DCF77

The low frequency T-aerial antennas of DCF77 in Mainflingen, Germany

Low cost DCF77 receiver

DCF77 range from Mainflingen

DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located at 50°0′56″N 9°00′39″E in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH (previously a subsidiary of Deutsche Telekom AG), on behalf of the Physikalisch-Technische Bundesanstalt, Germany's national physics laboratory. DCF77 has been in service as a standard-frequency station since 1959; date and time information was added in 1973. The timestamp sent is either in UTC+1 or UTC+2 depending on daylight saving time.

The 77.5 kHz carrier signal is generated from local atomic clocks that are linked with the German master clocks in Braunschweig. With a relatively high power of 50 kW, the station can be received in large parts of Europe, as far as 2000 km from Frankfurt (and further away depending on signal propagation and local interference). As an example, reception with high-grade consumer clocks is possible in Portugal and Gibraltar (during night hours). Its 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 repeats each minute.

• Current date and time;
• Leap second warning bit;
• Summer time bit;
• Primary/backup transmitter identification bit;
• Several parity bits.

Since 2003, 14 previously unused bits of the time code have been used for civil defence emergency signals. This is still an experimental service, aimed to one day replace the German network of civil defence sirens.

The call sign DCF77 stands for D=Deutschland (Germany), C=long wave signal, F=Frankfurt, 77=frequency: 77.5 kHz.

Radio clocks and watches have been very popular in Europe since the late 1980s and, in main-land Europe, most of them use the DCF77 signal to set their time automatically.

Time code details

Like most longwave time transmitters, DCF77 marks seconds by reducing carrier power for an interval beginning on the second. The duration of the reduction is varied to convey one bit of time code per second, repeating every minute. The carrier is synchronized so the rising zero-crossing occurs on the second. All modulation changes also occur at rising zero-crossings.

Amplitude modulation

Amplitude modulated signal of DCF77 as a function of time

The DCF77 signal uses amplitude-shift keying to transmit time signals. For this the signal is reduced to 15% power (−8¼ dBFS) 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, sent during minutes 19, 39 and 59 of each hour, however this was discontinued as the station is easily identifiable by the characteristic signal. 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.

Phase modulation

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 ±13° phase-shift keying. 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 (7.22 ms) of each second are not phase-modulated.

The chip sequence is generated by a 9-bit 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 +13° phase advance, while during 1 chips it is transmitted with a −13° 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 signals of the satellite navigation system Global Positioning System (GPS). Due to the GPS signal structure and the larger bandwidth available, the GPS reception would, in principle, achieve an uncertainty of the time transmission - GPS time is accurate to about ± 10 to 30 nanoseconds - which is lower by at least one order of magnitude than the uncertainty which can be achieved with DCF77 receivers.

Time code interpretation

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.

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.

DCF77 time code

Bit	Weight		Meaning		Bit	Weight		Meaning		Bit	Weight		Meaning
	PM	AM				PM	AM				PM	AM
:00	1	M	Start of minute, always 0.		:20	S		Start of encoded time, always 1.		:40	10		Day of month (continued)
:01	1	Civil warning bits, provided by the Bundesamt für Bevölkerungsschutz und Katastrophenwarnung (Federal Office of Civil protection and Disaster Relief). Also contains weather broadcasts.			:21	1		Minutes 00–59		:41	20
:02	1				:22	2				:42	1		Day of week Monday=1, Sunday=7
:03	1				:23	4				:43	2
:04	1				:24	8				:44	4
:05	1				:25	10				:45	1		Month number 01–12
:06	1				:26	20				:46	2
:07	1				:27	40				:47	4
:08	1				:28	P1		Even parity over minute bits 21–28.		:48	8
:09	1				:29	1		Hours 0–23		:49	10
:10	0				:30	2				:50	1		Year within century 00–99
:11	0				:31	4				:51	2
:12	0				:32	8				:52	4
:13	0				:33	10				:53	8
:14	0				:34	20				:54	10
:15	R		Call bit: abnormal transmitter operation. Previously: backup antenna in use.		: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. 01–31		:56	40
: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	A2		Leap second announcement. Set during hour before leap second.		:39	8				:59	0	Minute mark: no amplitude modulation.

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.

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.

The time zone bits can be considered a binary-coded representation of the GMT 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. The civil warnings and weather information is not included in the phase-modulated data.

Control

Atomic master clock CS2 in use at the PTB to check for deviations

The control signal is not transmitted by wire from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig to the transmitting radio station in Mainflingen but is generated at the place of emission using a control unit developed by the PTB. This control unit, which is housed in a room of the transmitting station, is shielded against high-frequency interferences and controlled from Braunschweig. Via the public telephone network operational data of the control unit can be called up with the aid of a telecontrol system. Furthermore, the carrier phase time and the states of the second markers are compared in Braunschweig with the setpoints specified by the PTB's atomic clocks. Of these atomic clocks the CS2 atomic clock in Braunschweig provides the German national legal time standard, and can be used as a highly accurate frequency standard. If there are deviations, the necessary corrections will be made via the telecontrol system.

The DCF77 transmitted carrier frequency relative uncertainty is 2 x 10^-12 over a 24-hour period and 2 x 10^-13 over 100 days, with a deviation in phase with respect to UTC that never exceeds more than 5.5 ± 0.3 microseconds. The four German caesium atomic clocks (CS1, CS2, CSF1 and CSF2) used by PTB in Braunschweig ensure significantly less long term clock drift than the atomic clocks used in the DCF77 facility in Mainflingen. 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.

Accuracy

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 reduced 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. In case of just ground wave reception a constant is included in the calculation if the distance is permanent. In case of just space wave reception the reception side cannot influence the time fluctuations. Time fluctuations are influenced directly by the changing altitude of the reflecting and bending layer of the ionosphere. Similar problems arise where ground and space waves overlap. This field is not constant but changes in the course of the day between approximately 600 km (373 mi) to 1,100 km (684 mi) from the transmitter position.

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 and can achieve a practical accuracy uncertainty better than ± 2 milliseconds.

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 PTB in 1987 measured in Braunschweig situated 273 km (170 mi) from the transmitter in Mainflingen inaccuracies, depending on the time in the day and season, of ± 2 to 22 microseconds.

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 can therefore only be expected to determine the beginning of a second with a practical accuracy uncertainty of ± 0.1 second.

See also

• Coordinated Universal Time
• Radio clock
• Time from NPL (MSF)

External links

• Official DCF77 web page at the PTB
• Template:De icon The atomic clock in Frankfurt/Main Germany
• DCF77 on the Global Frequency Database
• Time code description
• Observation of DCF77 during a leap second
• DCF77 Arduino implementation Hardware, Signal, Code
• Meteotime
• Time and frequency dissemination with DCF77: 1959–2009 and beyond" – a detailed description of the history of DCF77