Chapter 1. Air Navigation
Section 1. Navigation
a. Various types
of air navigation aids are in use today, each serving a special purpose. These
aids have varied owners and operators, namely: the Federal Aviation
Administration (FAA), the military services, private organizations, individual
states and foreign governments. The FAA has the statutory authority to
establish, operate, maintain air navigation facilities and to prescribe
standards for the operation of any of these aids which are used for instrument
flight in federally controlled airspace. These aids are tabulated in the
Airport/Facility Directory (A/FD).
b. Pilots should
be aware of the possibility of momentary erroneous indications on cockpit
displays when the primary signal generator for a ground-based navigational
transmitter (for example, a glideslope, VOR, or nondirectional beacon) is
inoperative. Pilots should disregard any navigation indication, regardless of
its apparent validity, if the particular transmitter was identified by NOTAM or
otherwise as unusable or inoperative.
Radio Beacon (NDB)
a. A low or medium
frequency radio beacon transmits nondirectional signals whereby the pilot of an
aircraft properly equipped can determine bearings and “home” on the station.
These facilities normally operate in a frequency band of 190 to 535 kilohertz
(kHz), according to ICAO Annex 10 the frequency range for NDBs is between 190
and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz
(Hz) modulation. All radio beacons except the compass locators transmit a
continuous three-letter identification in code except during voice
b. When a radio
beacon is used in conjunction with the Instrument Landing System markers, it is
called a Compass Locator.
transmissions are made on radio beacons unless the letter “W” (without voice) is
included in the class designator (HW).
d. Radio beacons
are subject to disturbances that may result in erroneous bearing information.
Such disturbances result from such factors as lightning, precipitation static,
etc. At night, radio beacons are vulnerable to interference from distant
stations. Nearly all disturbances which affect the Automatic Direction Finder
(ADF) bearing also affect the facility's identification. Noisy identification
usually occurs when the ADF needle is erratic. Voice, music or erroneous
identification may be heard when a steady false bearing is being displayed.
Since ADF receivers do not have a “flag” to warn the pilot when erroneous
bearing information is being displayed, the pilot should continuously monitor
the NDB's identification.
Omni-directional Range (VOR)
a. VORs operate
within the 108.0 to 117.95 MHz frequency band and have a power output necessary
to provide coverage within their assigned operational service volume. They are
subject to line-of-sight restrictions, and the range varies proportionally to
the altitude of the receiving equipment.
Normal service ranges for the various classes of VORs are given in Navigational
Aid (NAVAID) Service Volumes, paragraph 1-1-8.
b. Most VORs are
equipped for voice transmission on the VOR frequency. VORs without voice
capability are indicated by the letter “W” (without voice) included in the class
c. The only
positive method of identifying a VOR is by its Morse Code identification or by
the recorded automatic voice identification which is always indicated by use of
the word “VOR” following the range's name. Reliance on determining the
identification of an omnirange should never be placed on listening to voice
transmissions by the Flight Service Station (FSS) (or approach control facility)
involved. Many FSSs remotely operate several omniranges with different names. In
some cases, none of the VORs have the name of the “parent” FSS. During periods
of maintenance, the facility may radiate a T-E-S-T code (- D DDD -) or the code
may be removed.
d. Voice identification has been added to numerous VORs. The transmission
consists of a voice announcement, “AIRVILLE VOR” alternating with the usual
Morse Code identification.
effectiveness of the VOR depends upon proper use and adjustment of both ground
and airborne equipment.
1. Accuracy. The
accuracy of course alignment of the VOR is excellent, being generally plus or
minus 1 degree.
2. Roughness. On
some VORs, minor course roughness may be observed, evidenced by course needle or
brief flag alarm activity (some receivers are more susceptible to these
irregularities than others). At a few stations, usually in mountainous terrain,
the pilot may occasionally observe a brief course needle oscillation, similar to
the indication of “approaching station.” Pilots flying over unfamiliar routes
are cautioned to be on the alert for these vagaries, and in particular, to use
the “to/from” indicator to determine positive station passage.
propeller revolutions per minute (RPM) settings or helicopter rotor speeds can
cause the VOR Course Deviation Indicator to fluctuate as much as plus or minus
six degrees. Slight changes to the RPM setting will normally smooth out this
roughness. Pilots are urged to check for this modulation phenomenon prior to
reporting a VOR station or aircraft equipment for unsatisfactory operation.
1-1-4. VOR Receiver Check
a. The FAA VOR
test facility (VOT) transmits a test signal which provides users a convenient
means to determine the operational status and accuracy of a VOR receiver while
on the ground where a VOT is located. The airborne use of VOT is permitted;
however, its use is strictly limited to those areas/altitudes specifically
authorized in the A/FD or appropriate supplement.
b. To use the VOT
service, tune in the VOT frequency on your VOR receiver. With the Course
Deviation Indicator (CDI) centered, the omni-bearing selector should read 0
degrees with the to/from indication showing “from” or the omni-bearing selector
should read 180 degrees with the to/from indication showing “to.” Should the VOR
receiver operate an RMI (Radio Magnetic Indicator), it will indicate 180 degrees
on any omni-bearing selector (OBS) setting. Two means of identification are
used. One is a series of dots and the other is a continuous tone. Information
concerning an individual test signal can be obtained from the local FSS.
c. Periodic VOR
receiver calibration is most important. If a receiver's Automatic Gain Control
or modulation circuit deteriorates, it is possible for it to display acceptable
accuracy and sensitivity close into the VOR or VOT and display out-of-tolerance
readings when located at greater distances where weaker signal areas exist. The
likelihood of this deterioration varies between receivers, and is generally
considered a function of time. The best assurance of having an accurate receiver
is periodic calibration. Yearly intervals are recommended at which time an
authorized repair facility should recalibrate the receiver to the manufacturer's
Aviation Regulations (14 CFR Section 91.171) provides for certain VOR equipment
accuracy checks prior to flight under instrument flight rules. To comply with
this requirement and to ensure satisfactory operation of the airborne system,
the FAA has provided pilots with the following means of checking VOR receiver
1. VOT or a
radiated test signal from an appropriately rated radio repair station.
airborne check points.
3. Certified check
points on the airport surface.
e. A radiated VOT
from an appropriately rated radio repair station serves the same purpose as an
FAA VOR signal and the check is made in much the same manner as a VOT with the
1. The frequency
normally approved by the Federal Communications Commission is 108.0 MHz.
2. Repair stations
are not permitted to radiate the VOR test signal continuously; consequently, the
owner or operator must make arrangements with the repair station to have the
test signal transmitted. This service is not provided by all radio repair
stations. The aircraft owner or operator must determine which repair station in
the local area provides this service. A representative of the repair station
must make an entry into the aircraft logbook or other permanent record
certifying to the radial accuracy and the date of transmission. The owner,
operator or representative of the repair station may accomplish the necessary
checks in the aircraft and make a logbook entry stating the results. It is
necessary to verify which test radial is being transmitted and whether you
should get a “to” or “from” indication.
f. Airborne and ground check points consist of certified radials that should
be received at specific points on the airport surface or over specific landmarks
while airborne in the immediate vicinity of the airport.
1. Should an error
in excess of plus or minus 4 degrees be indicated through use of a ground check,
or plus or minus 6 degrees using the airborne check, Instrument Flight Rules
(IFR) flight must not be attempted without first correcting the source of the
No correction other than the correction card figures supplied by the
manufacturer should be applied in making these VOR receiver checks.
2. Locations of
airborne check points, ground check points and VOTs are published in the A/FD.
3. If a dual
system VOR (units independent of each other except for the antenna) is installed
in the aircraft, one system may be checked against the other. Turn both systems
to the same VOR ground facility and note the indicated bearing to that station.
The maximum permissible variations between the two indicated bearings is 4
1-1-5. Tactical Air
a. For reasons
peculiar to military or naval operations (unusual siting conditions, the
pitching and rolling of a naval vessel, etc.) the civil VOR/Distance Measuring
Equipment (DME) system of air navigation was considered unsuitable for military
or naval use. A new navigational system, TACAN, was therefore developed by the
military and naval forces to more readily lend itself to military and naval
requirements. As a result, the FAA has integrated TACAN facilities with the
civil VOR/DME program. Although the theoretical, or technical principles of
operation of TACAN equipment are quite different from those of VOR/DME
facilities, the end result, as far as the navigating pilot is concerned, is the
same. These integrated facilities are called VORTACs.
b. TACAN ground
equipment consists of either a fixed or mobile transmitting unit. The airborne
unit in conjunction with the ground unit reduces the transmitted signal to a
visual presentation of both azimuth and distance information. TACAN is a pulse
system and operates in the Ultrahigh Frequency (UHF) band of frequencies. Its
use requires TACAN airborne equipment and does not operate through conventional
Omni-directional Range/Tactical Air Navigation (VORTAC)
a. A VORTAC is a
facility consisting of two components, VOR and TACAN, which provides three
individual services: VOR azimuth, TACAN azimuth and TACAN distance (DME) at one
site. Although consisting of more than one component, incorporating more than
one operating frequency, and using more than one antenna system, a VORTAC is
considered to be a unified navigational aid. Both components of a VORTAC are
envisioned as operating simultaneously and providing the three services at all
signals of VOR and TACAN are each identified by three-letter code transmission
and are interlocked so that pilots using VOR azimuth with TACAN distance can be
assured that both signals being received are definitely from the same ground
station. The frequency channels of the VOR and the TACAN at each VORTAC facility
are “paired” in accordance with a national plan to simplify airborne operation.
1-1-7. Distance Measuring Equipment (DME)
a. In the
operation of DME, paired pulses at a specific spacing are sent out from the
aircraft (this is the interrogation) and are received at the ground station. The
ground station (transponder) then transmits paired pulses back to the aircraft
at the same pulse spacing but on a different frequency. The time required for
the round trip of this signal exchange is measured in the airborne DME unit and
is translated into distance (nautical miles) from the aircraft to the ground
b. Operating on the line-of-sight principle, DME furnishes distance
information with a very high degree of accuracy. Reliable signals may be
received at distances up to 199 NM at line-of-sight altitude with an accuracy of
better than 1/2 mile or 3 percent of the distance,
whichever is greater. Distance information received from DME equipment is SLANT
RANGE distance and not actual horizontal distance.
frequency range of a DME according to ICAO Annex 10 is from 960 MHz to 1215 MHz.
Aircraft equipped with TACAN equipment will receive distance information from a
VORTAC automatically, while aircraft equipped with VOR must have a separate DME
equipped with slaved compass systems may be susceptible to heading errors caused
by exposure to magnetic field disturbances (flux fields) found in materials that
are commonly located on the surface or buried under taxiways and ramps. These
materials generate a magnetic flux field that can be sensed by the aircraft's
compass system flux detector or “gate”, which can cause the aircraft's system to
align with the material's magnetic field rather than the earth's natural
magnetic field. The system's erroneous heading may not selfcorrect. Prior to
take off pilots should be aware that a heading misalignment may have occurred
during taxi. Pilots are encouraged to follow the manufacturer's or other
appropriate procedures to correct possible heading misalignment before take off
VORTAC, Instrument Landing System (ILS)/DME, and localizer (LOC)/DME navigation
facilities established by the FAA provide course and distance information from
collocated components under a frequency pairing plan. Aircraft receiving
equipment which provides for automatic DME selection assures reception of
azimuth and distance information from a common source when designated VOR/DME,
VORTAC, ILS/DME, and LOC/DME are selected.
to the limited number of available frequencies, assignment of paired frequencies
is required for certain military noncollocated VOR and TACAN facilities which
serve the same area but which may be separated by distances up to a few miles.
VORTAC, ILS/DME, and LOC/DME facilities are identified by synchronized
identifications which are transmitted on a time share basis. The VOR or
localizer portion of the facility is identified by a coded tone modulated at
1020 Hz or a combination of code and voice. The TACAN or DME is identified by a
coded tone modulated at 1350 Hz. The DME or TACAN coded identification is
transmitted one time for each three or four times that the VOR or localizer
coded identification is transmitted. When either the VOR or the DME is
inoperative, it is important to recognize which identifier is retained for the
operative facility. A single coded identification with a repetition interval of
approximately 30 seconds indicates that the DME is operative.
equipment which provides for automatic DME selection assures reception of
azimuth and distance information from a common source when designated VOR/DME,
VORTAC and ILS/DME navigation facilities are selected. Pilots are cautioned to
disregard any distance displays from automatically selected DME equipment when
VOR or ILS facilities, which do not have the DME feature installed, are being
used for position determination.
