Editing DCF77

{{Short description|German longwave time signal radio station}}
{{Use British English|date=December 2024}}
{{Use dmy dates|date=December 2024}}
{{Infobox
|above= DCF77 time code transmitter
|image=[[File:Sendeanlage Mainhausen Mainflingen DCF77 Zeitzeichen Luftbild.jpg|300px]]
|caption=The [[low frequency]] [[T-aerial]] antennas of DCF77 in Mainflingen
|label1=Location
|data1=[[Mainflingen longwave transmitter]], [[Mainflingen]], Germany
|label2=Coordinates
|data2={{coord|50.01556|N|9.01083|E|type:landmark_region:DE-HE|display=it}}
|label3=Elevation
|data3={{convert|113|m|ft|sigfig=2|abbr=on}}
|label4=Operator
|data4=Media Broadcast GmbH on behalf of the [[Physikalisch-Technische Bundesanstalt|PTB]]
|label5=Frequency
|data5=77.5 kHz
|label6=Power
|data6=50 kW
|label7=Began operation
|data7=1 January 1959; continuous date and time information was added in June 1973
|label8=Ceased operation
|data8=
|label9=Official range
|data9={{convert|2000|km|mi|sigfig=2|abbr=on}}
|label10=Website
|data10=[http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77.html DCF 77]
}}

[[Image:Low cost DCF77 receiver.jpg|thumb|Low cost DCF77 receiver]]

'''DCF77''' is a German [[longwave]] [[time signal]] and standard-[[frequency]] radio station. It started service as a standard-frequency station on 1&nbsp;January 1959. In June 1973, date and time information was added. Its primary and backup [[transmitter]] are located at {{coord|50|0|56|N|9|00|39|E}} in [[Mainflingen]], about {{convert|17|mi|km|sigfig=2|abbr=on}} south-east of [[Frankfurt am Main]],<ref>{{cite web|access-date=2025-01-27 |title=Air-line distance calculator |type=between primary antenna and Frankfurt/Main central train station |url=https://www.luftlinie.org/50.015528,%209.008515/50.107100,8.662312}}<!-- auto-translated from German by Module:CS1 translator --></ref> [[Germany]]. The transmitter generates a nominal power of 50&nbsp;kW, of which about 30 to 35&nbsp;kW can be radiated via a [[T-antenna]].

DCF77 is controlled by the ''[[Physikalisch-Technische Bundesanstalt]]'' (PTB), Germany's national [[physics]] laboratory and transmits in continuous operation (24&nbsp;hours). It is operated by ''Media Broadcast GmbH'' (previously a subsidiary of ''[[Deutsche Telekom AG]]''), on behalf of the PTB. With Media Broadcast GmbH, a temporal transmission availability of at least 99.7% per year or under 26.28 hours of annual downtime has been agreed upon. Most service interruptions are short-term disconnections of under two minutes. Longer lasting transmission service interruptions are generally caused by strong winds, freezing rain or snow-induced T-antenna movement. This manifests itself in electrical detuning of the antenna resonance circuit and hence a measurable [[phase modulation]] of the received signal. When the maladjustment is too large, the transmitter is taken out of service temporarily.<ref>{{cite web |url=http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-receiver-authorization-and-availability.html |title=DCF77 Receiver authorization and availability |publisher=Physikalisch-Technische Bundesanstalt (PTB) |date=2014-02-03 |access-date=2014-09-10 }}</ref> In the year 2002, almost 99.95% availability, or just over 4.38 hours of downtime, was realized.<ref>{{cite web |url=http://idw-online.de/pages/de/news59790%20 |title=Das abwechslungsreichste Radioprogramm der Welt |publisher=Physikalisch-Technische Bundesanstalt (PTB) |date=2003-02-21 |access-date=2013-10-02 |language= de}}</ref> The timestamp sent is either in [[Coordinated Universal Time]] (UTC)+1 or [[Coordinated Universal Time|UTC]]+2 depending on [[daylight saving time]].<!-- Link twice for esthetic reason --><ref name="eecis_dcf77">{{cite web|title=Time and Standard Frequency Station DCF77 (Germany)|url=http://www.eecis.udel.edu/~mills/ntp/dcf77.html}} 100503 eecis.udel.edu</ref>

The highly accurate 77.5&nbsp;kHz ({{val|{{#expr:299792458/77500 round 7}}|u=m}} wavelength) [[carrier signal]] is generated from local [[atomic clock]]s that are linked with the German master clocks at the PTB in [[Braunschweig]]. The DCF77 time signal is used for the dissemination of the [[Time in Germany|German national legal time]] to the public.<ref>{{cite web |url=http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html |title=Realisation of Legal Time in Germany |publisher=Physikalisch-Technische Bundesanstalt (PTB) |date=2013-08-08 |access-date=2013-09-26 }}</ref>

