ARDF Operating Areas

For ARDF tasking purposes, MACV partitioned Southeast Asia into 20 areas of operation (see Figure 6). The twentieth area, Cambodia, was added following the Lon Nol government's granting of permission for Camodian overflight. Within these areas, approximately 70 by 90 nautical miles in size, missions could be fragged for either "effective" or "absolute" coverage. The effective coverage missions flew a general, or random, type of reconnaissance -- intercepting, fixing, and analyzing any enemy transmission encountered within a specified geographical area. If, however, intelligence indicated a specific or general area of high interest, absolute coverage was assigned the fragged aircraft, the EC-47 nomally orbitting within 20 NM of a specified point until acquisition was achieved, then fixing from five to eight miles from the captive radio. Over 95 percent of all USAF/ARDF missions were fragged for absolute coverage.

Because of the superior performance of the EC-47 (compared with U.S. Army platforms), most of the overflights of Areas 10, 11, and 12, as well as all deep penetrations of Cambodia, were executed by the Air Force. Nakhon Phanom-based aircraft were responsible for ARDF coverage of permissive areas of Barrel Roll within Laos, while aircraft of the 362nd TEWS at Da Nang normally had the responsibility for coverage of Steel Tiger. All other permissive areas were flown by either Air Force and Army aircraft, although the limited performance of Army aircraft let to their being restricted to shallow penetrations of Cambodia.

FIXING TACTICS

As a normal course of action, frequency search was conducted to and from fragged effective or absolute areas; however, unless intercepted information was significant enough to merit delay, the aircraft continued on to its target area. If the target was highly significant, the aircraft fixed the target, then proceeded to it initial destination. If, because of weather or unforcast threat, the aircraft was unable to works its assigne area, radio contact was made through the ROC (Reconnaissance Operations Center) with the ACC for diversion instructions. Final approval to deviate from a fragged mission could be granted only by 7AF through DOCR, on the basis of his knowledge of a defined threat in the area of propossed deviation. Even with receipt of 7AF approval, the final decission to deviate rested with the aircraft commander, whose determination was made only after due consideration of the potential hazards to flight security.

Once in the fragged area, the procedure was fo the intercept operators to search the frequency spectrum continually for significant transmissions. Once found by either a "Y", "Z", or "X" operator, the "X" operator locked the ALR equipment on, and the ARDF equipment desplayed a relative bearing to the target. As of the first LOP, no substative range information was possible, although signal strength and needle movement could give an experienced navigator a fair approximation in many cases. Depending upon the information he had, the navigator positioned the aircraft in order to take subsequent LOPs. Two intersecting lines of position gave him a good idea of the transmitter's range, as well as its position. Although six to ten LOPs were considered desirable, a navigator could accept a fix based on only three if he considered then accurate.

An experienced "X" operator and navigator could and did take simultaneous fixes on more than one target, alternating frequencies as LOPs were taken and plotted. (See Figure 7 for methods of plotting fixes and types of fixes determined.) The most desirable method of fixing a target wat to fly a single heading "innocent track" past the target, taking LOPs as the swung from nose to wingtip to tail. This was considered ideal for several reasons: one, the DF plane looked like any ordinary airplane just flying by to a ground observer; two, the stand-off range was good; and three, Doppler errors were kept to a minimum. However, the navigator used whatever pattern would give him the necessary result. If a target were transmitting intermittently, a cirular or eliptical pattern might be used to keep the aircraft in a favorable position for taking an instantaneous LOP, should the radio come up for a short burst.

The judgement of the navigator was of prime importance in evaluating the accuracy (circular error probable) of the fix. Accuracy depended upon several factors, among them the known accuracy of the equipment, and the effects of weather and terrain upon given LOPs. Ideally, all LOPs should have intersected at a given point; in practice, however, the navigator usually had to do considerable editing, throwing out LOPs which, in his judgement, were faulty and retaining those which appeared accuate. The final step in a navigator's procedure was to assign a probable radius to the fix. ranging fro 250 to thousands of meters. Influencing this determination were the time since the last Doppler update, the type of update (driftmeter, MSQ-77, TACAN, etc.), terrain effect, weather, and stand-off range.

Once the fix and its CEP had been determined by the navigator, it was passed back to the "Y" console operator for encrypted transmittion to the ground-using agencies for action or inclusion into the intelligence data base. Since a large part of the data gathered, both fix and take, were highly perishable, speed was important, but not to the extent that it would degrade accuracy.