Radio clock
Encyclopedia
A radio clock or radio-controlled clock is a clock
that is synchronized
by a time code
bit stream 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 multiple transmitters, like the Global Positioning System
. Such systems may be used to set computer clocks or clocks meant for human readability, or for any purpose where accurate time is needed.
s can usually achieve an accuracy of around 1 millisecond relative to the time standard , generally limited by uncertainties and variability in radio propagation
.
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.
Many other countries can receive these signals (JJY can sometimes be received in Western Australia, Tasmania, and the Pacific Northwest of North America at night), but it 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 transit delay of approximately for every the receiver is from the transmitter. When operating properly and correctly synchronized, better brands of radio clocks are normally accurate to the second. (Product advertising often claims higher accuracy, but for many or most users that is only a theoretical possibility.)
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 in Colorado, USA whenever propagation conditions permitted, automatically switching between the 5, 10, and 15 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 frequency standard 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, US$400 preassembled, and was considered impressive at the time. Heath Company was granted a patent for their design.
In the 2000s radio-based "atomic clocks" became common in retail stores; prices start at around US$15 in many countries. 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 of similar accuracy to a non-radio-controlled quartz timepiece. Some clocks include an indicator to alert users to possible inaccuracy when synchronization has not been successful within the last 24 to 48 hours. In other cases, the indicator will indicate that synchronization has been achieved within the last few hours, and will go blank in the mid-morning.
Modern radio clocks can be referenced to atomic clocks, and provide access to high-quality atomic-derived time over a wide area using inexpensive equipment. They are suitable for scientific or other work which does not require higher accuracy than they can provide.
stratum, a traceable clock accuracy level.
Interval signal
s: Many analog broadcast stations also transmit a distinctive tone or tones at the precise top of every hour, derived from an official source. Most well known is the Greenwich Time Signal
, transmitted on BBC
radio since 1924. In the US, WTIC
in Hartford, Connecticut
has broadcast the Morse code
letter "V" every hour, on the hour, since 1943.
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 kHz, and some also transmit sub-audible time-code information, like the Radio France
longwave transmitter on 162 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.
Many digital radio
and digital television
schemes also include provisions for time-code transmission.
Digital Terrestrial Television : The 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 (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 BCD.
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 GPS-Glonass 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.
such as the Global Positioning System
. GPS, Galileo
and GLONASS
satellite navigation systems have a caesium or rubidium atomic clock on each satellite, referenced to a clock or clocks on the ground. Some navigation units can serve as local time standards, with a precision of about one microsecond
(µs). The recent revival and enhancement of the terrestrial based radio navigation system, LORAN
will provide another multiple source time distribution system.
to provide more accurate time than can be obtained from these 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 (OXCO) 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 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.
The planned establishment of the Galileo positioning system
by the EU (expected to be fully operational in 2013) is intended to provide a second source of time for GPS-compatible clocks that are also equipped to receive and decode the Galileo signals.
applications and development has recently appeared as an augmentation to GPS and other GNSS systems. Enhanced LORAN, also known as eLORAN or E-LORAN, comprises an advancement in receiver design and transmission characteristics which increase the accuracy and usefulness of traditional LORAN to that comparable with unenhanced GPS. eLoran also includes additional pulses which can transmit auxiliary data such as DGPS corrections and UTC information. eLoran receivers now use "all in view" reception, incorporating signals from all stations in range.
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 has detailed technical information about precision timekeeping for the amateur astronomer.
(DST). This often leads to clocks not updating themselves to the correct civil local time when DST transitions on to off or off to on. Clocks that interpret longwave stations tend to be the most affected due to very minimal low level software (often hand coded assembler
, or compiled C
) used to decode and display the time signal.
On some systems, notably WWVB, DST is much less an issue because part of the transmitted timecode contains a "DST is now in effect" flag. Therefore even if a change in law occurs which changes the transition days (the most common change), the transmitted timecode will be updated and the clock's software only needs to pay attention to this DST flag. The only change which would typically cause DST transition problems would be if the transition time were to change (e.g. 0200 hours local time).
