DCF77 is a longwave time signal broadcast station at Mainflingen, Germany, about 25 km south-east of Frankfurt (50°01′ North, 09°00′ East). The DCF77 radio signals are derived from an atomic clock, and broadcasted on 77.5 kHz. The signal can be received up to a distance of 2000 km and more. There are 2 transmitters installed, so in case of failure of the first one it is possible to use the reserve system.