Approach for evaluating the safety of a satellite-based train localisation system through the extended integrity concept

The integrity concept, safety quality criterion for satellite-based localisation systems used in aeronautics, is described in terms of levels (protection and alert levels), time (Time To Alarm) and probability (integrity risk). In land transport applications, the requirements in terms of integrity differ from aeronautics in their definition and values. Global Navigation Satellite Systems (GNSS) in railways suffer from additional weaknesses i.e. multipath and masking phenomena, which can degrade the localisation integrity. This situation cannot be tolerated in safety-related applications like train control and signalling. To mitigate these weaknesses, GNSS is usually combined with other localisation systems like inertial sensors. However, existing integrity monitoring processes are designed for GNSS integrity evaluation, i.e. to estimate the risk allocated to the position given by the GNSS receivers only. Our research work aims, first, to extend the integrity concept to such systems, and, secondly, to demonstrate how to evaluate, with this concept, the safety of a localisation system as expected in railways. The safety of GNSS-Based Localisation System is formalised and quantitatively evaluated.