EUROPEAN ORGANISATION FOR THE SAFETY OF AIR NAVIGATION EUROCONTROL EUROCONTROL EXPERIMENTAL CENTRE GNSS PERFORMANCE VALIDATION SUMMARY

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1 EUROPEAN ORGANISATION FOR THE SAFETY OF AIR NAVIGATION EUROCONTROL EUROCONTROL EXPERIMENTAL CENTRE GNSS PERFORMANCE VALIDATION SUMMARY EEC Report No. 345 Project NAV-4-E1 Issued: April 2000 The information contained in this document is the property of the EUROCONTROL Agency and no part should be reproduced in any form without the Agency s permission. The views expressed herein do not necessarily reflect the official views or policy of the Agency.

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3 REPORT DOCUMENTATION PAGE Reference: EEC Report No. 345 Originator: EEC SNA CoE (Satellite Navigation Applications Centre of Expertise) Sponsor: DSA (Directorate of Safety, Airspace, Airports & Information Services) Security Classification: Unclassified Originator (Corporate Author) Name/Location: NLR The Netherlands GMV -Spain European Space Agency Sponsor (Contract Authority) Name/Location: EUROCONTROL Agency Rue de la Fusée, 96 B BRUXELLES Telephone : TITLE: GNSS PERFORMANCE VALIDATION - SUMMARY Authors A. van den Berg J. Vermeij S. Lannelongue Date 04/00 Pages x+40 Figures 7 Tables 6 Appendix - References 5 EATCHIP Task Project Specification - NAV-4-E1 Distribution Statement: (a) Controlled by: Head of SNA CoE (b) Special Limitations: None (c) Copy to NTIS: YES / NO Descriptors (keywords): Navigation, GPS, GLONASS, GNSS, EGNOS Abstract: Task No. Sponsor - Period This is a summary of a report on the results of the GNSS Performance Validation (GPV) study carried out on behalf of the Eurocontrol Satellite Navigation Applications Centre of Expertise at he EEC. The work analysed the operational validation of the GNSS-1 concept from an aeronautical user point of view. The Total System Performance (TSP) was broken down into three topics: the Signal in Space (SIS), the onboard equipment and the dynamic system performance. This report describes the findings of this study focussing on requirements for measurement, simulation and analysis necessary to validate that GNSS is meeting the user requirements. In addition it describes the configuration options for a simulation environment that is proposed for performing some of the validation tests.

4 This document has been collated by mechanical means. Should there be missing pages, please report to: EUROCONTROL Experimental Centre Publications Office B.P BRETIGNY-SUR-ORGE CEDEX France

5 GNSS Performance Validation EUROCONTROL FOREWORD FOR GNSS PERFORMANCE VALIDATION STUDY This report presents a summary of the final report of a study into Global Navigation Satellite System (GNSS) Performance Validation. The study was initiated to provide background material to support a EUROCONTROL responsibility within the European Tripartite Group (ETG) to perform the validation of GNSS for civil aviation applications. The ETG consisting of the European Commission, the European Space Agency and EUROCONTROL is coordinating the development and implementation of a European component of GNSS called the European Geostationary Navigation Overlay Service (EGNOS). EGNOS is a Satellite Based Augmentation System (SBAS), to the GPS and GLONASS navigation constellations. There are currently three SBAS systems under development, the US WAAS system, the Japanese MSAS system and the European EGNOS system. These regional systems include a ground segment, which monitors the constellation of navigation satellites and transmits corrections and integrity information to the users through the geo-satellites (Figure 1-2). The geostationary satellites also provide GPS-like ranging signals. The GPV report proposes a validation strategy and detailed procedures for the validation of the core GNSS elements, GPS and GLONASS and the satellite based augmentation system. These validation methods are presented in the following structured way: 1. Analysis of critical parameters for the validation process 2. Definition of required tests 3. Definition of a simulation environment to validate: Signal-in-space User Equipment including integrated architectures Aircraft Performance The validation approach evaluates the systems from a user perspective. Procedures for the validation of different levels of performance are defined against the applicable user requirements. Much of the proposed validation methodology is generic to all GNSS and it will provide valuable input to future validation activities for Ground Based Augmentation Systems and GNSS-2. The complete four-volume set of final reports is available, on request, from the EUROCONTROL GNSS Programme. Edward Breeuwer Richard Farnworth EUROCONTROL Project Officers v

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7 GNSS Performance Validation EUROCONTROL EXECUTIVE SUMMARY The purpose of the GNSS Performance Validation (GPV) project was to investigate validation procedures for GNSS-based navigation systems. The results will be used in the definition of a future EUROCONTROL project for the operational validation of GNSS-1 in the ECAC region. The term GNSS-1 is used throughout this document to refer to the GPS and GLONASS satellite positioning systems augmented by the European Geostationary Navigation Overlay Service (EGNOS). Procedures have been developed for the validation of the required signal-in-space parameters and the performance of the user equipment, which together define the overall navigation system performance. Objectives The objective of the GPV study was to propose suitable validation procedures for the operational validation of the use of GNSS-1 (GPS + GLONASS + EGNOS) for civil aviation navigation purposes. This high-level objective was achieved by: analysing the critical parameters for the validation process the definition of adequate tests the definition of a simulation environment for the validation of the signal-in-space, the user equipment and the aircraft performance Validation strategy The validation strategy applied within the framework of this study was to approach the system from the user point of view. From this perspective, it is proposed that the operational validation of GNSS-1 be performed at different levels (SIS, user equipment and aircraft) against the applicable requirements. First the validation procedures which are required to assess the performance of the GNSS signal-in-space are investigated. Once the SIS has been validated, it is necessary to validate the performance of the receiver, possibly integrated with additional navigation sensors such as the inertial navigation system. Finally, the effect of the total system is investigated introducing the aircraft elements that are within the scope of the validation. The developed validation procedures are a combination of the following methods: flight trials static measurements simulation analysis data extrapolation Tests with real data and flight trials are very important and the final evidence of the GNSS performance, however they are very demanding and expensive in terms of resources. Therefore a large part of the system performance will be validated through the use of simulations based on reliable hardware and software models. Those models will need to be validated based on real data and flight trials. Once validated, the defined simulation environment allows many realistic scenarios to be investigated at a lower cost than flight trials. vii

