View source for Radio clock

{{Short description|Clock which synchronizes its time using radio transmitters}}
{{Distinguish|text=[[Alarm clock#Clock radio|clock radio]], an alarm clock incorporating a broadcast radio receiver or [[Broadcast clock]]}}
[[File:Atomic clock.jpg|thumb|245px|A modern [[Low frequency|LF]] radio-controlled clock]]

A '''radio clock''' or '''radio-controlled clock''' (RCC), and often colloquially (and incorrectly<ref name="Lombardi"/>) referred to as an "[[atomic clock]]", is a type of [[Quartz clock|quartz]] [[clock]] or [[watch]] that is automatically [[Synchronization|synchronized]] to a [[time code]] transmitted by a [[radio]] transmitter connected to a [[time standard]] such as an atomic clock. Such a clock may be synchronized to the time sent by a single transmitter, such as many national or regional time transmitters, or may use the multiple transmitters used by [[satellite navigation]] systems such as [[Global Positioning System]]. Such systems may be used to automatically set clocks or for any purpose where accurate time is needed. Radio clocks may include any feature available for a clock, such as alarm function, display of ambient temperature and humidity, broadcast radio reception, etc.

One common style of radio-controlled clock uses time signals transmitted by dedicated terrestrial [[longwave]] radio transmitters, which emit a time code that can be demodulated and displayed by the radio controlled clock. The radio controlled clock will contain an accurate time base oscillator to maintain timekeeping if the radio signal is momentarily unavailable. Other radio controlled clocks use the time signals transmitted by dedicated transmitters in the [[Shortwave radio|shortwave]] bands. Systems using dedicated time signal stations can achieve accuracy of a few tens of milliseconds.

GPS satellite receivers also internally generate accurate time information from the satellite signals. Dedicated GPS timing receivers are accurate to better than 1&nbsp;microsecond; however, general-purpose or consumer grade GPS may have an offset of up to one second between the internally calculated time, which is much more accurate than 1 second, and the time displayed on the screen.

Other broadcast services may include timekeeping information of varying accuracy within their signals. Timepieces with [[Bluetooth]] radio support, ranging from watches with basic control of functionality via a [[mobile app]] to full [[smartwatch]]es obtain time information from a connected [[Mobile phone|phone]], with no need to receive time signal broadcasts.

==Single transmitter==
Radio clocks synchronized to a terrestrial [[time signal]] can usually achieve an accuracy within a hundredth of a second relative to the time standard,<ref name="Lombardi">{{cite journal |title=How Accurate is a Radio Controlled Clock? |first=Michael A. |last=Lombardi |date=March 2010 |journal=Horological Journal |volume=152 |issue=3 |pages=108–111 |url=http://tf.nist.gov/general/pdf/2429.pdf |via= National Institute of Standards and Technology website |access-date=2023-12-01 |archive-date=2021-01-07 |archive-url=https://web.archive.org/web/20210107194406/https://tf.nist.gov/general/pdf/2429.pdf |url-status=live }}</ref> generally limited by uncertainties and variability in [[radio propagation]]. Some timekeepers, particularly watches such as some [[Casio Wave Ceptor#Multi-Band 6|Casio Wave Ceptors]] which are more likely than desk clocks to be used when travelling, can synchronise to any one of several different time signals transmitted in different regions.

===Longwave and shortwave transmissions===
Radio clocks depend on coded time signals from radio stations. The stations vary in broadcast frequency, in geographic location, and in how the signal is modulated to identify the current time. In general, each station has its own format for the time code.