1-1-8. Navigational Aid (NAVAID) Service Volumes
a. Most air
navigation radio aids which provide positive course guidance have a designated
standard service volume (SSV). The SSV defines the reception limits of
unrestricted NAVAIDs which are usable for random/unpublished route navigation.
b. A NAVAID will
be classified as restricted if it does not conform to flight inspection signal
strength and course quality standards throughout the published SSV. However, the
NAVAID should not be considered usable at altitudes below that which could be
flown while operating under random route IFR conditions (14 CFR Section 91.177),
even though these altitudes may lie within the designated SSV. Service volume
restrictions are first published in Notices to Airmen (NOTAMs) and then with the
alphabetical listing of the NAVAIDs in the A/FD.
Service Volume limitations do not apply to published IFR routes or procedures.
d. VOR/DME/TACAN Standard Service Volumes (SSV).
service volumes (SSVs) are graphically shown in FIG 1-1-1,
FIG 1-1-2, FIG 1-1-3,
FIG 1-1-4, and FIG 1-1-5. The
SSV of a station is indicated by using the class designator as a prefix to the
station type designation.
TVOR, LDME, and HVORTAC.
Standard High Altitude Service Volume
(See FIG 1-1-5 for altitudes below 1,000 feet).
Standard Low Altitude Service Volume
(See FIG 1-1-5 for altitudes below 1,000 feet).
Standard Terminal Service Volume
(See FIG 1-1-4 for altitudes below 1,000 feet).
2. Within 25 NM, the bottom of the T service volume is defined by the curve
in FIG 1-1-4. Within 40 NM, the bottoms of the L and H
service volumes are defined by the curve in FIG 1-1-5.
Radio Beacon (NDB)
1. NDBs are
classified according to their intended use.
2. The ranges of
NDB service volumes are shown in TBL 1-1-2. The
distances (radius) are the same at all altitudes.
VOR/DME/TACAN Standard Service Volumes
and Range Boundaries
1,000 feet above ground level (AGL) up to and including 12,000 feet AGL at
radial distances out to 25 NM.
L (Low Altitude)
1,000 feet AGL up to and including 18,000 feet AGL at radial distances out to 40
H (High Altitude)
1,000 feet AGL up to and including 14,500 feet AGL at radial distances out to 40
NM. From 14,500 AGL up to and including 60,000 feet at radial distances out to
100 NM. From 18,000 feet AGL up to and including 45,000 feet AGL at radial
distances out to 130 NM.
NDB Service Volumes
*Service ranges of individual facilities may be less than 50 nautical miles
(NM). Restrictions to service volumes are first published as a Notice to Airmen
and then with the alphabetical listing of the NAVAID in the A/FD.
Service Volume Lower Edge Terminal
Service Volume Lower Edge
Standard High and Low
1-1-9. Instrument Landing System (ILS)
1. The ILS is
designed to provide an approach path for exact alignment and descent of an
aircraft on final approach to a runway.
2. The ground
equipment consists of two highly directional transmitting systems and, along the
approach, three (or fewer) marker beacons. The directional transmitters are
known as the localizer and glide slope transmitters.
3. The system may
be divided functionally into three parts:
information: localizer, glide slope;
(b) Range information:
marker beacon, DME; and
information: approach lights, touchdown and centerline lights, runway
radar, or compass locators located at the Outer Marker (OM) or Middle Marker
(MM), may be substituted for marker beacons. DME, when specified in the
procedure, may be substituted for the OM.
5. Where a
complete ILS system is installed on each end of a runway; (i.e., the approach
end of Runway 4 and the approach end of Runway 22) the ILS systems are not in
1. The localizer
transmitter operates on one of 40 ILS channels within the frequency range of
108.10 to 111.95 MHz. Signals provide the pilot with course guidance to the
2. The approach
course of the localizer is called the front course and is used with other
functional parts, e.g., glide slope, marker beacons, etc. The localizer signal
is transmitted at the far end of the runway. It is adjusted for a course width
of (full scale fly-left to a full scale fly-right) of 700 feet at the runway
3. The course line
along the extended centerline of a runway, in the opposite direction to the
front course is called the back course.
Unless the aircraft's ILS equipment includes reverse sensing capability, when
flying inbound on the back course it is necessary to steer the aircraft in the
direction opposite the needle deflection when making corrections from off-course
to on-course. This “flying away from the needle” is also required when flying
outbound on the front course of the localizer. Do not use back course signals
for approach unless a back course approach procedure is published for that
particular runway and the approach is authorized by ATC.
4. Identification is in International Morse Code and consists of a
three-letter identifier preceded by the letter I (DD) transmitted on the
5. The localizer
provides course guidance throughout the descent path to the runway threshold
from a distance of 18 NM from the antenna between an altitude of 1,000 feet
above the highest terrain along the course line and 4,500 feet above the
elevation of the antenna site. Proper off-course indications are provided
throughout the following angular areas of the operational service volume:
(a) To 10 degrees
either side of the course along a radius of 18 NM from the antenna; and
(b) From 10 to 35
degrees either side of the course along a radius of 10 NM. (See
Limits of Localizer Coverage
signals may be received outside these areas.
c. Localizer Type
Directional Aid (LDA)
1. The LDA is of
comparable use and accuracy to a localizer but is not part of a complete ILS.
The LDA course usually provides a more precise approach course than the similar
Simplified Directional Facility (SDF) installation, which may have a course
width of 6 or 12 degrees.
2. The LDA is not
aligned with the runway. Straight-in minimums may be published where alignment
does not exceed 30 degrees between the course and runway. Circling minimums only
are published where this alignment exceeds 30 degrees.
3. A very limited
number of LDA approaches also incorporate a glideslope. These are annotated in
the plan view of the instrument approach chart with a note, “LDA/Glideslope.”
These procedures fall under a newly defined category of approaches called
Approach with Vertical Guidance (APV) described in paragraph
5-4-5, Instrument Approach Procedure Charts, subparagraph
Approach with Vertical Guidance (APV). LDA minima for with and without
glideslope is provided and annotated on the minima lines of the approach chart
as S-LDA/GS and S-LDA. Because the final approach course is not aligned with the
runway centerline, additional maneuvering will be required compared to an ILS
d. Glide Slope/Glide
1. The UHF glide
slope transmitter, operating on one of the 40 ILS channels within the frequency
range 329.15 MHz, to 335.00 MHz radiates its signals in the direction of the
localizer front course. The term “glide path” means that portion of the glide
slope that intersects the localizer.
False glide slope signals may exist in the area of the localizer back course
approach which can cause the glide slope flag alarm to disappear and present
unreliable glide slope information. Disregard all glide slope signal indications
when making a localizer back course approach unless a glide slope is specified
on the approach and landing chart.
2. The glide slope
transmitter is located between 750 feet and 1,250 feet from the approach end of
the runway (down the runway) and offset 250 to 650 feet from the runway
centerline. It transmits a glide path beam 1.4 degrees wide (vertically). The
signal provides descent information for navigation down to the lowest authorized
decision height (DH) specified in the approved ILS approach procedure. The
glidepath may not be suitable for navigation below the lowest authorized DH and
any reference to glidepath indications below that height must be supplemented by
visual reference to the runway environment. Glidepaths with no published DH are
usable to runway threshold.
3. The glide path
projection angle is normally adjusted to 3 degrees above horizontal so that it
intersects the MM at about 200 feet and the OM at about 1,400 feet above the
runway elevation. The glide slope is normally usable to the distance of 10 NM.
However, at some locations, the glide slope has been certified for an extended
service volume which exceeds 10 NM.
4. Pilots must be alert when approaching the glidepath interception. False
courses and reverse sensing will occur at angles considerably greater than the
5. Make every
effort to remain on the indicated glide path.
Avoid flying below the glide path to assure obstacle/terrain clearance is
6. The published
glide slope threshold crossing height (TCH) DOES NOT represent the height of the
actual glide path on-course indication above the runway threshold. It is used as
a reference for planning purposes which represents the height above the runway
threshold that an aircraft's glide slope antenna should be, if that aircraft
remains on a trajectory formed by the four-mile-to-middle marker glidepath
7. Pilots must be
aware of the vertical height between the aircraft's glide slope antenna and the
main gear in the landing configuration and, at the DH, plan to adjust the
descent angle accordingly if the published TCH indicates the wheel crossing
height over the runway threshold may not be satisfactory. Tests indicate a
comfortable wheel crossing height is approximately 20 to 30 feet, depending on
the type of aircraft.
The TCH for a runway is established based on several factors including the
largest aircraft category that normally uses the runway, how airport layout
effects the glide slope antenna placement, and terrain. A higher than optimum
TCH, with the same glide path angle, may cause the aircraft to touch down
further from the threshold if the trajectory of the approach is maintained until
the flare. Pilots should consider the effect of a high TCH on the runway
available for stopping the aircraft.
e. Distance Measuring
1. When installed
with the ILS and specified in the approach procedure, DME may be used:
(a) In lieu of the
(b) As a back
course (BC) final approach fix (FAF); and
(c) To establish
other fixes on the localizer course.
2. In some cases,
DME from a separate facility may be used within Terminal Instrument Procedures
(a) To provide ARC
initial approach segments;
(b) As a FAF for
BC approaches; and
(c) As a
substitute for the OM.
f. Marker Beacon
1. ILS marker
beacons have a rated power output of 3 watts or less and an antenna array
designed to produce an elliptical pattern with dimensions, at 1,000 feet above
the antenna, of approximately 2,400 feet in width and 4,200 feet in length.
Airborne marker beacon receivers with a selective sensitivity feature should
always be operated in the “low” sensitivity position for proper reception of ILS
there are two marker beacons associated with an ILS, the OM and MM. Locations
with a Category II ILS also have an Inner Marker (IM). When an aircraft passes
over a marker, the pilot will receive the indications shown in
(a) The OM
normally indicates a position at which an aircraft at the appropriate altitude
on the localizer course will intercept the ILS glide path.
(b) The MM
indicates a position approximately 3,500 feet from the landing threshold. This
is also the position where an aircraft on the glide path will be at an altitude
of approximately 200 feet above the elevation of the touchdown zone.
(c) The IM will
indicate a point at which an aircraft is at a designated decision height (DH) on
the glide path between the MM and landing threshold.
Marker Passage Indications
D ×− D −
D D D D
D D D D
3. A back course marker normally indicates the ILS back course final
approach fix where approach descent is commenced.
g. Compass Locator
1. Compass locator
transmitters are often situated at the MM and OM sites. The transmitters have a
power of less than 25 watts, a range of at least 15 miles and operate between
190 and 535 kHz. At some locations, higher powered radio beacons, up to 400
watts, are used as OM compass locators. These generally carry Transcribed
Weather Broadcast (TWEB) information.
locators transmit two letter identification groups. The outer locator transmits
the first two letters of the localizer identification group, and the middle
locator transmits the last two letters of the localizer identification group.
h. ILS Frequency
(See TBL 1-1-4.)
Frequency Pairs Allocated for ILS
i. ILS Minimums
1. The lowest
authorized ILS minimums, with all required ground and airborne systems
components operative, are:
(a) Category I. Decision
Height (DH) 200 feet and Runway Visual Range (RVR) 2,400 feet (with touchdown
zone and centerline lighting, RVR 1,800 feet), or (with Autopilot or FD or HUD,
RVR 1,800 feet);
Authorization Category I. DH 150 feet and Runway Visual Range (RVR) 1,400
feet, HUD to DH;
(c) Category II. DH
100 feet and RVR 1,200 feet (with autoland or HUD to touchdown and noted on
authorization, RVR 1,000 feet);
Authorization Category II with Reduced Lighting. DH 100 feet and RVR 1,200
feet with autoland or HUD to touchdown and noted on authorization (touchdown
zone, centerline lighting, and ALSF-2 are not required);
(e) Category IIIa. No
DH or DH below 100 feet and RVR not less than 700 feet;
(f) Category IIIb. No
DH or DH below 50 feet and RVR less than 700 feet but not less than 150 feet;
(g) Category IIIc. No
DH and no RVR limitation.
Special authorization and equipment required for Categories II and III.
j. Inoperative ILS Components
localizer. When the localizer fails, an ILS approach is not authorized.
2. Inoperative glide
slope. When the glide slope fails, the ILS reverts to a nonprecision
See the inoperative component table in the U.S. Government Terminal Procedures
Publication (TPP), for adjustments to minimums due to inoperative airborne or
ground system equipment.
k. ILS Course
1. All pilots
should be aware that disturbances to ILS localizer and glide slope courses may
occur when surface vehicles or aircraft are operated near the localizer or glide
slope antennas. Most ILS installations are subject to signal interference by
either surface vehicles, aircraft or both. ILS CRITICAL AREAS are established
near each localizer and glide slope antenna.
2. ATC issues
control instructions to avoid interfering operations within ILS critical areas
at controlled airports during the hours the Airport Traffic Control Tower (ATCT)
is in operation as follows:
Conditions. Less than ceiling 800 feet and/or visibility 2 miles.
