3

**Radionavigation System Use**

This section summarizes the plans of the Federal Government to provide
general-purpose and special-purpose radio aids to navigation for use by
the civil and military sectors. It focuses on three aspects of planning:
(1) the efforts needed to maintain existing systems in a satisfactory
operational configuration; (2) the development needed to improve
existing system performance or to meet unsatisfied user requirements in
the near term; and (3) the evaluation of existing and proposed
radionavigation systems to meet future user requirements. Thus, the plan
provides the framework for operation, development, and evolution of
systems.

The Government operates radionavigation systems that meet most of the
current and projected civil user requirements for safety of navigation,
promotion of reasonable economic efficiency, and positioning and timing
applications. These systems are adequate for the general navigation of
military craft as well, but none completely satisfies all the needs of
military missions or provides highly accurate, three-dimensional,
worldwide navigation capability. GPS satisfies many of these general and
special military requirements. GPS has broad potential for satisfying
current civil user needs or for responding to new requirements that
present systems do not satisfy. It could ultimately become the primary
worldwide system for military and civil navigation and position
location.

**3.1 Existing Systems Used in the Phases of Navigation**

It is generally accepted that the needs for navigation services derive
from the activities in which the users are engaged, the locations in
which these activities occur, the relation to other craft and physical
hazards and, to some extent, the type of craft. Because these
differences exist, navigation services are divided by classes or types
of users and the phases of navigation. Detailed descriptions of the
existing and proposed radionavigation systems are given in Appendix C.
Estimates of the current numbers of users of Federally provided
radionavigation systems are provided in Figure 3-1.

The following sections describe the approach employed to define the
needs, requirements, and degree to which existing systems satisfy user
needs.

***3.1.1 Air Navigation***

VOR/DME forms the basis of a safe, adequate, and trusted international
air navigation system, and there is a large investment in ground
equipment and avionics by both the Government and users. In view of
this, it is intended to maintain the VOR/DME system at its present
capability for a reasonable transition period for those systems being
phased out after augmented GPS SPS is approved as a primary navigation
system for domestic en route, terminal, nonprecision approach, and
precision approach phases of flight.

As evidenced by user conferences and aircraft equipage, there is
increasing interest and usage of GPS and Loran-C for air navigation.
Both systems are certified as supplemental systems. In 1994, unaugmented
GPS was also approved as a primary system for use in oceanic and remote
airspace. Incremental improvements to WAAS will allow the termination of
many existing ground-based radionavigation aids after an adequate
transition period to allow users to equip with WAAS avionics.

***Oceanic En Route:*** Oceanic en route air navigation is currently
accomplished using inertial reference system/flight management
computers, inertial navigation systems (INS), Loran-C, GPS, or a
combination of these systems. Use of Doppler and celestial navigation
are also approved. Use of VOR/DME, TACAN, and Loran-C is approved where
there is adequate coverage.

***Domestic En Route:*** Domestic en route air navigation services are
presently being provided, except in some remote and offshore areas. The
basic short-distance aid to navigation in the U.S. is VOR alone, or
collocated with either DME or TACAN to form a VOR/DME or a VORTAC
facility. This system is used for en route and terminal navigation for
flights conducted under Instrument Flight Rules. It is also used by
pilots operating under Visual Flight Rules. Loran-C and inertial systems
are also used for domestic en route navigation. When inertial systems
are used, their performance must be monitored through the use of an
approved externally referenced radio aid to navigation. Loran-C and GPS
both are approved as supplemental systems. GPS is also approved as a
primary system for use in remote areas, and distance information based
on GPS can be used to provide separation between aircraft in accordance
with current DME standards.

***Terminal:*** Terminal air navigation services are presently provided
using VOR, VOR/DME, VORTAC, TACAN, NDB, GPS, or Loran-C. Loran-C and GPS
are approved as supplemental systems.

![](media/image1.wmf)

**Figure 3-1. Estimated Current U.S. Radionavigation System User
Population**

***Approach and Landing:*** Nonprecision approach navigation services
are presently being provided using ILS localizer, VOR, VOR/DME, VORTAC,
TACAN, GPS, or NDB. GPS is approved as a supplemental system. Presently,
precision approach and landing requirements are met by ILS (Categories
I, II, and III) and MLS (a limited number of Category I systems only).

***3.1.2 Marine Navigation***

Marine navigation comprises four major phases: inland waterway, harbor
entrance and approach, coastal, and oceanic. The phase of navigation in
which a mariner operates determines which radionavigation system or
systems will be the most useful. While some radionavigation systems can
be used in more than one phase of marine navigation, the most promising
system to meet the most stringent requirements of the harbor entrance
and approach and inland waterway phases of marine navigation is DGPS.
With regard to the coastal phase of navigation, DGPS will provide the
navigation features currently being met by Loran-C as it is used in the
repeatable mode of navigation.

***Inland Waterway Phase:*** This phase of navigation is concerned
primarily with those vessels that are not oceangoing. Specific
quantitative requirements for navigation on rivers and other inland
waterways have been developed. Visual and audio aids to navigation,
radar, and intership communications are presently used to enable safe
navigation in those areas. However, DGPS is expected to play an
increasing role in this phase of navigation.

***Harbor Entrance and Approach Phase:*** Navigation in the harbor
entrance and approach areas is accomplished through use of fixed and
floating visual aids to navigation, radar, and audible warning signals.
The growing desire to reduce the incidence of accidents and to expedite
movement of traffic during periods of restricted visibility and ice
cover has resulted in the implementation of VTS along with AIS in
certain port areas and investigation of the use of radio aids to
navigation. DGPS coverage includes all coasts of the continental U.S.
and parts of Alaska, Hawaii, and the Great Lakes. The system provides
better than 10 meter accuracy.

***Coastal Phase:*** Navigation service for operation within the coastal
area is provided by Loran-C, GPS and DGPS. Radio Direction Finders
(RDF), required in some merchant ships by international agreement for
search and rescue purposes, are also used with the radiobeacon system
for navigation.

***Ocean Phase:*** Navigation on the high seas is accomplished by the
use of dead-reckoning, celestial fixes, self-contained navigation
systems (e.g., inertial systems), Loran-C and GPS. GPS is now the system
of choice. Worldwide coverage by most ground-based systems such as
Loran-C is not practicable.

***3.1.3 Space Applications***

There are numerous uses of GPS for space navigation; many are discussed
in Section 2. Several spacecraft including the ISS, the Space Shuttle,
and numerous small satellites are using or will be using GPS for
navigation. Some of these spacecraft will use GPS for support of
instrument pointing, scientific data processing and, in the case of
Space Shuttle and Reusable Launch Vehicles, for re-entry and landing as
well as during orbital operations. The private sector is also
implementing the use of GPS in space applications such as low Earth
orbiting communication satellites and Earth sensing satellites.

***3.1.4 Land Navigation***

GPS, in conjunction with other systems, is used in land vehicle
navigation. Government and industry have sponsored a number of projects
to evaluate the feasibility of using existing and proposed
radionavigation systems for land navigation. Operational tests have been
completed that use in-vehicle navigation systems and electronic mapping
systems to provide real-time route guidance information to drivers. GPS
is used for automatic vehicle location for bus scheduling and fleet
management. Operational tests are either planned or in progress to use
radionavigation for route guidance, in-vehicle navigation, providing
real-time traffic information to traffic information centers, and
improving emergency response. Several transit operational tests will use
automatic vehicle location for automated dispatch, vehicle re-routing,
schedule adherence, and traffic signal pre-emption. Railroads have
tested and continue to test GPS and DGPS as a part of positive train
control systems for freight as well as high-speed passenger train
operations. GPS and dead-reckoning/map-matching are being developed as
systems that take advantage of radionavigation systems and at the same
time improve safety and efficiency of land navigation.