{{anchor|timecode radio stations}}

==== List of radio time signal stations ====
{| class="wikitable sortable"
|+ List of radio time signal stations
! Frequency !! Callsign !! Country Authority !! Location !! Aerial type !! Power !! Remarks
|-
| rowspan=6 | {{ntsh|25}} {{nowrap|25 kHz}} 
| <!--1-->[[RJH69]] || {{BLR}}<br />[[VNIIFTRI]] || [[Vileyka]]<br />{{coord|54|27|47|N|26|46|37|E|region:BY-MI_type:landmark|name=RJH69}} || Triple [[umbrella antenna]]<ref group=lower-alpha>3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of {{cvt|305|m|ft}} and 15 guyed lattice masts with a height of {{cvt|270|m|ft}}</ref> || {{val|300|u=kW}} || rowspan=6 | This is [[Beta (time signal)|Beta time signal]].<ref name="vniiftri">{{Cite FTP |url=ftp://ftp.vniiftri.ru/BULLETINS/V/bull_b16_2018.pdf|title=Standard Time and Frequency Signals|access-date=2018-07-15|server=FTP server|url-status=dead|language=ru}} — official signal specification.</ref> The signal is transmitted in non-overlapping time:<br />02:00–02:20 UTC RAB99<br />04:00–04:25 UTC RJH86<br />06:00–06:20 UTC RAB99<br />07:00–07:25 UTC RJH69<br />08:00–08:25 UTC RJH90<br />09:00–09:25 UTC RJH77<br />10:00–10:25 UTC RJH86<br />11:00–11:20 UTC RJH63
|-
| <!--2-->RJH77 || {{RUS}}<br />[[VNIIFTRI]] || [[Arkhangelsk]]<br />{{coord|64|21|29|N|41|33|58|E|region:RU-ARK_type:landmark|name=RJH77}} || Triple umbrella antenna<ref group=lower-alpha>3 umbrella antennas, fixed on 18 guyed lattice masts, height of central masts: 305 metres</ref> || {{val|300|u=kW}} 
|-
| <!--3-->RJH63 || {{RUS}}<br />[[VNIIFTRI]] || [[Krasnodar]]<br />{{coord|44|46|25|N|39|32|50|E|name=RJH63}} || Umbrella antenna<ref group=lower-alpha>umbrella antenna, fixed on 13 guyed lattice masts, height of central mast: {{cvt|425|m|ft}}</ref> || {{val|300|u=kW}} 
|-
| <!--4-->RJH90 || {{RUS}}<br />[[VNIIFTRI]] || [[Nizhny Novgorod]]<br />{{coord|56|10|20|N|43|55|38|E|name=RJH90}} || Triple umbrella antenna<ref group=lower-alpha>3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of {{cvt|205|m|ft}} and 15 guyed lattice masts with a height of {{cvt|170|m|ft}}</ref> || {{val|300|u=kW}} 
|-
| <!--5-->RJH86<ref name="vniiftri" /><ref group=lower-alpha>in air RJH66</ref> || {{KGZ}}<br />[[VNIIFTRI]] || [[Bishkek]]<br />{{coord|43|02|29|N|73|37|09|E|name=RJH86}} || Triple umbrella antenna<ref group=lower-alpha>3 umbrella antennas, fixed on 18 guyed lattice masts, height of central masts: {{cvt|276|m|ft}}</ref> || {{val|300|u=kW}}
|-
| <!--6-->RAB99 || {{RUS}}<br />[[VNIIFTRI]] || [[Khabarovsk]]<br />{{coord|48|29|29|N|134|48|59|E|name=RAB99}} || Umbrella antenna<ref group=lower-alpha>umbrella antenna, fixed on 18 guyed lattice masts arranged in 3 rows, height of central masts: {{cvt|238|m|ft}}</ref> || {{val|300|u=kW}} 
|-
| {{ntsh|40}} {{nowrap|40 kHz}} || <!--7-->[[JJY]] || {{JPN}}<br />[[National Institute of Information and Communications Technology|NICT]] || [[Otakadoyayama Transmitter|Mount Otakadoya]], [[Fukushima Prefecture|Fukushima]]<br />{{coord|37|22|21|N|140|50|56|E|name=JJY}} || [[T-Antenna|Capacitance hat]], height {{cvt|250|m|ft}} || {{val|50|u=kW}} || Located near [[Fukushima, Fukushima|Fukushima]]<ref name=McCarthy09 />
|-
| {{ntsh|50}} {{nowrap|50 kHz}} || <!--8-->[[RTZ (radio station)|RTZ]]|| {{RUS}}<br />[[VNIIFTRI]] || [[Irkutsk]]<br />{{coord|52|25|41|N|103|41|12|E|name=RTZ}} || Umbrella antenna || {{val|10|u=kW}} || [[Phase modulation|PM]] time code
|-
| rowspan=3 | {{ntsh|60}} {{nowrap|60 kHz}}
| <!--9-->[[JJY]] || {{JPN}}<br />[[National Institute of Information and Communications Technology|NICT]] || [[Haganeyama Transmitter|Mount Hagane]], [[Kyushu]]<br />{{coord|33|27|54|N|130|10|32|E|name=JJY}} || Capacitance hat, height {{cvt|200|m|ft}} || {{val|50|u=kW}} || Located on [[Kyūshū|Kyūshū Island]]<ref name=McCarthy09 />
|-
| <!--10-->[[Time from NPL|MSF]] || {{GBR}}<br />[[National Physical Laboratory (United Kingdom)|NPL]] || [[Anthorn Radio Station|Anthorn, Cumbria]]<br />{{coord|54|54|27|N|03|16|24|W|name=MSF|notes={{efn|Before 1 April 2007, the signal was transmitted from [[Rugby Radio Station|Rugby, Warwickshire]] {{coord|52|21|33|N|01|11|21|W}}}}}} || Triple T-antenna<ref group=lower-alpha>3 T-antennas, spun {{cvt|150|m|ft}} above ground between two {{cvt|227|m|ft}} high guyed grounded masts in a distance of {{cvt|655|m|yd}}</ref> || {{val|17|u=kW}} || Range up to {{cvt|1500|km|mi}}
|-
| <!--11-->[[WWVB]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<ref>{{cite journal|title=NIST Radio Station WWVB|date=March 2010|url=https://www.nist.gov/pml/div688/grp40/wwvb.cfm|journal=NIST|access-date=18 March 2014|archive-date=25 March 2014|archive-url=https://web.archive.org/web/20140325181329/http://www.nist.gov/pml/div688/grp40/wwvb.cfm|url-status=live}}</ref><br />{{coord|40|40|41|N|105|02|48|W|name=WWVB}} || Two capacitance hats, height {{cvt|122|m|ft}} || {{val|70|u=kW}} || Received through most of mainland U.S.<ref name=McCarthy09 />
|-
| {{ntsh|66.66}} {{nowrap|66.66 kHz}} || <!--12-->[[RBU (radio station)|RBU]] || {{RUS}}<br />[[VNIIFTRI]] || [[Taldom]], [[Moscow]]<br />{{coord|56|43|59|N|37|39|47|E|name=RBU|notes={{efn|name=RBU|Before 2008, transmitter located at {{coord|55|44|14|N|38|09|04|E}}}}}} || Umbrella antenna<ref group=lower-alpha>umbrella antenna, fixed on a {{cvt|275|m|ft}} high central tower insulated against ground and five {{cvt|257|m|ft}} high lattice masts insulated against ground in a distance of {{convert|324|m|yd|abbr=off}} from the central tower</ref> || {{val|50|u=kW}} || [[Phase modulation|PM]] time code
|-
