EGNOS: The First Step in Europe’sContribution to the Global NavigationSatellite System

L. Gauthier, P. Michel & J. Ventura-TravesetESA Directorate of Application Programmes, Toulouse, France

J. Benedicto ESA Directorate of Application Programmes, ESTEC, Noordwijk, The Netherlands

IntroductionThe current capabilities of GPS and GLONASS,although very adequate for some user com-munities, present some shortfalls. The lack ofcivil international control represents a seriousproblem from the institutional point of view. Inaddition, there is a need for enhanced

performance. In particular, the civil-aviationrequirements for the precision and non-precision approach phases of flight cannot bemet by GPS or GLONASS only. Marine andland users may also require some sort ofaugmentation for improving GPS/GLONASSperformance.

The first-generation Global Navigation SatelliteSystem, GNSS-1, as defined by the experts ofthe ICAO/GNSS Panel, includes the basic GPS and GLONASS constellations and anysystem augmentation needed to achieve thelevel of performance suitable for civil-aviationapplications. EGNOS, which is a regionalsatellite-based augmentation equivalent to theAmerican Wide-Area Augmentation System(WAAS) or the Japanese Multi-transportSatellite-based Augmentation System (MSAS),is the first European implementation for GNSS.EGNOS will become operational during early2004. From 2006/2008 onwards, Europeshould also have available the independentGalileo system, which will be compatible andinteroperable with GPS/GLONASS/EGNOS.

The EGNOS missionGeneral objectivesThe purpose of EGNOS is to implement asystem that fulfils a range of user servicerequirements by means of an overlay augmen-tation to GPS and GLONASS, based on thebroadcasting through geostationary satellites ofGPS-like navigation signals containing integrityand differential-correction information applicableto the navigation signals of the GPS satellites,the GLONASS satellites, EGNOS’s own GEOOverlay satellites and the signals of other GEOOverlay systems (provided they can bereceived by a GNSS-1 user located within thedefined EGNOS service area). EGNOS willaddress the needs of all modes of transport,including civil aviation, maritime and land users.

The European Tripartite Group* - consisting of ESA, the EuropeanCommission and Eurocontrol - is implementing, via the EGNOSproject, the European contribution to the Global Navigation SatelliteSystem (GNSS-1), which will provide and guarantee the availability ofnavigation signals for aeronautical, maritime and land mobile trans-European network applications. On behalf of this Tripartite Group, ESAis responsible for the system design, development and qualification ofan Advanced Operational Capability (AOC) of the EGNOS system.

EGNOS will significantly improve the accuracy of GPS, typically from20 m to better than 5 m, will offer a service guarantee by means of the‘Integrity Signal’, and will also provide additional ranging signals. It willoperate on the GPS L1 frequency, and will thus be receivable withstandard GPS front-ends. EGNOS is one of three inter-regionalSatellite-Based Augmentation Services (SBAS) that complement GPSand GLONASS. The other two are the United States WAAS and theJapanese MSAS. The EGNOS coverage area will be the European CivilAviation Conference area, but could be readily extended to includeother regions within the broadcast area of the geostationary satellites,such as Africa, Eastern countries, and Russia. EGNOS will meet, incombination with GPS and GLONASS, many of the current positioning,velocity and timing requirements of the land, maritime andaeronautical modes of transport in the European region. It is the firstelement of the European satellite-navigation strategy and a majorstepping stone towards Galileo, Europe’s own global satellitenavigation system for the future.

This article summarises the EGNOS system requirements, the overallsystem design, as well as the current status of the on-goingdevelopment activities, including the EGNOS System Test Bed (ESTB).

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* A formal agreement based on Article 228 of the EC Treaty was concluded on 18 June 1996between the European Community, Eurocontrol and ESA, for the development of theEuropean Contribution to the first-generation Global Navigation Satellite System (GNSS-1).

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Aeronautical applicationsThe performance objectives for aeronauticalapplications are usually characterised by fourmain parameters: accuracy, integrity, availabilityand continuity of service. The values for theseparameters are highly dependent on thephases of flight, typical requirements for which are included in Table 1. Neither GPS norGLONASS can meet the required integrity,availability and continuity of service objectiveswithout a system augmentation, although theirperformance in terms of accuracy alone couldmeet the requirements of en-route andterminal-area navigation and non-precision

approaches. The actual requirements forEurope are currently being finalised by theInternational Civil Aviation Organisation (ICAO)in the form of SARPs (Standards andRecommended Practices).

