GPS

Table of Contents

1. Introduction.............................................................................................. 1-1

1.1 GPS..........................................................................................................................1-21.2 Benefits of GPS........................................................................................................1-3

2. GPS Augmentation for Aviation............................................................... 2-1

2.1 The Need for GPS Augmentation..............................................................................2-12.2 WAAS......................................................................................................................2-22.3 LAAS.......................................................................................................................2-5

3. Introduction of GPS-Based Services......................................................... 3-1

3.1 Early Operational Use...............................................................................................3-13.2 Introduction of WAAS..............................................................................................3-4

4. Transition to GPS-Based Navigationand Landing Guidance.............................................................................. 4-1

4.1 Transition Considerations..........................................................................................4-24.2 Projected User Equipage with GPS/WAAS Avionics.................................................4-34.3 Phaseout of Current Systems.....................................................................................4-5

4.3.1 Navigation System Phaseout..........................................................................4-54.3.2 Precision Approach and Landing System Phaseout.......................................4-11

5. Summary.................................................................................................. 5-1

Appendix A - Acronyms................................................................................ A-1Appendix B - Definitions................................................................................B-1

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1Introduction

The Federal Aviation Administration (FAA) hasembarked on an aggressive program to makesatellite-based navigation technology available foruse throughout the National Airspace System(NAS). Satellite-based navigation services willprovide significant economic and safety benefits tothe entire aviation community. The FAA isworking with the aviation industry to augment theGlobal Positioning System (GPS), developed by theDepartment of Defense (DOD), to providenavigation services adequate for all phases of flight.Together with improved computer-based decisionaids for controllers, these services will improve thesafety of flight operations, accommodate user-preferred flight profiles, and increase airport andairspace capacity to meet future air trafficdemands.

The transition to satellite navigation will permit theuse of a single type of navigation receiver onboardall aircraft rather than the current requirement for anumber of unique receivers to support differentphases of flight. New navigation, landing, andsurveillance services will be possible that are notcurrently economically feasible. In addition, therewill be significant reduction in the cost of equipageboth to the aircraft operator and to the groundservice provider. It will be possible to phase outboth the ground equipment and the associatedavionics for a large number of ground-basedsystems such as VHF omnidirectional range

(VOR), distance measuring equipment (DME),instrument landing system (ILS), nondirectionalbeacon (NDB), Omega, Loran-C, and markerbeacons.

This document provides an overview of the FAA’s15-year plan for transitioning to GPS-basedservices. The transition timelines presented in thisdocument are consistent with the 1994 FederalRadionavigation Plan (FRP) and with InternationalCivil Aviation Organization’s (ICAO) commitmentsto preserving ILS’s and transitioning to satellite-based navigation. The document reviews briefly theplan for implementing GPS-based navigation andlanding guidance (covered more fully in GPSImplementation Plan for Air Navigation andLanding [1]) and discusses in greater detail the planfor transitioning to the use of these services andphasing out the existing ground-based systems. Asubsequent effort will develop detaileddecommissioning criteria and a site-by-sitedecommissioning schedule.

This transition plan is based on the expectation thataugmented GPS will fully meet the re-quirements ofa sole-means aircraft navigation and landingguidance system, thereby allowing the phaseout ofexisting ground-based systems. The plannedtransition includes an extended period of overlap,during which both augmented GPS and the existingsystems will be available. This overlap period will

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give both the FAA and aircraft operators theopportunity to become comfortable that augmentedGPS meets the performance requirements before theexisting sole-means systems are decommissioned.If aug-mented GPS fails to meet the performancerequirements fully, the timetable for the phaseout ofthe existing systems will be modified as necessaryto ensure continuity of navigation and landingguidance services.

1.1. GPS

GPS is a satellite-based system used for navigation,position determination, and time-transferapplications. The system consists of a 24-satelliteconstellation (Figure 1-1), plus associated ground-based monitoring and control facilities; it isoperated and maintained by the DOD. Thesatellites radiate precisely timed signals coded sothat a receiver on or near the surface of the earthcan determine both the transmission time delay (orequivalently, distance) from the satellite to thereceiver and the precise satellite position. Bysimultaneously receiving such signals from at leastfour satellites, the receiver can determine itsposition and time.

GPS provides two levels of service: a precisepositioning service (PPS), available only to DODand other authorized users, and a standardpositioning service (SPS), available free of chargeto civil users worldwide. SPS provides a lowerlevel of position and time accuracy than PPS.

Through a technique termed selective availability,the accuracy of SPS is controlled to protect U.S.national security interests. The DOD hascommitted to operating the system so that itprovides a positioning accuracy of better than 100meters horizontal (150 meters vertical) 95 percentof the time, and better than 300 meters horizontal(450 meters vertical) 99.99 percent of the time.Time accuracy is within 340 nanoseconds ofCoordinated Universal Time (UTC).

The first of a series of research and developmentGPS satellites was launched in February 1978. InFebruary 1989, the DOD launched the first of theoperational GPS satellites. The GPS reached initialoperational capability (IOC) on December 8, 1993,and full operational capability (FOC) on July 17,1995; FOC means that the system fully meets itsspecified performance requirements.

To encourage both national and international civiluse of GPS, the United States has committed tomaintain the system for the foreseeable future andto provide a minimum of 6 years prior notice of anyintent to discontinue the system. Replacementsatellites (Block IIR) for the current constellationare in production, and the DOD is already initiatingprocurement of the Block IIF satellites as thefollow-on to the Block IIR satellites. Together, theBlock IIR and IIF satellites should provide formaintenance of the constellation to 2010 andbeyond.

Figure 1-1GPS Constellation

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1.2. Benefits of GPS

The advent of satellite-based navigation will have aprofound effect upon aviation. For the first time,aircraft will be able to determine their preciseposition anywhere in the world's airspace or on thesurface. Using line-of-sight or long-range digitalcommunications, aircraft will be able tocommunicate this satellite-derived position tonearby aircraft and to nearby or distant controlcenters. This will provide better situationalawareness to pilots and will permit extendingsurveillance-based air traffic control to areas whereit is not now technically or economically feasible,e.g., oceanic and remote airspace.Decommissioning some of the current en routeradar-based surveillance systems may also bepossible.

These capabilities will provide significant benefitsto both aircraft operators and to the air trafficcontrol systems which support their operations.Some of these benefits are:

• Precise 4-D (3 dimensions, plus time)navigation

• User-preferred flight paths

• Reduced separation standards for moreefficient use of the airspace

• Precision approach capability at all runways• Cost saving due to phasing out of ground-

based systems (for example, VOR, DME, ILS,NDB, Omega, Loran-C)

• Lower avionics equipment cost (single type ofavionics equipment supports all phases offlight)

• Reduced training costs, because ultimatelypilots will only have to be trained to fly GPS-based procedures

• New procedures and navigation techniques

These benefits fall into two categories: those due tothe greater operational efficiency which GPSpermits, and those resulting from the phasing out ofthe current ground-based systems which GPSfunctionally replaces. Benefits in the first categoryaccrue primarily to aircraft operators and areavailable as soon as GPS-based services areavailable. Benefits in the second group accrueprimarily to the service provider (the FAA), butalso to a lesser extent the aircraft operator; thesebenefits occur later, when equipage with GPSavionics has progressed to the point that theconventional systems can be decommissioned.

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2GPS Augmentation For Aviation

2.1. The Need for GPS Augmentation

GPS SPS, while suitable for many applications,including use as a supplemental means of aircraftnavigation, fails to provide the accuracy, integrity,availability, and continuity of service which arecurrently required for service as a primary-means1

or sole-means system in the

1 The ICAO definitions of supplemental, primary, and sole-means navigation systems appear in Appendix B.

NAS for aircraft navigation and landing guidance.The principal requirements for navigation andCategory I landing guidance are summarized inTable 2-1 [2]. Requirements for Category II/IIIprecision approach are specified in terms ofrequired navigation performance (RNP) and aresummarized in the Local Area AugmentationSystem Operational Requirements Document [3].

Table 2-1Navigation and Category I Landing Guidance Performance Requirements

En Route Through Precision ApproachNon-Precision Approach CAT I

Availability 0.99999 .999Accuracy (95%)

HorizontalVertical

100 mnot specified

7.6 m7.6 m

IntegrityProbability of

HMI*10 -7/hour 4x10-8/approach

Time to Alarm 8 sec 5.2 sec

Continuity 1-10-8/hour .99995/approach* Hazardously Misleading Information

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Accuracy is the degree of conformance of anaircraft’s measured position with its true position.Basic GPS meets the accuracy requirements for enroute through nonprecision approach (NPA), butnot for precision approach.