[[Radio clock]]s and [[Watch#Movement|watch]]es 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.<ref>{{cite book | url=https://books.google.com/books?id=EDXUGGnojasC&dq=german+radio+control+watch&pg=PA16 | title=Market Intelligence Report: Clocks | year=2004 | publisher=Global Sources | isbn=9789627853596 }}</ref><ref>{{cite book | url=https://books.google.com/books?id=m48wDwAAQBAJ | title=Collecting (Vintage) Watches: Wristwatches, antique- and vintage pocket watches | isbn=9783744894920 | last1=Meine | first1=Thomas M. | last2=Stannard | first2=Matthew | date=30 April 2021 | publisher=BoD – Books on Demand }}</ref> 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.<ref>{{cite web |url=https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/benefit-of-dcf77.html |title=Benefit of DCF77 |publisher=Physikalisch-Technische Bundesanstalt (PTB) |date=2021-11-29 |access-date=2021-11-29}}</ref> 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.<ref>{{cite web |url=http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/benefit-of-dcf77.html |title=Benefit of DCF77 |publisher=Physikalisch-Technische Bundesanstalt (PTB) |date=2010-03-12 |access-date=2014-09-10}}</ref>

<mapframe text="DCF77 primary and backup antenna" latitude="50.01539" longitude="9.01143" zoom="14" width="300" height="300">
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        "coordinates": [9.008515, 50.015528]
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        "title": "Backup antenna",
        "marker-color": "f00" 
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</mapframe>
== Location of antennas ==
According to the Air-Line, the distance is 350&nbsp;m between the primary and backup antenna.<ref>{{cite web|access-date=2025-01-25 |title=Air-Line distance calculator |type=between Primary and Backup Antenna of DCF77) |url=https://www.luftlinie.org/50.015528,%209.008515/50.012879,%209.011149}}<!-- auto-translated from German by Module:CS1 translator --></ref> At a receiver pointing northwest or southeast, there is a maximum transit time of <math>{\frac {\pm 350\,\mathrm{m}} c} \approx \pm 1{,}3\,\mathrm{\mu s}</math>, and at a minimum, when exactly orthogonal, the transit time difference vanishes. The value for other geographical direction is somewhere inbetween.

==Signal==
[[File:Bahnhof Kinding (Altmühltal), Bahnsteiguhr 2007.jpg|thumb|The DCF77 time signal is used by organizations like the [[Deutsche Bahn]] railway company to synchronize their [[station clock]]s.<ref>{{Cite web |url=http://www.deutschebahn.com/presse/duesseldorf/de/aktuell/im_blickpunkt/9108844/Zeitumstellung_bei_der_Bahn.html |title=Zeitumstellung - Zeit und Uhren bei der Bahn |language=de |access-date=2017-11-04 |archive-url=https://web.archive.org/web/20171107030603/http://www.deutschebahn.com/presse/duesseldorf/de/aktuell/im_blickpunkt/9108844/Zeitumstellung_bei_der_Bahn.html |archive-date=2017-11-07 |url-status=dead }}</ref>]]
[[File:2016-Mainflingen-DCF-1 1.jpg|thumb|The [[Mainflingen longwave transmitter|Mainflingen transmitter]] uses isolated [[Guyed mast|guyed]] [[Radio masts and towers|lattice masts]] to elevate the DCF77 antennas.]]
[[File:Sender Mainflingen 09032015 10.JPG|thumb|The low-frequency T-aerial antennas of the continuously  operated DCF77 signal in Mainflingen at night]]

===Time signal===
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 wave|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|Central European Time (CET)]] / [[European Summer Time|Central European Summer Time (CEST)]] bit;
* Abnormal transmitter operation identification bit;
* Several [[parity bit]]s.

===Experimental civil defence emergency signal===
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 siren]]s.