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
that is synchronized
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
by a time code
Time code
A timecode is a sequence of numeric codes generated at regular intervals by a timing system.- Video and film timecode :...
bit stream transmitted by a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
transmitter connected to a time standard
Time standard
A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time...
such as an atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
. 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 multiple transmitters, like the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
. Such systems may be used to set computer clocks or clocks meant for human readability, or for any purpose where accurate time is needed.
Single transmitter
Radio clocks synchronized to terrestrial time signalTime signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...
s can usually achieve an accuracy of around 1 millisecond relative to the time standard , generally limited by uncertainties and variability in radio propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
.
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.List of radio time signal stations
| Frequency | Callsign | Country | Location | Aerial type | Power | Remarks |
|---|---|---|---|---|---|---|
| 40 | JJY JJY JJY is the call sign of a low frequency time signal radio station.The station is located in Japan and broadcasts from two sites, one on Mount Otakadoya, near Fukushima, and the other on Mount Hagane, located on Kyushu Island... |
Mount Otakadoya, Fukushima Fukushima Prefecture is a prefecture of Japan located in the Tōhoku region on the island of Honshu. The capital is the city of Fukushima.-History:Until the Meiji Restoration, the area of Fukushima prefecture was known as Mutsu Province.... |
Capacitance hat, height | Located near Fukushima Fukushima, Fukushima is the capital city of Fukushima Prefecture in the Tōhoku Region of Japan. As of May 2011 the city has an estimated population of 290,064 and an area of 746.43 km².It lies about 250 km north of Tokyo and 80 km south of Sendai.-History:... and from Mount Hagane (located on Kyushu Island) |
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| 50 | RTZ |  Russia Russia |
Irkutsk Irkutsk Irkutsk is a city and the administrative center of Irkutsk Oblast, Russia, one of the largest cities in Siberia. Population: .-History:In 1652, Ivan Pokhabov built a zimovye near the site of Irkutsk for gold trading and for the collection of fur taxes from the Buryats. In 1661, Yakov Pokhabov... |
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| 60 | GBZ | Anthorn Anthorn Anthorn is a village in Cumbria, England. It is situated on the south side of the Solway Firth, on the Wampool estuary, about thirteen miles west of Carlisle... , Cumbria Cumbria Cumbria , is a non-metropolitan county in North West England. The county and Cumbria County Council, its local authority, came into existence in 1974 after the passage of the Local Government Act 1972. Cumbria's largest settlement and county town is Carlisle. It consists of six districts, and in... |
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| JJY JJY JJY is the call sign of a low frequency time signal radio station.The station is located in Japan and broadcasts from two sites, one on Mount Otakadoya, near Fukushima, and the other on Mount Hagane, located on Kyushu Island... |
Mount Hagane Haganeyama Transmitter Haganeyama Transmitter is an LF-time signal transmitter at Fuji-cho, Saga-city, Saga-ken, Japan used for transmitting the time signal JJY on 60 kHz... , Kyushu Kyushu is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands.... |
Capacitance hat, height | Located on Kyūshū Island Kyushu is the third largest island of Japan and most southwesterly of its four main islands. Its alternate ancient names include , , and . The historical regional name is referred to Kyushu and its surrounding islands.... |
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| WWVB WWVB WWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks in most of North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic... |
Fort Collins, Colorado Fort Collins, Colorado Fort Collins is a Home Rule Municipality situated on the Cache La Poudre River along the Colorado Front Range, and is the county seat and most populous city of Larimer County, Colorado, United States. Fort Collins is located north of the Colorado State Capitol in Denver. With a 2010 census... |
Two capacitance hats, height 122 m | Received through most of mainland USA. | |||
| MSF60 | Anthorn Anthorn Anthorn is a village in Cumbria, England. It is situated on the south side of the Solway Firth, on the Wampool estuary, about thirteen miles west of Carlisle... |
