One common style of radio-controlled clock uses time signals transmitted by dedicated terrestrial longwave radio transmitters, which emit a time code that can be demodulated and displayed by the radio controlled clock. The radio controlled clock will contain an accurate time base oscillator to maintain timekeeping if the radio signal is momentarily unavailable. Other radio controlled clocks use the time signals transmitted by dedicated transmitters in the shortwave bands. Systems using dedicated time signal stations can achieve accuracy of a few tens of milliseconds.
GPS satellite navigation receivers also internally generate accurate time information from the satellite signals. 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.
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
|
7001250000000000000♠ 25 kHz
|
RJH69
|
Belarus
|
Vileyka ( 54° 28' 8 N 26° 46' 23 E)
|
3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of 305 metres and 15 guyed lattice masts with a height of 270 metres
|
|
|
7001250000000000000♠ 25 kHz
|
RJH77
|
Russia
|
Arkhangelsk ( 64° 21' 51 N 41° 33' 52 E)
|
3 umbrella antennas, fixed on 18 guyed lattice masts, height of central masts: 305 metres
|
|
|
7001250000000000000♠ 25 kHz
|
RJH63
|
Russia
|
Imeretinskaya ( 44° 46' 25 N 39° 32' 50 E)
|
umbrella antenna, fixed on 13 guyed lattice masts, height of central mast: 425 metres
|
|
|
7001250000000000000♠ 25 kHz
|
RJH99
|
Russia
|
Nizhny Novgorod ( 56° 10' 20 N 43° 55' 38 E)
|
3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of 205 metres and 15 guyed lattice masts with a height of 170 metres
|
|
|
7001250000000000000♠ 25 kHz
|
RJH66
|
Kyrgyzstan
|
Bishkek ( 43° 2' 29 N 73° 37' 9 E)
|
3 umbrella antennas, fixed on 18 guyed lattice masts, height of central masts: 276 metres
|
|
|
7001250000000000000♠ 25 kHz
|
RAB99
|
Russia
|
Chabarowsk ( 48° 29' 29 N 134° 48' 59 E)
|
umbrella antenna, fixed on 18 guyed lattice masts arranged in 3 rows, height of central masts: 238 metres
|
|
|
7001400000000000000♠ 40 kHz
|
JJY
|
Japan
|
Mount Otakadoya, Fukushima ( 37° 22' 21 N 140° 50' 56 E)
|
Capacitance hat, height 250 m
|
7001500000000000000♠ 50 kW
|
[2] Located near Fukushima and from Mount Hagane (located on Kyushu Island)
|
7001500000000000000♠ 50 kHz
|
RTZ
|
Russia
|
Irkutsk ( 52° 25' 41 N 103° 41' 12 E)
|
|
7001100000000000000♠ 10 kW [3]
|
Inactive
|
7001600000000000000♠ 60 kHz
|
JJY
|
Japan
|
Mount Hagane, Kyushu ( 33° 27' 54 N 130° 10' 32 E)
|
Capacitance hat, height 200 m
|
7001500000000000000♠ 50 kW
|
[2] Located on Kyūshū Island
|
WWVB
|
United States
|
Near Fort Collins, Colorado[4] ( 40° 40' 41 N 105° 2' 48 W)
|
Two capacitance hats, height 122 m
|
7001700000000000000♠ 70 kW
|
[2] Received through most of mainland USA
|
MSF
|
United Kingdom
|
Anthorn ( 54° 54' 27 N 3° 16' 24 W)
|
Triple T-antenna, spun 150 metres above ground between two 227 metres high guyed grounded masts in a distance of 655 metres
|
7001170000000000000♠ 17 kW
|
Range up to 1500 km. Before 1 April 2007, the signal was transmitted from Rugby, Warwickshire ( 52° 21' 33 N 1° 11' 21 W)
