DCF77
DCF77 time code transmitter | |
---|---|
T-aerial antennas of DCF77 in Mainflingen | |
Location | Mainflingen longwave transmitter, Mainflingen, Germany |
Coordinates | 50°00′56″N 9°00′39″E / 50.01556°N 9.01083°E |
Elevation | 113 m (371 ft) |
Operator | Media Broadcast GmbH on behalf of the PTB |
Frequency | 77.5 kHz |
Power | 50 kW |
Began operation | 1 January 1959; continuous date and time information was added in June 1973 |
Official range | 2,000 km (1,243 mi) |
Website | DCF 77 |
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/Low_cost_DCF77_receiver.jpg/220px-Low_cost_DCF77_receiver.jpg)
DCF77 is a German
DCF77 is controlled by the
The highly accurate 77.5 kHz (3868.2897806 m wavelength)
Radio clocks and watches have been very popular in Europe since the late 1980s and, in mainland Europe, most of them use the DCF77 signal to set their time automatically.[5][6] The DCF77 longwave radio emission offers penetration into buildings and the time-transmissions can be received by small ferrite antennas incorporated in the case of radio-controlled low-cost time keepers without the help of exterior antennas.[7] The accuracy of the DCF77 amplitude-modulated time signals suffices for the every-day use of clocks and watches by consumers where primarily the long-term accuracy matters. Further industrial time-keeping systems at railway stations, in the field of telecommunication and information technology, and at radio and TV stations are radio-controlled by DCF77 as well as tariff change-over clocks of energy supply companies and clocks in traffic-light facilities.[8]
Signal
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/51/Bahnhof_Kinding_%28Altm%C3%BChltal%29%2C_Bahnsteiguhr_2007.jpg/220px-Bahnhof_Kinding_%28Altm%C3%BChltal%29%2C_Bahnsteiguhr_2007.jpg)
Time signal
The DCF77 station signal carries an amplitude-modulated, pulse-width coded 1 bit/s data signal. The same data signal is also
- Current date and time bits;
- Leap second warning bit;
- Imminent change of CET to CEST, or vice versa, Announcement bit;
- Central European Summer Time (CEST)bit;
- Abnormal transmitter operation identification bit;
- Several parity bits.
Experimental civil defence emergency signal
Since 2003, fourteen previously unused bits of the time code have been used for
Civil protection and weather forecast signal
Since 22 November 2006 the DCF77 transmitter uses bits 1–14 to transmit warning messages and weather information.
Future and call sign
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.
Time code details
Like most
Amplitude modulation
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/DCF77_code.svg/220px-DCF77_code.svg.png)
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.
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 ±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
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
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
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.[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.
Bit | Weight | Meaning | Bit | Weight | Meaning | Bit | Weight | Meaning | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
PhM | AM | PhM | AM | PhM | 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,[26] provided by the Bundesamt für Bevölkerungsschutz und Katastrophenwarnung (Federal Office of Civil protection and Disaster Relief). Also contains weather broadcasts.[25][27] |
: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.[25] 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.[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
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.