| {{ntsh|68.5}} {{nowrap|68.5 kHz}} || <!--13-->[[BPC (time signal)|BPC]] || {{CHN}}<br />[[National Time Service Center|NTSC]] || [[Shangqiu]], [[Henan]]<br />{{coord|34|27|25|N|115|50|13|E|name=BPC}} || 4 guyed masts, arranged in a square || {{val|90|u=kW}} || 21 hours per day, with a 3-hour break from 05:00–08:00 ([[China Standard Time]]) daily (21:00–24:00 UTC)<ref>{{cite web|title=BPC|url=http://www.time.ac.cn/serve/e_c.htm|work=National Time Service Center, Chinese Academy of Sciences|access-date=16 March 2013|archive-url=https://web.archive.org/web/20180214031330/http://www.time.ac.cn/serve/e_c.htm|archive-date=February 14, 2018 |url-status=dead}}</ref>
|-
| {{ntsh|75}} <s>{{nowrap|75 kHz}}</s> || <!--14--><s>[[HBG Prangins|HBG]]</s> || {{flagdeco|Switzerland}} <s>[[Switzerland]]</s><br /><s>[[Swiss Federal Office of Metrology|METAS]]</s> || <s>[[Prangins]]</s><br /><s>{{coord|46|24|24|N|06|15|04|E|name=HBG}}</s> || <s>T-antenna</s><ref group=lower-alpha>T-antenna spun between two {{cvt|125|m|ft}} tall, grounded free-standing lattice towers in a distance of {{cvt|227|m|yd}}</ref> || <s>{{val|20|u=kW}}</s> || Discontinued as of 1 January 2012
|-
| rowspan=2 | {{ntsh|77.5}} {{nowrap|77.5 kHz}} 
| <!--15-->[[DCF77]] || {{DEU}}<br />[[Physikalisch-Technische Bundesanstalt|PTB]] || [[Mainflingen]], [[Hessen]]<br />{{coord|50|00|58|N|09|00|29|E|name=DCF77}} || Vertical omni-directional antennas with top-loading capacity, height {{convert|150|m|ft}}<ref>{{cite web |url=http://www.ptb.de/en/org/4/44/442/dcf77_sende_e.htm |title=DCF77 transmitting facilities |author=Yvonne Zimber |date=2007-05-09 |access-date=2010-05-02 |archive-date=2010-05-14 |archive-url=https://web.archive.org/web/20100514002918/http://www.ptb.de/en/org/4/44/442/dcf77_sende_e.htm |url-status=live }}</ref> || {{val|50|u=kW}} || Located southeast of Frankfurt am Main with a range of up to {{cvt|2000|km|mi}}<ref name=McCarthy09>Dennis D. McCarthy, P. Kenneth Seidelmann ''Time: From Earth Rotation to Atomic Physics'' Wiley-VCH, 2009 {{ISBN|3-527-40780-4}} page 257</ref><ref name="compuphase_h0420">{{cite web|title=Synchronizing time with DCF77 and MSF60|url=http://www.compuphase.com/mp3/h0420_timecode.htm|access-date=2011-09-12|archive-date=2011-01-12|archive-url=https://web.archive.org/web/20110112034340/http://www.compuphase.com/mp3/h0420_timecode.htm|url-status=live}} 090917 compuphase.com</ref>
|-
| <!--16-->[[BSF (time service)|BSF]] || {{TWN}} || [[Zhongli District|Zhongli]]<br />{{coord|25|00|19|N|121|21|55|E|name=BSF}} || T-antenna<ref group=lower-alpha>T-antenna spun between two telecommunication towers in a distance of {{cvt|33|m|yd}}</ref> || ||<ref>{{cite web|url=http://lfintaiwan.bitbucket.io/overview.html|title=A Time Station Signal Project for Taiwan|access-date=2018-07-09|archive-date=2017-04-20|archive-url=https://web.archive.org/web/20170420145738/https://lfintaiwan.bitbucket.io/overview.html|url-status=live}}</ref>
|-
| bgcolor=silver rowspan=3 | {{ntsh|100}} {{nowrap|100 kHz}}<ref group=lower-alpha>Frequency for radio navigation system</ref>
| <!--17-->[[BPL (time service)|BPL]] || {{CHN}}<br />[[National Time Service Center|NTSC]] || [[Pucheng County, Shaanxi|Pucheng]], [[Shaanxi]]<br />{{coord|34|56|56|N|109|32|35|E|name=BPL}} || Single guyed lattice steel mast || {{val|800|u=kW}} || [[Loran-C]] compatible format signal on air from 05:30 to 13:30 UTC,<ref>{{cite web|title=长波授时 (Longwave time signal)|url=http://www.time.ac.cn/serve/BPL.htm|work=National Time Service Center, Chinese Academy of Sciences|access-date=16 March 2013|archive-date=10 January 2013|archive-url=https://web.archive.org/web/20130110210025/http://www.time.ac.cn/serve/BPL.htm|url-status=dead}}</ref> with a reception radius up to {{cvt|3000|km|mi}}<ref>{{cite web|title=科研成果 (Research achievements)|url=http://www.ntsc.cas.cn/kycg/|work=National Time Service Center, Chinese Academy of Sciences|access-date=16 March 2013|archive-date=17 April 2013|archive-url=https://web.archive.org/web/20130417182930/http://www.ntsc.cas.cn/kycg/|url-status=live}}</ref> 
|-
| <!--18-->RNS-E || {{RUS}}<br />[[VNIIFTRI]] || [[Bryansk]]<br />{{coord|53|08|00|N|34|55|00|E|name=RNS-E}} || 5 guyed masts || {{val|800|u=kW}} || [[CHAYKA]] compatible format signal<ref name="vniiftri" /><br />04:00–10:00 UTC and 14:00–18:00 UTC
|-
| <!--19-->RNS-V || {{RUS}}<br />[[VNIIFTRI]] || [[Alexandrovsk-Sakhalinsky (town)|Alexandrovsk-Sakhalinsky]]<br />{{coord|51|05|00|N|142|43|00|E|name=RNS-V}} || Single guyed mast || {{val|400|u=kW}} || [[CHAYKA]] compatible format signal<ref name="vniiftri" /><br />23:00–05:00 UTC and 11:00–17:00 UTC
|-
| bgcolor=lightgrey | {{ntsh|129.1}} {{nowrap|129.1 kHz}}<ref group=lower-alpha name="teleswitch">Frequency for radio teleswitch system</ref>|| <!--20-->DCF49 || {{DEU}}<br />[[Physikalisch-Technische Bundesanstalt|PTB]] || [[Mainflingen]]<br />{{coord|50|00|58|N|09|00|29|E|region:DE-HE_type:landmark|name=DCF49}} || T-antenna || {{val|100|u=kW}} || rowspan=3 | [[Radio teleswitch|EFR radio teleswitch]]<ref name="efr">{{cite web |title=PTB time monitor |url=http://www.efr.de/de/efr-system/#/PTB-Zeitmonitor |access-date=2018-07-16 |archive-date=2018-07-16 |archive-url=https://web.archive.org/web/20180716194814/http://www.efr.de/de/efr-system/#/PTB-Zeitmonitor |url-status=live }}&nbsp;— in German</ref><br />time signal only (no reference frequency)<br />FSK ± 170&nbsp;Hz 200 baud
|-
| bgcolor=lightgrey | {{ntsh|135.6}} {{nowrap|135.6 kHz}}<ref group=lower-alpha name="teleswitch" />|| <!--21-->HGA22 || {{HUN}}<br />[[Physikalisch-Technische Bundesanstalt|PTB]] || [[Lakihegy]]<br />{{coord|47|22|24|N|19|00|17|E|region:HU-PE_type:landmark|name=HGA22}} || Single guyed mast || {{val|100|u=kW}} 
|-
| bgcolor=lightgrey | {{ntsh|139}} {{nowrap|139 kHz}}<ref group=lower-alpha name="teleswitch" />|| <!--22-->DCF39 || {{DEU}}<br />[[Physikalisch-Technische Bundesanstalt|PTB]] || [[Burg bei Magdeburg]]<br />{{coord|52|17|13|N|11|53|49|E|region:DE-ST_type:landmark|name=DCF39}} || Single guyed mast || {{val|50|u=kW}} 
|-