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9 GNSS Performance Validation EUROCONTROL TABLE OF CONTENTS 1. INTRODUCTION BACKGROUND OBJECTIVE VALIDATION STRATEGY STRUCTURE OF THE DOCUMENT SCOPE OF THE GNSS PERFORMANCE VALIDATION PROJECT INTRODUCTION OPERATIONAL REQUIREMENTS FOR GNSS FINAL REPORT PART 2 - SIS PERFORMANCE VALIDATION FINAL REPORT PART 3 - (INTEGRATED) RECEIVER PERFORMANCE VALIDATION FINAL REPORT PART 4 - TOTAL SYSTEM PERFORMANCE VALIDATION SIGNAL-IN-SPACE PERFORMANCE VALIDATION GENERAL GPS AND GLONASS SIS PERFORMANCE VALIDATION GEO SIS PERFORMANCE VALIDATION SIS FAILURE MODES GNSS SIS PERFORMANCE VALIDATION (INTEGRATED) RECEIVER VALIDATION SAR SIMULATION ENVIRONMENT AND EXPERIMENTS DEFINITION SAR SIMULATION ENVIRONMENT VALIDATION STAND ALONE RECEIVER FAULT TREE ANALYSIS INTEGRATED RECEIVER ARCHITECTURES INTEGRATED RECEIVER SIMULATION ENVIRONMENT DEFINITION ADDED COMPLEXITY / LIMITATIONS TO THE FAULT TREE ANALYSIS RECOMMENDATIONS FOR FLIGHT TRIALS FOR (INTEGRATED) RECEIVER PERFORMANCE VALIDATION TOTAL SYSTEM PERFORMANCE VALIDATION DEFINITION OF SCENARIOS CONTRIBUTION OF THE NSE TO THE TSE CONTRIBUTION OF THE FTE TO THE TSE SIMULATION ENVIRONMENT DEFINITION RECOMMENDED SENSOR INTEGRATION RECOMMENDATIONS FOR ADDITIONAL FLIGHT TRIALS RECOMMENDATIONS FOR GNSS PERFORMANCE VALIDATION SIGNAL-IN-SPACE PERFORMANCE VALIDATION ix

10 EUROCONTROL GNSS Performance Validation 6.2. PERFORMANCE VALIDATION OF RECEIVERS AND INTEGRATED SYSTEMS TOTAL SYSTEM PERFORMANCE VALIDATION REFERENCES ABBREVIATIONS Green Pages: Pages Vertes: French translation of the foreword, the introduction, objectives, conclusions and recommendations. Traduction en langue française de l'avant-propos, de l'introduction, des objectifs, des conclusions et recommandations. VALIDATION DES PERFORMANCES DU GNSS INTRODUCTION OBJECTIFS STRATÉGIE DE VALIDATION PORTÉE DU PROJET DE VALIDATION DES PERFORMANCES DU GNSS VALIDATION DES PERFORMANCES DU SIS CONCLUSIONS ET RECOMMANDATIONS VALIDATION DES PERFORMANCES DU RÉCEPTEUR (INTÉGRÉ) CONCLUSIONS ET RECOMMANDATIONS VALIDATION DES PERFORMANCES DU SYSTÈME GLOBAL CONCLUSIONS ET RECOMMANDATIONS STRUCTURE DE LA DOCUMENTATION x

11 GNSS Performance Validation EUROCONTROL 1. INTRODUCTION 1.1. Background Phase 1 of the Global Navigation Satellite System (GNSS-1) is the first phase of the introduction of satellite navigation in civil aviation in Europe. The core of the system is the US GPS constellation and the Russian GLONASS constellation. The set up of GNSS-1 consists mainly in the development of three satellite based augmentation systems (SBAS) that cover a large part of the globe. The specific service areas of these systems, called EGNOS, WAAS and MSAS, cover respectively Europe, North America and Japan. Their function is both to provide additional ranging capability and to supply the user with differential corrections and integrity information on GPS and GLONASS, valid for a "wide" area. For critical operations or specific areas, this satellite based augmentation system can be complemented by ground based augmentation systems (GBAS) using multiple local reference stations, or airborne augmentation systems using other sensors available such as inertial navigation systems or altimeters. GPS GLONASS Local Area Systems GNSS-1 User Equipment Hybridisation & Processing EGNOS WAAS MTSAT Figure 1-1: GNSS-1 components A European Tripartite Group (ETG) encompassing the European Commission, the European Space Agency and Eurocontrol, agreed on the development of the European satellite based augmentation system (European Geostationary Navigation Overlay Service, EGNOS). This regional system includes a ground monitoring segment and geo-stationary satellites that are visible in the service zone. The ground segment monitors all the satellites of the navigation constellation and transmits corrections and integrity information to the users through the geo-satellites (Figure 1-2). Additionally these geostationary satellites also provide GPS-like ranging information. 1