Integrity is the ability to provide timely warningswhen part or all of the system is providingerroneous information and thus should not be usedfor navigation. Each GPS satellite broadcasts anintegrity message to assure users that the signalsbeing transmitted by the satellite are correct.However, one-half hour or more may elapse fromthe time that a fault occurs to the time that it isdetected and the integrity message changed toreflect it. This is too long for aviation use. Toensure timely integrity information, currentinstrument flight rules (IFR)-certified aircraft GPSreceivers use a technique termed receiverautonomous integrity monitoring (RAIM). Thisapproach involves the use of redundantmeasurements to test the validity of the receivedsignals. Four satellites in view are required tocompute a GPS-derived position. With fivesatellites, if one fails, the receiver can determinethat it is getting an inconsistent solution but cannotdetermine which has failed. If six or moresatellites are in view, the receiver has enoughinformation to determine which satellite has failedand use the remaining set in determining itsposition. While effective in providing integrity,RAIM reduces the availability, because now thesystem is available only when redundant satellitesare in view in an acceptable geometry.2 While thisis generally the case when all 24 satellites areworking, even then there are occasional “RAIMholes,” i.e., regions where RAIM is not availablefor some period of time due to an insufficientnumber of satellites in view. If one or moresatellites is out of service, for maintenance or dueto a failure, periods of unavailability due to RAIMoutages could become too numerous and too longto permit the use of GPS for aircraft navigation.

2 Altimeter aiding, i.e., the use of the aircraft’s altitudeas measured by its barometric altimeter, can substitutefor one satellite in the integrity assessment.

Availability is the probability that at any time thesystem will meet the accuracy and integrityrequirements for a specific phase of flight.

Continuity is the probability that a service willcontinue to be available for a specified period oftime, given that it is available at the beginning ofthe period (for example, that the system willcontinue to meet the requirements for approachguidance throughout an approach, given that it isavailable at the initiation of the approach). It is ofconcern primarily in the approach mode of flight.

To meet these requirements, the FAA hasundertaken programs to develop two systems toaugment GPS: the Wide Area Augmentation Sys-tem (WAAS) and the Local Area AugmentationSystem (LAAS).

2.2. WAAS

WAAS is an augmentation of GPS which includesintegrity broadcasts, differential corrections, andadditional ranging signals. It is being developed toprovide the accuracy, integrity, availability, andcontinuity required to support all phases of flightthrough Category I precision approach.

As illustrated in Figure 2-1, WAAS comprises anetwork of wide-area reference stations whichreceive and monitor the GPS signals. Data fromthese reference stations are transmitted to masterstations, where the validity of the signals from eachsatellite is assessed and wide-area corrections arecomputed. These validity (integrity) messages andwide-area corrections are transmitted to aircraftvia geostationary communications satellites, whichby serving as additional sources of GPS rangingsignals thereby increase the number of satellitesavailable to the system’s users. The WAAS signalwill be transmitted on the same frequency and withthe same type of code-division multiplexmodulation as the GPS SPS signal, so that thesame receiver can acquire and process both theGPS and WAAS broadcasts.

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The integrity message provided by WAAS, termeda ground-based integrity broadcast (GBIB),provides the user with a direct verification of theintegrity of the signal from each satellite in view.The user does not require the extra satellites whichare required for RAIM; in fact, since the WAASsatellite itself provides a ranging signal, generallyonly three GPS satellites will be required tocompute position. With this reduced requirementfor the number of satellites in view, GPS/WAASwill meet the availability and continuityrequirements for all phases of flight.

The wide-area correction signals transmitted byWAAS allow the aircraft’s GPS/WAAS receiverto correct for the timing and ephemeris (satelliteposition) errors in the signals from each GPS orWAAS satellite and the signal delay due to theEarth’s ionosphere. With these corrections,GPS/WAAS is expected to meet the accuracyrequirements of Category I precision approach.

The basic concept and operational feasibility ofWAAS has been demonstrated, and a contract forthe development of the operational system wassigned in August 1995. The system is scheduled toreach its initial operational capability termed InitialWAAS (IWAAS) in early 1998. The IWAAS willprovide dual coverage by geostationary satellites ofthe eastern and western parts of the continentalUnited States, with an area in the center of thecountry having only single coverage (Figure 2-2).

Although the IWAAS will have the capability forsupporting navigation and Category I precisionapproach, it will not have the level of internalredundancy, and thus guaranteed availability in theevent of failure of elements of the system, requiredof a sole-means system.

GeostationaryCommunicationSatellite

WAAS Signal

GroundEarthStation

Wide Area GroundStation Network

GPS

Figure 2-1Wide Area Augmentation System

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The WAAS contract contains several options forthe expansion of the system of both the number ofground stations and the number of satellites. Theseoptions will be exercised in the years followingIWAAS, with the goal that by 2001 WAAS willhave achieved a sufficient level of robustness toenable it to serve as a sole-means system for airnavigation and landing guidance.

In parallel with the development of WAAS, theavionics industry will be developing the requisiteaircraft equipment. The basic WAAS minimumoperational performance standard (MOPS), whichincludes the full specification of the navigationmodes, was completed on January 16, 1996 [4].Later in 1996, the WAAS MOPS will be updated toinclude definition of the precision approach modes.This will allow time for avionics to be developed byIWAAS.

Figure 2-2WAAS Coverage

Figure 2-3WAAS Implementation Schedule

Develop

1995 2000 2005 2010

EWAASIWAAS

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As soon as they are available, GPS/WAASavionics are expected to supplant technicalstandard order (TSO)-C129-based GPS avionics.The latter will continue to be useful forsupplemental navigation and TSO-C129-basedNPA’s, but unless they are upgraded to meet theGPS/WAAS TSO they will not be useable forprimary/sole-means navigation nor forGPS/WAAS nonprecision or precisionapproaches. The only foreseeable exception to thediminished value of TSO-C129 avionics will befor TSO-C129 equipment meeting the capabilitiesof FAA Notice 8110.60 and used for primary-means navigation in oceanic or remote areas.

2.3. LAAS

The accuracy provided by the WAAS will beadequate to support precision approaches toCategory I minimums but not to Category II/IIIminimums. Meeting the more stringentrequirements of Category II/III precisionapproaches will require a LAAS. As illustrated inFigure 2-4, under this concept the corrections tothe GPS (and WAAS) signals are broadcast toaircraft within line of sight of a ground referencestation. The range of this service will typically be25-30 nautical miles (nm).

In addition to providing a Category II/IIIcapability, LAAS may be used at some high-capacity airports to increase service availabilitybeyond that ensured by WAAS alone. LAAS mayalso be needed to support Category I approachesat a small number of airports whose specificlocations make it difficult to use GPS/WAASbecause of inadequate visibility of WAASsatellites. LAAS can also provide terminalnavigation, airport surface navigation, and guidedmissed approach and departure procedures.

The FAA is working with U.S. industry anduniversities to determine the technical feasibilityof using satellite-based systems for Category IIand III precision approaches. Several cooperativeprojects have already demonstrated the ability ofboth advanced code and kinematic carrier phasedifferential techniques to meet the accuracyrequirements of Category III autoland approaches.Several satisfactory integrity techniques have alsobeen demonstrated, but must be validated.

The work in this area is being closely coordi-natedwith the development of local area differentialGPS (LADGPS) systems for Special Category I(SCAT-I) precision approaches,

Figure 2-4Local Area Augmentation System (LAAS)

LAAS Ground Station

Differential Correction Message

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which is being funded by private industry. TheFAA will choose a LAAS architecture anddevelop LAAS and LAAS-compatible avionicsstandards by 1998. The method of acquiringLAAS is currently under review. The FAA maydefine the certification standards and let

manufacturers develop the equipment and requestits certification. The FAA is also conductingresearch on providing airport surface trafficsurveillance and guidance based on LAAS-augmented GPS.

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3Introduction of GPS-Based Services

3.1. Early Operational Use

Even with its limitations, unaugmented GPS isalready being used productively to enhance aircraft

operations. A number of the significant eventsrelating to this early use are listed in Table 3-1,illustrating the rapid pace at which GPS is beingintroduced into operational service.

