T-6A INSTRUMENT NAVIGATION
back, preferably from the opposite direction than the original setting, will sometimes correct this
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 aircrew is concerned, is
the same. As a result, the FAA has integrated many TACAN facilities with the civil VOR/DME
facilities. These integrated facilities are called VORTACs.
VHF OMNI-DIRECTIONAL RANGE/TACTICAL AIR NAVIGATION
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.
Even though they consist of more than one component, incorporate more than one operating
frequency, and use more than one antenna system, a VORTAC is considered to be a unified
NAVAID. Both components of a VORTAC are envisioned as operating simultaneously and
providing the three services at all times.
Transmitted signals of VOR and TACAN are each identified by three-letter code transmission
and are interlocked so that aircrews 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.
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 in NM 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 1/2 mile or 3% of the distance, whichever is greater.
It is important to note, the distance information received from DME equipment is SLANT
RANGE distance and not actual horizontal distance.
DME operates on frequencies in the UHF spectrum between 962 MHz and 1213 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.
VOR/DME, VORTAC, ILS/DME, and LOC/DME navigation facilities established by the FAA
provide course and distance information from collocated components under a frequency pairing
plan. Aircraft receiving equipment providing 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.
INTRODUCTION TO NAVIGATION SYSTEMS 1-5