View source for Physics:Radio clock

{{short description|Type of clock which self-synchronizes its time using dedicated radio transmitters}}
[[File:Atomic clock.jpg|thumb|245px|A modern LF radio-controlled clock]]

A '''radio clock''' or '''radio-controlled clock''' (RCC), and often colloquially (and incorrectly<ref name="Lombardi"/>) referred to as an '''"[[Engineering:Atomic clock|atomic clock]]"''', is a type of [[Engineering:Quartz clock|quartz]] [[Physics:Clock|clock]] or [[Engineering:Watch|watch]] that is automatically [[Synchronization|synchronized]] to a time code transmitted by a [[Social:Radio|radio]] transmitter connected to a [[Physics:Time standard|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 [[Earth:Satellite navigation|satellite navigation]] systems such as [[Engineering:Global Positioning System|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 [[Physics: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.

==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 [[Physics:Radio propagation|radio propagation]]. Some timekeepers, particularly watches such as some 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 [[Engineering:Umbrella antenna|umbrella antenna]]<ref group=lower-alpha>3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of 305 metres (1000') and 15 guyed lattice masts with a height of 270 metres (885')</ref> || {{val|300|u=kW}} || rowspan=6 | This is Beta time signal.<ref name="vniiftri">{{Citation|url=ftp://ftp.vniiftri.ru/BULLETINS/V/bull_b16_2018.pdf|title=Standard Time and Frequency Signals|access-date=2018-07-15|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 (1000')</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: 425 metres (1395')</ref> || {{val|300|u=kW}} 
|-
| <!--4-->RJH90 || {{RUS}}<br />VNIIFTRI || [[Place:Nizhny Novgorod|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 205 metres (673') and 15 guyed lattice masts with a height of 170 metres (558')</ref> || {{val|300|u=kW}} 
|-
| <!--5-->RJH86<ref name="vniiftri" /><ref group=lower-alpha>in air RJH66</ref> || {{KGZ}}<br />VNIIFTRI || [[Place:Bishkek|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: 276 metres (905')</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: 238 metres (780')</ref> || {{val|300|u=kW}} 
|-
| {{ntsh|40}} {{nowrap|40 kHz}} || <!--7-->JJY || {{JPN}}<br />[[Organization:National Institute of Information and Communications Technology|NICT]] || Mount Otakadoya, Fukushima<br />{{coord|37|22|21|N|140|50|56|E|name=JJY}} || Capacitance hat, height {{nowrap|250 metres}} (820') || {{val|50|u=kW}} || Located near Fukushima<ref name=McCarthy09 />
|-
| {{ntsh|50}} {{nowrap|50 kHz}} || <!--8-->RTZ|| {{RUS}}<br />VNIIFTRI || Irkutsk<br />{{coord|52|25|41|N|103|41|12|E|name=RTZ}} || Umbrella antenna || {{val|10|u=kW}} || [[Physics:Phase modulation|PM]] time code
|-
| rowspan=3 | {{ntsh|60}} {{nowrap|60 kHz}}
| <!--9-->JJY || {{JPN}}<br />[[Organization:National Institute of Information and Communications Technology|NICT]] || Mount Hagane, Kyushu<br />{{coord|33|27|54|N|130|10|32|E|name=JJY}} || Capacitance hat, height {{nowrap|200 metres}} (656') || {{val|50|u=kW}} || Located on Kyūshū Island<ref name=McCarthy09 />
|-
| <!--10-->MSF || {{GBR}}<br />[[Organization:National Physical Laboratory (United Kingdom)|NPL]] || 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, Warwickshire {{coord|52|21|33|N|01|11|21|W}}}}}} || Triple T-antenna<ref group=lower-alpha>3 T-antennas, spun 150 metres (492') above ground between two 227 metres (745') high guyed grounded masts in a distance of 655 metres (715 yards)</ref> || {{val|17|u=kW}} || Range up to {{nowrap|1,500 km}} (1000 miles)
|-
| <!--11-->WWVB || {{USA}}<br />[[Organization: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 122&nbsp;metres (400') || {{val|70|u=kW}} || Received through most of mainland U.S.<ref name=McCarthy09 />
|-
| {{ntsh|66.66}} {{nowrap|66.66 kHz}} || <!--12-->RBU || {{RUS}}<br />VNIIFTRI || Taldom, [[Place:Moscow|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 275 metres (902') high central tower insulated against ground and five 257 metres (843') high lattice masts insulated against ground in a distance of 324 metres (355 yards) from the central tower</ref> || {{val|50|u=kW}} || [[Physics:Phase modulation|PM]] time code
|-
| {{ntsh|68.5}} {{nowrap|68.5 kHz}} || <!--13-->BPC || {{CHN}}<br />[[Physics: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&nbsp;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|publisher=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</s> || {{flagdeco|Switzerland}} <s>{{wipe|Switzerland}}</s><br /><s>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 125 metres (410') tall, grounded free-standing lattice towers in a distance of 227 metres (248 yards)</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 />PTB || Mainflingen, Hessen<br />{{coord|50|00|58|N|09|00|29|E|name=DCF77}} || Vertical omni-directional antennas with top-loading capacity, height 150 metres (492')<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 {{nowrap|2,000 km}} (1250 miles)<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 || {{TWN}} || 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 33 metres (36 yards)</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 || {{CHN}}<br />[[Physics:National Time Service Center|NTSC]] || Pucheng, Shaanxi<br />{{coord|34|56|56|N|109|32|35|E|name=BPL}} || Single guyed lattice steel mast || {{val|800|u=kW}} || [[Engineering:Loran-C|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|publisher=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 3,000&nbsp;km (2000 miles)<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|publisher=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}} || [[Engineering:CHAYKA|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<br />{{coord|51|05|00|N|142|43|00|E|name=RNS-V}} || Single guyed mast || {{val|400|u=kW}} || [[Engineering:CHAYKA|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 />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 | [[Physics: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 Hz 200 baud
|-
| bgcolor=lightgrey | {{ntsh|135.6}} {{nowrap|135.6 kHz}}<ref group=lower-alpha name="teleswitch" />|| <!--21-->HGA22 || {{HUN}}<br />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 />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 || {{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 {{nowrap|350 metres}} (1150'), fed on the top || {{val|800|u=kW}}|| AM-broadcasting transmitter, located {{nowrap|150 km}} (100 miles) south of Paris with a range of up to 3,500&nbsp;km (2200 miles), using [[Physics: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 />[[Organization:National Physical Laboratory (United Kingdom)|NPL]] || Droitwich<br />{{coord|52.2955|N|2.1063|W|type:landmark|name=BBC4}} || [[Engineering:T-antenna|T-aerial]]<ref group=lower-alpha>Droitwich uses a T-aerial suspended between two {{nowrap|213 metres}} (699') guyed steel lattice [[Engineering:Radio masts and towers|radio masts]], which stand {{nowrap|180 metres}} (197 yards) 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 and 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 || {{CHN}}<br />[[Physics:National Time Service Center|NTSC]] || 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|publisher=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 || {{USA}}<br />[[Organization: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 />[[Organization: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 || {{CAN}}<br />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, [[Place:Moscow|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 || {{CHN}}<br />[[Physics:National Time Service Center|NTSC]] ||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 || {{KOR}}<br />KRISS || Daejeon<br />{{coord|36|23|14|N|127|21|59|E|name=HLA}} || || {{val|2|u=kW}} ||
|-
| <!--32-->WWV || {{USA}}<br />[[Organization: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 />[[Organization: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}} || [[Place:Caracas|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 || {{CAN}}<br />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, [[Place:Moscow|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 || {{CHN}}<br />[[Physics:National Time Service Center|NTSC]] || Pucheng, Shaanxi <br />{{coord|34|56|56|N|109|32|35|E|name=BPM}} || || || (BCD time code on 125 Hz sub-carrier not yet activated)<br />00:00–24:00 UTC<ref name=bpm />
|-
| <!--38-->LOL || {{ARG}}<br />SHN || [[Place:Buenos Aires|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 || {{USA}}<br />[[Organization: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 />[[Organization: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<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 [[Physics: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 || {{CAN}}<br />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, [[Place:Moscow|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 || {{CHN}}<br />[[Physics:National Time Service Center|NTSC]] || 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 || {{USA}}<br />[[Organization: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 />[[Organization: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 || {{USA}}<br />[[Organization: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 || {{USA}}<br />[[Organization: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</s> || <s>{{FIN}}<br />[[Organization: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 [[Engineering:Sloper antenna|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 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 – [ftp://ftp2.bipm.org/pub/tai/scale/TIMESIGNALS/timesignals.pdf Time Signals] {{Webarchive|url=https://web.archive.org/web/20211010023819/ftp://ftp2.bipm.org/pub/tai/scale/TIMESIGNALS/timesignals.pdf |date=2021-10-10 }}, 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>https://www.vttresearch.com/en/si-units-finland-time-and-frequency</ref>
|}