| bgcolor=silver | {{ntsh|162}} {{nowrap|162 kHz}}<ref group=lower-alpha name="AM">Frequency for AM-broadcasting</ref>|| <!--23-->[[ALS162 time signal|ALS162]] || {{FRA}}<br />{{ill|ANFR|fr|display=1}} || [[Allouis]]<br />{{coord|47|10|10|N|02|12|16|E|name=ALS162}} || Two guyed steel lattice masts, height {{cvt|350|m|ft}}, fed on the top || {{val|800|u=kW}}|| AM-broadcasting transmitter, located {{cvt|150|km|mi}} south of Paris with a range of up to {{cvt|3500|km|mi}}, using [[Phase modulation|PM]] with encoding similar to DCF77<ref group=lower-alpha>and requiring a more complex receiver for demodulating time signal</ref>
|-<!-- Frequency ! Callsign ! Country ! Location ! Aerial type ! Power ! Remarks -->
| bgcolor=silver | {{ntsh|198}} {{nowrap|198 kHz}}<ref group=lower-alpha name="AM" /><ref group=lower-alpha>since 1988, before 200 kHz</ref> || <!--24-->{{nowrap|[[BBC Radio 4]]}} || {{GBR}}<br />[[National Physical Laboratory (United Kingdom)|NPL]] || [[Droitwich Transmitting Station|Droitwich]]<br />{{coord|52.2955|N|2.1063|W|type:landmark|name=BBC4}} || [[T-antenna|T-aerial]]<ref group=lower-alpha>Droitwich uses a T-aerial suspended between two {{nowrap|213 metres}} (699') guyed steel lattice [[radio masts and towers|radio masts]], which stand {{cvt|180|m|yd}} apart.</ref> || {{val|500|u=kW}}<ref>{{cite web
| title=Radio stations in London, England
| access-date=2016-04-26
| url=https://radiomap.eu/uk/london
| quote=Birmingham, Droitwich, 500 kW + Blackwall Tunnel + Rotherhithe Tunnel
| archive-date=2016-04-19
| archive-url=https://web.archive.org/web/20160419145438/http://radiomap.eu/uk/london
| url-status=live
}}</ref> || Additional {{nowrap|(50 kW)}} transmitters is at [[Burghead Transmitting Station|Burghead]] and [[Westerglen transmitting station|Westerglen]]. The time signal is transmitted by {{nowrap|25-bit/s}} phase modulation.<ref>{{cite web
| title=L.F. RADIO-DATA: Specification of BBC phase-modulated transmissions on long-wave
| date=December 1984
| publication-date=2006-10-24
| url=https://downloads.bbc.co.uk/rd/pubs/reports/1984-19.pdf
| quote=The BBC long-wave a.m. transmitter network carries a low bit-rate data signal, in addition to the normal programme signal modulation. The data signal is conveyed by phase-modulation of the carrier
| access-date=2016-04-25
| archive-date=2016-03-04
| archive-url=https://web.archive.org/web/20160304194431/http://downloads.bbc.co.uk/rd/pubs/reports/1984-19.pdf
| url-status=live
}}</ref>
|-
| bgcolor=PaleGreen rowspan=3 | {{ntsh|2500}} {{nowrap|2.5 MHz}}
| <!--25-->[[BPM (time service)|BPM]] || {{CHN}}<br />[[National Time Service Center|NTSC]] || [[Pucheng County, Shaanxi|Pucheng]], [[Shaanxi]]<br />{{coord|34|56|56|N|109|32|35|E|name=BPM}} || || || (BCD time code on 125&nbsp;Hz sub-carrier not yet activated)<br />
07:30–01:00 UTC<ref name=bpm>{{cite web|title=短波授时 (Shortwave time signal)|url=http://www.time.ac.cn/serve/BPM.htm|work=National Time Service Center, Chinese Academy of Sciences|access-date=2013-03-16|archive-date=2013-01-15|archive-url=https://web.archive.org/web/20130115032133/http://www.time.ac.cn/serve/BPM.htm|url-status=live}}</ref> 
|-
| <!--26-->[[WWV (radio station)|WWV]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<br />{{coord|40|40|41|N|105|02|48|W|name=WWV}} || Broadband monopole || {{val|2.5|u=kW}} ||rowspan=2| [[Binary-coded decimal|Binary-coded decimal (BCD)]] time code on {{nowrap|100 Hz}} sub-carrier
|-
| <!--27-->[[WWVH]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || [[Kekaha, Hawaii]]<br />{{coord|21|59|16|N|159|45|46|W|name=WWVH}} || || {{val|5|u=kW}}
|-
| {{ntsh|3330}} {{nowrap|3.33 MHz}} || <!--28-->[[CHU (radio station)|CHU]] || {{CAN}}<br />[[National Research Council (Canada)|NRC]] || [[Ottawa, Ontario]]<br />{{coord|45|17|40|N|75|45|27|W|name=CHU}} || || {{val|3|u=kW}} || 300 baud [[Bell 103]] time code
|-
| bgcolor=LightGreen| {{ntsh|4996}} {{nowrap|4.996 MHz}} || <!--29-->[[RWM]] || {{RUS}}<br />[[VNIIFTRI]] || [[Taldom]], [[Moscow]]<br />{{coord|56|44|58|N|37|38|23|E|name=RWM|notes={{efn|name=RBU}}}} || || {{val|10|u=kW}} || [[Continuous wave|CW]] ({{Val|1|u=Hz}}, {{Val|10|u=Hz}})
|-
| bgcolor=LightGreen rowspan=5 | {{ntsh|5000}} {{nowrap|5 MHz}}
| <!--30-->[[BPM (time service)|BPM]] || {{CHN}}<br />[[National Time Service Center|NTSC]] ||[[Pucheng County, Shaanxi|Pucheng]], [[Shaanxi]]<br />{{coord|34|56|56|N|109|32|35|E|name=BPM}} || || || BCD time code on 125&nbsp;Hz sub-carrier.<br />00:00–24:00 UTC<ref name=bpm />
|-
| <!--31-->[[HLA (radio station)|HLA]] || {{KOR}}<br />[[Korea Research Institute of Standards and Science|KRISS]] || [[Daejeon]]<br />{{coord|36|23|14|N|127|21|59|E|name=HLA}} || || {{val|2|u=kW}} ||
|-
| <!--32-->[[WWV (radio station)|WWV]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<br />{{coord|40|40|41|N|105|02|48|W|name=WWV}} || Broadband monopole || {{val|10|u=kW}}<ref group=lower-alpha>Time signal article says 2.5 kW</ref> ||rowspan=2| BCD time code on {{nowrap|100 Hz}} sub-carrier
|-
| <!--33-->[[WWVH]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || [[Kekaha, Hawaii]]<br />{{coord|21|59|16|N|159|45|46|W|name=WWVH}} || || {{val|10|u=kW}}
|-
| <!--34-->[[YVTO]] || {{VEN}} || [[Caracas]]<br/>{{coord|10|30|13|N|66|55|44|W|region:VE-A_type:landmark|name=YVTO}} || || {{val|1|u=kW}} ||
|-
| {{ntsh|7850}} {{nowrap|7.85 MHz}} || <!--35-->[[CHU (radio station)|CHU]] || {{CAN}}<br />[[National Research Council (Canada)|NRC]] || [[Ottawa, Ontario]]<br />{{coord|45|17|40|N|75|45|27|W|name=CHU}} || || {{val|10|u=kW}} || {{nowrap|300 baud}} [[Bell 103]] time code
|-
| bgcolor=PaleGreen| {{ntsh|9996}} {{nowrap|9.996 MHz}} || <!--36-->[[RWM]] || {{RUS}}<br />[[VNIIFTRI]] || [[Taldom]], [[Moscow]]<br />{{coord|56|44|58|N|37|38|23|E|name=RWM|notes={{efn|name=RBU}}}} || || {{val|10|u=kW}} || [[Continuous wave|CW]] ({{Val|1|u=Hz}}, {{Val|10|u=Hz}})
