  The use of GNSS in the railway sector has been postulated on the notion of a
Virtual Balise (VB). The VB-based positioning system works by setting a VB
point on the railway track and determining the passage of the VB point using
the position solution from the GNSS receiver. Although augmentation systems
such as SBAS or GBAS are able to satisfy the integrity requirements of the
aviation standards down to the 10-7 level, it is difficult to satisfy the high
integrity requirements of the railway sector because firstly the railway users
located on the ground are affected by the ground environment such as terrain,
buildings and tunnels and secondly because the stringency of the railway sector
requirements extends below the 10-9 level. This paper proposes a method to
detect faults in the GNSS solution due to satellite failure or local effects.
Firstly, requirements for the monitoring performance are carefully derived
accounting for the specificities of GNSS, namely that the possibility of
consecutive VB faults cannot be discounted. The second contribution of the
paper is the proposed detection using both odometry and track geometry of the
onboard system. This enables to monitor all three-dimensional solution error so
that higher sensitivity for the fault detection can be achieved. Simulations
have been performed with both single and dual (GPS, GALILEO) solutions. It has
been found that the combinations of metrics are able to achieve very small
missed detection probabilities for mean failure rates from 5.0m/s down to
0.03m/s for most dual constellation geometries. The detection performance of
the odometer implementation varied according to the heading of the train. On
the other hand, when odometry and track geometry are used together, all the
three-directional monitors can obtain stable results regardless of the heading.
