Cesium Clock

Jan 1, 2022
The cesium fountain clock located in Fort Collins Colorado is certainly the most accurate and available time keeper we have. My question is when this clock was started for the first time how did they know exactly what time to set it to?
Jan 27, 2020
Signals were sent out by the Navy by 1904 from the US Naval Observatory, and in 1923 the National Bureau of Standards (NBS; now the National Institute of Standards and Technology, NIST) began broadcasting.

Until World War II, the U.S. Navy was the nation’s official timekeeper; NBS began broadcasting time signals in 1945.

NIST began building the new station on a large site north of Fort Collins in 1962. WWVB had begun as an experimental station in Boulder in 1956.

The site near Fort Collins was chosen for its relatively central location and flat ground. An atomic clock near Boulder is close to the Flatirons, which somewhat complicates transmission of the signals. Surprisingly, according to several sources, the alkalinity of the soil was also a factor in the placement of the stations. Alkaline soil aids in the grounding of the antennas.

NIST radio station WWVB, which today serves as the synchronization source for tens of millions of radio controlled clocks, began operation from its present location near Fort Collins, Colorado at 0 hours,
0 minutes Universal Time on July 5, 1963.
The two antenna arrays are powerful enough to reach nearly the entire contiguous country. The signal is weakest on the East Coast, where density of infrastructure interferes with reception. But the engineers who oversee the process continue working on ways to resolve the problem.

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Matt Deutsch, chief technician at the atomic clock radio station, WWVB, near Fort Collins, CO, explains the station's antenna system. Coloradoan Library

Because the United States encompasses several time zones, the broadcast is sent in coordinated universal time, allowing local clocks to adjust for time differences. The signal also lets radio-controlled clocks know when daylight saving time begins and ends. In 1965, a time code was added, allowing clock designs to incorporate automatic synchronization.

Once upon a time, people kept track of time by the passage of the sun across the sky. Today, we know that atomic clocks keep time much more accurately and can be adjusted to allow for the slight slowing of the Earth’s rotation around the sun. When necessary, the clock is stopped to add a leap second to accommodate that alteration.

The actual process of broadcasting is intricate, technical and complex, far beyond this lay person’s comprehension. Suffice it to say that the broadcast is low frequency and the power of the transmission has increased over the years.

At one point, scientific research and discussion centered on the length of a second, with a decision eventually reached.

This little-known slice of local history — those blinking red lights — have an impact much greater than one might initially think.

Contact history author Barbara Fleming at fcwriter@frii.com.

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The cesium fountain atomic clock. Credit: Copyright Geoffrey Wheeler

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 January 2013, the uncertainty has been reduced to about 3 x 10-16, which means it would neither gain nor lose a second in more than 100 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.

accurcy7 cesium clock.jpeg

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.

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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.

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.

Vladislav Gerginov

WWVB continuously broadcasts digital time codes on a 60 kHz carrier that may serve as a stable frequency reference traceable to the national standard at NIST. The time codes are synchronized with the 60 kHz carrier and are broadcast continuously in two different formats at a rate of 1 bit per second using pulse width modulation (PWM) as well as phase modulation (PM).

In the first of the two formats, based on PWM, which has been in use for several decades, the carrier power is reduced by 17 dB at the start of each second and restored to full power 0.2 s later for a binary "0", 0.5 s later for a binary "1", or 0.8 s later to convey a position marker. The pulse-width modulated time code contains the year, day of year, hour, minute, UT1 time correction and flags that indicate the status of Daylight Saving Time, leap years, and leap seconds, as listed in the legacy WWVB time code formatdescription and detailed in NIST Special Publication 432 (NIST Time and Frequency Services).

Since October 29, 2012, NIST Radio Station WWVB has been broadcasting a phase modulated (PM) time code that has been added to the legacy AM/pulse-width-modulation signal. This enhancement to the broadcast provides significantly improved performance in new products that are designed to receive it. Existing radio-controlled clocks and watches are not affected by this enhancement and continue to work as before.

In the PM format, binary-phase-shift-keying (BPSK) modulation is used, wherein the carrier's phase is unaffected when conveying a "0" and is inverted (i.e. 180-degree shifted) when conveying "1". This time code, also operating at a rate of 1 bit/sec, is delayed by 0.1 s with respect to the first time code described above, such that 180-degree transitions in the carrier phase can only occur 0.1 s after the 17 dB power reduction that is created by the pulse-width-modulation. The phase-modulated information may take several different forms, with the basic one having a frame duration of one minute, as in the legacy AM/PWM broadcast. The data content, physical properties and scheduling features of this BPSK time code may be found here:

Enhanced WWVB Broadcast Format (revised 2013-11-06)

Note: disciplined oscillator products that track and lock to the 60 kHz WWVB carrier and were designed to work as frequency standards, will not work with the PM signal and have become obsolete. Radio-controlled clocks that are based on synchronous AM demodulation (lock to the carrier), such as the Spectracom NetClock and receivers manufactured by True Time during the 1970s and 1980s, have also become obsolete.

