AIM
12/2/21
dimensions, at 1,000 feet above the antenna, of
approximately 2,400 feet in width and 4,200 feet in
length. Airborne marker beacon receivers with a
selective sensitivity feature should always be
operated in the “low” sensitivity position for proper
reception of ILS marker beacons.
2.
ILS systems may have an associated OM. An
MM is no longer required. Locations with a Category
II ILS also have an Inner Marker (IM). Due to
advances in both ground navigation equipment and
airborne avionics, as well as the numerous means that
may be used as a substitute for a marker beacon, the
current requirements for the use of marker beacons
are:
(a)
An OM or suitable substitute identifies the
Final Approach Fix (FAF) for nonprecision approach
(NPA) operations (for example, localizer only); and
(b)
The MM indicates a position approxi
mately 3,500 feet from the landing threshold. This is
also the position where an aircraft on the glide path
will be at an altitude of approximately 200 feet above
the elevation of the touchdown zone. A MM is no
longer operationally required. There are some MMs
still in use, but there are no MMs being installed at
new ILS sites by the FAA; and
(c)
An IM, where installed, indicates the point
at which an aircraft is at decision height on the glide
path during a Category II ILS approach. An IM is only
required for CAT II operations that do not have a
published radio altitude (RA) minimum.
TBL 1
−
1
−
3
Marker Passage Indications
Marker
Code
Light
OM
�
BLUE
MM
�
AMBER
IM
�
�
�
WHITE
BC
�
�
�
WHITE
3.
A back course marker normally indicates the
ILS back course final approach fix where approach
descent is commenced.
g. Compass Locator
1.
Compass locator transmitters are often
situated at the MM and OM sites. The transmitters
have a power of less than 25 watts, a range of at least
15 miles and operate between 190 and 535 kHz. At
some locations, higher powered radio beacons, up to
400 watts, are used as OM compass locators.
2.
Compass locators transmit two letter identi
fication groups. The outer locator transmits the first
two letters of the localizer identification group, and
the middle locator transmits the last two letters of the
localizer identification group.
h. ILS Frequency
(See TBL 1
−
1
−
4.)
TBL 1
−
1
−
4
Frequency Pairs Allocated for ILS
Localizer MHz
Glide Slope
108.10
334.70
108.15
334.55
108.3
334.10
108.35
333.95
108.5
329.90
108.55
329.75
108.7
330.50
Localizer MHz
Glide Slope
108.75
330.35
108.9
329.30
108.95
329.15
109.1
331.40
109.15
331.25
109.3
332.00
109.35
331.85
109.50
332.60
109.55
332.45
109.70
333.20
109.75
333.05
109.90
333.80
109.95
333.65
110.1
334.40
110.15
334.25
110.3
335.00
110.35
334.85
110.5
329.60
110.55
329.45
110.70
330.20
110.75
330.05
110.90
330.80
110.95
330.65
111.10
331.70
111.15
331.55
111.30
332.30
111.35
332.15
111.50
332.9
111.55
332.75
111.70
333.5
1
−
1
−
14
Navigation Aids