Among the least known army units serving in Vietnam were the super-secret forces of the Army Security Agency (ASA) Group, Vietnam. These included, the 8th ASA Field Station in Phu Bai (later Da Nang), the 224th ASA Battalion (Aviation) at Tan Son Nhut (later Long Than North), the 303th ASA Battalion at Long Binh, the 313th ASA Battalion at Nha Trang and the 20 ASA aviation, divisional support, operations, and security companies scattered throughout the country.
They were unknown because officially they did not exist. All were hidden undercover designations to mask their true identities, with Radio Research Unit (RRU) being the most common designator. Among the several missions of these ASA units was the collection of enemy intelligence through airborne radio direction finding (ARDF). By any measure, ARDF became the single most valuable intelligence resource available to American and Allied forces during the war in Vietnam.
Supporting the American units fighting in the southern part of South Vietnam was the RRU at Tan Son Nhut Air Base on the edge of Saigon. The unit occupied a small white stucco headquarters near the flight line. The ARDF mission involved finding two bits of information:
Where was the enemy radio transmitter and whose transmitter was it. These transmitters belonged to combat, administrative and logistical units of the Viet Cong, and in later years to elements of the Army of North Vietnam. The enemy operators used call-signs and techniques that were familiar to the men of the RRU, for some had been associated by the intelligence analysts with particular enemy units or headquarters.
Typically, the messages were sent by telegraph key ("CW" in military parlance) and encrypted, but this fact presented no problem for ARDF since message content was not the issue. The real problem was that the time a station remained on the air was often extremely short, presenting too brief a burst on which to get a "fix."
An account of a mission over Hau Nghia Province in April 1966 illustrates how the system worked. The flight would reach into the flat terrain near the Parrot’s Beak, the Cambodian salient that projects into the marshlands called Vietnam’s Plain of Reeds, and serves as the home base for a number of main-force Viet Cong formations.
The brown, high-winged, tail-dragging airplane that would fly the mission was a De Havilland "Beaver," what the Army then called the L-20 and later designated the U-6A. This was a venerable bird. It had been serving the Army for nearly 20 years by that time. There was nothing fast or fancy about it, but it was reliable. A big radial engine with a twobladed prop would pull it along at 160 mph, top speed in a crisis. The fat fuselage was shaded by a thick, wide wing, and the wing tips were pierced by vertical rods. These were the directional antennae that would give the intelligence analyst aboard the azimuth to any radio transmitter his receiver could pick up.
In the cargo space, bent over a steel desk backed by an instrument panel, and virtually surrounded by grey steel cabinets, was a soldier wearing earphones, testing his equipment and going over the collection plan for the day. The pilot settled into his seat and began his preflight routine. He fastened a map and the collection plan to a clipboard mounted above instruments in front of him, shouted "clear"—to no one in particular—out the window, and turned the switch.
The old Beaver followed a flight of F-104s down the runway and watched them turn northeast and recede into tiny dots as it gained altitude, quickly reaching for the clear air above the haze, smoke and dust that shrouded Saigon during the dry monsoon. It headed northwest over Gia Dinh, bound for Hau Nghia, cruising at 2,000 feet, above the effective range of machine guns but low enough to see the details of the green, pastoral countryside.
The course was set for Go Dau Ha, and now and then the ribbon of National Highway 1 could be seen far below. The pilot and the soldier-technician in the rear, heads clasped in earphones, had before them the broadcast "schedules" of a number of enemy transmitters. They knew when to expect these transmitters to come on the air, for the Viet Cong (VC) required its units and subordinate headquarters to send their reports according to an established routine. The VC would pass their messages quickly, and resume silence until their reporting time came around again.
The pilot kept a constant check on his position by reference to the map in front and the ground below. Abruptly, he kicked the right rudder and the plane swung around to the north like a weathervane in a strong gust. The soldier-technician had picked up one of the wanted call-signs and the pilot had turned the airplane’s nose around until the instruments told them that he was flying directly toward the transmitter. Checking his position with reference to the terrain below, he marked a dot on the map, noted the airplane’s heading with a glance at the compass and, with a protractor, drew the azimuth to the transmitter. He worked fast, flying the Beaver with his feet.
That accomplished, the next task was to fly as fast as the old bird would take him to another part of the sky to get another bearing before the transmitter fell silent. With the throttle to the firewall, he banked in a tight turn and headed southeast. They were lucky that morning, for this transmitter had a long message to send. Through the combined skills of the pilot and the technician, they managed three bearings. Where the bearings intersected was a "fix’ the location of an enemy transmitter belonging to a known enemy unit.
