A tactical air navigation system, commonly referred to by the acronym TACAN, is a navigation system used by military aircraft. It provides the user with bearing and distance (slant-range or hypotenuse) to a ground or ship-borne station. It is a more accurate version of the VOR/DME system that provides bearing and range information for civil aviation. The DME portion of the TACAN system is available for civil use; at VORTAC facilities where a VOR is combined with a TACAN, civil aircraft can receive VOR/DME readings. Aircraft equipped with TACAN avionics can use this system for en route navigation as well as non-precision approaches to landing fields. The Space Shuttle is one such vehicle that was designed to use TACAN navigation but later upgraded with GPS as a replacement. The typical TACAN onboard user panel has control switches for setting the channel (corresponding to the desired surface station's assigned frequency), the operation mode for either transmit/receive (T/R, to get both bearing and range) or receive only (REC, to get bearing but not range). Capability was later upgraded to include an air-to-air mode (A/A) where two airborne users can get relative slant-range information. Depending on the installation, Air-to-Air mode may provide range, closure (relative velocity of the other unit), and bearing, though an air-to-air bearing is noticeably less precise than a ground-to-air bearing. A TACAN only equipped aircraft cannot receive bearing information from a VOR only station. History The TACAN navigation system is an evolution of radio transponder navigation systems that date back to the British Oboe system of World War II. In the United States, many companies were involved with the development of TACAN for military aircraft. Hoffman Laboratories Div. of the Hoffman Electronics Corp.–Military Products Division (now NavCom Defense Electronics) was a leader in developing the present TACAN system in the US starting in the late 1950s. Operation 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, and because UHF transmissions are less prone to signal bending than VHF. The distance measurement component of TACAN operates with the same specifications as civil DMEs. Therefore, to reduce the number of required stations, TACAN stations are frequently co-located with VOR facilities. These