|
7001666600000000000♠ 66.66 kHz
|
RBU
|
Russia
|
Taldom, Moscow ( 56° 43' 59 N 37° 39' 47 E)
|
umbrella antenna, fixed on a 275 metres high central tower insulated against ground and five 257 metres high lattice masts insulated against ground in a distance of 324 metres from the central tower
|
7001100000000000000♠ 10 kW
|
before 2008, transmitter located at 55° 44' 14 N 38° 9' 4 E
|
7001685000000000000♠ 68.5 kHz
|
BPC
|
China
|
Shangqiu, Henan ( 34° 56' 54 N 109° 32' 34 E)
|
4 guyed masts, arranged in a square
|
7001900000000000000♠ 90 kW
|
21 hours per day, with a 3-hour break from 05:00–08:00 (China Standard Time) daily (21:00–24:00 UTC)[5]
|
7001750000000000000♠ 75 kHz
|
HBG
|
Switzerland
|
Prangins ( 46° 24' 24 N 6° 15' 4 E)
|
T-antenna spun between two 125 metres tall, grounded free-standing lattice towers in a distance of 227 metres
|
7001200000000000000♠20 kW
|
Discontinued as of 1 January 2012
|
7001775000000000000♠ 77.5 kHz
|
DCF77
|
Germany
|
Mainflingen, Hessen ( 50° 0' 58 N 9° 0' 29 E)
|
Vertical omni-directional antennas with top-loading capacity, height 150 m [6]
|
7001500000000000000♠ 50 kW
|
[2] Located southeast of Frankfurt am Main with a range of up to 2000 km[7]
|
BSF
|
Taiwan
|
Zhongli ( 25° 0' 19 N 121° 21' 55 E)
|
T-antenna spun between two telecommunication towers in a distance of 33 metres
|
|
[8]
|
7002100000000000000♠ 100 kHz
|
BPL
|
China
|
Pucheng, Shaanxi ( 34° 27' 23 N 115° 50' 13 E)
|
single guyed lattice steel mast
|
7002800000000000000♠ 800 kW
|
LORAN-C compatible format signal on air from 5:30 to 13:30 UTC,[9] with a reception radius up to 3000 km[10]
|
7002162000000000000♠ 162 kHz
|
TDF
|
France
|
Allouis ( 47° 10' 10 N 2° 12' 16 E )
|
Two guyed steel lattice masts, height 350 m, fed on the top
|
7003200000000000000♠ 2000 kW
|
AM-broadcasting transmitter, located 150 km south of Paris with a range of up to 3500 km, using an encoding similar to that of DCF77, but requiring a more complex receiver as time signal is transmitted by phase modulation
|
7003250000000000000♠ 2.5 MHz
|
BPM
|
China
|
Pucheng, Shaanxi ( 34° 56' 54 N 109° 32' 34 E )
|
|
|
7:30-1:00 UTC[11]
|
WWV
|
United States
|
Near Fort Collins, Colorado ( 40° 40' 41 N 105° 2' 48 W )
|
|
7000250000000000000♠ 2.5 kW
|
Binary-coded decimal (BCD) time code on 100 Hz sub-carrier
|
WWVH
|
United States
|
Kekaha, Hawaii ( 21° 59' 16 N 159° 45' 46 W )
|
|
7000500000000000000♠ 5 kW
|
7003333000000000000♠ 3.33 MHz
|
CHU
|
Canada
|
Ottawa, Ontario ( 45° 17' 40 N 75° 45' 27 W )
|
|
7000300000000000000♠ 3 kW
|
300 baud Bell 103 time code
|
7003499600000000000♠ 4.996 MHz
|
RWM
|
Russia
|
Moscow ( 55° 44' 14 N 38° 9' 4 E )
|
|
7000500000000000000♠ 5 kW [3]
|
SSB
|
7003500000000000000♠ 5 MHz
|
BPM
|
China
|
Pucheng, Shaanxi ( 34° 56' 54 N 109° 32' 34 E )
|
|
|
0:00-24:00 UTC[11]
|
BSF
|
Taiwan
|
Zhongli
|
|
|
[12]
|
WWV
|
United States
|
Near Fort Collins, Colorado ( 40° 40' 41 N 105° 2' 48 W)
|
|
7001100000000000000♠ 10 kW
|
BCD time code on 100 Hz sub-carrier
|
WWVH
|
United States
|
Kekaha, Hawaii ( 21° 59' 16 N 159° 45' 46 W )
|
|
