Radio-controlled and atomic clocks

by Chris Woodford. Last updated: October 7, 2022.

You might have the most expensive watch in the world, but if it's set to the wrong time to begin with, it's no use to you at all. Even really good quartz clocks struggle to keep time to better than a second a day; if they wander out by just a couple of seconds in 24 hours (an amazing accuracy of 99.998 percent), and the errors don't cancel out, that could add up to a minute a month or almost a quarter of an hour a year. That's why most people regularly check their watches against a reliable time signal—like the ones you hear before news broadcasts on radio stations. Now wouldn't it be neat if your watch could listen to those broadcasts and set itself to the right time automatically without you ever needing to worry? That's the basic idea behind radio-controlled clocks and watches, which set their time by super-accurate atomic clocks. Let's take a closer look at what these things are and how they work!

Artwork: Watches and clocks synchronized using radio signals mean anyone can own a watch as accurate as an atomic clock. Radio-controlled clocks and watches were popularized by such companies as Junghans in the late 1980s and early 1990s. Today, many different manufacturers make them and there are millions in use all over the world. Photo by Stefan Kühn courtesy of Wikimedia Commons.

What is an RCC?

An ordinary clock or watch is a time-counting device that adds up the number of seconds, minutes, hours, and days that have passed. But it doesn't actually know what time it is until you tell it: it's not a time-keeping device unless you set it to the right time to start with.

A radio-controlled clock (RCC) is different. It's similar to an ordinary electronic clock or watch but it has two extra components: an antenna that picks up radio signals and a circuit that decodes them. The circuit uses the radio signals to figure out the correct time and adjusts the time displayed by the clock or watch accordingly. Unlike an ordinary clock or watch, an RCC always knows what time it is—you never have to tell it!

The radio signals come from a unique radio "station" that doesn't broadcast any words or music. There's no DJ and no irritating advertisements for car insurance. All the station broadcasts is the time—over and over again—in the form of a special code that only radio-controlled clocks can understand. In the United States, these time signals are broadcast by a station called WWVB operated by the National Institute of Standards and Technology (NIST) from a base near Fort Collins, Colorado. (Other countries have equivalent radio stations. In the UK, for example, the station is called MSF and operated by the National Physical Laboratory, while China's station is called BPC and broadcast by the National Time Service Center.) The NIST time code contains the basic time and date, whether it's a leap-year, whether it's daylight-saving time, and so on and takes about a minute to broadcast in its entirety.

Artwork: The basic concept of RCC radio-controlled clocks: a radio transmitter hundreds or thousands of km/miles from your home beams regular signals to your quartz clock or watch to keep it in time.

Most RCCs synchronize themselves with a time broadcast signal once a day, at night, although some check themselves every few hours. Generally, that gives them an accuracy of better than plus or minus a half second (±0.5s) a day. Another advantage is that they automatically correct themselves for daylight-saving time, leap years, months with different numbers of days, and so on.

It's pretty obvious that an RCC is only going to be as accurate as the time signals it uses to regulate itself. How can you be sure those are accurate? The time-signal radio stations operated in different countries broadcast UTC (Coordinated Universal Time), the officially agreed time used worldwide that's informally known as GMT (Greenwich Mean Time). UTC is maintained by hundreds of atomic clocks (the world's most accurate timekeeping devices) around the world, all of which are synchronized with one another. It's because RCC radio signals are based on time kept by atomic clocks that you'll sometimes see RCC manufacturers describing their products as "atomic" clocks and watches (even though they're really no such thing).

What are atomic clocks?

Atomic clocks are actually quartz clocks—just like the ones you have at home. The difference is that an ordinary quartz clock relies purely on the oscillations of its quartz crystal to count seconds. As we've already seen, the rate at which quartz vibrates is affected by things like ambient temperature, so although a quartz clock is generally very accurate, it doesn't necessarily keep time as well as you might think. By contrast, an atomic clock has an extra mechanism—pulsating atoms—that it uses to keep an ordinary quartz clock to time.

Photo: Before atomic clocks were developed, highly accurate quartz clocks like this one were used as "time standards" to which other clocks could be synchronized. This is the 1941 Quartz Crystal Time Standard, used in the United States until 1949, including during World War II, when it helped to synchronize military forces all around the world. Photo courtesy of National Institute of Standards and Technology Digital Collections, Gaithersburg, MD 20899.

Cesium atomic clocks

This atomic mechanism is based on the idea that atoms have electrons in particular energy states. When an atom absorbs energy, electrons leap to higher energy states and become unstable. They then give out the same energy as photons of light (or some other kind of electromagnetic radiation such as X rays or radio waves), returning to their original or ground state. The cesium atoms used in many atomic clocks have 55 electrons arranged in orbitals. The very outermost electron can oscillate between two different energy states by spinning in two slightly different ways. When it shifts from the higher to the lower of these states, it gives out a photon that corresponds to microwaves with a frequency of exactly 9,192,631,770 Hz (roughly 9.2 billion hertz or 9.2 gigahertz). That means it can be stimulated from its lower to its higher state by exactly the same microwaves. We can use this neat fact to keep a quartz clock to very precise time.