|-
| bgcolor=PaleGreen rowspan=5 | {{ntsh|10000}} {{nowrap|10 MHz}}
| <!--37-->[[BPM (time service)|BPM]] || {{CHN}}<br />[[National Time Service Center|NTSC]] || [[Pucheng County, Shaanxi|Pucheng]], [[Shaanxi]] <br />{{coord|34|56|56|N|109|32|35|E|name=BPM}} || || || (BCD time code on 125&nbsp;Hz sub-carrier not yet activated)<br />00:00–24:00 UTC<ref name=bpm />
|-
| <!--38-->[[LOL (radio station)|LOL]] || {{ARG}}<br />[[Argentine Naval Hydrographic Service|SHN]] || [[Buenos Aires]]{{refn|group=lower-alpha|<ref name="lol">[http://www.hidro.gov.ar/Observatorio/LaHora.asp Information on the Official Time and Standard Frequency] {{Webarchive|url=https://web.archive.org/web/20180828134617/http://www.hidro.gov.ar/Observatorio/LaHora.asp |date=2018-08-28 }} — in Spanish</ref><!-- http://www.hidro.gov.ar/Observatorio/LaHora.asp (http://web.archive.org/web/20211211001540/http://www.hidro.gov.ar/Observatorio/LaHora.asp) --> says that the transmitter is located in Observatorio Naval Buenos Aires at Avenida España 2099, Buenos Aires; on Google Street View, some antenna structures can be seen both on and near the building, however, it's unclear where exactly the specific antenna is located. The coordinates here point to the building itself. <!--br/-->{{coord|34|37|19|S|58|21|18|W|name=LOL}}}} || || {{val|2|u=kW}} || Observatorio Naval Buenos Aires<ref name="lol" />
|-
| <!--39-->[[WWV (radio station)|WWV]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<br />{{coord|40|40|41|N|105|02|48|W|name=WWV}} || Broadband monopole || {{val|10|u=kW}} ||rowspan=2| BCD time code on {{nowrap|100 Hz}} sub-carrier
|-
| <!--40-->[[WWVH]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || [[Kekaha, Hawaii]]<br />{{coord|21|59|16|N|159|45|46|W|name=WWVH}} || || {{val|10|u=kW}}
|-
| <!--41-->[[PPE (radio station)|PPE]]<ref name="onrdsh">{{cite web|url=http://pcdsh01.on.br/RadioDifusaoSinaisHorarios.html|title=Rádio-Difusão de Sinais Horários|publisher=Observatório Nacional|access-date=2012-02-23|archive-date=2014-03-12|archive-url=https://web.archive.org/web/20140312212423/http://pcdsh01.on.br/RadioDifusaoSinaisHorarios.html|url-status=live}}</ref> || {{BRA}} || [[Rio de Janeiro, RJ]] {{coord|22|53|44|S|43|13|27|W|name=PPE}}<ref name="onrdsh" /> || Horizontal [[Dipole antenna#Half-wave antenna|half-wavelength dipole]]<ref name="onrdsh" /> || {{val|1|u=kW}}<ref name="onrdsh" /> || Maintained by [[National Observatory (Brazil)]]
|-
| {{ntsh|14670}} {{nowrap|14.67 MHz}} || <!--42-->[[CHU (radio station)|CHU]] || {{CAN}}<br />[[National Research Council (Canada)|NRC]] || [[Ottawa, Ontario]]<br />{{coord|45|17|40|N|75|45|27|W|name=CHU}} || || {{val|3|u=kW}} || 300 baud [[Bell 103]] time code
|-
| bgcolor=LightGreen| {{ntsh|14996}} {{nowrap|14.996 MHz}} || <!--43-->[[RWM]] || {{RUS}}<br />[[VNIIFTRI]] || [[Taldom]], [[Moscow]]<br />{{coord|56|44|58|N|37|38|23|E|name=RWM|notes={{efn|name=RBU}}}} || || {{val|10|u=kW}} || [[Continuous wave|CW]] ({{Val|1|u=Hz}}, {{Val|10|u=Hz}})
|-
| bgcolor=LightGreen rowspan=3 | {{ntsh|15000}} {{nowrap|15 MHz}}
| <!--44-->[[BPM (time service)|BPM]] || {{CHN}}<br />[[National Time Service Center|NTSC]] || [[Pucheng County, Shaanxi|Pucheng]], [[Shaanxi]] <br />{{coord|34|56|56|N|109|32|35|E|name=BPM}} || || || (BCD time code on 125&nbsp;Hz sub-carrier not yet activated)<br />01:00–09:00 UTC<ref name=bpm />
|-
| <!--45-->[[WWV (radio station)|WWV]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<br />{{coord|40|40|41|N|105|02|48|W|name=WWV}} || Broadband monopole || {{val|10|u=kW}} ||rowspan=2| BCD time code on {{nowrap|100 Hz}} sub-carrier
|-
| <!--46-->[[WWVH]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || [[Kekaha, Hawaii]]<br />{{coord|21|59|16|N|159|45|46|W|name=WWVH}} || || {{val|10|u=kW}}
|-
| bgcolor=PaleGreen| {{ntsh|20000}} {{nowrap|20 MHz}} || | <!--47-->[[WWV (radio station)|WWV]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<br />{{coord|40|40|41|N|105|02|48|W|name=WWV}} || Broadband monopole || {{val|2.5|u=kW}} || BCD time code on {{nowrap|100 Hz}} sub-carrier
|-
| bgcolor=LightGreen rowspan=2 | {{ntsh|25000}} {{nowrap|25 MHz}} || <!--48-->[[WWV (radio station)|WWV]] || {{USA}}<br />[[National Institute of Standards and Technology|NIST]] || Near [[Fort Collins, Colorado]]<br />{{coord|40|40|41|N|105|02|48|W|name=WWV}} || Broadband monopole || {{val|2.0|u=kW}} || Schedule: variable (experimental broadcast)
|-
| <!--49--><s>[[MIKES (radio station)|MIKES]]</s> || <s>{{FIN}}<br />[[Centre for Metrology and Accreditation|MIKES]]</s> || <s>[[Espoo, Finland]]<br /> {{coord|60|10|49|N|24|49|35|E|region:FI-18_type:landmark|name=MIKES time signal transmitter}}</s> || <s>λ/4 [[sloper antenna]]</s> || <s>{{val|0.2|u=kW}}</s><ref name=MIKES>{{cite web |title=QSL: MIKES Time Station, Espoo, Finland |date=14 May 2014 |url=http://swldx.us/blog/?p=821 |website=SWL DX Blog |access-date=2016-10-11 |archive-date=2016-10-12 |archive-url=https://web.archive.org/web/20161012151307/http://swldx.us/blog/?p=821 |url-status=live }} Reproduces a [[QSL card|QSL letter]] from MIKES with technical details.</ref> || <s>1&nbsp;kHz amplitude modulation similar to DCF77.</s><br />As of 2017 the transmission is discontinued until further notice.<ref name="BIPM">BIPM Annual Report on Time Activities – [https://web.archive.org/web/20211010023818/ftp://ftp2.bipm.org/pub/tai/scale/TIMESIGNALS/timesignals.pdf Time Signals] , retrieved 2018 July 31.</ref><br/>"MIKES has a transmitter for time code and precise 25 MHz frequency for those near the Helsinki metropolitan area who need precise time and frequency."<ref>{{cite web | url=https://www.vttresearch.com/en/si-units-finland-time-and-frequency | title=SI units in Finland, time and frequency &#124; }}</ref>
|}