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

{{Location map+ |World
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 <!-- 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=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<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">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,&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,&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</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<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 |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,&nbsp;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">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 |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 |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</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 |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<span style="visibility:hidden">MIKE</span> |position=top |mark=Black pog.svg}}
<!-- template for new lines:
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}}<!--
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|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 – [ftp://ftp2.bipm.org/pub/tai/scale/TIMESIGNALS/timesignals.pdf Time Signals] {{Webarchive|url=https://web.archive.org/web/20211010023819/ftp://ftp2.bipm.org/pub/tai/scale/TIMESIGNALS/timesignals.pdf |date=2021-10-10 }}, 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 {{nowrap|300 km}} (200 miles) 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 [[Company:Heathkit|Heathkit]] in late 1983. Their model GC-1000 "Most Accurate Clock" received shortwave time signals from 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 [[Engineering:LED|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>

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 [[Engineering:Weather station|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 [[Organization:National Institute of Standards and Technology|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>

===Other broadcasts===

; 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 [[Engineering:Digital television|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 [[Physics: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 (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 (DRM): DRM is able to send a clock signal, but one not as precise as [[Engineering:Navigation satellite|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|[[Physics: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:Bahnsteiguhr.jpg|The DCF77 time signal is used by organizations like the Deutsche Bahn railway company to synchronize their station clocks
</gallery>

==Multiple transmitters==
A radio clock receiver may combine multiple time sources to improve its accuracy. This is what is done in satellite navigation systems such as the [[Engineering:Global Positioning System|Global Positioning System]]. [[Engineering:GPS|GPS]], Galileo and [[Astronomy:GLONASS|GLONASS]] satellite navigation systems 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 [[Engineering:LORAN|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 [[Earth:Satellite navigation|satellite navigation]] systems such as [[Engineering:Global Positioning System|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 ([[Chemistry:Rubidium|rubidium]]) in some receivers used for [[Synchronization in telecommunications|synchronization in telecommunications]]. For this reason, these devices are technically referred to as GPS-disciplined oscillators.

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.
 | 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 [[Earth:Satellite navigation|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 [[Astronomy:Grazing lunar occultation|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
* [[Engineering:Clock network|Clock network]]
* Speaking clock
* Standard frequency and time signal service
* Time from NPL
* [[Time transfer]]
* [[Time synchronization in North America]]

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

==External links==
* [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 measurement and standards}}
{{Electric clock technology}}

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

{{Sourceattribution|Radio clock}}