Range up to . Before 1 April 2007, the signal was transmitted from Rugby, Warwickshire Rugby, Warwickshire Rugby is a market town in Warwickshire, England, located on the River Avon. The town has a population of 61,988 making it the second largest town in the county... |
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| RBU RBU (radio station) RBU is a time code radio station located in Moscow . It transmits a continuous 10 kW time code on 66⅔ kHz. According other sources the transmitter site may be or on the area of Taldom transmitter at... |
Elektrougli Elektrougli Elektrougli is a town in Noginsky District of Moscow Oblast, Russia, located east of Moscow and southeast of Noginsk, on the Moscow – Nizhny Novgorod railroad. Population:... , Moscow Moscow Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent... |
55° 44' N, 38° 12' E | ||||
| BPC BPC (time signal) BPC is the callsign of a time signal transmitter in China. It was inaugaurated on April 25, 2002 and transmits a time signal on 68.5 kHz, which can be used for steering radio controlled clocks. As opposed to to other time signal transmitters therefore, a special license is required. The TX-site is... |
Shangqiu Shangqiu Shangqiu is a city in eastern Henan province, Central China. It borders Kaifeng to the northwest, Zhoukou to the southwest, and the provinces of Shandong and Anhui to the northeast and southeast respectively. An ancient city with a rich history, Shangqiu was also the first capital of the Shang... |
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| 75 | HBG HBG Prangins HBG is a low frequency time signal transmitter for the Swiss time reference system. It currently operates on 75 kHz with 20 kW and is located in Prangins, Switzerland... |
Prangins Prangins Prangins is a municipality in the district of Nyon in the canton of Vaud in Switzerland. It is located on Lake Geneva.-Geography:Prangins has an area, , of . Of this area, or 54.7% is used for agricultural purposes, while or 13.1% is forested... |
Until 31 Dec 2011. Using a format compatible with DCF77. | |||
| DCF77 DCF77 DCF77 is a longwave time signal and standard-frequency radio station. Its primary and backup transmitter are located in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. It is operated by Media Broadcast GmbH , on behalf of the Physikalisch-Technische Bundesanstalt, Germany's... |
Mainflingen Mainflingen Mainflingen is a small settlement of about 3800 inhabitants, a constituent community of the municipality Mainhausen in Hesse, Germany.... , Hesse Hesse Hesse or Hessia is both a cultural region of Germany and the name of an individual German state.* The cultural region of Hesse includes both the State of Hesse and the area known as Rhenish Hesse in the neighbouring Rhineland-Palatinate state... |
Vertical omni-directional antennas with top-loading capacity, height 150 m | 50 kW | Located near Frankfurt with a range of up to | ||
| 162 | TDF Télé Distribution Francaise Télé Diffusion de France, or TDF, is a time signal service, broadcast on longwave radio by the French "Laboratoire primaire du temps et des frequences" . This time signal is generated by an extremely accurate caesium fountain atomic clock... |
Allouis Allouis Allouis is a commune in the Cher department of the Centre region of France.-Geography:An area of lakes, woods and farming comprising the village and several hamlets, situated in the valley of the river Yèvre, some northwest of Bourges at the junction of the D20, D79 and the D122... |
Two guyed steel lattice masts, height , fed on the top | Located south of Paris. Using an encoding similar to that of DCF77, but requiring a more complex receiver | ||
| 2,500 | BPM BPM (time service) BPM is the People's Republic of China's national time signal service, operated by the Chinese Academy of Sciences. It is located at .It is broadcast from the National Time Service Center in Pucheng county about 70 km northeast of Lintong... |
Xi'an Xi'an Xi'an is the capital of the Shaanxi province, and a sub-provincial city in the People's Republic of China. One of the oldest cities in China, with more than 3,100 years of history, the city was known as Chang'an before the Ming Dynasty... |
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| WWV | Fort Collins, Colorado | 2.5 kW | BCD time code on 100 Hz sub-carrier | |||
| WWVH WWVH WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station in Kekaha, on the island of Kauai in the state of Hawaii.... |
Kekaha, Hawaii Kekaha, Hawaii Kekaha is a census-designated place in Kauai County, Hawaii, United States. The population was 3,175 at the 2000 census.-History:... |
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| 3,330 | CHU CHU (radio station) CHU is the call sign of a shortwave time signal radio station operated by the Institute for National Measurement Standards of the National Research Council of Canada.- History :... |