(1) Localizer Critical
Area. Except for aircraft that land, exit a runway, depart or miss approach,
vehicles and aircraft are not authorized in or over the critical area when an
arriving aircraft is between the ILS final approach fix and the airport.
Additionally, when the ceiling is less than 200 feet and/or the visibility is
RVR 2,000 or less, vehicle and aircraft operations in or over the area are not
authorized when an arriving aircraft is inside the ILS MM.
(2) Glide Slope
Critical Area. Vehicles and aircraft are not authorized in the area when an
arriving aircraft is between the ILS final approach fix and the airport unless
the aircraft has reported the airport in sight and is circling or side stepping
to land on a runway other than the ILS runway.
Conditions. At or above ceiling 800 feet and/or visibility 2 miles.
(1) No critical
area protective action is provided under these conditions.
(2) A flight crew,
under these conditions, should advise the tower that it will conduct an AUTOLAND
or COUPLED approach to ensure that the ILS critical areas are protected when the
aircraft is inside the ILS MM.
Glide slope signal not protected.
holding below 5,000 feet between the outer marker and the airport may cause
localizer signal variations for aircraft conducting the ILS approach.
Accordingly, such holding is not authorized when weather or visibility
conditions are less than ceiling 800 feet and/or visibility 2 miles.
4. Pilots are
cautioned that vehicular traffic not subject to ATC may cause momentary
deviation to ILS course or glide slope signals. Also, critical areas are not
protected at uncontrolled airports or at airports with an operating control
tower when weather or visibility conditions are above those requiring protective
measures. Aircraft conducting coupled or autoland operations should be
especially alert in monitoring automatic flight control systems.
(See FIG 1-1-7.)
Unless otherwise coordinated through Flight Standards, ILS signals to Category I
runways are not flight inspected below 100 feet AGL. Guidance signal anomalies
may be encountered below this altitude.
Directional Facility (SDF)
a. The SDF
provides a final approach course similar to that of the ILS localizer. It does
not provide glide slope information. A clear understanding of the ILS localizer
and the additional factors listed below completely describe the operational
characteristics and use of the SDF.
b. The SDF
transmits signals within the range of 108.10 to 111.95 MHz.
c. The approach
techniques and procedures used in an SDF instrument approach are essentially the
same as those employed in executing a standard localizer approach except the SDF
course may not be aligned with the runway and the course may be wider, resulting
in less precision.
off-course indications are limited to 35 degrees either side of the course
centerline. Instrument indications received beyond 35 degrees should be
e. The SDF antenna may be offset from the runway centerline. Because of
this, the angle of convergence between the final approach course and the runway
bearing should be determined by reference to the instrument approach procedure
chart. This angle is generally not more than 3 degrees. However, it should be
noted that inasmuch as the approach course originates at the antenna site, an
approach which is continued beyond the runway threshold will lead the aircraft
to the SDF offset position rather than along the runway centerline.
FAA Instrument Landing Systems
f. The SDF signal is fixed at either 6 degrees or 12 degrees as necessary to
provide maximum flyability and optimum course quality.
consists of a three-letter identifier transmitted in Morse Code on the SDF
frequency. The appropriate instrument approach chart will indicate the
identifier used at a particular airport.
1-1-11. Microwave Landing
1. The MLS
provides precision navigation guidance for exact alignment and descent of
aircraft on approach to a runway. It provides azimuth, elevation, and distance.
2. Both lateral
and vertical guidance may be displayed on conventional course deviation
indicators or incorporated into multipurpose cockpit displays. Range information
can be displayed by conventional DME indicators and also incorporated into
3. The MLS
supplements the ILS as the standard landing system in the U.S. for civil,
military, and international civil aviation. At international airports, ILS
service is protected to 2010.
4. The system may
be divided into five functions:
(a) Approach azimuth;
(b) Back azimuth;
(c) Approach elevation;
(d) Range; and
(e) Data communications.
5. The standard
configuration of MLS ground equipment includes:
(a) An azimuth
station to perform functions
(a) and (e) above. In
addition to providing azimuth navigation guidance, the station transmits basic
data which consists of information associated directly with the operation of the
landing system, as well as advisory data on the performance of the ground
(b) An elevation
station to perform function (c).
Measuring Equipment (DME) to perform range guidance, both standard DME (DME/N)
and precision DME (DME/P).
6. MLS Expansion
Capabilities. The standard configuration can be expanded by adding one or
more of the following functions or characteristics.
(a) Back azimuth. Provides
lateral guidance for missed approach and departure navigation.
(b) Auxiliary data
transmissions. Provides additional data, including refined airborne
positioning, meteorological information, runway status, and other supplementary
Service Volume (ESV) proportional guidance to 60 degrees.
identification is a four-letter designation starting with the letter M. It is
transmitted in International Morse Code at least six times per minute by the
approach azimuth (and back azimuth) ground equipment.
b. Approach Azimuth
1. The azimuth
station transmits MLS angle and data on one of 200 channels within the frequency
range of 5031 to 5091 MHz.
2. The equipment
is normally located about 1,000 feet beyond the stop end of the runway, but
there is considerable flexibility in selecting sites. For example, for heliport
operations the azimuth transmitter can be collocated with the elevation
3. The azimuth
(See FIG 1-1-8.)
(a) Laterally, at
least 40 degrees on either side of the runway centerline in a standard
(b) In elevation,
up to an angle of 15 degrees and to at least 20,000 feet, and
(c) In range, to
at least 20 NM.
c. Elevation Guidance
1. The elevation
station transmits signals on the same frequency as the azimuth station. A single
frequency is time-shared between angle and data functions.
2. The elevation
transmitter is normally located about 400 feet from the side of the runway
between runway threshold and the touchdown zone.
coverage is provided in the same airspace as the azimuth guidance signals:
(a) In elevation,
to at least +15 degrees;
(b) Laterally, to
fill the Azimuth lateral coverage; and
(c) In range, to
at least 20 NM.
(See FIG 1-1-9.)
d. Range Guidance
1. The MLS
Precision Distance Measuring Equipment (DME/P) functions the same as the
navigation DME described in paragraph 1-1-7, Distance
Measuring Equipment (DME), but there are some technical differences. The beacon
transponder operates in the frequency band 962 to 1105 MHz and responds to an
aircraft interrogator. The MLS DME/P accuracy is improved to be consistent with
the accuracy provided by the MLS azimuth and elevation stations.
2. A DME/P channel
is paired with the azimuth and elevation channel. A complete listing of the
200 paired channels of the DME/P with the angle functions is contained in FAA
Standard 022 (MLS Interoperability and Performance Requirements).
3. The DME/N or
DME/P is an integral part of the MLS and is installed at all MLS facilities
unless a waiver is obtained. This occurs infrequently and only at outlying, low
density airports where marker beacons or compass locators are already in place.
e. Data Communications
1. The data
transmission can include both the basic and auxiliary data words. All MLS
facilities transmit basic data. Where needed, auxiliary data can be transmitted.
2. Coverage limits. MLS
data are transmitted throughout the azimuth (and back azimuth when provided)
3. Basic data
content. Representative data include:
(b) Exact locations of azimuth, elevation and DME/P stations (for MLS
receiver processing functions);
equipment performance level; and
(d) DME/P channel
4. Auxiliary data
content: Representative data include:
(a) 3-D locations
of MLS equipment;
(d) Weather (e.g.,
RVR, ceiling, altimeter setting, wind, wake vortex, wind shear).
1. The MLS has the
capability to fulfill a variety of needs in the approach, landing, missed
approach and departure phases of flight. For example:
(a) Curved and
glide path angles;
(c) Accurate 3-D
positioning of the aircraft in space; and
establishment of boundaries to ensure clearance from obstructions in the
2. While many of
these capabilities are available to any MLS-equipped aircraft, the more
sophisticated capabilities (such as curved and segmented approaches) are
dependent upon the particular capabilities of the airborne equipment.
1. Accuracy. The
MLS provides precision three-dimensional navigation guidance accurate enough for
all approach and landing maneuvers.
2. Coverage. Accuracy
is consistent throughout the coverage volumes. (See
3. Environment. The
system has low susceptibility to interference from weather conditions and
airport ground traffic.
4. Channels. MLS
has 200 channels- enough for any foreseeable need.
5. Data. The MLS
transmits ground-air data messages associated with the systems operation.
6. Range information. Continuous
range information is provided with an accuracy of about 100 feet.
1-1-12. NAVAID Identifier
Removal During Maintenance
During periods of routine
or emergency maintenance, coded identification (or code and voice, where
applicable) is removed from certain FAA NAVAIDs. Removal of identification
serves as a warning to pilots that the facility is officially off the air for
tune-up or repair and may be unreliable even though intermittent or constant
signals are received.
During periods of maintenance VHF ranges may radiate a T-E-S-T code (- D DDD -).
DO NOT attempt to fly a procedure that is NOTAMed out of service even if the
identification is present. In certain cases, the identification may be
transmitted for short periods as part of the testing.
1-1-13. NAVAIDs with
a. Voice equipped
en route radio navigational aids are under the operational control of either a
Flight Service Station (FSS) or an approach control facility. The voice
communication is available on some facilities. Hazardous Inflight Weather
Advisory Service (HIWAS) broadcast capability is available on selected VOR sites
throughout the conterminous U.S. and does not provide two‐way voice
communication. The availability of two‐way voice communication and HIWAS is
indicated in the A/FD and aeronautical charts.
otherwise noted on the chart, all radio navigation aids operate continuously
except during shutdowns for maintenance. Hours of operation of facilities not
operating continuously are annotated on charts and in the A/FD.
1-1-14. User Reports Requested on
NAVAID or Global Navigation Satellite System (GNSS) Performance or Interference
of the National Airspace System (NAS) can render valuable assistance in the
early correction of NAVAID malfunctions or GNSS problems and are encouraged to
report their observations of undesirable performance. Although NAVAIDs are
monitored by electronic detectors, adverse effects of electronic interference,
new obstructions or changes in terrain near the NAVAID can exist without
detection by the ground monitors. Some of the characteristics of malfunction or
deteriorating performance which should be reported are: erratic course or
bearing indications; intermittent, or full, flag alarm; garbled, missing or
obviously improper coded identification; poor quality communications reception;
or, in the case of frequency interference, an audible hum or tone accompanying
radio communications or NAVAID identification. GNSS problems are often
characterized by navigation degradation or service loss indications.
should identify the NAVAID (for example, VOR) malfunction or GNSS problem,
location of the aircraft (i.e., latitude, longitude or bearing/distance from a
NAVAID), altitude, date and time of the observation, type of aircraft and
description of the condition observed, and the type of receivers in use (i.e.,
make/model/software revision). Reports can be made in any of the following ways:
by radio communication to the controlling Air Route Traffic Control Center
(ARTCC), Control Tower, or FSS.
telephone to the nearest FAA facility.
FAA Form 8740-5, Safety Improvement Report, a postage-paid card designed for
this purpose. These cards may be obtained at FAA FSSs, Flight Standards District
Offices, and General Aviation Fixed Base Operations.
aircraft that have more than one receiver, there are many combinations of
possible interference between units. This can cause either erroneous navigation
indications or, complete or partial blanking out of the communications. Pilots
should be familiar enough with the radio installation of the particular
airplanes they fly to recognize this type of interference.
In accordance with the 2010 DHS Appropriations Act, the U.S. Coast Guard (USCG)
terminated the transmission of all U.S. LORAN-C signals on 08 Feb 2010. The USCG
also terminated the transmission of the Russian American signals on 01 Aug 2010,
and the Canadian LORAN-C signals on 03 Aug 2010. For more information, visit
http://www.navcen.uscg.gov. Operators should also note that TSO-C60b,
AIRBORNE AREA NAVIGATION EQUIPMENT USING LORAN-C INPUTS, has been canceled by
1-1-16. VHF Direction
a. The VHF
Direction Finder (VHF/DF) is one of the common systems that helps pilots without
their being aware of its operation. It is a ground-based radio receiver used by
the operator of the ground station. FAA facilities that provide VHF/DF service
are identified in the A/FD.
b. The equipment
consists of a directional antenna system and a VHF radio receiver.
c. The VHF/DF
receiver display indicates the magnetic direction of the aircraft from the
ground station each time the aircraft transmits.
d. DF equipment is
of particular value in locating lost aircraft and in helping to identify
aircraft on radar.
AIM, Direction Finding Instrument Approach Procedure, Paragraph 6-2-3.
Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading
Reference System (AHRS)
a. IRUs are
self-contained systems comprised of gyros and accelerometers that provide
aircraft attitude (pitch, roll, and heading), position, and velocity information
in response to signals resulting from inertial effects on system components.
