Time Synchronization in the Eurobalise Subsystem

The ETCS standard was released by the European Union to reach interoperability in European railway signaling systems. This standard uses so called Eurobalises to send locally stored information to the passing train. Eurobalises are track mounted devices that operate on transponder technology. In order to support the standard for the European train control system (ETCS), a subsystem on the train called balise transmission module (BTM) was developed. The duty of the BTM is to tele-power an Eurobalise as the train passes and to receive the information sent by the Eurobalise. This data has to be demodulated and passed to the European vital computer (EVC), which is the control unit of the locomotive ensuring safe operation. To be able to locate a Eurobalise independent of received telegrams, a Balise Detect signal has to be additionally created by the BTM and sent to the EVC as demanded by ETCS standard. This signal is generated if the received field strength exceeds a given reference level. All components within the ETCS are operating in different time domains. In order to be able to calculate correct timing and odometric data, different time domains have to be synchronized. This has even effect on the safety critical operations within the system. ETCS requires the whole system fulfilling the SIL4 criteria. Considering modularity, it was decided that the BTM has to fulfill SIL4 criteria on its own, too. The crucial aspect is combining safety demands and time synchronization. It affects the communication between the BTM and the EVC. This paper focuses on the synchronization mechanisms within the components of the BTM and the synchronization between BTM and EVC.