page_title: DCF77
section_title: Reception
hierarchical_section_title: DCF77 / Accuracy / Reception
caption_attribution_description: Deutsch: Funkuhr Wecker innen English: A radio clock. This alarm clock keeps time with a quartz movement, which is periodically synchronized to the correct time by coded radio time signals transmitted by the German Physikalisch Technische Bundesanstalt time radio station DCF77 on 77.5 kHz, received by a radio receiver (right) in the clock. The ferrite loopstick antenna which receives the radio waves is visible at the bottom left.
context_page_description: DCF77 is a German longwave time signal and standard-frequency radio station. It started service as a standard-frequency station on 1 January 1959. In June 1973 date and time information was added. Its primary and backup transmitter are located at 50°0′56″N 9°00′39″E in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. The transmitter generates a nominal power of 50 kW, of which about 30 to 35 kW can be radiated via a T-antenna.
  DCF77 is controlled by the Physikalisch-Technische Bundesanstalt, Germany's national physics laboratory and transmits in continuous operation. It is operated by Media Broadcast GmbH, on behalf of the PTB. With Media Broadcast GmbH, a temporal transmission availability of at least 99.7% per year or under 26.28 hours of annual downtime has been agreed upon. Most service interruptions are short-term disconnections of under two minutes. Longer lasting transmission service interruptions are generally caused by strong winds, freezing rain or snow induced T-antenna movement. This manifests itself in electrical detuning of the antenna resonance circuit and hence a measurable phase modulation of the received signal.
context_section_description: 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 with any time signal radio transmitter the precise establishment of time is affected by the distance to the transmitter, as the time signal propagates to a time signal receiver at the speed of light. For a DCF77 receiver located 1,000 km (621 mi) away from the DCF77 transmitter, due to transit delay the receiver will be set more than 3 milliseconds late. Such a small deviation will seldom be of interest and if desired instrument grade time receivers can be corrected for transit delay.
  Further inaccuracies may be caused by the type of wave the receiver records. In case of just ground wave reception a constant is included in the calculation if the distance is permanent. In case of just space wave reception the reception side cannot influence the time fluctuations. Time fluctuations are influenced directly by the changing altitude of the reflecting and bending layer of the ionosphere. Similar problems arise where ground and skywaves overlap. This field is not constant but changes in the course of the day between approximately 600 to 1,100 km (373 to 684 mi) from the transmitter position.
  Corrected instrument grade DCF77 receivers, using the amplitude-modulated time signals with accompanying antennas oriented tangential to the transmitter's antenna in Mainflingern to ensure the best possible interference-free time signal reception at fixed locations, can achieve a practical accuracy uncertainty better than ± 2 milliseconds.
  In addition to the amplitude-modulated time signal transmission this information is also transmitted since June 1983 by DCF77 via a phase modulation of the carrier wave with a pseudorandom noise sequence of 512 bits length. Using cross-correlation the reproduced signal at the receiving end can be used to determine the beginning of the second markers much more accurately. The drawback of using phase-modulated time signals lies in the complex instrument grade receiving hardware required for using this time signal reception method. Using this method the Physikalisch-Technische Bundesanstalt (PTB) measured standard deviations of ± 2 to 22 microseconds between UTC (PTB) and UTC (DCF77), depending on the time in the day and season. This was done in Braunschweig located 273 km (170 mi) from the transmitter in Mainflingen.
  Normal low cost consumer grade DCF77 receivers solely rely on the amplitude-modulated time signals and use narrow band receivers (with 10 Hz bandwidth) with small ferrite loopstick antennas and circuits with non optimal digital signal processing delay and can therefore only be expected to determine the beginning of a second with a practical accuracy uncertainty of ± 0.1 second. This is sufficient for radio controlled low cost consumer grade clocks and watches using standard-quality quartz clocks for timekeeping between daily DCF77 synchronization attempts, as they will be most accurate immediately after a successful synchronization and will become less accurate from that point forward until the next synchronization.
caption: Consumer grade radio clock movement with the DCF77 receiver (right) in the clock. The small ferrite loopstick antenna used in this alarm clock can be seen at the left.
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