Figure 9-26. A typical example (GNS 480) of a stand-alone GPS
receiver and display.
Figure 9-27. Typical GPS satellite array.
thereby. [Figure 9-26] The receiver that picks up multiple
signals from different satellites is able to triangulate its
position from these satellites.
Three GNSSs exist today: the GPS, a United States system; the
Russian GNSS (GLONASS); and Galileo, a European system.
1. GLONASS is a network of 24 satellites that can be
picked up by any GLONASS receiver, allowing the
user to pinpoint their position.
2. Galileo planned to be a network of 30 satellites that
continuously transmit high-frequency radio signals
containing time and distance data that can be picked
up by a Galileo receiver with operational expectancy
by 2013.
3. The GPS came on line in 1992 with 24 satellites and
today utilizes 30 satellites.
Global Positioning System (GPS)
The GPS is a satellite-based radio navigation system that
broadcasts a signal that is used by receivers to determine
precise position anywhere in the world. The receiver tracks
multiple satellites and determines a measurement that is then
used to determine the user location. [Figure 9-27]
The Department of Defense (DOD) developed and deployed
GPS as a space-based positioning, velocity, and time system.
The DOD is responsible for operation of the GPS satellite
constellation, and constantly monitors the satellites to ensure
proper operation. The GPS system permits Earth-centered
coordinates to be determined and provides aircraft position
referenced to the DOD World Geodetic System of 1984
(WGS-84). Satellite navigation systems are unaffected
by weather and provide global navigation coverage that
fully meets the civil requirements for use as the 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 IAPs. Navigational values,
such as distance and bearing to a WP and groundspeed, are
computed from the aircraft’s current position (latitude and
longitude) and the location of the next WP. Course guidance
is provided as a linear deviation from the desired track of a
Great Circle route between defined WPs.
GPS may not be approved for IFR use in other countries.
Prior to its use, pilots should ensure that GPS is authorized
by the appropriate countries.
GPS Components
GPS consists of three distinct functional elements: space,
control, and user.
The space element consists of over 30 Navstar satellites. This
group of satellites is called a constellation. The space element
consists of 24 Navigation System using Timing and Ranging
(NAVSTAR) satellites in 6 orbital planes. The satellites in
each plane are spaced 60° apart for complete coverage and
are located (nominally) at about 11,000 miles above the
Earth. The planes are arranged so that there are always five
satellites in view at any time on the Earth. Presently, there are
at least 31 Block II/IIA/IIR and IIR-M satellites in orbit with
the additional satellites representing replacement satellites
(upgraded systems) and spares. Recently, the Air Force
received funding for procurement of 31 Block IIF satellites.
The GPS constellation broadcasts a pseudo-random code
timing signal and data message that the aircraft equipment
processes to obtain satellite position and status data. By
knowing the precise location of each satellite and precisely
matching timing with the atomic clocks on the satellites, the
aircraft receiver/processor can accurately measure the time
each signal takes to arrive at the receiver and, therefore,
determine aircraft position.
The control element consists of a network of ground-based
GPS monitoring and control stations that ensure the accuracy
of satellite positions and their clocks. In its present form, it
has five monitoring stations, three ground antennas, and a
master control station.
The user element consists of antennas and receiver/processors
on board the aircraft that provide positioning, velocity,
and precise timing to the user. GPS equipment used while
operating under IFR must meet the standards set forth in
Technical Standard Order (TSO) C-129 (or equivalent); meet
the airworthiness installation requirements; be “approved” for
that type of IFR operation; and be operated in accordance with
the applicable POH/AFM or flight manual supplement.
An updatable GPS database that supports the appropriate
operations (e.g., en route, terminal, and instrument
approaches) is required when operating under IFR. The
aircraft GPS navigation database contains WPs from the
geographic areas where GPS navigation has been approved
for IFR operations. The pilot selects the desired WPs from
the database and may add user-defined WPs for the flight.
Equipment approved in accordance with TSO C-115a, visual
flight rules (VFR), and hand-held GPS systems do not meet
the requirements of TSO C-129 and are not authorized for
IFR navigation, instrument approaches, or as a principal
instrument flight reference. During IFR operations, these
units (TSO C-115a) may be considered only an aid to
situational awareness.
