Radio clock
A radio clock is a clock that is synchronized by a time code transmitted by a radio transmitter connected to a time standard such as an atomic clock.
Radio clocks depend on time signal radio stations, which usually have the following attributes:
- they refer their broadcast frequency to the frequency standard
- they broadcast seconds 'pips' to identify the start of second intervals
- also broadcast time codes as a way of identifying seconds intervals
- they publich exact geographic references for each antenna, so the radio signal's time-of-flight can be estimated.
| Table of contents |
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2 Loran Clocks 3 GPS clocks 4 Other access 5 See also 6 External links |
Terrestrial time signals
Radio clocks synchronized to terrestrial time signals can usually achieve an accuracy of around 1 millisecond relative to the time standard, generally limited by uncertainties and variability in radio propagation.
Time signals that can be used as references for radio clocks include:
- U.S. NIST Broadcasts:
- The U.S.'s NIST clocks are also available on longwave radio, station WWVB at 60KHz (binary coded decimal only) at 50,000W,
- and by shortwave radio stations WWV (a male voice, Fort Collins, Colorado, about 100 kilometers north of Denver at approximately 40ÃÂð40'49"Nx105ÃÂð02'27"W )at 2.5, 5, 10 , 15 and 20 MHz at 2,500 to 10,000W,
- and WWVH (a female voice, Kekaha, Hawaii, on Kauai near Kekaha, at about 21ÃÂð59'16"Nx159ÃÂð45'50"W., ) at 2.5, 5, 10 , 15 and 20 MHz at 2,500 to 10,000W.
- German Broadcasts: a time signal from DCF77, (Mainflingen, an atomic clock near Frankfurtat about 50ÃÂð01'Nx9ÃÂð00'E) can be obtained by DCF77 transmission on a standard frequency of 77.5 KHz to a range of about 2000 km.
- Canadian Broadcasts: The official time can be obtained by tuning to radio station CHU (Ottawa, Ontario) at 3.33, 7.335 and 14.67 MHz.
- UK Broadcasts: a time signal from an atomic clock near Rugby which can be obtained by MSF transmissions on 60 KHz.
- the JJY radio station in Japan
Loran Clocks
Loran-C time signals may also be used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of the offsets of LORAN navigation signals against time standards.
GPS clocks
Many modern radio clocks use the GPS satellite positioning 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. Because they compute the time and position simultaneously from readings from several sources, GPS clocks can automatically compensate for line-of-sight delay and many radio propagation defects, and can achieve sub-microsecond accuracy under ideal conditions.
GPS, Galileo and GLONASS: These satellite navigation systems, have caesium or rubidium atomic clocks on each satellite, rated from clocks on the ground. Some navigation units can serve as local time standards, with an accuracy of about one microsecond.
However, GPS clocks are dependent on the goodwill of the United States 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 by the EU (expected to be fully operational in 2008) is intended to provide a second source of time for GPS-compatible clocks that are also equipped to receive and decode the Galileo signals.
The radio frequencies are set by the clocks and are a precision standard, useful for adjusting receivers.
Other access
| Time signal stations |
| BPM | CHU | DCF77 | JJY | MSF | RWM | VNG | WWV | WWVB | WWVH | YVTO |