Table 3-1Significant Events Supporting Early Operational Use of GPS

{PRIVATE }Feb1991

GPS approved as an input to multisensor navigation systems

Dec 1992 TSO-C129 issued for GPS receiversJun 1993 GPS approved for supplemental use for en route through NPAJun 1993 NPA overlay program initiatedAug 1993 RTCA published minimum aviation system performance standard (MASPS)

for SCAT-I differential GPS systemDec 1993 First private GPS NPA in operationFeb 1994 The Administrator announced GPS to be operational and an integral part of

the U.S. air traffic control systemFeb 1994 Initiated approval of supplementary GPS receivers for oceanic, domestic en

route, terminal, and NPA’sMay 1994 First GPS route establishedJun 1994 First GPS helicopter approach approvedAug 1994 First stand-alone GPS NPA publishedAug 1994 Published FAA Order 8400.11 for the approval of SCAT-I systemsSep 1994 Letter from the Administrator to ICAO reiterating U.S. offer of GPS SPSDec 1994 Approval of GPS as a primary means of navigation in oceanic airspaceSep 1995 Use of future air navigation system (FANS)-1 in Pacific

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Domestic Navigation and NonprecisionApproach

Following issuance of TSO-C129 for GPSreceivers, GPS was approved in June 1993 for useas a supplemental system for navigation and NPA.Its status as a supplemental system means that aprimary- or sole-means system must be onboardand operational in case GPS is not useable.However, it allows the aircraft to realize some ofthe operational benefits of GPS, e.g., direct, off-airways navigation.

The overlay initiative, which permits the use ofGPS to fly most existing NPA procedures, hasbeen of particular significance in achieving earlyoperational benefits from GPS. The convenienceof GPS for executing the thousands of existingVOR- and NDB-based NPA’s was madeimmediately available to suitably equippedaircraft.

In addition to the “overlay” NPA’s, the FAA ismoving aggressively to produce and publish GPS-based NPA’s for runways for which approaches donot previously exist, as well as improvedapproaches (lower minimums) for runways withexisting NPA’s. The FAA de-veloped more than500 such approaches in 1995 (of which more than100 have since been published) and plans todevelop an additional 500 in 1996. Bothnonprecision and precision approaches producedafter 1996 will be designed for GPS/WAASavionics and will not be useable by unmodifiedTSO-C129 GPS avionics. Both overlay and stand-alone approaches designed for TSO-C129 avionicswill continue to be supported until at least 2005.

The increased navigational accuracy which GPSprovides, and the ability to define routes in threedimensions, will lead to much more efficient use ofthe airspace. Climbing and descending terminalarrival and departure routes can be preciselydefined and flown, improving the efficiency ofterminal area traffic flow and better allowing theavoidance of noise-sensitive areas. Separationstandards may be reduced. Realizing fulladvantage of these capabilities will requireimproved, data-link-based air-ground communi-cations and advanced automation-based control-leraids, such as automated en route air traffic control

(AERA) and center TRACON automa-tion system(CTAS). Airspace efficiencies will thus be pacedby the availability of the new hardware andsoftware required for these systems. The goal is toprovide the aircraft operator with increasingflexibility, evolving through easily changeableuser-preferred routing with optimized climb anddescent profiles to a nearly free-flight environment.In true free-flight, the operator will be able tochoose and vary his/her route at will, subject onlyto the constraints of conflict with other aircraft andrestricted airspace.

As an early initiative in providing more efficientrouting for aircraft, the FAA is gradually reducingthe altitude above which direct routing will beroutinely approved for suitably equipped (i.e., areanavigation capable) aircraft; the goal is to reducethis altitude to flight level 290 (29,000 feet).Flight management system (FMS)-equippedaircraft with scanning-DME area navigation(RNAV) capability can already take advantage ofthese direct routes; however, many older aircraftare not so equipped. A GPS navigator is a cost-effective means to achieve the RNAV capability,much lower in cost than equipping withFMS/scanning-DME.

FAA Order 7100.10, "Air Traffic Implementa-tionPlan For The Use Of The Global Position-ingSystem,” sets forth a number of specific steps theFAA is considering to provide benefits to theairspace user. Among these are:

En route

• Restructure existing airway system toaccommodate direct routings.

• Use GPS capabilities to reduce separationstandards in the domestic en routeenvironment.

• Develop a flexible offset route capability andprocedures that will relieve saturation on high-density routes.

• Restructure special-use airspace toaccommodate a GPS-based en route system

• Establish an altitude stratum in domesticairspace designated for GPS-equipped aircraft.

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Terminal

• Establish a GPS-based terminal routestructure.

• Use GPS capabilities to reduce terminalseparation standards.

• Use GPS to identify, track, and controlaircraft and vehicles on an airport surface toan accuracy of 1 to 3 meters.

While some operational benefits can be realized assoon as a single aircraft equips with GPS, many ofthe more significant benefits depend on a highdegree of equipage and/or providing segregatedairspace for GPS-equipped aircraft. The FAA willimplement these services in a way whichencourages equipage by maximizing benefits forthe equipped user, while minimizing theoperational penalty to the unequipped user.

Oceanic Navigation

GPS provides the basis for a revolution in oceanicoperations. Currently, aircraft are restricted tominimum lateral/longitudinal separations of60/120 nm because of the limited accuracy of theavailable means of oceanic navigation—Omegaand dead-reckoning based on inertial navigationsystems—and the poor pilot-controllercommunications. With GPS, precision navigationwill be available to aircraft out of range of land-based systems. Automatic dependent surveillance(ADS), based on reporting of GPS-derived positionby satellite or high frequency data link, willprovide the oceanic controller with a radar-likedisplay of aircraft position. Over time, oceanicoperations will evolve to resemble those over land,with much reduced separations and the flexibilityassociated with operating in a surveillance-basedair traffic control (ATC) environment.

The first step in this direction was the approval inDecember 1994 of the use of GPS as a primarymeans of navigation for oceanic operations; thiscapability was first used operationally in July1995. Also 1995 saw the initial operational use inthe Pacific of the FANS-1 "package,” whichincludes GPS-augmented navigation and satellitedata-link reporting of position and will result inreduced separations and more flexible flight paths.

The reduction of lateral/longitudinal separations to50/50 nm in the South Pacific is scheduled for1997, and further reductions to 30/30 nm arescheduled for 1999. Although initial implemen-tation of these separation standards is scheduledfor the South Pacific, the reduction of separationstandards in other regions is also expected foraircraft equipped with FANS-1 capabilities.

Foreign Use

Many countries with less developed navigationinfrastructures than the United States and WesternEurope have moved rapidly to make GPS anintegral part of their air navigation systems. Aprime (but not the only) example is Fiji, which,with U.S. assistance, now bases its internal aircraftoperations entirely on GPS.

Experiments, Demonstrations, and Private Use

In addition to the operational use described above,there have been numerous experimentaldemonstrations of GPS capabilities, especially ofthe use of differential GPS for precision approachguidance. Further, several specific operators havebeen authorized to use GPS guidance forcommercial operations; these include NPA’s bymedical helicopters and NPA and departureguidance at Aspen, Colorado, by ContinentalAirlines. Several locations and operators arecurrently seeking approval for the use of industry-

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developed LADGPS systems for Special Category-I (SCAT-I) precision approach.1

3.2. Introduction of WAAS

As soon as IWAAS is achieved, WAAS willincrease the availability of navigation and NPA’sthroughout its coverage volume. The combinationof additional ranging signals and ground integritybroadcast will allow GPS/WAAS to be used as theprimary radionavigation system.

In parallel with the operational use of WAAS fornavigation, intensive testing will be carried out toverify that the accuracy of the WAAS-provideddifferential corrections is adequate for precisionapproach. It is currently expected that within 3 to6 months after IWAAS, the use of WAAS will beapproved for precision approach. Initially,minimums may be somewhat higher than normalILS minimums while both the FAA and aircraftoperators gain additional experience in its use.

The use of WAAS for precision approach requiresnot only the availability of the signal, but also theproduction and flight testing of WAAS-basedapproach procedures. Producing these proceduresrequires the acquisition of new, high-precision databases of the approach waypoints. Production ofprocedures will be initiated in 1997, with the goalthat by the year 2000 procedures will be availablefor at least all runways currently equipped withILS.

In parallel with the development and certificationof GPS/WAAS-based Category I approacheswhere ILS approaches currently exist, approaches 3 A number of aircraft operators are interested inachieving Category I GPS operations before theavailability of WAAS. To satisfy this need, the FAAhas cooperated with RTCA in the development ofminimum aviation system performance standard(MASPS) for SCAT-I systems. Systems built to theseMASPS would be procured and installed by theoperator, and their use would usually be limited to thatoperator’s aircraft. Thus, SCAT-I approaches may notbe used by the general public, but can continue to beused by the private operators even after WAAS andLAAS are deployed. It is not anticipated that SCAT-Ireceivers would be compatible with WAAS/LAAS.

will be developed and certified for runways andheliports which do not currently have precisionapproaches.2 The technical capability will exist toprovide a precision approach to essentially allqualifying runways and heliports. The develop-ment of procedures will become the pacing item inmeeting the demand for new approaches, and thecurrent FAA resources and systems available forbuilding such procedures may become quicklyoverwhelmed with the demand.