7001100000000000000♠ 10 kW
|
HLA
|
South Korea
|
Taejon ( 36° 23' 14 N 127° 21' 59 E )
|
|
7000200000000000000♠ 2 kW
|
|
LOL1
|
Argentina
|
Buenos Aires
|
|
7000200000000000000♠ 2 kW
|
|
YVTO
|
Venezuela
|
Caracas
|
|
7000100000000000000♠ 1 kW
|
|
7003785000000000000♠ 7.85 MHz
|
CHU
|
Canada
|
Ottawa, Ontario ( 45° 17' 40 N 75° 45' 27 W )
|
|
7001100000000000000♠ 10 kW
|
300 baud Bell 103 time code
|
7003999600000000000♠ 9.996 MHz
|
RWM
|
Russia
|
Moscow ( 55° 44' 14 N 38° 9' 4 E )
|
|
7000500000000000000♠ 5 kW [3]
|
SSB
|
7004100000000000000♠ 10 MHz
|
BPM
|
China
|
Pucheng, Shaanxi ( 34° 56' 54 N 109° 32' 34 E )
|
|
|
0:00-24:00 UTC[11]
|
WWV
|
United States
|
Near Fort Collins, Colorado ( 40° 40' 41 N 105° 2' 48 W)
|
|
7001100000000000000♠ 10 kW
|
BCD time code on 100 Hz sub-carrier
|
WWVH
|
United States
|
Kekaha, Hawaii ( 21° 59' 16 N 159° 45' 46 W )
|
|
7001100000000000000♠ 10 kW
|
LOL1
|
Argentina
|
Buenos Aires
|
|
7000200000000000000♠ 2 kW
|
Observatorio Naval
|
PPE[13]
|
Brazil
|
Rio de Janeiro[13]
|
Horizontal half-wavelength dipole[13]
|
7000100000000000000♠ 1 kW[13]
|
|
7004110000000000000♠ 11 MHz
|
ATA
|
India
|
New Delhi, National Physical Laboratory of India
|
|
|
|
7004146700000000000♠ 14.67 MHz
|
CHU
|
Canada
|
Ottawa, Ontario ( 45° 17' 40 N 75° 45' 27 W )
|
|
7000300000000000000♠ 3 kW
|
300 baud Bell 103 time code
|
7004149960000000000♠ 14.996 MHz
|
RWM
|
Russia
|
Moscow ( 55° 44' 14 N 38° 9' 4 E )
|
|
7000800000000000000♠ 8 kW [3]
|
SSB
|
7004150000000000000♠ 15 MHz
|
BPM
|
China
|
Pucheng, Shaanxi ( 34° 56' 54 N 109° 32' 34 E )
|
|
|
1:00-9:00 UTC[11]
|
BSF
|
Taiwan
|
Zhongli
|
|
|
[12]
|
WWV
|
United States
|
Near Fort Collins, Colorado ( 40° 40' 41 N 105° 2' 48 W)
|
|
7001100000000000000♠ 10 kW
|
BCD time code on 100 Hz sub-carrier
|
WWVH
|
United States
|
Kekaha, Hawaii ( 21° 59' 16 N 159° 45' 46 W )
|
|
7001100000000000000♠ 10 kW
|
7004200000000000000♠ 20 MHz
|
WWV
|
United States
|
Near Fort Collins, Colorado ( 40° 40' 41 N 105° 2' 48 W)
|
|
7000250000000000000♠ 2.5 kW
|
BCD time code on 100 Hz sub-carrier
|
7004250000000000000♠ 25 MHz
|
WWV
|
United States
|
Near Fort Collins, Colorado ( 40° 40' 41 N 105° 2' 48 W)
|
Broadband monopole
|
7000100000000000000♠ 1.0 kW
|
Schedule: variable (experimental broadcast)
|
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.
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.[18] 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.
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.
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 Global Positioning System. GPS, Galileo and 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 ns.[20][21][22][23] The recent revival and enhancement of the terrestrial based radio navigation system, LORAN will provide another multiple source time distribution system.
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.[24]
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 [25] has detailed technical information about precision timekeeping for the amateur astronomer.
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