Prior to GPS/WAAS IFR operation, the pilot must review
appropriate NOTAMs and aeronautical information. This
information is available on request from an flight service
station (FSS). The FAA does provide NOTAMs to advise
pilots of the status of the WAAS and level of service
available.
Function of GPS
GPS operation is based on the concept of ranging and
triangulation from a group of satellites in space that act
as precise reference points. 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).
The aircraft GPS receiver measures distance from a satellite
using the travel time of a radio signal. Each satellite transmits
a specific code, called a course/acquisition (CA) code, which
contains information about satellite position, the GPS system
time, and the health and accuracy of the transmitted data.
Knowing the speed at which the signal traveled (approximately
186,000 miles per second) and the exact broadcast time,
the distance traveled by the signal can be computed from
the arrival time. The distance derived from this method of
computing distance is called a pseudo-range because it is not
a direct measurement of distance, but a measurement based
on time. In addition to knowing the distance to a satellite, a
receiver needs to know the satellite’s exact position in space,
its ephemeris. Each satellite transmits information about its
exact orbital location. The GPS receiver uses this information
to establish the precise position of the satellite.
Using the calculated pseudo-range and position information
supplied by the satellite, the GPS receiver/processor
mathematically determines its position by triangulation
from several satellites. The GPS receiver needs at least four
satellites to yield a three-dimensional position (latitude,
longitude, and altitude) and time solution. The GPS receiver
computes navigational values (distance and bearing to
a WP, groundspeed, etc.) by using the aircraft’s known
latitude/longitude and referencing these to a database built
into the receiver.
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. RAIM needs
a minimum of five satellites in view or four satellites and
a barometric altimeter baro-aiding to detect an integrity
anomaly. For receivers capable of doing so, RAIM needs
six satellites in view (or five satellites with baro-aiding)
to isolate a corrupt satellite signal and remove it from the
navigation solution.
Generally, there are two types of RAIM messages. One
type indicates that there are not enough satellites available
to provide RAIM and another type indicates that the RAIM
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.
Aircraft using GPS navigation equipment under IFR for
domestic en route, terminal operations, and certain IAPs,
must be equipped with an approved and operational alternate
means of navigation appropriate to the flight. The avionics
necessary to receive all of the ground-based facilities
appropriate for the route to the destination airport and any
required alternate airport must be installed and operational.
Ground-based facilities necessary for these routes must also
be operational. 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. In situations where the loss of RAIM
capability is predicted to occur, the flight must rely on other
approved equipment, delay departure, or cancel the flight.
GPS Substitution
IFR En Route and Terminal Operations
GPS systems, certified for IFR en route and terminal
operations, may be used as a substitute for ADF and DME
receivers when conducting the following operations within
the United States NAS.
1. Determining the aircraft position over a DME fix.
This includes en route operations at and above 24,000
feet mean sea level (MSL) (FL 240) when using GPS
for navigation.
2. Flying a DME arc.
3. Navigating TO/FROM an NDB/compass locator.
4. Determining the aircraft position over an NDB/
compass locator.
5. Determining the aircraft position over a fix defined
by an NDB/compass locator bearing crossing a VOR/
LOC course.
6. Holding over an NDB/compass locator.
GPS Substitution for ADF or DME
Using GPS as a substitute for ADF or DME is subject to the
following restrictions:
1. This equipment must be installed in accordance with
appropriate airworthiness installation requirements and
operated within the provisions of the applicable POH/
AFM or supplement.
2. The required integrity for these operations must be
provided by at least en route RAIM or equivalent.
3. WPs, fixes, intersections, and facility locations to be
used for these operations must be retrieved from the
GPS airborne database. The database must be current.
If the required positions cannot be retrieved from the
airborne database, the substitution of GPS for ADF and/
or DME is not authorized
4. Procedures must be established for use when RAIM
outages are predicted or occur. This may require the
flight to rely on other approved equipment or require
the aircraft to be equipped with operational NDB and/or
DME receivers. Otherwise, the flight must be rerouted,
delayed, canceled, or conducted under VFR.
5. The CDI must be set to terminal sensitivity (1
NM) when tracking GPS course guidance in the
terminal area.
6. A non-GPS approach procedure must exist at the
alternate airport when one is required. If the non-GPS
approaches on which the pilot must rely require DME
or ADF, the aircraft must be equipped with DME or
ADF avionics as appropriate.