Maritime applicationsThe performance objectives for maritimeapplications are generally broken down intoocean, coastal, inland-water and harbournavigation. Minimum performance require-ments for these four generic cases have beenquantified by the European Maritime Radio-Navigation Forum (see Table 2).

Land applicationsThere are a large number of applications underdevelopment worldwide related to the use ofsatellite navigation and land mobileapplications. These include: vehicle positioning,fleet management, position tracking,emergency services, theft protection,passenger information, road control, etc.Depending on the application, the accuracyneeded for the various systems ranges fromhundreds of metres to a few metres, requiringthe use of differential corrections. Integrity isalso required for some of these applications.

Other applicationsAnother important benefit of satellite navigationis the provision of a global time reference.EGNOS will provide a stable time reference towithin a few nanoseconds of Universal Time

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Table 1. Aviation GNSS signal-in-space performance requirements

Typical operation(s) Accuracy Alert Integrity Time Continuity Availability Associatedlateral/vertical limit to alert RNP type(s)

95% lateral/vertical

En-route 2.0 NM/ 4 NM/ 5 min 0.99 to 20 to 10N/A N/A 0.99999

En-route 0.4 NM/ 2 NM/ 5 to 2N/A N/A 1-10

-4/h 0.999

10-7

/h 15 s to to1-10

-8/h 0.99999

En-route, 0.4 NM/ 1 NM/ 1Terminal N/A N/A

Initial approach, 220 m/ 0.3 NM/ 0.5 to 0.3NPA, Departure N/A N/A

10 s

APV-I 220 m/ 0.3 NM/ 0.3/12520 m 50 m 0.99

2x10-7

toper 0.99999

APV-II 16 m/ 40 m/ approach 1-8x10-6

in 0.03/508 m 20 m any 15 s

Category-I 16.0 m/ 40 m/ 6 s 0.02/404-6 m 10-15 m

Table 2. Maritime GNSS typical performances

System-level Parameters

Predictable IntegrityAccuracy

Horizontal Alert limit Time to Integrity(m) (m) alarm (s) risk (/h)

Ocean 10 25 10 10-7

Coastal 10 25 10 10-7

Port approach and 10 25 10 10-7

restricted waters

Port 1 2.5 10 10-7

Inland waterways 10 25 10 10-7

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Figure 1. European Civil Aviation Conference (ECAC) approximate area coverage

(UTC). Related applications include timesynchronisation for cellular-phone networks,VSAT synchronisation, electric power synchro-nisation networks, Internet node synchro-nisation, etc. In addition, the combination ofsatellite navigation and mobile services willprovide a wide range of new services.

Performance objectives for the EGNOSsystemOf the three user communities, the civil-aviationrequirements are the most stringent (in terms ofintegrity and continuity) and hence the EGNOSperformance objectives are driven by thoseneeds, whilst still covering the needs of the landand maritime user communities.

The coverage area serviced by EGNOS will bethe European Civil Aviation Conference (ECAC)service Area (Fig. 1), comprising the FlightInstrument Regions (FIR) under theresponsibility of ECAC member states (mostEuropean countries, Turkey, the North Sea, andthe eastern part of the Atlantic Ocean).

The EGNOS AOC performance objectives areto provide a primary navigation service for allphases of flight, from en-route flying through toprecision approaches within the ECAC area. Inaddition, EGNOS has the potential to also offerservices over the full geostationary broadcastarea, and discussions are being pursued withinternational partners to provide this capabilityin order to offer users a seamless globalservice.

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The Main EGNOS Functionalities

• GEO Ranging (R-GEO): Transmission of GPS-like signals from threeGEO satellites (Inmarsat-3 AOR-E, Inmarsat-3 IOR, and ESA’s Artemissatellite (Fig. 2) for the AOC phase), will augment the number ofnavigation satellites available to users.

• GNSS Integrity Channel (GIC): Broadcasting of integrity information willincrease the availability of the GPS/GLONASS/EGNOS safe-navigationservice to the level required for civil-aviation non-precision.

