TDF time signal

TDF is a time signal controlled by the LNE–SYRTE and broadcast by Télédiffusion de France from the Allouis longwave transmitter at 162 kHz, with a power of 2 MW.^[1] It is also known as FI or France Inter because the transmitter primarily broadcasts the France Inter AM signal.

In 1980, the first atomic clock was installed to regulate the carrier frequency.^[2] The current time signal is generated by an extremely accurate caesium fountain atomic clock and phase-modulated on the 162 kHz carrier in a way that is inaudible when listening to the France Inter signal using a normal AM receivers. It requires a more complex receiver than the popular DCF77 service, but the much more powerful transmitter (40 to 80 times DCF77's 50 kW) give it a much greater range of 3500 km.

The signal is almost continuous but there is a regularly scheduled interruption for maintenance from 01:03 to 05:00 every Tuesday.^[2]

Signal format

TDF is actually an amplitude modulated longwave radio station, transmitting the programs of the France Inter network. Time signals are transmitted by phase-modulating the carrier by ±1 radian in 0.1 s every second except the 59th second of each minute. This modulation pattern is repeated to indicate a binary one.

The binary encoding of date and time data during seconds 15 through 59 is identical to that of DCF77; the numbers of the minute, hour, day of the month, day of the week, month and year are transmitted each minute from the 21st to the 58th second, in accordance with the French legal time scale. The time transmitted is the local time of the upcoming minute.

Also like DCF77, bit 20 is always 1, bit 19 indicates an upcoming leap second at the end of the current hour, bit 18 indicates that local time is UTC+1 (CET), bit 17 indicates that local time is UTC+2 (CEST), and bit 16 indicates that a change to local time will take place at the end of the current hour. Bit 15 is reserved to indicate abnormal transmitter operation.

As extensions to the DCF77 code, bit 14 is set during public holidays (14 July, Christmas, etc.), and bit 13 is set the day before public holidays.^[3]

The relative uncertainty of the carrier frequency is 2 parts in 10¹².

Phase modulation pattern

One signal element consists of the phase of the carrier shifted linearly by +1 rad in 25 ms (known as "ramp A"), then shifted linearly by −2 rad over 50 ms ("ramp B"), then shifted linearly again by +1 rad for another 25 ms ("ramp C"), returning the phase to zero. One signal element is always sent at each second between 0 and 58. Two signal elements are sent in sequence to represent a binary one; otherwise it is interpreted as binary zero. During ramp B of the initial signal element, the exact point the signal phase is at zero represents the top of the UTC second. Since the phase is the integral of the frequency, this triangular phase modulation at 40 rad/s corresponds to a square frequency modulation with a deviation of 20/π ≈ 6.37 Hz.

Both the average phase and the average frequency deviation are thus zero. Additional non-timing data is sent by phase modulation during the rest of each second. But the second marker (and data bit) is always preceded by 100 ms without any phase modulation. The signal is not phase-modulated at all during the 59th second past the minute.

See also

• Loop antenna
• Allouis longwave transmitter, the facility used for its transmission.
• TéléDiffusion de France (also known as the TDF Group)

References

Sources

• Betke, Klaus (2006-07-01). "Standard Frequency and Time Signal Stations On Longwave and Shortwave" (PDF). pp. 9–10. Archived from the original (PDF) on 2006-08-18. Retrieved 2006-10-27. 
• David L. Mills, Information on Time and Frequency — Time and Standard Frequency Station TDF (France)
• (German) Funkuhren—Vergleich DCF77 mit TDF ("Clocks—Compare DCF77 with TDF") Includes a map showing the different reception ranges.
• (French) http://pagesperso-orange.fr/tvignaud/am/allouis/allouis-heure.htm
• (French) Signaux Horaires Description of the TDF signal and a working receiver.

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