How do self-setting clocks work?

by | Mar 9, 2010 08:00 PM ET

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Marshall Brain

Hi, I'm Marshall Brain with today's question, how do the automatic self-setting clocks set themselves to the atomic clock in Colorado. Many gadget catalogues and high tech stores sell radio controlled clocks and wristwatches that are able to receive these radio signals. These clocks and watches truly are synchronizing themselves with the atomic clock in Colorado. This feature is made possible by a radio system set up and operating by NIST, the National Institute of Standards in Technology, located in Boulder, Colorado.

NIST operates radio stations WWVB, which is the station that transmits the time codes. WWVB is a very interesting radio station. It has high transmitter power, about 50,000 watts, a very efficient antennae and an extremely low frequency of only 60,000 Hz. For comparison, a typical AM radio station broadcasts at a frequency of a million Hz. The combination of high power and low frequency gives the radio waves from WWVB a lot of balance, and this single station can therefore cover all of the continental United States, much of Canada, and Central America.

The time codes are set from WWVB using one of the simplest systems possible, and at a very low data rate of one bit per second. For comparison, a typical modem transmits over phone lines at tens of thousands of bits per second. Imagine receiving a web page at one bit per second. The 60,000 Hz signal is always transmitted, but every second it's significantly reduced in power for a period of 0.2, 0.5, or 0.8 seconds. 0.2 seconds of reduced power means a binary zero. 0.5 seconds of reduced power is a binary one. And 0.8 seconds of reduced power is a separator.

The time code is set in BCD, or binary coded decimal, and it indicates minutes, hours, day of the year, and year along with information about daylight savings time and leap years. The time is transmitted using 53 bits and seven separators, and therefore takes exactly 60 seconds to transmit. A clock or watch can contain an extremely small and relatively simple antennae and receiver to decode the information in the signal and set the clock's time accurately. All you have to do is set the time zone and the clock can display a very accurate time.

The only thing more accurate that you can carry around easily is a GPS receiver, which derives atomic clock accuracy in real time from the atomic clocks in the orbiting GPS satellite.

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