• Wide-Area Differential (WAD): Broadcasting of differential correctionswill increase the GPS/GLONASS/EGNOS navigation service performance- mainly its accuracy - to the level required for precision approachesdown to CAT-I landings.

Figure 2. Artist’s impression of ESA’s Artemis satellite(J. Huart)

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Figure 3. The EGNOSsystem architecture

The EGNOS architecture and systemThe EGNOS reference architecture, depicted inFigure 3, is composed of four segments:

The EGNOS Ground Segment consists ofGNSS (GPS, GLONASS, GEO) Ranging andIntegrity Monitoring Stations (called RIMS),which are connected to a set of redundantcontrol and processing facilities called MissionControl Centres (MCCs). The system willdeploy 34 RIMS located mainly in Europe, andfour MCCs located at Torrejon (E), Gatwick(UK), Langen (D) and Ciampino (I). The MCCdetermines the integrity, pseudo-rangedifferential corrections for each monitoredsatellite, and ionospheric delays and generatesthe GEO satellite ephemeris. This information issent in a message to the Navigation Land EarthStation (NLES), to be uplinked along with theGEO ranging signal to GEO satellites. The latter downlink this data on the GPS Link 1 (L1)frequency with a modulation and codingscheme similar to the GPS one. All ground-segment components are interconnected bythe EGNOS Wide-Area CommunicationsNetwork (EWAN). The system will deploy twoNLESs (one primary and one back-up) perGEO navigation transponder and an NLES fortest and validation purposes, located atTorrejon (E), Fucino (I), Aussaguel (F), Raisting(D), Goonhilly (UK), and Sintra (P), respectively.Figure 4 shows the planned sites for the variousEGNOS ground-segment elements.

The EGNOS Space Segment is composed ofgeostationary transponders with global Earthcoverage. The EGNOS AOC system is basedon Inmarsat-3 AOR-E and IOR and the ESAArtemis navigation transponders (Fig. 5).

The EGNOS User Segment consists of anEGNOS standard receiver, to verify the signal-

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Figure 4. Planned sites for the various EGNOS Ground Segment elements

Figure 5. Inmarsat and Artemis EGNOS geostationary-satellite broadcast areas

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Figure 6. The EGNOSSystem Test Bed (ESTB)architecture

workshop aimed at fostering the use of ESTBand analysing the needs of potential users was organised on 6/7 July 2000. The verylarge number of participants represented awide variety of different users and countriesworldwide.

– The analysis of future EGNOS upgrades.

The ESTB architecture (Fig. 6) is made up of aspace segment comprising nominally twotransponders onboard the Inmarsat-III AtlanticOcean East and the Indian Ocean satellites, aground segment comprising a number ofreference stations spread across Europe andbeyond, a processing centre and the Inmarsatuplink stations. Communication lines interconnectall stations. During the ESTB’s development,the contributions from various providers havebeen integrated with the existing assets:– a network of RIMS, consisting of eight in a

first step, to be expandable in the near future,and which are permanently collecting GPS/GEO/GLONASS data

– a Central Processing Facility (CPF), generatingthe WAD (Wide Area Differential) usermessages. The CPF is located in Hønefoss(N), and hosted by the SATREF™ platform

– one Navigation Land Earth Station (NLES),forming part of the EURIDIS ranging system,located at Aussaguel (F) and allowing accessto the Inmarsat-III AOR-E satellite

– three EURIDIS RIMS for GEO rangingpurposes. These RIMS are distributed on anintercontinental basis to provide a wideobservation base for the geostationary orbit;they also collect GPS/GEO data

– a Processing Centre sited in Toulouse(France), devoted to the generation of theGEO ranging data, and which also acts as anode for the transmission of user messages

– a real-time communication network, allowingthe transfer of the RIMS data to the CPF, andnavigation messages from Hønefoss to theNLES.

in-space (SIS) performance, and a set ofprototype user equipment for civil-aviation, landand maritime applications. This prototypeequipment will be used to validate andeventually certify EGNOS for the differentapplications being considered.

Last but not least, the EGNOS SupportFacilities include the Development VerificationPlatform (DVP), the Application SpecificQualification Facility (ASQF) located in Torrejon(E), and the Performance Assessment andSystem Checkout Facility (PACF) located inToulouse (F). These facilities are needed tosupport system development, operations andqualification.