***3.1.5 Uses Other Than Navigation***

These uses are concerned primarily with the application of GPS for
geodesy and surveying, positioning in support of mapping, charting, and
geographical information systems, monitoring of Earth motions,
meteorological parameter determination position, and time and frequency
determination. Users with these applications represent a large
percentage of the GPS user community and involve all levels of
government, academia, and industry. Many of the products supported by
these applications are those traditionally provided by the Federal
government. These include the National Spatial Reference System,
nautical and aeronautical charts, weather prediction, earthquake
studies, and inland waterways management. In the Inland Waterways,
Harbor Entrance and Approach and Coastal Phases, DGPS is being used
extensively by the USCG to position floating aids as well as fixed aids
to navigation. Additionally, the USACE is using DGPS to conduct
surveying, aid positioning, dredging operations, and revetment
maintenance.

Many applications of GPS and augmented GPS are anticipated for Federal,
state, and local governments, industry, and consumers. The Government
does not have a responsibility under law to provide radionavigation
systems for these users. However, these applications represent a large
(and growing) percentage of the civil radionavigation user community and
are recognized in the radionavigation planning process.

**3.2 Existing and Developing Systems - Status and Plans**

Figure 3-2 shows the operating plans for Federally provided common-use
radionavigation systems.

***3.2.1 Global Positioning System (GPS)***

GPS is a space-based positioning and navigation system designed to
provide worldwide, all weather, passive, three-dimensional position,
velocity, and time data.

A. User Community

The GPS user community has grown exponentially in the past two years and
that growth is expected to continue. Rapid growth has occurred in all
modes of transportation. Non-transportation use is also growing at a
rapid rate and includes users employed in surveying, farming, resource
exploration, and law enforcement. The GPS signal, as defined in the
Standard Positioning Service Signal Specification (Ref. 10), is designed
to support civil GPS applications. The GPS PPS is restricted to U.S.
Armed Forces, U.S. Federal agencies, and selected allied armed forces
and governments. These restrictions are based on national security
considerations.

B. Operating Plan

GPS will be the primary Federally provided radionavigation system for
the foreseeable future. In certain regions of the world, GPS will be
augmented to satisfy additional civil requirements for accuracy,
coverage, availability, and integrity. The GPS constellation is
configured and operated to provide the SPS signals to civil users in
accordance with the GPS Standard Positioning Service Signal
Specification (Ref. 10). The DOD will maintain a 24-satellite
constellation. Replacement satellites will be launched on an expected
failure strategy (a replacement satellite is launched when there are
indications that a satellite should be replaced).

The DOD and DOT have agreed that representatives from the DOT will be
located within the Master Control Station (MCS) and at the GPS Joint
Program Office to participate in the day-to-day system operations,
system development, and future requirements definitions.

Any planned disruption of the SPS in peacetime, other than planned GPS
interference testing as described in Section 3.2.3, will be subject to a
minimum of 48-hour advance notice provided by the DOD to the USCG
Navigation Information Service (NIS) and the FAA Notice to Airman
(NOTAM) system. A disruption is defined as periods in which the GPS is
not capable of providing SPS as specified in the GPS Standard
Positioning Service Signal Specification (Ref. 10). Unplanned system
outages resulting from system malfunctions or unscheduled maintenance
will be announced by the NIS and NOTAM systems (see Appendix C) as they
become known.

![](media/image2.wmf){width="8.340277777777779in"
height="6.050694444444445in"}

**Figure 3-2. Radionavigation Systems Operating Plan**

The FAA's GPS overlay initiative, which permits use of GPS to fly most
existing NPA procedures, was of particular significance in achieving
early operational benefits from GPS. The convenience of GPS for
executing the thousands of existing VOR-and NDB-based NPAs was made
immediately available to suitable equipped aircraft. In addition to
"overlay" NPAs, the FAA moved aggressively to produce and publish
GPS-based NPAs for runways without existing approaches, as well as
improved approaches (lower minimums) for runways with existing NPAs.
More than 2200 stand-alone GPS approaches have been published. Initial
WAAS-based precision approach procedures are due to be published
coincident with WAAS achieving its initial operational capability in the
year 2000. A precision approach based on WAAS criteria will be designed
for each runway end that is currently served by an existing conventional
approach procedure. In addition, an NPA procedure will be developed with
each precision approach procedure. The NPA will be usable by both WAAS
and TSO-C129 receivers.

C. Spectrum

The L1 links of GPS and the Russian GLONASS system, the principal
elements of the ICAO GNSS, operate in the 1559-1610 MHz aeronautical
radionavigation/satellite navigation service frequency band. This is the
sole band that is identified worldwide for the satellite-based
aeronautical radionavigation requirements of civil aviation. The GPS L1
SPS ranging signal is a 2.046 MHz null-to-null bandwidth signal centered
about 1575.42 MHz. The transmitted ranging signal that compromises the
GPS-SPS is not limited to the null-to-null signal and extends through
the band 1563.42 to 1587.42 MHz. WAAS, when it becomes operational, will
utilize the same band and carrier frequency as GPS L1. Additionally,
systems of pseudolites that may share the GPS L1 frequency or operate on
an offset frequency have been proposed as an availability enhancement
for LAAS.

The GPS L2 link shares the 1215-1260 MHz frequency band with the GLONASS
L2 link and with the nationwide joint surveillance system radar network
operated by DOD and FAA. The GPS L2 carrier frequency is 1227.60 MHz.

Additional signals are planned to enhance the ability of GPS to support
civil users. These signals will assist in the mitigation of
ionospheric-delay estimation errors and serve as backups for the GPS L1
link. A second non-safety-of-life civil signal will be added at the GPS
L2 frequency (1227.60 MHz), and a third safety-of-life civil signal will
be added at 1176.45 MHz.

2.  ***GPS Modernization***

The utility of GPS to support civil and military positioning and timing
applications has grown tremendously during the 1990s. From hikers to
automotive direction finding, aviation to spacecraft applications, GPS
has become an integral part of our information infrastructure. Despite
its revolutionary impact on navigation and timing applications, some
improvements can make it significantly more useful and reduce the cost
of augmentation systems and receiver equipment being designed to enhance
and extend the current position and timing service provided by the GPS.

> In 1997, the Air Force initiated a review of the capabilities of GPS.
> In an unprecedented teaming of the Departments of Defense,
> Transportation, Commerce, Interior, and Agriculture, plus NASA, both
> military and civil user requirements were collected. Current system
> shortcomings relative to those requirements were identified, and
> changes were recommended to improve the GPS service.

The first element of GPS Modernization was the decision to provide a
civil signal at the L2 frequency. Civil users will be able to correct
for ionospheric errors using a second frequency in addition to the
current signal on L1. These corrections, when combined with setting
Selective Availability (SA) to zero, will enable user equipment that
meets benchmark standards to achieve horizontal accuracies in the 4
meter range. Vice President Gore announced the second civil signal
decision on March 30, 1998. In addition, the Vice President announced
that there would be a third civil signal for safety-of-life applications
implemented on the Block II F satellites. In January 1999, it was
announced that the second civil signal would be located at the L2
frequency (1227.60 MHz) and the third civil signal would be located at
1176.45 MHz, which is in an aeronautical radionavigation service
protected band.

> The GPS Modernization effort focuses on improving position and timing
> accuracy, availability, integrity monitoring support capability and
> enhancement to the control system to ensure a robust, highly
> dependable navigation and timing source for all users. As these system
> enhancements are introduced, users will be able to continue to use
> existing receivers, as signal backward compatibility is an absolute
> requirement for both the military and civil user community. Although
> current GPS users will be able to operate at the same, or better,
> levels of performance that they enjoy today, users will need to modify
> or procure new user equipment in order to take full advantage of any
> new signal structure enhancements.
>
> GPS modernization will apply the principles of electronic and
> information warfare to ensure uninterrupted access to the PPS signal
> by U.S., Allied, and coalition forces. In addition, SA will be
> replaced with other means to deny hostile exploitation of the GPS
> service.

