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The aviation industry is developing a new posi-tioning and landing system based on the GlobalNavigation Satellite System (GNSS). The GNSSlanding system (GLS) integrates satellite andground-based navigation information to providethe position information required for approach and landing guidance. Potential benefits of theGLS include significantly improved takeoff andlanding capability at airports worldwide and at reduced cost, improved instrument approachservice at additional airports and runways, and the eventual replacement of the InstrumentLanding System. Boeing plans to certify the airborne aspects of GLS on the 737, to supportCategory I operations, by the end of 2003.

TECHNOLOGY/PRODUCT DEVELOPMENT

JOHN ACKLAND

SENIOR TECHNICAL FELLOW

AIRPLANE SYSTEMS

BOEING COMMERCIAL AIRPLANES

TOM IMRICH

CHIEF PILOT

RESEARCH

BOEING COMMERCIAL AIRPLANES

TIM MURPHY

TECHNICAL FELLOW

SYSTEMS CONCEPT CENTER

BOEING COMMERCIAL AIRPLANES

GLOBAL NAVIGATION SATELLITE SYSTEM

LANDINGSYSTEM

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or more than 10 years, the aviation industry has been

developing a positioning and land-ing system based on the GlobalNavigation Satellite System (GNSS).These efforts culminated in late2001, when the International CivilAviation Organization (ICAO)approved an international standardfor a landing system based on localcorrection of GNSS data to a levelthat would support instrumentapproaches. The ICAO Standardsand Recommended Practices(SARPS) define the characteristicsof a Ground-Based AugmentationSystem (GBAS) service that can be provided by an airport authority

ELEMENTS OF THE GLS

The GLS consists of three major elements — a global satellite constel-lation that supports worldwide navi-gation position fixing, a GBAS facility at each equipped airport that provideslocal navigation satellite correctionsignals, and avionics in each airplanethat process and provide guidance and control based on the satellite andGBAS signals (fig.1).

The GLS uses a navigation satelliteconstellation (e.g., the U.S. GlobalPositioning System [GPS], the planned European Galileo System) for the basic positioning service. TheGPS constellation already is in placeand improvements are planned overthe coming decades. The Galileo con-stellation is scheduled to be availablein 2008.

The basic positioning service isaugmented locally — at or near theairport — through a GBAS radio transmitter facility. Because the groundfacility is located at a known surveyedpoint, the GBAS can estimate theerrors contained in the basic position-ing data. Reference receivers in theGBAS compare the basic positioningdata with the known position of thefacility and compute corrections on a satellite-by-satellite basis. Thecorrections are called pseudorange cor-rections because the primary parameterof interest is the distance between theGBAS facility and individual satellites.The satellite constellation is continu-ously in motion, and satellites ascendand descend over the horizon whenobserved from any point on Earth. The GBAS calculates corrections forall the satellites that meet the specifiedin-view criteria and transmits thatinformation to the nearby airplanesover a VHF Data Broadcast (VDB)data link.

Boeing airplanes that are currentlybeing produced contain Multi-ModeReceivers (MMR) that supportInstrument Landing System (ILS) andbasic GPS operations. These MMRs

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F or an Air Traffic Service provider.The GBAS service provides theradiated signal in space that can beused by suitably equipped airplanesas the basis of a GNSS landing system (GLS). The initial SARPS support an approach service. Futurerefinements should lead to full low-visibility service (i.e., takeoff,approach, and landing) and low-visibility taxi operations. This article describes

1. Elements of the GLS.

2. Operations using the GLS.

3. Benefits of the GLS.

4. Operational experience.

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can be modified to support GLS andpotentially Microwave Landing Systemoperations. The GLS capability is supported through the addition of areceiver and processing in the MMRsof the GBAS data provided through theVDB data link. The MMRs apply thelocal correction data received from the GBAS to each satellite that theairplane and GBAS share in common.Because of position and altitude differences and local terrain effects,the GBAS and the airplane may notnecessarily be observing the samecombination of satellites. The airplanesystems only use satellite informationthat is supported by correction datareceived from the GBAS. When the airplane is relatively close to the GBASstation, the corrections are most effective, and the MMRs can computea very accurate position. Typical lateralaccuracy should be ≤1 m.

OPERATIONS USING THE GLS

A single GBAS ground station typi-cally provides approach and landingservice to all runways at the airportwhere it is installed. The GBAS mayeven provide limited approach serviceto nearby airports. Each runwayapproach direction requires the defini-tion of a final approach segment (FAS)to establish the desired reference pathfor an approach, landing, and rollout.The FAS data for each approach aredetermined by the GBAS service pro-vider and typically are verified afterinstallation of the GBAS ground station.

