CHU (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 of Canada.

History 
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). Continuous transmission at 90 metres began at the end of 1929, with other wavelengths being used experimentally.  Time signals were generated from the observatory's own pendulum clocks. The transmitter oscillators were condenser-tuned and so frequency stability was not high until quartz crystal control was implemented in 1933.  In 1938 the call was changed to CHU, operating on frequencies of 3.33, 7.335 and 14.67 MHz, at a transmitter power of only 10 W. The 1,000 Hz tone imposed on the carrier was derived from the quartz oscillator that determined transmit frequency, but the seconds pulses were still derived from the observatory pendulum clocks. The station automatically sent its call sign in Morse code once per hour, and pulses were coded to identify the time of day. Since the CHU power was not high enough to cover much of Canada, including survey parties working in the North, observatory time signals were also transmitted by a Department of Transport station with 2 kW power. In 1947 three new transmitters with 300 W power were installed for the three frequencies. In 1951, a Collins transmitter rated for 3 kW was put in service on 7.335 MHz.

Since deciphering even a simple time code "by ear" was occasionally difficult under field conditions, voice announcements of time and station identification were added to CHU in 1952, using a speaking clock made by Ateliers Brillié Frères of France. Fredrick Martin Meach of the Canadian embassy in Paris recorded the time announcements in English, which were stored on strips of photographic film and played back under control of the observatory clocks. In 1960 the speaking clock was replaced with one manufactured by Audichron corporation and rented by the Dominion Observatory; this unit had more intelligible voice quality and lower maintenance. New English voice announcements were recorded by Harry Mannis of the Canadian Broadcasting Corporation.  Bilingual announcements started in 1964, with French speech provided by Miville Couture of CBC Montreal.

Until 1959 the carrier frequency, tone frequency and second pulses were derived from independent sources, and the carrier stability as that of any commercial short wave transmitter. A divider chain was put into service so that all of the CHU signals were derived from Western Electric standard crystal oscillators with pulses for seconds monitored by continuous comparison with the observatory clocks. By 1978 all parts of the CHU transmitted signal were derived from an NRC-designed cesium beam frequency standard.

Also in 1959, the 14.67 MHz transmitter was replaced with a new 20 kW unit. All site antennas were replaced with vertical antennas by 1971.

The station continued to be operated by the Observatory until 1970, when its operation was transferred to the Institute for National Measurement Standards at the National Research Council.

Effective January 1, 2009, CHU's 7 MHz frequency was changed to 7.85 MHz, due to an allocations change and interference on 7.335 MHz.

Broadcast format 
CHU's signal is used for continuous dissemination of official Canadian government time signals.  The CHU time signal and radio frequencies are derived from atomic clocks. CHU's announcers are Harry Mannis in English and Simon Durivage in French.

CHU will acknowledge listeners' reception reports with QSL cards.

A similar time signal from the National Research Council is used by CBC radio services daily at noon ET on Radio-Canada's Première Chaîne, and 1p.m. ET on CBC Radio One.

Transmission system 
CHU transmits 3 kW signals on 3.33 and 14.67 MHz, and a 10 kW signal on 7.85 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 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 "fixed service" within the band allocations of the International Telecommunications Union.

CHU's 10 kW signal has been transmitted on 7.85 MHz since January 1, 2009.  Before then, the signal was transmitted on 7.335 MHz, harmonically related to their 14.67 MHz frequency. The frequency change was necessary due to an ITU HF global reallocation at the 2003 World Radio Conference (WRC-03), where the frequency region around 7.3 MHz was turned over to different users.

Time signal format 

The primary time signal is a series of 300 ms-long  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 10 seconds of each minute (seconds 50 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.  The time of day (day of year through second) is transmitted twice during each second from 32 to 39.  During second 31, additional information (year, DUT1, daylight saving time, and leap second warning bits) is transmitted.

Western Canada signal coverage 

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.