7. Charted requirements for ADF and/or DME can be met
using the GPS system, except for use as the principal
instrument approach navigation source.
NOTE: The following provides guidance that is not specific
to any particular aircraft GPS system. For specific system
guidance, refer to the POH/AFM, or supplement, or contact
the system manufacturer.
To Determine Aircraft Position Over a DME Fix:
1. Verify aircraft GPS system integrity monitoring is
functioning properly and indicates satisfactory integrity.
2. If the fix is identified by a five-letter name that is
contained in the GPS airborne database, select either
the named fix as the active GPS WP or the facility
establishing the DME fix as the active GPS WP. When
using a facility as the active WP, the only acceptable
facility is the DME facility that is charted as the one
used to establish the DME fix. If this facility is not in
the airborne database, it is not authorized for use.
3. If the fix is identified by a five-letter name that is not
contained in the GPS airborne database, or if the fix
is not named, select the facility establishing the DME
fix or another named DME fix as the active GPS WP.
4. When selecting the named fix as the active GPS WP,
a pilot is over the fix when the GPS system indicates
the active WP.
5. If selecting the DME providing facility as the active
GPS WP, a pilot is over the fix when the GPS distance
from the active WP equals the charted DME value, and
the aircraft is established on the appropriate bearing
or course.
To Fly a DME Arc:
1. Verify aircraft GPS system integrity monitoring is
functioning properly and indicates satisfactory integrity.
2. Select from the airborne database the facility providing
the DME arc as the active GPS WP. The only
acceptable facility is the DME facility on which the arc
is based. If this facility is not in your airborne database,
you are not authorized to perform this operation.
3. Maintain position on the arc by reference to the GPS
distance instead of a DME readout.
To Navigate TO or FROM an NDB/Compass
Locator:
1. Verify aircraft GPS system integrity monitoring is
functioning properly and indicates satisfactory integrity.
2. Select the NDB/compass locator facility from the
airborne database as the active WP. If the chart depicts
the compass locator collocated with a fix of the same
name, use of that fix as the active WP in place of the
compass locator facility is authorized.
3. Select and navigate on the appropriate course to or
from the active WP.
To Determine Aircraft Position Over an NDB/
Compass Locator:
1. Verify aircraft GPS system integrity monitoring is
functioning properly and indicates satisfactory integrity.
2. Select the NDB/compass locator facility from the
airborne database. When using an NDB/compass
locator, the facility must be charted and be in the
airborne database. If the facility is not in the airborne
database, pilots are not authorized to use a facility WP
for this operation.
3. A pilot is over the NDB/compass locator when the
GPS system indicates arrival at the active WP.
To Determine Aircraft Position Over a Fix Made up
of an NDB/Compass Locator Bearing Crossing a
VOR/LOC Course:
1. Verify aircraft GPS system integrity monitoring is
functioning properly and indicates satisfactory integrity.
2. A fix made up by a crossing NDB/compass locator
bearing is identified by a five-letter fix name. Pilots
may select either the named fix or the NDB/compass
locator facility providing the crossing bearing to
establish the fix as the active GPS WP. When using
an NDB/compass locator, that facility must be charted
and be in the airborne database. If the facility is not
in the airborne database, pilots are not authorized to
use a facility WP for this operation.
3. When selecting the named fix as the active GPS WP,
pilot is over the fix when the GPS system indicates
the pilot is at the WP.
4. When selecting the NDB/compass locator facility
as the active GPS WP, pilots are over the fix when
the GPS bearing to the active WP is the same as
the charted NDB/compass locator bearing for the
fix flying the prescribed track from the non-GPS
navigation source.
To Hold Over an NDB/Compass Locator:
1. Verify aircraft GPS system integrity monitoring is
functioning properly and indicates satisfactory integrity.
2. Select the NDB/compass locator facility from the
airborne database as the active WP. When using a
facility as the active WP, the only acceptable facility
is the NDB/compass locator facility which is charted.
If this facility is not in the airborne database, its use
is not authorized.
3. Select nonsequencing (e.g., “HOLD” or “OBS”) mode
and the appropriate course in accordance with the
POH/AFM or supplement.
4. Hold using the GPS system in accordance with the
POH/AFM or supplement.
