Low-frequency radio time signals
Several national physics laboratories operate radio transmitters that broadcast a time code signal. The following transmitters operate in the 40–80 kHz range and are used to synchronize radio clocks over areas several hundred to some thousand kilometers across:
- JJY: The
first transmitter has 40 kHz, 50 kW and is located on the summit of
the Otakadoya-yama mountain near Miyakoji-mura/Fukushima/Japan (37°
22' N, 140° 51' E).
The second transmitter uses 60 kHz, 50 kW, is located on the summit of the Hagane-yama mountain, near the border of the Saga and Fukuoka prefectures of Japan (33° 28' N 130° 11' E), and was put into service in October 2001. (photos)
- (OMA: 50 kHz, 50 kW, Prague/Czechoslovakia. Was in operation from April 1958 to Spring 1995. See also Martin Poupa's page)
- RTZ: 50 kHz, 10 kW ERP, Irkutsk/Russia (52° 26' N, 103° 41' E).
- MSF: 60 kHz, 15 kW ERP, Britain (currently: Rugby 52° 22' N, 1° 11' W; soon: Anthorn 54° 55' N, 3° 15' W), received throughout much of Northern and Western Europe.
- WWVB, 60 kHz, 50 kW ERP, Ft. Collins/Colorado/USA (40° 40' N, 105° 03' W), received throughout most of mainland USA.
- RBU: 66.66 kHz, 10 kW, Moscow/Russia (55° 44' N, 38° 12' E).
- HBG: 75 kHz, 20 kW, Prangins/Switzerland (46° 24' N, 06° 15' E), DCF77 compatible code.
- DCF77: 77.5 kHz, 50 kW (30 kW ERP), Mainflingen/Germany (N 50° 01', E 09° 00'), receivable up to around 2000 km from Frankfurt/Main (Germany).
- BPC: China, 68.5 kHz, commercial time signal, started broadcasting 2002-04-25, data format apparently proprietary, receiver vendors require a commercial licence.
- Loran-C: 100 kHz, navigation system, offers standard frequency and location but no date/time information. (See also NELS for information on the Northwest European Loran-C.)
Advantages of LF time-signal broadcast
Compared to other time-signal transmissions in higher bands (e.g., GPS) long-wave signals have a number of advantages. With wavelengths of 3–6 km, edge diffraction helps such signals to go around obstacles such as mountains or buildings. The space between the ionosphere and the ground can act like a waveguide. Since no line-of-sight is necessary between the transmitter and receiver, a single very powerful station can cover a huge geographic area. Long-wave signals even penetrate the walls of most buildings quite well. Where propagation happens mostly in the form of a ground wave, transmission delay is less affected by the variability of the ionosphere.
Robust receivers can be constructed very easily for as little as 20–30 USD/EUR and are found today in many radio clocks.
Low-cost receiver components
A receiver consists of
- a tuned ferrite core antenna (e.g., from HKW, 60 kHz version sold by Maplin order no MK72P, 77.5 kHz version sold by Conrad order number 641138-62)
- a receiver IC for amplification, selection, AM detection and automatic gain control (e.g., Atmel T4227 40–120 kHz or U4223B 40–80 kHz, HKW UE6010/UE6011, GSG Semicon AK2124, AK2125 or AK2127, MAS MAS1016 or MAS1017)
- a microcontroller with ADC input for decoding the time signal and phase-locking a software-controlled local clock to it
Low-cost time-code receiver ICs, prebuilt modules and units, antennas and test equipment for DCF77/MSF/WWVB are available e.g. from HKW Elektronik.
Several enthusiasts recorded LF time-signal transmissions during the leap second at 2005-12-31 23:59:60Z:
- My MSF
recording covers 400 s, from about 23:57:44Z to 00:04:24Z,
in a ground-floor office in Cambridge. I used
a 50 Hz – 1.5 MHz active H-field antenna (coil with ferrite core), a
Dynamic Sciences R1250 Tempest receiver (set to: manual gain control,
linear AM demodulation, 60 kHz center frequency, 50 Hz IF bandwidth),
and a digital storage oscilloscope. The recording is available as
three different files:
- AM demodulator output, modulated with a 1 kHz tone (MP3, 8 kbit/s, mono, 8 kHz sampling frequency) – use this file if you want to listen to the signal with your sound card (like using a beat-frequency oscillator on an AM receiver)
- AM demodulator output (250 Hz sampling frequency) – pick this file if you want to look at the envelope of the modulated carrier wave with a graphical sound-file editor
- David Malone has made a similar recording in Dublin
- Pieter-Tjerk de Boer has recorded a waterfall diagram of the entire 58–80 kHz spectrum at about the same time, which nicely shows three LF time signals simultaneously, in which he noticed some oddities in the HBG and MSF signals
- Poul-Henning Kamp also noticed an odd representation of the leap second in HBG in his recording of the LF band
The Digital Radio Mondiale standard for long/medium/short-wave digital audio broadcasts (freely available for downloading as ETSI TS 101980) includes time data, but like with RDS and DVB, the data format specification is not really optimized towards high-precision clock synchronization and the DRM COFDM demodulator needed is significantly more complex than the AM receivers that decode the time signals listed above.
Thanks to Shuhei Amakawa, Dave Woolley, Tom Van Baak, Martin Poupa and Markus Prosch for providing information.
created 2001-04-16 – last modified 2006-05-10 – http://www.cl.cam.ac.uk/~mgk25/time/lf-clocks/