Artykuły :: Transport :: Conference papers
|Methods of train location based on satellite navigation systems|
Andrzej BIAĹOĹ, Andrzej TORUĹ, RafaĹ IWAĹSKI
System of space information were present on the railways since the beginning of their existence. At the beginning, the simple forms of this information were adapted to the present needs, where it was sufficient to establish the train location with an accuracy of one line between the stations etc, while this information indicated only the fact of presence of a vehicle at a certain place with an accuracy from several meters to 1200 - 1500 m. With time, the increasing requirements of customers enforced a development of train location systems transmitting not only information about the position, but also train identification data – train number.
In all contemporary railway traffic management (location) systems the processed information include: location of train, its composition (number of cars), information about hazardous packages, about the condition of rolling stock in the train etc.
Many railway lines in Poland still operate very obsolete mechanical railway traffic management systems (srk) and the statement of track and turnout occupancy is realized using worn-out and sometimes very unreliable devices. For this reason, following up the development of object space location techniques, especially based upon technologies of satellite positioning in other domains of life, and in particular the civil application in other public transport sections (air, sea, road transport), one may note a dynamic development of monitoring applications and systems using the space information from satellite systems also in the railway transport.
2. DEVELOPMENT TENDENCIES IN RAILWAY TRAFFIC MANAGEMENT SYSTEMS AND TRAIN POSITIONING SYSTEMS
The information about present location of the train on the line or station is a basic one used for making very important decisions in the railway traffic management systems i.e. safe establishing of route or issue of driving permit to the engine driver. Stating the nonoccupancy of tracks and turnouts in the railway traffic management systems used nowadays is basically realized using track circuits and axle counters. In accordance with their principle of operation they are bound to provide the interlocking systems with a safe information concerning their status “train present on the section” or “section is free”.
This key information about the train location is further processed in the interlocking systems that should ensure safe travel of trains on the station and open line, and in particular to ensure a safe distance between the trains on the line.
The new European Train Control System (ETCS) being presently implemented on many railway managements in Europe and worldwide gives enormous opportunities of interoperability implementation. There are two standardized components responsible for location of train in the system: euro-balise (transponder) and odometer. The balise is mounted at the rail and is responsible for transmitting its position to the train, when it moves over. The position of balise as received by the train is used for calibration of the odometer. The odometer measures the distance from balise the train has passed and velocity of the train.
Train location based upon GNSS constitutes an attractive supplement or total replacement of the existing technology. A proposal of such solutions is contained in the specification FRS 3.0 ERTMS Regional.
3. METHODS OF TRAIN LOCATION BASED UPON THE SATELLITE NAVIGATION
3.1. TECHNOLOGY AND SERVICES OF SATELLITE NAVIGATION SYSTEMS
Train location based upon GNSS practically does not require any track equipment and for this reason it is very advantageous from maintenance point of view. For over 20 years the GNSS technology was developed in many various applications (including railway ones). The GNSS receiver became very compact and cheap, resulting in the fact that location of the train based upon GNSS may be cheaper than the location systems presently being used by the railways.
The satellite navigation systems now being in use Global Positioning System (GPS) and Global Satellite Navigation System (GLONASS) because of its character (constructed for military purposes) although used for civil applications, still remain under control of military forces. Moreover, the above named systems without ground support have a relatively small accuracy and availability in civil applications, thus do not guarantee high quality of service and safety required in the railway traffic management process.
The GALILEO system being developed presently and fully civil, will give such quality in the Safety-of-Life service (SoL). The SoL signal of the GALILEO system will guarantee an appropriate accuracy and availability, it will be cross-checked, and thus its reliability will be monitored in a continuous way. Practically, as per today, we have at our disposal only GPS and GLONASS, which as a result requires adaptation of other, additional position-indication elements for the railway purposes. This will be discussed in the further part of this paper.
3.2. TRAIN LOCATION BASED UPON SATELLITE NAVIGATION
The implemented ETCS system for train location uses balises located along the track and an odometer installed on the board. Balises constitute reference points for odometer calibration and the odometer is responsible for determination of travel length and speed measurement.
Similarly, in the ETCS system with a “virtual balise” a hybrid location of vehicle position is realized, using information of a corrected GNSS signal. Although GNSS positioning is not available everywhere, due to signal deflection and “shadowing”, the satellite signal is still very available, and momentary deterioration of its availability (such as tunnels, dense forests) may be supplemented with the data of auxiliary detectors. For this reason the possibility of assurance of a required safety level (SIL) for a determined position is supported by information from other sources, such as Doppler’s radar or acceleration meter.
