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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?

The dissemination of time is one of PTB's legal tasks. Most clocks in the public sphere and in industry – but also private radio-controlled clocks – receive legal time via the long-wave transmitter DCF77 of the Media Broadcast GmbH. Within a radius of 2000 km around the location of the transmitter at Mainflingen near Frankfurt, coded signals can be detected 24 hours per day. 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 BCD-coded and transmitted by pulse duration modulation of the second markers. The 59th second marker is suppressed, thus indicating that the next marker is the minute marker. The control signal is not – as it is often assumed – transmitted by wire from Braunschweig to the transmitting radio station but is generated at the place of emission using a control unit developed by PTB. This control unit, which is housed in a room of the transmitting station, is shielded against high-frequency interferences and controlled from Braunschweig. Via the public telephone network operational data of the control unit can be called up with the aid of a telecontrol system. Furthermore, the carrier phase time and the states of the second markers are compared in Braunschweig with the setpoints specified by PTB's atomic clocks. If there are deviations, the necessary corrections will be made via the telecontrol system. The time announcements of radio and television stations as well as the clocks of the Deutsche Bahn AG and of the speaking clock service of the Deutsche Telekom AG are controlled by DCF77, as are many rate changing time switches of the energy supply companies and 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.