***3.2.3 Interference Testing Coordination***

In order to minimize service disruptions and prevent situations
threatening safety or efficient use of GPS, any government agency or
activity with a need to perform interference testing (i.e., transmit) in
the GPS spectrum must coordinate with the FAA Spectrum Policy and
Management Office. The FAA Spectrum Policy and Management Office acts as
coordinator for any and all GPS interference testing. Due to guidance in
the GPS Presidential Decision Directive (Ref. 2) that requires DOD to
"develop measures to prevent hostile use of GPS and its augmentations to
ensure that the United States retains a military advantage without
unduly disrupting or degrading civilian uses," the DOD has frequent need
to perform interference testing. However, any and all other agencies
with interference testing requirements must also coordinate through the
FAA.

***3.2.4 Augmentations to GPS***

Unaugmented GPS will not meet all performance requirements for aviation,
for the harbor entrance and approach phase of marine navigation, or for
many land transportation applications. For example, an aircraft must
have at least five satellites in view above a mask angle of 7.5 degrees
in order to provide Receiver Autonomous Integrity Monitoring (RAIM).
This condition is not always satisfied with the existing GPS
constellation, resulting in so-called "RAIM holes" and limiting GPS to
use as a supplemental navigation system. To meet the requirements for
Fault Detection and Exclusion (FDE), at least six satellites with good
geometry are necessary.

GPS may exhibit variances from a predicted grid established for
navigation, charting, or derivation of guidance information. This
variance may be caused by propagation anomalies, accidental
perturbations of signal timing, and the implementation of SA.

Adverse effects of these variances may be substantially reduced, if not
practically eliminated, by differential techniques. In such differential
operation, a reference station is located at a fixed point (or points)
within an area of interest. GPS signals are observed in real time and
compared with signals expected to be observed at the fixed point.
Differences between observed signals and predicted signals are
transmitted to users as differential corrections to upgrade the
precision and performance of the user's receiver.

Non-navigation users of GPS who require few-centimeter accuracy or
employ post processing to achieve few-decimeter to few-meter accuracy
often employ augmentation somewhat differently from navigation users.
For post processing applications using C/A code range, the actual
observations from a reference station (rather than corrections) are
provided to users. The users then compute corrections in their reduction
software. Surveyors and other users who need sub-centimeter to
few-centimeter accuracy in positioning from post-processing use
two-frequency carrier phase observations from reference stations, rather
than range data. The Continuously Operating Reference Stations (CORS)
system is designed to meet the needs of both of the above types of these
users.

***3.2.4.1 Maritime Differential GPS***

The USCG Maritime DGPS Service provides service for coastal coverage of
the continental U.S., the Great Lakes, Puerto Rico, portions of Alaska
and Hawaii, and portions of the Mississippi River Basin. Maritime DGPS
uses fixed GPS reference stations that broadcast pseudo-range
corrections using radionavigation radiobeacons. The Maritime DGPS
Service provides radionavigation accuracy better than 10 meters (2 drms)
for U.S. harbor entrance and approach areas.

A. User Community

Initially the U.S. Coast Guard identified four missions to be supported
by the implementation of DGPS:

• Harbor Entrance and Approach Phase navigation

• Vessel Traffic Services (VTS)

• Aids to Navigation (ATON) positioning

• Exclusive Economic Zone (EEZ) surveying

The first is the only listed mission that requires navigation capability
for both government and public users. The other three are government
missions requiring a positioning service. In addition to the four Coast
Guard identified missions, the USACE has partnered with the USCG to
establish DGPS along many of the navigable inland rivers of the U.S. As
a result, USACE surveying, positioning, dredging, revetment maintenance,
and other navigation related activities are to be accomplished with
improved levels of efficiency.

B. Operating Plan

The USCG declared Full Operational Capability (FOC) of the Maritime DGPS
Service on March 15, 1999. Necessary steps to include DGPS as a system
that meets the carriage requirements of the Navigation Safety
Regulations (33 CFR 164), for vessels operating on the navigable waters
of the U.S are being undertaken. In addition, the USCG on behalf of the
U.S. Government intends to offer the Maritime DGPS Service to the IMO
for recognition as a component of the worldwide radionavigation system

Recommended standards for maritime DGPS corrections have been developed
by the Radio Technical Commission for Maritime Services (RTCM) Special
Committee 104. The USCG is represented on this special committee and is
using the SC-104 standard for its Maritime DGPS Service.

C.  Spectrum

The Maritime DGPS Service operated by the USCG uses fixed GPS reference
stations that broadcast GPS pseudorange corrections in the 285-325 kHz
maritime radiobeacon band.

***3.2.4.2 Nationwide Differential GPS (NDGPS)***

A Nationwide DGPS (NDGPS) Service is being established to provide
coverage for all areas of the U.S. not currently covered by the USCG
Maritime DGPS Service.

This service is being established under the authority of P.L. 105-66
(Ref. 11) and is being implemented under a Memorandum of Agreement among
the FRA, FHWA, USCG, OSTDOT, USAF, NOAA, and USACE.

A. User Community

Positive Train Control, Intelligent Transportation Systems, and
precision agriculture are expected to receive benefits from the NDGPS
Service.

B.  Operating Plan

The NDGPS Service is expected to achieve IOC for land applications on
December 31, 2002. The IOC phase is identified by the system's ability
to provide accuracy, integrity, and single station broadcast coverage of
the continental U.S.

The NDGPS Service will achieve FOC when it is capable of meeting the
maritime broadcast standards of DGPS (Appendix C, section C.2.2.2) and
provides dual coverage of the continental U.S. and selected portions of
Hawaii and Alaska with single coverage elsewhere. FOC is expected
December 31, 2003.

The service uses RTCM SC-104 standards.

C.  Spectrum

NDGPS uses fixed GPS reference stations that broadcast pseudorange
corrections in the 285-325 kHz maritime radiobeacon band.

***3.2.4.3 Aeronautical GPS Wide Area Augmentation System (WAAS)***

The WAAS is a safety-critical system designed primarily for aviation
users consisting of the equipment and software that augments GPS. The
WAAS provides a signal-in-space to WAAS users to support en route
through precision approach navigation. The WAAS users include all
certified aircraft using the WAAS for any approved phase of flight. The
signal-in-space provides three services: (1) integrity data on GPS and
Geostationary Earth Orbit (GEO) satellites, (2) differential corrections
of GPS and GEO satellites to improve accuracy, and (3) a ranging
capability to improve availability and continuity.

The GPS satellite data are received and processed at widely dispersed
sites, referred to as Wide-area Reference Stations (WRS). These data are
forwarded to processing sites, referred to as Wide-area Master Stations
(WMS), which process the data to determine the integrity, differential
corrections, residual errors, and ionospheric information for each
monitored satellite and generate GEO satellite parameters. This
information is sent to a Ground Earth Station (GES) and uplinked along
with the GEO navigation message to the GEO satellites. The GEO
satellites downlink these data on the GPS L1 frequency with a modulation
similar to that used by GPS.

In addition to providing GPS integrity, the WAAS verifies its own
integrity and takes any necessary action to ensure that the system meets
performance requirements. The WAAS also has a system operations and
maintenance function that provides information to FAA maintenance
personnel.