'''Descriptions'''
{{Reflist|group=lower-alpha}}

{{Location map+ |World
|width          = 1000
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|places         = 
 <!-- 1-->{{Location map~ |World |coordinates={{coord|54|27|47|N| 26|46|37|E}} |label=<span style="position:relative; left:0.5em">[[RJH69]]<span style="visibility:hidden">RJH6</span><br/><span style="visibility:hidden">/</span>{{pipe}}<br/><span style="visibility:hidden">/</span>{{pipe}}<br/>/<span style="visibility:hidden">{{pipe}}</span></span> |position=top}}
 <!-- 2-->{{Location map~ |World |coordinates={{coord|64|21|29|N| 41|33|58|E}} |label=<span style="visibility:hidden">JH77</span>RJH77<!-- <br/>↓ --> |position=top}}
 <!-- 3-->{{Location map~ |World |coordinates={{coord|44|46|25|N| 39|32|50|E}} |label=RJH63 |position=}}
 <!-- 4-->{{Location map~ |World |coordinates={{coord|56|10|20|N| 43|55|38|E}} |label=←&nbsp;RJH90 |position=right}}
 <!-- 5-->{{Location map~ |World |coordinates={{coord|43| 2|29|N| 73|37| 9|E}} |label=RJH86 |position=right<!-- top -->}}
 <!-- 6-->{{Location map~ |World |coordinates={{coord|48|29|29|N|134|48|59|E}} |label=RAB99 |position=}}
 <!-- 7-->{{Location map~ |World |coordinates={{coord|37|22|21|N|140|50|56|E}} |label=[[Otakadoyayama Transmitter|JJY<br/><small>(Mt&nbsp;Otakadoya)</small>]] |position=right |mark=Green pog.svg}}
 <!-- 8-->{{Location map~ |World |coordinates={{coord|52|25|41|N|103|41|12|E}} |label=[[RTZ (radio station)|RTZ]]<span style="visibility:hidden">RT</span> |position=top |mark=Green pog.svg}}
 <!-- 9-->{{Location map~ |World |coordinates={{coord|33|27|54|N|130|10|32|E}} |label=<span style="position:relative; left:-5px; top:1em">[[Haganeyama Transmitter|JJY<br/><small>(Mt&nbsp;Hagane)</small>]]</span> |position=right |mark=Green pog.svg}}
 <!--10-->{{Location map~ |World |coordinates={{coord|54|54|27|N|  3|16|24|W}} |label=MSF<br/>↓ |position=top |mark=Green pog.svg}}
 <!--11, 26, 32, 39, 45, 47, 48
       -->{{Location map~ |World |coordinates={{coord|40|40|41|N|105|02|48|W}} |label=[[WWV (radio station)|WWV]],&nbsp;[[WWVB]] |position=bottom |mark=Green-blue dot.svg}}
 <!--12-->{{Location map~ |World |coordinates={{coord|56|43|59|N| 37|39|47|E}} |label=<!-- <span style="position:relative; left:-3px; top:-0.5em">↙︎<span style="position:relative; top:-0.25em> --><span style="position:relative; left:-2px; top:0.8em">↖︎<span style="position:relative; top:0.5em">[[RBU (radio station)|RBU]],&nbsp;[[RWM]]</span></span> |position=right |mark=Green-blue dot.svg}}
 <!--13-->{{Location map~ |World |coordinates={{coord|34|27|23|N|115|50|13|E}} |label=<span style="position:relative; left:3px; top:0.75em"><span style="position:relative; top:0.75em>[[BPC (time signal)|BPC]]</span>↗︎</span> |position=left |mark=Green pog.svg}}
 <!--14-->{{Location map~ |World |coordinates={{coord|46|24|24|N|  6|15| 4|E}} |label=↑&nbsp;<br/>[[HBG Prangins|HBG]]<span style="visibility:hidden">HBG</span> |position=bottom |mark=Black pog.svg}}
 <!--15, 207
       -->{{Location map~ |World |coordinates={{coord|50| 0|58|N|  9| 0|29|E}} |label={{pipe}}<br/>{{pipe}}<br/>{{pipe}}<br/>{{pipe}}<br/>DCF49,&nbsp;[[DCF77]]<span style="visibility:hidden">DCF49,&nbsp;DCF7</span> |position=bottom |mark=Red-green dot.svg}}
 <!--16-->{{Location map~ |World |coordinates={{coord|25| 0|19|N|121|21|55|E}} |label=[[BSF (time service)|BSF]] |position=bottom |mark=Green pog.svg}}
 <!--17, 25, 30, 37, 44
       -->{{Location map~ |World |coordinates={{coord|34|56|56|N|109|32|35|E}} |label=<span style="position:relative; top:-0.2em">[[BPL (time service)|BPL]],&nbsp;[[BPM (time service)|BPM]]</span> |position=left |mark=Red-blue dot.svg}}
 <!--18-->{{Location map~ |World |coordinates={{coord|53| 8| 0|N| 34|55| 0|E}} |label={{pipe}}<br/>{{pipe}}<br/><span style="visibility:hidden">NS-E</span>RNS-E |position=bottom}}
 <!--19-->{{Location map~ |World |coordinates={{coord|51| 5| 0|N|142|43| 0|E}} |label=<span style="position:relative; top:-0.6em">RNS-V</span> |position=right}}
 <!--20 — see above (15)-->
 <!--21-->{{Location map~ |World |coordinates={{coord|47|22|24|N| 19| 0|17|E}} |label=HGA22 |position=bottom}}
 <!--22-->{{Location map~ |World |coordinates={{coord|52|17|13|N| 11|53|49|E}} |label=DCF39 |position=top}}
 <!--23-->{{Location map~ |World |coordinates={{coord|47|10|10|N|  2|12|16|E}} |label=<span style="position:relative; left:4px; top:0.85em"><span style="position:relative; top:0.5em">[[Télé Distribution Française|TDF]]</span>↗︎</span> |position=left}}
 <!--24-->{{Location map~ |World |coordinates={{coord|52.2955|N|2.1063|W}} |label=<span style="position:relative; left:-3px; top:1em"><span style="position:relative; top:0.4em">[[BBC&nbsp;Radio&nbsp;4]]&nbsp;</span>↗︎</span> |position=left}}
 <!--25 — see above (17)-->
 <!--26 — see abobe (11)-->
 <!--27, 33, 40, 46
       -->{{Location map~ |World |coordinates={{coord|21|59|16|N|159|45|46|W}} |label=[[WWVH]] |position=top |mark=Blue pog.svg}}
 <!--28, 35, 42
       -->{{Location map~ |World |coordinates={{coord|45|17|40|N| 75|45|27|W}} |label=[[CHU (radio station)|CHU]] |position=top |mark=Blue pog.svg}}
 <!--29, 36, 43
       --><!-- {{Location map~ |World |coordinates={{coord|55|44|14|N| 38| 9| 4|E}} |label=<span style="position:relative; left:-3px; top:0.9em">↖︎<span style="position:relative; top:0.2em">[[RWM]]</span></span> |position=right}} -->
 <!--30 — see above (17)-->
 <!--31-->{{Location map~ |World |coordinates={{coord|36|23|14|N|127|21|59|E}} |label=[[HLA (radio station)|HLA]] |position=top |mark=Blue pog.svg}}
 <!--32 — see above (11)-->
 <!--33 — see above (27)-->
 <!--34-->{{Location map~ |World |coordinates={{coord|10|30|13|N| 66|55|44|W}} |label=<span style="visibility:hidden">VTO</span>[[YVTO]] |position=top |mark=Blue pog.svg}}
 <!--35 — see above (28)-->
 <!--36 — see above (29)-->
 <!--37 — see above (17)-->
 <!--38-->{{Location map~ |World |coordinates={{coord|34|37|19|S| 58|21|18|W}} |label=<span style="position:relative; top:0.75em">[[LOL (radio station)|LOL]]</span> |position= |mark=Blue pog.svg}}
 <!--39 — see above (11)-->
 <!--40 — see above (27)-->
 <!--41-->{{Location map~ |World |coordinates={{coord|22|53|44|S| 43|13|27|W}} |label=<span style="visibility:hidden">PE</span>[[PPE (radio station)|PPE]] |position=bottom |mark=Blue pog.svg}}
 <!--42 — see above (28)-->
 <!--43 — see above (29)-->
 <!--44 — see above (17)-->
 <!--45 — see above (11)-->
 <!--46 — see above (27)-->
 <!--47 — see above (11)-->
 <!--48 — see above (11)-->
 <!--49-->{{Location map~ |World |coordinates={{coord|60|10|49|N| 24|49|35|E}} |label=[[MIKES (radio station)|MIKES]]<span style="visibility:hidden">MIKE</span> |position=top |mark=Black pog.svg}}
<!-- template for new lines:
 {{Location map~ |World |coordinates= |label= |position=}}
-->
}}<!--
A current list of times signal stations is published by the [[BIPM]] as an appendix to their annual report; the appendix includes coordinates of transmitter sites, operating schedules for stations, and the uncertainty of the [[carrier frequency]] of transmitters.<ref>[http://www.bipm.org/utils/en/pdf/time_ann_rep/Time_annual_report_2010.pdf#page=85 BIPM Annual Report on Time Activities 2010] {{Webarchive|url=https://web.archive.org/web/20120331030327/http://www.bipm.org/utils/en/pdf/time_ann_rep/Time_annual_report_2010.pdf#page=85 |date=2012-03-31 }}, pages 85–93, retrieved 2011 September 12.</ref><ref name="BIPM">BIPM Annual Report on Time Activities – [https://web.archive.org/web/20211010023818/ftp://ftp2.bipm.org/pub/tai/scale/TIMESIGNALS/timesignals.pdf Time Signals] , retrieved 2018 July 31.</ref>-->

