Time Dissemination

Time Dissemination

Accurate TIME is disseminated or distributed by telecommunication systems to end users across Canada. Time and frequency references, traceable within stated limits to recognized standards, are available in Canada by ground and satellite based radio, television and telephone. By international agreement, the time disseminated is always based on the modern implementation of Greenwich time, UTC (or Coordinated Universal Time). Some services also allow Universal Time 1 (UT1) to be determined for the most precise celestial navigation and surveying.

Time Determination

The most accurate time and frequency determinations can be made at the NATIONAL RESEARCH COUNCIL OF CANADA (NRC) time laboratories, or across Canada by measuring radio signals from a satellite at the same moment as measurements are made at the NRC time laboratory. The easiest satellite system to use for these intercomparisons is the American military navigation satellite system, Global Positioning System (GPS). Intercomparisons can be made that are accurate to less than 10x10⁹ sec.

The ground-based Loran-C radio-navigation signals can be used in a similar way for parts of Canada within range of the Great Lakes Loran-C chain. Within common range of a fixed TV transmitter, time intercomparisons can also be done using TV broadcast signals. For less demanding time applications, direct reception of GPS signals can usually give the correct time to well within 10 microseconds. For applications that do not require the best accuracy or traceability documentation, the GPS system can also serve as a convenient frequency reference.

Using common modems, computer systems can read the time from shortwave broadcasts or via telephone with an accuracy that can be as good as a few milliseconds. Computer networks can determine the time uncertainty by using protocols which measure the time between a request for time and its reception of a time server's response.

Official Time Sources

Canadian official time is available from NRC at (613) 745-3900 (via 300 bit-per-sec Bell-103 compatible modems). Higher accuracy using these same modems and a different protocol is available across Canada via short wave radio reception of NRC's time and frequency radio station CHU. CHU broadcasts continuously at 3.330, 7.335 and 14.670 MHz, and depends on the Earth's upper atmosphere to reflect the signal to the user. Most locations in Canada will normally receive a satisfactory signal at some time of day. Short wave signals from the American time and frequency stations WWV (Colorado) and WWVH (Hawaii) at 2.5, 5, 10, 15 and 20 MHz can also be received, at different times than the CHU signals.

Talking clock signals giving UTC are available in Canada via short wave radio from CHU or WWV; or Eastern Time by telephone at (613) 745-1576 for service in English or (613) 745-9426 for service in French. Unless the phone call has to be routed via satellite, the signals are accurate to a few hundredths of a sec.

The Cable Parliamentary Channel, when not televising the proceedings of Parliament, often displays a video clock derived from NRC time laboratory signals, with an as-received accuracy of a few hundredths of a sec.

The English and French radio networks of the CANADIAN BROADCASTING CORPORATION (CBC) each broadcast the NRC time signal once per day, at 12 noon on the French network and at 1:00 pm Eastern Time on the English network. Generally the accuracy is a few hundredths of a sec, but for radio stations linked by satellite, there is a delay of about one quarter of a sec per satellite hop.

For precise astronomical observations, UTC time signals can be used for determining the rotational position of Earth. NRC can provide interested Canadians with up-to-date information about impending leap seconds; about the number and dates of past leap seconds; about International Atomic Time (IAT) and Terrestrial Dynamic Time (TDT); about the correction to be applied to UTC to obtain UT1; and about methods for obtaining this information automatically.