Tactical air navigation system 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 ) 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 combine d with a TACAN , civil aircraft can receive VOR / DME readings . Aircraft equipped with TACAN avionics can use this system for enroute navigation as well as non - precision approach es to landing fields . The space shuttle is one such vehicle that was designed to use TACAN navigation ( although it has since been 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 . 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 Electronics - Military Products Division 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 . 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 co - located stations are known as VORTAC s . 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 , co - located 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 . Accuracy . Theoretically a TACAN should provide a ninefold increase in accuracy compared to a VOR but operational use has shown only an approximate 3 - fold increase . Accuracy of the 135 Hz azimuth component is ±1° or ±63 m at 3 . 47 km . Accuracy of the DME portion is 926 m ( ±0 . 5 nautical mile ) or 3 percent of slant range distance , whichever is greater - see FAA 9840 . 1 1982 . TACAN stations can provide distance up to 390NM Current Modern TACANs are much more accurate . The requirement now is to have portable TACAN that is IFR certifiable , both Station and Portable systems . The latest modern version of TACAN has been tested to an average error of 0 . 00 in both range and azimuth , and could be a feasible back - up to future Air Traffic Control Systems and may even be integrated into systems for a seamless back up . Past TACANs have relied on high output power ( up to 10 , 000 Watts ) to ensure good signal in space to overcome nulls present in antenna design and to provide their required 200 mile range . With the advancement of technology , antenna design has improved with higher gain antennas , much shallower nulls , and lighter construction . Now it ' s feasible to have a 200 nm range with a 400 Watt TACAN DME Transmitter , making the TACAN package much smaller , more portable and more reliable ( Power = Heat which shortens the life of electronics ) . TACAN is getting smaller : Full TACAN coverage can now be provided in a system that can be carried on a single trailer weighing less than 4000 lbs , and set up by two people in less than an hour . TACAN Transceivers can now be as small as lunch boxes ( with full coverage and range ) and the antennas can be reduced from 800 pounds to less than 100 pounds . Benefits . 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 . An airborne TACAN receiver can be used in air - to - air mode which allows two cooperating aircraft to find their relative bearings and distance . Drawbacks . 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 ) with no moving parts . Future . Like all other forms of ground - based aircraft radio navigation currently used , it is likely that TACAN will be replaced by some form of space based navigational system such as GPS .