12 EUROCONTROL GNSS Performance Validation GLONASS Geo GPS Master Control Center Monitoring network Figure 1-2: GNSS-1 Architecture The goal of GNSS-1 is to be approved as a basis for the navigation function of civil aviation. Since the Safety of life concept is in the balance, the system has to fulfil very stringent requirements in terms of accuracy, integrity, continuity and availability. Within the tripartite group, Eurocontrol is in charge of the organisation of the validation of GNSS for civil aviation applications. For this purpose, Eurocontrol has initiated the GPV project at the beginning of 1998 in order to develop a validation methodology. A team lead by the Netherlands Aerospace Laboratory NLR and including the Spanish company Grupo de Mecánica del Vuelo GMV and specialised sections within the European Space Agency ESA/ESTEC, were selected to carry out this task of which this report is the main outcome Objective The objective of the GNSS Performance Validation (GPV) study is to propose suitable validation procedures for the operational validation of the use of GNSS-1 (GPS + GLONASS + SBAS (EGNOS)) for civil aviation navigation purposes. This high-level objective is achieved by: analysing all the critical parameters for the validation process the definition of adequate tests the definition of a simulation environment for the validation of the signal-in-space, the user equipment and the aircraft performance 1.3. Validation strategy The validation strategy applied within the framework of this study is to approach the system strictly from the user point of view. From this perspective, the operational validation of GNSS-1 is measured at different levels (SIS, user equipment and aircraft) against the applicable operational requirements from the user as laid down in various standard documents. First the validation procedures, required to assess the performance of the GNSS Signal-In-Space (SIS) that arrives at the user, are investigated. Once the SIS has been validated, it is necessary to validate the performance of the user receiver, possibly integrated with additional navigation sensors such as inertial navigation systems. Finally, the effect of the total system is investigated, introducing aircraft elements in the scope of the validation. 2

13 GNSS Performance Validation EUROCONTROL The developed validation procedures are a combination of the following methods: Flight trials Static measurements Simulation Analysis Data evaluation and extrapolation Tests with real data and flight trials are very important and constitute the final evidence of the GNSS performance. However, they are very demanding and expensive in terms of resources. Therefore a large part of the system performance will be validated with simulations based on reliable hardware and software models. These models will need to be validated with real data and flight trials. Once validated, the defined simulation environment allows many realistic validation scenarios at reduced costs Structure of the document The results of this study have been combined into one document containing four Parts, each consisting of several Volumes. Document title: GNSS Performance Validation Part 1: Executive Part 2: GNSS Signal-In-Space Performance Validation - Volume I: of GNSS SIS Performance Validation - Volume II: GPS/GLONASS SIS Performance Validation - Volume III: Geostationary Overlay Satellite SIS Performance Validation - Volume IV: SIS Failure Modes - Volume V: GNSS SIS Performance Validation Part 3: (Integrated) Receiver Performance Validation - Volume I: of (Integrated) Receiver Performance Validation - Volume II: SAR Simulation Environment and Experiments Definition - Volume III: SAR Simulation Environment Validation - Volume IV: SAR Fault Tree Analysis - Volume V: IR x Architecture Definition - Volume VI: IR x Simulation Environment Definition - Volume VII: IR x Fault Tree Analysis - Volume VIII: Recommendations for Flight Trials for (Integrated) Receiver Performance Validation 3

14 EUROCONTROL GNSS Performance Validation Part 4: Total System Performance Validation - Volume I: of the Total System Performance Validation - Volume II: Definition of Scenarios - Volume III: Contribution of the Navigation System Error to the Total System Performance - Volume IV: Contribution of the Flight Technical Error to the Total System Performance - Volume V: Total System Simulation Environment Definition - Volume VI: Recommended Sensor Integration - Volume VII: Recommendations for Additional Flight Trials 4

15 GNSS Performance Validation EUROCONTROL 2. SCOPE OF THE GNSS PERFORMANCE VALIDATION PROJECT 2.1. Introduction Within the framework of the European Tripartite Group agreement Eurocontrol is responsible for the organisation of validation activities to allow the operational approval of Global Navigation Satellite Systems (GNSS) for air navigation purposes. The GNSS system performance must be validated against the user requirements, which are described by the International Civil Aviation Organisation (ICAO) in terms of Required Navigation Performance (RNP) levels, related to different flight procedures. As mentioned, the strategy within the framework of this project is to validate the navigation system from the user point of view. From this perspective, the system can be divided into four parts (Figure 2-1): the various components of the GNSS Signal-In-Space (SIS) the GNSS receiver other navigation sensors the aircraft GNSS SIS Receiver Other sensors Navigation system performance Aircraft Total system performance Figure 2-1: GNSS system performance In a first step, the operational requirements for GNSS-1 are assessed together with their impact on signal-inspace, receiver and aircraft performance Operational requirements for GNSS-1 Over the past ten years different ICAO panels - RGCSP and AWOP - have attempted to define generic operational requirements for navigation systems in the different phases of flight. The applicable airspace requirements were used as a baseline associated with the operations that were expected in a future ATM environment. These activities have resulted in the endorsement of the concept of Required Navigation Performance (RNP) by ICAO. The RNP parameter values, as proposed by RGCSP and AWOP, are reviewed in Table

