Integrating GNSS with Digital Route Map for Train Positioning and Integrity System

Abstract

The use of Global Navigation Satellite Systems (GNSS) for railway applications is complicated by environmental features such as deep cuttings, buildings, overhead bridges, vegetation and tunnels which block satellite signals and cause multipath and non-line-of-sight effects. This presents a significant obstacle to the use of GNSS for safety-critical train control applications. However by combining GNSS with other sensors and sources of information through a hybridised approach, performance can be significantly enhanced. A new method of integrating GNSS with Digital Route Maps (DRM) has been developed, referred to as Map Aiding. It is based around using the geospatial track data from the DRM in order to constrain train location computations. Choosing the correct track data is critical but challenging because at any one point in time the current train position is not known. The Weighted Topological Map Matching (WeTMM) algorithm for railway application is developed in order to determine prior and post track points in real time. Once the prior and post track points are found by the WeTMM algorithm, a rigorous mathematical model for GNSS/DRM integration is used to calculate train position. A key feature of this model is its ability to model errors in both GNSS observables and the DRM in order to achieve realistic performance statistics for the combined system. A novel Receiver Autonomous Integrity Monitoring (RAIM) algorithm is developed for the GNSS/DRM integration system. The system is further enhanced through the integration of Dead Reckoning (DR), primarily in order to increase the availability of positioning. For testing the integration system, data from a bespoke data collection unit has been used. The unit provides raw GNSS and DR measurements, as well as reference solutions of different grades. DRM data is provided by Network Rail (NR). The results demonstrate significant performance improvements by using DRM and DR to complement GNSS for safety-critical railway applications. When GNSS is integrated with a DRM, the accuracy, integrity and availability are all improved. With DR introduced, positioning availability can reach 100%.