===Civil protection and weather forecast signal===
Since 22 November 2006, the DCF77 transmitter uses bits 1–14 to transmit warning messages and weather information.<ref>{{cite web |url=http://www.bbk.bund.de/EN/Home/home_node.html |title=BBK homepage |publisher=German Federal Office of Civil Protection and Disaster Assistance (BBK) |date=2013-03-09 |access-date=2013-03-09}}</ref><ref name="ptb.de">{{cite web |url=http://www.ptb.de/de/aktuelles/archiv/presseinfos/pi2006/pitext/pi061212.htm |title=DCF77 fit für die Zukunft. PTB-Zeitsignal-Aussendung per Langwellensender ist "runderneuert" worden |publisher=[[Physikalisch-Technische Bundesanstalt]] PTB |date=2006-12-12 |access-date=2010-12-12 |language= de}}</ref> 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 clock]]s 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.<ref>{{usurped|1=[https://web.archive.org/web/20160314161158/http://arduino-projects4u.com/meteotime/ Meteotime]}} A blog post describing decoding the weather information with an [[Arduino]].</ref><ref>{{Citation |title=Wetterdatenbeschreibung des Systems Meteotime |author=Meteo Time GmbH |date=2006-10-27 |url=http://www.hkw-elektronik.de/pdf/DB%20W-Protokoll-V%201.pdf |language=de |trans-title=Weather data description of the Meteotime System |url-status=dead |archive-url=https://web.archive.org/web/20091229063350/http://www.hkw-elektronik.de/pdf/DB%20W-Protokoll-V%201.pdf |archive-date=2009-12-29 }}</ref> 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.<ref name="ptb.de"/><ref>{{cite web |url=http://www.meteotime.com/Web/en/Home/Default.htm |title=Welcome at Meteotime.com |publisher=Meteo Time GmbH |date=2013-03-09 |access-date=2013-03-09}}</ref> Since the bits previously reserved for the PTB are used, older radio clocks should not be affected by the weather data signal.

===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.<ref>{{cite web|url=https://www.pro-physik.de/nachrichten/auch-weiterhin-auf-sendung-das-zeitsignal-auf-langwelle |title=Auch weiterhin auf Sendung: das Zeitsignal auf Langwelle |publisher=pro-physik.de |date=2021-10-26 |access-date=2021-11-29 |language=de}}</ref><ref>{{cite web|url=https://www.ptb.de/cms/en/presseaktuelles/journalisten/news-press-releases/press-release.html?tx_news_pi1%5Bnews%5D=11219&tx_news_pi1%5Bcontroller%5D=News&tx_news_pi1%5Baction%5D=detail&tx_news_pi1%5Bday%5D=25&tx_news_pi1%5Bmonth%5D=10&tx_news_pi1%5Byear%5D=2021&cHash=1e27d53fe127612d4f55345fbf7a40b8 |title=Die Zeit auf Langwelle gibt es auch weiterhin  |publisher=Physikalisch-Technischen Bundesanstalt (PTB) |date=2021-10-25 |access-date=2021-11-29 |language=de}}</ref>

The [[call sign]] DCF77 stands for D = ''Deutschland'' (Germany), C = long wave signal, F = Frankfurt am Main as the umbrella regional unit, 77 = frequency: 77.5&nbsp;kHz.

== Time code details ==
Like most [[longwave]] time transmitters (akin to the 162&nbsp;kHz 800&nbsp;kW [[TDF time signal]] broadcast from France), 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 ===
[[File:DCF77 code.svg|thumb|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&nbsp;[[decibel|dB]]) for 0.1 or 0.2 seconds at the beginning of each second. A 0.1 second reduction (7750 cycles of the 77500&nbsp;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.<ref>{{Citation|url=http://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/4.42/dcf77.pdf |title=Zeit- und Normalfrequenzverbreitung mit DCF77 |page=6 |language=de |publisher=[[Physikalisch-Technische Bundesanstalt]] |access-date=2009-08-12}}</ref> A 250&nbsp;Hz tone was generated by [[Square wave (waveform)|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 (CDMA)|chip]] sequence and encoded on the carrier using ±15.6° [[phase-shift keying]].<ref>{{Citation |url=http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-phase-modulation.html |title=DCF77 phase modulation |publisher=Physikalisch-Technische Bundesanstalt}}</ref>  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.<ref name="phasemod">{{cite conference |url=https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/pdf/5_1988_Hetzel_-_Proc_EFTF_88.pdf |title=Time dissemination via the LF transmitter DCF77 using a pseudo-random phase-shift keying of the carrier |first=P. |last=Hetzel |date=16 March 1988 |conference=2nd European Frequency and Time Forum |location=Neuchâtel |pages=351–364 |access-date=11 October 2011}}</ref>

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:
<syntaxhighlight lang="c">
  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;
  }
</syntaxhighlight>