A. User Community

Substantial benefits will accrue to both users and providers as the WAAS
becomes operational and the aviation community transitions to WAAS
avionics. Near-term user benefits will result from the use of a single
navigation receiver that provides area navigation for all phases of
flight and a significant increase in runways approved for precision
approaches. When combined with necessary improvements in air traffic
control automation, additional user benefits are expected to be derived
from reduced IFR separations and more efficient routings. Near-term
provider benefits will be derived from the decommissioning of redundant
navigation systems and more cost-effective instrument approaches. The
WAAS is also expected to be used extensively for numerous other civil
applications where improved accuracy, integrity and availability are
needed.

B. Operating Plan

The FAA is conducting a major system acquisition consisting of the WAAS
operational system and functional verification system. The program
strategy is to quickly field an initial WAAS that meets the basic
requirements, and to enhance the system to meet the full WAAS
requirements through a series of contract options.

WAAS is planned to achieve its initial operational capability by the end
of 2000, and will provide en route through nonprecision approach service
as well as a limited precision approach capability. After achieving
initial operational capability, the WAAS will then be incrementally
improved over the next six years to expand the area of coverage,
increase the availability of precision approaches, increase signal
redundancy, and reduce operational restrictions. The result of these
incremental improvements will enable pilots equipped exclusively with
WAAS avionics to execute all phases of flight in the NAS including
Category I precision approach.

C. Spectrum

The WAAS will operate as an overlay on the GPS L1 link in the 1559-1610
MHz ARNS/RNSS frequency band. WAAS reference stations will also require
codeless access to GPS L2 signals in the 1215-1260 MHz band to enhance
system accuracy until such time as the second coded civil GPS signal is
operational. The exact timeline and conditions will be specified in a
jointly developed DOD/DOT transition plan.

4.  ***Aeronautical GPS Local Area Augmentation System (LAAS)***

A. User Community

The LAAS is a local GPS augmentation where the corrections to GPS (and
WAAS) signals are broadcast to aircraft within line of sight of a ground
reference station. LAAS is expected to support Category II/III
applications. The system is also expected to provide Category I
precision approaches at some high capacity airports which require
increased availability and at locations where WAAS is unable to provide
Category I precision approach services. LAAS may be used to support
runway incursion warnings, high-speed turnoffs, missed approaches,
departures, vertical takeoffs and surface operations.

B. Operating Plan

The FAA completed the development of Category I LAAS specifications in
1999 and plans to develop prototype systems to validate the ground
station specification. There will be a subsequent effort to specify and
validate CAT II/III LAAS performance. The FAA is also conducting
research on providing airport surface traffic surveillance and guidance
based on LAAS-augmented GPS.

C. Spectrum

The international community has evaluated spectral alternatives and has
agreed with the FAA that the 108-117.975 MHz ARNS frequency band,
currently populated by VORs and ILSs, is the candidate of choice for
LAAS. Pseudolites sharing the GPS L1 frequency in a low duty cycle
pulsed mode have been proposed as an availability enhancement for CAT
II/III LAAS ground facilities.

5.  ***The Continuously Operating Reference Station (CORS) System***

The CORS system is a GPS augmentation being established by the NGS to
support non-navigation, post-processing applications of GPS. The CORS
system provides code range and carrier phase data from a nationwide
network of GPS stations for access by the Internet. As of November 1998,
data were being provided from about 144 stations.

A. User Community

The observational data provided by the CORS system are being used by
government, academia, and industry groups to support most of the
applications described in section 2.6. Currently, users are downloading
about 1.6 gigabytes of data per day. The largest user groups, in terms
of number of bytes downloaded, are academic and government research
groups involved in geophysical studies of Earth movement. However, the
largest numbers of users are private industry and Federal, state and
local government users involved in surveying, mapping, charting, and GIS
applications. These users require lesser quantities of data to support
their applications.

National Geodetic Survey (NGS) has implemented CORS by making use of
stations established by other groups, rather than by building an
independent network of reference stations. In particular, use is being
made of data from stations operated by components of DOT to support
real-time navigation requirements. More than half of the stations now
providing data for the CORS system are the stations of the USCG Maritime
DGPS Service described in section 3.2.4.1. Stations of the WAAS network
(described in section 3.2.4.3 above) will be CORS compatible, as well as
the NDGPS stations being established by DOT to support land navigation.
Other stations currently contributing data to the CORS system include
stations operated by the National Oceanic and Atmospheric Administration
(NOAA) and NASA in support of crustal motion activities, stations
operated by state and local governments in support of surveying
applications, and stations operated by NOAA's Forecast Systems
Laboratory in support of meteorological applications.

B. Operating Plan

The CORS system takes data to a Central Facility from the contributing
stations using either the Internet or a telephone packet service (such
as x.25). At the Central Data Facility, the data are converted to a
common format, quality controlled, and place in files for access via
Internet. The data are available via Internet for 50 days, after which
they are archived on CD ROM. In addition to the data, the Central Data
Facility provides software to support extraction, manipulation, and
interpolation of the data. The precise positions of the CORS antennas
are computed and monitored. In the future it is planned to compute and
provide ancillary data, such as multipath models and tropospheric and
ionospheric refraction models, to improve the accuracy of the CORS data.

C. Spectrum

Not applicable.

6.  ***Vulnerability of GPS in the National Transportation
    Infrastructure***

Appendix G of the Final Report of the President's Commission on Critical
Infrastructure Protection was entitled, "Vulnerabilities of the NAVSTAR
Global Positioning System and its Augmentations." This report concluded
that GPS services and applications are susceptible to various types of
interference, and that the effects of these vulnerabilities on civilian
applications should be studied in detail. As a result of the report,
Presidential Decision Directive 63 gave the Department of Transportation
the following directive:

> The Department of Transportation, in consultation with the Department
> of Defense, shall undertake a thorough evaluation of the vulnerability
> of the national transportation infrastructure that relies on the
> Global Positioning System. This evaluation shall include sponsoring an
> independent, integrated assessment of risks to civilian users of
> GPS-based systems, with a view to basing decisions on the ultimate
> architecture of the modernized NAS on these evaluations.

This evaluation will assist the DOT in developing a plan for protecting
the national transportation infrastructure. The focus of the study will
be on the civilian user of the national transportation infrastructure,
although the scope will include other civilian users and applications
with appropriate authorities being notified of vulnerabilities as
necessary.

DOT is expected to produce a report of current studies, a recommended
plan of action for additional studies, a report of vulnerabilities to
the national transportation infrastructure relying on GPS, and a
recommendation as to priorities of risks and potential mitigation
actions. The report is expected in 2000.

Presidential Decision Directive 63 also issued the following directive
to the Federal Aviation Administration (FAA):

> The Federal Aviation Administration shall develop and implement a
> comprehensive National Airspace System Security Program to protect the
> modernized NAS from information-based and other disruptions and
> attacks.

Although not mentioned specifically, the security of GPS-reliant systems
in the NAS is included. The FAA worked with the Air Transport
Association of America (ATA) and the Aircraft Owners and Pilots
Association (AOPA) to perform an independent GPS risk assessment. This
study assesses the risks associated with the use of GPS and GPS enhanced
by the Wide Area Augmentation System (WAAS) and the Local Area
Augmentation System (LAAS) as the only navigation system required in
aircraft operating within the NAS. The final report was delivered in
January 1999. The main conclusions of the study are as follows:

-   GPS with appropriate WAAS/LAAS configurations can satisfy navigation
    performance requirements as the only navigation system installed in
    the aircraft and the only navigation service provided by the U.S.
    Federal Government for aviation.

-   Risks to GPS signal reception can be managed, but steps must be
    taken to minimize the effects of intentional interference.

-   A definitive national GPS plan and management commitment is needed
    to establish system improvements with civil aviation users and to
    provide greater informational access to the civil aviation
    community.

The final report's findings are being assessed.

