An Analytical Evaluation for Hazardous Failure Rate in a Satellite-based Train Positioning System with reference to the ERTMS Train Control Systems

"ERTMS-ETCS developed in Europe for high speed lines is de facto the railways standard train control system being adopted in most new lines and major upgrades. Satellite localization and IP-based TLC have been identified as the new technologies to be adopted by the ERTMS-ETCS since they will reduce the investments and maintenance costs on track-side components to provide cost-effective solutions for the increasing demand of the local, regional and new freight lines in rural and environmental critical areas. A major challenge in the adoption of GNSS-based Location Determination Systems (LDS) is to guarantee the same Hazardous Failure Rate (HFR) achieved with the mechanical odometry systems which has to be less than 10-9 during 1 hour of operation according to the CENELEC SIL 4 requirements. To reach this target, we have investigated an LDS architecture, based on (i) a multi-constellation space segment to increase both the accuracy and the number of satellites in visibility , (ii) the deployment of a proper Track Area Augmentation and Integrity Monitoring Network with very high availability, and (iii) an independent on-board capability to further mitigate GNSS errors and autonomously assess the GNSS location integrity when augmentation data are unavailable. In this paper we exploit the railway characteristics for an analytical evaluation of the performance of Integrity Monitoring and, Detection and Exclusion algorithms, expressed in terms of impact on Protection Level (PL). Since LDS may operate in several operational modes, the analytical model, as well the simulation results, will describe the LDS behavior in each different case outlining the advantege of using the multi-constellation capability respect to the single GPS constellation."