One feature that differentiates theGLS from a traditional landing systemsuch as the ILS is the potential for mul-tiple final approach paths, glideslopeangles, and missed approach paths for a given runway. Each approach is

2given a unique identifier for a particularFAS, glideslope, and missed approachcombination. FAS data for all approach-es supported by the particular GBASfacility are transmitted to the airplanethrough the same high-integrity datalink as the satellite range correctiondata (i.e., through the VDB data link).The MMRs process the pseudorangecorrection and FAS data to produce anILS-like deviation indication from thefinal approach path. These deviationsare then displayed on the pilot’s flight instruments (e.g., Primary FlightDisplay [PFD] ) and are used by air-plane systems such as the flight guid-ance system (e.g., autopilot and flightdirector) for landing guidance.

The ILS-like implementation of theGLS was selected to support commonflight deck and airplane systems in-tegration for both safety and economicreasons. This implementation helps

Multi-Mode Receiver VDB data link

Corrections and final approach segment data

Ground-BasedAugmentation System

THE GLS

FIGURE

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provide an optimal pilot and systeminterface while introducing the GLS at a reasonable cost. The use of operational procedures similar to thoseestablished for ILS approach and land-ing systems minimizes crew training,facilitates the use of familiar instru-ment and flight deck procedures, sim-plifies flight crew operations planning,and ensures consistent use of flightdeck displays and annunciations. For example, the source of guidance information (shown on the PFD in fig. 2) is the GLS rather than the ILS.The scaling of the path deviation information on the pilot’s displays for a GLS approach can be equivalentto that currently provided for an ILSapproach. Hence, the pilot can monitora GLS approach by using a display that is equivalent to that used duringan ILS approach.

Figure 2 shows a typical PFD presentation for a GLS approach. TheFlight Mode Annunciation on the PFDis “GLS” for a GLS approach and“ILS” for an ILS approach.

To prepare for a GLS approach,the pilot selects GLS as the navigationsource and chooses the particularapproach to be flown. This is accom-plished by selecting a GLS approachthrough the FMS (fig. 3) or by enteringan approach designator on a dedicatednavigation control panel (fig. 4). Ineither case, a unique five-digit channelnumber is associated with eachapproach. With the FMS interface, thepilot does not need to enter a channelnumber; tuning is accomplished automatically based on the approachselected, just as is now done for ILS.However, for an airplane equippedwith separate navigation tuning panels,the pilot tunes the MMRs by entering a GLS channel number in that panel.This is similar to the equivalent ILSflight deck interface where a pilot tunes the ILS by using a designatedVHF navigation frequency. As with the ILS, certain GLS identification data are available on other FMS pagessuch as the APPROACH REF page,which shows the runway identifier,

GLS channel, and associated approachidentifier (fig. 3).

Regardless of the selection method,the five-digit GLS channel number isencoded with the frequency to be usedfor the VDB data link receiver and withan identifier for the particular approachand missed approach path (FAS dataset) that corresponds to the desiredapproach.

Figure 4 shows a navigation controlpanel used to tune navigation radios,including GLS, for the 737-600/-700/-800/-900.

The approach plate shows thechannel number for each approach anda four-character approach identifier toensure consistency between the selectedchannel and the approach procedurechosen by the pilot. The approach identifier is read from the FAS datablock and displayed to the pilot on thePFD to provide positive confirmationthat the desired approach has indeedbeen selected.

Figure 5 shows a typical GLSapproach procedure. The procedure issimilar to that used for ILS except forthe channel selection method and theGLS-unique identifier. The approachchart is an example of a Boeing flight-test procedure and is similar to a chartthat would be used for air carrieroperations, with appropriate specifi-cation of the landing minima.

Figure 6 is an example of a possiblefuture complex approach procedureusing area navigation (RNAV),Required Navigation Performance(RNP), and GLS procedures in com-bination. Pilots could use such proce-dures to conduct approaches in areasof difficult terrain, in adverse weather,or where significant nearby airspacerestrictions are unavoidable. These procedures would combine an RNPtransition path to a GLS FAS to therunway. These procedures can also use GBAS position, velocity, and time(PVT) information to improve RNP

PFD WITH GLS

FIGURE

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capability and more accurately deliverthe airplane to the FAS.

