The DCF-77 (German) Time and Frequency Broadcast

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The German national time and frequency transmission is broadcast from Mainflingen near Frankfurt, Germany. It's radio call sign is known as DCF-77, it is broadcast at 77.5 Khz, hence the name DCF-77. The signal can be used by timing equipment to provide a precise time and date information. Network timing equipment, such as NTP time servers, which provide an accurate time reference for computer networks can utilise the DCF-77 signal as an accurate source of time. This article describes how the DCF-77 transmission is encoded and how it is used to provide a precise time reference for NTP servers.

The DCF-77 radio transmission has been in operation since 1959. The transmitters are currently maintained by T-Systems, a sub-division of the German Telecoms group Deuche Telecom. Caesium atomic clocks maintained by the German National Physics Laboratory provide the timing reference for the transmission. The broadcast has a relatively high power output of 50W, which can be received up to 2000 km from Frankfurt. Signal reception can quite easily be obtained throughout much of central and north-western Europe.

The transmission is broadcast as an amplitude-modulated, pulse-width encoded data signal. Accurate time and date information is continuously broadcast, repeated each minute. Timing data is transmitted as a series of 59 pulses, one pulse each second. Each pulse represents a data bit. A 100 millisecond pulse represents a binary zero, while a 200 millisecond pulse represents a binary one. At the end of each minue the pulses can be decoded into date and time information.

To conclude, the DCF-77 time transmission is a reliable and precise external time reference for NTP servers. The signal can be received indoors close to the host NTP time server itself, thus reducing installation costs. Also, the simple encoding of time and date information within the transmission enables easy implementation into NTP server software.

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