4/20/23 

AIM 

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 approximately 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

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1

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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 identification 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

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1

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4.) 

Navigation Aids 

1

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1

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