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AIM

10/12/17

1−1−5

Navigation Aids

incorporating more than one operating frequency,

and using more than one antenna system, a VORTAC

is considered to be a unified navigational aid. Both

components of a VORTAC are envisioned as

operating simultaneously and providing the three

services at all times.

b. Transmitted signals of VOR and TACAN are

each identified by three−letter code transmission and

are interlocked so that pilots using VOR azimuth with

TACAN distance can be assured that both signals

being received are definitely from the same ground

station. The frequency channels of the VOR and the

TACAN at each VORTAC facility are “paired” in

accordance with a national plan to simplify airborne

operation.

1−1−7. Distance Measuring Equipment

(DME)

a. In the operation of DME, paired pulses at a

specific spacing are sent out from the aircraft (this is

the interrogation) and are received at the ground

station. The ground station (transponder) then

transmits paired pulses back to the aircraft at the same

pulse spacing but on a different frequency. The time

required for the round trip of this signal exchange is

measured in the airborne DME unit and is translated

into distance (nautical miles) from the aircraft to the

ground station.

b. 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 

1

/

2

 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.

c. 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.

d. 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.

e. 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.

f. 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.

g. 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.

1−1−8. Navigational Aid (NAVAID) Service

Volumes

a. 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.

b. 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