Each time code bit to be transmitted is [[exclusive-or]]ed 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 [[GNSS applications|(precise time reference) signals]] transmitted by [[satellite navigation system|global navigation satellite system]]s like the [[Global Positioning System]] (GPS), [[GLONASS]], [[Galileo (satellite navigation)|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 [[orders of magnitude (time)|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 [[nanosecond]]s<ref>{{cite web|author=David W. Allan |url=http://www.allanstime.com/Publications/DWA/Science_Timekeeping/index.html |title=The Science of Timekeeping |publisher=Hewlett Packard |year=1997}}</ref><ref>{{cite web|url=http://tf.nist.gov/time/commonviewgps.htm |title=Common View GPS Time Transfer |publisher=nist.gov |access-date=2011-07-23 |url-status=dead |archive-url=https://web.archive.org/web/20121028043917/http://tf.nist.gov/time/commonviewgps.htm |archive-date=2012-10-28 }}</ref> and the Galileo April, May, June 2021 Quarterly Performance Report by the European GNSS Service Centre reported the UTC Time Dissemination Service Accuracy was ≤&nbsp;4.3 ns, computed by accumulating samples over the previous 12 months and exceeding the ≤&nbsp;30 ns target<ref name="Timing accuracy2">{{cite web|url=https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-OS-Quarterly-Performance_Report-Q2-2021.pdf|title=EUROPEAN GNSS (GALILEO) SERVICES OPEN SERVICE QUARTERLY PERFORMANCE REPORT APRIL - JUNE 2021|publisher=European GNSS Service Centre|date=1 July 2021|access-date=30 November 2021|url-status=live|archive-url=https://web.archive.org/web/20211130104613/https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-OS-Quarterly-Performance_Report-Q2-2021.pdf |archive-date=2021-11-30 }}</ref><ref>{{Cite web |url=https://www.gsc-europa.eu/electronic-library/performance-reports |title=Galileo Open Service and Search and Rescue – Quarterly Performance Reports, containing measured performance statistics |access-date=3 March 2019 |archive-date=26 August 2019 |archive-url=https://web.archive.org/web/20190826170704/https://www.gsc-europa.eu/electronic-library/performance-reports |url-status=dead }}</ref><ref name="Timing accuracy">{{cite web|url=https://www.gsa.europa.eu/galileo/services/initial-services|title=Galileo Goes Live|publisher=European GNSS Agency|date=15 December 2016|access-date=1 February 2017|archive-date=15 January 2021|archive-url=https://web.archive.org/web/20210115130957/http://www.gsa.europa.eu/galileo/services/initial-services|url-status=live}}</ref>).

=== 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.&nbsp;g. during 31&nbsp;December, 23:59, the transmitted time encodes 1&nbsp;January, 00:00.<ref name=timecode>{{Citation |url=https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-time-code.html|title=DCF77 time code |publisher=Physikalisch-Technische Bundesanstalt |date=2022-07-14}}</ref>

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.