Photo: The NIST-F1 Cesium fountain atomic clock: the amazingly accurate clock by which pretty much every other clock and watch in the United States is set! Photo by courtesy of National Institute of Standards and Technology (NIST) Physics Laboratory.

In a cesium atomic clock, there's a quartz oscillator tuned to exactly the same frequency, 9,192,631,770 Hz, which makes microwaves and fires them at a bunch of cesium atoms. If its frequency is correct, and hasn't drifted at all, these microwaves will have exactly the right amount of energy to shift the electrons in the atoms to their higher energy state. A magnetic detector in the clock measures how many atoms are in the higher and lower energy states. If most are in the higher state, it means most have been excited by the waves from the quartz oscillator. And that means those waves are exactly the right frequency, so the quartz oscillator must be telling time correctly. However, if the atoms are mostly in the lower state, it means the oscillator has drifted away from its correct frequency and isn't giving out the right amount of energy to promote electrons in the cesium atoms. A feedback mechanism in the clock detects this and adjusts the frequency of the oscillator so it's correct again. In this way, the quartz oscillator is constantly regulated so it's always exactly set to 9,192,631,770 Hz. An electronic circuit converts this exact frequency into one-per-second pulses that can be used to drive a relatively ordinary quartz clock mechanism with amazing accuracy. "Amazing" in this case means just that: the best atomic clocks are accurate to within 2 nanoseconds per day, or one second in 1.4 million years!

Photo: Not all atomic clocks are as huge as NIST-F1. Here you can see a miniaturized, chip-scale atomic clock (CSAC) designed for use in missiles and other weapons. Photo by Edric Thompson courtesy of US Army and DVIDS.

Other types of atomic clocks

Other atomic clocks work in broadly the same way but using atoms of different gases to regulate the quartz oscillator. In a hydrogen clock, atoms of hydrogen gas are stimulated with a microwave-frequency laser (maser), but they're less practical because hydrogen is a fairly hard gas to contain. Rubidium clocks are simpler, and therefore more compact and portable; they use microwaves to excite the atoms in rubidium glass. The world's most advanced atomic clocks, such as NIST-7 at the National Institute of Standards and Technology in Boulder, Colorado, use what are called atomic fountains. They use six laser beams to contain cesium atoms, cool them almost to absolute zero, bounce them upward, and let them fall back down through gravity (hence the name "atomic fountain"). This process makes them oscillate between two precise energy states that can be measured, in a broadly similar way to how we explored above, and used to keep a quartz clock to time.

Who invented radio-controlled clocks?

According to Michael Lombardi of the NIST, one of the world's authorities on radio-controlled clocks: "There is no true consensus on who invented the first RCC that could synchronize to a wireless signal." He suggests the first such device may have been the Horophone invented by Frank Hope-Jones (1887–1950) and sold from 1913 by his Synchronome Company of London, England.

I looked through numerous patents covering RCCs on the US Patent and Trademark Office database and the earliest one I found was filed on March 24, 1921 (granted February 5, 1925) by Thaddeus Casner for the Radio Electric Clock Corporation of New York City. Casner explains that his invention covers a "... mechanism by means of which a clock may be periodically corrected by electrical impulses transmitted through space... [by] Hertzian waves [what we now call radio waves]..." You can read a full description and browse numerous detailed drawings (including the one shown here) in US Patent: # 1,575,096: Mechanism for Synchronizing Clocks (via Google Patents).

Artwork: One of the drawings of Thaddeus Casner's early radio-controlled clock. It keeps time using a traditional gear mechanism (which I've roughly indicated in blue), but also uses electromagnets (red) controlled by radio signals to keep the time correct. Artwork courtesy of US Patent and Trademark Office.

Can clocks really be this accurate?

We can now tell time with an incredible degree of accuracy—a nanosecond or two each day. While you might think that's wonderful, it really just swaps one problem for another. In past times, the problem was that we couldn't tell time well enough to keep up with the "natural accuracy" of the real world. So while the heavens turned and the planets whizzed round the Sun, our clocks struggled to keep time as effectively as the natural clock high in the sky.

