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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

How is time transmitted?

antenna mast of the long-wave transmitter DCF77

The dissemination of time is one of PTB's legal tasks. Since decades PTB employs the long-wave transmitter DCF77 of the Media Broadcast GmbH for that purpose. During every minute the numbers of minute, hour, calendar day, day of the week, calendar month, and the two last figures of the calendar years are transmitted. Details how this is done can be found here. The transmitted signal is not – as it is often assumed – transported by wire from Braunschweig to the radio station but is generated at the place of emission using a signal generator unit (including atomic clocks) developed by PTB, which is controlled from Braunschweig. Within a radius of about 2000 km around the location of the transmitter at Mainflingen, about  40 km south-east of Frankfurt/Main, signals can be received 24 hours per day. The time announcements of radio and television stations as well as the clocks of the Deutsche Bahn AG are controlled by DCF77. Industrial applications comprise installations of energy supply companies, traffic control and traffic light systems. Furthermore, the time signals control and monitor industrial manufacturing processes and last but not least keep private radio-controlled clocks up to date.
Compared with time transmission using satellites, long-wave signals have a decisive advantage: They penetrate buildings to a large extend unobstructed and their detection is not significantly affected by obstacles such as trees or high-rise buildings. They can be received without an exterior antenna using small ferrite antennas incorporated in the case of radio-controlled clocks. Compact battery- or solar cell-operated radio-controlled clocks without any cable connections are feasible. The long-wave dissemination of time will remain thus attractive also in the future and continue to be a useful supplement to time transmission via telephone, internet and satellite signals.