2/19/04 AIM
Air Navigation Aids 1−1−1
Chapter 1. Navigation Aids
Section 1. Air 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 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 Airport/Facility
Directory (A/FD).
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
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