Many other countries can receive these signals ([[JJY]] can sometimes be received in New Zealand, Western Australia, Tasmania, Southeast Asia, parts of Western Europe and the Pacific Northwest of North America at night), but success depends on the time of day, atmospheric conditions, and interference from intervening buildings. Reception is generally better if the clock is placed near a window facing the transmitter. There is also a propagation delay of approximately {{nowrap|1 ms}} for every {{cvt|300|km|mi}} the receiver is from the transmitter.

====Clock receivers====
A number of manufacturers and retailers sell radio clocks that receive coded time signals from a radio station, which, in turn, derives the time from a true atomic clock.

One of the first radio clocks was offered by [[Heathkit]] in late 1983. Their model GC-1000 "Most Accurate Clock" received shortwave time signals from radio station [[WWV (radio station)|WWV]] in [[Fort Collins, Colorado]]. It automatically switched between WWV's 5, 10, and 15&nbsp;MHz frequencies to find the strongest signal as conditions changed through the day and year. It kept time during periods of poor reception with a quartz-crystal [[oscillator]]. This oscillator was disciplined, meaning that the microprocessor-based clock used the highly accurate time signal received from WWV to trim the crystal oscillator. The timekeeping between updates was thus considerably more accurate than the crystal alone could have achieved. Time down to the tenth of a second was shown on an [[LED]] display. The GC-1000 originally sold for US$250 in kit form and US$400 preassembled, and was considered impressive at the time. Heath Company was granted a [http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4582434.PN.&OS=PN/4582434&RS=PN/4582434 patent] for its design.<ref>{{cite web |title=Heathkit GC-1000-H Most Accurate Clock |url=https://www.pestingers.net/pages-images/heathkit/radio-equipment/gc1000/gc1000.htm |website=Pestingers |archive-url=https://web.archive.org/web/20200214054036/https://www.pestingers.net/pages-images/heathkit/radio-equipment/gc1000/gc1000.htm |archive-date=February 14, 2020 |url-status=live}}</ref><ref>{{US patent reference |number = 4582434 |issue-date = April 15, 1986 |inventor = David Plangger and Wayne K. Wilson, Heath Company |title = Time corrected, continuously updated clock}}</ref>

By 1990, engineers from German watchmaker [[Junghans]] had miniaturized this technology to fit into the case of a digital wristwatch. The following year the analog version [[Junghans Mega|Junghans MEGA]] with hands was launched.

In the 2000s (decade) radio-based "atomic clocks" became common in retail stores; as of 2010 prices start at around US$15 in many countries.<ref>[http://www.kleenezeshop.com/products/2988-radio-controlled-clock.aspx/?affiliateid=779" Radio controlled clock £19.95] {{webarchive|url=https://archive.today/20130216084404/http://www.kleenezeshop.com/products/2988-radio-controlled-clock.aspx/?affiliateid=779 |date=2013-02-16}}</ref> Clocks may have other features such as indoor thermometers and [[weather station]] functionality. These use signals transmitted by the appropriate transmitter for the country in which they are to be used. Depending upon signal strength they may require placement in a location with a relatively unobstructed path to the transmitter and need fair to good atmospheric conditions to successfully update the time. Inexpensive clocks keep track of the time between updates, or in their absence, with a non-disciplined [[quartz-crystal clock]], with the accuracy typical of non-radio-controlled quartz timepieces. Some clocks include indicators to alert users to possible inaccuracy when synchronization has not been recently successful.

The United States [[National Institute of Standards and Technology]] (NIST) has published guidelines recommending that radio clock movements keep time between synchronizations to within ±0.5 seconds to keep time correct when rounded to the nearest second.<ref name="lombardi2" >[https://tf.nist.gov/general/pdf/2429.pdf "How Accurate is a Radio Controlled Clock?"] by Michael Lombardi (2010).</ref> Some of these movements can keep time between synchronizations to within ±0.2 seconds by synchronizing 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>

Timepieces with [[Bluetooth]] radio support, ranging from watches with basic control of functionality via a [[mobile app]] to full [[smartwatch]]es<ref>{{cite web | title=Bluetooth| publisher=Casio| url=https://www.casio.com/europe/watches/technology/bluetooth/ | access-date=16 July 2024}}</ref> obtain time information from a connected [[Mobile phone|phone]], with no need to receive time signal broadcasts.

===Other broadcasts===
{{Main|Time signal}}

; Attached to other broadcast stations: Broadcast stations in many countries have carriers precisely synchronized to a standard phase and frequency, such as the [[BBC Radio 4]] [[longwave]] service on 198&nbsp;kHz, and some also transmit sub-audible or even inaudible time-code information, like the [[Radio France]] longwave transmitter on 162&nbsp;kHz. Attached time signal systems generally use audible tones or phase modulation of the carrier wave.

; [[Teletext]] (TTX): Digital text pages embedded in television video also provide accurate time. Many modern TV sets and VCRs with TTX decoders can obtain accurate time from Teletext and set the internal clock. However, the TTX time can vary up to 5 minutes.<ref name="digitalspy_co_uk-showthread_php_p_11057588">{{cite web|title=How's your GHD8015F2 operating?&nbsp;— Personal Video Recorders&nbsp;— Digital Spy Forums| website=[[Digital Spy]] |url=http://www.digitalspy.co.uk/forums/showthread.php?p=11057588}} 100506 digitalspy.co.uk</ref><!-- Simple test, check your clock against the TTX clock.. -->
{{clear}}
Many [[digital radio]] and [[digital television]] schemes also include provisions for time-code transmission.

; Digital Terrestrial Television : The [[Digital Video Broadcasting|DVB]] and [[ATSC]] standards have 2 packet types that send time and date information to the receiver. Digital television systems can equal GPS stratum 2 accuracy (with short term clock discipline) and stratum 1 (with long term clock discipline) provided the transmitter site (or network) supports that level of functionality.

; VHF FM [[Radio Data System|Radio Data System (RDS)]]: RDS can send a clock signal with sub-second precision but with an accuracy no greater than 100 ms and with no indication of clock stratum. Not all RDS networks or stations using RDS send accurate time signals. The time stamp format for this technology is Modified Julian Date (MJD) plus UTC hours, UTC minutes and a local time offset.

; L-band and VHF [[Digital Audio Broadcasting]] : DAB systems provide a time signal that has a precision equal to or better than [[Digital Radio Mondiale|Digital Radio Mondiale (DRM)]] but like FM RDS do not indicate clock stratum. DAB systems can equal GPS stratum 2 accuracy (short term clock discipline) and stratum 1 (long term clock discipline) provided the transmitter site (or network) supports that level of functionality. The time stamp format for this technology is BCD.