WWVB uses two identical antennas that were originally constructed in 1962, and refurbished in 1999. The north antenna was originally built for the WWVL 20 kHz broadcast (discontinued in 1972), and the south antenna was built for the WWVB 60 kHz broadcast. The antennas are spaced 857 m apart. Each antenna is a top loaded monopole consisting of four 122-m towers arranged in a diamond shape. A system of cables, often called a capacitance hat or top hat, is suspended between the four towers. This top hat is electrically isolated from the towers, and is electrically connected to a downlead suspended from the center of the top hat. The downlead serves as the radiating element.

North antenna coordinates: 40° 40' 51.3" N, 105° 03' 00.0" W

South antenna coordinates: 40° 40' 28.3" N, 105° 02' 39.5" W

Ideally, an efficient antenna system requires a radiating element that is at least one-quarter wavelength long. At 60 kHz, this becomes difficult. The wavelength is 5000 m, so a one-quarter wavelength antenna would be 1250 m tall, or about 10 times the height of the WWVB antenna towers. As a compromise, some of the missing length was added horizontally to the top hats of this vertical dipole, and the downlead of each antenna is terminated at its own helix house under the top hats. Each helix house contains a large inductor to cancel the capacitance of the short antenna and a variometer (variable inductor) to tune the antenna system. Energy is fed from the transmitters to the helix houses using underground cables housed in two concrete trenches. Each trench is about 435 m long.

A computer is used to automatically tune the antennas during icy and/or windy conditions. This automatic tuning provides a dynamic match between the transmitter and the antenna system. The computer looks for a phase difference between voltage and current at the transmitter. If one is detected, an error signal is sent to a 3-phase motor in the helix house that rotates the rotor inside the variometer. This retunes the antenna and restores the match between the antenna and transmitter.

There are three transmitters at the WWVB site. Two are in constant operation and one serves as a standby transmitter that is activated if one of the primary transmitters fail. Each transmitter consists of two identical power amplifiers which are combined to produce the greatly amplified signal sent to the antenna. One transmitter delivering an amplified time code signal into the north antenna system, and one transmitter feeds the south antenna system. The time code is fed to a console where it passes through a control system and then is delivered to the transmitters.

Using two transmitters and two antennas allows the station to be more efficient. As mentioned earlier, the WWVB antennas are physically much smaller than one quarter wavelength. As the length of a vertical radiator becomes shorter compared to wavelength, the efficiency of the antenna goes down. In other words, it requires more and more transmitter power to increase the effective radiated power. The north antenna system at WWVB has an efficiency of about 56.3%, and the south antenna has an efficiency of about 54%. However, the combined efficiency of the two antennas is about 68.8%. As a result, each transmitter has to produce a forward power of about 51 kW to produce an effective radiated power of 70 kW.

The frequency uncertainty of the WWVB signal as transmitted is less than 1 part in 1012. If the path delay is removed, WWVB can provide UTC with an uncertainty of about 100 microseconds. The variations in path delay are minor compared to those of WWV and WWVH. When proper receiving and averaging techniques are used, the uncertainty of the received signal should be nearly as small as the uncertainty of the transmitted signal.

Other Information about WWVB ...
WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers
A 64-page booklet containing recommended practices for WWVB radio controlled clock manufacturers, plus tips for consumers attempting to troubleshoot reception problems.
NIST Time and Frequency Services (NIST Special Publication 432)
A detailed 80 page overview of NIST time and frequency services and how to use them. Chapter 2 is all about WWVB.
WWVB Time Signal Broadcast: An Enhanced Broadcast Format and Multi-Mode Receiver
A description of the new enhanced BPSK-based WWVB broadcast and its benefits over the legacy AM/PWM-Based broadcast Format
New Improved System For WWVB Broadcast
A more in-depth explanation of WWVB PWM-Based Broadcast Format
We Help Move Time Through The Air
A Radio World commentary: Managers of WWVB explore options to improve the service further, by John Lowe
WWVB: A Half Century of Delivering Accurate Frequency and Time By Radio
50th Anniversary of broadcasting from WWVB and its history
How Accurate is a Radio Controlled Clock?
A discussion of WWVB radio controlled clock accuracy from the Horological Journal, March 2010.
Frequently Asked Questions
Answers to frequently asked questions about the NIST Radio Stations
Manufacturers of Time and Frequency Receivers
Links to manufacturers of WWVB and other time and frequency receivers
Questions? Send mail to: nist.radio@boulder.nist.gov

See: https://nvlpubs.nist.gov/nistpubs/jres/119/jres.119.004.pdf

When I actively listened to Shortwave radio I often tuned in to WWVB for the exact time. For some reason it was always reassuring to pick up the time signal out of the surrounding static and then set my watch and clock to the signal.