Methods and Tools for the Certification of GALILEO Localisation for Railway Applications

Jan Poliak1, Juliette Marais2, Frank Hänsel1, Uwe Becker1, Eckehard Schnieder1 1Institute for Traffic Safety and Automation Engineering (iVA), Technical University of Braunschweig, 2Transport Electronics and Signal Processing laboratory, French National Institute for Transport and Safety Research (INRETS-LEOST)

Abstract Positioning information is used in many railway typical applications: for track, fleet or wagon management as well as for level crossings and many others. Today, this function is provided by track side equipments. However, in the context of European harmonisation but also for saving costs (in particular to help low density traffic lines survive) GNSS (Global Navigation Satellite Systems) seems to fulfil some of the railway localisation requirements – or at least can be one of the bricks of a localisation system. In order to introduce this technology, many applications have been developed over the last few years in this sector and supported by European Commission and national programs. They integrate and use available as well as innovative technologies with the main focus on satellite based positioning (GPS and EGNOS already available, GLONASS and Galileo under re- deployment and development). But this introduction will not be possible without any validation of the performances. The basic idea that will be developed in this paper consists of developing generic reference measurement platforms for the evaluation and validation of applications in the transportation sector. The upcoming European satellite based localisation system Galileo will provide five different services with different performances and characteristics that will be suitable for different ranges of applications. These services will be: Open Services, Safety of Life Services, Commercial Services, Public Regulated Service and Rescue Service. The Safety of Life Service is the key service for most safety related applications due to its guaranteed characteristics of integrity, availability and accuracy. For the concrete validation or testing of applications and services, it is expedient to conclude simulation tests with an evaluation of the real system [4]. For this evaluation, the environmental conditions of the real operation have to be met as good as possible (or sensible) to make the results significant. For this, adequate reference platforms have to be set up and used. These platforms have to offer a very accurate positioning system independent from the positioning systems under test (here independent form satellite based localisation). In this paper, we will first introduce what kind of applications can benefit from Galileo before presenting a methodology devoted to validate the performances in the particular railway environment. An experimental platform is developed that will be presented in the last part, before conclusions.

Introduction: GNSS and Railways

The Context Most of the positioning functions in railways today rely on track side equipments: balises, track circuits or transponders, and other odometry methods [7]. These sensors are efficient, but offer mainly discrete positioning and lead to high maintenance costs (especially on low density lines compared with the often very low revenue on these tracks). Moreover, historically and technically, railway networks differ from one country to another on infrastructure, energy (Electrification 25, 15, 3, 1.5 KV and 750 V), rolling stock, maintenance and exploitation rules, as well as control-command systems and signalling. This is obviously also the case for positioning functions. In Europe in particular, the cross-border interoperability became a major problem for train circulation. The ETCS (European Train Control System) aims to solve it by

defining a standard on European level. The first level of its development relies on track-mounted Eurobalises, augmented by odometry. However, some railways regard them as too costly. First experimental lines GNSS based localisation will be the low density traffic lines, which represent more than 110 000 km single lines in all Europe (approximately 50% of all lines). Indeed, lots of such lines in Europe are threatened of closing for economical reasons. In Germany, for example, 5199 km lines have been close in the last 16 years (approximately 10% of the total track length). GNSS solutions are today expected to reduce or eliminate the need for infrastructure by offering a worldwide solution that is independent of the trackside elements.

Challenges During the last years, initiatives have been funded by European Commission, ESA and GJU as well as by national research programs in order to evaluate GNSS-based applications in railways. First operational solutions focus on non safety critical applications with relaxed requirements. This is the case of the SNCF locomotives tracking and tracing system or the DB Cargo initiative for wagon survey. Safety critical applications are today still subject of research and experimentation. The projects APOLO [1], GADEROS [6], LOCOPROL [10] have opened the way, the GRAIL consortium is currently working on availability and maturity of different “service enablers” (appropriate standards, availability of user terminal, economic and regulatory aspects, awareness, etc) [2]. It aims to propose a strategy, consistent with the current deployment process of ERTMS/ETCS in Europe. The issues to deal with for a good introduction of GNSS in railways are various. We can mention: • The interoperability of GNSS with ERTMS/ERTCS is mandatory in order not to define new standards resulting in the need of new investments. • Of course, performances will have to reach the specific requirements. • Some railways are uncomfortable about relying on outside parties. In particular, the certification process will have to be understood and acceptable compared to the railway certification usages. For all these reasons, the validation stage will be an important step that will demonstrate the expected GNSS performance in the real railway environment. This will be the purpose of the contents of this paper.