Fig.1 presents an instance of hybrid GNSS-based location system architecture. In this instance measurement data from three various detectors are synchronized and compared before the connecting detector.
An adder receives a combination of data from these detectors, estimates errors of each detector and assigns them appropriate weights, and yields the actual position at the output.
Fig.1. Instance of a hybrid locator
3.3. INTEROPERAILITY WITH ON-BOARD ETCS EQUIPMENT
The presently approved ETCS specification interfaces: FFFS (Form Fit Functional Specification) and FFFIS (Form Fit Functional Interface Specificaction), there is no mention about the hybrid GNSS locator.
Fig.2. Proposed location of GNSS module in the ETCS system
Thus, it is possible to include the hybrid GNSS locator as a module of on-board ETCS equipment, expanding the presently existing odometer module or replacing it by the hybrid GNSS locator. Thus we may avoid odometer’ s travel and speed measurement errors during starting/acceleration and braking. The locator with its functionality is able to replace not only the odometer, but also balises being reference points for the former.
4. INTEGRAIL AS AN INSTANCE OF PROJECT USING THE SATELLITE NAVIGATION IN RAILWAY TRAFFIC MANAGEMENT AND CONTROL
4.1. OBJECTIVE OF THE PROJECT
In the applications related with passenger information or vehicle fleet management the requirements concerning accuracy or availability of the location system are low and may be met by application of simple systems using the GPS signal and a standard ground communication or GSM. In safe applications, related with the railway traffic management these requirements are much higher (SIL-4) and they may be met only by using location systems of much higher complexity, such as mentioned before hybrid positioning system using : GNSS-I, additional measurement detectors, odometer, digital line map and a very flexible and mobile communication (GSM, GSM-R, GPRS, satcom).
In the INTEGRAIL project realized by European Space Agency (ESA) with participation of Bombardier Transportation, iFEN and Kayser-Threde, the main objective that was set, was to develop a low-cost hardware and software platform for hybrid train positioning using EGNOS/GPS system combination as well as odometer, angle speed meter, digital line map, acceleration meter, digital compass. Moreover, the assumption behind the project is to create a mobile module, fulfilling a function of train positioning subsystem functionally compatible with the location and speed measurement equipment as listed and specified in ETCS specifications, in order to enable integration of positioning based on GNSS-I with ERTMS/ETCS.
The INTEGRAIL project was aimed at allowing a precise positioning of the train for safe application purposes, jointly with a safe differentiation between parallel tracks and a change of track when passing through turnouts. This project assumed also a creation of combined positioning multi-detector (mobile locator) using combined data from GPS and EGNOS and additional speed measurement detectors, determining the location. Module of such a hybrid locator would not require such a continuous operator’s service but still it would give a possibility to watch the situation and control.
4.2. SAFETY ASSUMPTIONS
The INTEGRAIL project aims at establishing a location device using the GNSS signal for the purpose of railway traffic management and designed for cooperation with ERTMS/ETCS system.
In accordance with requirements for ETCS system the accuracy of measurement for a train location device is defined as:
± 5m+ 5%*s,
where s is a distance traveled since the last calibration of odometer.
The writings of requirements specification RAMS concerning ERTMS mention a confidence level for train location 6*(10-11/h) and availability for balises and odometer about 10-7 ÷ 10-8.
Because of the above, the design requirements for INTEGRAIL were as follows:
• Accuracy of positioning
- <5m+ 5%*s; s – traveled road, (95%, for a single track),
- <1m (95%, for track cross-sections),
• reliability of the system
- switching on the alarm: <20m (terminals), <50m (loaded lines), <125m (lines in rural areas),
- time from switching the alarm:
<6 seconds for EGNOS;
<1 second – target value in the project,
- risk: <10-7/h (target <10-9/h)
• availability of the system: >99,99999% (non-availability ) for each 20 seconds or travel of 2km,
• accordance with CENELEC standards: thermal, EMC, vibrations.
4.3. SYSTEM STRUCTURE
INTEGRAIL enables the user to preview the situation and control of operation correctness on a PC. Due to a GSM module located in INTEGRAIL, such data as present position, speed, condition of specific INTEGRAIL components etc are sent to the central server using GSM communication. Between the server and the users these data may be exchanged using LAN network or internet.
