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Low-frequency radio time signals
Stations
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)
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.
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).
Related services:
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.)
Compared to other time-signal transmissions in higher bands (WWV,
GPS, etc.) long-wave signals have a number of advantages. They can go
around obstacles such as mountains or buildings. 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. Propagation happens mostly in the form of a ground wave, such
that 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 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.
Complete WWVB receivers are also available from Ultralink. MSF and DCF77 wrist watches
are distributed in the UK for example by
watch-heaven.com.
Example signals
Leap second – December 2005
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
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
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.