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systems in use in the United States are the ILS, simplified
directional facility (SDF), localizer 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 7-34] The system consists of the following
components:
1. A localizer providing horizontal (left/right) guidance
along the extended centerline of the runway.
2. A glide slope (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 standalone),
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
7-38
Figure 7-34. Instrument Landing Systems.
7-39
Figure 7-35. Localizer Coverage Limits.
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
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 7-35]
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
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 onequarter
scale deflection maintained, the aircraft will be
aligned with the runway.
Glide Slope (GS)
GS describes the systems that generate, receive, and indicate
　
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