TACAN is an acronym for TACtical Air Navigation. It is the military version of the VOR/DME systems used in Civil Aviation, with additional features which offer both increased accuracy as well as in-band beacon identification.

TACAN consists of beacon stations, which can operation from fixed locations on the ground as well as on board vehicles (ships, ground vehicles, or even aircraft) as well as the TACAN navigation units. Note that both the beacon and navigation units contain both transmitter and receivers.

TACAN provides slant range and bearing from the beacon to the user's navigation unit. In addition, the beacon will identify itself through coded pulse transmissions, allowing the user to determine their position relatively accurately through the use of a single TACAN station, assuming they have position information for the TACAN station they are attempting to utilize via charts or tables. For convenience and efficiency in peacetime, most TACAN stations in the U.S. and Europe are colocate with civilian VOR/DME stations - such installations are known as VORTAC stations in both civil and military aviation.

In addition to its use for navigation, TACAN is used for aerial rendezvous. This is the reason TACAN beacons can be mounted aboard vehicles; the most common use of TACAN in this manner is to support aerial refueling missions. Tanker aircraft mount a single TACAN beacon whose identification code is distributed to their 'client' aircraft before a mission; by tuning in the tanker's TACAN signal when they are ready to refuel, aircraft can 'home in' on the tanker as needed.

How it Works
TACAN beacons consist of a central emitter ("radiator") around which are placed ten unpowered parasite radiators; nine in a ring around the central, and one adjacent to it. This entire assembly is then rotated at 900 RPM. The result is that a receiver will 'hear' a signal which will vary according to a known 'pattern' determined by the shape of the emitters (which is standard and well-known) and the speed (which is standard). When the assembly is 'facing' Magnetic East, it emits a standard sequence of pulses, known as the 'North Reference Burst' or 'Main Reference Burst.' It transmits reference pulses every 40 degrees of rotation otherwise. Given that indicator, and the position on the reference signal waveform from the rotation that the nav unit is receiving when the MRB occurs, it should be able to determine what its bearing from the assembly is.

In addition to its direction-finding capabilities, the TACAN beacon listens for interrogative transmissions from navigation units. When it receives them, it responds with an 'addressed' pulse. The navigation unit is responsible for timing the return pulse to determine slant range to the beacon, assuming a propagation speed of approximately 12.359 microseconds per nautical mile for the signal in atmosphere and a fixed delay between receipt and transmit.

Finally, the beacon randomly transmits identification pulses. These can be coded for security purposes, and the whole process can (and does, in more modern units) incorporate IFF security transactions.

Sources:
http://www.radarpages.co.uk/mob/navaids/tacan/tacan1.htm
http://www.rockwellcollins.com/ecat/gs/TCN-550_Tacan_PrintFriendly.html

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