Validation Methodology At next, the first approach to the methodology will be shown. Figure 1 shows the workflow in which the GALILEO receiver will be considered as a “black box”. The performance of the typical GALILEO receiver will be characterised based on GALILEO specifications and existing documents.

EN 50126 EN 50129 EN 50128

Figure 1: Requirements relation between transport application types and GNSS

In a first step, this paper will deal with general definitions and general requirements [11]. The main task will be first to describe each feature of quality: accuracy, integrity, continuity, availability [3]. Each of these features of quality has to be defined and described (using a formal language if necessary) [5], so that everybody can agree on the definition. Indeed, discussions between representatives of different transport modes make quickly appear the languages and interpretations different (Figure 2). In particular, the goal is to make them understandable for every transport mode community and make transposable the specification definitions to the transport. In the next steps, the work will define performance requirements for application classes. Each class will represent a certain level of accuracy, integrity, availability independently of the application. The railway applications will then be associated to the different classes previously defined. From this classification, different evaluations can be done according to the appropriate class using some simulation or “real life” tests, as shown in an example in Figure 2.

Figure 2: Schematic representation of the validation process

Requirements will be summarised based on existing documents from the specification of the GALILEO system. Some other identified documents are: the report of the GNSS rail user forum (2000), LOCOPROL reports, UIC working group reports. The GRAIL project has already presented some first results [2], classifying some applications function of integrity and accuracy. A state of the art on standards will be begun and if necessary, a standardised classification could be proposed (see also Figure 3). A necessary step will be to specify a standardised course and predefined trajectory for each class previously defined (comparable to the “EuroCycle” for fuel consumption). Some examples of applications will be chosen, related to safety. Scenarios will be defined and described that will be used in a second phase of work in order to validate the GNSS answer to these particular requirements. The validation process will then be applied on these examples, based on experimental use of both CaRail and PREDISSAT tools (which are described in the next section), which will be combined in order to validate the scenarios for the selected applications. The performance of a GNSS receiver will be analysed based on a reference trajectory and the knowledge of the reception environment.

Train protection Trackman warning

< 1 < 1 s Track protection Maintenance Reliability

Time to Alarm Alarm Time to Tilting technology

GSM-R Train integrity Communikation

Power supply Dispatching

1s < Alarm < 10 s < 10 Alarm 1s < Standstill detection

Infrastructure inspection Tracking and Tracing (track, catenary wire)

Passenger Panthograph information lowering > 10 s

10 km 1 km 100 m 10 m 1 m 10 cm 1 cm

Discrete Continous Accuracy

Figure 3: Railway applications function of reliability and accuracy requirements

Approaches for validation Galileo has engaged its own certification process. The SIS (Signal in Space) will be certified. An integrity flag is transmitted by augmentation systems. However, some difficulties remain for transport applications. It has to be questioned, whether Galileo specifications can be reached in the operational environment of these transport applications. Indeed, satellite based positioning requires at least four distinct satellites to obtain a four- dimensional positioning, consisting of three coordinates in space and one in time (fourth coordinate). However, when used on the surface of the earth, the reception of the necessary amount of satellites (namely four) can be difficult due to environmental obstacles (buildings, trees, etc.) in close range of the vehicle. This problem arises, because quasi-optical wave propagation occurs in the frequency range used for satellite positioning. If an object obstructs the necessary direct line of sight to the satellite, no correct signal can be received. This fact reduces the availability of satellite based positioning in places shadowed by other objects, which cannot be avoided in a railway environment. Furthermore, for the usage in railway systems, a high reliability is necessary for safety related functions (e.g. the train control system). On the other hand, in railway systems, the special constraint exists that the vehicle cannot leave the track. For a valildation of the performance of a positioning system, two main approaches exist, that are not to be seen as oppositional but as complementary. One way is to perform simulations. They have the advantages of being controlled test environments, with modelled scenes, repeatable scenarios, etc. [9]. The other way is to build a referemce measurement platform to perform tests for different kinds of localisation equipment. The main goal of the experiments is to come near to simulation advantages but offer the evaluation of the real (positioning) system under reproducible conditions near those encountered in the intended use. Next parts of this paper will briefly describe the CaRail and PREDISSAT tools more detailed, that will be combined in order to provide this experimental tool.

Reference measurement platform – CaRail A reference measurement platform for the evaluation of satellite based applications in rail has been designed after an evaluation and set up and its validation is in progress. The platform, named CaRail uses two different sensor systems together with a precise map of the track. The localisation system is independent of the satellite system. A schematic view is represented in Figure 4.

