transmitter when a malfunction occurs. The monitor is located in the control tower or
RADAR room and provides a warning when an out-of-tolerance condition exists.
The TACAN system readily lends itself to unique military and naval requirements, and
the Federal Aviation Administration (FAA) has integrated TACAN facilities with the civil
VOR/DME program. Although the theoretical or technical principles of operation of
TACAN are quite different from those of the VOR facilities, the end result is the same.
VORTAC
Integrated VOR and TACAN facilities are called VORTACs. A VORTAC provides the
following services:
VOR azimuth
TACAN azimuth
TACAN distance (DME)
The VOR and TACAN of a VORTAC system are each identified by a three-letter code
transmission. In addition, the VOR and TACAN are assigned paired frequency channels
so that pilots using VOR azimuth with TACAN distance can be assured that both signals
being received are from the same ground station.
Distance Measuring Equipment
The distance from a known ground point is essential information necessary to accurate
navigation.
Distance Measuring Equipment
The availability of Distance Measuring Equipment (DME) to a pilot depends on whether
there is DME associated with the facility being used and whether the equipment needed
is onboard the aircraft. The two parts to every DME system are the ground station,
called the transponder, and the airborne portion, called the interrogator. Inquiries are
sent from the interrogator to the transponder, which replies with data that the
interrogator can process and display to the pilot as distance from the transponder site.
DME operates on the line-of-sight principle and furnishes information with a 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.
DME operates on frequencies in the UHF band between 962 to 1213 MHz. Aircraft
equipped with TACAN equipment will receive distance information from a VORTAC
automatically, but aircraft equipped with VOR must have separate DME equipment.
DME can also be associated with an instrument landing system (ILS). The ILS provides
the pilot with distance information from the touchdown point on the runway. In such an
arrangement, the DME transponder is located at the ILS glide slope site.
2-29
