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radio receiver used by the operator of the ground
station. FAA facilities that provide VHF/DF service
are identified in the Airport/Facility Directory
(A/FD).
1.2The equipment consists of a directional antenna
system and a VHF radio receiver.
1.3The VHF/DF receiver display indicates the
magnetic direction of the aircraft from the ground
station each time the aircraft transmits.
1.4DF equipment is of particular value in locating
lost aircraft and in helping to identify aircraft on
radar.
2. Nondirectional Radio Beacon (NDB)
2.1A 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
1020hertz (Hz) modulation. All radio beacons
except the compass locators transmit a continuous
three−letter identification in code except during voice
transmissions.
2.2When a radio beacon is used in conjunction with
the Instrument Landing System markers, it is called
a Compass Locator.
2.3Voice transmissions are made on radio beacons
unless the letter W" (without voice) is included in
the class designator (HW).
2.4Radio 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 aircraft’s
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 will usually
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.
3. VHF Omni−directional Range (VOR)
3.1VORs operate within the 108.0 − 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 range varies proportionally to the
altitude of the receiving equipment.
NOTE−
Normal service ranges for the various classes of
VORs are given in GEN 3.4, TBL GEN 3.4−1,
VOR/DME/TACAN Standard Service Volumes.
3.2Most 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).
3.3The effectiveness of the VOR depends upon
proper use and adjustment of both ground and
airborne equipment.
3.3.1Accuracy.The accuracy of course alignment
of the VOR is excellent, being generally plus or
minus 1 degree.
3.3.2Roughness.On some VORs, minor course
roughness may be observed, evidenced by course
needle or brief flag alarm activity (some receivers are
more subject to these irregularities than others). At a
few stations, usually in mountainous terrain, the pilot
may occasionally observe a brief course needle
oscillation, similar to the indication of approaching
station." Pilots flying over unfamiliar routes are
cautioned to be on the alert of these vagaries, and, in
particular, to use the to−from" indicator to determine
positive station passage.
ENR 4.1−2 AIP
15 MAR 07 United States of America
Nineteenth Edition Federal Aviation Administration
3.3.2.1Certain propeller RPM settings or helicopter
rotor speeds can cause the VOR Course Deviation
Indicator (CDI) to fluctuate as much as plus or minus
six degrees. Slight changes to the RPM setting will
normally smooth out this roughness. Pilots are urged
to check for this modulation phenomenon prior to
reporting a VOR station or aircraft equipment for
unsatisfactory operation.
3.4The 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 identification
of an omnirange should never be placed on
listening to voice transmissions by the FSS (or
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