{|class=wikitable style="text-align:center"
|+ DCF77 time code
!rowspan=2| Bit ||colspan=2| Weight ||rowspan=2| Meaning
|rowspan=22|
!rowspan=2| Bit ||colspan=2| Weight ||rowspan=2| Meaning
|rowspan=22|
!rowspan=2| Bit ||colspan=2| Weight ||rowspan=2| Meaning
|-
! PhM || AM || PhM || AM || PhM || AM
|-
|bgcolor=#ffccff| :00 ||bgcolor=#ffccff| 1 ||bgcolor=#ffccff| M ||bgcolor=#ffccff| Start of minute. Always 0.
|bgcolor=#ffccff| :20 ||colspan=2 bgcolor=#ffccff| S ||bgcolor=#ffccff| Start of encoded time. Always 1.
|bgcolor=#ccffcc| :40 ||colspan=2 bgcolor=#ccffcc| 10 ||rowspan=2 bgcolor=#ccffcc|Day of month (continued)
|-
| :01 ||bgcolor=#ffccff| 1 ||colspan=2 rowspan=14| Civil warning bits,<ref>{{Citation |url=http://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/warnings-to-the-general-public-by-means-of-dcf77.html |title=Warnings to the general public by means of CF77? |publisher=Physikalisch-Technische Bundesanstalt |date=2007-05-09}}</ref> provided by the<br/>''Bundesamt für Bevölkerungsschutz<br/>und Katastrophenwarnung'' (Federal Office<br/>of Civil protection and Disaster Relief).<br/>Also contains weather broadcasts.<ref name=timecode/><ref>{{Citation |url=http://www.piester.de/dp200701.pdf |title=Proc. 38th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting |contribution=PTB's Time and Frequency Activities in 2006 |first1=D. |last1=Piester |first2=A. |last2=Bauch |first3=J. |last3=Becker |first4=T. |last4=Polewka |first5=M. |last5=Rost |first6=D. |last6=Sibold |first7=E. |last7=Staliuniene |pages=37–47 |date=2006-12-05 |access-date=2009-03-24}}</ref>
|bgcolor=#ffffcc| :21 ||colspan=2 bgcolor=#ffffcc| 1 ||rowspan=7 bgcolor=#ffffcc| Minutes<br/>00–59
|bgcolor=#ccffcc| :41 ||colspan=2 bgcolor=#ccffcc| 20
|-
| :02 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :22 ||colspan=2 bgcolor=#ffffcc| 2
|bgcolor=#ccffcc| :42 ||colspan=2 bgcolor=#ccffcc| 1 ||rowspan=3 bgcolor=#ccffcc| Day of week<br/>Monday=1, Sunday=7
|-
| :03 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :23 ||colspan=2 bgcolor=#ffffcc| 4
|bgcolor=#ccffcc| :43 ||colspan=2 bgcolor=#ccffcc| 2
|-
| :04 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :24 ||colspan=2 bgcolor=#ffffcc| 8
|bgcolor=#ccffcc| :44 ||colspan=2 bgcolor=#ccffcc| 4
|-
| :05 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :25 ||colspan=2 bgcolor=#ffffcc| 10
|bgcolor=#ccffcc| :45 ||colspan=2 bgcolor=#ccffcc| 1 ||rowspan=5 bgcolor=#ccffcc| Month number<br/>01–12
|-
| :06 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :26 ||colspan=2 bgcolor=#ffffcc| 20
|bgcolor=#ccffcc| :46 ||colspan=2 bgcolor=#ccffcc| 2
|-
| :07 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :27 ||colspan=2 bgcolor=#ffffcc| 40
|bgcolor=#ccffcc| :47 ||colspan=2 bgcolor=#ccffcc| 4
|-
| :08 ||bgcolor=#ffccff| 1
|bgcolor=#ffffcc| :28 ||colspan=2 bgcolor=#ffffcc| P1 ||bgcolor=#ffffcc| [[Even parity]] over minute bits 21–27.
|bgcolor=#ccffcc| :48 ||colspan=2 bgcolor=#ccffcc| 8
|-
| :09 ||bgcolor=#ffccff| 1
|bgcolor=#ffcccc| :29 ||colspan=2 bgcolor=#ffcccc| 1 ||rowspan=6 bgcolor=#ffcccc| Hours<br/>0–23
|bgcolor=#ccffcc| :49 ||colspan=2 bgcolor=#ccffcc| 10
|-
| :10 ||bgcolor=#ffccff| 0
|bgcolor=#ffcccc| :30 ||colspan=2 bgcolor=#ffcccc| 2
|bgcolor=#ccffcc| :50 ||colspan=2 bgcolor=#ccffcc| 1 ||rowspan=8 bgcolor=#ccffcc| Year within century<br/>00–99
|-
| :11 ||bgcolor=#ffccff| 0
|bgcolor=#ffcccc| :31 ||colspan=2 bgcolor=#ffcccc| 4
|bgcolor=#ccffcc| :51 ||colspan=2 bgcolor=#ccffcc| 2
|-
| :12 ||bgcolor=#ffccff| 0
|bgcolor=#ffcccc| :32 ||colspan=2 bgcolor=#ffcccc| 8
|bgcolor=#ccffcc| :52 ||colspan=2 bgcolor=#ccffcc| 4
|-
| :13 ||bgcolor=#ffccff| 0
|bgcolor=#ffcccc| :33 ||colspan=2 bgcolor=#ffcccc| 10
|bgcolor=#ccffcc| :53 ||colspan=2 bgcolor=#ccffcc| 8
|-
| :14 ||bgcolor=#ffccff| 0
|bgcolor=#ffcccc| :34 ||colspan=2 bgcolor=#ffcccc| 20
|bgcolor=#ccffcc| :54 ||colspan=2 bgcolor=#ccffcc| 10
|-
|bgcolor=#ccffff| :15 ||colspan=2 bgcolor=#ccffff| R ||bgcolor=#ccffff| Call bit: abnormal transmitter operation.<ref name=timecode/> Previously: backup antenna in use.
|bgcolor=#ffcccc| :35 ||colspan=2 bgcolor=#ffcccc| P2 ||bgcolor=#ffcccc| Even parity over hour bits 29–34.
|bgcolor=#ccffcc| :55 ||colspan=2 bgcolor=#ccffcc| 20
|-
|bgcolor=#ccffff| :16 ||colspan=2 bgcolor=#ccffff| A1 ||bgcolor=#ccffff| [[Daylight saving time|Summer time]]/standard time changeover announcement.<br/>Set during hour before change.
|bgcolor=#ccffcc| :36 ||colspan=2 bgcolor=#ccffcc| 1 ||rowspan=4 bgcolor=#ccffcc| Day of month.<br/>01–31
|bgcolor=#ccffcc| :56 ||colspan=2 bgcolor=#ccffcc| 40
|-
|bgcolor=#ccffff| :17 ||colspan=2 bgcolor=#ccffff| Z1 ||bgcolor=#ccffff| Set to 1 when [[Central European Summer Time|CEST]] is in effect.
|bgcolor=#ccffcc| :37 ||colspan=2 bgcolor=#ccffcc| 2
|bgcolor=#ccffcc| :57 ||colspan=2 bgcolor=#ccffcc| 80
|-
|bgcolor=#ccffff| :18 ||colspan=2 bgcolor=#ccffff| Z2 ||bgcolor=#ccffff| Set to 1 when [[Central European Time|CET]] is in effect.
|bgcolor=#ccffcc| :38 ||colspan=2 bgcolor=#ccffcc| 4
|bgcolor=#ccffcc| :58 ||colspan=2 bgcolor=#ccffcc| P3 ||bgcolor=#ccffcc| Even parity over date bits 36–57.
|-
|bgcolor=#ccffff| :19 ||colspan=2 bgcolor=#ccffff| A2 ||bgcolor=#ccffff| [[Leap second]] announcement.<br/>Set during hour before leap second.
|bgcolor=#ccffcc| :39 ||colspan=2 bgcolor=#ccffcc| 8
|bgcolor=#ff88ff| :59  ||bgcolor=#ffccff| 0 ||colspan=2 bgcolor=#ff88ff| 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.<ref name=timecode/>

