Radio-controlled watch

A radio-controlled watch is an ordinary quartz watch with a receiver added. NIST, describing radio-controlled clocks that use its station WWVB, explains that the device has "a miniature radio receiver inside, which is permanently tuned to receive the 60 kHz signal", and that the broadcast sends bits at "1 bit per second, and it takes a full minute to send a complete time code". The watch decodes that code and resets its display.

The signals are national metrology broadcasts, not a service of the watch industry. Germany's Physikalisch-Technische Bundesanstalt transmits DCF77 on a 77.5 kHz carrier from Mainflingen, to "disseminate the legal time realized by PTB for Germany". The United Kingdom's MSF, in the National Physical Laboratory's words, "operates on a frequency of 60 kHz" and is "transmitted from Anthorn Radio Station in Cumbria". Japan's NICT broadcasts under the "call sign JJY" on "40 kHz" and "60 kHz" from its Ohtakadoyayama and Haganeyama stations.

A radio-controlled watch is therefore only as good as its last successful reception. NIST states that "most clocks only decode the signal once per day, but some do it more often (for example, every 6 hours)", usually "at midnight or in the very early hours of the morning, because the signal is easiest to receive when it is dark", and that "in between synchronizations, the clocks keep time using their quartz crystal oscillators".

The time carried by these signals comes from the atomic clocks the institutes operate, not from anything inside the watch: "atomic watch" describes where the time came from rather than what the watch is. NICT publishes an "accuracy of frequency and time interval" for JJY of "+-1*10-12"; the watch inherits none of that between receptions.