Once aligned with a known position, IRUs continuously calculate position and
velocity. IRU position accuracy decays with time. This degradation is known as
b. INSs combine
the components of an IRU with an internal navigation computer. By programming a
series of waypoints, these systems will navigate along a predetermined track.
c. AHRSs are
electronic devices that provide attitude information to aircraft systems such as
weather radar and autopilot, but do not directly compute position information.
1-1-18. Doppler Radar
Doppler Radar is a
semiautomatic self-contained dead reckoning navigation system (radar sensor plus
computer) which is not continuously dependent on information derived from ground
based or external aids. The system employs radar signals to detect and measure
ground speed and drift angle, using the aircraft compass system as its
directional reference. Doppler is less accurate than INS, however, and the use
of an external reference is required for periodic updates if acceptable position
accuracy is to be achieved on long range flights.
1-1-19. Global Positioning System (GPS)
a. System Overview
Description. The Global Positioning System is a satellite-based radio navigation
system, which broadcasts a signal that is used by receivers to determine precise
position anywhere in the world. The receiver tracks multiple satellites and
determines a pseudorange measurement that is then used to determine the user
location. A minimum of four satellites is necessary to establish an accurate
three-dimensional position. The Department of Defense (DOD) is responsible for
operating the GPS satellite constellation and monitors the GPS satellites to
ensure proper operation. Every satellite's orbital parameters (ephemeris data)
are sent to each satellite for broadcast as part of the data message embedded in
the GPS signal. The GPS coordinate system is the Cartesian earth-centered
earth-fixed coordinates as specified in the World Geodetic System 1984 (WGS-84).
2. System Availability and Reliability
(a) The status of
GPS satellites is broadcast as part of the data message transmitted by the GPS
satellites. GPS status information is also available by means of the U.S. Coast
Guard navigation information service: (703) 313-5907, Internet:
http://www.navcen.uscg.gov/. Additionally, satellite status is available
through the Notice to Airmen (NOTAM) system.
operational status of GNSS operations depends upon the type of equipment being
used. For GPS-only equipment TSO-C129(a), the operational status of nonprecision
approach capability for flight planning purposes is provided through a
prediction program that is embedded in the receiver or provided separately.
Autonomous Integrity Monitoring (RAIM). When GNSS equipment is not using
integrity information from WAAS or LAAS, the GPS navigation receiver using RAIM
provides GPS signal integrity monitoring. RAIM is necessary since delays of up
to two hours can occur before an erroneous satellite transmission can be
detected and corrected by the satellite control segment. The RAIM function is
also referred to as fault detection. Another capability, fault exclusion, refers
to the ability of the receiver to exclude a failed satellite from the position
solution and is provided by some GPS receivers and by WAAS receivers.
4. The GPS receiver verifies the integrity (usability) of the signals
received from the GPS constellation through receiver autonomous integrity
monitoring (RAIM) to determine if a satellite is providing corrupted
information. At least one satellite, in addition to those required for
navigation, must be in view for the receiver to perform the RAIM function; thus,
RAIM needs a minimum of 5 satellites in view, or 4 satellites and a barometric
altimeter (baro-aiding) to detect an integrity anomaly. [Baro-aiding satisfies
the RAIM requirement in lieu of a fifth satellite.] For receivers capable of
doing so, RAIM needs 6 satellites in view (or 5 satellites with baro-aiding) to
isolate the corrupt satellite signal and remove it from the navigation solution.
Baro-aiding is a method of augmenting the GPS integrity solution by using a
nonsatellite input source. GPS derived altitude should not be relied upon to
determine aircraft altitude since the vertical error can be quite large and no
integrity is provided. To ensure that baro-aiding is available, the current
altimeter setting must be entered into the receiver as described in the
5. RAIM messages vary somewhat between receivers; however, generally
there are two types. One type indicates that there are not enough satellites
available to provide RAIM integrity monitoring and another type indicates that
the RAIM integrity monitor has detected a potential error that exceeds the limit
for the current phase of flight. Without RAIM capability, the pilot has no
assurance of the accuracy of the GPS position.
Availability. Selective Availability (SA) is a method by which the accuracy of
GPS is intentionally degraded. This feature is designed to deny hostile use of
precise GPS positioning data. SA was discontinued on May 1, 2000, but many GPS
receivers are designed to assume that SA is still active. New receivers may take
advantage of the discontinuance of SA based on the performance values in ICAO
Annex 10, and do not need to be designed to operate outside of that performance.
7. The GPS
constellation of 24 satellites is designed so that a minimum of five is always
observable by a user anywhere on earth. The receiver uses data from a minimum of
four satellites above the mask angle (the lowest angle above the horizon at
which it can use a satellite).
8. The DOD
declared initial operational capability (IOC) of the U.S. GPS on December 8,
1993. The FAA has granted approval for U.S. civil operators to use properly
certified GPS equipment as a primary means of navigation in oceanic airspace and
certain remote areas. Properly certified GPS equipment may be used as a
supplemental means of IFR navigation for domestic en route, terminal operations,
and certain instrument approach procedures (IAPs). This approval permits the use
of GPS in a manner that is consistent with current navigation requirements as
well as approved air carrier operations specifications.
b. VFR Use of GPS
1. GPS navigation
has become a great asset to VFR pilots, providing increased navigation
capability and enhanced situational awareness, while reducing operating costs
due to greater ease in flying direct routes. While GPS has many benefits to the
VFR pilot, care must be exercised to ensure that system capabilities are not
2. Types of receivers used for GPS navigation under VFR are varied, from a
full IFR installation being used to support a VFR flight, to a VFR only
installation (in either a VFR or IFR capable aircraft) to a hand-held receiver.
The limitations of each type of receiver installation or use must be understood
by the pilot to avoid misusing navigation information. (See
TBL 1-1-6.) In all cases, VFR pilots should never rely solely on one system
of navigation. GPS navigation must be integrated with other forms of electronic
navigation (when possible), as well as pilotage and dead reckoning. Only through
the integration of these techniques can the VFR pilot ensure accuracy in
3. Some critical
concerns in VFR use of GPS include RAIM capability, database currency and
Capability. Many VFR GPS receivers and all hand-held units have no RAIM alerting
capability. Loss of the required number of satellites in view, or the detection
of a position error, cannot be displayed to the pilot by such receivers. In
receivers with no RAIM capability, no alert would be provided to the pilot that
the navigation solution had deteriorated, and an undetected navigation error
could occur. A systematic cross-check with other navigation techniques would
identify this failure, and prevent a serious deviation. See subparagraphs
a5 for more information
(1) In many
receivers, an up-datable database is used for navigation fixes, airports, and
instrument procedures. These databases must be maintained to the current update
for IFR operation, but no such requirement exists for VFR use.
(2) However, in
many cases, the database drives a moving map display which indicates Special Use
Airspace and the various classes of airspace, in addition to other operational
information. Without a current database the moving map display may be outdated
and offer erroneous information to VFR pilots wishing to fly around critical
airspace areas, such as a Restricted Area or a Class B airspace segment.
Numerous pilots have ventured into airspace they were trying to avoid by using
an outdated database. If you don't have a current database in the receiver,
disregard the moving map display for critical navigation decisions.
(3) In addition,
waypoints are added, removed, relocated, or re-named as required to meet
operational needs. When using GPS to navigate relative to a named fix, a current
database must be used to properly locate a named waypoint. Without the update,
it is the pilot's responsibility to verify the waypoint location referencing to
an official current source, such as the Airport/Facility Directory, Sectional
Chart, or En Route Chart.
(1) In many VFR
installations of GPS receivers, antenna location is more a matter of convenience
than performance. In IFR installations, care is exercised to ensure that an
adequate clear view is provided for the antenna to see satellites. If an
alternate location is used, some portion of the aircraft may block the view of
the antenna, causing a greater opportunity to lose navigation signal.
(2) This is
especially true in the case of hand-helds. The use of hand-held receivers for
VFR operations is a growing trend, especially among rental pilots. Typically,
suction cups are used to place the GPS antennas on the inside of cockpit
windows. While this method has great utility, the antenna location is limited to
the cockpit or cabin only and is rarely optimized to provide a clear view of
available satellites. Consequently, signal losses may occur in certain
situations of aircraft-satellite geometry, causing a loss of navigation signal.
These losses, coupled with a lack of RAIM capability, could present erroneous
position and navigation information with no warning to the pilot.
(3) While the use
of a hand-held GPS for VFR operations is not limited by regulation, modification
of the aircraft, such as installing a panel- or yoke-mounted holder, is governed
by 14 CFR Part 43. Consult with your mechanic to ensure compliance with the
regulation, and a safe installation.
4. As a result of
these and other concerns, here are some tips for using GPS for VFR operations:
(a) Always check
to see if your unit has RAIM capability. If no RAIM capability exists, be
suspicious of your GPS position when any disagreement exists with the position
derived from other radio navigation systems, pilotage, or dead reckoning.
(b) Check the
currency of the database, if any. If expired, update the database using the
current revision. If an update of an expired database is not possible, disregard
any moving map display of airspace for critical navigation decisions. Be aware
that named waypoints may no longer exist or may have been relocated since the
database expired. At a minimum, the waypoints planned to be used should be
checked against a current official source, such as the Airport/Facility
Directory, or a Sectional Aeronautical Chart.
(c) While hand-helds can provide excellent navigation capability to VFR
pilots, be prepared for intermittent loss of navigation signal, possibly with no
RAIM warning to the pilot. If mounting the receiver in the aircraft, be sure to
comply with 14 CFR Part 43.
(d) Plan flights
carefully before taking off. If you wish to navigate to user-defined waypoints,
enter them before flight, not on-the-fly. Verify your planned flight against a
current source, such as a current sectional chart. There have been cases in
which one pilot used waypoints created by another pilot that were not where the
pilot flying was expecting. This generally resulted in a navigation error.
Minimize head-down time in the aircraft and keep a sharp lookout for traffic,
terrain, and obstacles. Just a few minutes of preparation and planning on the
ground will make a great difference in the air.
(e) Another way to
minimize head-down time is to become very familiar with your receiver's
operation. Most receivers are not intuitive. The pilot must take the time to
learn the various keystrokes, knob functions, and displays that are used in the
operation of the receiver. Some manufacturers provide computer-based tutorials
or simulations of their receivers. Take the time to learn about your particular
unit before you try to use it in flight.
5. In summary, be
careful not to rely on GPS to solve all your VFR navigational problems. Unless
an IFR receiver is installed in accordance with IFR requirements, no standard of
accuracy or integrity has been assured. While the practicality of GPS is
compelling, the fact remains that only the pilot can navigate the aircraft, and
GPS is just one of the pilot's tools to do the job.
c. VFR Waypoints
1. VFR waypoints
provide VFR pilots with a supplementary tool to assist with position awareness
while navigating visually in aircraft equipped with area navigation receivers.
VFR waypoints should be used as a tool to supplement current navigation
procedures. The uses of VFR waypoints include providing navigational aids for
pilots unfamiliar with an area, waypoint definition of existing reporting
points, enhanced navigation in and around Class B and Class C airspace, and
enhanced navigation around Special Use Airspace. VFR pilots should rely on
appropriate and current aeronautical charts published specifically for visual
navigation. If operating in a terminal area, pilots should take advantage of the
Terminal Area Chart available for that area, if published. The use of VFR
waypoints does not relieve the pilot of any responsibility to comply with the
operational requirements of 14 CFR Part 91.
2. VFR waypoint
names (for computer-entry and flight plans) consist of five letters beginning
with the letters “VP” and are retrievable from navigation databases. The VFR
waypoint names are not intended to be pronounceable, and they are not for use in
ATC communications. On VFR charts, stand-alone VFR waypoints will be portrayed
using the same four-point star symbol used for IFR waypoints. VFR waypoints
collocated with visual check points on the chart will be identified by small
magenta flag symbols. VFR waypoints collocated with visual check points will be
pronounceable based on the name of the visual check point and may be used for
ATC communications. Each VFR waypoint name will appear in parentheses adjacent
to the geographic location on the chart. Latitude/longitude data for all
established VFR waypoints may be found in the appropriate regional
Airport/Facility Directory (A/FD).
3. VFR waypoints
must not be used to plan flights under IFR. VFR waypoints will not be recognized
by the IFR system and will be rejected for IFR routing purposes.
4. When filing VFR
flight plans, pilots may use the five letter identifier as a waypoint in the
route of flight section if there is an intended course change at that point or
if used to describe the planned route of flight. This VFR filing would be
similar to how a VOR would be used in a route of flight. Pilots must use the VFR
waypoints only when operating under VFR conditions.
5. Any VFR
waypoints intended for use during a flight should be loaded into the receiver
while on the ground and prior to departure. Once airborne, pilots should avoid
programming routes or VFR waypoint chains into their receivers.
