DCF77

The DCF77 station signal carries an amplitude-modulated, pulse-width coded 1 bit/s data signal. The same data signal is also phase modulated onto the carrier using a 512 bit long pseudorandom sequence (direct-sequence spread spectrum modulation). The transmitted data signal is repeated every minute.

• Current date and time bits;
• Leap second warning bit;
• Imminent change of CET to CEST, or vice versa, Announcement bit;
• Central European Time (CET) / Central European Summer Time (CEST) bit;
• Abnormal transmitter operation identification bit;
• Several parity bits.

Since 2003, fourteen previously unused bits of the time code have been used for civil defence emergency signals. This is an experimental service, aimed to one day replace the German network of civil defence sirens.

Since 22 November 2006 the DCF77 transmitter uses bits 1–14 to transmit warning messages and weather information.[10][11] Under responsibility of the German Federal Office of Civil Protection and Disaster Assistance (the German Bundesamt für Bevölkerungsschutz und Katastrophenhilfe, BBK), warnings to the population can be transmitted using these 14 bits. As a further extension of the information content transmitted by DCF77, appropriately equipped radio clocks can provide a four-day weather forecast for 60 different regions in Europe. The forecast data is provided by and under responsibility of the Swiss company Meteo Time GmbH and is transferred in a proprietary transfer protocol.[12][13] The same 14 bits are employed in a way that ensures compatibility with the transmission protocols of the warning messages. For decoding the weather forecast data a license is required.[11][14] Since the bits previously reserved for the PTB are used, older radio clocks should not be affected by the weather data signal.

The signal distribution contract between the PTB and the DCF77 transmitter operator Media Broadcast GmbH is periodically renewed. After negotiations in 2021 the PTB and Media Broadcast GmbH agreed to continue the dissemination of the German national legal time for the next 10 years. In order to improve the reliability of the broadcast and also the ease of maintenance by the operator, Media Broadcast GmbH has announced that it will build a second remote-controllable high-performance transmitter in 2022. The facilities will then be completely duplicated on site. In the past the PTB expressed it will initialize new negotiations if modernization activities at the transmitting station to improve the signal reception reliability throughout Europe are deemed necessary.[15][16]

The call sign DCF77 stands for D = Deutschland (Germany), C = long wave signal, F = the longwave transmitters on the premises of the transmitting station Mainflingen (due to its vicinity to Frankfurt am Main), 77 = frequency: 77.5 kHz.

Amplitude modulated signal of DCF77 as a function of time

The DCF77 signal uses amplitude-shift keying to transmit digitally coded time information by reducing the amplitude of the carrier to 15% of normal (−16½ dB) for 0.1 or 0.2 seconds at the beginning of each second. A 0.1 second reduction (7750 cycles of the 77500 Hz carrier amplitude) denotes a binary 0; a 0.2 second reduction denotes a binary 1. As a special case, the last second of every minute is marked with no carrier power reduction.

There was also a Morse code station identification until 2006, sent during minutes 19, 39 and 59 of each hour, however this was discontinued as the station is easily identifiable by the characteristic signal.[17] A 250 Hz tone was generated by square wave modulating the carrier between 100% and 85% power, and that tone was used to send one letter per second, between the second marks. During seconds 20–32, the call sign "DCF77" was transmitted twice.

In addition, for 793 ms beginning at 200 ms, each time code bit is transmitted using direct-sequence spread spectrum. The bit is mixed with a 512-bit pseudo-random chip sequence and encoded on the carrier using ±15.6° phase-shift keying.[18] The chip sequence contains equal amounts of each phase, so the average phase remains unchanged. Each chip spans 120 cycles of the carrier, so the exact duration is cycles 15500 through 76940 out of 77500. The last 560 cycles (7.23 ms) of each second are not phase-modulated.[19]

The chip sequence is generated by a 9-bit linear feedback shift register (LFSR), repeats every second, and begins with 00000100011000010011100101010110000….

A software implementation of a Galois LFSR can generate the full chip sequence:

  unsigned int i, lfsr;
  lfsr = 0;

  for (i = 0; i < 512; i++) {
    unsigned int chip;

    chip = lfsr & 1;
    output_chip(chip);

    lfsr >>= 1;
    if (chip || !lfsr)
      lfsr ^= 0x110;
  }

Each time code bit to be transmitted is exclusive-ored with the LFSR output. The final chipped sequence is used to modulate the transmitter phase. During 0 chips the carrier is transmitted with a +15.6° phase advance, while during 1 chips it is transmitted with a −15.6° phase lag.

In lieu of the special minute marker used in the amplitude code, bit 59 is transmitted as an ordinary 0-bit, and the first 10 bits (seconds 0–9) are transmitted as binary 1.

When compared to amplitude modulation, phase modulation makes better use of the available frequency spectrum and results in a more precise low frequency time distribution with less sensitivity to interferences. Phase modulation is however not used by many DCF77 receivers. The reason for this is the worldwide availability of the (precise time reference) signals transmitted by global navigation satellite systems like the Global Positioning System (GPS), GLONASS, Galileo and BeiDou. Due to the GPS signal structure and the larger bandwidth available, the GPS reception would, in principle, achieve an uncertainty of the time transmission that is lower by at least one order of magnitude than the uncertainty that can be achieved with DCF77 phase modulation receiving hardware (GPS time is accurate to about ± 10 to 30 nanoseconds[20][21] and the Galileo April, May, June 2021 Quarterly Performance Report by the European GNSS Service Centre reported the UTC Time Dissemination Service Accuracy was ≤ 4.3 ns, computed by accumulating samples over the previous 12 months and exceeding the ≤ 30 ns target[22][23][24]).