***3.2.5 Loran-C***

Loran-C was developed to provide military users with a radionavigation
capability having much greater coverage and accuracy than its
predecessor (Loran-A). It was subsequently selected as the Federally
provided radionavigation system for civil marine use in the U.S. coastal
areas. It is currently designated by the FAA as a supplemental system in
the NAS. Loran-C can also be used for precise time interval and highly
accurate frequency applications.

A. User Community

Although there is a steady trend towards the use of GPS, there remains a
significant number of both maritime and aviation users of the Loran-C
system. In addition, telecommunications and weather services use Loran-C
as an economical timing device and weather services use it to determine
upper air wind speed and direction by determining the change of position
of radiosonde flights with time.

B. Operating Plan

While the Administration continues to evaluate the long-term need for
continuation of the Loran-C radionavigation system, the Government will
operate the Loran-C system in the short term. The U.S. Government will
give users reasonable notice if it concludes that Loran-C is not needed
or is not cost effective, so that users will have the opportunity to
transition to alternative navigation aids. With this continued
sustainment of the Loran-C service, users will be able to realize
additional benefits. Improvement of GPS time synchronization of the
Loran-C chains and the use of digital receivers may support improved
accuracy and coverage of the service. Loran-C will continue to provide a
supplemental means of navigation. Current Loran-C receivers do not
support nonprecision instrument approach operations.

C. Spectrum

Loran-C operates in the 90-110 kHz frequency band.

***3.2.6 VOR and VOR/DME***

VOR was developed as a replacement for the Low-Frequency Radio Range to
provide a bearing from an aircraft to the VOR transmitter. A collocated
DME provides the distance from the aircraft to the DME transmitter. At
most sites, the DME function is provided by the TACAN system that also
provides azimuth guidance to military users. Such combined facilities
are called VORTAC stations. Some VOR stations are used for broadcast of
weather information.

A. User Community

VOR is the primary radionavigation aid in the National Airspace System
and is the internationally designated standard short-distance
radionavigation aid for air carrier and general aviation IFR operations.
Because it forms the basis for defining the airways, its use is an
integral part of the air traffic control procedures.

B. Operating Plan

The FAA operates 1012 VOR, VOR/DME, and VORTAC stations including 150
VOR-only stations. The number of stations is expected to remain stable
until the VOR/DMEs begin to be decommissioned in 2008. The DOD also
operates stations in the U.S. and overseas which are available to all
users.

A small increase in the number of users equipped with VOR is expected
over the next several years due to an increase in the aircraft
population operating in the U.S. During this time, many users that are
equipping their aircraft for VFR operation may choose to equip with GPS
in preference to VOR. VOR/DME will still be required for IFR flight
until the WAAS is approved for primary means navigation. It is then
expected that VOR equipage will begin to rapidly decrease.

The current VOR/DME network will be maintained until 2008 to enable
aircraft to become equipped with WAAS avionics and to allow the aviation
community to become familiar with the system. Plans for expansion of the
network are limited to site modernization or facility relocation, and
the conversion of sub-standard VORs to a Doppler VOR configuration. The
phase-down of the VOR/DME and TACAN network is expected to begin in
2008.

From today's full coverage network, the phase-down will transition
through an interim network and then to a minimum operational network.
This phased approach will allow for more efficient transition of
airspace routings, encourage user equipage for area navigation, and
maintain nonprecision approach alternatives. The minimum operational
network will support IFR operations for the busiest airports in the NAS.
A further reduction is then planned to the level of a basic backup
network. Section 3.3 discusses the transition in more detail.

C. Spectrum

VOR operates in the 108-117.975 MHz frequency band. It shares the
108-111.975 MHz portion of that band with ILS. The FAA and the rest of
the civil aviation community are investigating several potential
aeronautical applications of the 108-117.975 MHz band for possible
implementation after VOR and ILS have been partially or completely
decommissioned. One of those future applications is LAAS. Another is the
expansion of the present 117.975-137 MHz air/ground (A/G) communications
band to support the transition to, and future growth of, the
next-generation VHF A/G communications system for air traffic services.

DME operates in the 960-1027, 1033-1087, and 1093-1215 MHz sub-bands of
the 960-1215 MHz ARNS band. It shares those sub-bands with TACAN. The
frequency 1176.45 MHz has been selected as the third civil frequency
(L5) for GPS. Location of GPS L5 in this protected ARNS band meets the
needs of critical safety-of-life applications. The DOD's Joint Tactical
Information Distribution System/Multi-function Information Distribution
System (JTIDS/MIDS) also operates in this band on a non-interference
basis. The civil aviation community is investigating potential
aeronautical applications of those sub-bands for implementation after
DME and TACAN have been partially or completely decommissioned. These
potential future applications include:

• Automatic Dependent Surveillance, Broadcast (ADS-B), a function in
which aircraft transmit position and altitude data derived from onboard
navigation systems.

• Traffic Information Services (TIS), in which processed surveillance
data will be reported automatically from ground stations to aircraft in
flight.

• A/G transfer of voice and data traffic for CNS services.

• Potential future CNS applications to support Free Flight.

The FAA is also considering the retention of a subset of the nationwide
VOR/DME network. Continued use of some of the 108-117.975 MHz band would
be needed to sustain the VOR elements of such a network. A substantial
portion of the 960-1215 MHz ARNS band would be required to support its
DME elements.

***3.2.7 TACAN***

TACAN is a UHF radionavigation system that is the military counterpart
of VOR/DME. TACAN is the primary tactical air navigation system for the
military services ashore and afloat. TACAN is often collocated with the
civil VOR stations (VORTAC facilities) to permit military aircraft to
operate in civil airspace.

A. User Community

There are presently approximately 14,500 aircraft that are equipped to
determine bearing and distance to TACAN. These consist primarily of
Navy, Air Force, and to a lesser extent, Army aircraft. Additionally,
allied and third world military aircraft use TACAN extensively.

Because of propagation characteristics and radiated power, TACAN is
limited to line-of-sight and is limited to approximately 180 miles at
higher altitudes. As with VOR/DME, special consideration must be given
to location of ground-based TACAN facilities, especially in areas where
mountainous terrain is involved due to its line-of-sight coverage.

B. Operating Plan

DOD presently operates 173 TACANs and the FAA operates 609 TACANs for
DOD. Present TACAN coverage ashore will be maintained until phased out
in favor of GPS. However, the sea-based function of TACAN cannot be
replaced by GPS unless combined with an appropriate data link function
(ship to aircraft) with consideration for security, detection,
classification, and exploitation threats. The potential to replace TACAN
is being studied as a part of the Joint Precision Approach and Landing
System (JPALS) program. The requirement for sea-based TACAN will
continue until a suitable replacement is operational. Civil DME and the
distance-measuring functions of TACAN will continue to be the same.

The DOD requirement for and use of land-based TACAN will continue until
aircraft are properly integrated with GPS and when GPS is approved for
all operations in national and international controlled airspace. Proper
integration requires hardware and software modifications to GPS user
equipment to meet navigation accuracy, integrity, availability, and
continuity of service requirements. These modifications as well as
development of operational procedures and navigation databases will
require a transition period where TACAN must be retained. The target
date to begin TACAN phase-down is 2008.

> The FAA and DOD are conducting a NAS-wide prioritization review of
> FAA-operated TACAN facilities based on DOD mission requirements. The
> objective of the review is to identify and support critical facilities
> to ensure continued operation of these facilities to meet DOD needs.
> The prioritization assigns a class category to each facility.
>
> • Class I - Critical, facilities essential for DOD operations. Class I
> facilities will continue to be maintained and operated with the
> support of a standby power source. Standby power may be provided by
> either an engine generator or a four-hour battery system.
>
> • Class II - Non-Critical, facilities required, but not essential, for
> DOD operations. Class II facilities will continue to be maintained and
> operated but will not require a standby power source.
>
> • Class III - Facility Not Required for DOD operations. Class III
> facilities will reduce service by eliminating TACAN azimuth service
> from operation.