6. Pilots should be especially vigilant for other traffic while operating
near VFR waypoints. The same effort to see and avoid other aircraft near VFR
waypoints will be necessary, as was the case with VORs and NDBs in the past. In
fact, the increased accuracy of navigation through the use of GPS will demand
even greater vigilance, as off-course deviations among different pilots and
receivers will be less. When operating near a VFR waypoint, use whatever ATC
services are available, even if outside a class of airspace where communications
are required. Regardless of the class of airspace, monitor the available ATC
frequency closely for information on other aircraft operating in the vicinity.
It is also a good idea to turn on your landing light(s) when operating near a
VFR waypoint to make your aircraft more conspicuous to other pilots, especially
when visibility is reduced. See paragraph
7-5-2, VFR in Congested Areas, for more information.
d. General Requirements
to conduct any GPS operation under IFR requires that:
navigation equipment used must be approved in accordance with the requirements
specified in Technical Standard Order (TSO) TSO-C129 (as revised), TSOC196 (as
revised), TSOC145 (as revised), or TSOC146 (as revised), and the installation
must be done in accordance with Advisory Circular AC 20-138, Airworthiness
Approval of Positioning and Navigation Systems. Equipment approved in accordance
with TSO-C115a does not meet the requirements of TSO-C129. Visual flight rules
(VFR) and hand-held GPS systems are not authorized for IFR navigation,
instrument approaches, or as a principal instrument flight reference. During IFR
operations they may be considered only as an aid to situational awareness.
(b) Aircraft using GPS (TSOC129 (as revised) or
TSOC196 (as revised)) navigation equipment under IFR must be equipped with an
approved and operational alternate means of navigation appropriate to the
flight. Active monitoring of alternative navigation equipment is not required if
the GPS receiver uses RAIM for integrity monitoring. Active monitoring of an
alternate means of navigation is required when the RAIM capability of the GPS
equipment is lost.
must be established for use in the event that the loss of RAIM capability is
predicted to occur. In situations where this is encountered, the flight must
rely on other approved equipment, delay departure, or cancel the flight.
GPS operation must be conducted in accordance with the FAA-approved aircraft
flight manual (AFM) or flight manual supplement. Flight crew members must be
thoroughly familiar with the particular GPS equipment installed in the aircraft,
the receiver operation manual, and the AFM or flight manual supplement. Unlike
ILS and VOR, the basic operation, receiver presentation to the pilot and some
capabilities of the equipment can vary greatly. Due to these differences,
operation of different brands, or even models of the same brand of GPS receiver,
under IFR should not be attempted without thorough study of the operation of
that particular receiver and installation. Most receivers have a built-in
simulator mode which will allow the pilot to become familiar with operation
prior to attempting operation in the aircraft. Using the equipment in flight
under VFR conditions prior to attempting IFR operation will allow further
navigating by IFR approved GPS are considered to be area navigation (RNAV)
aircraft and have special equipment suffixes. File the appropriate equipment
suffix in accordance with
TBL 5-1-2, on the
ATC flight plan. If GPS avionics become inoperative, the pilot should advise ATC
and amend the equipment suffix.
to any GPS IFR operation, the pilot must review appropriate NOTAMs and
aeronautical information. (See GPS NOTAMs/Aeronautical Information.)
carrier and commercial operators must meet the appropriate provisions of their
approved operations specifications.
domestic operations for commerce or for hire, operators must have a second
navigation system capable of reversion or contingency operations.
must have two independent navigation systems appropriate to the route to be
flown, or one system that is suitable and a second, independent backup
capability that allows the operator to proceed safely and land at a different
airport, and the aircraft must have sufficient fuel (reference 14 CFR 121.349,
125.203, 129.17, and 135.165). These rules ensure the safety of the operation by
preventing a single point of failure.
An aircraft approved for multisensor navigation and equipped with a single FMS
must maintain an ability to navigate or proceed safely in the event that any one
component of the navigation system fails, including the flight management system
(FMS). Retaining a FMSindependent VOR capability would satisfy this
requirements for a second system apply to the entire set of equipment needed to
achieve the navigation capability, not just the individual components of the
system such as the radio navigation receiver. For example, to use two RNAV
systems (e.g., GPS and DME/DME/IRU) to comply with the requirements, the
aircraft must be equipped with two independent radio navigation receivers and
two independent navigation computers (e.g., flight management systems (FMS)).
Alternatively, to comply with the requirements using a single RNAV system with
an installed and operable VOR capability, the VOR capability must be independent
of the FMS.
satisfy the requirement for two independent navigation systems, if the primary
navigation system is GPSbased, the second system must be independent of GPS
(for example, VOR or DME/DME/IRU). This allows continued navigation in case of
failure of the GPS or WAAS services. Recognizing that GPS interference and test
events resulting in the loss of GPS services have become more common, the FAA
requires operators conducting IFR operations under 14 CFR 121.349, 125.203,
129.17 and 135.65 to retain a nonGPS navigation capability consisting of either
DME/DME, IRU or VOR for en route and terminal operations, and VOR and ILS for
final approach. Since this system is to be used as a reversionary capability,
single equipage is sufficient.
e. Use of GPS for IFR Oceanic,
Domestic En Route, Terminal Area, and Approach Operations
IFR operations in oceanic areas can be conducted as soon as the proper avionics
systems are installed, provided all general requirements are met. A GPS
installation with TSO-C129 (as revised) authorization in class A1, A2, B1, B2,
C1, or C2 or TSOC196 (as revised) may be used to replace one of the other
approved means of long-range navigation, such as dual INS. (See
TBL 1-1-5 and TBL 1-1-6.)
A single TSOC129 GPS installation meeting the certification requirements in AC
20138C, Appendix 1 may be used on oceanic routes as the only means of long
range navigation. TSOC196 (as revised) equipment is inherently capable of
supporting oceanic operation if the operator obtains a Fault Detection and
Exclusion (FDE) Prediction Program as outlined in AC 20138C, Appendix 1. A
single GPS/WAAS receiver (TSOC145 (as revised) or TSOC146 (as revised)) is
inherently capable of supporting oceanic operation if the operator obtains a FDE
Prediction Program as outlined in AC 20138C, Appendix 1.
(TSOC129 (as revised) or TSOC196 (as revised)) domestic en route and terminal
IFR operations can be conducted as soon as proper avionics systems are
installed, provided all general requirements are met. For required backup
navigation, the avionics necessary to receive all of the ground-based facilities
appropriate for the flight to the destination airport and any required alternate
airport must be installed and operational. Ground-based facilities necessary for
en route and terminal operations must also be in service.
single GPS/WAAS receiver (TSOC145 (as revised) or TSOC146 (as revised)) may
also be used for these domestic en route and terminal IFR operations. Though not
required, operators may consider retaining backup navigation equipment in their
aircraft to guard against potential outages or interference.
Alaska, GPS en route IFR RNAV operations may be conducted outside the
operational service volume of groundbased navigation aids when a GPS/WAAS
(TSO-C145 (as revised) or TSO-C146 (as revised)) system is installed and
operating. Groundbased navigation equipment is not required to be installed and
operating. Though not required, operators may consider retaining backup
navigation equipment in their aircraft to guard against potential outages or
may operate on GNSS Qroutes with GPS (TSOC129 (as revised) or TSOC196 (as
revised)) or GPS/WAAS equipment while the aircraft remains in Air Traffic
Control radar surveillance.
may operate on GNSS Troutes with GPS/WAAS (TSOC145(as revised) or TSOC146 (as
to fly approaches under IFR using GPS or GPS/WAAS avionics systems requires that
a pilot use avionics with:
TSO-C129, (as revised) authorization in class A1,
B1, B3, C1, or C3;
TSOC196 (as revised) authorization; or
TSOC145 (as revised) or TSOC146 (as revised) authorization.
GPS IFR Equipment Classes/Categories
Nav. Sys. to Prov. RAIM Equiv.
Nonprecision Approach Capable
- GPS sensor and navigation capability.
- GPS sensor data to an integrated navigation system (i.e., FMS, multi-sensor
navigation system, etc.).
- GPS sensor data to an integrated navigation system (as in Class B) which
provides enhanced guidance to an autopilot, or flight director, to reduce flight
tech. errors. Limited to 14 CFR Part 121 or equivalent criteria.
GPS Approval Required/Authorized Use
Installation Approval Required
Operational Approval Required
In Lieu of ADF and/or DME3
Route and Terminal
Route, Terminal, and Approach
1To determine equipment approvals and
limitations, refer to the AFM, AFM supplements, or pilot guides.
2Requires verification of data for
correctness if database is expired.
3Requires current database or
verification that the procedure has not been amended since the expiration of the
4VFR and hand-held GPS systems are not
authorized for IFR navigation, instrument approaches, or as a primary instrument
flight reference. During IFR operations they may be considered only an aid to
5Hand-held receivers require no
approval. However, any aircraft modification to support the hand-held receiver;
i.e., installation of an external antenna or a permanent mounting bracket, does
the production of standalone GPS approaches has progressed, many of the
original overlay approaches have been replaced with standalone procedures
specifically designed for use by GPS systems.
A GPS approach overlay allows pilots to use GPS avionics under IFR for flying
designated nonprecision instrument approach procedures, except LOC, LDA, and
simplified directional facility (SDF) procedures. These procedures are
identified by the name of the procedure and “or GPS” (for example, VOR/DME or
GPS RWY15). Other previous types of overlays have either been converted to this
format or replaced with standalone procedures. Only approaches contained in the
current onboard navigation database are authorized. The navigation database may
contain information about nonoverlay approach procedures that is intended to be
used to enhance position orientation, generally by providing a map, while flying
these approaches using conventional NAVAIDs. This approach information should
not be confused with a GPS overlay approach. (See the receiver operating manual,
AFM, or AFM Supplement for details on how to identify these approaches in the
f. General Database Requirements
onboard navigation data must be current and appropriate for the region of
intended operation and should include the navigation aids, waypoints, and
relevant coded terminal airspace procedures for the departure, arrival, and
database guidance for terminal and en route requirements may be found in AC
90100, U.S. Terminal and En Route Area Navigation (RNAV) Operations.
database guidance on Required Navigation Performance (RNP) instrument approach
operations, RNP terminal, and RNP en route requirements may be found in AC
90105, Approval Guidance for RNP Operations and Barometric Vertical Navigation
in the U.S. National Airspace System.
approach procedures to be flown must be retrievable from the current airborne
navigation database supplied by the equipment manufacturer or other FAA approved
source. The system must be able to retrieve the procedure by name from the
aircraft navigation database, not just as a manually entered series of
waypoints. Manual entry of waypoints using latitude/longitude or place/bearing
is not permitted for approach procedures.
to using a procedure or waypoint retrieved from the airborne navigation
database, the pilot should verify the validity of the database. This
verification should include the following preflight and inflight steps:
the date of database issuance, and verify that the date/time of proposed use is
before the expiration date/time.
that the database provider has not published a notice limiting the use of the
specific waypoint or procedure.
that the waypoints and transition names coincide with names found on the
procedure chart. Do not use waypoints which do not exactly match the spelling
shown on published procedure charts.
that the waypoints are generally logical in location, in the correct order, and
that their orientation to each other is as found on the procedure chart, both
laterally and vertically.
There is no specific requirement to check each waypoint latitude and longitude,
type of waypoint and/or altitude constraint, only the general relationship of
waypoints in the procedure, or the logic of an individual waypoint's location.
the cursory check of procedure logic or individual waypoint location, specified
in [b] above, indicates a potential error, do not
use the retrieved procedure or waypoint until a verification of latitude and
longitude, waypoint type, and altitude constraints indicate full conformity with
the published data.
g. GPS Approach
As the production of
stand-alone GPS approaches has progressed, many of the original overlay
approaches have been replaced with stand-alone procedures specifically designed
for use by GPS systems. The title of the remaining GPS overlay procedures has
been revised on the approach chart to “or GPS” (e.g., VOR or GPS RWY 24).
Therefore, all the approaches that can be used by GPS now contain “GPS” in the
title (e.g., “VOR or GPS RWY 24,” “GPS RWY 24,” or “RNAV (GPS) RWY 24”). During
these GPS approaches, underlying ground-based NAVAIDs are not required to be
operational and associated aircraft avionics need not be installed, operational,
turned on or monitored (monitoring of the underlying approach is suggested when
equipment is available and functional). Existing overlay approaches may be
requested using the GPS title, such as “GPS RWY 24” for the VOR or GPS RWY 24.
Any required alternate airport must have an approved instrument approach
procedure other than GPS that is anticipated to be operational and available at
the estimated time of arrival, and which the aircraft is equipped to fly.
