GNSS/Galileo Global And Regional Integrity Performance .

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GNSS/Galileo Global and Regional IntegrityPerformance AnalysisHelmut Blomenhofer, Walter Ehret, Arian Leonard, THALES ATM GmbHEduarda Blomenhofer, NavPos Systems GmbHBIOGRAPHYABSTRACTDr. Helmut BlomenhoferAfter finishing University he was Research Associate at theInstitute of Geodesy and Navigation (IfEN) of theUniversity FAF Munich from 1990 to 1995 and didresearch and software development in high-precisionkinematic Differential-GPS.From March 1995 to December 1997 he was at DaimlerChrysler Aerospace AG (Dasa); NFS Navigations- undFlugführungs-Systeme being responsible for thedevelopment of an Integrated Navigation and LandingSystem (INLS) for aircraft precision approaches andautomatic landings. From January 1998 to 2001 he was theEGNOS Programme Manager at the EADS subsidiaryAstrium GmbH located at Friedrichshafen.Since 2002 he is GNSS Business Development Director atThales ATM, Germany.Eduarda Blomenhofer is Managing Director of NavPosSystems GmbH, a German SME which specialised in thesatellite navigation related systems engineering, softwaredevelopment and consultancy. She owns an EngineerDegree in Surveying/Geodesy from the Porto University,Portugal. She is working in satellite navigation since 1990,with activities on high precision differential GPSalgorithms and software for real time applications, dataprocessing and service volume simulation for GPS,Glonass, GBAS, SBAS and Galileo.Walter Ehret graduated as an Aeronautical and SpaceEngineer from the Technical University (TU) ofBraunschweig, Germany in 1996. Since 1996 he is involvedin research and engineering activities related with SatelliteNavigation. He is currently working as Systems Engineer atTHALES ATM in Langen where he is involved in Galileorelated tasks and particularly Integrity related issues.Arian Leonard graduated in 1991 with an MSc. inAerospace Engineering from the University of Stuttgart.Following his R&D work at German Aerospace and ATMBiD activities in North America, he joined Thales ATM in1997 as EGNOS Deputy PM. Since 1999 he has beencontributing as Galileo Project Manager to various EC andESA contracts and is currently Galileo BusinessDevelopment Manager.1Augmented GPS and Galileo are expected to serve asnavigation sources for a variety of applications. The moststringent performance requirements are derived from safetycritical applications including aviation precision approachoperations.Where GPS integrity is determined by augmentationsystems like WAAS or EGNOS the Galileo baselinearchitecture specifies a global integrity concept. This meanse.g. that the accuracy and integrity performance must beachieved globally keeping the Time-To-Alert thresholds.Further major performance measures are the availability ofthe Accuracy and Integrity figures and the Continuity ofService.The GPS augmentation systems provide besides the widearea differential corrections also related residual errorswhich are used to compute Protection Levels. This is donein near real-time with latency times of maximum 6 seconds(UDRE concept), which grants a timely warning if failuresin the GPS system occur.The Galileo concept uses a combination of predicted errors(caused by satellite, clock, and non-precise navigationmessage) which are validated in real-time by integritymonitoring taking into consideration the monitoringaccuracy. The latter is considered by a new introducedparameter, the SISMA (SIS Monitored Accuracy) Thepredicted component which is transmitted with thenavigation message is called SISA (Signal In SpaceAccuracy) and is a quantitative estimation of the orbit andclock prediction of the Galileo Control Centre. SISA isupdated with every clock correction update in theNavigation Message (Ephemeris Set). The two parametersSISA and SISMA will be used by the user to compute aProtection Level associated to the positioning result. If anerror occurs in the satellites, clocks, signal, navigationmessage or in the processing itself, then it has to bedetected by the Integrity Processing Facility (IPF) in realtime and a warning flag IF has to be raised. This leads tothe exclusion of the related (Faulty) satellite from both, thepositioning equations and from Protection level equations.In the Galileo Global concept there will be a globalmonitoring network. However Regions will be given thepossibility to built up there own regional Monitoring andIntegrity Determination Network. Previous assessments andanalysis have demonstrated the integrity performance

