DCF77 is a longwave time signal radio station. The callsign stands for D=Deutschland (Germany), C=long wave signal, F=Frankfurt, 77=frequency: 77.5 kHz. It is run by The Physikalisch-Technische Bundesanstalt.
DCF77 radio signals (set by an atomic clock) are broadcast on 77.5 kHz from Mainflingen, Germany, about 25 km south-east of Frankfurt. The signal can be received from a distance of 2000 km. There are two transmitters so it is rarely off the air.
Radio clocks are popular in this region and use DCF77's signal to set their time automatically.
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