Ottawa, Ontario | 300 baud Bell 103 time code | |||
| 4,996 | RWM RWM RWM is the callsign of a high frequency standard frequency and time signal radio station in Moscow, Russia. RWM transmits on 4996 kHz with 5 kW and on 9996 and 14996 kHz with 8 kW.... |
Russia Russia |
Moscow | SSB Single-sideband modulation Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth.... |
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| 5,000 | BSF BSF BSF could mean* Bachelor of Science in Finance* Basic Sliding Fee* Basal Skull Fracture, a head injury* Bean Scripting Framework, in relation to Java programming language* Bibliothécaires Sans Frontières, a French charitable organisation... |
Chung-Li | ||||
| BPM BPM (time service) BPM is the People's Republic of China's national time signal service, operated by the Chinese Academy of Sciences. It is located at .It is broadcast from the National Time Service Center in Pucheng county about 70 km northeast of Lintong... |
Xi'an Xi'an Xi'an is the capital of the Shaanxi province, and a sub-provincial city in the People's Republic of China. One of the oldest cities in China, with more than 3,100 years of history, the city was known as Chang'an before the Ming Dynasty... |
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| WWV | Fort Collins, Colorado | BCD time code on sub-carrier | ||||
| WWVH WWVH WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station in Kekaha, on the island of Kauai in the state of Hawaii.... |
Kekaha, Hawaii | |||||
| HLA HLA -Biochemistry:*Human leukocyte antigen, a key part of the human immune system, or the paternity test based upon it*Hyaluronic Acid, an important molecule that plays a role throughout the body's skin and connective tissues.-Computing:... |
Taejon | |||||
| LOL1 | Buenos Aires Buenos Aires Buenos Aires is the capital and largest city of Argentina, and the second-largest metropolitan area in South America, after São Paulo. It is located on the western shore of the estuary of the Río de la Plata, on the southeastern coast of the South American continent... |
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| YVTO YVTO YVTO is the callsign of the official time signal from the Juan Manuel Cagigal Naval Observatory in Caracas, Venezuela. The content of YVTO's signal, which is a continuous 1 kW amplitude modulated carrier wave at 5.000 MHz, is much simpler than that broadcast by some of the other time signal... |
Caracas Caracas Caracas , officially Santiago de León de Caracas, is the capital and largest city of Venezuela; natives or residents are known as Caraquenians in English . It is located in the northern part of the country, following the contours of the narrow Caracas Valley on the Venezuelan coastal mountain range... |
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| 7,850 | CHU CHU (radio station) CHU is the call sign of a shortwave time signal radio station operated by the Institute for National Measurement Standards of the National Research Council of Canada.- History :... |
Ottawa, Ontario | Bell 103 time code | |||
| 9,996 | RWM RWM RWM is the callsign of a high frequency standard frequency and time signal radio station in Moscow, Russia. RWM transmits on 4996 kHz with 5 kW and on 9996 and 14996 kHz with 8 kW.... |
Russia Russia |
Moscow | SSB Single-sideband modulation Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth.... |
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| 10,000 | BPM BPM (time service) BPM is the People's Republic of China's national time signal service, operated by the Chinese Academy of Sciences. It is located at .It is broadcast from the National Time Service Center in Pucheng county about 70 km northeast of Lintong... |
Xi'an Xi'an Xi'an is the capital of the Shaanxi province, and a sub-provincial city in the People's Republic of China. One of the oldest cities in China, with more than 3,100 years of history, the city was known as Chang'an before the Ming Dynasty... |
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| WWV | Fort Collins, Colorado | 10 kW | BCD time code on sub-carrier | |||
| WWVH WWVH WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station in Kekaha, on the island of Kauai in the state of Hawaii.... |
Kekaha, Hawaii | |||||
| LOL1 | Buenos Aires Buenos Aires Buenos Aires is the capital and largest city of Argentina, and the second-largest metropolitan area in South America, after São Paulo. It is located on the western shore of the estuary of the Río de la Plata, on the southeastern coast of the South American continent... |
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| 11,000 | ATA | New Delhi, National Physical Laboratory of India | ||||
| 14,670 | CHU CHU (radio station) CHU is the call sign of a shortwave time signal radio station operated by the Institute for National Measurement Standards of the National Research Council of Canada.- History :... |
