Verification of time signals

Abstract

A continuous broadcast transmission of time information is actually common. In every data network the accurate system time has to be available in any devices. This paper describes a method to verify received time signals over different existing time transmission systems. By this approach, the time between two received time signals is continuously measured and compared with the time difference calculated by the time information contained in the time signals. In other words, physical and logical information is compared. The physical time difference is directly calculated using the recovered or reconstructed carrier frequency of the transmission system (for example: the DCF77 time distribution system in Germany uses a carrier frequency of 77.5 kHz with ASK/PSK modulation). A time counter is clocked by the recovered or reconstructed carrier frequency of the system. The physical time difference between the transmission of two time signals can be derived from the number of oscillations. The logical time difference is given by the time information contained in the received time signals. The time counter value of the time counter is stored, evaluated, restarted and compared to the logical time difference after any correct received time signal in the receiver. The comparison of physical and logical time differences is continuously verified to detect time leaps, which may appear during the transmission of time signals. Manipulated or delayed time signals can be identified accurately. This method can be applied without changing the time distribution protocol and can applied to other time distribution services. The verification process takes place exclusively in the receiver and does not disturb the reception of the present recipients in the reception area of the transmitter station.