1. GPS satellite
outages are issued as GPS NOTAMs both domestically and internationally. However,
the effect of an outage on the intended operation cannot be determined unless
the pilot has a RAIM availability prediction program which allows excluding a
satellite which is predicted to be out of service based on the NOTAM
term UNRELIABLE is used in conjunction with GPS NOTAMs. The term UNRELIABLE is
an advisory to pilots indicating the expected level of service may not be
available. UNRELIABLE does not mean there is a problem with GPS signal
integrity. If GPS service is available, pilots may continue operations. If the
LNAV or LNAV/VNAV service is available, pilots may use the displayed level of
service to fly the approach. GPS operation may be NOTAMed UNRELIABLE due to
testing or anomalies. (Pilots are encouraged to report GPS anomalies, including
degraded operation and/or loss of service, as soon as possible, reference
paragraph 1-1-14.) Air Traffic Control will advise
pilots requesting a GPS or RNAV (GPS) approach of GPS UNRELIABLE for:
(a) NOTAMs not
contained in the ATIS broadcast.
(b) Pilot reports
of GPS anomalies received within the preceding 15 minutes.
3. Civilian pilots may obtain GPS RAIM availability information for
nonprecision approach procedures by specifically requesting GPS aeronautical
information from a Flight Service Station during preflight briefings. GPS RAIM
aeronautical information can be obtained for a period of 3 hours (for example,
if you are scheduled to arrive at 1215 hours, then the GPS RAIM information is
available from 1100 to 1400 hours) or a 24 hour time frame at a particular
airport. FAA briefers will provide RAIM information for a period of 1 hour
before to 1 hour after the ETA hour, unless a specific time frame is requested
by the pilot. If flying a published GPS departure, a RAIM prediction should also
be requested for the departure airport.
4. The military
provides airfield specific GPS RAIM NOTAMs for nonprecision approach procedures
at military airfields. The RAIM outages are issued as M-series NOTAMs and may be
obtained for up to 24 hours from the time of request.
manufacturers and/or database suppliers may supply “NOTAM” type information
concerning database errors. Pilots should check these sources, when available,
to ensure that they have the most current information concerning their
i. Receiver Autonomous
Integrity Monitoring (RAIM)
1. RAIM outages
may occur due to an insufficient number of satellites or due to unsuitable
satellite geometry which causes the error in the position solution to become too
large. Loss of satellite reception and RAIM warnings may occur due to aircraft
dynamics (changes in pitch or bank angle). Antenna location on the aircraft,
satellite position relative to the horizon, and aircraft attitude may affect
reception of one or more satellites. Since the relative positions of the
satellites are constantly changing, prior experience with the airport does not
guarantee reception at all times, and RAIM availability should always be
2. If RAIM is not
available, another type of navigation and approach system must be used, another
destination selected, or the trip delayed until RAIM is predicted to be
available on arrival. On longer flights, pilots should consider rechecking the
RAIM prediction for the destination during the flight. This may provide early
indications that an unscheduled satellite outage has occurred since takeoff.
3. If a RAIM
failure/status annunciation occurs prior to the final approach waypoint
(FAWP), the approach should not be completed since GPS may no longer provide
the required accuracy. The receiver performs a RAIM prediction by 2 NM prior
to the FAWP to ensure that RAIM is available at the FAWP as a condition for
entering the approach mode. The pilot should ensure that the receiver has
sequenced from “Armed” to “Approach” prior to the FAWP (normally occurs 2 NM
prior). Failure to sequence may be an indication of the detection of a satellite
anomaly, failure to arm the receiver (if required), or other problems which
preclude completing the approach.
4. If the receiver
does not sequence into the approach mode or a RAIM failure/status annunciation
occurs prior to the FAWP, the pilot should not descend to Minimum Descent
Altitude (MDA), but should proceed to the missed approach waypoint (MAWP) via
the FAWP, perform a missed approach, and contact ATC as soon as practical. Refer
to the receiver operating manual for specific indications and instructions
associated with loss of RAIM prior to the FAF.
5. If a RAIM
failure occurs after the FAWP, the receiver is allowed to continue operating
without an annunciation for up to 5 minutes to allow completion of the approach
(see receiver operating manual). If the RAIM flag/status annunciation appears
after the FAWP, the missed approach should be executed immediately.
1. GPS approaches
make use of both fly-over and fly-by waypoints. Fly-by waypoints are used when
an aircraft should begin a turn to the next course prior to reaching the
waypoint separating the two route segments. This is known as turn anticipation
and is compensated for in the airspace and terrain clearances. Approach
waypoints, except for the MAWP and the missed approach holding waypoint (MAHWP),
are normally fly-by waypoints. Fly-over waypoints are used when the aircraft
must fly over the point prior to starting a turn. New approach charts depict
fly-over waypoints as a circled waypoint symbol. Overlay approach charts and
some early stand alone GPS approach charts may not reflect this convention.
2. Since GPS receivers are basically “To-To”
navigators, they must always be navigating to a
defined point. On overlay approaches, if no
pronounceable five-character name is published for
an approach waypoint or fix, it was given a database
identifier consisting of letters and numbers. These
points will appear in the list of waypoints in the
approach procedure database, but may not appear on
the approach chart. A point used for the purpose of
defining the navigation track for an airborne
computer system (i.e., GPS or FMS) is called a
Computer Navigation Fix (CNF). CNFs include
unnamed DME fixes, beginning and ending points of
DME arcs and sensor final approach fixes (FAFs) on
some GPS overlay approaches. To aid in the approach
chart/database correlation process, the FAA has
begun a program to assign five-letter names to CNFs
and to chart CNFs on various FAA Aeronautical
Navigation Products (AeroNav Products). These
CNFs are not to be used for any air traffic control
(ATC) application, such as holding for which the fix
has not already been assessed. CNFs will be charted
to distinguish them from conventional reporting
points, fixes, intersections, and waypoints. The CNF
name will be enclosed in parenthesis, e.g., (CFBCD),
and the name will be placed next to the CNF it
defines. If the CNF is not at an existing point defined
by means such as crossing radials or radial/DME, the
point will be indicated by an “X.” The CNF name will
not be used in filing a flight plan or in aircraft/ATC
communications. Use current phraseology, e.g.,
facility name, radial, distance, to describe these fixes.
waypoints in the database will be uniquely identified for each airport but may
be repeated for another airport (e.g., RW36 will be used at each airport with a
runway 36 but will be at the same location for all approaches at a given
4. The runway
threshold waypoint, which is normally the MAWP, may have a five letter
identifier (e.g., SNEEZ) or be coded as RW## (e.g., RW36, RW36L). Those
thresholds which are coded as five letter identifiers are being changed to the
RW## designation. This may cause the approach chart and database to differ until
all changes are complete. The runway threshold waypoint is also used as the
center of the Minimum Safe Altitude (MSA) on most GPS approaches. MAWPs not
located at the threshold will have a five letter identifier.
As with most RNAV
systems, pilots should pay particular attention to position orientation while
using GPS. Distance and track information are provided to the next active
waypoint, not to a fixed navigation aid. Receivers may sequence when the pilot
is not flying along an active route, such as when being vectored or deviating
for weather, due to the proximity to another waypoint in the route. This can be
prevented by placing the receiver in the nonsequencing mode. When the receiver
is in the nonsequencing mode, bearing and distance are provided to the selected
waypoint and the receiver will not sequence to the next waypoint in the route
until placed back in the auto sequence mode or the pilot selects a different
waypoint. On overlay approaches, the pilot may have to compute the along-track
distance to stepdown fixes and other points due to the receiver showing
along-track distance to the next waypoint rather than DME to the VOR or ILS
l. Conventional Versus
GPS Navigation Data
There may be slight
differences between the course information portrayed on navigational charts and
a GPS navigation display when flying authorized GPS instrument procedures or
along an airway. All magnetic tracks defined by any conventional navigation aids
are determined by the application of the station magnetic variation. In
contrast, GPS RNAV systems may use an algorithm, which applies the local
magnetic variation and may produce small differences in the displayed course.
However, both methods of navigation should produce the same desired ground track
when using approved, IFR navigation system. Should significant differences
between the approach chart and the GPS avionics' application of the navigation
database arise, the published approach chart, supplemented by NOTAMs, holds
Due to the GPS avionics'
computation of great circle courses, and the variations in magnetic variation,
the bearing to the next waypoint and the course from the last waypoint (if
available) may not be exactly 180° apart when long distances are involved.
Variations in distances will occur since GPS distance-to-waypoint values are
along-track distances (ATD) computed to the next waypoint and the DME values
published on underlying procedures are slant-range distances measured to the
station. This difference increases with aircraft altitude and proximity to the
m. Departures and Instrument Departure Procedures (DPs)
The GPS receiver must be
set to terminal (±1 NM) CDI sensitivity and the navigation routes contained in
the database in order to fly published IFR charted departures and DPs. Terminal
RAIM should be automatically provided by the receiver. (Terminal RAIM for
departure may not be available unless the waypoints are part of the active
flight plan rather than proceeding direct to the first destination.) Certain
segments of a DP may require some manual intervention by the pilot, especially
when radar vectored to a course or required to intercept a specific course to a
waypoint. The database may not contain all of the transitions or departures from
all runways and some GPS receivers do not contain DPs in the database. It
is necessary that helicopter procedures be flown at 70 knots or less since
helicopter departure procedures and missed approaches use a 20:1 obstacle
clearance surface (OCS), which is double the fixed-wing OCS, and turning areas
are based on this speed as well.
n. Flying GPS
which area of the TAA the aircraft will enter when flying a “T” with a TAA must
be accomplished using the bearing and distance to the IF(IAF). This is most
critical when entering the TAA in the vicinity of the extended runway centerline
and determining whether you will be entering the right or left base area. Once
inside the TAA, all sectors and stepdowns are based on the bearing and distance
to the IAF for that area, which the aircraft should be proceeding direct to at
that time, unless on vectors. (See FIG 5-4-3
and FIG 5-4-4.)
2. Pilots should
fly the full approach from an Initial Approach Waypoint (IAWP) or feeder fix
unless specifically cleared otherwise. Randomly joining an approach at an
intermediate fix does not assure terrain clearance.
3. When an
approach has been loaded in the flight plan, GPS receivers will give an “arm”
annunciation 30 NM straight line distance from the airport/heliport reference
point. Pilots should arm the approach mode at this time, if it has not already
been armed (some receivers arm automatically). Without arming, the receiver will
not change from en route CDI and RAIM sensitivity of ±5 NM either side of
centerline to ±1 NM terminal sensitivity. Where the IAWP is inside this 30 mile
point, a CDI sensitivity change will occur once the approach mode is armed and
the aircraft is inside 30 NM. Where the IAWP is beyond 30 NM from the
airport/heliport reference point, CDI sensitivity will not change until the
aircraft is within 30 miles of the airport/heliport reference point even if the
approach is armed earlier. Feeder route obstacle clearance is predicated on the
receiver being in terminal (±1 NM) CDI sensitivity and RAIM within 30 NM of the
airport/heliport reference point, therefore, the receiver should always be armed
(if required) not later than the 30 NM annunciation.
4. The pilot must
be aware of what bank angle/turn rate the particular receiver uses to compute
turn anticipation, and whether wind and airspeed are included in the receiver's
calculations. This information should be in the receiver operating manual. Over
or under banking the turn onto the final approach course may significantly delay
getting on course and may result in high descent rates to achieve the next
5. When within 2
NM of the FAWP with the approach mode armed, the approach mode will switch to
active, which results in RAIM changing to approach sensitivity and a change in
CDI sensitivity. Beginning 2 NM prior to the FAWP, the full scale CDI
sensitivity will smoothly change from ±1 NM to ±0.3 NM at the FAWP. As
sensitivity changes from ±1 NM to ±0.3 NM approaching the FAWP, with the CDI not
centered, the corresponding increase in CDI displacement may give the impression
that the aircraft is moving further away from the intended course even though it
is on an acceptable intercept heading. Referencing the digital track
displacement information (cross track error), if it is available in the approach
mode, may help the pilot remain position oriented in this situation. Being
established on the final approach course prior to the beginning of the
sensitivity change at 2 NM will help prevent problems in interpreting the CDI
display during ramp down. Therefore, requesting or accepting vectors which will
cause the aircraft to intercept the final approach course within 2 NM of the
FAWP is not recommended.
6. When receiving
vectors to final, most receiver operating manuals suggest placing the receiver
in the nonsequencing mode on the FAWP and manually setting the course. This
provides an extended final approach course in cases where the aircraft is
vectored onto the final approach course outside of any existing segment which is
aligned with the runway. Assigned altitudes must be maintained until established
on a published segment of the approach. Required altitudes at waypoints outside
the FAWP or stepdown fixes must be considered. Calculating the distance to the
FAWP may be required in order to descend at the proper location.
