Elevation 129 m (423 ft) Began operation July 1980 | Frequency 162 kHz | |
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Location Allouis longwave transmitter, Allouis, France Operator TéléDiffusion de France on behalf of the LNE–SYRTE Power 1500 kW (1000 kW at night 00 - 06h) | ||
TéléDiffusion de France broadcast the TDF time signal, controlled by LNE–SYRTE, from the Allouis longwave transmitter at 162 kHz, with a power of 2 MW.
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It was also known as FI or France Inter because the signal was formerly best known for broadcasting the France Inter AM signal. This signal ceased at the end of 2016, but the transmitter remains in use for its time signal and other digital signals.
In 1980, the first atomic clock was installed to regulate the carrier frequency. 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 (22 to 40 times DCF77's 50 kW) gives it a much greater range of 3500 km.
The signal is almost continuous but there is a regularly scheduled interruption for maintenance every Tuesday. This used to be from 01:03 to 05:00, but with the cessation of audio signals, it has been moved to 08:00 to 12:00.
The signal was formerly 2000 kW, but has been reduced to 1500 kW, and tests are in progress of a further reduction to 1100&kbsp;kW for cost savings.
Signal format
TéléDiffusion de France (TDF) uses an amplitude modulated longwave transmitter station. 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 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.
Unlike DCF77, bit 19 is not used for leap second warnings, but is always zero. Instead, bit 1 is used to warn of a positive leap second, and bit 2 is used to warn of a (very unlikely) negative leap second. In case of a leap second, an additional zero bit is inserted between bits 2 and 3. This is supposed to be inserted at 23:59:03, during minute 59 of the hour (during which the timestamp for minute :00 is transmitted), so that the minute markers are all broadcast at the correct times, but for the leap second at the end of December 2016, it was apparently inserted at 23:58:03.
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.