Contributions to the foundations of a safety case for the use of GNSS in railway environments

The use of GNSS in the railways for passenger information services and selective door opening is already commonplace but the advancement of this increasingly popular navigation technique into safety of life rail applications has been hindered by the unknown level of measurement error caused by the local rail environment, especially that due to multipath. Current state of the art receiver technologies are discussed along with the additional advantages of signal differencing using local base stations. Limiting factors for hardware in a kinematic environment are also discussed and specific examples to the rail environment highlighted. Safety critical analysis techniques such as FMEA, HAZOP and FTA are reviewed to illustrate the evaluation of safety integrity values and the possibility of system risk, leading to the formation of a structured safety case. Three main data sets from electrified, rural and urban rail environments have been collected using dual frequency geodetic receivers in order to enable analysis of multipath effects in normal railway operations. The code and phase data have been combined to compute fluctuations in multipath errors and these have been used to characterise this effect in both space and time. Where phase positioning is possible comparisons with standard code-based positions have been made to assess the overall quality of the type of GNSS positioning expected to be operationally-viable on the railways. Experiments have also been undertaken to evaluate the possible effects of electromagnetic radiation from overhead cables used to power the trains. Finally, the ways in which the results of these experiments can be used to help build a safety case for the use of GNSS on the railways are discussed. Overall it is concluded that it is unlikely that multipath errors or electromagnetic interference will be the limiting factors in utilising GNSS for safety-critical railway applications