IFR Flight Using GPS
Preflight preparations should ensure that the GPS is properly
installed and certified with a current database for the type
of operation. The GPS operation must be conducted in
accordance with the FAA-approved POH/AFM or flight
manual supplement. Flightcrew members must be thoroughly
familiar with the particular GPS equipment installed in the
aircraft, the receiver operation manual, and the POH/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. Using the equipment in
flight under VFR conditions prior to attempting IFR operation
allows for further familiarization.
Required preflight preparations should include checking
NOTAMs relating to the IFR flight when using GPS as a
supplemental method of navigation. GPS satellite outages
are issued as GPS NOTAMs both domestically and
internationally. Pilots may obtain GPS RAIM availability
information for an airport by specifically requesting GPS
aeronautical information from an FSS during preflight
briefings. GPS RAIM aeronautical information can be
obtained for a 3-hour period: the estimated time of arrival
(ETA), and 1 hour before to 1 hour after the ETA hour, or
a 24-hour time frame for a specific airport. FAA briefers
provide RAIM information for a period of 1 hour before to 1
hour after the ETA, unless a specific timeframe is requested
by the pilot. If flying a published GPS departure, the pilot
should also request a RAIM prediction for the departure
airport. Some GPS receivers have the capability to predict
RAIM availability. The pilot should also ensure that the
Figure 7-26. GPS approach.
-
SE-3, 16 DEC 2010 to 13 JAN 2011
Figure 9-28. A GPS stand-alone approach.
required underlying ground-based navigation facilities and
related aircraft equipment appropriate to the route of flight,
terminal operations, instrument approaches for the destination,
and alternate airports/heliports are operational for the ETA.
If the required ground-based facilities and equipment are
not available, the flight should be rerouted, rescheduled,
canceled, or conducted under VFR.
Except for programming and retrieving information from
the GPS receiver, planning the flight is accomplished in a
similar manner to conventional NAVAIDs. Departure WP,
DP, route, STAR, desired approach, IAF, and destination
airport are entered into the GPS receiver according to the
manufacturer’s instructions. During preflight, additional
information may be entered for functions such as ETA, fuel
planning, winds aloft, etc.
When the GPS receiver is turned on, it begins an internal
process of test and initialization. When the receiver is
initialized, the user develops the route by selecting a WP
or series of WPs, verifies the data, and selects the active
flight plan. This procedure varies widely among receivers
made by different manufacturers. GPS is a complex system,
offering little standardization between receiver models. It is
the pilot’s responsibility to be familiar with the operation of
the equipment in the aircraft.
The GPS receiver provides navigational values such as track,
bearing, groundspeed, and distance. These are computed from
the aircraft’s present latitude and longitude to the location of
the next WP. Course guidance is provided between WPs. The
pilot has the advantage of knowing the aircraft’s actual track
over the ground. As long as track and bearing to the WP are
matched up (by selecting the correct aircraft heading), the
aircraft is going directly to the WP.
GPS Instrument Approaches
There is a mixture of GPS overlay approaches (approaches
with “or GPS” in the title) and GPS stand-alone approaches
in the United States.
NOTE: GPS instrument approach operations outside the United
States must be authorized by the appropriate country authority.
While conducting these IAPs, ground-based NAVAIDs are
not required to be operational and associated aircraft avionics
need not be installed, operational, turned on, or monitored;
however, monitoring backup navigation systems is always
recommended when available.
Pilots should have a basic understanding of GPS approach
procedures and practice GPS IAPs under visual meteorological
conditions (VMC) until thoroughly proficient with all
aspects of their equipment (receiver and installation) prior
to attempting flight in instrument meteorological conditions
(IMC). [Figure 9-28]
All IAPs must be retrievable from the current GPS database
supplied by the manufacturer or other FAA-approved
source. Flying point to point on the approach does not
assure compliance with the published approach procedure.
The proper RAIM sensitivity is not available and the CDI
sensitivity does 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 are not available as overlays.
GPS approaches are requested and approved by ATC using
the GPS title, such as “GPS RWY 24” or “RNAV RWY 35.”
Using the manufacturer’s recommended procedures, the
desired approach and the appropriate IAF are selected from
the GPS receiver database. 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 ensure terrain clearance.
When an approach has been loaded in the flight plan, GPS
receivers give an “arm” annunciation 30 NM straight
line distance from the airport/heliport reference point.
