Science: Tacan Unveiled

The Air Navigation Development Board last week took the technical wraps off the controversial Tacan (Tactical Air Navigation), electronic distance-direction guide for aircraft. Backed by the Navy and Air Force as a replacement for the Omni-DME (Omnirange and Distance Measuring Equipment) system adopted by the Civil Aeronautics Administration in 1948, Tacan stirred up a storm in the flying industry (TIME, May 30), but it will probably become the system for all U.S. aircraft.

In essence, Tacan and Omni-DME are much alike, but though Tacan and DME use the same ultra-high-frequency radio band (960-1215), they cannot be read on the same instruments. Both are short-range (200 miles) systems; both show true distance from the ground station within 500-600 ft. Nevertheless, the new details bring out differences. Tacan can provide compass direction with less than one degree error; Omni-DME is accurate within three degrees. Tacan, not bothered by superstructure interference or a ship's roll, is better fitted for Navy carriers.

With its "search and track" Tacan equipment, an airplane sends out "query" signals to a Tacan ground beacon. The ground beacon, identifying itself in International code (which the pilot can hear), sends out signals in "reply" to the aircraft. To determine distance, the plane's Tacan continuously measures the time interval between its own "interrogation" signal and the reply, computes the time delay into miles, and indicates the figure on a dial on the instrument board. The same radio pulses are simultaneously performing a more complicated process. To determine direction, the ground beacon's pulses pass through a revolving (15 revolutions per second) antenna system that relies on two concentric cylindrical rings, one mounted with a single rod-shaped element, the other fitted with nine rods. Whirling around the antenna core, these rods, set at different modulations, "tag" (modulate) the signals as they go out. Every time the inner rod passes "zero" (north), a regulating signal is transmitted. In a sense, this signal is Tacan's compass needle. The airplane's Tacan separates all the signals, computes their differences, and, all without a sound, converts the result into a degree reading on a dial for the pilot to steer by.