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.<ref>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.</ref>

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.<ref name="Engeler">{{Citation |title=Performance analysis and receiver architectures of DCF77 radio-controlled clocks |first=Daniel |last=Engeler |journal=IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control |volume=59 |issue=5 |date=May 2012 |pages=869–884 |doi=10.1109/TUFFC.2012.2272 |pmid=22622972 |s2cid=11484317 |url=http://caxapa.ru/thumbs/417284/Engeler_DCF77.pdf |access-date=2013-12-05}}</ref>  The civil protection warnings and weather information is not included in the phase-modulated data.

==Reception area==
[[File:Europe DCF77 iso.svg|thumb|DCF77 reception area from Mainflingen]]
[[File:DCF77_field_strength.png|thumb|DCF77 signal strength over a 24-hour period measured in [[Nerja]], on the south coast of Spain {{convert|1800|km|mi|sigfig=2|abbr=on}} from the transmitter. Around 1 AM, it peaks at ≈ 100 μV/m signal strength. During the day, the signal is weakened by ionization of the ionosphere due to solar activity.]]

With a relatively high power of {{nowrap|50 kW}}, the DCF77 transmissions can reliably be received in large parts of Europe, as far as {{convert|2000|km|mi|sigfig=2|abbr=on}} from the transmitter in Mainflingen. Within this range the [[signal strength]] of the DCF77 signal as specified by the [[Physikalisch-Technische Bundesanstalt]] (PTB) is ≥ 100 μV/m. This signal strength assessment was made according to the reflection model with a reflection (one hop) on the [[ionospheric]] D-layer. As an example, reception with consumer grade clocks&nbsp;— assuming the employed radio clock can manage reception with ≈ 100 μV/m signal strength&nbsp;— is possible in Norway (Bodø), Russia (Moscow), Turkey (Istanbul), Gibraltar and Portugal (during night hours). Metal structures or interference caused by other electronic devices can cause reception problems within this range.<ref>[https://www.meinbergglobal.com/english/faq/faq_21.htm In which area can the DCF77 time signal be received?]</ref> At shorter distances the DCF77 signal strength is much higher. As an example, under {{convert|500|km|mi|sigfig=1|abbr=on}} from the transmitter in Mainflingen the expected signal strength of the [[ground wave]] is ≥ 1 mV/m.<ref name="Reach of DCF77">[http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/reach-of-dcf77.html Reach of DCF77]</ref>

Depending on signal [[radio propagation|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.<ref>{{cite web |url=http://www.heret.de/funkuhr/reichw.htm |title=Reichweite des DCF77-Senders |work=Die Funk-Uhr Homepage |publisher=Robert Heret und Thomas Losert |access-date=2010-12-12 |language=de}}</ref>

