Low-frequency radio time signals

[This old web page from 2001 has not been updated much since 2007. You may now find on the the Wikipedia article radio clock a more up-to-date list of transmitter 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:

Related services:

See also Time Stations by KH6BZF, William Hepburn's Time Signals Information Site, NPL, JJY, as well as ITU-R Recommendations TF.583 (time signal formats) and TF.768 (station list).

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

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:

Related information

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.

Signal identification guide: time signals

Markus Kuhn

created 2001-04-16 – last modified 2018-08-23 – http://www.cl.cam.ac.uk/~mgk25/time/lf-clocks/