16 EUROCONTROL GNSS Performance Validation Procedure Expected RNP Type Accuracy - 95%TSE Integrity Continuity Availability Lateral Vertical Risk HAL VAL TTA En-Route 20 37,000 m Not specified ,310 m B-RNAV m N/A 10-6 / h P-RNAV m NPA m IPV 3 0.3/ m 41 m 10-5 / h CAT I 0.03/50 37m 12m per oper. Not specified 1110 m N/A 1110 m 82 m Not specified Not specified / h 167 m 50 m 6 s in any 15s Not specified The original ICAO concept uses RNP 4. 2 Currently the usefulness of RNP 1 operations is being investigated, possibly P-RNAV operations may require RNP The IPV is not yet an existing operation in European airspace Table 2-1: ICAO RNP parameter values for total system performance derived from the ICAO manuals related to performance types associated with GNSS operations currently expected in European airspace. Using the RNP manuals for the en-route and approach phases of flight, the ICAO GNSS Panel has attempted to actually translate these top-level requirements into world-wide signal-in-space requirements. Moreover EUROCONTROL (in co-operation with ESA in Europe) has attempted to translate the GNSS requirements into GNSS system performance parameters applicable to the European Region. This process has been very difficult, illustrated by the fact that it is now very difficult to re-establish the link between the GNSS signal-inspace requirements, as currently listed in the draft ICAO SARPs - for ECAC, and the original numbers in Table 2-1 RNP Type (nmi) Accuracy (95%) Integrity Continuity Availability Lateral Vertical Risk TTA HAL VAL Global Local 20 to m 300 s 7400 m /h to m 15 s 3700 m /h N/A 10-7 /h N/A m 15 s 1850 m /h to m 10 s 555 m /h N/A 0.3/ m 9.1 m 2x s 555 m 22.8 m x10-6 in 0.03/50 to 16.0 m 7.7 m to in any 6 s 40 m 20m to any 15 s / m 150 s 10 m Table 2-2: Proposed performance parameters for GNSS-1 in Europe [EUROCONTROL, June 1998] (nmi - nautical mile 1850 m; approach duration = 150 s) The link between the GNSS SIS specifications and the service performance specifications from ICAO are the receiver MOPS [RD-02]. RTCA and EUROCAE have put together minimum performance requirements for GNSS receivers, again in terms of RNP, including testing procedures. The requirements for SBAS receivers, intended to serve as a primary means of navigation, are shown in Table

17 GNSS Performance Validation EUROCONTROL Phase of flight Accuracy Lateral Vertical UEE Integrity FDE SBAS Integrity P MD P FA TTA FD availability FE availability En-route/ Terminal NPA 100m N/A /hr 8 s 99.80% % 99.80% % 94.55% % 94.55% % PA 0.4/0.8/ 0.55 m 1 No req / approach 4 s 95% No req. 1 DO-229 Range error to GPS/GLONASS/WAAS satellite, excluding multipath/troposphere Table 2-3: RTCA/EUROCAE MOPS for EGNOS/WAAS receiver Ideally there should be a direct link between Table 2-1 and Table 2-3. Unfortunately, in the process of drafting all the requirements the traceability between the tables has been lost. It would be desirable to re-establish this traceability between the tables. This is a difficult task and the first issue is to start from a clear set of definitions Final Report Part 2 - SIS Performance validation The GNSS signal-in-space performance is specified via the parameters: accuracy, integrity, continuity, and availability. These parameters are discussed with respect to the different elements involved in the emission of the signals to the end-users. In the frame of the GPV project the SIS validation part has been divided in the following elements: GPS & GLONASS SIS performance validation GEO SIS performance validation SIS failure modes GNSS SIS performance validation The first element aims at the definition of procedures to assess the performance of the existing GPS and GLONASS satellites that are the core of the future GNSS-1. The GEO SIS performance is considered separately because of the new complexity of the augmentation performance. The purpose of the SIS failure mode assessment is to gain a better insight into the non-nominal performance and the high level implications on the user of the performance of the system in terms of accuracy and especially integrity, continuity and availability. As the first three elements only cover the performance of individual satellite signals, the final element on total GNSS SIS Performance combines the contributions of the different elements including their failure modes. In this element the performance of the system is assessed in terms of navigation system error (NSE) i.e. the service provided to the user, including the geometrical aspects of the satellite constellation, whereas in the former ones the focus was on satellite pseudorange performance only. 7