C. Spectrum

TACAN operates in the 960-1027, 1033-1087, and 1093-1215 MHz sub-bands
of the 960-1215 MHz ARNS frequency band. It shares those sub-bands with
DME. The DOD's JTIDS/MIDS also operates in this band on a
non-interference basis. The civil aviation community is investigating
potential aeronautical applications of those sub-bands for
implementation after DME and TACAN have been partially or completely
decommissioned. Possible future applications are noted in Section 3.2.6.

***3.2.8 ILS***

ILS provides aircraft with precision vertical and lateral navigation
(guidance) information during approach and landing. Associated marker
beacons or DME equipment identify the final approach fix, the point
where the final descent to the runway is initiated.

A. User Community

Federal regulations require U.S. part 121 air carrier aircraft to be
equipped with ILS avionics. ILS also is extensively used by general
aviation aircraft. A slight increase in the number of users equipped
with ILS is expected over the next several years due to an increase in
the aircraft population operating in the U.S. ILS equipage rates are
then expected to rapidly decrease once the WAAS is approved for Category
I approaches.

Because ILS is an ICAO standard landing system, it is extensively used
by air carrier and general aviation aircraft of other countries.

B. Operating Plan

ILS is a standard civil precision approach system in the U.S. and
abroad, and is protected by ICAO agreement to January 1, 2010. The FAA
operates 1062 ILS systems in the NAS, of which 99 are Category II or
Category III systems. In addition, the DOD operates 165 ILS facilities
in the U.S.

For Category I precision approaches, ILS will remain in service together
with WAAS to allow users an opportunity to equip with WAAS receivers and
to become comfortable with its service. The phase-down of Category I ILS
is expected to begin in 2008. For Category II/III precision approaches,
new and upgrade requirements will continue to be met with ILS until LAAS
systems are available. The FAA does not anticipate phasing out any
Category II/III ILS systems prior to 2015.

As the GPS-based precision approach systems (WAAS/LAAS) are integrated
into the NAS, and user equipage and acceptance grows, the ILS systems
will be phased down. From today's full coverage network the phase-down
will transition through an interim network and then to a minimum
operational network. This phased approach will encourage user changeover
to GPS-based approaches and maintain precision approach alternatives.
The minimum operational network will support IFR operations for the
busiest airports in the NAS. A further reduction is then planned to the
level of a basic backup network. Section 3.3 discusses the transition in
more detail.

As the ILS phase-down occurs, non-Federal sponsors may wish to continue
their operation of their non-Federal ILS systems. Additionally,
non-Federal sponsors may wish to take over operations and maintenance of
some systems planned for decommissioning by the FAA.

C. Spectrum

ILS marker beacons operate in the 74.8-75.2 MHz frequency band. Since
all ILS marker beacons operate on a single frequency (75 MHz), the
aeronautical requirements for this band will remain unchanged until ILS
has been completely phased out. No future aeronautical uses are
envisioned for this band after ILS has been fully decommissioned.

ILS localizers share the 108-111.975 MHz portion of the 108-117.975 MHz
ARNS band with VOR. As noted in Section 3.2.6, the FAA and the rest of
the civil aviation community are investigating several potential
aeronautical applications of this band for possible implementation after
VOR and ILS have been partially or completely decommissioned. One of
those future applications is LAAS. Another is the expansion of the
present 117.925-137 MHz A/G communications band to support the
transition to, and future growth of, the next-generation VHF A/G
communications system for air traffic services. Substantial amounts of
spectrum in the 108-111.975 MHz sub-band will continue to be needed to
operate Category II and III localizers even after Category I ILS has
been decommissioned.

ILS glide slope subsystems operate in the 328-335.4 MHz band. The FAA
and the rest of the civil aviation community are investigating several
potential aeronautical applications of this band for possible
implementation after ILS has been partially or completely
decommissioned. The inherent physical characteristics of this band, like
those of the 108-111.975 MHz band, are quite favorable to long-range
terrestrial line-of-sight A/G communications and data-link applications
like LAAS, ADS-B and TIS. Consequently, this band is well suited to
provide multiband diversity to such services or to serve as an overflow
band for them if they cannot be accommodated entirely in other bands.
Substantial amounts of spectrum in this band will continue to be needed
to operate Category II and III ILS glide slope subsystems even after
Category I ILS has been decommissioned.

***3.2.9 MLS***

MLS applications are limited to precision approach and landing. MLS is
easier to cite than ILS and offers higher accuracy and greater
flexibility, permitting precision approaches at more airports. MLS
provides USAF tactical flexibility due to its ease in siting and
adaptability to mobile operations. While there is limited user support
for MLS in the U.S., it has continued to be a factor in other countries.

The USAF has implemented MLS capability on its fleet of C-130 aircraft
for use with 37 Mobile MLS (MMLS) ground systems. The C-17 fleet is in
the process of being equipped with a Multi-Mode Receiver (MMR) with
enhanced ILS (radio interference protection), MLS, and
GPS/JPALS/LAAS/WAAS growth capabilities. Additional fielding of MLS
capability in the USAF will be driven by the extent of international
civil and NATO Allied implementation. The U.S. Army and U.S. Navy
currently have no plans to implement MLS.

A. User Community

FAA initiated a limited procurement of Category I MLS equipment in 1992.
Twenty-nine Category I MLS systems have been installed. The FAA
terminated the development of Category II and III MLS equipment based on
favorable GPS test results.

B. Operating Plan

The U.S. does not anticipate additional MLS development. The phase-down
of MLS is expected to begin in 2008.

C. Spectrum

MLS operates in the 5000-5250 MHz frequency band. The FAA and the rest
of the civil aviation community are investigating potential aeronautical
applications of this band for implementation after MLS has been
partially or completely decommissioned. These include:

• An extension of the tuning range of the Terminal Doppler Weather Radar
(TDWR) in order to relieve spectral congestion within its present
limited operating band.

• Weather functions of the planned multipurpose primary terminal radar
that will become operational around the year 2013.

***3.2.10 Aeronautical Nondirectional Beacons (NDBs)***

Aeronautical nondirectional beacons are used for transition from en
route to precision terminal approach facilities and as nonprecision
approach aids at many airports. In addition, some state and locally
owned NDBs are used to provide weather information to pilots. However,
GPS and the FAA's automated weather observing system (AWOS) and
automated surface observing system (ASOS) are providing the navigation
and weather broadcast services currently met by NDBs.

A. User Community

All air carriers, most military, and many general aviation aircraft
carry automatic direction finders (ADF). However, the importance of ADF
is expected to decline with the increasing popularity of GPS.

Aircraft use radiobeacons as compass locators to aid in finding the
initial approach point of an instrument landing system, for nonprecision
approaches at low traffic airports without convenient VOR approaches,
and for en route operations in some remote areas.

The large number of general aviation aircraft that are equipped with
radio direction finders attests to the wide acceptance of radiobeacons
by the user community. The primary reason for this acceptance is that
adequate accuracy can be achieved with low-cost user equipment. However,
now that GPS-based nonprecision approaches are available, transition
from the NDB network can begin.

B. Operating Plan

The FAA operates over 700 NDBs. This number is expected to decline
steadily over the next decade due to the increasing popularity of GPS.
In addition, there are about 200 military NDBs and 800 non-Federally
operated NDBs. During the next 10 years, FAA expenditures for beacons
are planned to be limited to the replacement of deteriorated components,
modernization of selected facilities, and an occasional establishment or
relocation of an NDB used for ILS transition.

The FAA expects to decommission stand-alone NDBs starting in 2008.
However, there may be cases where operation and maintenance of an NDB
will be taken over by an individual operator or community desiring to
delay its phaseout.

