GNSS-based Location Determination System Architecture for Railway Performance Assessment in Presence of Local Effects
Cosimo Stallo, Pietro Salvatori, Andrea Coluccia, Alessandro Neri RadioLabs, Italy; Roberto Capua, Giorgia Olivieri, Luca Gattuso Sogei, Lukasz Bonenberg, Terry Moore, University of Nottingham, UK, Francesco Rispoli, Ansaldo STS
Date/Time: Tuesday, Apr. 24, 4:46 p.m.
The introduction of the GNSS (Global Navigation Satellite System) technology into the ERTMS (European Rail Traffic Management System) is receiving many interests in the last years. Thanks to GNSS, it could be possible to realize cost-effective solutions to increase the safety in the regional lines, where the traffic density is lower. Focusing on ERTMS, we can identify two main operational phases called Start of Mission (SoM) and Full Supervision (FS). A Performance Analysis is here presented. Mitigations that are implemented by the ERTMS at system level are not considered, and the attention is focused here on GNSS only. The train position estimation is implemented taking into account that the train is constrained to lie on the track (i.e. track constraint). In this way, we can express the position in terms of the curvilinear abscissa (progressive mileage) of the track corresponding to the train position. Considering a railway scenario, the impact of local effects such as multipath, foliage attenuation and shadowing plays a crucial role due to the presence of infrastructures like platform roofs, side walls, tunnel entrances, buildings and so on close to the trackside. In the paper, we analyze the impact of the multipath on the GNSS position estimation and Rail Requirements performances. At this aim, a Test Bed based on real railway scenario and data corrupted by means of a synthetic source multipath and foliage models have been used. Several Scenarios in presence of local effects have been generated through a GNSS simulator and data handling. A Sensitivity Analysis has been conducted, varying main Scenarios parameters (e.g. height of obstacles, presence of trees and shadowing). The result of the performed Analysis, in terms of Availability, Accuracy and Integrity, are here presented.
According with , , , the introduction of GNSS (Global Navigation Satellite Systems) technology and the IP-based communication is the next frontier for the European standard for train control: ERTMS (European Railway Traffic Management System / European Train Control System). Major benefits of such innovation rely in the possibility to reduce the maintenance and operational costs without losing in terms of system safety. This consideration leads to the possibility of a cost effective solution for the modernization of the regional low traffic density lines that in Europe represent a big market slice . The big challenge in the adoption of the GNSS technology is represented by the fulfilment of the SIL-4 (Safety Integrity Level) requirement defined by Comité Européen de Normalisation Électrotechnique (CENELEC). With this scope, we have to achieve the same total hazardous failure rate of the traditional mechanical odometers , ,,,. To reach such high performance the use of augmentation systems is highly recommended. In literature, different architectures have been proposed , . Another important issue is related to SIS (Signal In Space) integrity assessment. A position solution by a satellite fault or strongly prejudiced by atmospheric or local effect as multipath, can lead to a misleading information. In this framework, an Integrity Monitoring network, is mandatory. In literature, the integrity monitoring, has been studied both in terms of RAIM (Receiver Autonomous Integrity Monitoring) , ,, and in terms of IMN (Integrity Monitoring Network) algorithms ,,,.
In this paper, we analyse the impact of the local effects (multipath, foliage and shadowing) on the GNSS position estimation calculated through GNSS-based LDS (Location Determination System) taking into account the rail requirements. At this aim, a Test Bed based on real ailway scenario (along Cagliari-San Gavino railway track in Italy) and data corrupted by means of a synthetic source multipath and foliage models has been used. Several Scenarios have been generated through a GNSS simulator and data handling to simulate the different local effects. A Sensitivity Analysis has been conducted, varying main Scenarios parameters (e.g. height of obstacles, presence of trees, shadowing). The result of the performed Analysis, in terms of Availability, Accuracy and Integrity, are here presented.
GNSS LDS REFERENCE AUGMENTATION AND INTEGRITY MONITORING AND BOARD UNIT ARCHITECTURE FOR RAILWAY
The Reference Architecture GNSS based LDS is composed of the following components:
• On Board Unit (OBU): it applies the Augmentation messages coming from the Radio Block Center (RBC) for calculating the train position;
• Radio Block Constituent (RBC): knowing the train position, it is in charge of define and send the Movement Authority to the OBU;
• SBAS Ground Services: the terrestrial services provided to the user by the SBAS Ground Segment (e.g. EDAS Service for EGNOS);
• Radio Communication Network: it is the Rail Communication Network used by the Rail Operator for the communication between the RBC and the OBU
• GPS and Galileo Ground Services: ancillary services provided to the user by the GNSS Ground Segment (e.g. precise ephemeris, almanacs, geodetic reference frame parameters); an example of that is the IGS-RTS system for GPS and the GRSP (Geodetic Reference Service Provider) for Galileo
• High QoS/Security Communication Network: a high Quality of Services (low latency, low number of lost packets) is needed between the GPS and Galileo Ground Services operators, SBAS Ground Services operators, External Networks operator and the RBC; furthermore, an high level of security is needed for Cyber-attacks counteraction
High Integrity GNSS Reference Station Network: they implement an FDE algorithm in order to perform the first level of Integrity Monitoring on GNSS SIS and Reference Stations; in the current Test-Bed, a Real-Time 2-Tiers algorithm has been here operated (see next paragraph)The Local Augmentation and Integrity Monitoring (AIMN) architecture takes GNSS raw data from SBAS Ground Services (e.g. EDAS) and Reference Stations belonging to External Augmentation Networks (External Network) to be used for densification of a Rail backbone Network. Raw data are gathered through the Data Acquisition block and fed into the Integrity Monitoring block. It is in charge of performing Reference Stations and SIS Fault Detection and Exclusion (FDE). The candidate mean is the 2-Tiers algorithm , able to integrate SBAS RIMS Raw data and Reference Stations raw data for performing the FDE. Relevant Integrity Parameters (e.g. B-Values, Probability of fault of Reference Stations, Satellites and Constellations) are calculated by the Integrity Parameters block. Anomalous Ionospheric conditions are detected by the Local Atmospheric Monitoring block.