==Control==
[[File:Atomuhr-CS2.jpg|thumb|Atomic master clock CS2 in use at the [[Physikalisch-Technische Bundesanstalt|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 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 standard|caesium atomic clock]]. In addition a [[rubidium standard|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.<ref>{{Cite web |url=http://www.ptb.de/cms/en/themenrundgaenge/wegweiser/fragenzurzeit/fragenzurzeit12.html |title=With what accuracy do PTB's atomic clocks work? |access-date=2012-05-29 |archive-url=https://web.archive.org/web/20120925065435/http://www.ptb.de/cms/en/themenrundgaenge/wegweiser/fragenzurzeit/fragenzurzeit12.html |archive-date=2012-09-25 |url-status=dead }}</ref>
If there are deviations, the necessary corrections will be made via the telecontrol system.<ref>{{Cite web |url=http://www.ptb.de/cms/en/themenrundgaenge/wegweiser/fragenzurzeit/fragenzurzeit10.html |title=How is time transmitted? |access-date=2012-05-29 |archive-url=https://web.archive.org/web/20120602055020/http://www.ptb.de/cms/en/themenrundgaenge/wegweiser/fragenzurzeit/fragenzurzeit10.html |archive-date=2012-06-02 |url-status=dead }}</ref>

==Accuracy==

===Transmission===
The DCF77 transmitted [[carrier frequency]] relative uncertainty is 2 × 10<sup>−12</sup> over a 24-hour period and 2 × 10<sup>−13</sup> over 100 days, with a deviation in phase with respect to UTC that never exceeds 5.5 ± 0.3 [[microsecond]]s.<ref>[http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-carrier-frequency.html DCF77 carrier frequency]</ref> 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.<ref>[https://www.ptb.de/cms/en/presseaktuelles/journals-magazines/ptb-news/ptb-news-ausgaben/archivederptb-news/ptb-news-archives.html?tx_news_pi1%5Bnews%5D=9851&tx_news_pi1%5Bcontroller%5D=News&tx_news_pi1%5Baction%5D=detail&tx_news_pi1%5Bday%5D=24&tx_news_pi1%5Bmonth%5D=9&tx_news_pi1%5Byear%5D=2019&cHash=fbf49e1c2f12573f70424f2fa6f13428 SI seconds ever more accurate, PTB-News 3-2019]</ref> 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&nbsp;kHz carrier frequency of the DCF77 signal, or within ± 6.452 × 10<sup>−6</sup> s or ± 6.452 microseconds.<ref name="phasemod"/>

===Reception===
[[File:Inside radio clock.jpg|thumb|Consumer grade radio clock movement with the DCF77 receiver (right) in the clock. The small ferrite [[Loop antenna#AM broadcast receiving antennas|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 [[Atomic clock#Time signal radio transmitters|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 {{convert|1000|km|mi|sigfig=1|lk=on|abbr=on}} away from the DCF77 transmitter, due to transit delay the receiver will be set more than 3 [[millisecond]]s 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 [[surface wave|ground]] and [[skywave]]s overlap. This field is not constant but changes in the course of the day between approximately {{convert|600|to|1100|km|mi|sigfig=1|lk=on|abbr=on}} from the transmitter position.<ref name="Reach of DCF77"/>

Corrected instrument grade DCF77 receivers, using the amplitude-modulated time signals with accompanying antennas oriented [[tangent]]ial 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.<ref>{{Cite web |url=http://www.hopf-time.com/en/dcf-info.htm |title=How the DCF77-receiver works |access-date=29 May 2012 |archive-date=2 October 2012 |archive-url=https://web.archive.org/web/20121002050042/http://www.hopf-time.com/en/dcf-info.htm |url-status=usurped }}</ref>

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 {{convert|273|km|mi|sigfig=2|lk=on|abbr=on}} from the transmitter in Mainflingen.<ref name="phasemod"/>

Normal low cost consumer grade DCF77 receivers solely rely on the amplitude-modulated time signals and use narrow band receivers (with 10&nbsp;Hz bandwidth) with small ferrite [[Loop antenna#AM broadcast receiving antennas|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 clock]]s 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.<ref>Michael A. Lombardi, [https://tf.nist.gov/general/pdf/2429.pdf "How Accurate is a Radio Controlled Clock?], National Institute of Standards and Technology, 2010.</ref> Some DCF77 controlled consumer grade quartz movements promote accurate time keeping by [[Synchronization|synchronizing]] and [[Quartz_clock#External_time_signal_correction|correcting]] their time automatically more than once spread over a day.<ref>[https://cdn.nedis.com/datasheets/MAN_HE-CLOCK-89_EN.PDF RADIO-CONTROLLED WALLCLOCK INSTRUCTION MANUAL]</ref>

== Network Time Protocol reference clock use ==
{{Main|Network Time Protocol}}

Network Time Protocol time servers display the reference ID (REFID) ''.DCFa.'' (amplitude modulation) or ''.DCFp.'' (phase modulation) when a standard DCF77 time receiver is used as the reference time source.<ref>{{cite web |url=http://www.meinbergglobal.com/english/info/ntp.htm#ntp_status |title=Checking the NTP Status |work=Network Time Protocol (NTP) |publisher=Meinberg Radio Clocks GmbH & Co. KG |access-date=2011-08-29 |language=de}}</ref>

