| |
DCF77 |
TDF (France Inter) |
Operator |
PTB |
LPTF |
Time coordinate |
Caesium clock in Brunswick, Germany |
Caesium fountain in Paris, France |
Transmitter operated by |
Deutsche Telekom |
France Inter |
Location of time transmitter |
Mainflingen
(near Frankfurt) |
Allouis
(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 |
Notes
- 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.
