DCF77

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DCF77 in Mainflingen

DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt, Germany. It is operated by T-Systems Media Broadcast, a subsidiary of Deutsche Telekom AG, on behalf of the Physikalisch-Technische Bundesanstalt, Germany's national physics laboratory. DCF77 has been in service as a standard-frequency station since 1959; date and time information was added in 1973.

The 77.5 kHz carrier signal is generated from local atomic clocks that are linked with the German master clocks in Braunschweig. The 50 kW strong transmitter can be received in large parts of Europe, as far as 2000 km from Frankfurt. Its signal carries an amplitude-modulated, pulse-width coded 1 bit/s data signal. The same data signal is also phase modulated onto the carrier using a 511-bit long pseudorandom sequence (direct-sequence spread spectrum modulation). The transmitted data repeats each minute

Map showing the range of the DCF77 signal.

the current date and time;
a leap second warning bit;
a summer time bit;
a primary/backup transmitter identification bit;
several parity bits.

Since 2003, 14 previously unused bits of the time code have been used for civil defence emergency signals. This is still an experimental service, aimed to replace one day the German network of civil defense sirens.

The callsign stands for D=Deutschland (Germany), C=long wave signal, F=Frankfurt, 77=frequency: 77.5 kHz. It is transmitted three times per hour in morse code.

Radio clocks have been very popular in Europe since the late 1980s and most of them use the DCF77 signal to set their time automatically.

Time signal stations