potential of the Galileo global Integrity concept usingsimplified Service Volume Simulations (SVS).allowed. This is mainly due to limitations of the GPSRAIM availability.This paper shows the results determined by moresophisticated SVS using e.g. inclusion of operationaloutages as well as the consideration of the critical satelliteconcept. The SISMA driven performance is analysed forthe Global and an exemplary Regional case. The achievableIntegrity performance is then mapped to the aviationperformance standards as currently discussed by the ICAO.Finally a trade-off will be performed on the basis ofpreviously introduced results for the other Integrityconcepts like RAIM for combined GNSS systems or spacebased augmented GPS.INTRODUCTIONThe provision of integrity is essential to support criticaloperations in the safety-critical application domain withindifferent modes of transport and in particularly in aviation.Regions/States will have the option of using the GalileoGlobal Integrity service (Galileo SoL service) which will beoffered by the Galileo Operating Company or to implementtheir own regional concept for the Galileo integrityprovision. The two options for implementing GalileoRegional Integrity are the ERIS concept based onbroadcasting Regional Integrity through Galileo satellitesand the updated SBAS concept, where Galileo RegionalIntegrity will be broadcast by Geostationary satellites. Thedecision of a Region/State about it’s Regional Integrityprovision concept for Galileo, will depend on differentcriteria (e.g. geopolitical, institutional, technical, industrialpolicy, ).In the case of the North-Atlantic and according to IATA,North American, Trans-Atlantic and European Air Trafficsums up to a total of 63% of World-wide Air Traffic.Global Navigational Satellite Systems (GNSS), such asGPS and Galileo, are hence independently importantelements of the future aeronautical navigationinfrastructure. Ensuring these systems are functionallyinteroperable will further increase their collective value.Although GPS and Galileo must retain their basic authority,mission and operations, comprehensively addressing crosssystem interoperability, will eventually provide realizationof combined systems’ capabilities and a considerablenumber of benefits as illustrated in the example of theaviation community.REQUIREMENTS ANALYSIS AND SIMULATIONINPUT PARAMETERSPerformance RequirementsICAO Annex 10 (SARPS Radionavigation Aids) definesthe requirements for the different phases of flight [1].Figure 1 compares the Required Navigation Performance(RNP) per phase of flight with the existing or expectedGNSS system performance.The use of GPS together with RAIM fulfills requirementsdown to the Non-Precision flight phases. However thesereceivers are to be used as supplemental means ofnavigation with the exception of Remote En Route(Oceanic and domestic routes) where primary use is2Figure 1 Aviation Phases of Flight versus GNSSPerformanceThe introduction of Satellite Based Augmentation Systems(SBAS) like WAAS in US, MSAS in Japan and EGNOS inEurope will improve the capability of GPS in terms ofaccuracy but especially in terms of System Integrity suchthat GPS/SBAS devices can fulfill at least APV-IIrequirements.Accuracy (95%)AvailabilityContinuity RiskIntegrityhorizontal: 4mvertical:8m99.5 % of service life time 10-5 / 15sHAL: 12mVAL: 20mTTA: 6 secondsIntegrity Risk: 3.5x10-7 / 150sTable 1: Galileo Performance Requirements for theSafety of Life ServiceAccuracy (95%)AvailabilityContinuity RiskIntegrityhorizontal: 16 mvertical:8m99.0% to 99.999% 8x10-6 / 15sHAL: 40mVAL: 20mTTA: 6 secondsIntegrity Risk: 2x10-7 / approachTable 2: ICAO APV-II RequirementsTable 1 shows the Galileo System requirements for theGalileo Safety Of Life Service as stated in the MissionRequirements Document [2]. The comparison of Table 1and Table 2 yields that the Galileo System aims to be usedas a certified navigation means for the flight phasesRemote/Oceanic En Route down to non precision approachplus the new defined approach categories with verticalguidance APV-I and APV-II without the need for local orregional augmentation. The Galileo MRD requirements forhorizontal navigation are even more stringent than theICAO GNSS SARPS requirements for APV-II [1].

