+S Last Update: 15.12.2004


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DCF77 and TDF Radio Clock Systems: a Comparison

Server radio clock for networks


Price list





TDF (France Inter)




Time coordinate

Caesium clock in Brunswick, Germany

Caesium fountain in Paris, France

Transmitter operated by

Deutsche Telekom

France Inter

Location of time transmitter

(near Frankfurt)

(150 km South of Paris)

Transmission frequency

77.5 kHz

162 kHz

Transmitter power

50 kW

2000 kW
(2 megawatts)

Transmitter type

Dedicated time transmitter

Radio transmitter with additional time signal

Modulation type

Amplitude modulation

Phase modulation

Range (official)

2000 km

3500 km

Transmitter availability

99.7 % guaranteed by Deutsche Telekom; no interruption in recent years

Weekly disconnection for maintenance (Tuesdays 1.00 to 5.00 a.m.)

Short-term deviations due to reception conditions

Approx. 30 to 60 ms
depending on receiver and location

Approx. 10 to 40 ms
depending on location


  • PTB: "Physikalisch-Technische Bundesanstalt" (Federal Institute of Physics and Metrology) – the German national testing and certification authority, based in Brunswick, Germany.
  • LPTF – "Laboratoire Primaire du Temps et des Fréquences" (now renamed BNM – SYRTE) in the "Observatoire de Paris", France. The world's most accurate atomic clock (a caesium fountain) is located here.
  • Both systems have about the same accuracy and reliability, and both institutions play a leading role in UTC. The deviation from UTC of both time signals can be measured in mere millionths of a second, and both systems work round the clock with high availability.
  • Although the two systems have a similar time signal structure, the modulation methods are not compatible with each other, so that different receivers are needed for each.

Range comparison





DCF77 transmitter of the PTB in Mainflingen, Germany

Three atomic clocks are located here. If needed, they are readjusted remotely by the PTB in Brunswick. The transmitter is operated by Deutsche Telekom – a German telecommunications provider. Deutsche Telekom guarantees a long-term average availability of 99.7%. The transmitter has recently been fitted with lightning protection, largely eliminating the need to stop transmission during thunderstorms.

The easily decoded amplitude modulation method has contributed to the DCF77 system's popularity. Beside commercial and public applications, millions of wristwatches and alarm clocks throughout central Europe are based on this system.

TDF transmitter in Allouis, France

The long range of the TDF system is the result of its enormous transmitter power of two times one megawatt and its two transmission masts of 350 metres height. A reserve transmitter with a considerable 600 kW can be connected to one of the two aerials during maintenance work and in the event of a fault.

TDF is really a radio transmitter that also sends a modulated time signal. The decoding method for phase-modulated signals is complex, and the required demodulation equipment in the time receiver was therefore expensive until now. That is why TDF is not as popular as DCF77.

Operators France Inter do not officially guarantee the transmitter's availability, which should nevertheless easily exceed 99%.

The time information is supplied by the LPTF (Laboratoire Primaire du Temps et des Fréquences, now renamed BNM – SYRTE) – an international institution responsible for the central coordination of the UTC (universal time coordinate) and operators of the world's currently most accurate clock, a so-called caesium fountain. The deviation between DCF77 and TDF are in the order of only millionths of a second.

The future of long-wave in time signal transmission
It has been speculated that long-wave systems will be replaced GPS. This is highly unlikely: long-wave-based time systems have significant technical advantages over GPS and their use is rapidly growing. GPS receivers are expensive and need an outdoor aerial. Furthermore, GPS is a US military installation, made available for civil use only on a goodwill basis, without any guarantees. The European civil alternative – Galileo – has recently been given the go-ahead but will not be operational for some time to come.

Photos by kind permission of PTB and Thierry Vignaud's private website about TDF.