The approach mode should be “armed” when within 30
NM distance so the receiver changes from en route CDI
(±5 NM) and RAIM (±2 NM) sensitivity to ±1 NM terminal
sensitivity. Where the IAWP is within 30 NM, a CDI
sensitivity change occurs once the approach mode is armed
and the aircraft is within 30 NM. Where the IAWP is beyond
the 30 NM point, CDI sensitivity does not change until the
aircraft is within 30 NM even if the approach is armed earlier.
Feeder route obstacle clearance is predicated on the receiver
CDI and RAIM being in terminal CDI sensitivity within 30 NM
of the airport/heliport reference point; therefore, the receiver
should always be armed no later than the 30 NM annunciation.
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 WPs by the receiver and to resume
automatic GPS navigation sequencing once the maneuver
is complete. The same WP may appear in the route of flight
more than once and consecutively (e.g., IAWP, final approach
waypoint (FAWP), missed approach waypoint (MAWP) on a
procedure turn). Care must be exercised to ensure the receiver
is sequenced to the appropriate WP for the segment of the
procedure being flown, especially if one or more fly-over WPs
are skipped (e.g., FAWP rather than IAWP if the procedure
turn is not flown). The pilot may need to sequence past one or
more fly-overs of the same WP in order to start GPS automatic
sequencing at the proper place in the sequence of WPs.
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 that is aligned with the runway.
Assigned altitudes must be maintained until established on a
published segment of the approach. Required altitudes at WPs
outside the FAWP or step-down fixes must be considered.
Calculating the distance to the FAWP may be required in
order to descend at the proper location.
When within 2 NM of the FAWP with the approach mode
armed, the approach mode switches to active, which results
in RAIM and CDI sensitivity changing to the approach
mode. Beginning 2 NM prior to the FAWP, the full scale
CDI sensitivity changes smoothly from ±1 NM to ±0.3 NM
at the FAWP. As sensitivity changes from ±1 NM to ±0.3
NM approaching the FAWP, and the CDI not centered,
the corresponding increase in CDI displacement may give
the impression the aircraft is moving further away from
the intended course even though it is on an acceptable
intercept heading. If digital track displacement information
(cross-track error) is available in the approach mode, it 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 helps prevent
problems in interpreting the CDI display during ramp-down.
Requesting or accepting vectors, which causes the aircraft
to intercept the final approach course within 2 NM of the
FAWP, is not recommended.
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. Overriding an
automatically selected sensitivity during an approach cancels
the approach mode annunciation. If the approach mode is
not armed by 2 NM prior to the FAWP, the approach mode
does not become active at 2 NM prior to the FAWP and
the equipment will flag. In these conditions, the RAIM and
CDI sensitivity do not ramp down, and the pilot should not
descend to minimum descent altitude (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.
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
causes CDI sensitivity to change immediately to terminal
(±1 NM) sensitivity, and the receiver continues to navigate
to the MAWP. The receiver does not sequence past the
MAWP. Turns should not begin prior to the MAWP. If the
missed approach is not activated, the GPS receiver displays
an extension of the inbound final approach course and the
along track distance (ATD) increases from the MAWP until
it is manually sequenced after crossing the MAWP.
Missed approach routings in which the first track is via a course
rather than direct to the next WP require additional action
by the pilot to set the course. Being familiar with all of the
required inputs is especially critical during this phase of flight.
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 provided automatically by the
receiver. (Terminal RAIM for departure may not be available
unless the WPs 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 WP. 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. Turning areas are based
on this speed also. Missed approach routings in which the
first track is via a course rather than direct to the next WP
require additional action by the pilot to set the course. Being
familiar with all of the required inputs is especially critical
during this phase of flight.
GPS Errors
Normally, with 30 satellites in operation, the GPS
constellation is expected to be available continuously
worldwide. Whenever there are fewer than 24 operational
satellites, GPS navigational capability may not be available
at certain geographic locations. Loss of signals may also
occur in valleys surrounded by high terrain, and any time
the aircraft’s GPS antenna is “shadowed” by the aircraft’s
structure (e.g., when the aircraft is banked).
Certain receivers, transceivers, mobile radios, and portable
receivers can cause signal interference. Some VHF
transmissions may cause “harmonic interference.” Pilots
can isolate the interference by relocating nearby portable
receivers, changing frequencies, or turning off suspected
causes of the interference while monitoring the receiver’s
signal quality data page.
