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Does Anybody Really Know What Time It Is?

June 7, 2013

There are laboratories with atomic clocks capable of keeping time to a precision of one part in 1014, for an accuracy of about one second in a billion years. Not many people need such accuracy. When I ask for the time, "about three o'clock" is often good enough for me.

As I was growing up, there were evolving ways of getting time to the people. The telephone company had a calling number you could dial to hear the spoken time, as did a local bank, which provided the temperature as well. Radio stations were a good source of time information, and they usually subscribed to a time service that synchronized their mechanical clocks by an electric pulse distributed each hour via telephone lines.

Clock of the Church of Saint Martin, Busskirch, Jona, Switzerland

Clock of the Church of Saint Martin, Busskirch, Jona, Switzerland

Clock faces use IIII as the Roman numeral for the number four, and not IV.

This was actually the preferred way to write four through most of history.

(Photo by Roland zh, via Wikimedia Commons.)


As a tween technology enthusiast of my generation, I had a shortwave radio, and I was able to receive the time signal radio broadcasts from the National Bureau of Standards (now National Institute of Standards and Technology, NIST) time station, WWV, on 5 MHz. The "tick-tock" signal of WWV was often too weak, so I would opt for the "beep-beep" signal of Canada's time station, CHU, operated by the National Research Council, at 3.330 MHz.

The speed of light is an important factor in reception of time signals, but direct reception of a radio signal from a thousand miles away results in just a five millisecond delay, which wasn't too important half a century ago. The speed of light in a wire, however, is smaller, and when you put amplifiers and loading coils in the path, delays start to add up.

I observed the delay effect of wire transmission first hand in the late 1960s while working at a radio station. In what was a good simulation of Internet radio, the engineers on our network news feed, which originated from Chicago, were using the off-air signal of Chicago radio station, WCFL, as a test signal. I could hear a considerable delay between the broadcast signal, received on an AM radio, and the signal on the network feed.

My wife had an Atomic Watch; or, at least, that's what her colleagues called it. It was one of the first digital wrist watches. Since it had a power-hungry LED display, you needed to push a button to see the time, which was in hours and minutes, only. We went through a lot of watch batteries, which were not that common at the time, since her colleagues were always asking her for the time.

Nowadays, we can use the Global Positioning System as our time reference, by which we can easily get a tenth of a microsecond accuracy. The traditional time stations have tried to keep pace with the digital world, with WWV providing a digital time code in BCD format on a 100 Hz subcarrier; and CHU providing digital signals in the more computer compatible Bell 103, 300 baud format. The Bell 103 format was used by early telephone modems and is used in the US caller-ID system.

One problem that the legacy time signal stations have is the same problem television broadcasters had many years ago when they wanted to broadcast color video and still have their signals remain compatible with the older monochrome television receivers. NIST has recently upgraded the WWV signal by adding a phase modulated signal.[1] This signal supplements the amplitude modulated signal of the carrier wave, it allows better signal reception on newer devices, but it allows older devices to function as before.

NIST scientists have just demonstrated accurate time signal synchronization between distant locations using free-space optical communication. The reason for going optical is simple. You can't use microwave radio signals to achieve the femtosecond accuracy inherent in the present generation of atomic clocks. You need a higher carrier frequency to get that resolution.[2-3]

Such a free-space optical connection, implemented using lasers, has application in improved geodesy on the ground, and for satellite-based relativity experiments in space.[2] The demonstration was conducted using eye-safe, infrared laser pulses reflected from a mirror on a hillside, as shown in the figure.

NIST freespace optical time signal distribution

Ultraprecise time signals were transferred on an infrared laser beam by NIST researchers between a laboratory on NIST's campus in Boulder, Colorado, and nearby Kohler Mesa. Signals from the laboratory were reflected off a mirror on the mesa, and returned approximately two kilometers to the laboratory. The two-way technique overcomes timing distortions from atmospheric turbulence. (Talbott/NIST image.)[3)]


Synchronization of oscillators at each end of the path was by phase-locking to a coherent frequency comb carried on the laser beam. The frequency comb was a stream of ultrashort optical pulses, the spacing of which was synchronized precisely with a resonant optical cavity that served as a stand-in for an atomic clock.

The measured timing difference between the two path ends was infinitesimal, just one million-billionths (10-15) of a second per hour.[3] This femtosecond precision was obtained despite signal fading from atmospheric turbulence and obstructions across the two kilometer optical link.[2] This research was funded in part by the Defense Advanced Research Projects Agency (DARPA).[3]

Does Anybody Really Know What Time It Is? is a 1969 song by the music group, Chicago. The group was originally called The Chicago Transit Authority, until the actual transit authority objected. Apparently, the Windy City posed no objection to the use of its Chicago name.

References:

  1. James Burrus, "New NIST time code to boost reception for radio-controlled clocks," NIST Tech Beat, National Institute of Standards and Technology, March 5, 2013
  2. Fabrizio R. Giorgetta, William C. Swann, Laura C. Sinclair, Esther Baumann, Ian Coddington and Nathan R. Newbury, "Optical two-way time and frequency transfer over free space," Nature Photonics, Online Before Print, April 28, 2013, doi:10.1038/nphoton.2013.69.
  3. Laura Ost, "NIST Demonstrates Transfer of Ultraprecise Time Signals over a Wireless Optical Channel, NIST Tech Beat, National Institute of Standards and Technology, April 30, 2013.
  4. NIST Physical Measurement Laboratory, Time and Frequency Division , Time and Frequency Services Web Site.

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