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Atomic Sync Signal

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Atomic Timekeeping FAQ:

1. What is the range for receiving the time calibration signal?

  • North America Image:North_america_range.gif
  • Europe Image:Europe_range.gif
  • China Image:China-Atomic-Sync-Signal.jpg
  • Japan Image:Japan.gif

These diagrams show the recommended range for a good signal. But at night the radio signal has ben known to bounce off the ionosphere and sync up G-Shocks hundreds of miles outside of the official range shown in these images.

2. Is there anything I can do to improve time calibration signal reception?

  • Position the G-Shock in a window facing the closest atomic sync signal (Colorado for the US).
  • Signal reception is normally better at night and when the weather is clear.

3. Why is the time off by one hour?

  • This is because the watch is not making allowances for Daylight Saving Time

4. How do I find the 3-letter location code for cities?

How Atomic Clocks Work:

‘Atomic’ G-Shocks

The “Atomic” G-Shock watches maintain superb accuracy over extended periods, and can even change to and from Daylight Saving Time on the correct day. The moniker “Atomic” is a bit misleading, as the atomic clock itself is actually located in a building some distance from the watch. However, it is a useful shorthand which is used by G-philes, and so is used in this book too.

The atomic clock itself is a masterpiece of precision. Rather than relying upon the tolerance of a human-made device such as a pendulum, with all its inherent imperfections, the atomic clock exploits a fundamental physical property of matter. The property chosen by physicists is the frequency of radiation emitted by cesium atoms in a particular state (See

The frequency of the radiation is used as the raw timekeeping source of the clock, exactly as a pendulum is used in an old mechanical clock, or a quartz crystal in a digital watch. (See Appendix C on page 13.)

This extremely accurate time signal is then turned into a digital code which is broadcast from a radio transmitter. It is this code that is picked up by your atomic G, which enables the watch to set itself to the correct time.

There are several of these atomic clock transmitters around the world (see Table B.1). Some G’s can use as many as five or six of these signals.

Table B.1: Atomic Clock Signals

Location Frequency Callsign
USA (Fort Collins, CO) 60kHz WWVB
United Kingdom (Anthorn) 60kHz MSF
Germany (Mainflingen) 77.5kHz DCF77
Japan (Fukushima) 40kHz JJY
Japan (Kyushu Island) 60kHz JJY
China (Xi’an) 2.5/5/10/15MHz BPM

Note that these signals generally use different protocols. (A ‘protocol’ is the electronic version of a language.) If you have a watch that can use the WWVB signal in the USA, then it can receive the MSF signal in the UK, but it can not understand it unless it has been explicitly designed to do so. It’s the same as taking a radio receiver from one country to another; you may be able to receive a radio broadcast, but unless you have been taught the language in that country you won’t know what is being said.

That said, many “Multi-Band” G’s are designed to receive and use more than one signal. They generally do this by referring to the ‘home city’ that the user of the watch sets. If you set your home city to New York City, then it will try to sync with WWVB. If you set your home city to Tokyo, then it will try to sync with JJY.

In addition, some of these signals are physically close to each other, so multi-band G’s can try two different signals. If your home city is London, the watch will try to sync with MSF first. If that fails, then it will try to sync with DCF77 in Germany. If your home city is Berlin, it will try DCF77 first, then MSF. If your home city is Tokyo, it will try both the JJY signals.

The maximum range of reception of the signal varies. These transmitters have different power outputs, and local conditions can have an impact on reception.

Signal reception is generally much better at night than in the day. That’s why the watch tries to sync overnight, when a successful sync is more likely. The reason for this nocturnal improvement is the behavior of the ionosphere (see, the uppermost layer of the atmosphere.

During the day, the ionosphere is ionized by solar radiation. At night, this ionized layer becomes a very effective radio mirror, allowing radio signals to bounce back to Earth. This is the reason why you can listen to shortwave radio stations from distant countries at night, even though they cannot be heard in the day. The radio signals can even bounce between the Earth and the ionosphere several times, allowing the signal to propagate thousands of miles. If it weren’t for this bounce (or ‘DX’ as it is known in radio circles), then the signal would be severely limited in range.