18 EUROCONTROL GNSS Performance Validation 2.4. Final Report Part 3 - (Integrated) receiver performance validation Once the Signal-In-Space (SIS) has been validated, the second important point with considerable impact on the navigation system performance is the GNSS stand-alone receiver (SAR). The GNSS receiver has to fulfil certain requirements in terms of accuracy, time to first fix, ability of interpreting the satellite navigation messages. The third point is the validation of the navigation system performance after the integration of the GNSS receiver with possible other navigation sensors. As there are many different options for sensor integration, selected integrated architectures have been investigated. The strategy to develop a validation environment for an (integrated) GNSS receiver is divided in three steps: Definition of validation scenarios Definition of simulation environment Validation of the simulation environment First the test scenarios needed for the validation of the stand alone and integrated receiver are defined. For the stand-alone receiver, those scenarios are mainly based on the tests as described in the MOPS [RD-02]. For the stand-alone receiver validation, these tests are critically reviewed and specific test scenarios are defined for integrated receivers. The second step is to deduce from the previously defined test scenarios, the required simulation environment. Finally it is necessary that the simulation environment is validated to ensure that it will represent the real environment in a very accurate and reliable way in order to produce meaningful results. This step consists of two elements: The first element is to validate that the individual theoretical models included in the simulator reflect the real environment of the receiver. Those models are e.g. the multipath and interference models and the orbits, satellite error, SA models. Secondly, the final implementation of the complete set of models in the simulation environment must be validated. The only way to do this is to compare the simulation results with real data. Depending on the model, this validation may demand different kinds of real data (e.g. Figure 2-2). The three possibilities are tests using a static receiver in a laboratory, tests with a receiver onboard of a static aircraft and tests with receivers on dynamic aircraft. Recommendations are made about the necessity for flight trials in order to validate the defined simulation facilities. Static Receiver in lab environment Models Receiver on board of a static plane Receiver on board of a dynamic plane Figure 2-2: Types of tests with real data The performance of the receiver under nominal conditions (i.e. without receiver failures) can be validated with the simulation facilities. But, as GNSS performance is defined in terms of accuracy, integrity, continuity and availability, procedures concerning the availability of the receiver are necessary. The establishment of a receiver fault-tree analysis for both a stand-alone as well as an integrated receiver can deal with this problem and is therefore described. 8

19 GNSS Performance Validation EUROCONTROL 2.5. Final Report Part 4 - Total System Performance validation The main topic of this part is to assess the scope of validation of Total System Performance (TSP) of GNSS usage in civil aviation. The requirements on TSP are determined by the concept of Required Navigation Performance and are established by the International Civil Aviation Organisation (ICAO). These requirements are given in terms of accuracy, integrity, availability, and continuity of the Total System for the different phases of flight (see Table 2-1). The NSE is not the only source of error that has to be accounted for in the total system performance of GNSS- 1. The FTE, which is the error introduced by the guidance system (pilot or autopilot), is also quite important for civil aviation applications. FTE and NSE together determine the TSE, which is the difference between the true position and the desired position of the aircraft (Figure 2-3) NSE FTE Figure 2-3: TSE, NSE and FTE combination The FTE depends on many factors such as phase of flight, actual guidance system used, aircraft type, pilot workload and operational environment. As the GNSS-1 system must meet the requirements in any combination of the possible variables, all of these variables must be input for the validation of the total system. The validation strategy for the total system performance consists of the following steps: To identify the different contributing elements to the total system performance To define of validation scenarios To define the simulation environment To validate the simulation environment Performing the validation using the simulation environment First the identification of the different contributing elements in the total system performance is described. Then the contribution of the NSE and the FTE to the TSP are characterised. Using these inputs different scenarios have been identified for validation. From the scenario description, the requirements on the simulation environment are identified. With respect to the simulation environment for the (integrated) receiver, additional software models are required specifically for the aircraft dynamics and the guidance and control elements. Consequently different applicable scenario environments are proposed. Individual recommendations are made for flight trials to validate the different added models. Finally additional recommendation are made about what is necessary in terms of flight trials in order to validate the full simulation facilities. 9

20 EUROCONTROL GNSS Performance Validation 3. SIGNAL-IN-SPACE PERFORMANCE VALIDATION 3.1. General For Signal in Space (SIS) performance validation it has been found important to have a clear set of definitions. Especially ambiguousness in the terms: accuracy, integrity, availability and continuity contribute to a lot of confusion. Furthermore, one of the main problems to validate the GNSS SIS is to differentiate the performance of the SIS from the performance of the receiver. Performance of the SIS can only be measured at the output of the receiver, and furthermore, the GNSS requirements are expressed in terms of NSE, which means after processing in a receiver. Since this study aims at validating the SIS some options to remove the receiver failures from the validation procedure are proposed. The ICAO GNSSP selected one option, which is used in the proposed validation procedures. Therefore, the way the SARPS definition of the SIS including a fault-free receiver should be interpreted is shown in Table 3-1. Included in Signal in Space: Physical performance of the signal Quality of information contained within the signal (i.e. clock, ephemeris, integrity, etc.) Environment (tropo- and ionosphere) Receiver accuracy and time-to-alert Included in Receiver budget: Multi-path Interference Masking Receiver failures Table 3-1: Signal in Space error budgets GPS and GLONASS SIS Performance Validation For GPS and GLONASS the RNP parameters have been analysed and a list of critical parameters that require further validation is proposed. As a general rule, those elements which have never been reported as a potential source of SIS errors will not require any validation, for instance the carrier frequency. This list of critical SIS parameters can be summarised as follows: The integrity of GPS and GLONASS SIS is the most critical parameter, since little information exists on this parameter. Accuracy is also a critical parameter, although a lot of information exists on both GPS and GLONASS SIS accuracy, the detailed characteristics of some of the individual sources of SIS accuracy errors are not very well known. Additional information is required on the following components: Space segment errors, especially SV ephemeris and clocks. Environmental errors affecting SIS, especially the ionosphere will require much attention and it strongly recommended to set-up a data collection effort as soon as possible. To a lesser extent also tropospheric effects will need to be considered. SIS availability is probably the least critical parameter, as official information published on GPS NANUs and GLONASS NAGUs characterise the SIS availability very well. However, this official information is known to contain errors. In addition, in the case of GLONASS, the satellite constellation has deteriorated in the past year and a half, and therefore availability figures need to be updated. In the case of GPS, the introduction of a new generation of satellites (Block II-F) will require an additional validation effort to characterise the new satellites. 10

