How does a radio-controlled watch (or radio controlled clock) work?

Radio controlled watch (photo by RBerteig - CC-BY)

Radio controlled watch (photo by RBerteig - CC-BY)

In many countries of the world you can buy a clock or watch that will always show the correct time to the nearest second, and which never needs to be set. It will even automatically switch to daylight savings time when applicable. How is this possible?

The timepiece includes a radio receiver tuned to a radio station which broadcasts a continuous time signal. Instead of broadcasting voice or music, it broadcasts the current time in coded form. You can think of it as sending the time in Morse code, although the code used is different.

The clock or watch receives the time signal, decodes the time, and sets itself to the correct time. When daylight savings time applies, the transmitter indicates this by sending an extra code bit, and the timepiece knows to add an extra hour.

A clock powered by mains electricity might continuously monitor the time signal, but a battery-operated clock would only check a few times a day (to prolong the life of the batteries). A watch with radio-controlled time setting has a very small battery and might check and adjust the time only once per day.

The United Kingdom time signal is transmitted on a frequency of 60kHz in the longwave band. Its call signal is MSF and it transmits from Anthorn in Cumbria (from 1927 to 2007, it transmitted from Rugby). There is a European transmitter in Mainfligen, Frankfurt, Germany with callsign DCF77. For the United States, the time signal is transmitted on 60kHz by WWVB in Fort Collins Colorado. Japan also broadcasts a time signal.

Radio time signals are not the only source of time information. The time is also broadcast:

  • by some FM radio stations and TV stations
  • as part of caller-ID in the telephone system of some countries
  • as part of the signal from GPS satellites

A computer that has internet access can set its time automatically using NTP, the Network Time Protocol.


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  • Lawrence says:

    Hi Eiffel,

    “Radio time signals are not the only source of time information. The time is also broadcast:

    * by some FM radio stations and TV stations”

    True, but here in Germany I have discovered that sometimes when I switch from one station after hearing the “beep, beep, beep, beeeep”, I can hear it again on another station, i.e., one of the time signals can’t be correct. An hour ago, while post time signal news was being broadcast, suddenly the time signal beeped, five minutes after the hour. Apparently, the signal sent is a recording that is manually clicked on, this time by accident after the hour.
    Hmm. I just recognize that this station just (usually?) sends a gong tone, but the mistaken signal was the four beeps. ??

    Regards, Myo

    • eiffel says:

      It used to be that the “hourly pips” were the definitive time signal for civilian use. They still should be, but as you point out the system doesn’t always work.
      Certainly in the UK the time signal comes from a “feed” rather than being activated manually at the station, but even so there can be a short time delay between the FM and DAB outlets of the same station so the precision has been lost.

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