VORs operate on frequencies between 108.0 and 117.95 MHz and are used by all
types of aircraft for navigation and approach guidance. Courses produced by VOR
facilities compare to the 360° points on a compass. These courses, known as radials,
are identified by their magnetic bearing from the station. Regardless of heading, an
aircraft on the 090° radial is physically located due east of the station. When an aircraft
flies to the station on this radial, its magnetic course is 270°. Since the transmitting
equipment is in the VHF band, the signals are free of atmospheric disturbances but are
subject to line-of-sight reception. Reception range varies with the altitude of the aircraft.
VORs are identified by their Morse code identification or by the recorded automatic
voice identification which is always indicated by the use of the word "VOR" following the
range s name. The accuracy of the VOR s course alignment is excellent, generally ±1°.
Some VOR sites are equipped with a DME feature. The VOR/DME site furnishes
azimuth information from the VOR and distance information from the DME facility.
These are two separate types of equipment located at the same site. A pilot's access to
azimuth and distance information from such a site is limited only by the aircraft's
Tactical Air Navigation (TACAN) System
Although VOR was a great improvement over earlier navigational systems, a gap still
existed in the information presented to the pilot. The TACAN system was developed to
fill this gap by providing the pilot with the information needed for precise, geographical
fixing of the aircraft s position at all times.
TACAN added a continuous display of range information to the course information
already available. An integral part of TACAN is DME, which provides continuous slant
range distance information. Like VOR, TACAN provides 360 courses radiating from the
station. Also, because the ground equipment is compact and relatively easy to
transport, it provides greater versatility in installation and mobility than the VOR system.
TACAN operates in the UHF frequency band and has a total of 126 two-way channels.
Air-to-ground frequencies (DME) for these channels are in the 1025 to 1150 MHz range,
and associated ground-to-air frequencies are in the 962 to 1024 MHz and 1151 to 1213
MHz ranges. Channels are spaced at 1-MHz intervals in these ranges. The TACAN
identifies itself aurally through Morse code every 35 seconds.
TACAN ground equipment has either a fixed or mobile transmitting unit capable of
providing bearing information to an unlimited number of aircraft but is limited to 120
simultaneous replies for distance information. The airborne unit (interrogator), in
conjunction with the ground unit (transponder), reduces the transmitted signal to a
visual presentation of both azimuth and distance information.
TACAN facilities are usually dual-transmitter equipped (one operating and one in
standby), fully monitored installations which automatically switch to the standby