BENEFITS OF COMBINED GPS/GALILEO USEPerformance Benefits of Interoperability :Due to the improved availability of integrity for thecombination of SBAS/GPS Galileo it is expected toachieve the navigation performance required for CAT-Iduring all phases of flight. Here preliminary simulationshave been shown, whereby the Vertical Navigation SystemError VNSE and the Vertical Protection Level VPL to staywell below the APV-II vertical alert limit.Safety Benefits of Interoperability :One of the most promising benefits of GPS and GalileoInteroperability to the Aviation Community could be thatthe level of navigation performance usually reserved tolarger airports only, could be made available continuouslyfor unequipped ground locations as well as throughout theentire airspace at all flight levels and during all phases offlight. This would significantly increase the overall aviationsafety in regions that are not covered by on ground systemand in parts of the world where currently there is less ATCinfrastructure available. Further, at less developed airports,safety could be improved by enabling vertically guidedapproaches to all runway ends. Search and rescueoperations could be supported with improved accuracy andintegrity of the navigation equipment. (e.g. rescuehelicopter approaches to the home base under rmination capability in conjunction with ADS-B couldenhance situation awareness (surveillance) and therebyfurther improve safety.Robustness Benefits of Interoperability :It is expected that a two frequency Galileo (E1 & E5a, E5b)and a two frequency SBAS will be operational by 2010,followed by a two frequency GPS III (L1 & L5) in 2015.Regarding the interoperability of GPS and Galileo anumber of mid-term scenarios with single frequency GPS(L1) and long-term scenarios with two frequency GPS (L1& L5) can be considered. In the latter significant levels ofoperational redundancy at the CAT-I level are expected forthe combination of Galileo(L1/E5) GPS(L1/L5) SBAS(L1/L5). This combination provides a very robustarchitecture which should be capable of providing therequired navigation performance even in degraded modesof the GNSS services.Benefits according to ICAO ref. GNSS P/4-WP/5 :“The increasing number of GNSS signals and constellationsoffer significant benefits to civil aviation in terms of GNSSground architecture simplification and alleviation ofinstitutional concerns.” “, the introduction of newconstellation and additional signals will facilitate thetransition to GNSS as a global system for all phases offlight.” Further in this regard, the FAA and Eurocontrol arein the unique position as leaders of US and Europeanairspace systems to affect global ATM solutions and arekey to achieving the benefits of GPS and GalileoInteroperability for the aviation community.Benefits to Airports :The combination of SBAS/GPS Galileo could provide allweather CAT-I precision approach guidance to virtuallyevery runway in the US and Europe as early as 2010, when3both L1 and E5 signals become available via Galileo and ata later stage via GPS III. By achieving primary meansnavigation at core high-density areas, at regional airfields,and at remote regions the service areas could be extended.Usually, CAT-I precision approach capability is reserved tolarger airports. GNSS (GPS and Galileo) Interoperabilitywill also support less developed regional airports, whichcannot afford conventional ILS and MLS systems. With thehelp of GNSS, regional airports will become fully usableand competitive, allowing the same level of service to beestablished at relatively low investment costs. Additionally,due to the enhanced availability of accuracy throughout theentire airspace and at all flight altitudes, aircraft separationscould be optimised thereby enabling improved use ofairspace capacity in the terminal area. Separation can befurther reduced with the introduction of ADS-Btechniques/equipment once adapted by a majority ofairspace users. In conjunction with new operationalprocedures for all users, enabling closely spaced parallelapproaches, converging approaches in marginal visualmeteorological conditions (VMC), curved approachedbased on Precision Area Navigation (P-RNAV) as well asguided missed approaches, system capability and capacitycan be significantly improved. Finally, follow-onoperational enhancements could be enabled including:trajectory planning and management, thereby extending thedaily window as well as the take-off and landing frequency,precision gate-to-gate operations, and achievingseamlessness of air navigation through standardisationbetween the US and European airspace systems. By meansof these measures, based ultimately upon the improvedcapabilities of interoperable systems, many of today’s ATClimitations could be overcome and the consequentialfinancial losses could be mitigated.Benefits to Airlines :An improved navigation performance, possibly incombination with ADS-B techniques, is a further steptowards Precision Area Navigation (P-RNAV). Hence, theresulting efficiency and relaxation of air traffic congestionswould be considerable. As a consequence of the increasedtraffic throughput delay times could be drastically reduced,achieving substantial cost savings for both airlines andpassengers. Additionally, the reduction of the number ofdifferent equipment for navigation, in airspace and onground and for landing as well as the provision of a singlehuman machine interface for all flight phases, allows costreduction in terms of both equipment and pilot training. Aneasy to use navigation equipment is also more favorable tothe pilot. The future multimode receiver MMR could bebased upon multiple and independent navigation sourceslike GPS, Galileo, Inertials, etc., thus providing enhancedredundancy, safety and integrity on board the aircraft. Tomake the cost saving accessible to the airlines industry acertified equipment would be necessary which could beused for all phases of flight. It is expected that certificationof the combined on-board equipment would benefit fromthe similarity of the GPS and Galileo systems. In addition,airlines could operate more frequently form regionalairports and benefit from reduced airport taxes that resultfrom navigation infrastructure cost reduction. This wouldenable commercial GA and business/company flights toachieve significant growth rates and commercial benefits.

