Aeronautical Information Manual (AIM)

AIM

4/3/14

1−1−4

Navigation Aids

measured in the airborne DME unit and is translated

into distance (nautical miles) from the aircraft to the

ground station.

Operating on the line−of−sight principle, DME

furnishes distance information with a very high

degree of accuracy. Reliable signals may be received

at distances up to 199 NM at line−of−sight altitude

with an accuracy of better than

mile or 3 percent

of the distance, whichever is greater. Distance

information received from DME equipment is

SLANT RANGE distance and not actual horizontal

distance.

Operating frequency range of a DME according

to ICAO Annex 10 is from 960 MHz to 1215 MHz.

Aircraft equipped with TACAN equipment will

receive distance information from a VORTAC

automatically, while aircraft equipped with VOR

must have a separate DME airborne unit.

Aircraft equipped with slaved compass systems

may be susceptible to heading errors caused by

exposure to magnetic field disturbances (flux fields)

found in materials that are commonly located on the

surface or buried under taxiways and ramps. These

materials generate a magnetic flux field that can be

sensed by the aircraft’s compass system flux detector

or “gate”, which can cause the aircraft’s system to

align with the material’s magnetic field rather than

the earth’s natural magnetic field. The system’s

erroneous heading may not self-correct. Prior to take

off pilots should be aware that a heading

misalignment may have occurred during taxi. Pilots

are encouraged to follow the manufacturer’s or other

appropriate procedures to correct possible heading

misalignment before take off is commenced.

VOR/DME, VORTAC, Instrument Landing

System (ILS)/DME, and localizer (LOC)/DME

navigation facilities established by the FAA provide

course and distance information from collocated

components under a frequency pairing plan. Aircraft

receiving equipment which provides for automatic

DME selection assures reception of azimuth and

distance information from a common source when

designated VOR/DME, VORTAC, ILS/DME, and

LOC/DME are selected.

Due to the limited number of available

frequencies, assignment of paired frequencies is

required for certain military noncollocated VOR and

TACAN facilities which serve the same area but

which may be separated by distances up to a few

miles.

VOR/DME, VORTAC, ILS/DME, and LOC/

DME facilities are identified by synchronized

identifications which are transmitted on a time share

basis. The VOR or localizer portion of the facility is

identified by a coded tone modulated at 1020 Hz or

a combination of code and voice. The TACAN or

DME is identified by a coded tone modulated at

1350 Hz. The DME or TACAN coded identification

is transmitted one time for each three or four times

that the VOR or localizer coded identification is

transmitted. When either the VOR or the DME is

inoperative, it is important to recognize which

identifier is retained for the operative facility. A

single coded identification with a repetition interval

of approximately 30 seconds indicates that the DME

is operative.

Aircraft equipment which provides for auto-

matic DME selection assures reception of azimuth

and distance information from a common source

when designated VOR/DME, VORTAC and ILS/

DME navigation facilities are selected. Pilots are

cautioned to disregard any distance displays from

automatically selected DME equipment when VOR

or ILS facilities, which do not have the DME feature

installed, are being used for position determination.

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8. Navigational Aid (NAVAID) Service

Volumes

Most air navigation radio aids which provide

positive course guidance have a designated standard

service volume (SSV). The SSV defines the reception

limits of unrestricted NAVAIDs which are usable for

random/unpublished route navigation.

A NAVAID will be classified as restricted if it

does not conform to flight inspection signal strength

and course quality standards throughout the

published SSV. However, the NAVAID should not be

considered usable at altitudes below that which could

be flown while operating under random route IFR

conditions (14 CFR Section 91.177), even though

these altitudes may lie within the designated SSV.

Service volume restrictions are first published in

Notices to Airmen (NOTAMs) and then with the

alphabetical listing of the NAVAIDs in the A/FD.

Standard Service Volume limitations do not

apply to published IFR routes or procedures.