Aeronautical Information Manual (AIM), page 55

Index   54 -- Page 55 -- 56


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
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


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.
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

Navigation Aids 1-1-1

Page 55 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   54 -- Page 55 -- 56