Wide Area Reference Station
receives GPS signal
GPS
Satellites
Wide Area Reference Station
Wide Area Reference Station
data is sent to a Wide Area
Master Station for correction
GPS
Satellites
Wide Area Reference Station
Wide Area Master station uplinks corrected
signal to GEO Synchronous Satellites
GUS
GEO Synchronous
Satellites
Wide Area Reference Station
Wide Area Master station
GUS
GEO Synchronous Satellites sends updated
WAAS signal to aircraft equipped with WAAS
receivers
GEO Synchronous
Satellites
Figure 7-27a. LAAS representation.
Wide Area Augmentation System (WAAS)
Figure 9-29. WAAS satellite representation.
of installing ground stations at each airport. A new class
of approach procedures, which provide vertical guidance
requirements for precision approaches, has been developed
to support satellite navigation use for aviation applications.
These new procedures called Approach with Vertical
Guidance (APV) include approaches such as the LNAV/
VNAV procedures presently being flown with barometric
vertical navigation.
Local Area Augmentation System (LAAS)
LAAS is a ground-based augmentation system that uses a
GPS-reference facility located on or in the vicinity of the
airport being serviced. This facility has a reference receiver
that measures GPS satellite pseudo-range and timing and
retransmits the signal. Aircraft landing at LAAS-equipped
airports are able to conduct approaches to Category I level and
above for properly equipped aircraft. [Figures 9-31 and 9-32]
The local area augmentation system (LAAS) ground facility
receives the data from the GPS ground reference receivers.
GPS reference receivers, which are serving the local area
(such as an airport) receive the GPS signal from the GPS
constellation (one or more satellites).
The corrected signal is then sent and transmitted as a VHF
signal, called a VHF data broadcast.
The broadcast signal is received by appropriately equipped
aircraft which provide the pilot with highly refined GPS
guidance.
GPS
Reference
Receiver
GPS
Reference
Receiver
LAAS
Ground
Facility
VHF Data
Broadcast
LAAS
Ground
Facility
VHF Data
Broadcast
Figure 7-28a. LAAS representation.
GPS
Reference
Receiver
Figure 9-31. LAAS representation.
50 ft.
10 ft.
Figure 7-27b. WAAS satellite representation
You are here
GPS Accuracy WAAS Accuracy
Figure 9-30. WAAS provides performance enhancement for GPS
approach procedures through real-time monitoring.
Inertial Navigation System (INS)
Inertial Navigation System (INS) is a system that navigates
precisely without any input from outside of the aircraft. It is
fully self-contained. The INS is initialized by the pilot, who
enters into the system the exact location of the aircraft on the
ground before the flight. The INS is also programmed with
WPs along the desired route of flight.
INS Components
INS is considered a stand-alone navigation system, especially
when more than one independent unit is onboard. The
airborne equipment consists of an accelerometer to measure
acceleration—which, when integrated with time, gives
velocity—and gyros to measure direction.
Later versions of the INS, called inertial reference systems
(IRS), utilize laser gyros and more powerful computers;
therefore, the accelerometer mountings no longer need to
be kept level and aligned with true north. The computer
system can handle the added workload of dealing with the
computations necessary to correct for gravitational and
directional errors. Consequently, these newer systems are
sometimes called strap down systems, as the accelerometers
and gyros are strapped down to the airframe rather than being
mounted on a structure that stays fixed with respect to the
horizon and true north.
INS Errors
The principal error associated with INS is degradation of
position with time. INS computes position by starting with
accurate position input which is changed continuously as
accelerometers and gyros provide speed and direction inputs.
Both accelerometers and gyros are subject to very small
errors; as time passes, those errors probably accumulate.
Figure 7-28. LAAS representation.
Reference Receiver
GPS Satellites
LAAS Facility
VHF Transmitter
Figure 9-32. The LAAS system working with GPS satellites,
reference receivers and radio transmitters which are located on
or in the vicinity of the airport.