The EGNOS AOC Pre-Operational Implemen-tation involves the detailed design, development,deployment and verification of three elements:– the EGNOS System Test Bed (ESTB)– the EGNOS Advanced Operational Capability

(AOC) System– the AOC Complementary Activities.

The EGNOS System Test Bed (ESTB)The ESTB, which became operational inJanuary 2000, is a real-time prototype ofEGNOS, providing the first continuous GPSaugmentation service within Europe. It hasbeen developed under ESA contract by anindustrial consortium involving key Europeansatellite-navigation companies such as AlcatelSpace Industries, Astrium, GMV, Racal, Seatexand DLR. To optimise the overall ESTB effort,existing assets have been taken into account inbuilding up the ESTB. These include theSATREF™ system from NMA (NorwegianMapping Authority) and the EURIDIS rangingsystem from CNES. In early 2001, the ESTB willalso be fully connected with the ItalianMediterranean Test Bed (MTB) being providedby ENAV (Italian Civil Aviation Authority).

The ESTB therefore constitutes a great stepforward in terms of the European strategy fordeveloping the future European EGNOS andGalileo satellite-navigation systems. The drivingobjectives in its development include:– The support to EGNOS design: In particular,

algorithm design benefits from the ESTBexperience in both design and usage.

– The demonstration of the capabilities of thesystem to users: The ESTB constitutes astrategic tool for the European TripartiteGroup (ETG). The ETG is promoting the useof EGNOS and analysing its capabilities fordifferent applications. In particular, ESTBavailability will allow Civil Aviation Authoritiesto adapt their infrastructure and operationalprocedures for future EGNOS use when itbecomes operational. An ETG-sponsored

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Figure 7. Typical ESTB vertical-error histogram

Figure 8. Estimated ESTB accuracy (2-sigma value) performances (including MTB)

Figure 9. Typical vertical protection levels achieved by the ESTB

By using GPS and the ESTB Signal-In-Space,users within Europe can nowadays determinetheir positions with an error of less than 3 mhorizontally and 5 m vertically, for 95% of thetime. A typical ESTB vertical error distribution isrepresented in Figure 7, where the histogram ofthe errors is shown together with theassociated statistical values (mean, standarddeviation and 95th percentile). The area withinwhich the test signal can be exploited isdetermined primarily by the locations of thereference stations. The present accuracyperformances are illustrated in Figure 8.

The ESTB is also providing an integrity service,represented by the vertical and horizontalprotection levels computed by the user with theESTB information data, which are to boundwith a probability of 2x10-7/150 sec the Alertlimits associated with a particular operation.Figure 9 shows typical vertical protection levels(reflecting the guaranteed maximum errorprovided by the system ensuring the requiredlevel of safety) achieved throughout Europethrough the ESTB. The values required foraircraft precision-approach landing are ensuredacross most of Europe. These results provideadditional confidence in the current EGNOSdesign, especially given the reduced number ofreference stations available and the currenthigh solar activity.

The ESTB has already supported a number ofapplication demonstrations during 2000. Theyincluded landing planes at several airports,guiding ships into harbours, but also navigatingcars. The European Commission, nationalagencies and ESA are supporting suchdemonstration initiatives by European industryand operators in a number of ways.

The ESTB is still evolving to include the latestICAO standards. Moreover, early in 2001 it willbe operational 24 hours/day, 7 days/week, andwill embrace capabilities for service expansion(outside Europe) and interoperability analysis(with other augmentation systems such asWAAS). In addition, the ESTB will be fullyconnected with the Italian Mediterranean TestBed (MTB) being provided by ENAV (ItalianCAA) and will incorporate additional referencestations provided in co-operation with AENA(Spanish CAA).

The ESTB Help Desk service can be reachedvia the E-mail address ESTB@esa.int. Generalinformation on ESTB scheduling, signalstandards and the like can be found at:http://www.esa.int /navigation.