The GBAS is intended to supportmultiple levels of service to an unlimitednumber of airplanes within radio range of the VDB data link. Currently,ICAO has defined two levels of service: Performance Type 1 (PT 1)service and GBAS Positioning Service(GBAS PS). PT 1 service supports ILS-like deviations for an instrumentapproach. The accuracy, integrity, andcontinuity of service for the PT 1 levelhave been specified to be the same asor better than ICAO standards for an

ILS ground station supporting CategoryI approaches. The PT 1 level wasdeveloped to initially support approachand landing operations for Category Iinstrument approach procedures.However, this level also may supportother operations such as guided takeoffand airport surface position determina-tion for low-visibility taxi.

The GBAS PS provides for veryaccurate PVT measurements within theterminal area. This service is intendedto support FMS-based RNAV andRNP-based procedures. The improvedaccuracy will benefit other future

FMS APPROACH SELECTION INTERFACE EXAMPLES

FIGURE

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Example APPROACH REF page with GLS

Example ARRIVALS page with GLS

uses of GNSS positioning such asAutomatic Dependent Surveillance —Broadcast and Surface MovementGuidance and Control Systems.

The accuracy of the GBAS servicemay support future safety enhance-ments such as a high-quality electronictaxi map display for pilot use in badweather. This could help reduce run-way incursion incidents and facilitateairport movements in low visibility.The service also may support auto-mated systems for runway incursiondetection or alerting.

As important as the accuracy of theGBAS service is the integrity moni-toring provided by the GBAS facility.Any specific level of RNP operationwithin GBAS coverage should be moreavailable because the user receivers no longer will require redundant satellites for satellite failure detection(e.g., Receiver Autonomous IntegrityMonitoring).

Because the GBAS PS is optionalfor ground stations under the ICAOstandards, some ground stations mayonly provide PT 1 service. The messages uplinked through the VDBdata link indicate whether or not theground station supports the GBAS PSand specify the level of service foreach approach for which a channelnumber has been assigned. When the GBAS PS is provided, a specific five-digit channel number is assignedto allow selection of a non-approach-specific GBAS PS from that station. Consequently, the channel selectionprocess allows different users toselect different approaches and levelsof service.

The GBAS PS and the PT 1 serviceare not exclusive. If the ground stationprovides the GBAS PS, selecting achannel number associated with anyparticular approach automatically will enable the GBAS PS service. Thereceiver provides corrected PVT in-formation to intended airplane systemsas long as the GBAS PS is enabled.ILS-like deviations also are availablewhen the airplane is close enough tothe selected approach path.

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ICAO is continuing development of a specification for service levels that would support Category II and IIIapproaches.

BENEFITS OF THE GLS

From the user perspective, the GBASservice can offer significantly betterperformance than an ILS. The guidancesignal has much less noise becausethere are no beam bends caused byreflective interference (from buildingsand vehicles). However, the real valueof the GLS is the promise of addi-tional or improved capabilities that the ILS cannot provide. For example,the GLS can

■ Provide approach and takeoff guid-ance service to multiple runwaysthrough a single GBAS facility.

■ Optimize runway use by reducingthe size of critical protection areasfor approach and takeoff operationscompared with those needed for ILS.

■ Provide more flexible taxiway orhold line placement choices.

■ Simplify runway protection constraints.

■ Provide more efficient airplane separation or spacing standards forair traffic service provision.

■ Provide takeoff and departure guid-ance with a single GBAS facility.

From the service provider perspec-tive, the GBAS can potentially provideseveral significant advantages over theILS. First, significant cost savings maybe realized because a single systemmay be able to support all runways atan airport. With the ILS, each runwayserved requires an ILS and a frequencyassignment for that ILS, which can be difficult because of the limited numbers of available frequencies.Operational constraints often occurwith the ILS when an Air TrafficService provider needs to switch acommonly used ILS frequency to servea different runway direction. This is notan issue with the GBAS because amplechannels are available for assignmentto each approach. In addition, becausethe GBAS serves all runway ends witha single VHF frequency, the limitednavigation frequency spectrum is used much more efficiently. In fact,a GBAS may even be able to support asignificant level of instrument approachand departure operations at other nearby airports.

The siting of GBAS ground stationsis considerably simpler than for the ILSbecause GBAS service accuracy is notdegraded by any radio frequency prop-agation effects in the VHF band.Unlike the ILS, which requires levelground and clear areas on the runway,the siting of a GBAS VHF transmittercan be more flexible than ILS. Theremoval of the requirement to provide

a large flat area in front of the ILS glideslopealone can represent a very significant savings in sitepreparation cost and opensup many more locationsfor low-minima instrumentapproach service.