While the best INS/IRS display errors of 0.1 to 0.4 NM after
flights across the North Atlantic of 4 to 6 hours, smaller and
less expensive systems are being built that show errors of 1
to 2 NM per hour. This accuracy is more than sufficient for
a navigation system that can be combined with and updated
by GPS. The synergy of a navigation system consisting of an
INS/IRS unit in combination with a GPS resolves the errors
and weaknesses of both systems. GPS is accurate all the time
it is working but may be subject to short and periodic outages.
INS is made more accurate because it is continually updated
and continues to function with good accuracy if the GPS has
moments of lost signal.
Instrument Approach Systems
Most navigation systems approved for en route and terminal
operations under IFR, such as VOR, NDB, and GPS, may also
be approved to conduct IAPs. The most common systems in
use in the United States are the ILS, simplified directional
facility (SDF), localizer-type directional aid (LDA), and
microwave landing system (MLS). These systems operate
independently of other navigation systems. There are new
systems being developed, such as WAAS and LAAS. Other
systems have been developed for special use.
Instrument Landing Systems (ILS)
The ILS system provides both course and altitude guidance
to a specific runway. The ILS system is used to execute a
precision instrument approach procedure or precision approach.
[Figure 9-33] The system consists of the following components:
1. A localizer providing horizontal (left/right) guidance
along the extended centerline of the runway.
2. A glideslope (GS) providing vertical (up/down)
guidance toward the runway touchdown point, usually
at a 3° slope.
3. Marker beacons providing range information along
the approach path.
4. Approach lights assisting in the transition from
instrument to visual flight.
The following supplementary elements, though not specific
components of the system, may be incorporated to increase
safety and utility:
1. Compass locators providing transition from en route
NAVAIDs to the ILS system and assisting in holding
procedures, tracking the localizer course, identifying
the marker beacon sites, and providing a FAF for
ADF approaches.
2. DME collocated with the GS transmitter providing
positive distance-to-touchdown information or DME
associated with another nearby facility (VOR or stand-
alone), if specified in the approach procedure.
ILS approaches are categorized into three different types of
approaches based on the equipment at the airport and the
experience level of the pilot. Category I approaches provide
for approach height above touchdown of not less than 200 feet.
Category II approaches provide for approach to a height above
touchdown of not less than 100 feet. Category III approaches
provide lower minimums for approaches without a decision
height minimum. While pilots need only be instrument rated
and the aircraft be equipped with the appropriate airborne
equipment to execute Category I approaches, Category II
and III approaches require special certification for the pilots,
ground equipment, and airborne equipment.
ILS Components
Ground Components
The ILS uses a number of different ground facilities. These
facilities may be used as a part of the ILS system, as well as
part of another approach. For example, the compass locator
may be used with NDB approaches.
Localizer
The localizer (LOC) ground antenna array is located on the
extended centerline of the instrument runway of an airport,
located at the departure end of the runway to prevent it from
being a collision hazard. This unit radiates a field pattern,
which develops a course down the centerline of the runway
toward the middle markers (MMs) and outer markers
(OMs) and a similar course along the runway centerline in
the opposite direction. These are called the front and back
OBS
3 33
21 15
GS
NAV
OBS
3 33
21 15
GS
NAV
OBS
3 33
21 15
GS
NAV
KNOTS
I40 I20
KNOTS
KNOTS 240
I50 I20
KNOTS
I60 I40 I20
KNOTS
–
–
OBS
3 33
21 15
GS
NAV
90Hz
150Hz
OBS
3 33
21 15
GS
NAV
Figure 7-29. Instrument landing systems.
Point of intersection
runway and glideslope
extended.
Figure 9-33. Instrument landing systems.
courses, respectively. The localizer provides course guidance,
transmitted at 108.1 to 111.95 MHz (odd tenths only),
throughout the descent path to the runway threshold from a
distance of 18 NM from the antenna to an altitude of 4,500
feet above the elevation of the antenna site. [Figure 9-34]
The localizer course width is defined as the angular
displacement at any point along the course between a full
“fly-left” (CDI needle fully deflected to the left) and a full
“fly-right” indication (CDI needle fully deflected to the right).
