AIM

4/20/23

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Navigation Aids

Chapter 1. Air Navigation

Section 1. Navigation Aids

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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 services, private
organizations, individual states and foreign governments. The FAA has the statutory authority to establish,
operate, maintain air navigation 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

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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.

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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

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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.

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3. VHF Omni

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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

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sight restrictions,

and the range varies proportionally to the altitude of the receiving equipment.

NOTE

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Normal service ranges for the various classes of VORs are given in Navigational Aid (NAVAID) Service Volumes, Paragraph
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b.

Most VORs are equipped for voice transmission on the VOR frequency. VORs without voice capability

are indicated by the letter “W” (without voice) included in the class designator (VORW).