Reception area
![](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b4/Dcf_weite.jpg/220px-Dcf_weite.jpg)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9a/DCF77_field_strength.png/220px-DCF77_field_strength.png)
With a relatively high power of 50 kW, the DCF77 transmissions can reliably be received in large parts of Europe, as far as 2,000 km (1,243 mi) from the transmitter in Mainflingen. Within this range the
Depending on signal propagation and multiple reflections (hops) and local interference the DCF77 signal can sometimes be received further away (see tropospheric propagation). This is associated with a significant decrease in the signal strength and depends on many factors, e.g., the daytime and season, the angle of incidence of the skywave on the D-layer and the solar activity.[32]
Control
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/Atomuhr-CS2.jpg/220px-Atomuhr-CS2.jpg)
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 an air conditioned room of the transmitting station, is shielded against high-frequency interferences and controlled from Braunschweig. For reasons of operational reliability, the control signal is generated by three independent control channels all equipped with their own caesium atomic clock. In addition a rubidium atomic clock is available on site. To avoid incorrect emissions the output of these three channels are compared in two electronic switch circuits on site. Output for transmission is only generated when at least two of the three channels are in agreement. 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 master clocks that provide the UTC (PTB). 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.[33] If there are deviations, the necessary corrections will be made via the telecontrol system.[34]
Accuracy
Transmission
The DCF77 transmitted
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]
Reception
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3d/Inside_radio_clock.jpg/220px-Inside_radio_clock.jpg)
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]
Network Time Protocol reference clock use
Network Time Protocol time servers display the refid identifier .DCFa. (amplitude modulation) or .DCFp. (phase modulation) when a standard DCF77 time receiver is used as the reference time source.[40]
References
- ^ "DCF77 Receiver authorization and availability". Physikalisch-Technische Bundesanstalt (PTB). 2014-02-03. Retrieved 2014-09-10.
- ^ "Das abwechslungsreichste Radioprogramm der Welt" (in German). Physikalisch-Technische Bundesanstalt (PTB). 2003-02-21. Retrieved 2013-10-02.
- ^ "Time and Standard Frequency Station DCF77 (Germany)". 100503 eecis.udel.edu
- ^ "Realisation of Legal Time in Germany". Physikalisch-Technische Bundesanstalt (PTB). 2013-08-08. Retrieved 2013-09-26.
- ISBN 9789627853596.
- ISBN 9783744894920.
- ^ "Benefit of DCF77". Physikalisch-Technische Bundesanstalt (PTB). 2021-11-29. Retrieved 2021-11-29.
- ^ "Benefit of DCF77". Physikalisch-Technische Bundesanstalt (PTB). 2010-03-12. Retrieved 2014-09-10.
- ^ "Zeitumstellung - Zeit und Uhren bei der Bahn" (in German). Archived from the original on 2017-11-07. Retrieved 2017-11-04.
- ^ "BBK homepage". German Federal Office of Civil Protection and Disaster Assistance (BBK). 2013-03-09. Retrieved 2013-03-09.
- ^ a b "DCF77 fit für die Zukunft. PTB-Zeitsignal-Aussendung per Langwellensender ist "runderneuert" worden" (in German). Physikalisch-Technische Bundesanstalt PTB. 2006-12-12. Retrieved 2010-12-12.
- ^ Meteotime A blog post describing decoding the weather information with an Arduino.
- ^ Meteo Time GmbH (2006-10-27), Wetterdatenbeschreibung des Systems Meteotime [Weather data description of the Meteotime System] (PDF) (in German), archived from the original (PDF) on 2009-12-29
- ^ "Welcome at Meteotime.com". Meteo Time GmbH. 2013-03-09. Retrieved 2013-03-09.
- ^ "Auch weiterhin auf Sendung: das Zeitsignal auf Langwelle" (in German). pro-physik.de. 2021-10-26. Retrieved 2021-11-29.
- ^ "Die Zeit auf Langwelle gibt es auch weiterhin" (in German). Physikalisch-Technischen Bundesanstalt (PTB). 2021-10-25. Retrieved 2021-11-29.
- ^ Zeit- und Normalfrequenzverbreitung mit DCF77 (PDF) (in German), Physikalisch-Technische Bundesanstalt, p. 6, retrieved 2009-08-12
- ^ DCF77 phase modulation, Physikalisch-Technische Bundesanstalt
- ^ a b c Hetzel, P. (16 March 1988). Time dissemination via the LF transmitter DCF77 using a pseudo-random phase-shift keying of the carrier (PDF). 2nd European Frequency and Time Forum. Neuchâtel. pp. 351–364. Retrieved 11 October 2011.
- ^ David W. Allan (1997). "The Science of Timekeeping". Hewlett Packard.