; [[Digital Radio Mondiale|Digital Radio Mondiale (DRM)]]: DRM is able to send a clock signal, but one not as precise as [[navigation satellite]] clock signals. DRM timestamps received via shortwave (or multiple hop mediumwave) can be up to 200 ms off due to path delay. The time stamp format for this technology is BCD.

===Gallery===
<gallery>
Low cost DCF77 receiver.jpg|[[Low frequency|LF]] time signal receiver
Junghans Mega.jpg|World's first radio clock wrist watch, [[Junghans Mega]] (analog model)
File:Funkuhr.JPG|Radio controlled analog [[wall clock]]
File:Bahnhof Kinding (Altmühltal), Bahnsteiguhr 2007.jpg|The DCF77 time signal is used by organizations like the [[Deutsche Bahn]] railway company to synchronize their [[station clock]]s
</gallery>

==Multiple transmitters==
A radio clock receiver may combine multiple time sources to improve its accuracy. This is what is done in [[Global Navigation Satellite System|satellite navigation systems]] such as the [[Global Positioning System]], [[Galileo positioning system|Galileo]], and [[GLONASS]]. [[Satellite navigation system]]s have one or more caesium, rubidium or hydrogen maser atomic clocks on each satellite, referenced to a clock or clocks on the ground. Dedicated timing receivers can serve as local time standards, with a precision better than 50&nbsp;ns.<ref>{{cite web|url=http://www.ilotus.com.sg/sites/all/themes/zeropoint/pdf/tx/TX%20Oncore%20-%20TDS%20(Ver%203.5.0).pdf|title=datasheet i-Lotus TX Oncore|access-date=2014-01-22|archive-date=2015-10-16|archive-url=https://web.archive.org/web/20151016222646/http://www.ilotus.com.sg/sites/all/themes/zeropoint/pdf/tx/TX%20Oncore%20-%20TDS%20(Ver%203.5.0).pdf|url-status=live}}</ref><ref>{{cite web|url=http://www.symmetricom.com/products/time-frequency-distribution/gps-instruments/xl-gps/|title=Symmetricom XL-GPS|access-date=2014-01-22|archive-date=2014-02-01|archive-url=https://web.archive.org/web/20140201153456/http://www.symmetricom.com/products/time-frequency-distribution/gps-instruments/xl-gps/|url-status=live}}</ref><ref>{{cite web|url=http://www.trimble.com/timing/pdf/022542-039A_Resolution_SMT_GG_DS_0412_US_LR.pdf|title=datasheet Trimble Resolution SMT GG|access-date=2014-01-22|archive-date=2013-06-22|archive-url=https://web.archive.org/web/20130622223112/http://www.trimble.com/timing/pdf/022542-039A_Resolution_SMT_GG_DS_0412_US_LR.pdf|url-status=live}}</ref><ref>{{cite web|url=https://www.u-blox.com/sites/default/files/NEO-LEA-M8T-FW3_DataSheet_%28UBX-15025193%29.pdf|title=datasheet u-blox NEO/LEA-M8T|access-date=2017-04-11|archive-date=2017-04-12|archive-url=https://web.archive.org/web/20170412061545/https://www.u-blox.com/sites/default/files/NEO-LEA-M8T-FW3_DataSheet_%28UBX-15025193%29.pdf|url-status=live}}</ref> The recent revival and enhancement of [[LORAN]], a land-based radio navigation system, will provide another multiple source time distribution system.

===GPS clocks===
{{Main|GPS disciplined oscillator}}
Many modern radio clocks use [[satellite navigation]] systems such as [[Global Positioning System]] to provide more accurate time than can be obtained from terrestrial radio stations. These ''GPS clocks'' combine time estimates from multiple satellite atomic clocks with error estimates maintained by a network of ground stations. Due to effects inherent in radio propagation and ionospheric spread and delay, GPS timing requires averaging of these phenomena over several periods. No GPS receiver directly computes time or frequency, rather they use GPS to discipline an oscillator that may range from a quartz crystal in a low-end navigation receiver, through oven-controlled [[crystal oscillators]] (OCXO) in specialized units, to atomic oscillators ([[rubidium]]) in some receivers used for [[synchronization in telecommunications]]. For this reason, these devices are technically referred to as [[GPSDO|GPS-disciplined oscillator]]s.

GPS units intended primarily for time measurement as opposed to navigation can be set to assume the antenna position is fixed. In this mode, the device will average its position fixes. After approximately a day of operation, it will know its position to within a few meters. Once it has averaged its position, it can determine accurate time even if it can pick up signals from only one or two satellites.

GPS clocks provide the precise time needed for [[synchrophasor]] measurement of voltage and current on the commercial power grid to determine the health of the system.<ref>{{Cite journal
 | author = KEMA, Inc.
 | author-link = KEMA
 | title = Substation Communications: Enabler of Automation / An Assessment of Communications Technologies
 | publisher = UTC&nbsp;— United Telecom Council
 | date = November 2006
 | pages = 3}}</ref>

===Astronomy timekeeping===
Although any [[satellite navigation]] receiver that is performing its primary navigational function must have an internal time reference accurate to a small fraction of a second, the displayed time is often not as precise as the internal clock. Most inexpensive navigation receivers have one [[CPU]] that is multitasking. The highest-priority task for the CPU is maintaining satellite lock—not updating the display. Multicore CPUs for navigation systems can only be found on high end products.

For serious precision timekeeping, a more specialized GPS device is needed. Some amateur astronomers, most notably those who time [[grazing lunar occultation]] events when the moon blocks the light from stars and planets, require the highest precision available for persons working outside large research institutions. The Web site of the International Occultation Timing Association<ref>{{Cite web |url=http://www.lunar-occultations.com/iota/ |title=International Occultation Timing Association |access-date=2006-07-19 |archive-date=2006-07-20 |archive-url=https://web.archive.org/web/20060720062504/http://www.lunar-occultations.com/iota/ |url-status=live }}</ref> has detailed technical information about precision timekeeping for the amateur astronomer.

==Daylight saving time==
Various formats listed above include a flag indicating the status of [[daylight saving time]] (DST) in the home country of the transmitter. This signal is typically used by clocks to adjust the displayed time to meet user expectations.

==See also==
* [[Casio Wave Ceptor]]
* [[Clock network]]
* [[Speaking clock]]
* [[Standard frequency and time signal service]]
* [[Time from NPL]]
* [[Time and frequency transfer]]
* [[Time synchronization in North America]]

==References==
{{Reflist|33em}}

==External links==
{{Wiktionary}}
{{Commons category|Radio clocks}}
* [http://www.poyntsource.com/IOTAmanual/IOTA_Observers_Manual_all_pages.pdf IOTA Observers Manual] This manual from the ''International Occultation Timing Association'' has very extensive details on methods of accurate time measurement.
* [https://www.nist.gov/pml/div688/grp40/radioclocks.cfm NIST website: WWVB Radio Controlled Clocks]
* [http://ntp.org NTP Project Development Website]

{{Time signal stations}}
{{Time signal authorities}}
{{Time topics}}
{{Time measurement and standards}}
{{UTC time offsets}}
{{Electric clock technology}}
{{Authority control}}

[[Category:Clocks]]
[[Category:Watches]]
[[Category:Radio technology|Clock]]