To satisfy these new requirements in a timelyfashion, and to take full advantage of the accuracyand other capabilities obtainable with satellite-based systems, instrument approach proceduredevelopment time must be reduced to keep paceand to be responsive to the demand. That places ahigh priority on the new instrument approachprocedures automation upgrade currentlyunderway. The upgrade offers significant potentialfor developing faster terminal instrument approachprocedures; the program will be aggressivelypursued to achieve full operational utility fromGPS/WAAS in a timely manner, while maintainingthe highest level of safety.

2 WAAS-based precision approaches can beimplemented quickly, and at relatively low cost, aslong as full approach lighting systems are notrequired. Deploying the approach without a standardapproach lighting system will mean that an additional1/4 nm visibility (3/4 nm visibility) will be required toexecute the approach. The FAA will develop newestablishment criteria for precision approaches withand without approach lighting systems. Airports withrunways which do not meet the establishment criteriafor a federally provided approach lighting system willhave the option of acquiring and installing such asystem using airport funds in order to obtain the lowerapproach minimums. Unless purchased with AirportImprovement Program (AIP) or Passenger FacilityCharge (PFC) funds, maintenance would be theresponsibility of the airport. If the approach lightingsystem is purchased with AIP or PFC funds, the FAAwill only assume maintenance responsibility if (1) thesystem is designed to an FAA specification, or (2) thesystem is certified to Part 171 of the Federal AviationRegulations and is or can be 100 percent supportableby the FAA Logistics Center.

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4Transition to GPS/WAAS-Based Navigation and

Landing Guidance

Today, aircraft navigation and landing guidancefunctions are provided by a multiplicity of systemsincluding VOR/DME,TACAN, Omega, Loran-C,NDB, ILS, and INS. In the United States alone,the ground-based infrastructure for navigation andfor precision approach guidance each represent abillion-dollar-level investment. In addition, thenavigation-related avionics investments in each ofthe three principal user communities—air carrier,general aviation, and military—themselvesrepresent billion-dollar-level investments. Totransition from this massive in-place infrastructure,which enjoys great user confidence based upondecades of operational experience, to a totally newsystem represents a substantial undertaking—onewhich will require a major investment of resourcesby both the service provider and the aircraftoperator. The required investment by each of thethree elements of the user community will be on theorder of a billion dollars, excluding the costs ofaircraft downtime and the retraining of air crewsand maintenance personnel.

Before such a transition can take place, threeessential prerequisites must be met:

Operational Benefit - The aircraft operator mustperceive sufficient operational benefit tomotivate the investment in the newtechnology.

System Performance - Through analyses,flight tests, and operational experience,aircraft operators must be convinced that thenew system meets their requirements foraccuracy, integrity, and reliability. This canonly be finally proven through extensiveoperational experience.

Transition Period - The aircraft operatorsmust have time to recoup their investment inconventional avionics. While many avionicssystems have been used for 15 to 20 years ormore, a transition period of approximately 10years appears to be a reasonable compromisebetween the FAA’s desire for a rapidtransition and the aircraft operator’s desire touse current equipment as long as possible.

The transition will be a three-phase process. In thefirst phase, the new system will be available on asupplemental basis. This phase allows the users to

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gain confidence through operational experience andto begin to realize substantial operational benefits,while the conventional systems are still fullyoperational. During this period even new aircraftmust still be equipped with avionics for theground-based systems.

In the second phase, the new system will becertified as primary/sole-means for navigationand/or landing guidance. During this phase boththe old and new are primary/sole-means systems;aircraft can operate with either or both. Duringthis period new aircraft could be equipped onlywith GPS/WAAS, and existing aircraft would begradually re-equipped with GPS/WAAS. The userwould no longer be required to be equipped withavionics for the ground-based systems.

Finally, in the third phase of the transition, theconventional systems will be decommissioned. Atthis point, users must be equipped withGPS/WAAS in order to operate their aircraft usingelectronic navigation.

As pointed out above, a 10-year dual-primary/sole-means period (phase 2) is felt to be areasonable compromise between the aircraftoperators’ desire to realize maximum economicbenefit from their investment in existing avionicsand the service providers’ need to decommissionexisting systems to save sustainment costs.

4.1. Transition Considerations

Before the current sole-means systems can bedecommissioned, two principal events must occur.First, aircraft must be equipped with GPS/WAAS;and second, both the ground system operators andthe aircraft operators must be convinced thatGPS/WAAS-based operation meets requiredstandards of safety and reliability. The latter issuewill be addressed through a combination ofextensive analyses, flight tests, and operationalexperience.

A number of issues need to be considered inassessing the overall ability of the system to meetaviation's needs.

Reliability

GPS/WAAS must have, and demonstrate, theoverall level of reliability needed to support civilaviation operations. This involves not onlyreliability in day-to-day operations but also ademonstrated level of redundancy to cope withfailures in elements of the system (for example,satellites). In addition, a credible plan must be inplace for system sustainment, especially satellitereplenishment.

Electromagnetic Interference

Radio-frequency interference (RFI) is a matter ofconcern in any radionavigation system. While allsystems which depend on radio transmission aresusceptible to both accidental and intentionalinterference, GPS/WAAS is especially vulnerablebecause of the very low level of signal powerreceived from the satellites.

Interference to GPS/WAAS is being evaluated bya number of agencies, including the RTCA SpecialCommittee 159. Analyses, field measurements, andoperational experience to date give confidence thatall naturally occurring (i.e., non-intentional) RFIcan be adequately suppressed at the source.Continuing analytical and experimentalinvestigations and operational experience willvalidate this conclusion well before GPS/WAAS isdesignated as a sole-means system.

A prerequisite for making GPS/WAAS a sole-means system will be to develop the ability todetect, locate, and suppress any interferencerapidly, intentional or non-intentional, which mayoccur. Procedures will also need to be in place tomaintain separation safely and recover aircraftwhich are affected by such interference during thetime between when the interference occurs andwhen it can be suppressed. This is not a newsituation; procedures are in place today to dealwith the temporary loss of a major system elementsuch as a regional radar or control facility.

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Accuracy

Some naturally occurring phenomena, especiallyionospheric disturbances, are known to affectGPS/WAAS accuracy. There is some continuingconcern related to the magnitude of this effect onthe accuracy of the WAAS during the peaks of the11-year sunspot cycle. Experience of GPS usersduring the most recent peak period of 1989/90gives confidence that these effects are manageable.However, data during the upcoming peak periodaround 2001 will be important to finalizing thesystem parameters, especially the number ofWAAS reference stations needed to maintain therequisite system accuracy during ionosphericdisturbances, and the number of local area systemsrequired to maintain the very high level ofavailability required at high capacity airports.

Operation During National Emergencies

Concern has been expressed that during a nationalemergency the DOD might use its control of GPSto deny the signal to civil users or to degrade GPSto the point where it could no longer support civilaviation. All U.S. navigation facilities, and in factall electronic emitters, are subject to control at thedirection of the National Command Authority (i.e.,the President). Ever since World War II a plan,termed SCATANA (Security Control of AirTraffic and Navigation Aids), has existed toexercise such control if needed. However, theUnited States is committed to making GPSavailable for both national and worldwide civilapplications, and only in a dire national emergency(for example, a direct attack on the United States)would it deny the availability of GPS along withany other navigation systems which could assist anattacker.

Operation during GPS Signal Disruption

The FAA's plan is that GPS/WAAS/LAAS willbecome the sole-means radionavigation andlanding guidance system; this, by definition, meansthat no back-up radionavigation system will berequired. During an extended transition period, thecurrent navigation and landing guidance systems,especially the VOR/DME and ILS, will provide a

backup while the aviation community becomesconvinced, through extensive operationalexperience, that GPS/WAAS provides the level ofavailability and integrity required of a sole-meanssystem. Only when this has been accomplishedwill the current ground-based systems bedecommissioned.

As discussed above, after VOR/DME and ILS aredecommissioned, there will need to be proceduresfor coping with a possible temporary interruptionof GPS/WAAS, for example due to the occurrenceof unintentional or intentional inter-ference. Aprincipal option under consideration is for ATC tomaintain separation of the affected aircraft usingsurveillance which is independent of GPS/WAAS(e.g. primary and/or secondary radar), vectoringthe aircraft to visual conditions or to a regionunaffected by the interference.

4.2. Projected User Equipage with GPS/WAASAvionics

Achieving widespread user equipage with GPS/WAAS avionics is critical to the transition toGPS/WAAS-based navigation and landingguidance. Only when essentially all aircraft areequipped can extensive decommissioning of currentground-based systems take place.

It is expected that for most aircraft, equipage withGPS/WAAS avionics will occur in two steps, thefirst motivated by the operational benefits ofGPS/WAAS and the second by the reducedmaintenance and training costs of beingGPS/WAAS-only equipped and by the expectedphaseout of ground-based navigation and landingguidance aids.

