Accuracy
Due to the propagation process, phase and/or frequency shifts observed in received signals the practical obtainable accuracy is reduced 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 space waves overlap. This field is not constant but changes in the course of the day between approximately 600 km (373 mi) to 1,100 km (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 and 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 PTB in 1987 measured in Braunschweig situated 273 km (170 mi) from the transmitter in Mainflingen inaccuracies, depending on the time in the day and season, of ± 2 to 22 microseconds.
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 can therefore only be expected to determine the beginning of a second with a practical accuracy uncertainty of ± 0.1 second.
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