ILS Components

Ground Components
The ILS uses a number of different ground facilities. These
facilities maybe used as a part of the ILS system, as well as
part of another approach. For example, the compass locator
may he 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 hack
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 Jell) and a full
fly-tight" 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 "T" (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 50°. This
results in high needle sensitivity. With this course width,
a fall-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 he
aligned with the runway.

Glide Slope (GS)

GS describes the systems that generate, receive, and indicate
the ground facility radiation pattern. The glide path 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.