Almost all aircraft used regularly for IFRoperations are equipped with redundant avionics,3

both to provide a backup in the event of a failureof one of the units and for the convenience of beingable to tune to two VOR/DME's at one time.However, essentially all of the operational benefits

3 Redundant avionics are required for Part 121 (air carrier)and Part 135 (regional) operators, and optional for Part 91(general aviation) operators.

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of GPS/WAAS can be achieved with a singleGPS/WAAS receiver. It is expected, therefore,that most aircraft being retrofitted withGPS/WAAS will initially be equipped with asingle unit, with the conventional avionics left inplace as a backup both to possible failure of theon-board GPS/WAAS unit as well as to thepossible unavailability of the GPS/WAAS signal.(GPS/TSO-C129 avionics may also be retained ina backup role.) In many cases, such equipage willtake place—and provide benefits—even beforeGPS/WAAS is certified as a sole-means system.The second step in the process, equipage with dualGPS/WAAS avionics, will most likely occur onlyafter GPS/WAAS is certified as sole-means,aircraft operators are fully convinced of its abilityto serve as a sole-means system, and the time fordecommissioning of the ground-based systems isimminent.

The aircraft operator achieves several benefitsfrom this approach. First, the added cost of dualequipage is deferred for a considerable period,perhaps 5 to 10 years. Second, by deferring theacquisition of the second unit the operator has theadvantage of additional years of design maturity,likely providing additional features and/or lowercost based on the years of experience with theconstruction and use of the early units.

Until essentially all operators are dual-equipped,there will be continued dependence on theconventional ground systems as back-up. This hasbeen a major factor in determining the time-scalefor system decommissioning.

Several factors will pace the rate of equipage withGPS/WAAS avionics during both phases ofequipage. Principal among these is the aircraftoperators’ perception of the tradeoff betweenoperational benefit and cost. The rate of equipageitself has a significant effect on cost, especially for

operators of scheduled services. The cost ofhaving an aircraft out of service can be a majorpart of the equipage cost; one airline has estimateda typical cost of $35,000 per day for an aircraftout of service. Thus there is a strong desire toperform the installation of any new avionics at atime when the aircraft is already out of service formajor scheduled maintenance actions. For anairline fleet this itself can spread equipage over a4- to 6-year period.

Similar factors affecting the equipage rate forprivate and corporate aircraft include theproduction rate of avionics and the installation ratewhich the avionics service industry can support.Both manufacturing and service industries aresized to meet a relatively steady demand. Theycannot economically expand to meet a one-timepeak load, for example to equip the generalaviation fleet with GPS/WAAS avionics in a 1- to2- year period, and then revert to the size needed tosupport a reduced steady-state load. Thus, even ifthere were a demand for rapid equipage of thelarge general aviation fleet with GPS/WAASavionics, it would take a number of years to satisfythat demand.

The operational benefits of GPS/WAAS,especially increased routing flexibility and manymore precision approaches, will motivate mostoperators of aircraft used extensively for IFRoperations to equip with GPS/WAAS in the 5- to6-year period following the availability of services.Thus, with expected GPS/WAAS implementationschedules, most aircraft will be at least single-GPS/WAAS-equipped by 2005. At that point, thecurrent sole-means ground systems—VOR, DME,and ILS—will become backup systems for theseoperators. Since most aircraft will be navigatingusing GPS/WAAS, substantial reductions can thenbe made in the number of VOR/DME and ILSground facilities.

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Figure 4-1Projected Equipage with GPS/WAAS Avionics

Although a sufficient number of ground facilitieswill be maintained to allow users to complete theirflight without GPS/WAAS avionics, there may besome loss in flexibility and efficiency. Forexample, for aircraft not equipped with aGPS/WAAS receiver, the reduction in the numberof VOR/DME facilities may lead to less-directrouting, and the reduction in the number of ILSfacilities will mean that ILS-based precisionapproach may not always be available or may berestricted to a single runway at airports nowhaving multiple ILS approaches.

As operators gain increasing confidence inGPS/WAAS, and as GPS/WAAS avionics withnew features and reduced cost become available,equipage with dual GPS/WAAS avionics willincrease. By the end of the transition period—nominally 2010—all operators who requireessentially 100 percent avionics reliability will bedual-GPS/WAAS-equipped. (Just as today manyoperators who fly IFR only occasionally are notdual avionics equipped, the same can be expectedto be the case when GPS/WAAS is sole-means.)This projected equipage strategy is illustrated inFigure 4-1; as indicated above, dual-equipagenever reaches 100 percent, reflecting that someoperators who do not use their aircraft forextensive IFR operations will choose not to equipwith redundant avionics.

4.3. Phaseout of Current Systems

The following sections outline the current plans forphasing out each of the existing ground-basednavigation and landing guidance systems. Thedates indicated are consistent with the1994 FRP [5] and are based on the currentschedules for when GPS-based capabilities willprovide a level of service equivalent to the systembeing phased out. Thus, for example, the phaseoutof the current sole-means systems—VOR/DME,TACAN and ILS—would be delayedif WAAS were delayed; however, the phaseout ofOmega and Loran-C would not be affected by adelay in the introduction of WAAS, as basic GPSalready provides an equivalent level of service.

4.3.1 Navigation System Phaseout

The NAS currently provides several systems tosupport en route and terminal area navigation,including NPA. These include VOR with associ-ated DME, TACAN, NDB, Omega, Loran-C, andGPS. Omega and Loran-C are operated andmaintained by the U.S. Coast Guard. Omega isused by a limited number of aircraft for oceanicand domestic en route navigation. Loran-C iswidely used by general aviation for en route andterminal area navigation.

Single

Dual

Per

cent

Equ

ippe

d

2000 2005 2010

100

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The current inventory of the FAA-operatedsystems is shown in Table 4-1.

Table 4-1FAA-Operated Ground-Based NAVAIDS4

NAVAID TYPE INVENTORY VORTAC 640 VOR/DME 256 VOR 36 NDB 725

VOR, DME, and TACAN

VOR is the principal aircraft navigation system inthe United States and, to a large extent, the rest ofthe world. It is the basis of the current low- andhigh- altitude aviation route (airways) structure;airways usually consist of direct lines connectingthe VOR’s.

The VOR system (i.e., the combination oftransmitting station and aircraft receiver) typicallyhas an accuracy of a few degrees, resulting incross-track errors on the order of a mile at 20miles from the station. This is the principal factordefining both the width of airways and how closeadjacent independent airways can be spaced.

VOR navigation normally consists of flying theseairways. Because of the location of the VOR's,this often leads to indirect, inefficient flight pathsbetween an aircraft's origin and destination.

Of the 932 FAA-operated VOR stations in theUnited States, all but 36 have an associated DMEto allow an aircraft to determine its distance aswell as bearing from the station and thus define itstwo-dimensional position in space. By using acombination of two or more VOR’s and/or DME’sto determine position, specially equipped aircraftcan navigate off airways. This is referred to asarea navigation (RNAV) and allows more direct

4 In addition, there are approximately 100 non-FAA VORfacilities, and 1,000 non-FAA NDB’s.

routing. Most new air carrier and similarlyequipped aircraft have a flight management system(FMS) which uses multiple DME’s to determineposition; with this equipment, RNAV with aprecision of 0.3 nm or better is possible.

VOR/DME-based navigation is used for almost allaircraft navigation in the United States, includingen route, terminal area, and non-precisionapproach. Exceptions are direct high-altitudenavigation flown using Omega or on-board INS’sand a few remaining low-frequency airways inAlaska and other parts of the world defined as linesconnecting low-frequency beacons.

In addition to forming the basic en route navigationnetwork, in recent years many VOR’s have beenadded to assist in organizing arrivals anddepartures at major terminal areas. This has beennecessary because without the special avionicswhich allow RNAV capability, aircraft using VORnavigation must fly on radials to or from a VORstation. Thus, wherever a route is desired, one ormore VOR's must be installed to define it. This isone of the principal limitations of VOR navigation.

Most VOR sites are part of the airways structure;i.e., they are a navigation fix for one or moreairways. Many of these also provide NPAguidance to nearby airports. In addition, there area small number of VOR’s whose function is solelyas an NPA aid.

TACAN is functionally similar to VOR/DME.Both use the same ranging component (DME), butthe TACAN azimuth component operates in adifferent radio-frequency band than VOR.TACAN is widely used by DOD aircraft; in fact,most fighters and bombers are not VOR-equippedand depend on TACAN for airway navigation andNPA. The FAA-operated VORTAC’s combineVOR, DME, and TACAN in a single facility.

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Phaseout of VOR and DME

Maintaining the current VOR/DME system isexpensive. To provide the current level of service,the equipment costs are estimated at $139 millionover the next 10 years, and the operations andmaintenance costs are estimated at $80 million peryear. Thus, there is considerable financialincentive to reduce the number and ultimatelyphase out VOR/DME.

