Chapter 1

Early ARDF: German and French Efforts

The dependence of the Viet Cong and the North Vietnamese Army upon radio communications offered the opening which technology could exploit in behalf of tactical air reconnaissance. During World War II, the Germans used both airborne and ground radio direction finding equipment to track down partisan forces in occupied areas such as Norway. The French apparently used some form of this technique during their struggle in Vietnam. A recently captured VC document calls for radio discipline to counter detection techniques such as the French had used against them.

USA ARDF

The long-time interest of the Signal Corps of the U.S. Army in ground radio detection finding appears to have led to the Army's early work in the airborne field. In 1961, Army ARDF experimented with ARDF in RVN. By 1 January 1966, it had 15 aircraft in Vietnam to satisfy weekly COMUSMACV ARDF requirement for 240 hours of coverage of enemy operations.

Ever since its experiments in 1961, Army ARDF has depended upon the so-called "aural null" technique. When one of its ARDF crews picks up an enemy signal, the pilot maneuvers his aircraft until he obtains an aural null (i.e., he hears nothing on that aircraft heading). The aural null means that the aircraft is heading directly toward or away from the enemy transmitter. The aircraft's heading can be plotted as a line of position (LOP), if the exact position of the aircraft is known, and the ambiguity of its heading toward or away from the transmitter is resolved. The Army crew repeats this aural null procedure until several widely spread LOPs are obtained. The LOPs should cross at a common point, or fix, which is the location of the enemy transmitter. (See Chapter VI.)