Although GBAS accuracy can be affectedby multipath interference,careful siting of GBASreceivers can readilyeliminate multipath concerns because GBASreceivers do not need tobe placed near a runway

in a specific geometry, as is the casewith the ILS or MLS. Hence, this virtually eliminates the requirementsfor critical protection areas or restrictedareas to protect against signal interfer-ence on runways and nearby taxiways.

Finally, the GBAS should have lessfrequent and less costly flight inspec-tion requirements than the ILS becausethe role of flight inspection for GBASis different. Traditional flight inspec-tion, if needed at all, primarily wouldapply only during the initial installationand ground station commissioning.This flight inspection would verify thesuitability of the various approach path(FAS) definitions and ensure that theGBAS-to-runway geometry definitionsare correct. Because verifying the coverage of the VDB data link prin-cipally is a continuity of service issuerather than an accuracy or integrityissue, it typically would not requireperiodic inspection.

GBAS systems capable of sup-porting Category II and III operations internationally are envisioned. Airbornesystem elements that would be neces-sary for the enhanced GLS capability(e.g., MMR and GLS automatic land-ing provisions) already are well on theway to certification or operationalauthorization. Airborne systems andflight deck displays eventually willevolve to take full advantage of the linear characteristic of the GLS overthe angular aspects of the ILS.

GLS AS ACTIVE

FIGURE

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OPERATIONAL EXPERIENCE

To date, flight-test and operationalexperience with the GLS hasbeen excellent. Many GLS-guidedapproaches and landings havebeen conducted successfully at a variety of airports and undervarious runway conditions.

Both automatic landings and landings using head-up displayshave been accomplished safelythrough landing rollout, in bothroutine and non-normal conditions.

On the pilot’s flight displays,the GLS has been unusually steadyand smooth when compared withthe current ILS systems even whencritical areas necessary for theILS approaches were unprotectedduring the GLS approaches.

The Boeing TechnologyDemonstrator program has used a 737-900 to demonstrate successful GLS operations toairline customers, airplane andavionics manufacturers, airportauthorities, Air Traffic Serviceproviders, and regulatoryauthority representatives.

The GLS represents a maturecapability ready for widespreadoperational implementation.When implemented, the GLS willimprove safety, increase capacity,and provide operational benefitsto airlines, pilots, passengers,airports, and Air Traffic Serviceproviders. Boeing plans to certifythe airborne aspects of the GLSon the 737 by the end of 2003 to support Category I operations,with other models to follow. Work is continuing for the airborne certification of the GLSto support Category II and IIIoperations when suitable GBASground facilities are specifiedand made available.

4TYPICAL GLS APPROACH PROCEDURE

FIGURE

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SUMMARY

The aviation industryis developing the GLS, a new positioning and landingsystem that integratessatellite and ground-basednavigation information.Potential benefits of the GLS include significantlyimproved takeoff andlanding capability at air-ports worldwide at reducedcost, instrument approachservice at additional airports and runways, andeventual replacement of the ILS. Boeing plans to certify the airborne aspects of the GLS on the 737 by the end of 2003 to supportCategory I operations, withother models to follow.

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POTENTIAL FUTURE APPROACH PROCEDURE USING BOTH RNP AND GLS CAPABILITY

FIGURE

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About the Authors

Capt. Thomas Imrichhas held a variety of positionsin the FAA, including nationalresource specialist for AirCarrier Operations, where hehelped formulate and imple-ment international flight safetyand operations policies for areassuch as navigation systems, all-weather operations, and crew qualification. He currentlyserves as Chief Pilot, Researchfor Boeing as a qualified airlinetransport pilot and flightinstructor who is type rated inthe B737 through A340, MD-11,B747-400, and B777.

John Acklandhas worked on airplane system design and devel-opment for the Concorde, L-1011, DC-10, and all current Boeing productionairplanes. A Boeing em-ployee since 1980, he is aSenior Technical Fellow andparticipates in a number ofaviation industry committeesand organizations that areworking to improve the aviation business.

Tim Murphyis a Technical Fellow supporting the AirplaneSystems Concept Center with work on next-generationcommunications, navigation,and surveillance (CNS) technologies for Air TrafficManagement with a focus on GPS landing systems, modernization, and augmen-tation. He is active in devel-oping international satellitenavigation standards for commercial aviation.

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