The National Institute of Standards and Technology (NIST) maintains the time standard for the United States. It operates a collection of clocks and measuring tools that together help determine UTC — known internationally as UTC(NIST).
These tools are located at a laboratory in Boulder, CO. In addition to the laboratory, NIST uses a nearby radio station, WWVB, to broadcast a time signal relayed to it from the laboratory. A radio-controlled clock (RCC) has a tiny listener that can pick up this signal and translate it into the current time according to the atomic clock; the firmware inside the clock mechanism then uses that information to adjust the clock to match the signaled time.
I have an RCC, have had it for quite a few years. As is typical for these clocks, besides keeping accurate time, the clock also captures internal and external temperatures, and relative humidity. Unfortunately, enough years have passed that my clock’s external temperature monitoring unit has bit it. I just ordered a new, improved model of RCC. I don’t have a real need for a clock that is accurate to ±0.5 second per day. I just like the idea.
The current model synchronizes the time only once, at 0200. If the sync fails, then it waits 24 hours for the next sync, during which period it will lose/gain as much as 0.5 sec — OR MORE!! The clock time would then be as much as 1 second off. OR MORE!! The new model will sync every hour, 0000 to 0600, until it is successful, or fails at 0600.
Among other things, the RCC demonstrates how artificial our computation of “time” has become. We often associate UTC, or Greenwich Mean Time (GMT), with scientific and technical measurements. But UTC is actually a kludge — an average of times from 70 clock laboratories around the world. UTC exists to maintain a clock that is timed with the Earth’s rotation. This requires that it periodically have leap seconds shoved into its time cycle. Scientific time is maintained as a separate clock that is not munged in this way.
“The oscillator found inside an RCC is based on the mechanical vibrations of a quartz crystal, typically counting 32,768 vibrations of the crystal to mark one second.” 1
A mere 32,768 vibrations per second! Not nearly good enough!
“… the second is defined internationally as the duration of 9,192,631,770 energy transitions of a cesium atom.” 2
Bwahaha! Now you’re talking accuracy!
Marcus du Sautoy has done a 3-part documentary, Precision: The Measure of All Things, for BBC Four, it’s quite good. Part one is on time and distance.
Oh, BTW, the international standard is maintained by the French, the Bureau International des Poids et Mesures. 3
Time-scales and the International Bureau of Weights and Measures, Elisa Felicitas Arias, Director, Time Department, International Bureau of Weights and Measures. ITU News, 2013
From now on, four PTB primary atomic clocks will contribute to UTC. Press release, 2010
- How Accurate is a Radio Controlled Clock?, Michael A Lombardi, Time and Frequency Division of the National Institute of Standards and Technology, March 2010 ↩
- Ibid. ↩
- And, did you know that Frenchmen first conceived of the idea of latitude and longitude, and measured them? ↩