- ^ "Common View GPS Time Transfer". nist.gov. Archived from the original on 2012-10-28. Retrieved 2011-07-23.
- ^ "EUROPEAN GNSS (GALILEO) SERVICES OPEN SERVICE QUARTERLY PERFORMANCE REPORT APRIL - JUNE 2021" (PDF). European GNSS Service Centre. 1 July 2021. Archived (PDF) from the original on 2021-11-30. Retrieved 30 November 2021.
- ^ "Galileo Open Service and Search and Rescue – Quarterly Performance Reports, containing measured performance statistics". Archived from the original on 26 August 2019. Retrieved 3 March 2019.
- ^ "Galileo Goes Live". European GNSS Agency. 15 December 2016. Archived from the original on 15 January 2021. Retrieved 1 February 2017.
- ^ a b c d DCF77 time code, Physikalisch-Technische Bundesanstalt, 2022-07-14
- ^ Warnings to the general public by means of CF77?, Physikalisch-Technische Bundesanstalt, 2007-05-09
- ^ Piester, D.; Bauch, A.; Becker, J.; Polewka, T.; Rost, M.; Sibold, D.; Staliuniene, E. (2006-12-05), "PTB's Time and Frequency Activities in 2006", Proc. 38th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting (PDF), pp. 37–47, retrieved 2009-03-24
- ^ The date XX00-02-28 must fall on a Monday, Sunday, Friday, or Wednesday. Only the first case is a leap year, followed by Tuesday the 29th. In the other three cases, the next day is March 1.
- S2CID 11484317, retrieved 2013-12-05
- ^ In which area can the DCF77 time signal be received?
- ^ a b Reach of DCF77
- ^ "Reichweite des DCF77-Senders". Die Funk-Uhr Homepage (in German). Robert Heret und Thomas Losert. Retrieved 2010-12-12.
- ^ "With what accuracy do PTB's atomic clocks work?". Archived from the original on 2012-09-25. Retrieved 2012-05-29.
- ^ "How is time transmitted?". Archived from the original on 2012-06-02. Retrieved 2012-05-29.
- ^ DCF77 carrier frequency
- ^ SI seconds ever more accurate, PTB-News 3-2019
- ^ How the DCF77-receiver works
- ^ Michael A. Lombardi, "How Accurate is a Radio Controlled Clock?, National Institute of Standards and Technology, 2010.
- ^ RADIO-CONTROLLED WALLCLOCK INSTRUCTION MANUAL
- ^ "Checking the NTP Status". Network Time Protocol (NTP) (in German). Meinberg Radio Clocks GmbH & Co. KG. Retrieved 2011-08-29.
Further reading
- Dietze, Andreas; Kriedt, Hans; Wondra, Jürgen (August 1981). "Anregung zum Nachbau: Einfacher Empfänger für DCF77-Zeitzeichensignale" (PDF). Applikationsseiten. Funk-Technik - Fachzeitschrift für die gesamte Unterhaltungselektronik (in German). Vol. 36, no. 8. Heidelberg, Germany: Dr. Alfred Hüthig Verlag GmbH. pp. 279–282. (PDF) from the original on 2021-07-10. Retrieved 2021-07-10. (4 pages)
External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Official DCF77 web page at the PTB
- (in German) The atomic clock in Frankfurt/Main Germany
- DCF77 Archived 2012-03-31 at the Wayback Machine on the Global Frequency Database Archived 2013-03-30 at the Wayback Machine
- Time code description
- Observation of DCF77 during a leap second
- DCF77 Arduino implementation Hardware Archived 2013-07-22 at the Wayback Machine, Signal Archived 2013-07-24 at the Wayback Machine, Code Archived 2013-07-25 at the Wayback Machine
- Meteotime
- Time and frequency dissemination with DCF77: 1959–2009 and beyond" – a detailed description of the history of DCF77
- How to build a clock DCF (archive.org copy)
- A historical visit at DCF77 Mainflingen by Ekkehard Plicht