Figure 1: AN/TRN-26 used by the German Luftwaffe.
TACAN (TACtical Air Navigation System) is a fixed navigation system primarily used by military, not civilian aircraft. It provides electronic information from a single ground station to aircraft during approach/departure to/from a particular airfield or enroute to a distant airfield. Information transmitted by the station consists of angular bearing relative to magnetic north, station identification, and distance information relative to that ground station. This all serves to determine the geographic position of the aircraft.
The TACAN operates on a fixed channel in the 962 … 1213 MHz range. The TACAN periodically transmits its identification call sign in Morse code to enable aircraft to determine which ground station they are using. Distance reply signals are transmitted upon request. TACAN receives an interrogation code from an aircraft, delays it and retransmits the same code back to the aircraft. The time, measured in microseconds, it takes for the code to reach the aircraft determines the distance from the ground station. Approximately 100 aircraft can interrogate the TACAN at any given moment. The usable distance of the TACAN is usually 40 nautical miles (NM), but it can be received up to 200 NM ( Line-Of-Sight) away.