Figure 4: The CaRail vehicle

The first sensor is a Doppler radar sensor for continuous position measuring along the track. This sensor has the disadvantage of a relative high drift (0.2%) that has to be stabilised. The RFID-based absolute positioning is used as second sensor to stabilise the drift of the radar sensor. For the RFID system, transponders are located along the (test) track, which represent absolute marks. The position of the transponders (or at least some) are known with high accuracy (about 0,5 cm) and they are unambiguously identifiable. These positions are stored in an XML based electronic track map to obtain information about the matching between the topology of the track and the (geographic) position of the transponders.

Prediction of satellite availability with PREDISAT As described above, the performances will depend on the propagation environment. In order to benefit from a completely described test environment, it is necessary to have a good knowledge of it before analysing the GNSS measurements. The PREDISSAT tool (PREDIctive Software for Satellite Availability in the field of Transport) has been initially developed in order to predict the availability of a satellite system along a known trajectory. The tool allows determining the number of satellites that will be received and available for localisation, at any time and anywhere along a given trajectory. It includes a simulation stage for the knowledge of the satellite positions and an image-processing stage based on a video record of the optical environment surrounding the receiving antenna. Its process is represented in Figure 5.

Figure 5: PREDISAT tool representation

The optical approach allows us to define an “optical horizon line” so that a satellite is “visible” above it, and “blocked” underneath it. Nevertheless satellites can be received without the direct ray. The satellite elevation is then under the optical mask horizon line and the signal is received by a reflected ray. As suggested in telecommunication studies, we classify the satellite signal into three states: direct path, alternate path, signal blocked. The PREDISSAT tool compares satellite positions with positions and elevations of the masks. From the antenna point of view, the mask can be represented like a “fish-eye” view of the sky (Figure 6). The intersection of satellite positions and image determines whether each satellite is over or under the mask. A detailed discussion of the tool can be found in [8].

Azimut 0 10 90 20 30

Elevation

270 10 90

180 Figure 6: Schematic representation of the process and fish-eye view of the sky over the GNSS antenna

By applying the optical geometrical laws like in the case of a “Ray tracing” method, the tool is able to determine not only the satellites received directly and those not received at all (blocked), but also the ones that could be received after one reflection (alternate path). The differentiation between “direct” and “alternate path” is mandatory to predict the validity or reliability of the receiver’s measurements.

Proposed system architecture None of the both tools discussed above is able to validate the performance of a localisation system or GNSS receiver on its own. Only the combination of both can lead to an architecture that is promising for this purpose. By this combination, the disadvantages of each tool can be compensated by the other and the advantages are strengthened in the scope of data evaluation, generation of reference position information and evaluation of the local environmental properties regarding the reception conditions. The resulting architecture is depicted in Figure 7.

Figure 7: Proposed system architecture for qualification of GNSS

For the expressiveness of the results of such validations, not only the procedures and processes have to be specified in an exact way, but also the environmental conditions have to meet some predefined levels to make the tests comparable, for a successful validation of GNSS based application for safety critically applications.

Conclusions and perspectives Satellite-based positioning systems are promising solutions for the future of railways as they can surely satisfy most of the requirements. They are not dependant of national systems and they can be embedded onboard instead of relying on infrastructure and thus, can reduce maintenance cost. The main challenge for a good penetration of the domain will be to prove that performance answers to railway requirements in real railway conditions are of use in particular for safety related applications especially on low density secondary lines. The paper has presented a method being developed commonly by INRETS and the Technical University of Braunschweig in order to provide the necessary tools for this validation. First, we have given an overview of railway requirements for positioning and Galileo specifications. Both communities are using their own definitions and their own language. The aim of the project will be their comparison in order to help them to understand themselves and make railways accept Galileo as a reliable system. The second part of the paper has concentrated on the methodology proposed in order to validate the GALILEO performance in a real railway environment. The work to do will be described in the context of the already existing GALILEO certification process. From the railway point of view, a GALILEO receiver will be used as certified black box equipment. However the reception conditions that impact on positioning accuracy and availability will be studied in typical railway environments. These typical environments will have to be defined and described. This work will combine the PREDISSAT tool developed at INRETS that analyses reception conditions extracted from a video record along lines and the tools CaRail and RailGate from the Technical University of Braunschweig that allows the comparison of measurements from an

experimental reference platform with those from a GNSS receiver. The use of both tools will give some important information for precise quality and performance analyses. This work will be performed in close contact with railway managers via the UIC working group. Their support gives the guarantee that the answers will be as close as possible to the real conditions of use and will require that the language can be understand and accepted by the usual railway processes. The certification question will have to be answered and explained in order to convince users and to find a common language between the two communities who are part of a common certification chain.

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