Principles of tacan

Tacan is a polar coordinate aid to air navigation working in the 1000 Mc/s band. A transmitter - receiver in the aircraft inter-rogates a beacon on the ground and the time interval between transmission and the reception of the reply pulse gives a measure of the aircraft's distance from the beacon. To prevent interference by replies to other interrogating aircraft, the p.r.f. of each aircraft's transmission is jittered and the replies are strobed by a narrow range gate. This is automatically advanced step by step during the process of searching, and at each range interval - measured from the interrogation pulse - a count is made for a given number of replies in a fixed time. When the count shows that regular replies are being received the search stops and the range gate follows the very slow variations in time delay due to the aircraft's motion. The position of the delay control is used to control a meter which displays in figures the distance in miles. Bearing information is introduced at the ground beacon by a reflector rotating round a stationary aerial at 15 rev/sec. The pulses radiated in any direction are thus amplitude modulated with a 15 c/s sinusoidal envelope. To give a smooth envelope, the beacon always transmits at the same rate. If less than 100 aircraft (the beacon capacity) are interrogating, a.g.c. causes the transponder to be triggered by noise to make up the const. number of pulses. Phase reference pulse groups are inserted at an appropriate instant in the aerial rotation. At the aircraft the phase of the demodulated pulse envelope is measured by a rotary phase-shifter the shaft of which controls the bearing indicator. An outer array of nine reflectors at the ground aerial provides a 135 c/s modulation which enables fine bearing measurements to be made, further reference pulses being inserted for this purpose. This facility also reduces the effects of site errors. The average system error is 3/4deg. The service range is 200 n.m. See also abstracts 1957B03170, 1957B03171.