The time is represented in binary-coded decimal. It represents civil time, including summer time adjustments. The time transmitted is the time of the following minute; e.g. during December 31 23:59, the transmitted time encodes January 1 00:00.[25]

The first 20 seconds are special flags. The minutes are encoded in seconds 21–28, hours during seconds 29–34, and the date during seconds 36–58.

Two flags warn of changes to occur at the end of the current hour: a change of time zones, and a leap second insertion. These flags are set during the hour up to the event. This includes the last minute before the event, during which the other time code bits (including the time zone indicator bits) encode the time of the first minute after the event.

In the event of an added leap second, a 0-bit is inserted during second 59, and the special missing bit is transmitted during the leap second itself, second 60.[25]

Although the time code only includes two digits of year, it is possible to deduce two bits of century using the day of week. There is still a 400-year ambiguity, as the Gregorian calendar repeats weeks every 400 years, but this is sufficient to determine which years ending in 00 are leap years.[28]

The time zone bits can be considered a binary-coded representation of the UTC offset. Z1 set indicates UTC+2, while Z2 indicates UTC+1.

The phase modulation generally encodes the same data as the amplitude modulation, but differs for bits 59 through 14, inclusive. Bit 59 (no amplitude modulation) is phase-modulated as a 0-bit. Bits 0–9 are phase modulated as 1 bits, and bits 10–14 are phase modulated as 0 bits.[29] The civil protection warnings and weather information is not included in the phase-modulated data.

The DCF77 transmitted carrier frequency relative uncertainty is 2 × 10⁻¹² over a 24-hour period and 2 × 10⁻¹³ over 100 days, with a deviation in phase with respect to UTC that never exceeds 5.5 ± 0.3 microseconds.[35] The four German primary caesium (fountain) atomic clocks (CS1, CS2, CSF1 and CSF2) in use by the PTB in Braunschweig ensure significantly less long term clock drift than the atomic clocks used in the DCF77 facility in Mainflingen.[36] With the aid of external corrections from Braunschweig the control unit of DCF77 in Mainflingen is expected to neither gain nor lose a second in approximately 300,000 years.

In theory, a DCF77 controlled external clock should be able to synchronize to within one half of the period of the transmitted 77.5 kHz carrier frequency of the DCF77 signal, or within ± 6.452 × 10⁻⁶ s or ± 6.452 microseconds.[19]

Consumer grade radio clock movement with the DCF77 receiver (right) in the clock. The small ferrite loopstick antenna used in this alarm clock can be seen at the left.

Due to the propagation process, phase and/or frequency shifts observed in received signals the practical obtainable accuracy is lower than originally realized with the atomic clocks at the place of transmission. As with any time signal radio transmitter the precise establishment of time is affected by the distance to the transmitter, as the time signal propagates to a time signal receiver at the speed of light. For a DCF77 receiver located 1,000 km (621 mi) away from the DCF77 transmitter, due to transit delay the receiver will be set more than 3 milliseconds late. Such a small deviation will seldom be of interest and if desired instrument grade time receivers can be corrected for transit delay.

Further inaccuracies may be caused by the type of wave the receiver records. If pure ground wave reception is anticipated and the reception location is permanent a constant may be included in the calculation, while in the case of pure space waves the receiver cannot compensate for the fluctuations since these are the result of the changing altitude of the reflecting and bending layer of the ionosphere. Similar problems arise where ground and skywaves overlap. This field is not constant but changes in the course of the day between approximately 600 to 1,100 km (373 to 684 mi) from the transmitter position.[31]

Corrected instrument grade DCF77 receivers, using the amplitude-modulated time signals with accompanying antennas oriented tangential to the transmitter's antenna in Mainflingern to ensure the best possible interference-free time signal reception at fixed locations, can achieve a practical accuracy uncertainty better than ± 2 milliseconds.[37]

In addition to the amplitude-modulated time signal transmission this information is also transmitted since June 1983 by DCF77 via a phase modulation of the carrier wave with a pseudorandom noise sequence of 512 bits length. Using cross-correlation the reproduced signal at the receiving end can be used to determine the beginning of the second markers much more accurately. The drawback of using phase-modulated time signals lies in the complex instrument grade receiving hardware required for using this time signal reception method. Using this method the Physikalisch-Technische Bundesanstalt (PTB) measured standard deviations of ± 2 to 22 microseconds between UTC (PTB) and UTC (DCF77), depending on the time in the day and season. This was done in Braunschweig located 273 km (170 mi) from the transmitter in Mainflingen.[19]

Normal low cost consumer grade DCF77 receivers solely rely on the amplitude-modulated time signals and use narrow band receivers (with 10 Hz bandwidth) with small ferrite loopstick antennas and circuits with non optimal digital signal processing delay and can therefore only be expected to determine the beginning of a second with a practical accuracy uncertainty of ± 0.1 second. This is sufficient for radio controlled low cost consumer grade clocks and watches using standard-quality quartz clocks for timekeeping between daily DCF77 synchronization attempts, as they will be most accurate immediately after a successful synchronization and will become less accurate from that point forward until the next synchronization.[38] Some DCF77 controlled consumer grade quartz movements promote accurate time keeping by synchronizing and correcting their time automatically more than once spread over a day.[39]