Interoperability Benefits at No Extra CostA further improvement of the combined performance androbustness can be achieved at no extra cost. Preliminarysimulation results have shown that the angle that separatesthe Galileo and GPS planes, i.e. the phase angle betweenthe two independent constellations has an impact on thecombined performance. Optimisation of this parameterknown as the Right Ascension of Ascending Node RAANfor both constellations will improve the combined GNSSperformance. A further advantage is the increase ofrobustness in case of GPS and/or Galileo satellite failuresallowing continuity of operation in degraded modes. It mustbe noted that this kind of harmonisation is purely aplanning issue (function of the launch date) and that anyrelative RAAN angle can be achieved for Galileo and GPSIII without any impact on individual system designs orcosts. Within the remaining window of opportunity untilSystems Requirements are frozen for Galileo ( 2005) andin future for GPS III, this parameter could still bedetermined for maximum interoperability.UERE – User Equivalent Range ErrorA UERE budget (see Table 3) in dependence of the satelliteelevation angle was used, which was defined in the GalileoB2C study [3]. The simulation duration was one to threedays to account for the repetition of the Galileo satelliteconstellation. The grid resolution is 1 x1 i.e. 65400 virtualuser positions. The used time step in the simulation runswas 501.01601.00901.00Table 3: UERE budgets for E1/E5b Galileo signal forPL calculations (B2C phase)Operational OutagesThe AVIGA tool allows to predict GNSS constellationavailabilities. It is also able to take into account outagecharacteristics like manoeuvres (frequency and duration)and satellite failures in form of Short term outages (satellite failure repair) Long term outages (satellite replacement)to evaluate the constellation availability.Critical SatellitesFor the allocation of continuity requirements in the LAASMASPS [4] the possibility is outlined that the protectionlevel during a 15 seconds period could jump over thespecified alarm limit due to loss of signals to one or moresatellites (PL AL risk). For the reduction of the approachcontinuity risk it is reasonable to analyse the availableconstellation for the number of so called "critical satellites".These are the satellites which when being removed from thexPL computations would cause the xPL to rise above thelimit. Acceptance of one or more of the critical satelliteswould have an effect on the continuity risk in such a waythat it will be reduced if more critical satellites are allowed.The introduction of the number of critical satellites willhave also an effect on the availability of PL AL. With a4low allowed number of critical satellites the availabilitywill be lower but the continuity will be higher.The consideration of critical satellites is also part of theGalileo baseline. In the frame of this paper simulationshave been performed showing the availability of protectionlevels taking into account a number of allowable criticalsatellites.RAIM - RECEIVER AUTONOMOUS INTEGRITYMONITORINGThe natural redundancy of ranging sources in satellitenavigation makes RAIM (Receiver Autonomous IntegrityMonitoring) an important contributor to the provision of arequired integrity level on user side.One basic method how to use RAIM in a certifiableairborne receiver is given in RTCA Do-208 MOPS forAirborne Supplemental Navigation using the GPS [5].However, since then, RTCA SC-159 proposed an improvedalgorithm and associated parameters [6]. This is thereference RAIM algorithm as used in the simulationsshown in this paper. The ability to detect anomalies in apseudorange measurement is highly dependent on theobservation geometry. A measure for the sensitivity of theFault Detection algorithm is the protection level (xPL)either in the horizontal (HPL) or in the vertical (VPL)plane.Figure 2 presents results of a RAIM VPL computation forthe snapshot fault-detection algorithm with fixed FalseAlarm and Missed Detection rates used as input parameters.The RAIM protection level was computed according [6]and the suggested specification of the UERE for Safety ofLife applications (see Table 3). The Pfa and Pmdparameters are derived from Precision Approachrequirements.vertVPL Slopemax pbias σ UERE(Eq. 1)where Slopemax is the maximum amplifying coefficientvertfor pseudorange offsets which would cause increase invertical position error, pbias is a threshold computed offline and linked to the number of satellites in view. There islow instantaneous RAIM FD availability for APV-II modein the vertical plane apparently, seen from Figure 2, asmany light-blue peaks breach the alert limit plane.Figure 2: Galileo Protectio

Table 1 shows the Galileo System requirements for the Galileo Safety Of Life Service as stated in the Mission Requirements Document [2]. The comparison of Table 1 and Table 2 yields that the Galileo System aims to be used as a certified navigation means for the flight phases Remote/Oceanic En Route down to non precision approach

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