Each localizer facility is audibly identified by a three-letter
designator transmitted at frequent regular intervals. The ILS
identification is preceded by the letter “I” (two dots). For
example, the ILS localizer at Springfield, Missouri, transmits
the identifier ISGF. The localizer includes a voice feature on
35°
10°
10°
10 NM
35°
18 NM
Figure 7-30. Localizer coverage limits. Figure 9-34. Localizer coverage limits.
2.5°
2.5°
Outer Marker Beacons
Middle Marker Beacons
Figure 7-31. Localizer receiver indications and aircraft
displacement.
´ ´
Figure 9-35. Localizer receiver indications and aircraft displacement.
its frequency for use by the associated ATC facility in issuing
approach and landing instructions.
The localizer course is very narrow, normally 5°. This
results in high needle sensitivity. With this course width,
a full-scale deflection shows when the aircraft is 2.5° to
either side of the centerline. This sensitivity permits accurate
orientation to the landing runway. With no more than one-
quarter scale deflection maintained, the aircraft will be
aligned with the runway.
Glideslope (GS)
GS describes the systems that generate, receive, and indicate
the ground facility radiation pattern. The glidepath is the
straight, sloped line the aircraft should fly in its descent from
where the GS intersects the altitude used for approaching the
FAF to the runway touchdown zone. The GS equipment is
housed in a building approximately 750 to 1,250 feet down
the runway from the approach end of the runway and between
400 and 600 feet to one side of the centerline.
The course projected by the GS equipment is essentially the
same as would be generated by a localizer operating on its
side. The GS projection angle is normally adjusted to 2.5°
to 3.5° above horizontal, so it intersects the MM at about
200 feet and the OM at about 1,400 feet above the runway
elevation. At locations where standard minimum obstruction
clearance cannot be obtained with the normal maximum GS
angle, the GS equipment is displaced farther from the approach
end of the runway if the length of the runway permits; or the GS
angle may be increased up to 4°.
Unlike the localizer, the GS transmitter radiates signals only
in the direction of the final approach on the front course. The
system provides no vertical guidance for approaches on the back
course. The glidepath is normally 1.4° thick. At 10 NM from
the point of touchdown, this represents a vertical distance of
approximately 1,500 feet, narrowing to a few feet at touchdown.
Marker Beacons
Two VHF marker beacons, outer and middle, are normally
used in the ILS system. [Figure 9-35] A third beacon, the
inner, is used where Category II operations are certified. A
marker beacon may also be installed to indicate the FAF on
the ILS back course.
The OM is located on the localizer front course 4–7 miles
from the airport to indicate a position at which an aircraft, at
the appropriate altitude on the localizer course, will intercept
the glidepath. The MM is located approximately 3,500 feet
from the landing threshold on the centerline of the localizer
front course at a position where the GS centerline is about 200
feet above the touchdown zone elevation. The inner marker
(IM), where installed, is located on the front course between
the MM and the landing threshold. It indicates the point at
which an aircraft is at the decision height on the glidepath
during a Category II ILS approach. The back-course marker,
where installed, indicates the back-course FAF.
Compass Locator
Compass locators are low-powered NDBs and are received
and indicated by the ADF receiver. When used in conjunction
with an ILS front course, the compass locator facilities are
collocated with the outer and/or MM facilities. The coding
identification of the outer locator consists of the first two
letters of the three-letter identifier of the associated LOC.
For example, the outer locator at Dallas/Love Field (DAL) is
identified as “DA.” The middle locator at DAL is identified
by the last two letters “AL.”
Approach Lighting Systems (ALS)
Normal approach and letdown on the ILS is divided into two
distinct stages: the instrument approach stage using only radio
guidance, and the visual stage, when visual contact with the
ground runway environment is necessary for accuracy and
safety. The most critical period of an instrument approach,
particularly during low ceiling/visibility conditions, is the
point at which the pilot must decide whether to land or
execute a missed approach. As the runway threshold is
approached, the visual glidepath separates into individual
lights. At this point, the approach should be continued by
reference to the runway touchdown zone markers. The
approach lighting system (ALS) provides lights that will
penetrate the atmosphere far enough from touchdown to
give directional, distance, and glidepath information for safe
visual transition.