7. Overriding an automatically selected sensitivity during an approach will
cancel the approach mode annunciation. If the approach mode is not armed by 2 NM
prior to the FAWP, the approach mode will not become active at 2 NM prior to the
FAWP, and the equipment will flag. In these conditions, the RAIM and CDI
sensitivity will not ramp down, and the pilot should not descend to MDA, but fly
to the MAWP and execute a missed approach. The approach active annunciator
and/or the receiver should be checked to ensure the approach mode is active
prior to the FAWP.
8. Do not attempt
to fly an approach unless the procedure in the on-board database is current and
identified as “GPS" on the approach chart. The navigation database may contain
information about nonoverlay approach procedures that is intended to be used to
enhance position orientation, generally by providing a map, while flying these
approaches using conventional NAVAIDs. This approach information should not be
confused with a GPS overlay approach (see the receiver operating manual, AFM, or
AFM Supplement for details on how to identify these procedures in the navigation
database). Flying point to point on the approach does not assure compliance with
the published approach procedure. The proper RAIM sensitivity will not be
available and the CDI sensitivity will not automatically change to ±0.3 NM.
Manually setting CDI sensitivity does not automatically change the RAIM
sensitivity on some receivers. Some existing nonprecision approach procedures
cannot be coded for use with GPS and will not be available as overlays.
9. Pilots should
pay particular attention to the exact operation of their GPS receivers for
performing holding patterns and in the case of overlay approaches, operations
such as procedure turns. These procedures may require manual intervention by the
pilot to stop the sequencing of waypoints by the receiver and to resume
automatic GPS navigation sequencing once the maneuver is complete. The same
waypoint may appear in the route of flight more than once consecutively (e.g.,
IAWP, FAWP, MAHWP on a procedure turn). Care must be exercised to ensure that
the receiver is sequenced to the appropriate waypoint for the segment of the
procedure being flown, especially if one or more fly-overs are skipped (e.g.,
FAWP rather than IAWP if the procedure turn is not flown). The pilot may have to
sequence past one or more fly-overs of the same waypoint in order to start GPS
automatic sequencing at the proper place in the sequence of waypoints.
inputs into the GPS receiver are especially critical during approaches. In some
cases, an incorrect entry can cause the receiver to leave the approach mode.
11. A fix on an
overlay approach identified by a DME fix will not be in the waypoint sequence on
the GPS receiver unless there is a published name assigned to it. When a name is
assigned, the along track to the waypoint may be zero rather than the DME stated
on the approach chart. The pilot should be alert for this on any overlay
procedure where the original approach used DME.
12. If a visual
descent point (VDP) is published, it will not be included in the sequence of
waypoints. Pilots are expected to use normal piloting techniques for beginning
the visual descent, such as ATD.
stepdown fixes in the final approach segment will not be coded in the waypoint
sequence of the aircraft's navigation database and must be identified using ATD.
Stepdown fixes in the final approach segment of RNAV (GPS) approaches are being
named, in addition to being identified by ATD. However, since most GPS avionics
do not accommodate waypoints between the FAF and MAP, even when the waypoint is
named, the waypoints for these stepdown fixes may not appear in the sequence of
waypoints in the navigation database. Pilots must continue to identify these
stepdown fixes using ATD.
o. Missed Approach
1. A GPS missed
approach requires pilot action to sequence the receiver past the MAWP to the
missed approach portion of the procedure. The pilot must be thoroughly familiar
with the activation procedure for the particular GPS receiver installed in the
aircraft and must initiate appropriate action after the MAWP. Activating
the missed approach prior to the MAWP will cause CDI sensitivity to immediately
change to terminal (±1NM) sensitivity and the receiver will continue to navigate
to the MAWP. The receiver will not sequence past the MAWP. Turns should not
begin prior to the MAWP. If the missed approach is not activated, the GPS
receiver will display an extension of the inbound final approach course and the
ATD will increase from the MAWP until it is manually sequenced after crossing
2. Missed approach routings in which the first track is via a course rather
than direct to the next waypoint require additional action by the pilot
to set the course. Being familiar with all of the inputs required is especially
critical during this phase of flight.
p. GPS Familiarization
Pilots should practice
GPS approaches under visual meteorological conditions (VMC) until thoroughly
proficient with all aspects of their equipment (receiver and installation) prior
to attempting flight by IFR in instrument meteorological conditions (IMC). Some
of the areas which the pilot should practice are:
1. Utilizing the
receiver autonomous integrity monitoring (RAIM) prediction function;
2. Inserting a DP
into the flight plan, including setting terminal CDI sensitivity, if required,
and the conditions under which terminal RAIM is available for departure (some
receivers are not DP or STAR capable);
3. Programming the
4. Programming and
flying the overlay approaches (especially procedure turns and arcs);
5. Changing to
another approach after selecting an approach;
6. Programming and
flying “direct” missed approaches;
7. Programming and
flying “routed” missed approaches;
flying, and exiting holding patterns, particularly on overlay approaches with a
second waypoint in the holding pattern;
9. Programming and
flying a “route” from a holding pattern;
and flying an approach with radar vectors to the intermediate segment;
11. Indication of
the actions required for RAIM failure both before and after the FAWP; and
12. Programming a
radial and distance from a VOR (often used in departure instructions).
1-1-20. Wide Area Augmentation System (WAAS)
1. The FAA
developed the Wide Area Augmentation System (WAAS) to improve the accuracy,
integrity and availability of GPS signals. WAAS will allow GPS to be used, as
the aviation navigation system, from takeoff through Category I precision
approach when it is complete. WAAS is a critical component of the FAA's
strategic objective for a seamless satellite navigation system for civil
aviation, improving capacity and safety.
International Civil Aviation Organization (ICAO) has defined Standards and
Recommended Practices (SARPs) for satellite-based augmentation systems (SBAS)
such as WAAS. Japan and Europe are building similar systems that are planned to
be interoperable with WAAS: EGNOS, the European Geostationary Navigation Overlay
System, and MSAS, the Japan Multifunctional Transport Satellite (MTSAT)
Satellite-based Augmentation System. The merging of these systems will create a
worldwide seamless navigation capability similar to GPS but with greater
accuracy, availability and integrity.
traditional ground-based navigation aids, WAAS will cover a more extensive
service area. Precisely surveyed wide-area ground reference stations (WRS) are
linked to form the U.S. WAAS network. Signals from the GPS satellites are
monitored by these WRSs to determine satellite clock and ephemeris corrections
and to model the propagation effects of the ionosphere. Each station in the
network relays the data to a wide-area master station (WMS) where the correction
information is computed. A correction message is prepared and uplinked to a
geostationary satellite (GEO) via a ground uplink station (GUS). The message is
then broadcast on the same frequency as GPS (L1, 1575.42 MHz) to WAAS receivers
within the broadcast coverage area of the WAAS GEO.
4. In addition to providing the correction signal, the WAAS GEO provides an
additional pseudorange measurement to the aircraft receiver, improving the
availability of GPS by providing, in effect, an additional GPS satellite in
view. The integrity of GPS is improved through real-time monitoring, and the
accuracy is improved by providing differential corrections to reduce errors. The
performance improvement is sufficient to enable approach procedures with
GPS/WAAS glide paths (vertical guidance).
5. The FAA has
completed installation of 25 WRSs, 2 WMSs, 4 GUSs, and the required terrestrial
communications to support the WAAS network. Prior to the commissioning of the
WAAS for public use, the FAA has been conducting a series of test and validation
activities. Enhancements to the initial phase of WAAS will include additional
master and reference stations, communication satellites, and transmission
frequencies as needed.
navigation, including GPS and WAAS, is referenced to the WGS-84 coordinate
system. It should only be used where the Aeronautical Information Publications
(including electronic data and aeronautical charts) conform to WGS-84 or
equivalent. Other countries civil aviation authorities may impose additional
limitations on the use of their SBAS systems.
b. Instrument Approach
1. A new class of
approach procedures which provide vertical guidance, but which do not meet the
ICAO Annex 10 requirements for precision approaches has been developed to
support satellite navigation use for aviation applications worldwide. These new
procedures called Approach with Vertical Guidance (APV), are defined in ICAO
Annex 6, and include approaches such as the LNAV/VNAV procedures presently being
flown with barometric vertical navigation (Baro-VNAV). These approaches provide
vertical guidance, but do not meet the more stringent standards of a precision
approach. Properly certified WAAS receivers will be able to fly these LNAV/VNAV
procedures using a WAAS electronic glide path, which eliminates the errors that
can be introduced by using Barometric altimetery.
2. A new type of
APV approach procedure, in addition to LNAV/VNAV, is being implemented to take
advantage of the high accuracy guidance and increased integrity provided by
WAAS. This WAAS generated angular guidance allows the use of the same TERPS
approach criteria used for ILS approaches. The resulting approach procedure
minima, titled LPV (localizer performance with vertical guidance), may have a
decision altitude as low as 200 feet height above touchdown with visibility
minimums as low as 1/2 mile, when the terrain and airport
infrastructure support the lowest minima. LPV minima is published on the RNAV
(GPS) approach charts (see paragraph
5-4-5, Instrument Approach Procedure
3. A new
nonprecision WAAS approach, called Localizer Performance (LP) is being added in
locations where the terrain or obstructions do not allow publication of
vertically guided LPV procedures. This new approach takes advantage of the
angular lateral guidance and smaller position errors provided by WAAS to provide
a lateral only procedure similar to an ILS Localizer. LP procedures may provide
lower minima than a LNAV procedure due to the narrower obstacle clearance
WAAS receivers certified prior to TSO-C145b and
TSO-C146b, even if they have LPV capability, do not contain LP capability unless
the receiver has been upgraded. Receivers capable of flying LP procedures must
contain a statement in the Flight Manual Supplement or Approved Supplemental
Flight Manual stating that the receiver has LP capability, as well as the
capability for the other WAAS and GPS approach procedure types.
4. WAAS provides a
level of service that supports all phases of flight, including RNAV (GPS)
approaches to LNAV, LP, LNAV/VNAV and LPV lines of minima, within system
coverage. Some locations close to the edge of the coverage may have a lower
availability of vertical guidance.
1. WAAS avionics
must be certified in accordance with Technical Standard Order (TSO) TSO-C145A,
Airborne Navigation Sensors Using the (GPS) Augmented by the Wide Area
Augmentation System (WAAS); or TSO-146A, Stand-Alone Airborne Navigation
Equipment Using the Global Positioning System (GPS) Augmented by the Wide Area
Augmentation System (WAAS), and installed in accordance with Advisory Circular
(AC) 20-130A, Airworthiness Approval of Navigation or Flight Management Systems
Integrating Multiple Navigation Sensors, or AC 20-138A, Airworthiness Approval
of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR
2. GPS/WAAS operation must be conducted in accordance with the FAA-approved
aircraft flight manual (AFM) and flight manual supplements. Flight manual
supplements will state the level of approach procedure that the receiver
supports. IFR approved WAAS receivers support all GPS only operations as long as
lateral capability at the appropriate level is functional. WAAS monitors both
GPS and WAAS satellites and provides integrity.
equipment is inherently capable of supporting oceanic and remote operations if
the operator obtains a fault detection and exclusion (FDE) prediction program.
4. Air carrier and
commercial operators must meet the appropriate provisions of their approved
5. Prior to
GPS/WAAS IFR operation, the pilot must review appropriate Notices to Airmen
(NOTAMs) and aeronautical information. This information is available on request
from a Flight Service Station. The FAA will provide NOTAMs to advise pilots of
the status of the WAAS and level of service available.
(a) The term
UNRELIABLE is used in conjunction with GPS and WAAS NOTAMs. The term UNRELIABLE
is an advisory to pilots indicating the expected level of WAAS service
(LNAV/VNAV, LPV) may not be available; e.g., !BOS BOS WAAS LPV AND LNAV/VNAV
MNM UNREL WEF 0305231700 - 0305231815. WAAS UNRELIABLE NOTAMs are predictive
in nature and published for flight planning purposes. Upon commencing an
approach at locations NOTAMed WAAS UNRELIABLE, if the WAAS avionics indicate
LNAV/VNAV or LPV service is available, then vertical guidance may be used to
complete the approach using the displayed level of service. Should an outage
occur during the approach, reversion to LNAV minima may be required.
(1) Area-wide WAAS
UNAVAILABLE NOTAMs indicate loss or malfunction of the WAAS system. In flight,
Air Traffic Control will advise pilots requesting a GPS or RNAV (GPS) approach
of WAAS UNAVAILABLE NOTAMs if not contained in the ATIS broadcast.
