partially or completely decommissioned. One of those future applications is LAAS.
Another is the expansion of the present 117.975-137 MHz air/ground (A/G)
communications band to support the transition to, and future growth of, the next-generation
VHF A/G communications system for air traffic services.

DME operates in the 960-1027, 1033-1087, and 1093-1215 MHz subbands of the
960-1215 MHz band. It shares those subbands with TACAN. The FAA and the rest
of the civil aviation community are investigating potential aeronautical applications
of those subbands for implementation after DME and TACAN have been partially or
completely decommissioned. These future applications include:

• Automatic Dependent Surveillance, Broadcast (ADS-B), a system that will
  enable aircraft to report their positions, velocities, and related data
  automatically during flight.
• Traffic Information Services (TIS), in which processed surveillance data will
  be reported automatically from ground stations to aircraft in flight.
• Terrestrial or satellite-based A/G transfer of voice and data traffic for CNS
  services.
• Potential future CNS applications to support Free Flight.

The FAA is also considering the retention of a subset of the nationwide VOR/DME
network. Continued use of some of the 108-117.975 MHz band would be needed to
sustain the VOR elements of such a network. A substantial portion of the 960-1215
MHz band would be required to support its DME elements.

TACAN is a UHF radionavigation system which is the military counterpart of
VOR/DME. TACAN is the primary tactical air navigation system for the military
services ashore and afloat. TACAN is often collocated with the civil VOR stations
(VORTAC facilities) to permit military aircraft to operate in civil airspace.

There are presently approximately 14,500 aircraft which are equipped to determine
bearing and distance to TACAN beacons. These consist primarily of Navy, Air
Force, and to a lesser extent, Army aircraft. Additionally, allied and third world
military aircraft use TACAN extensively.

Because of propagation characteristics and radiated power, TACAN is limited to
line-of-sight and is limited to approximately 180 miles at higher altitudes. As with
VOR/DME, special consideration must be given to location of ground-based
TACAN facilities, especially in areas where mountainous terrain is involved due to
its line-of-sight coverage.