NDBs used as compass locators for ILS approaches, where no equivalent
ground-based means for transition to the ILS course exists, will be
maintained until the underlying ILS is itself phased out. A separate
transition timeline will be developed for NDBs that define low frequency
airways in Alaska.

C. Spectrum

Aeronautical NDBs operate in the 190-435 and 510-535 kHz frequency
bands, portions of which it shares with maritime NDBs. Except in Alaskan
airspace, no future civil aeronautical uses are envisioned for these
bands after the aeronautical NBD system has been decommissioned
throughout the rest of the NAS.

***3.2.11 Maritime Radiobeacons***

Maritime radiobeacons have remained as a backup to more sophisticated
radionavigation systems and as a low-cost, medium accuracy system for
vessels equipped with only minimal radionavigation equipment. Use and
number of these beacons is dwindling very rapidly.

A. User Community

Radiobeacons are primarily used as homing devices for recreational
boaters, but they also act as a backup for those users having more
sophisticated radionavigation capability. As selected radiobeacons are
modified to broadcast DGPS corrections, those radiobeacons will become a
primary element in the harbor entrance and approach and coastal phases
of navigation, used by all vessels, and required for certain classes of
vessels. Due to single carrier operations, that eliminate the Morse tone
identifier, maritime DGPS beacons do not conform to traditional
radiobeacon standards.

Maritime radiobeacons have been an acceptable radionavigation tool for
pleasure boaters using them for homing purposes, largely due to the
adequate service with low-cost user equipment.

Marine radiobeacons provide a bearing accuracy relative to vehicle
heading on the order of [+]{.underline}3 to [+]{.underline}10 degrees.
This might be considered a systemic limitation but, in actual use, it is
satisfactory for many navigation purposes. Radiobeacons are not
satisfactory for marine navigation within restricted channels or
harbors. They do not provide sufficient accuracy or coverage to be used
as a primary aid to navigation for large vessels in U.S. coastal areas.

B. Operating Plan

Four maritime radiobeacons continue to be operated by the USCG. Many of
the previously configured maritime radiobeacons have been modified to
broadcast DGPS corrections for the Maritime DGPS Service; therefore,
they no longer provide service as traditional homing devices.

With the availability of low-cost Loran-C and GPS receivers that provide
far more flexible use to the boater, the use of radiobeacons has been
continually declining. As the USCG conducts evaluation of the need for
beacons, those with no identifiable user base will be discontinued.
Maritime radiobeacons not modified to carry DGPS correction signals are
expected to be phased out by the year 2000.

Although some aviation users have benefited from maritime radiobeacons,
modulation of maritime radiobeacons with DGPS corrections will make
these beacons unusable by digital aviation ADFs and may make their use
by analog ADFs difficult.

**3.3 Phase-Down of Ground-Based Aeronautical Navaids**

***3.3.1 Transition to Satellite-Based Navigation (Satnav)***

The FAA is planning to transition into providing Satnav services based
primarily on GPS augmented by the Wide Area Augmentation System (WAAS)
and the Local Area Augmentation System (LAAS). As a result of this
transition, the number of Federally provided ground-based navigation
facilities will be reduced to coincide with a reduction in the need for
ground-based navigation services. Transition to a totally new system
represents a substantial undertaking---one that will require a major
investment of resources by both the FAA and the aircraft owners and
operators. Three essential prerequisites must be met for such a massive
transition to take place:

• *System Performance:* Through analyses, flight tests, and substantial
operational experience, aircraft operators and the FAA must be convinced
that the new system meets their requirements for accuracy, integrity,
availability, and continuity of service.

• *Operational Benefit:* The aircraft operator must perceive sufficient
operational benefit to warrant an investment in the new technology.

• *Transition Period:* The aircraft operators must have sufficient time
to recoup their investment in conventional avionics. Although many
avionics systems have been used for 15 to 20 years or more, a reasonable
compromise must be reached between the FAA's desire for a rapid
transition (to avoid further investment in ground-based Navaids) and the
aircraft operators' desire to use current avionics as long as possible.

The transition period begins when the capability is established for a
pilot to perform navigation procedures throughout the NAS using Satnav
as the only means of radionavigation aboard an aircraft. This will occur
when WAAS achieves its full operational capability and procedures to use
the new capability have been published (i.e., precision and nonprecision
Satnav instrument approach procedures). Prior to this, even new aircraft
must be equipped with avionics for the conventional ground-based
Navaids. The transition period ends when the conventional Navaids have
been reduced to the extent that they are unnecessary for routine NAS
operations.

The reduction in Federally provided Navaid services can be performed in
several distinct steps. This approach would allow the FAA to begin the
phaseout gradually, providing users sufficient time to equip with Satnav
avionics. A more abrupt transition would be too disruptive to NAS
operations and would place too great a burden on the users. The proposed
phase-down strategy is depicted in Figure 3-3. The FAA is evaluating
alternatives for the future navigation architecture and will update the
transition plans as Satnav program milestones are achieved; as the
actual performance of Satnav systems is documented; and as users equip
with Satnav avionics.

The proposed transition strategy involves a two-step phase-down from the
current full coverage network of Navaids to a reduced network that
supports a substantial number of currently certified airways, jet
routes, and instrument approach procedures. This network, termed the
Minimum Operational Network, is a scaled-down version of the current
infrastructure of VORs, ILSs, MLSs, and NDBs. The phase-down strategy
would provide the FAA and the airspace users with a safe recovery and
sustained operations capability in the event of a disruption in Satnav
service. A follow-on step to a basic backup network is also depicted in
Figure 3-3.

The Navaid phase-down can be initiated after the following conditions
have been met:

• WAAS has achieved FOC and has been approved as an only means of
radionavigation for a given flight operation (at FOC, WAAS will comply
with its end-state requirements, providing a level of availability
sufficient to replace existing VOR/DME and NDB facilities, and many
Category I ILS facilities).

• Procedures to use the new WAAS capability have been published.

![](media/image3.wmf)• A majority of the airspace users have equipped
with appropriate WAAS avionics.

# 

# 

# Figure 3-3. Proposed Civil Aeronautical Navaid Phase-Down Steps

The phase-down is planned to begin in 2008 based on projected Satnav
program milestones and anticipated user equipage rates.

The specific Navaids that would no longer qualify for Federal support,
at each step of the phase-down, would be determined based on specific
criteria, currently under development. Navaids supporting en route
procedures would be decommissioned. Navaids supporting terminal
procedures could be decommissioned or transitioned to a non-Federal
sponsor.

The discontinuance criteria would be published as early as possible and
well ahead of the phase-down. A site-specific list of Navaids fitting
the discontinuance criteria would be published later---perhaps at the
time of WAAS FOC. The advanced site-specific notice would afford users
the opportunity to plan their transition to Satnav based upon the
operational schedule for the specific Navaids they use most often.

• *Phase I* -- Many currently under-utilized VORs and ILSs would be
discontinued at the first step of the phase-down. Preliminary analysis
indicates that approximately 350 VORs and 300 ILSs would no longer
qualify for Federal support at this first step. (The population of NDBs
would remain essentially intact to support ILS approaches.) Although
this represents an approximate 30 percent reduction in the number of
Navaids, it would be expected to cause a relatively minor impact on the
NAS.

• *Phase II* -- The second step, planned to occur in 2011, would further
reduce the population of ground-based Navaids to the level of the
proposed Minimum Operational Network. The Phase II Navaids are intended
to support continued operation in the NAS by those aircraft not yet
equipped with Satnav avionics, albeit at a reduced level of efficiency.
Although this represents an approximate 50 percent reduction in the
number of Federally supported Navaids, the remaining network would
continue to support a robust set of IFR operations.