Ottawa, Ontario | 300 baud Bell 103 time code | |||
| 14,996 | RWM RWM RWM is the callsign of a high frequency standard frequency and time signal radio station in Moscow, Russia. RWM transmits on 4996 kHz with 5 kW and on 9996 and 14996 kHz with 8 kW.... |
Russia Russia |
Moscow | SSB Single-sideband modulation Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth.... |
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| 15,000 | BPM BPM (time service) BPM is the People's Republic of China's national time signal service, operated by the Chinese Academy of Sciences. It is located at .It is broadcast from the National Time Service Center in Pucheng county about 70 km northeast of Lintong... |
Xi'an Xi'an Xi'an is the capital of the Shaanxi province, and a sub-provincial city in the People's Republic of China. One of the oldest cities in China, with more than 3,100 years of history, the city was known as Chang'an before the Ming Dynasty... |
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| BSF BSF BSF could mean* Bachelor of Science in Finance* Basic Sliding Fee* Basal Skull Fracture, a head injury* Bean Scripting Framework, in relation to Java programming language* Bibliothécaires Sans Frontières, a French charitable organisation... |
Chung-Li | |||||
| WWV | Fort Collins, Colorado | BCD time code on sub-carrier | ||||
| WWVH WWVH WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station in Kekaha, on the island of Kauai in the state of Hawaii.... |
Kekaha, Hawaii | |||||
| 20,000 | WWV | Fort Collins, Colorado | BCD time code on sub-carrier |
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.
Many other countries can receive these signals (JJY can sometimes be received in Western Australia, Tasmania, and the Pacific Northwest of North America at night), but it 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 transit delay of approximately for every the receiver is from the transmitter. When operating properly and correctly synchronized, better brands of radio clocks are normally accurate to the second. (Product advertising often claims higher accuracy, but for many or most users that is only a theoretical possibility.)
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
Heathkit
Heathkits were products of the Heath Company, Benton Harbor, Michigan. Their products included electronic test equipment, high fidelity home audio equipment, television receivers, amateur radio equipment, electronic ignition conversion modules for early model cars with point style ignitions, and...
in late 1983. Their model GC-1000 "Most Accurate Clock" received shortwave time signals from radio station WWV in Colorado, USA whenever propagation conditions permitted, automatically switching between the 5, 10, and 15 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 frequency standard 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
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
display. The GC-1000 originally sold for US$250 in kit form, US$400 preassembled, and was considered impressive at the time. Heath Company was granted a patent for their design.
In the 2000s radio-based "atomic clocks" became common in retail stores; prices start at around US$15 in many countries. Clocks may have other features such as indoor thermometers and weather station
Weather station
A weather station is a facility, either on land or sea, with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind...
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 of similar accuracy to a non-radio-controlled quartz timepiece. Some clocks include an indicator to alert users to possible inaccuracy when synchronization has not been successful within the last 24 to 48 hours. In other cases, the indicator will indicate that synchronization has been achieved within the last few hours, and will go blank in the mid-morning.
Modern radio clocks can be referenced to atomic clocks, and provide access to high-quality atomic-derived time over a wide area using inexpensive equipment. They are suitable for scientific or other work which does not require higher accuracy than they can provide.
Other broadcasts
When stratum is referred to it means NTPNetwork Time Protocol
The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
stratum, a traceable clock accuracy level.
Interval signal
Interval signal
An interval signal, or tuning signal, is a characteristic sound or musical phrase used in international broadcasting and by some domestic broadcasters...
s: Many analog broadcast stations also transmit a distinctive tone or tones at the precise top of every hour, derived from an official source. Most well known is the Greenwich Time Signal
Greenwich Time Signal
The Greenwich Time Signal , popularly known as the pips, is a series of six short tones broadcast at one-second intervals by many BBC Radio stations to mark the precise start of each hour...
