Tactical air navigation
TACAN is the usual way of referring to tactical air navigation an ultrahigh frequency electronic air (and sometimes shipboard) navigation system, able to provide aircraft with both the direction to, and distance from, a ground beacon.  It was introduced in 1955, , but, in many applications, it has been replaced by GPS-based techniques. Nevertheless, reports of its demise may be premature, as a new generation of "deployable TACAN" (D-TACAN) is useful for applications where it may be useful to put a beacon at, for example, a temporary military airfield. It also is seen as an alternative, in military use, if GPS is jammed. 
TACAN beacons are located at precisely surveyed ground locations, and their radio signals carry the beacon's identifier. An aircraft can determine its bearing to the TACAN transmitter by passive means, but, to determine the range between the aircraft and the beacon, there must be an active exchange of pulses between the aircraft and the retransmitting function of the beacon. 
While an unlimited number of aircraft can use TACAN as a bearing reference, there is a limit to the number of aircraft that can use it as a distance-measuring technique. The limit comes from the beacon's ability to retransmit interrogations from a finite number of transponder.
TACAN signals are an integral part of the Joint Tactical Information Distribution System (JTIDS).
- US Department of Defense (12 July 2007), Joint Publication 1-02 Department of Defense Dictionary of Military and Associated Terms
- "Tacan Unveiled", Time, August 29, 1955
- "Moog Selected by U.S. Navy for TACAN Upgrade", Defense News, February 19, 2008
- , Navigational Aids: Tacan, The Radar Pages: All you ever wanted to know about British air defence radar