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AIM

10/12/17

1−1−1

Navigation Aids

Chapter 1. Air Navigation

Section 1. Navigation Aids

1−1−1. General

a. Various types of air navigation aids are in use

today, each serving a special purpose. These aids have

varied owners and operators, namely: the Federal

Aviation Administration (FAA), the military ser-

vices, private organizations, individual states and

foreign governments. The FAA has the statutory

authority to establish, operate, maintain air naviga-

tion facilities and to prescribe standards for the

operation of any of these aids which are used for

instrument flight in federally controlled airspace.

These aids are tabulated in the Chart Supplement U.S.

b. Pilots should be aware of the possibility of

momentary erroneous indications on cockpit displays

when the primary signal generator for a ground−

based navigational transmitter (for example, a

glideslope, VOR, or nondirectional beacon) is

inoperative. Pilots should disregard any navigation

indication, regardless of its apparent validity, if the

particular transmitter was identified by NOTAM or

otherwise as unusable or inoperative.

1−1−2. Nondirectional Radio Beacon (NDB)

a. A low or medium frequency radio beacon

transmits nondirectional signals whereby the pilot of

an aircraft properly equipped can determine bearings

and “home” on the station. These facilities normally

operate in a frequency band of 190 to 535 kilohertz

(kHz), according to ICAO Annex 10 the frequency

range for NDBs is between 190 and 1750 kHz, and

transmit a continuous carrier with either 400 or

1020 hertz (Hz) modulation. All radio beacons

except the compass locators transmit a continuous

three−letter identification in code except during voice

transmissions.

b. When a radio beacon is used in conjunction with

the Instrument Landing System markers, it is called

a Compass Locator.

c. Voice transmissions are made on radio beacons

unless the letter “W” (without voice) is included in

the class designator (HW).

d. Radio beacons are subject to disturbances that

may result in erroneous bearing information. Such

disturbances result from such factors as lightning,

precipitation static, etc. At night, radio beacons are

vulnerable to interference from distant stations.

Nearly all disturbances which affect the Automatic

Direction Finder (ADF) bearing also affect the

facility’s identification. Noisy identification usually

occurs when the ADF needle is erratic. Voice, music

or erroneous identification may be heard when a

steady false bearing is being displayed. Since ADF

receivers do not have a “flag” to warn the pilot when

erroneous bearing information is being displayed, the

pilot should continuously monitor the NDB’s

identification.

1−1−3. VHF Omni−directional Range (VOR)

a. VORs operate within the 108.0 to 117.95 MHz

frequency band and have a power output necessary to

provide coverage within their assigned operational

service volume. They are subject to line−of−sight

restrictions, and the range varies proportionally to the

altitude of the receiving equipment.

NOTE−

Normal service ranges for the various classes of VORs are

given in Navigational Aid (NAVAID) Service Volumes,

Paragraph 1−1−8.

b. Most VORs are equipped for voice transmis-

sion on the VOR frequency. VORs without voice

capability are indicated by the letter “W” (without

voice) included in the class designator (VORW).

c. The only positive method of identifying a VOR

is by its Morse Code identification or by the recorded

automatic voice identification which is always

indicated by use of the word “VOR” following the

range’s name. Reliance on determining the identifica-

tion of an omnirange should never be placed on

listening to voice transmissions by the Flight Service

Station (FSS) (or approach control facility) involved.

Many FSSs remotely operate several omniranges

with different names. In some cases, none of the

VORs have the name of the “parent” FSS. During

periods of maintenance, the facility may radiate a

T−E−S−T code (-

  -) or the code may be