However, until GPS/WAAS is approved as aprimary means of navigation in the NAS(estimated to occur by 1998/99), all aircraft whichwish to operate under IFR will have to be equippedwith the avionics for VOR navigation. This ingeneral requires at least two VOR receivers andfrequently one or more associated DME’s. Theaircraft operator will want to be able to use thisequipment for a reasonable service life beforebeing forced to re-equip.

As soon as GPS/WAAS avionics are available,operators are anticipated to begin equipping with itto achieve the associated operational benefits andconvenience. Because of its accuracy andflexibility, GPS/WAAS will be the navigation aid

of choice. An operator who equips or re-equips anaircraft during this period is likely to equip withone GPS/WAAS system in addition to retainingone or two conventional VOR system(s). Thelatter will allow completing a flight in the event ofa temporary unavailability of GPS/WAAS, albeitwith less convenience. But to do this, the VORground environment must still be in place. Theconventional system is now relegated to the role ofbackup. Even when GPS/WAAS becomescertified as sole-means, decommissioning of theVOR ground environment would require theaircraft to have dual GPS/WAAS equipage tomaintain avionics redundancy.

The basic phaseout strategy will be to gain, asquickly as possible, the cost savings from reducingthe number of VOR facilities while at the sametime minimizing adverse financial impact onaircraft operators. A transition period ofapproximately 10 years during which both VORand GPS/WAAS can be used as a sole means ofnavigation is viewed as a reasonable compromisebetween the FAA's desire to minimize its cost formaintaining and replacing VOR and the aircraftoperators’ desire to get maximum utilization fromtheir investment in conventional avionics.

Figure 4-2Phaseout of VOR/DME

1995 2000 2005 2010

VOR/DME

GPS Supplemental Primary Sole-Means

DecommissionedNum

ber

of V

OR

/DM

E

IWAAS EWAAS

Sole-Means Primary Means

4-8

For the first 5 years of this 10-year period, theVOR/DME system will be maintained at its fullcapability. In the second 5 years, VOR/DMEfacilities will be selectively phased out in such away that aircraft operators will still be able tocomplete their flight using VOR-based navigation,but with some efficiency penalty. This willincentivize the frequent operator to equip withGPS/WAAS, while minimizing the financialpenalty to the occasional system user. Forexample, the removal of selected VOR's interminal or en route environments would mean thatthe VOR-only-equipped aircraft would need tofollow more circuitous routes than theGPS/WAAS RNAV-equipped aircraft, but theformer would still be able to get from origin todestination.

Some VOR’s which are still in relatively goodcondition when decommissioned could be used toreplace critical stations which have reached the endof their service life and are no longer maintainable.

At the end of the transition period (nominally2010), remaining VOR/DME facilities will berapidly phased out. Since it is expected that therewill be few or no new installations of VOR/DMEavionics following the time when GPS/WAAS isdeclared sole-means, all such avionics will by thattime have had a service life of at least 10 years.

Throughout the decommissioning period, the FAAwill work closely with aircraft and airportoperators to minimize financial impact. Theimpact on individual operators will be balancedagainst the financial cost to the system as a whole,recognizing that the aviation system is ultimatelypaid for primarily by its users.

Phaseout of TACAN

The TACAN equipment at the FAA-operatedVORTAC’s (especially the rotating antenna) isexpensive to operate and maintain. The FAA isworking with the DOD to decommission theTACAN azimuth component of as manyVORTAC’s as possible while still supporting theDOD's operational requirements. The remainingVORTAC’s will be operated until 2005, by whichtime all DOD aircraft are expected to be GPS-equipped.

Nondirectional Beacons

NDB’s serve two principal functions in the NAS:first, as a stand-alone NPA aid at small airports;and second, as a compass locator, generallycollocated with the outer marker of an ILS to assistpilots in getting on the ILS course in a non-radarenvironment. Currently there are 232 NDB’s inthe first category and 493 in the second. Almostall of the approximately 1,000 non-FAA NDB’sare in the first category, i.e., they are stand-alonefacilities to support NPA’s.

In addition to these uses of NDB’s, a few are usedin Alaska to define low frequency airways.Because of this heavy reliance on NDB’s inAlaska, a separate transition plan will bedeveloped for Alaskan airspace which considers itsunique operating environment.

To make use of an NDB for en route navigation orNPA guidance requires an automatic directionfinder (ADF) in the aircraft.

NDB’s are a relatively low-cost navigation aid.The typical cost for an FAA-installed NDB usedas an approach aid at a small airport is $100,000.In many cases, NDB’s have been purchased andinstalled by a community and then turned over tothe FAA for maintenance. The annual sustain-ment cost of the existing system is estimated to beapproximately $9 million.

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Phaseout of NDB’s

As a result of the overlay program, GPS today cansubstitute for an NDB in carrying out an NPA.Thus, the overlapping transition period betweenGPS and NDB can be considered to have begun inFebruary 1994.

The phaseout strategy for NDB’s will be tomaintain the current level of capability through theyear 2005, decommissioning prior to that dateonly redundant facilities where essentiallyequivalent capability is provided by VOR. Afterthe year 2005, the remaining stand-alone NDB’swill be rapidly phased out. However, in each case,through consultation with the user community,aircraft operator desires for continued NDBservice will be weighed against the cost ofcontinuing to provide that service. There may becases where operation and maintenance of an NDBwill be taken over by an individual operator orcommunity desiring to delay its phaseout.

NDB’s required as the compass locator for ILSapproaches where no equivalent ground-basedmeans for transition to the ILS course exists, willbe maintained until the underlying ILS is itselfphased out, as discussed below.

Omega

Omega is a long-range navigation system operat-ing in the very-low-frequency band. Eighttransmitting stations radiate signals between 10.2and 13.6 kHz. A receiver determines its positionbased on the phase differences between the variousreceived signals.

The Omega stations are located in Norway,Liberia, North Dakota, Hawaii, La Reunion Island,Argentina, Australia, and Japan. The U.S. CoastGuard operates the two U.S.-based stations.

Omega provides two-dimensional position accu-racy of 2 to 4 nm with an availability of 99 percentand is approved for long-range navigation and asan RNAV system. Omega may be used as a sole-means system (i.e., the only installed long-rangenavigation system) for class II navigation(commonly referred to as operation in oceanic andremote airspace). It may also be used as asupplemental system for RNAV in U.S. domesticairspace.Omega is used for navigation by approximately1,400 air carrier aircraft (600 U.S. and 800foreign) and for the tracking of radiosondeballoons by international weather services.

1990 1995 2000 2005

NDB

GPS

Num

ber

of F

AA

Ope

rate

d N

DB

s

IWAAS EWAAS2010

Sole-Means

Sole-Means

PrimaryMeans

Approved for NPA Overlay Primary

Decommissioned

Figure 4-3Phaseout of NDB’s

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Phaseout of Omega

The United States plans to discontinue support forOmega at the end of 1997 [5]. Australia is alsoplanning to terminate operations at its Omegastation on September 30, 1997. Since GPS cannow substitute for the long-range RNAV functionof Omega, there are no plans to continue operatingthe Omega system after that date.

Loran-C

Loran-C is a navigation system installed, operated,and maintained by the U.S. Coast Guard. Loran-Cwas first installed primarily to serve maritimeusers in coastal and harbor areas (as well asmilitary operations), but in recent years it hasgained widespread use by general aviation aircraft.To serve the needs of general aviation, Loran-Ccoverage has been extended so that it now coversthe entire United States except for Hawaii andparts of Alaska.

Loran-C is a low-frequency navigation system inwhich the receiver determines its position bymeasuring the time-of-arrival differences fromsignals received from several (three or more)ground stations. It provides substantially greateraccuracy than VOR/DME, typically 0.3 mileswithin its primary coverage area. Because itscoverage is not limited to line-of-sight transmis-sion, it provides better coverage than VOR forlow-altitude general aviation and helicopteroperations. Since users can determine theirposition anywhere within the coverage area, Loran-C is inherently an RNAV system.

Loran-C is widely used by general aviation aircraftfor its convenience and accuracy; however, it hasnever been a primary- or sole-means navigationsystem in the NAS. With IFR-certified avionics(most general aviation Loran-C avionics are notIFR-certified), it is approved for supplemental IFRnavigation but not for NPA.

Phaseout of Loran-C

The annual operating cost of Loran-C isapproximately $18 million. In addition, since

much of the ground station equipment is nearingthe end of its useful life, operation beyond the year2000 would require extensive refurbish-ment,estimated to cost more than $100 million in capitalinvestment over a 10-year period.

