Aeronautical Information Manual (AIM), page 62

Index   61 -- Page 62 -- 63

4. Identification is in International Morse Code
and consists of a three-letter identifier preceded by
the letter I (  ) transmitted on the localizer


5. The localizer provides course guidance
throughout the descent path to the runway threshold
from a distance of 18 NM from the antenna between
an altitude of 1,000 feet above the highest terrain
along the course line and 4,500 feet above the
elevation of the antenna site. Proper off-course
indications are provided throughout the following
angular areas of the operational service volume:

(a) To 10 degrees either side of the course
along a radius of 18 NM from the antenna; and
(b) From 10 to 35 degrees either side of the
course along a radius of 10 NM. (See FIG 1-1-6.)

FIG 1-1-6

Limits of Localizer Coverage



0 NM

8 NM






6. Unreliable signals may be received outside
these areas.
c. Localizer Type Directional Aid (LDA)

1. The LDA is of comparable use and accuracy
to a localizer but is not part of a complete ILS. The
LDA course usually provides a more precise
approach course than the similar Simplified
Directional Facility (SDF) installation, which may
have a course width of 6 or 12 degrees.
2. The LDA is not aligned with the runway.
Straight-in minimums may be published where
alignment does not exceed 30 degrees between the

course and runway. Circling minimums only are
published where this alignment exceeds 30 degrees.

3. A very limited number of LDA approaches

also incorporate a glideslope. These are annotated in
the plan view of the instrument approach chart with
a note, "LDA/Glideslope." These procedures fall
under a newly defined category of approaches called
Approach with Vertical Guidance (APV) described in
paragraph 5-4-5, Instrument Approach Procedure
Charts, subparagraph a7(b), Approach with Vertical
Guidance (APV). LDA minima for with and without
glideslope is provided and annotated on the minima
lines of the approach chart as S-LDA/GS and
S-LDA. Because the final approach course is not
aligned with the runway centerline, additional
maneuvering will be required compared to an ILS
d. Glide Slope/Glide Path
1. The UHF glide slope transmitter, operating
on one of the 40 ILS channels within the frequency

range 329.15 MHz, to 335.00 MHz radiates its signals

in the direction of the localizer front course. The term
"glide path" means that portion of the glide slope that

intersects the localizer.
False glide slope signals may exist in the area of the
localizer back course approach which can cause the glide
slope flag alarm to disappear and present unreliable glide

slope information. Disregard all glide slope signal

indications when making a localizer back course

approach unless a glide slope is specified on the approach
and landing chart.

2. The glide slope transmitter is located between
750 feet and 1,250 feet from the approach end of the
runway (down the runway) and offset 250 to 650 feet
from the runway centerline. It transmits a glide path
beam 1.4 degrees wide (vertically). The signal
provides descent information for navigation down to
the lowest authorized decision height (DH) specified
in the approved ILS approach procedure. The
glidepath may not be suitable for navigation below
the lowest authorized DH and any reference to
glidepath indications below that height must be
supplemented by visual reference to the runway
environment. Glidepaths with no published DH are
usable to runway threshold.
3. The glide path projection angle is normally
adjusted to 3 degrees above horizontal so that it
intersects the MM at about 200 feet and the OM at

1-1-8 Navigation Aids

Page 62 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   61 -- Page 62 -- 63