A Generalized Architecture, able to cover any kind of available High Precision System in the future, starting from current GBAS toward current Network RTK and the incoming Real-Time PPP, has been studied. With this scope, Network-ambiguities can be solved by the Control Centre. In the future, precise Orbit and Clocks estimation in Real-Time, as well as Local Ionospheric STEC (Slant Electron Content) estimation can be performed for PPP and Integrity Monitoring issues, respectively. A Measurement Correction component is finally in charge of calculating final corrections and sending them to the Augmentation Processing and Messages Generation for final corrections formatting in standard mode and sending to the TALS. GNSS Precise Ephemeris and Clocks will be gathered from the IGS sites (sp3 files). GRSP is currently not available.
Standard Communication data formats (e.g. RTCM) and new proposed RTCM messages for integrity parameters messages, as well GBAS ones) are used for the communication among blocks.
The Reference Architecture is a full instance able to cover any kind of available High Precision System in the future, starting from current GBAS toward current Network RTK and the incoming Real-Time PPP.
The Local Augmentation architecture used for the Performance Analysis is a subset of the full architecture. It provides relevant safety-related functions needed for implementing high demanding SIL-4 applications in different Rail Operation modes, e.g. Start of Mission (SoM) Full Supervision (FS) and to assist the Track Discrimination.
Fault Tree Analysis has been carried out, taking into account relevant rail standards. It showed the importance of Local effects mitigation, with particular reference to Multipath. A Function B mitigation branch has been created for dealing with innovative multipath mitigation techniques.
The current Test Result document reports the characterization of GNSS Performances in a Rail operational environment. Tests have been developed in Post-Processing mode collecting data in a real railway scenario along Cagliari-San Gavino track in Sardinia, through simulated data. For this second task, a GNSS Simulator has been produced. The ERSAT Local Augmentation infrastructure, composed by five Reference Stations and a Control Centre in Rome (implementing the 2-Tiers Integrity Monitor), was used for FDE and providing relevant corrections.
Tests were grouped by the two main Rail operational conditions: SoM, starting from an unknown position, and FS. In order to simulate a SoM scenario a synthetic track have been used, OBU position and Parallel Track Discrimination have been carried out through the classical RTK method, the Alert Limit has been set to 3 m. For the FS case the DGNSS mode has been adopted and the Alert Limit has been set to 12 m. Furthermore a second Track Discrimination algorithm, based on a Hypothesis Testing using Melbourne-Wübbena combination , has been tested.
A Tolerable Hazard Rate (THR) of 10-9/h for the whole system (satisfying SIL-4 requirements) has been imposed for Protection Level calculation and relevant Integrity Analysis. Local Augmentation can therefore achieve the 4.5*e-10/h THR requirement, as reported in .
Local Effects have been investigated through Sensitivity Analysis (SA) about Shadowing, Multipath, and Foliage attenuation.
The impact of Shadowing has been analysed in SoM through an incrementally increasing cutoff angle with three possible levels. For the three cases (Light, Medium, Severe), a full availability is estimated, while the position error can be in the order of 0.2-0.4m, with a standard deviation ranging from 0.1 to 0.3m. Also if not relevant for the Integrity performances, a High level of shadowing can provide a significant contribution to the error budget. In the performed tests, the Stanford analysis revealed that till 32 degrees of shadowing, the SoM Safety requirements can still be met.
Concerning the most relevant Local Effect, the Multipath, raw data corrupted by means of a synthetic multipath source have been simulated for the Cagliari-San Gavino rail track. In order to carry out a SA for the Multipath Scenario, an increasing wall has been placed on both sides of the Rail Track, ranging from 0.5 to 2 m above the antenna height. The Analysis confirmed the relevant impact of Multipath on Integrity. The System Availability decrease to 0.99735, while the percentage of MI is in the order of 6.3%. The induced position error is in the order of 4.7 m, with a standard deviation of 2.7m. It confirms the relevant impact of Multipath on System Availability and Position Errors.
The Foliage attenuation has been simulated adding trees on top of the wall of the previous analysis. The Availability is in the order of 0.996005, while the MI is 6.72% and the system is in Nominal operations for the 92.88% of the epochs. The introduced error is in the order of 4.5 m, with a standard deviation of 2.7 m.
The foliage disturbance, together with multipath, as expected by the GNSS literature and surveying experiences, introduces a relevant error.
It is therefore highly recommended to choose the location of Virtual Balises in areas not highly affected by Multipath and in presence of trees around the rail track.
SUMMARY AND FUTURE WORK
A GNSS Performance Analysis in a Rail operational environment has been carried out to investigate the impact of Local Effects in terms of Availability, Accuracy and Integrity of the System. The results pointed out that the Shadowing was not significantly impacting on Performances till 32 degrees for the SoM operations.
The tests concerning the introduction of Multipath effect and Foliage attenuation confirmed them as a relevant Fault Source (MI>6%) in GNSS PVT computation.
Therefore, the main recommendation for ERTMS is to design is to position Virtual Balise location by design in sites with low multipath and presence of tree and a continuous monitoring is recommended to mitigating Local Effects. Innovative mitigation techniques (named Function B) have to be identified for this cope.
A second run of the Performance Analysis will be carried on for deeply analysing Local Augmentation System Faults and Local Effects mitigation for ERTMS implementation purposes.
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