Learn More About This
Directory
This directory sponsored by SIQL, a Spider Makers company...
Radio clock
(Redirected from Time signal)
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.
A variety of frequencies helps reception no matter what the ionospheric weather.
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 40°40'49"Nx105°02'27"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 21°59'16"Nx159°45'50"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 50°01'Nx9°00'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-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.
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.
- News radio: The easiest method to access standard time is to listen to the news on radio. National radio news programs set their clocks to the transmissions from the standards departments of their respective countries.
- Internet: Some standards are available on the net. U.S. Government atomic clocks are available to the public on the NIST website (see below) with a time-of day display accurate to about 0.3S. They also provide downloads of a program to set your computer's clock via the internet or a modem using NTP. The time jitter of NTP ranges from tens of milliseconds to tens of microseconds, depending on the quality of the Internet links and the local computer clock.
- Telephone: The U.S. clocks are also available by phone at +1 303 499 7111 (WWV), +1 808 335 4363 (WWVH), or +1 202 762 1401, +1 202 762 1069, and +1 719 567 6742 (USNO).
| Time signal stations
|
| BPM | CHU | DCF77 | JJY | MSF | RWM | VNG | WWV | WWVB | WWVH | YVTO
|
- Time transfer
- Network time protocol
- Speaking clock
- atomic clock
From http://en.wikipedia.org/wiki/Time_signal
This custom Guide To The Internet produced by
Siql. Visit us today, and find out how to get your own
custom guide to the Internet, and how to get your site
listed in our guides.
Copyright 1995-2004 by Siql. All
Rights Reserved.
