|TDF time code transmitter|
The 350 m high low frequency antennas of TDF time signal in Allouis
|Location||Allouis longwave transmitter, Allouis, France|
|Elevation||129 m (423 ft)|
|Operator||TéléDiffusion de France on behalf of the LNE–SYRTE|
|Power||1,500 kW (1,000 kW at night 00-06h)|
|Began operation||July 1980|
|Official range||3,500 km (2,175 mi)|
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 3,500 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 2,000 kW, but has been reduced to 1,500 kW, and tests are in progress of a further reduction to 1,100 kW for cost savings.
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 18 and 20 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.
Bits 7–12 are unused and always transmitted as 0.
Bits 3 through 6 provide additional error checking; they encode the total number of bits set (the Hamming weight of) bits 21 through 58. Because this includes the even parity bits, the sum is always even. Also, although there are 38 bits in that range, they may not all be set. The possible values are even numbers from 4 (on Tuesday 2000-01-04 at 00:00) through 24 (on Sunday 2177-07-27 at 17:37).
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 at the end of the current hour, 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.
|:00||M||Start of minute, always 0.||:20||S||Start of encoded time, always 1.||:40||10||Day of month (continued)|
|:01||A2||Positive leap second warning,
set during previous hour
|:02||A3||Negative leap second warning,
set during previous hour
|:22||2||:42||1||Day of week|
|:03||2||Hamming weight (number
of 1 bits) of bits 21–58
|:07||0||Unused, always 0||:27||40||:47||4|
|:08||0||:28||P1||Even parity over minute bits 21–28.||:48||8|
|:10||0||:30||2||:50||1||Year within century|
|:13||F1||Following day is a public holiday.||:33||10||:53||8|
|:14||F2||Current day is a public holiday.||:34||20||:54||10|
|:15||0||To be ignored.||:35||P2||Even parity over hour bits 29–35.||:55||20|
|:16||A1||Summer time announcement.
Set during hour before change.
|:36||1||Day of month.
|:17||Z1||Set to 1 when CEST is in effect.||:37||2||:57||80|
|:18||Z2||Set to 1 when CET is in effect.||:38||4||:58||P3||Even parity over date bits 36–58.|
|:19||0||Unused, always 0||:39||8||:59||No bit transmitted during last second of each minute.|
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.
Thanks to people at Agence Nationale des Fréquences (ANFR) and Chambre française de l'horlogerie et des microtechniques (CFHM) I got a copy of Norme Française NF C 90-002 from August 1988, describing the format of the TDF time signal. The leap second announcement bits are in fact the 2nd (for positive leapsecond) and 3rd (for negative) bits in the minute. The positive leap second is to be inserted by inserting an extra 0 bit after the 3rd bit of the last minute of the hour.