[ntp:questions] CHU Public Notice : http://inms-ienm.nrc-cnrc.gc.ca/time_services/shortwave_broadcasts_e.html

Max Power mikehack at u.washington.edu
Tue Dec 12 21:24:41 UTC 2006

NRC Short Wave Station Broadcasts (CHU)

About the New Messages on CHU - October, 2006

The added messages on CHU are:

"On April 1, 2007, CHU needs to stop operating, change frequencies, or 
re-licence. Contact radio.chu at nrc.gc.ca or mail CHU Canada K1A 0R6," and

« En avril 2007, CHU doit soit cesser ses opérations, soit changer de 
fréquence, soit renouveler sa licence. Contactez radio.chu at cnrc.gc.ca ou 
écrivez à CHU Canada, Conseil national de recherches, K1A 0R6. »

This outreach is to collect information from users of CHU to help shape 
recommendations concerning what should be done concerning changes to CHU 
that will have to be in place by April 2007.

In April 2007 the licence on 7.335 MHz will have to be modified to reflect 
changes on the status of the band allocation by the International 
Telecommunications Union. This frequency has been changed from "fixed 
service" to "broadcast". (The ITU decision does not affect the frequencies 
3.33 MHz and 14.67 MHz.)

Some alternatives are:

Re-licencing just might be possible, calling the 7.335 MHz a "broadcast".

It is also possible to stop using that frequency (the most useful of the 
three we use). Stopping one signal is the easiest solution but could create 
problems for some clients who are counting on this particular signal.

Change the frequency from 7.335 MHz to a nearby fixed-service frequency. It 
would need some investment from our part in new hardware and in manpower. It 
could also create problems for clients, and likely not all radios will be 
able to tune to the new frequency.

Closure of the entire CHU operation, as discussed below.

To be seriously considered, any of the above alternatives will need to have 
a zero-based budgeting justification prepared, comparing it against the 
least expensive alternative of closing CHU entirely. CHU is entering a phase 
where major investment in new transmitters will be required if it is to be 
kept operating. In the absence of input from the CHU user community, 
concerning the importance of CHU's contribution in the modern world, this 
last option is an inescapable recommendation.

The CHU code is also used as a radio clock, which can be used as a reference 
clock for an NTP time server. Software drivers have been written that can 
obtain the date and time from the code and that tune a digitally tuned radio 
to one of our 3 frequencies, to get the best signal. Users of this service 
generally don't listen to the audio broadcast. So we cannot gauge the usage 
by sending this announcement.

Please, if you know of anyone using CHU but not aware of the possible 
changes to its frequency usage, let them know and ask them to contact us 
about any essential uses. Also if you have an important use for CHU signals, 
please tell us how you use our signals.

We are preparing the case to keep CHU in operation.

CHU Time Service

Time accuracy superior to telephone time accuracy is available throughout 
Canada and in many other parts of the world by means of NRC's radio time 
signals broadcast continuously from short wave radio station CHU. If 
corrections are made for the propagation delay from CHU to the user, and for 
delays in the user's receiver, an accuracy of better than 1 ms can be 
obtained. Signal availability at a user's location depends on ionospheric 
conditions. CHU also broadcasts a time code which can be decoded with common 
computers and modems.

Three frequencies are used: 3330, 7335, and 14 670 kHz. The transmission 
mode, upper single sideband with carrier re-inserted, provides time signal 
service without requiring a special SSB radio, and also provides three 
standard frequencies. The frequencies are derived from one of a trio of 
closely synchronized atomic clocks located at the transmitter site. Three 
clocks are employed to permit majority logic checking. CHU time signals are 
also derived from these clocks. The clocks at the CHU transmitter site, 
about 20 km from NRC's time laboratory, are compared daily with the NRC 
primary cesium clocks.

Normally CHU's emission times are accurate to 10-4 s, with carrier frequency 
accuracy of 5x10-12, compared to NRC's primary clocks, which are usually 
within 10 microseconds and 1x10-13 compared to UTC. UTC is the international 
official time reference. It is constructed by the Bureau International des 
Poids et Mesures (BIPM), based on the average of laboratory and commercial 
atomic clocks located in laboratories around the world. It is steered in 
frequency using the primary cesium standards (such as those at NRC) located 
at some of the major time laboratories. UTC loosely follows the 
irregularities of the astronomical time scale UT1, which is needed in 
astronomical observations and in celestial navigation. Since 1972, leap 
seconds have been used to keep UTC within 0.9 s of UT1. The difference [UT1- 
UTC] is called DUT1, and this fraction of a second [-0.8 s to +0.8 s] is 
broadcast by means of an internationally accepted code. To decode the size 
of DUT1, in tenths of a second, a user counts the number of emphasized 
seconds markers in one minute. For CHU, the emphasized seconds pulses are 
split, so that a double tone is heard. When the emphasis is on seconds 1 
through 8, DUT1 is positive; and when DUT1 is negative, seconds 9 through 16 
are used.