GPS can already provide the aircraft operator withall of the functions of Loran-C (plus, withappropriately certified equipment, NPAcapability). As indicated above, most of theapproximately 130,000 Loran-C receiversestimated to be in use in general aviation are VFR-only units, functionally equivalent to the low-costnon-TSO GPS receivers (hand-held or panel-mounted). Ever since GPS avionics have beenavailable at comparable cost and greatercapability, there has been little or no new equipagewith Loran-C. The United States intends todiscontinue Loran-C service in the year 2000 [5].This will give aircraft operators generally 10 ormore years to amortize their investment in Loran-Cavionics before they are no longer functional (mostof the Loran receivers in service were installedbefore 1990).

Global Positioning System

As described earlier, GPS is experiencingincreasing use in the NAS as a supplementalnavigation system, using avionics certified underTSO-C129, Airborne Supplemental NavigationEquipment Using the Global Positioning System.TSO-C129 defines several classes of GPS avionicswhich provide different levels of service, from enroute navigation through NPA. When certainadditional approval criteria are met (as defined inNotice 8110.60, GPS as a Primary Means ofNavigation for Oceanic/Remote Operations, dated12/04/95), GPS can be certified for use as aprimary navigation system in oceanic airspace.

Phaseout of GPS Avionics

GPS avionics which only meet the requirements ofTSO-C129 can never be approved for general useas a sole-means system in the NAS and cannotprovide precision approach guidance. Therefore itis intended that the use of these avionics, and theassociated NPA’s, will be phased out, to be

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replaced by GPS/WAAS avionics and approaches.(It is expected that it will be possible to upgradesome, but probably not all, TSO-C129 avionics tomeet GPS/WAAS requirements.) Non-upgradedTSO-C129 avionics will continue to be usefulindefinitely as a backup to GPS/WAAS avionicsfor en route and terminal navigation, and for NPAuntil the TSO-C129-based approaches are decom-missioned. It is currently planned to maintainthese approaches at least through 2005. After2005, GPS overlay approaches will be canceledwhen the associated ground-based navaid (VOR orNDB) is decommissioned; a GPS/WAAS NPAwill be provided in its place. (If the associatedground-based navaid for an overlay approach isdecommissioned before 2005, a stand-aloneGPS/TSO-C129 approach will be provided forthat airport.) Also after 2005, stand-aloneGPS/TSO-C129 approaches will gradually bedecommissioned; in all cases, a substituteGPS/WAAS approach will have beencommissioned prior to decommissioning the TSO-C129 approach. TSO-C129 approaches, overlayand stand-alone, will be decommissioned by 2010.

Inertial Navigation System

The Inertial Navigation Systems are self-containednavigation systems which use onboard gyros andaccelerometers to measure precisely the changes inaircraft speed and direction. At the beginning ofeach flight the pilot initializes the INS with theaircraft’s exact location. Based on its continuousmeasurement of the aircraft’s speed and direction,the INS continuously computes the aircraft’sposition. The positional accuracy of an INSdegrades with time at the rates from approximately0.25 to 2.0 nm per hour. As in the case of Omega,INS is approved as a sole-means navigation systemfor oceanic operation and as a supplemental meansfor domestic en route navigation.

With the introduction of GPS, the role of INSshifts from navigation to flight control. Since thisrelaxes the requirements for very low drift, it willallow the use of lower-cost Inertial ReferenceSystems (IRS). Several aircraft operators haveindicated their intent to replace existing INS with

GPS because of the high maintenance cost of olderinertial systems.

4.3.2 Precision Approach and Landing SystemPhaseout

A precision approach and landing system is onewhich provides a landing aircraft with electronicvertical as well as horizontal guidance. ILS is thecurrent worldwide standard for precision approachand landing. ILS provides lateral guidance by afixed "localizer" beam transmitted at a VHFfrequency (in the band 108-112 MHz) and verticalguidance by a fixed "glideslope" beam transmittedat a UHF frequency (in the band 328.6-335.4MHz).

Because of operational and technical limitations ofILS (especially frequency congestion, interferencefrom adjacent broadcast services, and sitingdifficulties), a microwave landing system (MLS)was developed in the 1970's as a replacement forILS. MLS was designated by ICAO to be the newworld standard for precision landing, beginning in1998. However, because of the reluctance of bothservice providers and aircraft operators to equipwith MLS (because of its high cost), and theadvent of satellite-based guidance technology, theUnited States recom-mended at the ICAOCommunications/Opera-tions Divisional meeting inthe spring of 1995 that the mandatory transition toMLS by 1998 be repealed and that ILS be retainedas an alternate until satellite-based precisionlanding technology could be fully evaluated. Thisrecommendation was adopted and will lead to thecontinuation of ILS for several years, with thegradual introduction of GPS/WAAS-basedprecision approach as it becomes available. In themeantime, the United States has canceled itsprogram to develop MLS systems.

There are three categories of ILS, differing in theirassociated landing minimums, expressed asdecision height and visibility (or runway visualrange (RVR)) requirements. Decision height is theheight above a runway to which a pilot candescend by electronic guidance, after which thepilot must be able to complete the approach andland visually. Visibility or RVR is a measure of

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the current "seeing" conditions, determined eitherby visual observation by a tower controller(visibility) or by measurement with electronicinstrumentation beside the runway RVR.

The current inventory of ILS and MLS systemswithin the United States is shown in Table 4-2.Although the United States has canceled its MLSdevelopment program, there are currently 26Category I MLS systems under contract. Thecurrent plan is to deploy these systems to providean interim capability to meet special needs atselected airports, pending the availability ofGPS/WAAS.

While costs vary somewhat from site to site, thetypical cost to procure and install a Category I ILSsystem at an airport is $0.8 million and CategoryII/III ILS is $1.1 million. These costs are for theILS electronics, associated monitoring systems,and RVR’s as required. In addition, approachlighting systems are required, which themselvescost (including installation) on the order of $0.4million for a Category I approach and $0.9 millionfor a Category II/III approach. Note that in thecase of Category I ILS, an approach lightingsystem is required only to achieve the fullcapability of the Category I ILS

5 Plus about 200 non-Federal systems.6 Includes planned installations.

(200 foot decision height, 1/2 mile visibility).Without an approach lighting system the standardvisibility minimums are raised from 1/2 milevisibility to 3/4 mile visibility.

The operation and maintenance costs for the cur-rently installed ILS systems total approximately$80 million per year.

Phaseout of Category I ILS

Up to the time (estimated to be the year 2001) thatWAAS is certified as a sole-means approach aidand approaches exist for essentially all airportswhich have Category I ILS, aircraft needingprecision approach capability will have to beequipped with ILS receivers. That time will thenbe the starting point for an approximately 10-yeardual-sole-means transition period. As in the caseof VOR/DME, essentially all Category I ILS’s willremain in service until 2005. After that date it willbe assumed that most IFR aircraft are at leastsingle-GPS/WAAS equipped, and Category IILS’s will begin to be decommissioned; enoughwill be retained, however, to serve as a backup incase of failure of the single GPS/WAAS avionicsunit. In 2010, the remaining Category I ILS’s willbe rapidly phased out. This transition strategy isdepicted in Figure 4-4.

Table 4-2FAA-Operated Precision Approach Systems

NAVAID TYPE INVENTORY

ILS (CAT I) 8715

MLS (CAT I) 296

ILS (CAT II/III) 80

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In the years prior to 2001, Category I ILS will beinstalled at newly qualifying runways only if thereis clear indication that the benefits to be realizedby 2005 exceed the costs. It is not expected thatthere would be any deployment of new Category IILS systems once GPS/WAAS is available tosupport Category I approaches. During the dual-sole-means transition period, ILS’s may bedecommissioned where redundant: for example, ifthere are multiple ILS’s at an airport, or wherethere is an indication of so little use that it is notcost-effective to maintain. As in the case of VOR,the strategy will be to minimize sustainment costsconsistent with continuing to provide the capabilityfor the aircraft operators to complete their flights.

Phaseout of Category I MLS

The Category I MLS systems will be phased outon a schedule similar to that of the Category I ILS:decommissioning beginning in 2005, with allsystems decommissioned by 2010.

Phaseout of Category II/III ILS

The date when GPS-based Category II/IIIapproaches will become available for public use isless certain. Extensive testing has demonstrat-edthe ability of LAAS to meet Category II/IIIaccuracy requirements. Analyses and field tests arecurrently in progress to demonstrate that theintegrity requirements can also be achieved.Following this, several years will be required toselect among the available techniques and developand certify an operational system.

Until certified GPS-based systems are available,the FAA plans to meet Category II/IIIrequirements with ILS. This will entail sustainingthe existing Category II/III systems and providingnew systems to meet the requirements forupgrading the capability of a system (Category I toCategory II or Category II to Category III), and fornew establishments. Upgrades and new systemestablishments will be done only if cost-benefitanalyses indicate that they are cost beneficial,given the expected availability of GPS-basedCategory II/III approaches by 2005 and theprojected 2010 ILS decommissioning date.