21 GNSS Performance Validation EUROCONTROL Validation methods have been proposed for each of the identified critical parameters. These are mostly based on measurements, though data extrapolation (especially for integrity and availability) and simulation (especially for ionosphere and troposphere) will also be needed. A network of GPS/GLONASS receivers all over the world seems to be the best solution to have continuous access to GPS/GLONASS SIS. Advantage could be taken from existing infrastructure, e.g. IGS, though this does not yet cover GLONASS satellites GEO SIS performance validation For GEO SIS the same considerations apply as for GPS and GLONASS SIS performance characteristics. The main difference in the SIS is the data-rate, the information in the message and the fact that the messages are generated on the ground instead of onboard the satellite. The Ranging function of the GEO can be treated similar to GPS, but especially the Wide Area Differential (WAD) function adds extra complexity to the validation due to the fact that they can not be treated independent from the corrected errors. Accuracy has three different aspects regarding the GEO SIS performance: The accuracy of the ranging function. This refers to the accuracy of the pseudorange measurement of the GEO SV itself; The accuracy of the fast and long term corrections. This refers to the accuracy of the pseudorange measurement of any navigation satellite after application of the basic corrections provided by the GEO; The accuracy of the ionospheric vertical delay estimates as they are provided per Ionospheric Grid Point (IGP). Most of the accuracy tests are quite similar to the tests for GPS/GLONASS validation. Though now differential corrections are added. It is advised to combine these tests as much as possible. The term integrity is used for the probability that the augmented system accuracy performance is less than what is communicated via the GEO. This occurrence is called an integrity failure for the SIS. Only integrity that is provided through the SIS is assessed here. Other integrity sources (i.e. Receiver Autonomous Integrity Monitoring (RAIM)) are not part of the SIS performance. Validation of integrity is very difficult, as achieving confidence in performance figures of requires very long periods of measurements. Therefore it is advised to complement this with a detailed analysis of the safety implications and the failure modes leading to these integrity failures. However the quality of the integrity parameters (UDRE and GIVE) can be validated through simulation. Signal availability is considered to depend only on the coverage area and on failures, resulting in a loss of service (both signal loss and unusable contents). For validation of GEO SIS availability only the message update rate is considered as a critical parameter. In Figure 3-1 a functional breakdown is shown of the GEO SIS performance, including the parameters to be validated. The figure illustrates that the GEO SIS can not be validated completely separated from the core systems (GPS and GLONASS). 11

22 EUROCONTROL GNSS Performance Validation GEO SIS Performance - System - Signal - Message RF Signal Characteristics Signal quality - received C/N o Availability - Failure rates and modes - Coverage Ranging (GEO) - Clock + ephemeris - URA Basic differential (GPS/Glonass/GEO) - Fast corrections - Long term corr. - UDRE Precise differential (IGP) - Ionospheric corrections - GIVE Accuracy (GEO) - clock/ephemeris - ionospheric (without corr.) - tropospheric Integrity - URA Accuracy (GPS/Glonass/GEO) - clock/ephemeris (with correction) Integrity (GPS/Glonass/GEO) - UDRE - Delay Accuracy (IGP) - ionospheric (with correction) Integrity (IGP) - GIVE - Delay Figure 3-1: Performance breakdown of GEO SIS SIS Failure modes For civil aviation, safety is one of the most important design aspects driving the development of the European Geostationary Navigation Overlay System (EGNOS). Therefore not only baseline requirements such as accuracy, integrity, availability and continuity are important. Also a thorough knowledge of possible failure modes of the Global Navigation Satellite System (GNSS) and their effects is indispensable. The main goal of this part of the project is to identify: Signal In Space (SIS) failure modes, failure effects, and failure statistical figures Procedures to characterise failures. It must be clear that this report cannot cover all the possible failure modes. One of the reasons for this is that the Global Positioning Service (GPS) and the Global Navigation Satellite System (GLONASS) are under military control with information hard to get at. The aim of this document is to structure the current information available on GNSS SIS failures, to identify what is still missing and to provide insight from the user point of view in the way that SIS failure modes can affect the performance. Also, methodologies are proposed to gather this additional information and to characterise failure modes. 12

