transmitted by wire from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig to the transmitting radio station in Mainflingen but is generated at the place of emission using a control unit developed by the PTB. This control unit, which is housed in an air conditioned room of the transmitting station, is shielded against high-frequency interferences and controlled from Braunschweig. For reasons of operational reliability, the control signal is generated by three independent control channels all equipped with their own caesium atomic clock. In addition a rubidium atomic clock is available on site. To avoid incorrect emissions the output of these three channels are compared in two electronic switch circuits on site. Output for transmission is only generated when at least two of the three channels are in agreement. 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 the PTB's atomic master clocks that provide the UTC (PTB). Of these atomic clocks the CS2 atomic clock in Braunschweig provides the German national legal time standard, and can be used as a highly accurate frequency standard. If there are deviations, the necessary corrections will be made via the telecontrol system. Accuracy Transmission The DCF77 transmitted carrier frequency relative uncertainty is 2 × 10−12 over a 24-hour period and 2 × 10−13 over 100 days, with a deviation in phase with respect to UTC that never exceeds 5.5 ± 0.3 microseconds. The four German primary caesium (fountain) atomic clocks (CS1, CS2, CSF1 and CSF2) in use by the PTB in Braunschweig ensure significantly less long term clock drift than the atomic clocks used in the DCF77 facility in Mainflingen. With the aid of external corrections from Braunschweig the control unit of DCF77 in Mainflingen is expected to neither gain nor lose a second in approximately 300,000 years. In theory, a DCF77 controlled external clock should be able to synchronize to within one half of the period of the transmitted 77.5 kHz carrier frequency of the DCF77 signal, or within ± 6.452 × 10−6 s or ± 6.452 microseconds. Reception Due to the propagation process, phase and/or frequency shifts observed in received signals the practical obtainable accuracy is lower than originally realized with the atomic clocks at the place of transmission. As