, transmitted on BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
radio since 1924. In the US, WTIC
WTIC (AM)
WTIC is a 50,000-watt radio station operating out of Hartford, Connecticut, broadcasting news and talk radio. Its signal, located at 1080 kHz, can be picked up throughout southern New England by day and over several states as well as parts of Canada by night...
in Hartford, Connecticut
Hartford, Connecticut
Hartford is the capital of the U.S. state of Connecticut. The seat of Hartford County until Connecticut disbanded county government in 1960, it is the second most populous city on New England's largest river, the Connecticut River. As of the 2010 Census, Hartford's population was 124,775, making...
has broadcast the Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
letter "V" every hour, on the hour, since 1943.
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
BBC Radio 4
BBC Radio 4 is a British domestic radio station, operated and owned by the BBC, that broadcasts a wide variety of spoken-word programmes, including news, drama, comedy, science and history. It replaced the BBC Home Service in 1967. The station controller is currently Gwyneth Williams, and the...
longwave
Longwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
service on 198 kHz, and some also transmit sub-audible time-code information, like the Radio France
Radio France
Radio France is a French public service radio broadcaster.-Mission:Radio France's two principal missions are:* To create and expand the programming on all of their stations; and...
longwave transmitter on 162 kHz. Attached time signal systems generally use audible tones or phase modulation of the carrier wave.
Teletext
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
(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.
Many digital radio
Digital radio
Digital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
and digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
schemes also include provisions for time-code transmission.
Digital Terrestrial Television : The DVB and ATSC
ATSC
ATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks....
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 (RDS)
Radio Data System
Radio Data System, or RDS, is a communications protocol standard for embedding small amounts of digital information in conventional FM radio broadcasts. RDS standardises several types of information transmitted, including time, station identification and programme information.Radio Broadcast Data...
: 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 BCD.
L-band and VHF Digital Audio Broadcasting
Digital audio broadcasting
Digital Audio Broadcasting is a digital radio technology for broadcasting radio stations, used in several countries, particularly in Europe. As of 2006, approximately 1,000 stations worldwide broadcast in the DAB format....
: DAB systems provide a time signal that has a precision equal to or better than Digital Radio Mondiale (DRM)
Digital Radio Mondiale
Digital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave...
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)
Digital Radio Mondiale
Digital Radio Mondiale is a set of digital audio broadcasting technologies designed to work over the bands currently used for AM broadcasting, particularly shortwave...
: DRM is able to send a clock signal, but one not as precise as GPS-Glonass 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.
Multiple transmitters
Multiple time sources may be combined to derive a more accurate time synchronization sources. This is what is done in satellite navigation systemsGlobal Navigation Satellite System
A satellite navigation or SAT NAV system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location to within a few metres using time signals transmitted along a line-of-sight by radio from...
such as the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
. GPS, Galileo
Galileo positioning system
Galileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...
and GLONASS
GLONASS
GLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...
satellite navigation systems have a caesium or rubidium atomic clock on each satellite, referenced to a clock or clocks on the ground. Some navigation units can serve as local time standards, with a precision of about one microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
(µs). The recent revival and enhancement of the terrestrial based radio navigation system, LORAN
LORAN
LORAN is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment to determine the location and speed of the receiver....
will provide another multiple source time distribution system.
GPS clocks
Many modern radio clocks use the Global Positioning SystemGlobal Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
to provide more accurate time than can be obtained from these 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 (OXCO) in specialized units, to atomic oscillators (rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
) in some receivers used for synchronization in telecommunications
Synchronization in telecommunications
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if switches do not operate with the same clock rates, then slips will occur and degrade performance...
. 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
Synchrophasor
A phasor measurement unit is a device which measures the electrical waves on an electricity grid, using a common time source for synchronization. Time synchronization allows synchronized real-time measurements of multiple remote measurement points on the grid...
measurement of voltage and current on the commercial power grid to determine the health of the system.