This means that increased solar radiation results in better signal propagation (and thus increased maximum sync range for G’s). The sun does in fact go through an approximately 11-year cycle of radiation output, known as the “sunspot cycle” (because the greater the number of sunspots, the higher the radiation -

2008 is at the end of cycle 23 (numbering began in the 18th century), so is just about the worst time for radio propagation. As the sunspot count increases in cycle 24, the maximum sync range should likewise increase, reaching a maximum in 2013 or 2014. If you want to try for the sync range record, those are the years to make your attempt! Then the sunspots should start to decrease again, with cycle 24 ending in around 2019.

Even at sunspot minimum, some impressive ranges are possible. The Japanese signal JJY can be successfully received in Brisbane and Sydney, Australia, even though they are outside the official range of the signal. Unfortunately, atomic G’s have a pre-set list of signals that ‘can’ be received in the different locations, and Casio has decided that there is no available signal in Australia. So if you set your home city to Sydney, the G will not even try to sync.

Oz G-philes get around this problem by setting their home city to Tokyo overnight, allowing the G to sync, then resetting the home city to Sydney the next morning.

NIST-F1 Cesium Fountain Atomic Clock

NIST-F1, the nation's primary time and frequency standard, is a cesium fountain atomic clock developed at the NIST laboratories in Boulder, Colorado. NIST-F1 contributes to the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time. Because NIST-F1 is among the most accurate clocks in the world, it makes UTC more accurate than ever before.

The uncertainty of NIST-F1 is continually improving. In 2000 the uncertainty was about 1 x 10-15, but as of the summer of 2005, the uncertainty has been reduced to about 5 x 10-16, which means it would neither gain nor lose a second in more than 60 million years! The graph below shows how NIST-F1 compares to previous atomic clocks built by NIST. It is now approximately ten times more accurate than NIST-7, a cesium beam atomic clock that served as the United State's primary time and frequency standard from 1993-1999.

NIST-F1 is referred to as a fountain clock because it uses a fountain-like movement of atoms to measure frequency and time interval. First, a gas of cesium atoms is introduced into the clock's vacuum chamber. Six infrared laser beams then are directed at right angles to each other at the center of the chamber. The lasers gently push the cesium atoms together into a ball. In the process of creating this ball, the lasers slow down the movement of the atoms and cool them to temperatures near absolute zero.

Two vertical lasers are used to gently toss the ball upward (the "fountain" action), and then all of the lasers are turned off. This little push is just enough to loft the ball about a meter high through a microwave-filled cavity. Under the influence of gravity, the ball then falls back down through the microwave cavity.


The round trip up and down through the microwave cavity lasts for about 1 second. During the trip, the atomic states of the atoms might or might not be altered as they interact with the microwave signal. When their trip is finished, another laser is pointed at the atoms. Those atoms whose atomic state were altered by the microwave signal emit light (a state known as fluorescence). The photons, or the tiny packets of light that they emit, are measured by a detector.

This process is repeated many times while the microwave signal in the cavity is tuned to different frequencies. Eventually, a microwave frequency is found that alters the states of most of the cesium atoms and maximizes their fluorescence. This frequency is the natural resonance frequency of the cesium atom (9,192,631,770 Hz), or the frequency used to define the second.

Video of How a Cesium Fountain Works

The combination of laser cooling and the fountain design allows NIST-F1 to observe cesium atoms for longer periods, and thus achieve its unprecedented accuracy. Traditional cesium clocks measure room-temperature atoms moving at several hundred meters per second. Since the atoms are moving so fast, the observation time is limited to a few milliseconds. NIST-F1 uses a different approach. Laser cooling drops the temperature of the atoms to a few millionths of a degree above absolute zero, and reduces their thermal velocity to a few centimeters per second. The laser cooled atoms are launched vertically and pass twice through a microwave cavity, once on the way up and once on the way down. The result is an observation time of about one second, which is limited only by the force of gravity pulling the atoms to the ground.

As you might guess, the longer observation times make it easier to tune the microwave frequency. The improved tuning of the microwave frequency leads to a better realization and control of the resonance frequency of cesium. And of course, the improved frequency control leads to what is one of the world's most accurate clocks.


NIST-F1 was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physics Laboratory in Boulder, Colorado. It was constructed and tested in less than four years. The current NIST-F1 team includes physicists Steve Jefferts, Tom Heavner, and Elizabeth Donley.

NIST Radio Station WWVB

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