According to the regulations of the ITU, in particular of article 19 of the
Radio Regulations, “Identification of Stations“, all stations whose ranges exceed national frontiers must be identified by a call sign in accordance with the series of call signs assigned to each country and be registered in the International Frequency List. The series of call signs assigned to the Federal Republic of Germany begins with DAA and ends with DRZ. For the DCF77 transmitter, the call sign has been determined as follows: In accordance with the series of call signs assigned, D stands for Germany. The letter C was selected to identify long-wave transmitters. As a third letter, the letter F was determined for the long-wave transmitters on the premises of the transmitting radio station Mainflingen (due to the vicinity to Frankfurt). As the transmitting radio station Mainflingen uses several frequencies in the long-wave region, the numbers 77 corresponding to the carrier frequency 77.5 kHz were added to the three letters DCF to distinguish between the different transmitters of that station.
At present, the DCF77 call sign is still transmitted in Morse signals three times an hour, each time twice in succession, in minutes 19, 39, 59 (during seconds 20 to 32). One letter or one number each has been emitted between two second marks, the Morse signals being given in the form of 250 Hz rectangular carrier reductions from 100% to approx. 85%. Although the call sign generation was performed electronically, without interruption of the time marker sequence, their emission caused an impairment of the signal-to-noise ratio of the respective second markers. As a clear assignment of signals to the DCF77 transmitter is always possible due to the characteristics of the DCF77 signals, emission of this call sign will in future be dispensed with in agreement with the Radio Regulations.
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