European GNSS Navigation Safety Service for Rail

Background & Objectives

Background

Localisation systems, also known in railway as signalling systems, are necessary to ensure train safe traffic, i.e., no collision or derailment. Historically, signalling systems were specific to each area of operation. Interoperability across Europe will be achieved through the ERTMS (European Rail Traffic Management System) norm, under ERA (European Union Agency for Railways) patronage.

The current ERTMS standard relies on train on-board balise reader and odometer coupled with trackside detection systems to estimate trains speed and track occupancy that are computed on the trackside. To increase railway traffic capacity and sustainability, trains should instead continuously compute on-board their position and speed (defined in ERTMS level 3). This technological step can be achieved through multi-sensor localisation systems using Satellite Navigation services (EGNSS in Europe, EGNOS and Galileo services). Such EGNSS-enabled multi-sensor localisation systems could already be used in ERTMS level 2.

In this new paradigm, a specific concept of integrity for train safe localisation is necessary, that can be designed in reference to the integrity concept developed for the usage of EGNOS in civil aviation.

On the basis of the needs stated by the user groups, including Railways, the European Commission (EC) is developing a roadmap for the evolution of the European GNSS programme, including additional services for Galileo (the European GNSS constellation) and EGNOS. EGNOS is the European satellite-based augmentation system (SBAS) designed to enhance GNSS (GPS and Galileo) performance in terms of accuracy and integrity information for GNSS positioning with continuous monitoring of the continuity and availability of the signal in space and associated services, enabling Safety-of-Life positioning applications.

Former projects like STARS (https://www.stars-rail.eu/) or CLUG (https://clugproject.eu/fr) were conducted to close the gap between ERTMS requirements for safety-critical applications and GNSS services and to perform preliminary feasibility analyses of an on-board localisation unit using EGNSS.

Two approaches are considered for the usage of EGNSS for on-board localisation of trains:

Virtual Balise: this method aims at minimizing evolutions of the on-board architecture by adding a GNSS-based virtual balise reader that detects pre-defined locations instead of physical balises. The on-board odometer is kept unchanged and as well the overall performance.
Continuous use of EGNSS (the CLUG approach): this innovative and modular method promises to improve overall performance while reducing the amount of trackside equipment and functionally replace the odometer and the balise reader. This solution requires more updates of the ERTMS standard.

An EGNSS Safety Service for Rail must be defined for GNSS adoption in both approaches.

The key problem when using GNSS for train localisation, as in aviation, is to demonstrate compliance to availability and integrity performance requirements, i.e., the level of trust that can be placed in the safe bounds on the position, velocity, or time estimate provided by the localisation function.

Project objectives and activities

The main aim of the EGNSS-R project was to propose a future EGNSS-based safety service that would enable the introduction of GNSS in railway localisation.

The main objectives of the project were:

to collect and define Mission requirements and definition of the EGNSS SoL service for railway applications;
to define a novel EGNSS-based Rail Safety Service;
to perform a preliminary assessment of the service;
to deliver recommendations on the evolution process for the future ERTMS TSI.

The EGNSS-R project studied a new EGNSS service dedicated to the rail user allowing a mix of EGNSS and navigation techniques that would enable safe and continuous train localisation for safety-critical applications. This approach includes handling satellite signals outages caused by the railway operational environment (local feared events such as satellite masking, multipath, signal interferences or diffraction). Based on the design of an adequate integrity concept, a preliminary definition of an EGNSS-based safety service has been proposed, associated with an incremental implementation roadmap.

The project was composed of five main tasks.

The first step was the identification of operational requirements, safety requirements and system feared events based on inputs from former European projects, European railway operators and from a working group set-up by EUSPA, called ERGO (Experts in Rail for EGNSS Operational use), that involved key representatives from railway operators, railway and GNSS industry. These requirements were then translated into a user-level integrity concept and a preliminary EGNSS-based service definition for safety critical railway applications. The overall designed concept was then analysed to determine the decision criteria for the EGNSS-based safety service acceptance by all stakeholders. Finally, a roadmap for the implementation of this future service was elaborated.

The consortium organised four main meetings with the collaboration of the ERGO experts, where their feedback and suggestions were collected so in the end they could validate each of the main tasks of the project:

ERGO #1 (16/12/2020): Presentation of the study: organisation, objectives and work plan.
ERGO #2 (01/02/2021): Discussion and validation of user needs and requirements for rail signalling and train control applications
ERGO #3 (05/10/2021): Discussion and validation of the proposed integrity concept proposed and high-level service implementation roadmap
ERGO #4 (16/02/2022): Presentation of Service definition, decision criteria for service adoption and implementation roadmaps