DCF77 is a long-wave transmitter located in Mainflingen near Frankfurt/Main and transmits the time signal of the Physikalisch-Technische Bundesanstalt(PTB) on the frequency 77.5 kHz. The transmitter has a power of 30 kW and a 200 m high antenna.
Transmission by means of long wave has the advantage that the long wave propagates as a ground wave and can be received everywhere, even indoors. This signal can also be received by radio clocks, station clocks or radio-controlled church tower clocks.
PTB operates three atomic clocks at the transmitter and transmits the time signalsdigitally. Each second marker transmitted within one minute characterizes certain times: the minute, the hour, the day, the week etc. The deviation of the carrier frequency from the nominal value is `10^-12` per day.
The time information is broadcast as a sequence of time stamps. The second marks are transmitted with a logical "0" with the duration of 0.1 second or with the logical "1" with the duration of 0.2 s by lowering the transmission level by 25%. Each second marker within a minute has a certain meaning. With seven bits the minute is coded and with six bits the hour, with five the day of the week and with four the month.