Today, ironically, it's just the opposite. We now define time not in terms of moving planets but using oscillating atoms. Since 1967, the second has been defined as 9,192,631,770 oscillations of the atom cesium-133 between two energy states (for reasons we saw up above). Keep time with an atomic clock (as the world's various national standards organizations now do) and you'll find that the stars and planets gradually get out of step—because they're not moving accurately enough! Earth's rotational speed isn't constant, for example: it has random blips and it's gradually slowing down. All this means that we have to "correct" our super-accurate atomic clocks, from time to time, so they keep step with the much less accurate world around us. We do that by adding occasional "leap seconds" to the official, scientifically measured world time (International Atomic Time, TAI) so that it always agrees with the official time that people actually use (Coordinated Universal Time, UTC).

A brief history of atomic time

Photo: An early electromechanical mechanism for synchronizing a clock by radio control, developed by U.S. Dunmore Bureau of Standards. Photo by Harris & Ewing courtesy of US Library of Congress.

• 1879: British physicist Lord Kelvin (William Thomson) suggests that the energy transitions of sodium and hydrogen atoms might be used for telling time.
• 1937: American physicist Isidor Rabi pioneers a technique called atomic beam magnetic resonance (ABMR), which uses magnetism to measure properties of an atom. The work earns him the 1944 Nobel Prize in Physics.
• 1945: Rabi suggests how a practical atomic clock can be built.
• 1949: Rabi's student and collaborator Norman Ramsey builds on and improves Rabi's ABMR method with discoveries that gradually lead to the development of the cesium atomic clock. For this work, Ramsey is later awarded the 1989 Nobel Prize in Physics.
• 1949: The US National Bureau of Standards (the official US standards body, renamed the National Institute of Standards and Technology, NIST, in 1988), builds the world's first atomic clock using ammonia gas and a maser (microwave laser). It was not very accurate, but it proves that atomic clocks can be built.
• 1952: NBS builds a prototype cesium atomic clock, NBS-1. Political problems halt further research.
• 1953: Meanwhile, at the National Physical Laboratory in the UK (the British equivalent of NBS/NIST), Louis Essen and Jack Parry begin work on atomic timekeeping.
• 1955: Essen and Parry build the first highly accurate cesium atomic clock, Cesium-1.
• 1956/1958: Atomichron, the first commercial atomic clock, goes on sale.
• 1959: Essen and Parry's improved clock, Cesium-2, can tell time to an unheard accuracy of one second in 2000 years.
• 1967: The International System of Units (SI) is revised to define the second as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom." For the first time in human history, the measurement of time is no longer based on the movement of the stars and planets.
• 1993: NIST build NIST-7, a cesium beam atomic clock used for official timekeeping in the United States until 1999.
• 1999: NIST builds NIST-F1, a replacement for NIST-7 that is 10 times more accurate. Based on cesium fountain technology, it is accurate to about one second in 100 million years.

Find out more

On this website

• Tools, instruments, and measurement
• Pendulum clocks
• Quartz clocks and watches
• Radio control and remote control

On other sites

• WWVB Radio Controlled Clocks An introductory page from the National Institute of Standards and Technology (NIST).
• Radio Controlled Clocks by Michael A. Lombardi, National Institute of Standards and Technology (NIST): This is a longer, more detailed, and more interesting introduction to how RCCs work. It traces the technology all the way back to Marconi's radio experiments in the early 20th century! (PDF format, approx 1 MB).
• NIST-F1 Cesium Fountain Atomic Clock: The Primary Time and Frequency Standard for the United States: A good overview of the United States' most important clock.
• MSF Radio Time Signal: The National Physical Laboratory's official explanation of how the UK's MSF time signal works.
• A Brief History of Atomic Clocks at NIST: A handy timeline that charts the development of atomic clocks in the United States.
• Atomic Clocks: The UK Science Museum narrates the story of atomic clocks from a British perspective (with more credit for Essen). [Archived via the Wayback Machine.]
• Oral-History:Norman Ramsey (1995): A fascinating insight into the development of atomic clocks from one of their pioneers.

Books

• WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers by Michael A. Lombardi. The official NIST guide to developing clocks that use WWVB broadcasts for timekeeping. (PDF format)
• Time: From Earth Rotation to Atomic Physics by Dennis D. McCarthy, P. Kenneth Seidelmann. Cambridge University Press, 2018. A detailed scientific exploration of time and timekeeping.
• The Quantum Beat: Principles and Applications of Atomic Clocks by F.G. Major. Springer, 2007.

News articles

• The Most Important Radio Station You've Never Heard of Marks 50 Years On The Air by Joe Hanson, Wired, 2 July 2013. Marking the first half-century of WWVB, the time-signal radio station run by the NIST.
• Time change marks end of an era: BBC News, 25 March 2007. How radio-controlled clocks using MSF time signals keep Britain synchronized.
• Technology Resets the Clock by Joe Hanson, Wired, 2 July 2013. Marking the first half-century of WWVB, the time-signal radio station run by the NIST.
• How it Works: Clocks That Won't Miss a Second in 20 Million Years by Catherine Greenman, The New York Times, January 17, 2002.