• *Phase III --* Previous plans were to complete the transition to
Satnav with the phase-out of all the remaining ground-based Navaids.
However, a more conservative approach now planned by the FAA is to
step-down to a subset of ground-based Navaids that would continue to
support Satnav operations beyond 2012. A candidate "Basic Backup
Network" composed of several hundred conventional VOR/DME Navaids would
allow aircraft equipped with DME/DME avionics to continue en route
navigation using dual-DME position updates. It would also provide a
nonprecision instrument approach capability at selected airports. A
limited number of Category I ILSs, and virtually all existing Category
II/III ILSs, would also be retained to support prevision instrument
approaches at major airports.

***3.3.2 Satnav as an Only Means of Radionavigation***

The FAA's goal is to approve Satnav as the only radionavigation system
required to be installed in an aircraft to support operations anywhere
in the NAS. The Air Transport Association of America and the Aircraft
Owners and Pilots Association conducted a risk assessment of using
Satnav technology as an only means of radionavigation in the NAS. The
study was conducted by the Johns Hopkins University Applied Physics Lab
under the oversight of the RTCA Free Flight Steering Committee and with
FAA funding. The final report concluded that GPS with appropriate
WAAS/LAAS configurations can satisfy the required navigation performance
as the only navigation system installed in the aircraft and the only
navigation service provided by the FAA (see Section 3.2.4.6). However,
from a service provider perspective, the FAA plans to continue operating
a subset of conventional ground-based Navaids for those users choosing
to remain equipped with conventional avionics.

There is concern about potential disruptions to Satnav service,
primarily due to the relatively weak signals received from the GPS
satellites. As one example, the President's Commission on Critical
Infrastructure Protection highlighted GPS vulnerability and questioned
its use as the only means of radionavigation in the NAS. The predominant
concerns relate to a potential loss of service from intentional jamming,
unintentional radio frequency interference, or ionospheric scintillation
during severe solar storms. Intentional jamming is the most difficult
threat to overcome.

• The effects of jamming and unintentional interference are primarily to
increase the workload of both the users and the air traffic controllers.
Pilots and controllers will work together to assure safety, but a loss
of navigation and landing capabilities increases the demand for
services. Operational restrictions would likely be necessary in the
event of an outage to balance demand and assure safety.

• Solar effects are expected to have only minimal impact on CONUS
airspace. The greatest impact is expected in the polar regions and near
the equator. Most aircraft operating on polar routes are equipped with
inertial systems and can operate for many hours between radionavigation
updates before violating separation requirements. Some care will be
needed in high-latitude and equatorial zone Satnav-based instrument
approaches at night.

A loss of GPS service in the absence of any other means of
radionavigation would have varying negative effects on air traffic
operations. These effects range from nuisance events requiring standard
restoration of capabilities to an inability to provide service within
one or more sectors of airspace for a significant period of time.

***3.3.3 Mitigation of Potential Satnav Disruptions***

Several solutions have been identified to help mitigate the effects of a
Satnav service disruption, but each has its limitations.

• The L5 civil frequency planned for GPS will help alleviate the impacts
of both solar activity and unintentional interference, but it may be
2013 or later before a full constellation of dual-frequency satellites
is available. The cost implications of the L5 civil frequency are not
yet defined.

• Modern transport-category turbojet aircraft, when engaged in
relatively stable en route flight, may be able to continue navigating
safely an hour or more after losing radionavigation position updating.
In some cases, this capability may prove adequate to depart an area with
localized jamming or proceed under visual flight rules during good
visibility and high ceilings. However, inertial performance without
radionavigation updates degrades substantially faster on a maneuvering
aircraft, and the viability of continued terminal-area navigation is
unclear. There is no assurance of compliance with airspace requirements
after executing a procedural turn or entering a holding pattern, even in
en route airspace.

• Integrated GPS/inertial avionics having significant anti-jam
capability could greatly reduce the area affected by a GPS jammer or by
unintentional interference. Industry research is proceeding to develop
this technology, with an expectation that it might be marketed to the
general aviation community. However, significant certification
challenges will be encountered, and some users may still find this
technology to be unaffordable.

• A basic backup network composed of several hundred conventional
VOR/DME Navaids would allow aircraft equipped with DME/DME avionics to
continue en route navigation using dual-DME position updates. It would
also provide a nonprecision instrument approach capability at selected
airports. However, low-altitude users may need to be vectored by air
traffic controllers into an area with VOR coverage or to an area in
visual meteorological conditions. Additional Navaids (where required)
may also be needed for missed approaches and departures where terrain or
obstruction clearances must be maintained---particularly in non-radar
environments.

• Users may have an option to equip with IFR-certified Loran-C avionics,
pending the improvements needed to achieve a nonprecision instrument
approach capability with Loran. A combined Loran/Satnav receiver could
provide navigation and nonprecision instrument approach service
throughout any disruption to Satnav service.

• If a majority of operations are conducted by aircraft equipped with an
additional navigation capability (e.g., inertial or Loran), then the
balance should be able to be managed with air traffic control vectors
based on an independent (radar) surveillance system. Additional research
may be necessary to validate this concept in terms of the impact to air
traffic controller workload and the sensitivity to the proportion of
backup-equipped aircraft.

• An ILS (or MLS) may need to be retained at major airports to provide a
backup precision approach capability, and where necessary to support
international compatibility. ILSs may also be needed at a few remote
airports where the distance to the closest major (ILS-equipped) airports
is excessive.

***3.3.4 Long-Term Transition Plans***

The pace and extent of the transition to Satnav will depend upon a
number of factors related to system performance and user acceptance. The
FAA plans to reduce ground-based Navaids subject to these factors.

A decision to retain a selected subset of ground-based Navaids to
support satellite navigation does not need to be made until well after
experience is gained with Satnav technology. Some site-specific Navaids
will face the end of their serviceable life before 2010. The need to
replace selected Navaids will require investment analysis and investment
decisions on what specific Navaids to retain.

The FAA's plans for the transition to Satnav and for the phase-down of
ground-based Navaids will be periodically reevaluated. These plans need
to remain flexible, and may need to be adjusted as Satnav program
milestones are achieved, as the actual performance of Satnav systems is
demonstrated, and as users equip with Satnav avionics. The transition
plans will continue to be closely coordinated with airspace users and
with the FAA's air traffic control community.

**3.4 Interoperability of Radionavigation Systems**

***3.4.1 Overview***

Radionavigation systems are sometimes used in combination with each
other or with other systems. These combined systems are often
implemented so that a major attribute of one system will offset a
weakness of another. For example, a system having high accuracy and a
low fix rate might be combined with a system with a lower accuracy and
higher fix rate. The combined system would demonstrate characteristics
of a system with both high accuracy and a high fix rate.

***3.4.2 GPS/GLONASS***

Manufacturers of navigation and positioning equipment are beginning to
develop and manufacture combined GPS/GLONASS receivers to take advantage
of these benefits. Some receivers are on the market with others in the
planning stage. The RTCA SC 159 is developing a Minimum Operation
Performance Standard (MOPS) for a combined GPS and GLONASS system. The
Airlines Electronic Engineering Committee (AEEC) is developing
specifications for a multimode receiver that includes GLONASS. The
satellite communications MOPS and SARPs provide for both GPS and GLONASS
protection.

A combination of GPS and GLONASS has several potential benefits over
either system alone. Combining the capability in one receiver to
navigate using satellites from the GPS and GLONASS constellations
results in a receiver with improved navigation and positioning
availability worldwide, improved polar coverage, improved resistance to
interference and jamming and improved RAIM and FDE. The FAA has entered
into a bilateral agreement with the Russian Federation to investigate a
combined GPS/GLONASS avionics receiver that could take advantage of the
two constellations.