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35° 

10° 

10° 

35° 

6/17/21 

AIM 

3. 

The system may be divided functionally into 

three parts: 

(a) Guidance information: 

localizer, glide 

slope. 

(b)  Range information:

 marker beacon, 

DME. 

(c)  Visual information: 

approach lights, 

touchdown and centerline lights, runway lights. 

4. 

The following means may be used to 

substitute for the OM: 

(a) 

Compass locator; or 

(b) 

Precision Approach Radar (PAR); or 

(c) 

Airport Surveillance Radar (ASR); or 

(d) 

Distance Measuring Equipment (DME), 

Very High Frequency Omni

directional Range 

(VOR), or Nondirectional beacon fixes authorized in 
the Standard Instrument Approach Procedure; or 

(e) 

Very High Frequency Omni

directional 

Radio Range (VOR); or 

(f) 

Nondirectional beacon fixes authorized in 

the Standard Instrument Approach Procedure; or 

(g) 

A suitable RNAV system with Global 

Positioning System (GPS), capable of fix identifica-
tion on a Standard Instrument Approach Procedure. 

5. 

Where a complete ILS system is installed on 

each end of a runway; (i.e., the approach end of 
Runway 4 and the approach end of Runway 22) the 

CAUTION

 

Unless the aircraft’s ILS equipment includes reverse 
sensing capability, when flying inbound on the back 
course it is necessary to steer the aircraft in the direction 
opposite the needle deflection when making corrections 
from off

course to on

course. This “flying away from the 

needle” is also required when flying outbound on the 
front course of the localizer. Do not use back course 
signals for approach unless a back course approach 
procedure is published for that particular runway and the 
approach is authorized by ATC. 

4. 

Identification is in International Morse Code 

and consists of a three

letter identifier preceded by 

the letter I ( 

 

) transmitted on the localizer 

frequency. 

EXAMPLE

 

I

DIA 

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

Limits of Localizer Coverage 

ILS systems are not in service simultaneously. 

b.  Localizer 

1. 

The localizer transmitter operates on one of 

40 ILS channels within the frequency range of 
108.10 to 111.95 MHz. Signals provide the pilot with 

NORMAL LIMITS OF LOCALIZER 

NORMAL LIMITS OF LOCALIZER 

the front course and is used with other functional 

RUNWAY

RUNWAY 

LOCALIZER 

LOCALIZER 

course guidance to the runway centerline. 

ANTENNA 

ANTENNA 

2. 

The approach course of the localizer is called 

parts, e.g., glide slope, marker beacons, etc. The 

COVERAGE:  THE SAME AREA 

COVERAGE:  THE SAME AREA 
APPLIES TO A BACK COURSE 

APPLIES TO A BACK COURSE 

WHEN PROVIDED. 

WHEN PROVIDED. 

localizer signal is transmitted at the far end of the 
runway. It is adjusted for a course width of (full scale 
fly

left to a full scale fly

right) of 700 feet at the 

6. 

Unreliable signals may be received outside 

runway threshold. 

these areas. ATC may clear aircraft on procedures 

3. 

The course line along the extended centerline 

beyond the service volume when the controller 

of a runway, in the opposite direction to the front 

initiates the action or upon pilot request when radar 

course is called the back course. 

monitoring is provided. Pilots should use caution 

Navigation Aids 

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