1995 2000 2005 2010

ILS

GPS

Decommissioned

Number of ILS Number of CAT IGPS Approaches

IWAAS EWAAS

Sole-Means

Sole-MeansPrimary

Primary Means

Figure 4-4Phaseout of Category I ILS

4-14

5Summary

GPS, augmented by WAAS and LAAS, offerssubstantial benefits both to aircraft operators andto the FAA. For the aircraft operators, the benefitsinclude increased operational efficiency and safetyand reduced equipage, maintenance, and trainingcosts associated with having a single system fornavigation and landing guidance rather than themultiplicity of systems required today to performthose functions. For the FAA, the primary benefitscome from eliminating the sustainment costs of theexisting ground-based navigation and landingguidance systems, which today total approximately$200 million per year just for operations andmaintenance (O&M) compared to a projectedO&M cost of about $80 million per year forWAAS/LAAS. Realizing these benefits requiresthat the WAAS and LAAS be developed andfielded, that aircraft be equipped with GPSavionics, and that the ground-based systems bedecommissioned.

This document has described the FAA’s plan toaccomplish this transition. The plan represents abalance, or compromise, between the aircraftoperators’ desire to get maximum return oninvestment in avionics for the existing ground-based systems, and the FAA’s desire todecommission the ground equipment for thesesystems as rapidly as possible to minimizesustainment costs. For the primary navigation andlanding guidance systems, the plan has beendesigned around a 10-year transition period fromthe time that a service is available from GPS untilthe corresponding ground-based system isdecommissioned. For the first half of thistransition period, the ground-based system ismaintained at full functionality; during the secondhalf, functionality is reduced commensurate withthe reduced number of users, but sufficientfunctionality is retained to permit continuedoperation by aircraft which are not yet equippedwith GPS.

REFERENCES

1. GPS Implementation Plan for Air Navigation and Landing, Federal Aviation Administration, August 1994

2. Operational Requirements Document: Wide Area Augmentation System (WAAS), Federal AviationAdministration, June 10, 1994.

3. Operational Requirements Document: Local Area Augmentation System (LAAS), Federal AviationAdministration, February 28, 1995.

5-2

4. Minimum Operational Performance Standard for Global Positioning System/Wide Area Augmentation SystemAirborne Equipment, RTCA Document No. DO-229, January 16, 1996.

5. 1994 Federal Radionavigation Plan, Document Number DOT-VNTSC-RSPA-95-1/DOD-4650.5, NationalTechnical Information Service, Springfield, VA 22161.

A-1

Appendix AAcronyms

ADF Automatic Direction Finder

ADS Automatic Dependent Surveillance

AERA Automated En Route Air Traffic Control

ATC Air Traffic Control

CAT Category

CTAS Center TRACON Automation System

DME Distance Measuring Equipment

DOD Department of Defense

EMC Electromagnetic Compatibility

EWAAS End-State WAAS

FAA Federal Aviation Administration

FANS Future Air Navigation System

FMS Flight Management System

FOC Full Operational Capability

FRP Federal Radionavigation Plan

GBIB Ground-Based Integrity Broadcast

GNSS Global Navigation Satellite System

GPS Global Positioning System

HMI Hazardously Misleading Information

ICAO International Civil Aviation Organization

IFR Instrument Flight Rules

ILS Instrument Landing System

INS Inertial Navigation System

IOC Initial Operational Capability

IRS Inertial Reference Systems

A-2

IWAAS Initial WAAS

LAAS Local Area Augmentation System

LADGPS Local Area Differential Global Positioning System

MASPS Minimum Aviation System Performance Standard

MLS Microwave Landing System

MOPS Minimum Operational Performance Standard

NAS National Airspace System

Navaid Navigation Aid

NDB Nondirectional Beacon

nm Nautical Mile

NPA Nonprecision Approach

PPS Precise Positioning Service

RAIM Receiver Autonomous Integrity Monitoring

RFI Radio Frequency Interference

RNAV Area Navigation (Radio)

RNP Required Navigation Performance

RVR Runway Visual Range

SCAT-I Special Category I

SCATANA Security Control of Air Traffic and Navigation Aids

SPS Standard Positioning System

TACAN Tactical Air Navigation

TRACON Terminal Radar Control

TSO Technical Standard Order

U.S. United States

UTC Coordinated Universal Time

VFR Visual Flight Rules

VHF Very High Frequency

VOR VHF Omnidirectional Range

VORTAC Collocated VOR and TACAN

WAAS Wide Area Augmentation System

B-1

B-2

Appendix BDefinitions

Area Navigation (RNAV) - A method of navigation that permits aircraft operations on any desired coursewithin the coverage of station-referenced navigation signals or within the limits of self-contained systemcapability.

Autoland Approach - A precision instrument approach to touchdown and, in some cases, through thelanding rollout. An autoland approach is performed by the aircraft autopilot which is receiving positioninformation and/or steering commands from onboard navigation equipment.

Category I (CAT I) precision approach - A precision approach procedure which provides for approachto a height above touchdown of not less than 200 feet and with runway visual range of not less than 2,400feet (with touchdown zone and centerline lighting 1,800 feet Category A,B,C; 2,000 feet Category D).

Category II (CAT II) precision approach - A precision approach procedure which provides for approachto a height above touchdown of not less than 100 feet and with runway visual range of not less than 1,200feet.

Category III (CAT III) precision approach - A precision approach procedure which provides forapproach without a decision height minimum and:

IIIA - with runway visual range of not less than 700 feet.IIIB - with runway visual range of not less than 150 feetIIIC - without runway visual range minimum

Differential - A technique used to improve radionavigation system accuracy by determining positioningerror at a known location and subsequently transmitting the determined error, or corrective factors, to usersof the same radionavigation system, operating in the same area.

End-State WAAS (EWAAS) - Final stage of WAAS which is capable of supporting navigation andCategory I precision approach with internal redundancy and guaranteed availability in the event of failureof elements in the system.

En Route - A phase of navigation covering operations between a point of departure and termination of amission. For airborne missions the en route phase of navigation has two subcategories, en route domesticand en route oceanic.

Full Operational Capability (FOC) - For GPS, this is defined as the capability that occurred when 24GPS satellites (Block II/IIA) were operating in their assigned orbits and were tested for militaryfunctionality and met military requirements.Initial Operational Capability (IOC) - For GPS, this is defined as the capability that occurred when 24GPS satellites (Block I/II/III) were operating in their assigned orbits and were available for navigation use.

B-3

Initial WAAS (IWAAS) - Initial stage of WAAS which is capable of supporting navigation and categoryI precision approach but lacks internal redundancy and guaranteed availability in the event of failure ofelements in the system.

Minimum Aviation System Performance Standard (MASPS) - A set of standards that specifycharacteristics that should be used to designers, installers, manufacturers, service providers, and users forsystems intended for operational use within the United States National Airspace System

Minimum Operational Performance Standard (MOPS) - A set of standards that define minimumperformance, functions, and features for Area Navigation (RNAV), and optionally Vertical Navigation(VNAV) equipment to be certified in order to serve in the NAS.

Nonprecision Approach (NPA) - A standard instrument approach procedure in which no electronic glideslope is provided.

Precision Approach - A standard instrument approach procedure in which a course and glideslope/glidepath are provided.

Primary Means of Navigation - A navigation system approved for a given operation or phase of flight thatmust meet accuracy and integrity requirements, but need not meet full availability and continuity-of-servicerequirements. Procedural restrictions apply to the given phase of flight since there is no requirement tohave a sole-means system onboard to support the primary system.

Pseudorange - The distance between a user and a ground-based and/or space-based signal source plus anunknown user clock offset distance.

Required Navigation Performance (RNP) - A statement of the navigation performance accuracynecessary for operation within a defined airspace, including the operating parameters of the navigationsystems used within that airspace.

RTCA, Inc. - An association of aeronautical organizations of the United States from both Government andindustry that seeks sound technical solutions to problems involving the application of electronics andtelecommunications to aeronautical operations.

Sole Means of Navigation - An approved navigation system for a given operation or phase of flight thatmust allow the aircraft to meet, for that operation or phase of flight, all four navigation system performancerequirements: accuracy, integrity, availability, and continuity of service.

Supplemental Means of Navigation - An approved navigation system that can be used in controlledairspace of the NAS in conjunction with a sole means of navigation.

Terminal - A phase of navigation covering operations required to initiate or terminate a planned mission orfunction at appropriate facilities. For airborne missions, the terminal phase is used to describe airspace inwhich approach control service or airport traffic control service is provided.

Technical Standard Order (TSO) - A set of standards that avionics must meet in order to be identifiedwith the applicable TSO marking.