The first minute of each hour commences with a full 1 s pulse of 1000 Hz 
tone, followed by 9 s of silence, and then the normal pattern of 0.3 s 
pulses of 1000 Hz at one-second intervals. The normal pattern for each of 
the next 59 minutes starts with a 0.5 s 1000 Hz pulse, followed by the DUT1 
code employing split 0.3 s pulses where required, and normal 0.3 s pulses up 
to and including that at 28 seconds. The pulse at 29 seconds is omitted. 
Following the normal pulse at 30 seconds, for a 9 s period, 1000 Hz pulses 
of 0.01 s occur, each followed by the CHU FSK digital time code described in 
CHU Broadcast Codes. The pulses between 40 and 50 seconds are of normal 
length. In the final 10 s period of each minute a bilingual station 
identification and time announcement is made, with the 1000 Hz seconds 
pulses shortened to "ticks". Each minute's announced time refers to the 
beginning of the pulse which follows. Since April 1, 1990, the announced 
time is always UTC.

The CHU station is located 15 km southwest of Ottawa at 45º 17' 47" N, 75º 
45' 22" W. Main transmitter powers are 3 kW at 3330 and 14 670 kHz, and 10 
kW at 7335 kHz. Individual vertical antennas are used for each frequency. 
The electronics 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 by a talking 
clock using digitally recorded voices.
Historical Information

Radio station CHU is operated by the Institute for National Measurement 
Standards at the National Research Council of Canada.

The call letters CHU were first used for Canadian time transmission in 1938, 
on the modern frequencies, 3330 KHz, 7335 KHz and 14670 KHz. Before that the 
call letters of essentially the same transmissions were VE9OB. The carrier 
frequency has been the specified standard since 1934; before that the quartz 
oscillators had been tuned to standard wavelengths. Continuous transmissions 
at a wavelength of 20.4 m had started in 1933, joining the 40.8 m and 90 m 
transmissions, which began in 1929 (daytime only).

Daily transmission on a wavelength of 52.5 m had begun in 1928 under the 
call letters 9CC (later VE9CC), but ceased with the startup of 40.8 m 
operation. 9CC had started experimental time transmission in 1923 on 275 m, 
and license 3AF had operated in 1922. Thus there is quite a range of 
possible dates to assign to the establishment of CHU; we lean towards 1929 
as being the start of daily time transmissions at essentially the modern 
frequencies. Of course there has been quite a change in equipment and 
accuracy over the years, but the largest improvement was with the change to 
cesium atomic clocks in 1967. In 1970 the responsibility of operating CHU 
was transferred from the astronomers at the Dominion Observatory, to the 
physicists at the National Research Council.

Since 1970, the National Research Council has been charged with maintaining 
official time for Canada. The short wave radio station CHU is one, but only 
one of the ways that official time is disseminated across Canada.

Following internationally accepted recommendations, Canada and other 
countries have official time scales in agreement within 10µs. Since CHU's 
transmissions are well within 100µs of official Canadian time, for all 
distant users of CHU, the dominant source of time error comes from the radio 
wave path reflecting off the ionosphere as the radio signal travels from the 
transmitter (in Ottawa, Ontario, Canada) to the user. The time delay is 
3.3µs per km of path, and generally varies by less than 1ms, due to 
uncertainties in path including the uncertainty in the number of skips made 
by the radio wave (reflections down from the ionosphere and back up from the 
surface of the Earth). For a fixed receiver when the number of skips does 
not change, the variation in the path delay will usually be less than 100µs. 
A small additional delay comes from the radio receiver, and may be 

Before April 1, 1990, CHU's time announcements were given as Eastern 
Standard Time. Since that time CHU's time announcements have been given as 
Coordinated Universal Time (UTC). The change from EST to UTC was done to 
remain in the spirit of the recommendations of the International 
Consultative Committee on Radio: 'that the standard time broadcasts on 
standard frequencies be given in UTC'. In a narrow sense, since CHU does not 
broadcast on the frequencies allocated for frequency standards, one might 
argue that these recommendations do not necessarily apply to CHU. However, 
since CHU is received across Canada's six time zones and around the world, 
we made the change when it became possible technically to change from EST to 
UTC without difficulty.

The warble tone at seconds 31 to 39 allow any computer with a Bell 103 
compatible 300 bps modem to receive and decode an accurate source of time. 
The details on the CHU broadcast code can be found here.

Reception reports from around the world, are gladly accepted from listeners. 
We will respond with a QSL card. Please send reception reports to:

Radio Station CHU
National Research Council of Canada
1200 Montreal Road, Bldg M-36
Ottawa, Ontario, Canada K1A 0R6

Or by e-mail to radio.chu at nrc-cnrc.gc.ca

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