radio-controlled-clock
A radio-controlled clock receives longwave radio signals transmitted by a national time-standard station and automatically corrects its built-in quartz oscillator. In Japan the signal is JJY (40 kHz / 60 kHz), operated by NICT. As long as the clock successfully receives the signal, it maintains accuracy on the order of 10^-12, matching the precision of the broadcast standard.
The time signal is transmitted via amplitude modulation (AM) of the carrier wave, encoding one bit per second. A complete time frame spanning year, month, day, hour, minute, and day of week is assembled over 60 seconds (one minute). Most radio-controlled clocks attempt automatic reception once or several times during the night, when signal conditions are best. If reception fails, the clock continues to run as a standard quartz clock with accuracy of roughly plus or minus 15 seconds per month.
Several countries operate their own longwave time signal stations: Germany's DCF77 (77.5 kHz), the United Kingdom's MSF (60 kHz), the United States' WWVB (60 kHz), and China's BPC (68.5 kHz). Multi-band radio-controlled clocks can receive more than one of these signals, automatically switching to the correct local time when the wearer travels abroad.
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nict
NICT is Japan's national research institute responsible for generating, maintaining, and distributing Japan Standard Time (JST), operating a fleet of atomic clocks and transmitting the JJY time signal.
atomic clock
An atomic clock measures time by using the quantum-mechanical transition frequency of atoms as its reference, achieving accuracies on the order of one second in hundreds of millions of years and forming the backbone of modern timekeeping.
jst
JST (Japan Standard Time) is the standard time of Japan at UTC+9, based on the 135th meridian east. Japan does not observe daylight saving time.
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