I took a look at WiKipedia and they explained it this way
TACtical Air Navigation, or TACAN, is a navigation system used by military aircraft. It provides the user with a distance and bearing from a ground station. It is a more accurate version of the VHF omnidirectional range / Distance Measuring Equipment (VOR/DME) system that provides range and bearing information for civil aviation. At VORTAC facilities, the DME portion of the TACAN system is available for civil use.
TACAN in general can be described as the military version of the VOR/DME system. It operates in the frequency band 960-1215 MHz. The bearing unit of TACAN is more accurate than a standard VOR since it makes use of a two frequency principle, with 15 Hz and 135 Hz components.
The distance measurement component of TACAN operates with the same specifications as do civil DMEs. Therefore to reduce the number of required stations, TACAN stations are frequently co-located with VOR facilities. These co-located stations are known as VORTACs. This is a station composed of a VOR for civil bearing information and a TACAN for military bearing information and military/civil distance measuring information. The TACAN transponder performs the function of a DME without the need for a separate, collocated DME. Because the rotation of the antenna creates a large portion of the azimuth signal, if the antenna fails, the azimuth component is no longer available and the TACAN downgrades to a DME only mode.
Theoretically a TACAN should provide a ninefold increase in accuracy compared to a VOR but operational use has shown only a 1.5 to 2 fold increase
Accuracy of the 135 Hz azimuth component is Â±1Â° or Â±63 m at 3.47 km. Accuracy of the DME portion is 185 m (Â±0.1 nautical mile).
Because the azimuth and range units are combined in one system it provides for simpler installation. Less space is required than a VOR because a VOR requires a large counterpoise and a fairly complex phased antenna system. A TACAN system theoretically might be placed on a building, a large truck, an airplane, or a ship, and be operational in a short period of time. TACAN, for example, are used on air refueling tankers.
For military usage a primary drawback is lack of the ability to control emissions (EMCON) and stealth. Naval TACAN operations are designed so an aircraft can find the ship and land. There is no encryption involved, an enemy can simply use the range and bearing provided to attack a ship equipped with a TACAN. Some TACANs have the ability to employ a "Demand Only" mode wherein they will only transmit when interrogated by an aircraft on-channel. It is likely that TACAN will be replaced with a differential GPS system similar to the Local Area Augmentation System called JPALS. The Joint Precision Approach and Landing System has a low probability of intercept to prevent enemy detection and an aircraft carrier version can be used for autoland operations.
Some systems used in the United States modulate the transmitted signal by using a 900 RPM rotating antenna. Since this antenna is fairly large and must rotate 24 hours a day, it can cause reliability issues. Modern systems have antennas that use electronic rotation (instead of mechanical rotation) and have no moving parts.
Of course this is not the whole story well it never is, is it, what it used to do when I worked for ATC in the 1970's uless my memory is playing tricks on me, was it plotted a trace on a circular display giving the compass bearing and range of aircraft when they made a call.
Hope you havr all had a merry ol' Christmas
W/Cmdr Percival Prune (hic!)