Visual identification of the ALS by the pilot must be
instantaneous, so it is important to know the type of ALS
before the approach is started. Check the instrument approach
chart and the A/FD for the particular type of lighting facilities
at the destination airport before any instrument flight. With
reduced visibility, rapid orientation to a strange runway can
be difficult, especially during a circling approach to an airport
with minimum lighting facilities or to a large terminal airport
located in the midst of distracting city and ground facility
lights. Some of the most common ALS systems are shown
in Figure 9-36.
A high-intensity flasher system, often referred to as “the
rabbit,” is installed at many large airports. The flashers consist
of a series of brilliant blue-white bursts of light flashing in
sequence along the approach lights, giving the effect of a ball
of light traveling towards the runway. Typically, “the rabbit”
makes two trips toward the runway per second.
Runway end identifier lights (REIL) are installed for rapid and
positive identification of the approach end of an instrument
runway. The system consists of a pair of synchronized
flashing lights placed laterally on each side of the runway
threshold facing the approach area.
The visual approach slope indicator (VASI) gives visual
descent guidance information during the approach to a
runway. The standard VASI consists of light bars that
project a visual glidepath, which provides safe obstruction
clearance within the approach zone. The normal GS angle
is 3°; however, the angle may be as high as 4.5° for proper
obstacle clearance. On runways served by ILS, the VASI
angle normally coincides with the electronic GS angle.
Visual left/right course guidance is obtained by alignment
with the runway lights. The standard VASI installation
consists of either 2-, 3-, 4-, 6-, 12-, or 16-light units arranged
in downwind and upwind light bars. Some airports serving
long-bodied aircraft have three-bar VASIs that provide two
visual glidepaths to the same runway. The first glidepath
encountered is the same as provided by the standard VASI.
The second glidepath is about 25 percent higher than the first
and is designed for the use of pilots of long-bodied aircraft.
The basic principle of VASI is that of color differentiation
between red and white. Each light projects a beam having
a white segment in the upper part and a red segment in the
lower part of the beam. From a position above the glidepath
the pilot sees both bars as white. Lowering the aircraft with
respect to the glidepath, the color of the upwind bars changes
from white to pink to red. When on the proper glidepath,
the landing aircraft will overshoot the downwind bars and
undershoot the upwind bars. Thus the downwind (closer)
bars are seen as white and the upwind bars as red. From
a position below the glidepath, both light bars are seen as
red. Moving up to the glidepath, the color of the downwind
bars changes from red to pink to white. When below the
glidepath, as indicated by a distinct all-red signal, a safe
obstruction clearance might not exist. A standard two-bar
VASI is illustrated in Figure 9-37.
ILS Airborne Components
Airborne equipment for the ILS system includes receivers
for the localizer, GS, marker beacons, ADF, DME, and the
respective indicator instruments.
The typical VOR receiver is also a localizer receiver with
common tuning and indicating equipment. Some receivers
have separate function selector switches, but most switch
between VOR and LOC automatically by sensing if odd
ALSF-2
ALSF-1
SSALR/MALSR
MALSF
ODALS
REIL
THRESHOLD
Figure 9-36. Precision and nonprecision ALS configuration.
Below glidepath On glidepath Above glidepath
Far Bar
Near Bar
Far Bar
Near Bar
Far Bar
Near Bar
Figure 7-33. Standard 2-bar VASI. Figure 9-37. Standard two-bar VASI.
tenths between 108 and 111.95 MHz have been selected.
Otherwise, tuning of VOR and localizer frequencies is
accomplished with the same knobs and switches, and the CDI
indicates “on course” as it does on a VOR radial.
Though some GS receivers are tuned separately, in a typical
installation the GS is tuned automatically to the proper
frequency when the localizer is tuned. Each of the 40 localizer
channels in the 108.10 to 111.95 MHz band is paired with a
corresponding GS frequency.
When the localizer indicator also includes a GS needle, the
instrument is often called a cross-pointer indicator. The
crossed horizontal (GS) and vertical (localizer) needles are
free to move through standard five-dot deflections to indicate
position on the localizer course and glidepath.