Galileo positioning system
Using the Global Positioning System is dependent on the goodwill of the United States government for the operation of the GPS satellite constellation. This is not acceptable for many critical non-US civilian and military systems, although it may be acceptable for many civilian purposes, as it is assumed by most users that the civilian GPS signal would not be switched off except in the event of a global crisis of unprecedented proportions.The planned establishment of the Galileo positioning system
Galileo positioning system
Galileo is a global navigation satellite system currently being built by the European Union and European Space Agency . The €20 billion project is named after the famous Italian astronomer Galileo Galilei...
by the EU (expected to be fully operational in 2013) is intended to provide a second source of time for GPS-compatible clocks that are also equipped to receive and decode the Galileo signals.
LORAN
Renewed interest in LORANLORAN
LORAN is a terrestrial radio navigation system using low frequency radio transmitters in multiple deployment to determine the location and speed of the receiver....
applications and development has recently appeared as an augmentation to GPS and other GNSS systems. Enhanced LORAN, also known as eLORAN or E-LORAN, comprises an advancement in receiver design and transmission characteristics which increase the accuracy and usefulness of traditional LORAN to that comparable with unenhanced GPS. eLoran also includes additional pulses which can transmit auxiliary data such as DGPS corrections and UTC information. eLoran receivers now use "all in view" reception, incorporating signals from all stations in range.
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
Grazing lunar occultation
A lunar occultation occurs when the Moon, moving along its orbital path, passes in front of a star or other celestial object, as seen by an observer...
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 has detailed technical information about precision timekeeping for the amateur astronomer.
Daylight Saving Time
Very often radio clocks have bugs in their software relating to daylight saving timeDaylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...
(DST). This often leads to clocks not updating themselves to the correct civil local time when DST transitions on to off or off to on. Clocks that interpret longwave stations tend to be the most affected due to very minimal low level software (often hand coded assembler
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
, or compiled C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
) used to decode and display the time signal.
On some systems, notably WWVB, DST is much less an issue because part of the transmitted timecode contains a "DST is now in effect" flag. Therefore even if a change in law occurs which changes the transition days (the most common change), the transmitted timecode will be updated and the clock's software only needs to pay attention to this DST flag. The only change which would typically cause DST transition problems would be if the transition time were to change (e.g. 0200 hours local time).
See also
- Time signalTime signalA time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...
- Time transferTime transferTime transfer is a scheme where multiple sites share a precise reference time. Time transfer solves problems such as astronomical observatories correlating observed flashes or other phenomenon with each other, as well as cell phone towers coordinating handoffs as a phone moves from one cell to...
- Network Time ProtocolNetwork Time ProtocolThe Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
(NTP) - Speaking clockSpeaking clockA speaking clock service is a recorded or simulated human voice service, usually accessed by telephone, that gives the correct time. The first telephone speaking clock service was introduced in France, in association with the Paris Observatory on 14 February 1933.The format of the service is...
- Atomic clockAtomic clockAn atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
- Greenwich Time SignalGreenwich Time SignalThe Greenwich Time Signal , popularly known as the pips, is a series of six short tones broadcast at one-second intervals by many BBC Radio stations to mark the precise start of each hour...
- Time from NPL
- Clock networkClock networkA clock network or clock system is a set of clocks designed to always show exactly the same time by communicating with each other. Clock networks usually include a central master clock kept in sync with an official time source, and one or more slave clocks which receive and display the time from...
- Casio Wave Ceptor wristwatches
External links
- IOTA Observers Manual This manual from the International Occultation Timing Association has very extensive details on methods of accurate time measurement for astronomical research purposes
- NPL list of Standard Time and Frequency Transmissions
- List of long- and short-wave time-stations and their transmission codes
- NIST website
- NRC Canada clock
- Greenwich Mean Time and world time
- German PTB clock
- UTC and TAI time service from BIPM, Paris
- NIST Internet Time Service (ITS): Set Your Computer Clock Via the Internet
- Informative site from a hobbyist who has built his own clock
- NTP Public Services Project
- NTP Project Development Website
- International Loran Association