== See also ==

*[[Time from NPL (MSF)]], similar time service in the United Kingdom
*[[WWVB]], similar time service in the United States

==References==
{{Reflist}}

== Further reading ==
* <!-- <ref name="Dietze-Kriedt-Wondra_1981"> -->{{cite magazine |title=Anregung zum Nachbau: Einfacher Empfänger für DCF77-Zeitzeichensignale |language=de |department=Applikationsseiten |author-first1=Andreas |author-last1=Dietze |author-first2=Hans |author-last2=Kriedt |author-first3=Jürgen |author-last3=Wondra |magazine=Funk-Technik - Fachzeitschrift für die gesamte Unterhaltungselektronik |issn=0016-2825 |publisher=Dr. Alfred Hüthig Verlag GmbH |publication-place=Heidelberg, Germany |date=August 1981 |volume=36 |issue=8 |pages=279–282 |url=https://nvhrbiblio.nl/biblio/tijdschrift/Funktechnik/1981/FT_1981_Heft_08_OCR.pdf |access-date=2021-07-10 |url-status=live |archive-url=https://web.archive.org/web/20210710212427/https://nvhrbiblio.nl/biblio/tijdschrift/Funktechnik/1981/FT_1981_Heft_08_OCR.pdf |archive-date=2021-07-10}} (4 pages)<!-- </ref> -->

== External links ==
{{Commons category|DCF77}}
* [http://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77.html Official DCF77 web page at the PTB]
* {{in lang|de}} [http://www.dcf77.de/ The atomic clock in Frankfurt/Main Germany]
* [http://qrg.globaltuners.com/details.php?id=10730 DCF77] {{Webarchive|url=https://web.archive.org/web/20120331191542/http://qrg.globaltuners.com/details.php?id=10730 |date=2012-03-31 }} on the [http://qrg.globaltuners.com/ Global Frequency Database] {{Webarchive|url=https://web.archive.org/web/20130330214029/http://qrg.globaltuners.com/ |date=2013-03-30 }}
* [http://www.eecis.udel.edu/~mills/ntp/dcf77.html Time code description]
* {{usurped|1=[https://web.archive.org/web/20100526030302/http://inter-actief.cs.utwente.nl/~ptdeboer/ham/sdr/leapsecond.html Observation of DCF77 during a leap second]}}
* DCF77 Arduino implementation [http://thijs.elenbaas.net/2012/04/arduino-dcf77-radio-clock-receiver-hardware-2/ Hardware] {{Webarchive|url=https://web.archive.org/web/20130722190156/http://thijs.elenbaas.net/2012/04/arduino-dcf77-radio-clock-receiver-hardware-2/ |date=2013-07-22 }}, [http://thijs.elenbaas.net/2012/04/arduino-dcf77-radio-clock-receiver-signal/ Signal] {{Webarchive|url=https://web.archive.org/web/20130724063339/http://thijs.elenbaas.net/2012/04/arduino-dcf77-radio-clock-receiver-signal/ |date=2013-07-24 }}, [http://thijs.elenbaas.net/2012/04/arduino-dcf77-radio-clock-receiver-library/ Code] {{Webarchive|url=https://web.archive.org/web/20130725084711/http://thijs.elenbaas.net/2012/04/arduino-dcf77-radio-clock-receiver-library/ |date=2013-07-25 }}
* [http://www.meteotime.com/Web/en/Home/Default.htm Meteotime]
* {{usurped|1=[https://web.archive.org/web/20120304222048/http://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/pdf/PTBM_50a_DCF77_engl.pdf Time and frequency dissemination with DCF77: 1959–2009 and beyond" – a detailed description of the history of DCF77]}}
* {{usurped|1=[https://web.archive.org/web/20161203062346/http://www.redesmadrid.com/?page_id=4 How to build a clock DCF (archive.org copy) ]}}
*[http://www.plicht.de/ekki/varia/mainflingen.html A historical visit at DCF77 Mainflingen by Ekkehard Plicht] {{Webarchive|url=https://web.archive.org/web/20190703001938/http://www.plicht.de/ekki/varia/mainflingen.html |date=3 July 2019 }}

{{Time signal stations}}
{{Time signal authorities}}
{{Time topics}}
{{Time measurement and standards}}
{{Authority control}}

[[Category:Time signal radio stations]]
[[Category:Time in Germany]]
[[Category:Transmitter sites in Germany]]
[[Category:Call signs]]
[[Category:1959 establishments in West Germany]]