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GNSS has been recognized a strategic asset for favoring the evolution of the modern railways signaling systems that play a major role to prevent accidents due to human errors. The European ERTMS-ETCS train control system has already envisaged the utilization of GNSS for supporting the odometry function in its evolving path to provide cost-effective solutions for the low traffic lines and, more in general, for those lines which are in semi desert areas where it is imperative to limit the wayside equipment along the line ...

The main challenge for the introduction of GNSS-based localization systems on the train signaling systems is to guarantee the same safety levels of the current systems based on wayside equipment (balyse) to estimate the exact train position and the rail where the train is travelling. This latter requirement is at least one order of magnitude more stringent respect to the determination of the train position along the track and, for that reason it requires today the support of the train driver who has to confirm in which rail the train is located. Based on these requirements and since the GNSS constellations are rapidly evolving towards a redundant and resilient global infrastructure, we have developed a novel multi-constellation PVT algorithm for train localization determination, specifically designed to handle the case of multiple track scenarios. This solution is driven by the requirements of the ERTMS-ETCS train control system and compliant with the SIL-4 safety requirements of CENELEC railways norms i.e., Tolerable Hazard Rate (THR), 10(-9) <= THR <= 10(-8).

To meet the SIL 4 requirements, we adopted a GNSS architecture comprising of a dedicated integrity monitoring and augmentation network, as well as the use of multi-constellation receivers.

The PVT algorithm estimates the train location by explicitly accounting for the fact that the train is constrained to lie on a railway track. Basically, exploiting this constraint allows to estimate train location even when only two satellites are in view. Effective reduction in the number of required satellites to make a fix, when track constraint is applied, depends on the track-satellite geometry. In essence, satellites aligned along the track give more information than those at the cross-over. By implementing this technique, the satellites in excess are taken into consideration for the elaboration of the PVT only to achieve the desired accuracy, integrity and availability specifically for the rail application.

In this paper, we address the problem of PVT estimation of the train in presence of multiple tracks. This can be formulated as a combination of hypothesis testing (i.e., which is the current track where the train lies on, or, better, which is the probability of a train lying on a given track?) and parameter estimation (i.e. given a track, which is the curvilinear abscissa of the train receiver?).

A detailed description of the overall PVT process is given. The performance of the track detector versus intertrack distance, observation time duration, signal-to-noise ratio and observables (i.e., pseudoranges and carrier phase) are discussed. Then, the impact of satellite failures on the PVT error magnitude and track detection is exploited. Finally, the assessment of the performance is also provided by means of simulation results making use of both synthetic data (Monte Carlo simulations), and measures recorded on a railway test bed environment as a part of the 3InSat project co-funded by the European Space Agency (ESA) in the framework of the ARTES 20 programme.