WAAS UNRELIABLE NOTAMs indicate an expected level of service, e.g., LNAV/VNAV or
LPV may not be available. Pilots must request site-specific WAAS NOTAMs during
flight planning. In flight, Air Traffic Control will not advise pilots of WAAS
(3) When the
approach chart is annotated with the
symbol, site-specific WAAS UNRELIABLE NOTAMs or Air Traffic advisories are not
provided for outages in WAAS LNAV/VNAV and LPV vertical service. Vertical
outages may occur daily at these locations due to being close to the edge of
WAAS system coverage. Use LNAV minima for flight planning at these locations,
whether as a destination or alternate. For flight operations at these locations,
when the WAAS avionics indicate that LNAV/VNAV or LPV service is available, then
the vertical guidance may be used to complete the approach using the displayed
level of service. Should an outage occur during the procedure, reversion to LNAV
minima may be required.
Area-wide WAAS UNAVAILABLE NOTAMs apply to all airports in the WAAS UNAVAILABLE
area designated in the NOTAM, including approaches at airports where an approach
chart is annotated with the
6. GPS/WAAS was
developed to be used within SBAS GEO coverage (WAAS or other interoperable
system) without the need for other radio navigation equipment appropriate to the
route of flight to be flown. Outside the SBAS coverage or in the event of a WAAS
failure, GPS/WAAS equipment reverts to GPS-only operation and satisfies the
requirements for basic GPS equipment.
7. Unlike TSO-C129
avionics, which were certified as a supplement to other means of navigation,
WAAS avionics are evaluated without reliance on other navigation systems. As
such, installation of WAAS avionics does not require the aircraft to have other
equipment appropriate to the route to be flown.
(a) Pilots with
WAAS receivers may flight plan to use any instrument approach procedure
authorized for use with their WAAS avionics as the planned approach at a
required alternate, with the following restrictions. When using WAAS at
an alternate airport, flight planning must be based on flying the RNAV (GPS)
LNAV minima line, or minima on a GPS approach procedure, or conventional
approach procedure with “or GPS” in the title. Code of Federal Regulation (CFR)
Part 91 nonprecision weather requirements must be used for planning. Upon
arrival at an alternate, when the WAAS navigation system indicates that
LNAV/VNAV or LPV service is available, then vertical guidance may be used to
complete the approach using the displayed level of service. The FAA has begun
NA (Alternate Minimums Not Authorized) symbol from select RNAV (GPS) and GPS
approach procedures so they may be used by approach approved WAAS receivers at
alternate airports. Some approach procedures will still require the
NA for other reasons, such as no weather reporting, so it cannot be removed
from all procedures. Since every procedure must be individually evaluated,
removal of the NA
from RNAV (GPS) and GPS procedures will take some time.
d. Flying Procedures with WAAS
1. WAAS receivers
support all basic GPS approach functions and provide additional capabilities.
One of the major improvements is the ability to generate glide path guidance,
independent of ground equipment or barometric aiding. This eliminates several
problems such as hot and cold temperature effects, incorrect altimeter setting
or lack of a local altimeter source. It also allows approach procedures to be
built without the cost of installing ground stations at each airport or runway.
Some approach certified receivers may only generate a glide path with
performance similar to Baro-VNAV and are only approved to fly the LNAV/VNAV line
of minima on the RNAV (GPS) approach charts. Receivers with additional
capability (including faster update rates and smaller integrity limits) are
approved to fly the LPV line of minima. The lateral integrity changes
dramatically from the 0.3 NM (556 meter) limit for GPS, LNAV and LNAV/VNAV
approach mode, to 40 meters for LPV. It also provides vertical integrity
monitoring, which bounds the vertical error to 50 meters for LNAV/VNAV and LPVs
with minima of 250' or above, and bounds the vertical error to 35 meters for
LPVs with minima below 250'.
2. When an
approach procedure is selected and active, the receiver will notify the pilot of
the most accurate level of service supported by the combination of the WAAS
signal, the receiver, and the selected approach, using the naming conventions on
the minima lines of the selected approach procedure. For example, if an approach
is published with LPV minima and the receiver is only certified for LNAV/VNAV,
the equipment would indicate “LNAV/VNAV available,” even though the WAAS signal
would support LPV. If flying an existing LNAV/VNAV procedure with no LPV minima,
the receiver will notify the pilot “LNAV/VNAV available,” even if the receiver
is certified for LPV and the signal supports LPV. If the signal does not support
vertical guidance on procedures with LPV and/or LNAV/VNAV minima, the receiver
annunciation will read “LNAV available.” On lateral only procedures with LP and
LNAV minima the receiver will indicate “LP available” or “LNAV available” based
on the level of lateral service available. Once the level of service
notification has been given, the receiver will operate in this mode for the
duration of the approach procedure, unless that level of service becomes
unavailable. The receiver cannot change back to a more accurate level of service
until the next time an approach is activated.
Receivers do not “fail down” to lower levels of service once the approach has
been activated. If only the vertical off flag appears, the pilot may elect to
use the LNAV minima if the rules under which the flight is operating allow
changing the type of approach being flown after commencing the procedure. If the
lateral integrity limit is exceeded on an LP approach, a missed approach will be
necessary since there is no way to reset the lateral alarm limit while the
approach is active.
additional feature of WAAS receivers is the ability to exclude a bad GPS signal
and continue operating normally. This is normally accomplished by the WAAS
correction information. Outside WAAS coverage or when WAAS is not available, it
is accomplished through a receiver algorithm called FDE. In most cases this
operation will be invisible to the pilot since the receiver will continue to
operate with other available satellites after excluding the “bad” signal. This
capability increases the reliability of navigation.
4. Both lateral
and vertical scaling for the LNAV/VNAV and LPV approach procedures are different
than the linear scaling of basic GPS. When the complete published procedure is
flown, +/-1 NM linear scaling is provided until two (2) NM prior to the FAF,
where the sensitivity increases to be similar to the angular scaling of an ILS.
There are two differences in the WAAS scaling and ILS: 1) on long final approach
segments, the initial scaling will be +/-0.3 NM to achieve equivalent
performance to GPS (and better than ILS, which is less sensitive far from the
runway); 2) close to the runway threshold, the scaling changes to linear instead
of continuing to become more sensitive. The width of the final approach course
is tailored so that the total width is usually 700 feet at the runway threshold.
Since the origin point of the lateral splay for the angular portion of the final
is not fixed due to antenna placement like localizer, the splay angle can remain
fixed, making a consistent width of final for aircraft being vectored onto the
final approach course on different length runways. When the complete published
procedure is not flown, and instead the aircraft needs to capture the extended
final approach course similar to ILS, the vector to final (VTF) mode is used.
Under VTF the scaling is linear at +/-1 NM until the point where the ILS angular
splay reaches a width of +/-1 NM regardless of the distance from the FAWP.
5. The WAAS scaling is also different than GPS TSO-C129 in the initial
portion of the missed approach. Two differences occur here. First, the scaling
abruptly changes from the approach scaling to the missed approach scaling, at
approximately the departure end of the runway or when the pilot requests missed
approach guidance rather than ramping as GPS does. Second, when the first leg of
the missed approach is a Track to Fix (TF) leg aligned within 3 degrees of the
inbound course, the receiver will change to 0.3 NM linear sensitivity until the
turn initiation point for the first waypoint in the missed approach procedure,
at which time it will abruptly change to terminal (+/-1 NM) sensitivity. This
allows the elimination of close in obstacles in the early part of the missed
approach that may cause the DA to be raised.
6. A new method
has been added for selecting the final approach segment of an instrument
approach. Along with the current method used by most receivers using menus where
the pilot selects the airport, the runway, the specific approach procedure and
finally the IAF, there is also a channel number selection method. The pilot
enters a unique 5-digit number provided on the approach chart, and the receiver
recalls the matching final approach segment from the aircraft database. A list
of information including the available IAFs is displayed and the pilot selects
the appropriate IAF. The pilot should confirm that the correct final approach
segment was loaded by cross checking the Approach ID, which is also provided on
the approach chart.
7. The Along-Track
Distance (ATD) during the final approach segment of an LNAV procedure (with a
minimum descent altitude) will be to the MAWP. On LNAV/VNAV and LPV approaches
to a decision altitude, there is no missed approach waypoint so the along-track
distance is displayed to a point normally located at the runway threshold. In
most cases the MAWP for the LNAV approach is located on the runway threshold at
the centerline, so these distances will be the same. This distance will always
vary slightly from any ILS DME that may be present, since the ILS DME is located
further down the runway. Initiation of the missed approach on the LNAV/VNAV and
LPV approaches is still based on reaching the decision altitude without any of
the items listed in 14 CFR Section 91.175 being visible, and must not be delayed
until the ATD reaches zero. The WAAS receiver, unlike a GPS receiver, will
automatically sequence past the MAWP if the missed approach procedure has been
designed for RNAV. The pilot may also select missed approach prior to the MAWP,
however, navigation will continue to the MAWP prior to waypoint sequencing
1-1-21. Ground Based
Augmentation System (GBAS) Landing System (GLS)
1. The GLS
provides precision navigation guidance for exact alignment and descent of
aircraft on approach to a runway. It provides differential augmentation to the
Global Navigation Satellite System (GNSS).
GBAS is the ICAO term for Local Area Augmentation System (LAAS).
2. LAAS was
developed as an “ILS look-alike” system from the pilot perspective. LAAS is
based on GPS signals augmented by ground equipment and has been developed to
provide GLS precision approaches similar to ILS at airfields.
3. GLS provides
guidance similar to ILS approaches for the final approach segment; portions of
the GLS approach prior to and after the final approach segment will be based on
Area Navigation (RNAV) or Required Navigation Performance (RNP).
4. The equipment
consists of a GBAS Ground Facility (GGF), four reference stations, a VHF Data
Broadcast (VDB) uplink antenna, and an aircraft GBAS receiver.
1. Pilots will
select the five digit GBAS channel number of the associated approach within the
Flight Management System (FMS) menu or manually select the five digits (system
dependent). Selection of the GBAS channel number also tunes the VDB.
procedure selection, confirmation that the correct LAAS procedure is loaded can
be accomplished by cross checking the charted Reference Path Indicator (RPI) or
approach ID with the cockpit displayed RPI or audio identification of the RPI
with Morse Code (for some systems).
3. The pilot will
fly the GLS approach using the same techniques as an ILS, once selected and
Approach Systems other than ILS, GLS, and MLS
Approval and use of
precision approach systems other than ILS, GLS and MLS require the issuance of
special instrument approach procedures.
b. Special Instrument
instrument approach procedures must be issued to the aircraft operator if pilot
training, aircraft equipment, and/or aircraft performance is different than
published procedures. Special instrument approach procedures are not distributed
for general public use. These procedures are issued to an aircraft operator when
the conditions for operations approval are satisfied.
aviation operators requesting approval for special procedures should contact the
local Flight Standards District Office to obtain a letter of authorization. Air
carrier operators requesting approval for use of special procedures should
contact their Certificate Holding District Office for authorization through
their Operations Specification.
c. Transponder Landing
1. The TLS is
designed to provide approach guidance utilizing existing airborne ILS localizer,
glide slope, and transponder equipment.
equipment consists of a transponder interrogator, sensor arrays to detect
lateral and vertical position, and ILS frequency transmitters. The TLS detects
the aircraft's position by interrogating its transponder. It then broadcasts ILS
frequency signals to guide the aircraft along the desired approach path.
3. TLS instrument
approach procedures are designated Special Instrument Approach Procedures.
Special aircrew training is required. TLS ground equipment provides approach
guidance for only one aircraft at a time. Even though the TLS signal is received
using the ILS receiver, no fixed course or glidepath is generated. The concept
of operation is very similar to an air traffic controller providing radar
vectors, and just as with radar vectors, the guidance is valid only for the
intended aircraft. The TLS ground equipment tracks one aircraft, based on its
transponder code, and provides correction signals to course and glidepath based
on the position of the tracked aircraft. Flying the TLS corrections computed for
another aircraft will not provide guidance relative to the approach; therefore,
aircrews must not use the TLS signal for navigation unless they have received
approach clearance and completed the required coordination with the TLS ground
equipment operator. Navigation fixes based on conventional NAVAIDs or GPS are
provided in the special instrument approach procedure to allow aircrews to
verify the TLS guidance.
d. Special Category I
Differential GPS (SCAT-I DGPS)
1. The SCAT-I DGPS
is designed to provide approach guidance by broadcasting differential correction
2. SCAT-I DGPS
procedures require aircraft equipment and pilot training.
equipment consists of GPS receivers and a VHF digital radio transmitter. The
SCAT-I DGPS detects the position of GPS satellites relative to GPS receiver
equipment and broadcasts differential corrections over the VHF digital radio.
4. Category I
Ground Based Augmentation System (GBAS) will displace SCAT-I DGPS as the public
AIM, Para 5-4-7f,
Instrument Approach Procedures.