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considered unsuitable for military or naval use. A new navigational
system, TACAN, was therefore developed

by the military and naval forces to more readily lend itself to
military and naval requirements. As a result, the

FAA has integrated TACAN facilities with the civil VOR/DME program.
Although the theoretical, or technical

principles of operation of TACAN equipment are quite different from
those of VOR/DME facilities, the end

result, as far as the navigating pilot is concerned, is the same. These
integrated facilities are called VORTACs.

b.

TACAN ground equipment consists of either a fixed or mobile
transmitting unit. The airborne unit in

conjunction with the ground unit reduces the transmitted signal to a
visual presentation of both azimuth and

distance information. TACAN is a pulse system and operates in the
Ultrahigh Frequency (UHF) band of

frequencies. Its use requires TACAN airborne equipment and does not
operate through conventional VOR

equipment.

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6.  VHF Omni

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directional Range/Tactical Air Navigation (VORTAC)

a.

A VORTAC is a facility consisting of two components, VOR and TACAN,
which provides three individual

services: VOR azimuth, TACAN azimuth and TACAN distance (DME) at one
site. Although consisting of more

than one component, 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

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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.

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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

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of

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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

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of

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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.

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

Navigation Aids

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