Communication with the central server enables:
- SMS information about status from the mobile module in order to perform the check of achievements
- All information with compatible headers or compatible data
- Two defined messages: configuration status and communication address
- The messages received may contain the following information: position from GNSS, speed, hybrid position, condition of the device, configuration data
- The messages may be transmitted periodically or depending of situation.
- Defined messages – SMS commands to the mobile module about change of configuration parameters
- Three defined messages: change of communication address, change of communication parameters, status of configuration
- Presenting the incoming, outcoming messages, commands sent, data and preview of situation through a graphic user interface availabe through LAN network and Internet for the users (password-protected) on the PCs and on the central server (Apache, PHP ver 4)
Fig.3. Communication diagram in the INTEGRAIL
4.4. CONCLUSIONS FROM SITE TESTS
The site tests were performed mainly on private and non-urban lines, but also on main lines in Austria (LogServ/CargoServ, VoestAlpine) and Belgium (SNCB).
Main criteria used for selection of test sites were as follows:
• Topography – the landscape enabling observation of various conditions of satellite signal visibility starting from very good ones (flat lands) up till very limited (in the cities, forests, mountains) where reflections and shadowing of the signal were often present.
• Track and turnout system – railway lines where the tests were performed, included many parallel tracks and turnouts in a close proximity.
• Railway operators are inclined to use modern positioning technologies based on satellite navigation.
Below is presented an assessment of operation of various elements and detectors used in INTEGRAIL.
• Receiver GNSS-1
- Accuracy of positioning not less than 3 m without reflections and shadowing
- Signal EGNOS was unavailable mainly in the mountains
- Signal EGNOS was temporarily lost by inclination of the antenna on the bends
• Additional detectors:
- odometer: relatively good accuracy (in normal conditions, disregarding the sleeping statuses during startup/braking)
- acceleration meter: good accuracy, existence of momentum phenomenon when measuring speed and travel
- rotational speed detector: good accuracy.
- improves availability and accuracy of train location
Use of additional detectors for road measurement has shown that:
• Odometer measures the travel with accuracy 300- 500 m after traveling of more than 140 km, the error of odometer measurement of travel in normal conditions is less than 1%.
• Momentum of the acceleration meter was fixed and lasted ca 1 minute, and the measurement of travel with the acceleration meter when compared with the travel measurement performed with odometer indicated less than 2% error (usually about 1%).
In the difficult cases of train location, on the areas with several parallel tracks, the error resulting from reflection may be very high, especially during the standstill. Situations that may arise especially on the hump yards where high accuracy of measurement of both acceleration and travel is necessary and the high number of tracks and turnouts renders the task very difficult.
Summarizing the project results we may define the conclusions as below.
• Accuracy of positioning from EGNOS without reflections and shadowing obtained during the site tests with errors about 3 m and system availability of 99.99999% was sufficient for non-urban lines, especially for one-track lines whereas it was insufficient for areas with several parallel tracks.
• In mountain areas, EGNOS should be amplified by ground communication.
• In the positioning process based upon a GNSS receiver it is necessary to have the
auxiliary detectors (equipment) necessary for determination of distance and direction of travel.
• In order to state reliably the occupancy of the track also information about the turnout location may be necessary.
The key element of a good system operation is a very good digital map of railway lines.
We have to state that the development of satellite navigation systems as well as results of research projects in progress related with use of GNSS systems in railway traffic management enable the conclusion as formulated below.
• The positioning signal used from GNSS systems may be used in the railway traffic management systems but presently, with the availability of practically GPS system only it requires comparison of information delivered to the location system from other sources such as additional detectors, odometer, etc.
• Application of hybrid solutions enables achievement of required high safety level.
• Use of GNSS signal is competitive in relation to the existing non-occupancy check and location control systems as it features lack of track infrastructure, which means lesser costs of supervision and positioning. This is a solution that is particularly attractive in the low-burden lines where the maintenance of track infrastructure of traditional equipment generates high costs for the line owner (repairs, theft, devastation).
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Faculty of Transport, Silesian University of Technology, Railway Scientific and Technical Centre
Andrzej TORUĹ, RafaĹ IWAĹSKI
Railway Scientific and Technical Centre