23 GNSS Performance Validation EUROCONTROL A SIS failure is defined here as the state where the SIS is not working according to its design specifications. The design specifications are derived from the user performance requirements, in this case mainly from the aviation community where they are defined in terms of accuracy, availability, continuity and integrity. The individual SIS specifications can be defined in the following terms: Pseudorange accuracy of individual satellite signals Integrity information on pseudorange accuracy Physical presence of individual signals With these terms a failure state matrix can be defined: States Integrity information Correct Wrong Unavailable Pseudorange measurement Nominal Nominal conditions False alarm Degraded Correct alarm Hazardous situation Unavailable Unavailability Unavailability Table 3-2: Failure state matrix. The matrix shows that there is only one real dangerous situation for GNSS users: when integrity shows everything is correct while the pseudorange accuracy is severely degraded (combination of two failures). In the matrix, physical presence of the signal is a part of pseudorange unavailability. Depending on a user s requirements both alarm conditions (correct or false) could affect his availability. Failure modes are identified by grouping them into accuracy, integrity and availability failures. Also, to structure analysis, a distinction is made between the stand-alone failure modes and failures due to the augmentation system (SBAS). For these failure modes, duration and frequency figures are partly derived. Establishing these figures however, is not the prime goal of this report. The information on GPS and GLONASS mainly stems from literature, with emphasis on outages and pseudorange degraded performance. The EGNOS system is analysed more specifically, dividing the system into major elements. To gain more insight in the character of SIS failures, data acquisition procedures are established. Real time monitoring using an observation station network is needed to identify real failure effects. To reduce the amount of data handling, data for post-processing and failure analysis should only be preserved over a small period before and during a failure. Until the EGNOS system becomes available, failure injection in a simulated environment is the only other option to support validation. During the EGNOS pre-operational phase, real failure injection might be a useful but remains a limited method for characterising the failure modes already identified GNSS SIS performance validation In the previous sections, GPS and GLONASS SIS performance validation and GEO SIS validation the performance of the signal emitted by individual GPS, GLONASS and GEO satellites have been assessed. The performance has been studied in terms of pseudo-range errors. Here, the geometry aspect of the constellation will be included in order to assess the performance of the system in terms of navigation system performance at the user level. Firstly different architectures that may be included in the future GNSS-1 have been identified. The GNSS-1 elements GBAS and ABAS are not part of this study. Then, procedures to validate the GNSS SIS are proposed. This validation is done against the same RNP requirement parameters: accuracy, integrity, availability and continuity, but now at user level. 13

24 EUROCONTROL GNSS Performance Validation The main problem with the validation of accuracy is that this parameter is not stationary. This is due to the fact that accuracy varies with satellite geometry (which changes with time). Therefore, it should not be averaged over a long period of time. An option to validate accuracy via the measurement of a stationary parameter is proposed. For integrity, the critical point is the stringency of the requirements. This makes it very difficult to validate it with real data only. However, real data is selected as a basis for two reasons. The first one is that no models exist that can represent the real system with the degree of confidence required. This is especially the case for ionospheric models and errors due to the ionosphere will be most likely the main threat for the integrity of the SIS. Therefore, in order to take advantage of the ionospheric peak activity that will occur in , a measurement campaign should be undertaken during this period. The GPS outages observed recently in the tropics due to scintillation problems confirms that an action of this kind is necessary. The second reason is that the goal of this study is to validate the signal-in-space from a user point of view, therefore the validation shall rely as much as possible on what is available to the user, that is to say the real data. For continuity and availability, validation procedures rely partly on software tools and partly on real measurements. This is due to the fact that these parameters have to be estimated for the whole lifetime of the system. Therefore, measurements recorded have to be extrapolated to other possible scenarios using software simulations. The result of this is a set of procedures that validates the performance of the GNSS SIS against the RNP requirements from En Route to the Precision Approach phase of flight. 14

25 GNSS Performance Validation EUROCONTROL 4. (INTEGRATED) RECEIVER VALIDATION Part 3 of the Final Report, addresses the validation methods and procedures and the simulation environment required for the validation of the stand-alone and integrated receiver performance SAR simulation environment and experiments definition According to the validation strategy followed in this project, once the SIS has been validated (using a standard receiver), it is necessary to validate the performance at the output of a standard receiver. This validation consists of testing the GNSS stand-alone receiver against the tests that are defined in the MOPS (RD-02). These tests require a GNSS radio frequency (RF) signal simulator that can generate the RF signal as it would be received by the receiver in a real operational environment (Figure 4-1). This means, that the simulator must be able to emulate all the disturbances that could affect the receiver performance in a real environment (i.e. interference, multipath, obstacles, dynamics, etc). Therefore, the currently available simulation models have been analysed. SIS simulator Local disturbances (mpath, interf) GNSS RF simulator Receiver Navigation Performances Receiver Performances Figure 4-1: Receiver performance validation The receiver (Rx) behaviour in its environment can be divided in three error sources: Thermal noise Multipath Interference Thermal noise and RF interference (RFI) are different phenomena, but, in general, they both have a similar effect on the receiver performance. The models used to analyse them share many commonalties. The effects of these disturbances are analysed in terms of acquisition, code tracking loop (code measurement error and tracking losses), carrier tracking loop (phase measurement error and cycle slips), demodulation (data-bit error) and quantisation. These analytical models can predict the effect of interference in the measurement noise. The accuracy of these models to predict the performance of the receiver in the presence of interference can be assessed in the laboratory using a well-characterised signal and a receiver for which the detailed design parameters are available. This is the case of the Multistandard Receiver, for which both the receiver itself and the detailed design are available at ESTEC. These kind of tests can be used to validate the protection masks as described in the MOPS [RD-02]. However, it is important to point out that there is still a lot of work to be performed in order to have really accurate models on the effects of interference on GNSS receivers. 15

