Канадская коротковолновая станция точного времени CHU

Canadian shortwave time signal radio station

CHU is the call sign of a shortwave time signal radio station operated by the Institute for National Measurement Standards of the National Research Council. CHU's signal is used for continuous dissemination of official Canadian government time signals, derived from atomic clocks.

Radio time signals allowed accurate and rapid distribution of time signals beyond the range of the telegraph or visual signals. This was of particular value in surveying remote areas, where time signals allowed accurate determination of longitude. In the summer of 1914, a survey party at Quinze Dam in the Ottawa River watershed attempted to receive time signals transmitted from Kingston; however, signals were not resolvable and the time signal from NAA in Arlington, Virginia was used instead. The station was started in 1923 by the Dominion Observatory in Ottawa, Ontario, Canada, with a call sign of 9CC on an experimental basis until 1928. Regular daytime transmission began under the callsign of VE9OB in January 1929 on a wavelength of 40.8 metres (about 7.353 MHz). These nonstandard time signal frequencies were chosen to avoid interference from WWV and WWVH. The signal is amplitude modulated, with the lower sideband suppressed (emission type H3E). The same information is carried on all three frequencies simultaneously including announcements every minute, alternating between English and French. The CHU transmitter is located near Barrhaven, Ontario, 15 km (10 miles) southwest of Ottawa's central business district. The systems feeding the transmitters are duplicated for reliability, and have both battery and generator protection. The generator can also supply the transmitters. The announcements are made using digitally recorded voices. Individual vertical dipole antennas are used for each frequency. CHU has long been licensed as a "fixed service" within the band allocations of the International Telecommunication Union. as of 2020, CHU has three atomic clocks at the station, contained in a special enclosure to eliminate possible electromagnetic interference and compared with the atomic clocks at NRC's headquarters.

The primary time signal is a series of 300 ms long 1,000 Hz tones, transmitted once per second, on the second. The following exceptions to the pattern provide additional information:
The top of the minute is marked by a half second long beep. The top of the hour is marked with a one second long beep, followed by nine seconds of silence. The 29th second of a minute is always omitted (no beep). Between one and sixteen seconds past the minute (except at the top of the hour), CHU transmits the difference between UT1 and Coordinated Universal Time (UTC) by using split tones. For positive DUT1 values from +0.1 to +0.8 s, seconds 1 through 8 are split. For negative DUT1 values from −0.1 to −0.8 s, seconds 9 through 16 are split. Between 31 and 39 seconds past the minute inclusive, the once per second tones are reduced to 10 millisecond "ticks" while a digital time code is transmitted. The digital time code is formatted so that a Bell 103 compatible 300 baud modem can decode it, and CHU is the only time signal station that uses this format for its time code transmissions. For the last 9 seconds of each minute (seconds 51 to 59), the once per second tones are again cut to 10 milliseconds each, while CHU transmits a brief voice station identification, followed by voice announcements of the next minute in UTC, alternating between French and English. French announcements, using the voice of Radio Canada news anchor Simon Durivage, are transmitted first on the odd minutes, while English announcements, voiced by late CBC Radio announcer Harry Mannis, come first on the even minutes. The digital time code sends 10 characters at 300 bits per second using 8N2 asynchronous serial communication. This follows the Bell 103 standard, a 2,225 Hz tone to represent a mark (1 bit) and 2,025 Hz tone for a space (0 bit). Immediately after the 10 ms tick, a mark tone is sent until 133.3 ms, then 110 data bits, ending at precisely 500 ms. The final stop bit is extended by 10 ms of mark tone to ensure it is detected reliably, and the final 490 ms of the second are silent.

CHU often cannot be received in Western Canada on any of its broadcast frequencies. Propagation conditions, low transmitter power coupled with the typical two ionospheric hops distances from Ottawa result in relatively weak time signals for Western Canada. Electromagnetic interference can further aggravate reception difficulty in urban areas in the West. CHU can be practically unusable in most of Western Canada, Nunavut, and the Northwest Territories, for significant stretches of time. U. S. stations WWV and WWVH are the fallback in Western Canada. In the high Arctic, however, both the U. S. shortwave time stations and CHU become essentially unusable or unreliable. Canada has no longwave time signal transmitters. The American station WWVB is the only option for reliable time signals during geomagnetic storms in the Western Arctic, based on WWVB's published pattern maps. If WWVB is not available, those who need precision time transfer may be able to use GPS time transfer instead.