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Session B6: Frontiers of GNSS

Use of SBAS Corrections with Local-Area Monitoring for Railway Guidance and Control Applications
Pietro Salvatori and Cosimo Stallo; RadioLabs, Italy; Sam Pullen, Sherman Lo, and Per Enge; Stanford University, USA
Location: Cypress

The use of augmented GNSS to support aviation, land, and marine applications with demanding safety requirements has become common over the last 15 years. One particular application of interest is railway positioning within an automated control system such as the European Train Control System (ETCS) component of the European Rail Traffic Management System (ERTMS). Augmented GNSS has been proposed as a means of replacing fixed transponders (“balises”) within ETCS so as to reduce installation and maintenance cost and make full ETCS capability possible on many more train lines in Europe and elsewhere [1].

A multi-nation research project funded by the European “Horizon 2020” research effort known as RHINOS has proposed an ERTMS/ETCS system architecture that makes use of augmented GNSS [2]. Local-area differential GNSS (LADGNSS) corrections based on carrier-smoothed code measurements are generated and broadcast from a network of reference receivers placed alongside each track every 30 to 50 km. Reference receivers that are located within SBAS coverage make use of SBAS correction messages to confirm the health of each GNSS satellite and the smoothness of the local ionosphere. However, the network of local-area reference stations computes the differential corrections and performs integrity monitoring that meets all requirements with or without SBAS. Because SBAS information is not required, this approach can be used almost anywhere, but the need for many reference receivers along the entire length of each track to be covered limits the cost reduction that can be achieved relative to today’s transponder-based ERTMS.
This paper describes variations of the RHINOS architecture for tracks within SBAS coverage in which SBAS plays the primary role. Instead of the large network of local-area reference receivers mentioned above, a sparser network of receivers at train stations or at already-existing Radio Block Center (RBC) locations (every 100 km or so) decodes SBAS messages and translates the included correction messages into LADGNSS-formatted corrections and integrity information that are relevant to the location of each reference station (a similar technique was proposed for civil aviation in [3]). The resulting information can then be broadcast to trains using the protocols already defined for RHINOS. Local measurements made at each reference station are used to confirm the health of the derived SBAS corrections, which allows each reference station to improve the level of integrity (e.g., lower the values of UDRE and UIRE) compared to what would be experienced by a normal SBAS user without local augmentation. Trains that receive corrections and bounding error standard deviations (“sigmas”) under this approach will see no difference from the current RHINOS architecture except that the bounding error sigmas will be slightly larger to reflect the more-limited observability of SBAS compared to a dense network of local-area reference stations.
This paper demonstrates the effectiveness of this approach for the “full supervision” mode of ERTMS using Stanford MAAST software simulations and compares the results to those obtained in [2] for the LADGNSS-based architecture to confirm that the loss of availability due to larger error bounds is insignificant. It also examines the cost savings made possible by reliance on SBAS within the European SBAS (EGNOS) coverage region.

[1] A. Neri, F. Rispoli, P. Salvatori, “The Perspective of Adopting the GNSS for the Evolution of the European Train Control System (ERTMS): A Roadmap for a Standardized and Certifiable Platform,” Proceedings of ION GNSS+ 2015, Tampa, FL, Sept. 14-18, 2015, pp. 542-552.
[2] S. Lo, S. Pullen, J. Blanch, P. Enge, V. Palma, M. Salvitti. C. Stallo, "Projected Performance of a Baseline High Integrity GNSS Railway Architecture under Nominal and Faulted Conditions," Proceedings of ION GNSS+ 2017, Portland, OR, Sept. 26-30, 2017.
[3] J. Rife, S. Pullen, T. Walter, E. Phelts, B. Pervan, P. Enge, “WAAS-Based Threat Monitoring for a Local Airport Monitor (LAM) that Supports Category I Precision Approach,” Proceedings of IEEE/ION PLANS 2006, San Diego, CA, April 2006, pp. 468-482.

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