GPS position data can be affected by equipment characteristics
and various geometric factors, which typically cause errors
of less than 100 feet. Satellite atomic clock inaccuracies,
receiver/processors, signals reflected from hard objects
(multi-path), ionospheric and tropospheric delays, and
satellite data transmission errors may cause small position
errors or momentary loss of the GPS signal.
System Status
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 United States
Coast Guard navigation information service: (703) 313-5907
or on the internet at www.navcen.uscg.gov. Additionally,
satellite status is available through the NOTAM system.
The GPS receiver verifies the integrity (usability) of the
signals received from the GPS constellation through 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 five
satellites in view or four satellites and a barometric altimeter
(baro-aiding) to detect an integrity anomaly. For receivers
capable of doing so, RAIM needs six satellites in view (or
five satellites with baro-aiding) to isolate the corrupt satellite
signal and remove it from the navigation solution.
RAIM messages vary somewhat between receivers; however,
there are two most commonly used 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.
Selective Availability. Selective availability 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. Selective availability was discontinued on
May 1, 2000, but many GPS receivers are designed to assume
that selective availability is still active. New receivers may
take advantage of the discontinuance of selective availability
based on the performance values in ICAO Annex 10 and do
not need to be designed to operate outside of that performance.
GPS Familiarization
Pilots should practice GPS approaches under VMC until
thoroughly proficient with all aspects of their equipment
(receiver and installation) prior to attempting flight by IFR in
IMC. Some of the tasks which the pilot should practice are:
1. Utilizing the 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 WP
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).
Differential Global Positioning Systems (DGPS)
Differential global positioning systems (DGPS) are designed
to improve the accuracy of GNSS by measuring changes in
variables to provide satellite positioning corrections.
Because multiple receivers receiving the same set of satellites
produce similar errors, a reference receiver placed at a known
location can compute its theoretical position accurately and
can compare that value to the measurements provided by the
navigation satellite signals. The difference in measurement
between the two signals is an error that can be corrected by
providing a reference signal correction.
As a result of this differential input accuracy of the
satellite system can be increased to meters. The Wide
Area Augmentation System (WAAS) and Local Area
Augmentation System (LAAS) are examples of differential
global positioning systems.
Wide Area Augmentation System (WAAS)
The WAAS is designed to improve the accuracy, integrity,
and availability of GPS signals. WAAS allows GPS to be
used as the aviation navigation system from takeoff through
Category I precision approaches. ICAO has defined Standards
for satellite-based augmentation systems (SBAS), and 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 Japanese Multifunctional Transport Satellite (MTSAT)
Satellite-based Augmentation System. The result will be a
worldwide seamless navigation capability similar to GPS but
with greater accuracy, availability, and integrity.
Unlike traditional ground-based navigation aids, WAAS
will cover a more extensive service area in which surveyed
wide-area ground reference stations are linked to the WAAS
network. Signals from the GPS satellites are monitored by
these stations to determine satellite clock and ephemeris
corrections. Each station in the network relays the data to a
wide-area master station where the correction information is
computed. A correction message is prepared and uplinked to
a geostationary satellite (GEO) via a ground uplink and then
broadcast on the same frequency as GPS to WAAS receivers
within the broadcast coverage area. [Figure 9-29]
In addition to providing the correction signal, WAAS
provides an additional 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. [Figure 9-30] As a result, performance improvement
is sufficient to enable approach procedures with GPS/WAAS
glidepaths. At this time the FAA has completed installation of
25 wide area ground reference systems, two master stations,
and four ground uplink stations.
General Requirements
WAAS avionics must be certified in accordance with
TSO-C145A, Airborne Navigation Sensors Using the GPS
Augmented by the WAAS; or TSO-146A for stand-alone
systems. GPS/WAAS operation must be conducted in
accordance with the FAA-approved aircraft flight manual
(AFM) and flight manual supplements. Flight manual
supplements must state the level of approach procedure that
the receiver supports.
Instrument Approach Capabilities
WAAS receivers support all basic GPS approach functions
and provide additional capabilities with the key benefit to
generate an electronic glidepath, independent of ground
equipment or barometric aiding. This eliminates several
problems, such as cold temperature effects, incorrect
altimeter setting, or lack of a local altimeter source, and
allows approach procedures to be built without the cost