely on the definition of the second as part of their own definitions.

A microwave radio transmitter fills the chamber with a standing wave of radio waves. The cesium atoms absorb the radio waves and emit light. The radio waves make the electrons move farther from their nuclei. When the electrons are attracted back closer by the opposite charge of the nucleus, the electrons wiggle before they settle down in their new location. This moving charge causes the light, which is a wave of alternating electricity and magnetism.

A photocell looks at the light. When the light gets dimmer, electronics between the photocell and radio transmitter adjusts the frequency of the radio transmitter. This adjustment process is where most of the work and complexity of the clock lies. When a clock is first turned on, it takes a while for it to settle down before it can be trusted.

A counter counts the waves made by the radio transmitter. A computer reads the counter, and does math to convert the number to something that looks like a digital clock, or a radio wave that is transmitted. Of course, the real clock is the original counter.

Public access

  • News radio: The easiest method to access standard time is to listen to the news on radio. National radio news programs set their clocks to the transmissions from the standards departments of their respective countries.

  • Internet: Some standards are available on the net. U.S. Government atomic clocks are available to the public on the NIST website (see below) with a time-of day display accurate to about 0.3S. They also provide downloads of a program to set your computer's clock via the internet or a modem using NTP. The time jitter of NTP ranges from tens of milliseconds to tens of microseconds, depending on the quality of the Internet links and the local computer clock.

  • Telephone: If you lack a radio or computer, the U.S. clocks are also available by phone at 1-303-499-7111 (WWV) or 1-808-335-4363 (WWVH).

  • Radio clock broadcasts
    • U.S. NIST Broadcasts: The U.S.'s NIST clocks are also available on longwave radio, station WWVB at 60KHz (binary coded decimal only) at 30,000W, and by shortwave radio stations WWV (Fort Collins, Colorado) and WWVH (Kekaha, Hawaii, on Kauai) at 2.5, 5, 10 and 15 Mhz at 20,000W and 10,000W respectively.
    • German Broadcasts: In Europe the signal from atomic clock can be obtained by DCF77[?] transmission. DCF77 stands for D=German, C=long wave signal, F=Frankfurt, 77=frequency. This signal is transmitted from Mainflingen, about 25 km south-east of Frankfurt and has a standard frequency of 77.5 kHz. It can be obtained by receivers within about 2000 km radius around Mainflingen.

The variety of frequencies helps reception no matter what the ionospheric weather. A binary coded decimal transmission is made once per second, and on the shortwave stations, a computerized voice announcement is made every ten seconds. The radio frequencies are set by the clocks and are a precision standard, useful for adjusting receivers. The shortwave broadcast information also includes standard time intervals, UT1 time corrections, geophysical alerts (e.g. tsunami warnings), marine storm warnings, and Global Positioning System (GPS) status reports.

  • GPS, Galileo and GLONASS: These satellite navigation systems, have a Caesium atomic clock on each satellite, rated from clocks on the ground. The time available is exact, and some instrument-quality navigation units can serve as local time standards.

One can obtain a close approximation of UTC from using a GPS receiver with suitable timecode output. GPS satellites contain atomic clocks and broadcast a precise time signal based on these clocks, with time corrections to UTC updated from the ground component of the GPS system.

GPS timestamp signals can have a hardware-based precision measured in tens of nanoseconds, given a suitably equipped GPS receiver.


If you need to rate a clock by distance: WWVB and WWV are located in Fort Collins, Colorado, about 100 kilometers north of Denver at approximately 40°40'49"Nx105°02'27"W. (The antennas are all at slightly different locations.) WWV's announcements are in a male voice.

WWVH's time annoucements have a female voice. WWVH is located on Kauai Hawaii, near Kekaha, at about 21°59'16"Nx159°45'50"W.

Mainflingen's DCF77 is at about 50°01'Nx9°00'E.

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