Radio clock

From Academic Kids

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Atomic_clock.JPG
A 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 publish 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.

Contents

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:
  • German Broadcasts: a time signal from DCF77, (Mainflingen, an atomic clock near Frankfurt at about Template:Coor dm) 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, with FSK digital time data sent once per minute at 300 baud
  • 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
  • the BPM (http://www.time.ac.cn/jianjie/3.htm) radio station in Xi'an, China
  • Swiss Broadcasts: the legal Swiss time is can be picked up from the HBG longwave transmitter in Prangins on 75 kHz. The time code is compatible with that of the German DCF-77 transmitter.

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 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 so that after a day or so of operation it will know its position to within a few meters. Once it has averaged its position, it can then determine accurate time even if it can only pick up signals from one or two satellites. The highest-quality GPS clocks have their own precision clocks -- either an atomic clock or a temperature-controlled crystal clock -- so they can maintain accurate time during any interruption of GPS signals.

Note that although any GPS 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 on most consumer GPS units may not be as exact. This is because an inexpensive GPS unit typically has one CPU that is multitasking; the highest-priority task for the CPU is maintaining satellite lock, while updating the display gets the lowest priority. Therefore, the displayed time of most consumer handheld GPS units will be accurate to around half a second.

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

  • 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.

In the era when national broadcasting networks operated over point-to-point terrestrial microwave links, the time announcements were very accurate. Today, however, satellite and digital networks often have latencies on the order of a half second or a second. In places where a car radio can receive more than one station broadcasting the same national news program, when switching between them one often either misses part of a word or hears part of the same word twice due to such variations.

Also, once upon a time every radio station had a local full-time engineer who took considerable pride in keeping their clocks accurate; today many stations do not care as much about such details. Some stations still do provide highly accurate time beeps, such as WTIC (noted below), or WCBS AM 88 in New York City.

  • Interval signals: 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 subaudible timecode information, like the Radio France longwave transmitter on 162 kHz. Many digital radio and digital television schemes also include provisions for timecode transmission.
  • FM Radio Data Systm (RDS): RDS can send a clock signal accurate to sub second accuracy, but not all RDS networks [or stations using RDS] send accurate time signals.
  • Digital Radio Mondial (DRM): DRM is able to send a clock signal accuracy as RDS, but not as accurate as GPS-Glonass clock signals.
  • Mobile telephones: Some mobile telephone technologies, such as Qualcomm's CDMA, are designed to distribute high-quality standard time signals (referenced to GPS in the case of CDMA).

CDMA clocks are increasingly popular for providing reference time to computer networks; their accuracy is nearly as good as GPS, but since the signal comes from a nearby cell phone base station rather than a distant satellite, a CDMA clock will work better inside buildings. So in many cases, when a GPS reference clock would require installing the antenna on the outside wall of the building, a CDMA clock can eliminate the requirement for the outdoor antenna.

  • Internet: Some standards are available on the net. U.S. Government (NIST/USNO) and BIPM atomic clocks are available to the public on their website (see below) with a time-of day display accurate to about 300 ms, depending on the round-trip travel time of IP packets between your computer and the server. Both NIST and BIPM use applets to provide this service: the applet running in your web browser exchanges packets with their server; both also display an accuracy estimate based on the round-trip time of the data packets. NIST also provides 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). Canadian clocks are available by phone at +1 (613) 745-1576 (English) and +1 (613) 745-9426 (French).


Time signal stations
 BPM | CHU | DCF77 | JJY | MSF | RWM | VNG | WWV | WWVB | WWVH | YVTO 

See also

External links

da:Radiour de:Funkuhr ja:電波時計

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