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
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Navigation Aids
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 sensi-
tivity 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 is contained in the current, on-board
navigation database 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 Supple-
ment 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.
10. Incorrect 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.
13. Unnamed 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
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1-1-36 Navigation Aids
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
the MAWP.
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 destination airport;
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;
8. Entering, 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;
10. Programming 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)
a. General
1. The FAA developed the Wide Area Aug-
mentation 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.
2. The International Civil Aviation Organiza-
tion (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 Aug-
mentation System. The merging of these systems will
create a worldwide seamless navigation capability
similar to GPS but with greater accuracy, availability
and integrity.
3. Unlike 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.
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Navigation Aids
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.
6. GNSS 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 Capabilities
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 Charts).
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 surface.
NOTE-
WAAS receivers certified prior to TSO C-145b and TSO
C-146b, 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 LNAV, LP,
LNAV/VNAV and LPV approaches, within system
coverage. Some locations close to the edge of the
coverage may have a lower availability of vertical
guidance.
c. General Requirements
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
7/31/08 AIM
AIM 2/14/1-1-38 Navigation Aids
Approval of Global Positioning System (GPS)
Navigation Equipment for Use as a VFR and IFR
Navigation System.
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.
3. GPS/WAAS 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
operations specifications.
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 an
Automated 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.
(2) Site-specific 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 UNRELIABLE
NOTAMs.
(3) When the approach chart is annotated
with the symbol, site-specific WAAS UNRELI-
ABLE 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.
NOTE-
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 symbol.
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
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Navigation Aids
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
removing the 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.
NOTE-
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.
3. Another 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
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AIM 2/14/1-1-40 Navigation Aids
+/-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.