26 EUROCONTROL GNSS Performance Validation Concerning multipath, different models, which can be found in the technical literature, have been reviewed. This review covered the complete spectrum. From simple models (statistical) to high fidelity models (including electromagnetic field and receiver correlation effects). From these models, a high-level overview can be obtained on which multipath model should be applied in any particular analysis. Multipath models can be divided into a signal propagation model, an antenna model and a receiver model. For the signal propagation model, the Geometrical Theory of Diffraction is recommended to be implemented in the GNSS RF simulator and to be included in the MOPS tests description [RD-02]. Concerning the receiver antenna models, the two aspects, which need to be considered, are depolarisation and the antenna gain pattern. A GNSS receiver antenna model proposed by Eisfeller covers both aspects and should therefore be used. The majority of the GNSS receiver models, except the Eisfeller model, model the stationary behaviour of the receiver by neglecting the dynamic effects. This approach leads to less complex and more manageable models, which could be implemented in the receiver S/W model as part of the TSE simulator. The RF simulator aims at reproducing the real environment of a receiver. The advantage of this is that one can reproduce all kinds of situations such as dynamic manoeuvres of the aircraft, multipath or interference scenarios or signal-in-space failures. This kind of tool is thus clearly more flexible than the real environment of the receiver. Therefore, the receiver can be tested in many representative situations. This section of the report contains also a review of the RTCA MOPS [RD-02] resulting in a total of 26 recommendations. Some of them are proposing test methods to be included. Other recommendations concern missing information that must be available in order to complete or clarify some dark points. Finally, recommendations are made to include an additional paragraph in section 2.5 of the MOPS [RD-02] SAR simulation environment validation To obtain correct results from the stand-alone receiver validation it is necessary to validate the simulation environment. This means validation of the GNSS RF simulator and the models involved. In order to be able to validate the GNSS RF Simulator, a clear set of requirements against which to validate the simulator is required. These requirements have been derived from the following sources: The Interface Control Document (ICD) for GPS and GLONASS [RD-03, RD-04] for the signal requirements. The Civil Aviation Requirements as expressed in the ICAO GNSSP SARPs [RD-01] are used for the accuracy requirements of GNSS including SBAS (Table 4-1). The RTCA/DO-229 (MOPS) document, [RD-02], describes the tests for stand-alone receivers. From these tests the different capability requirements are derived. (E.g. Selective Availability (SA), satellite errors and WAD corrections). The aircraft dynamics that must be taken into account in the GNSS RF Simulator are retrieved from the ICAO document 8168 [RD-05]. Simulator Accuracy GPS GLONASS GEO Pseudorange (m) <0.15 <0.3 <0.15 Pseudorange Rate (m/s) <0.21 <0.42 <0.21 Delta Range (mm) < 3 (maximum dynamics) < 0.1 (static user) Table 4-1: GNSS SAR Simulator Accuracy Requirements 16

27 GNSS Performance Validation EUROCONTROL Concerning the detailed models applied in the RF simulator, each of the software models and RF parameters are evaluated and some of these parameters require additional validation methods when considered critical for the validation of the simulation environment Stand alone receiver fault tree analysis In this volume a fault-tree analysis has been performed for a generic GNSS airborne stand-alone receiver. The analysis is based on a generic GNSS receiver as described in the RTCA MOPS Do 229A [RD-02], from the input signals until reconstruction of navigation data (power supply and data presentation are not taken into account). Presuming reasonable reliability figures for the distinguished receiver components as provided in literature, an illustrative example of quantitative GNSS receiver analysis by means of a fault-tree is given. The failure modes and rates used in the fault tree analysis presented are based on collected field data. Of course the failure rates of a particular component used at a certain receiver could differ from the field-determined average. Nevertheless this approach has led to a fault tree, appropriate to obtain a sensible first indication of the receiver reliability, with the consideration of practical failure modes for the distinguished functions. Assuming practical values for the unknown model parameters, the fault tree resulted in the following Mean Time Between Failure (MTBF) for a generic GNSS receiver: MTBF hrs (order of magnitude) It is obvious that this is a rough indication of receiver reliability. On basis of studying one particular receiver it is not possible to establish the MTBF for other receivers, without additional analysis for the differing parts. Due to bringing in only one very unreliable component at the one hand or the introduction of redundancy at the other hand, receiver reliability could decrease or respectively increase enormously. Nevertheless the outcome for the theoretical receiver denotes the plausibility that the receiver MTBF requirement can be fulfilled. For detailed evaluation of a particular receiver implementation, the fault-tree should be used with care. For a conscious use of the fault-tree, knowledge of both the receiver and the fault-tree are necessary. The normal procedure applied at reliability studies, is to consider worst case values for the unknown parameters. This results in conservative outcomes for the expected MTBF values, but will never be a guarantee for the actual reliability Integrated receiver architectures The need to provide navigation during periods of shading of the GNSS receiver s antenna and through periods of interference, will be the impetus for integrating GNSS with various additional navigation sensors. This section focuses on the most promising integration options, including their architectures, performance and benefits. Furthermore the civil aviation user perspective is taken into account in the study. There are various ways to improve the integrated receiver performance in terms of availability and continuity, especially in cases of signal loss. The integrated receiver architectures are discussed concisely and an analysis in terms of accuracy performance, improvements in availability, integrity and continuity are provided. An important aspect is also the capability to detect and isolate failures in the combined system. The augmentation options considered are barometric altimeter aiding, GNSS with high stability clock and the use of the radio navigation system, Loran-C. Although various architectures of GNSS integration with different sensors are discussed, the most promising solution assessed is the integration of INS with GNSS, where accuracy is not compromised at all. Although most of the benefits can be discussed in general, certain benefits depend specifically on how the INS is integrated with GNSS. An important advantage that makes the INS such a good candidate is that many commercial aircraft are already equipped with INS and the integration with GNSS equipment can be conducted in several ways, allowing lower costs of a retrofit. 17