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AIM 

6/17/21 

(b) 

As a back course (BC) final approach fix 

(FAF); and 

(c) 

To establish other fixes on the localizer 

course. 

2. 

In some cases, DME from a separate facility 

may be used within Terminal Instrument Procedures 
(TERPS) limitations: 

(a) 

To provide ARC initial approach seg-

ments; 

(b) 

As a FAF for BC approaches; and 

(c) 

As a substitute for the OM. 

f.  Marker Beacon 

1. 

ILS marker beacons have a rated power 

output of 3 watts or less and an antenna array 
designed to produce an elliptical pattern with 
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

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

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 

1

1

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