Interoperability of SBAS systemsIn addition to EGNOS, there are currently two

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Figure 11. EGNOS AOCservice broadcast areas

Figure 12. The industrialteam in charge of EGNOSAOC

Implementation Phase. The EGNOS InitialPhase was successfully concluded in November1998 with the System Preliminary DesignReview (PDR). The relevant ESA ProgrammeBoard approved full implementation of theEGNOS AOC System in December 1998, andthe prime contract was signed with AlcatelSpace Industries (F) on 16 June 1999 (Fig. 12).

other Satellite-Based Augmentation Systems(SBASs) under development: the Wide-AreaAugmentation System (WAAS) in the USA, andthe multi-functional MTSAT-based augmentationsystem (MSAS) in Japan. Although all SBASsare currently defined as regional systems, it iscommonly recognised that there is a need toestablish adequate co-operation/co-ordinationbetween the different systems so that theirimplementation becomes more effective andpart of a seamless worldwide navigation system.

To guarantee a seamless worldwide service, itis essential that the three systems meet somecommon interoperability requirements. Theservice providers of those SBAS systems meetregularly in so-called ‘Interoperability WorkingGroup (IWG)’ meetings to arrive at a preciseunderstanding of the term interoperability, andto identify the necessary interfaces betweenSBASs. The EGNOS system includes specificrequirements so that interoperability mayindeed be achieved. In parallel, severalinitiatives are in progress for performing testbedinteroperability demonstrations and flight trialsin the near future.

In addition to interoperability, EGNOS has abuilt-in expansion capability to enableextension of the services over regions withinthe Geostationary Broadcast Areas of the GEOsatellites used, such as Africa, Easterncountries, and Russia (Fig. 10). Thiscombination of SBAS interoperability andexpansion possibilities should allow theprovision of a truly global and seamlessworldwide navigation service (Fig. 11).

Current EGNOS programme statusThe EGNOS programme comprises twodifferent phases: the Initial Phase and the AOC

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SERVICE EXPANSION AND INTEROPERABILITY

CWAAS

US WAAS EGNOS MSAS

Figure 10. SBAS global interoperability and potential service expansion

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Figure 13. EGNOS AOCimplementation schedule

Since then, all subsystem activities have beenkicked-off and detailed design of thosesubsystems is currently in progress, afterprogressive completion of all subsystem PDRsduring the course of 2000. The OperationalReadiness Review is scheduled for December2003.

The next milestones in the EGNOS AOCImplementation Phase (Fig. 13) are the SystemCDR in the second half of 2001, the SystemFactory Qualification Review (FQR) in early2003, and the EGNOS AOC OperationalReadiness Review (ORR) in December 2003.

The EGNOS project includes significantcontributions from the French Space Agency(CNES), the Norwegian Mapping Authority(NMA), and the main European Air TrafficManagement service providers such as AENA(E), NAV-EP (P), DFS (D), ENAV (I), DGAC (F),NATS (UK) and SwissControl (CH). Thosepartners will in particular provide ESA with in-kind deliveries, including the infrastructure tohost a number of the necessary EGNOSground stations. Some other hosting sites arebeing finalised by ESA via specific agreements

with potential hosting entities. Site-surveyactivities started in mid-2000 and will last untilmid-2001.

In parallel with those on-going developmentefforts, the actual integration of EGNOS into theGalileo mission is currently under detailedassessment. The results to date are verypromising, and demonstrate that the EGNOSsystem can be used as a sound foundation onwhich the Galileo system architecture cancapitalise.

ConclusionEGNOS is the main European contribution toGNSS-1 to serve the needs of maritime, landtransport and aeronautical applications in theEuropean and neighbouring regions. Foraviation, EGNOS AOC will be used in the ECACRegion as a primary means of navigation for allphases of flight down to CAT-I. EGNOS will beinteroperable with equivalent US (WAAS) andJapanese (MSAS) SBAS systems, with the aimof contributing to a truly worldwide globalnavigation system.

The EGNOS Test Bed signal-in-space has beenavailable since early 2000, and is being used tosupport demonstrations and trials in Europe,Africa and South America, and interoperabilitytrials with Japan and the USA. The ESTB providesa unique opportunity for validating new applicationdevelopments in a realistic environment, inpreparation not only for the EGNOS operations,but also for future Galileo services.

EGNOS AOC development will be completedby the end of 2003, enabling operations to startin 2004. s

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Figure 14. Artist’simpression of the Galileo

satellite-navigationconstellation

(J. Huart)

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