When the aircraft is on the glidepath, the needle is horizontal,
overlying the reference dots. Since the glidepath is much
narrower than the localizer course (approximately 1.4° from
full up to full down deflection), the needle is very sensitive
to displacement of the aircraft from on-path alignment. With
the proper rate of descent established upon GS interception,
very small corrections keep the aircraft aligned.
The localizer and GS warning flags disappear from view on
the indicator when sufficient voltage is received to actuate the
needles. The flags show when an unstable signal or receiver
malfunction occurs.
The OM is identified by a low-pitched tone, continuous dashes
at the rate of two per second, and a purple/blue marker beacon
light. The MM is identified by an intermediate tone, alternate
dots and dashes at the rate of 95 dot/dash combinations per
minute, and an amber marker beacon light. The IM, where
installed, is identified by a high-pitched tone, continuous dots
at the rate of six per second, and a white marker beacon light.
The back-course marker (BCM), where installed, is identified
by a high-pitched tone with two dots at a rate of 72 to 75 two-
dot combinations per minute and a white marker beacon light.
Marker beacon receiver sensitivity is selectable as high or low
on many units. The low-sensitivity position gives the sharpest
indication of position and should be used during an approach.
The high-sensitivity position provides an earlier warning that
the aircraft is approaching the marker beacon site.
ILS Function
The localizer needle indicates, by deflection, whether the
aircraft is right or left of the localizer centerline, regardless of
the position or heading of the aircraft. Rotating the OBS has
no effect on the operation of the localizer needle, although
it is useful to rotate the OBS to put the LOC inbound course
under the course index. When inbound on the front course, or
outbound on the back course, the course indication remains
directional. (See Figure 9-38, aircraft C, D, and E.)
Unless the aircraft has reverse sensing capability and it is in
use, when flying inbound on the back course or outbound on
the front course, heading corrections to on-course are made
opposite the needle deflection. This is commonly described
as “flying away from the needle.” (See Figure 9-38, aircraft A
and B.) Back course signals should not be used for an approach
unless a back course approach procedure is published for that
particular runway and the approach is authorized by ATC.
Once you have reached the localizer centerline, maintain
the inbound heading until the CDI moves off center. Drift
corrections should be small and reduced proportionately as
the course narrows. By the time you reach the OM, your drift
correction should be established accurately enough on a well-
executed approach to permit completion of the approach,
with heading corrections no greater than 2°.
The heaviest demand on pilot technique occurs during
descent from the OM to the MM, when you maintain
the localizer course, adjust pitch attitude to maintain the
proper rate of descent, and adjust power to maintain proper
airspeed. Simultaneously, the altimeter must be checked
and preparation made for visual transition to land or for a
missed approach. You can appreciate the need for accurate
instrument interpretation and aircraft control within the ILS
as a whole, when you notice the relationship between CDI
and glidepath needle indications and aircraft displacement
from the localizer and glidepath centerlines.
Deflection of the GS needle indicates the position of the
aircraft with respect to the glidepath. When the aircraft is
above the glidepath, the needle is deflected downward. When
the aircraft is below the glidepath, the needle is deflected
upward. [Figure 9-39]
ILS Errors
The ILS and its components are subject to certain errors,
which are listed below. Localizer and GS signals are subject to
the same type of bounce from hard objects as space waves.
1. Reflection. Surface vehicles and even other aircraft
flying below 5,000 feet above ground level (AGL)
may disturb the signal for aircraft on the approach.
2. False courses. In addition to the desired course, GS
facilities inherently produce additional courses at
higher vertical angles. The angle of the lowest of
these false courses occurs at approximately 9°– 12°.
An aircraft flying the LOC/GS course at a constant
altitude would observe gyrations of both the GS needle
and GS warning flag as the aircraft passed through the
various false courses. Getting established on one of
these false courses results in either confusion (reversed
GS needle indications) or in the need for a very high
descent rate. However, if the approach is conducted
at the altitudes specified on the appropriate approach
chart, these false courses are not encountered.
Marker Beacons
The very low power and directional antenna of the marker
beacon transmitter ensures that the signal is not received
any distance from the transmitter site. Problems with signal
reception are usually caused by the airborne receiver not
being turned on or by incorrect receiver sensitivity.