Unfortunately, the equipment then in the hands of the RRU denied them the capability to "fix" with precision. Error was built into the system and the size of the error depended on the relative skills and experience of the pilots, operators and analysts, and the accuracy to which the equipment was calibrated. Nevertheless, correlated with intelligence gained from prisoners-of-war, captured documents, and agents, ARDF intelligence became the best means for following enemy movement. From his movements, intelligence officers and commanders could estimate the enemy’s intentions and plan fire and maneuvers against him with telling effect.
In early 1966, the G2 of the U.S. 1st Infantry Division (the "Big Red One") initiated a program of direct liaison with the RRU that rapidly developed into a remarkably symbiotic relationship, an association that significantly helped the division succeed in its battles against the Viet Cong’s main force regiments in War Zones C and D north of Saigon. Enemy units posing the greatest threat in the division’s area of operations were designed priority targets for the RRU. Most of these belonged to the Viet Cong’s veteran 9th Division, whose headquarters lurked in the jungles of northern Tay Ninh Province. Priority "fixes" plotted by the RRU flight would be passed immediately to "Danger Forward’ the 1st Infantry Divison Command Post (CP). How this procedure paid off is illustrated by the intelligence story behind the Battle of Minh Thanh Road on July 9,1966. Minh Thanh was a small rubber plantation in War Zone C, just east of the Saigon River in southern Binh Long Province. Situated as it was on one of the main communications-liaison routes between War Zones C and D, it had long been a locus of violence. The 1st Division became interested in Minh Thanh during Operation Birmingham in northwestern Tay Ninh (April 24 to May 16, 1966).
When the campaign in Tay Ninh wound down, Division Commander Maj. Gen. William E. Depuy moved his division east to Binh Long Province for operations along National Route 13, the "ambush alley" called "Thunder Road" that ran from Saigon, north through jungles and plantations, to cross the Cambodian border above Loc Ninh. The 9th Viet Cong Division, which consisted of the 271st, 272nd and 273rd regiments, was headquartered somewhere in or near the "Fishhook," a bend of the Cambodian border about 20 kilometers north of Minh Thanh.
From its headquarters at the Quan Loi plantation above An Loc, the division staff planned and controlled a series of fierce encounters with the regiments of the 9th Viet Cong Division throughout the summer of 1966. These battles, five in all, culminated in the massacre on the Minh Thanh road.
The move to Binh Long had proved to be a wise maneuver for it had immediate results. On the day the command post was opened at Quan Loi, June 8, the 272nd hit a troop of the division’s cavalry squadron, the 1st of the 4th Cavalry, at the Cam Le Bridge on Route 13 south of An Loc. [See "Double Ambush on Route 13" Vietnam, Spring, 1989] The 272nd left 150 dead behind, withdrew into the jungles to the west, and left blood-trails as it limped toward Tay Ninh. Three days later, the 273rd attacked the 2nd Battalion, 28th Infantry, in the Loc Ninh plantation. Following a brief hiatus, on June 30 the 271st ambushed B Troop, 1/4 Cavalry at Srok Dong on Route 13 north of An Loc. The troopers were well prepared for this one, and they and their artillery and air support demolished the 271st. It withdrew, leaving~270 dead beside the road. Then the 2nd Battalion, 28th Infantry, went looking for action west of Loc Ninh and found it on July 2 in a clash with the 273rd near the "Fishhook."
It was then, around the first of July, that the RRU reported that the 272nd regimental headquarters was on the move eastward from its base in the jungles of Tay Ninh toward the Saigon River near Minh Thanh. About the same time, a Minh Thanh patrol killed a Viet Cong soldier in a fire-fight on the northeast edge of the plantation. A letter found on the body contained the letter-box number of the 272nd Regiment. Then the RRU reported that the 272nd headquarters had crossed the Saigon River into Binh Long north of Minh Thanh. That did it! There was going to be another meeting with the 272nd, but this time the division commander had a golden opportunity to choose the time and place. Bill Depuy recognized that the situation contained the ingredients for a convincing deception plan. He could exploit these ingredients to convince the Viet Cong that they could destroy a high-value target at low risk on the Minh Thanh road. The essence of the commander’s concept was to lead the 272nd to believe that it could ambush and destroy an American mechanized column on the Minh Thanh road and sustain only light losses while doing so. That is just what the 9th Viet Cong Division had been trying to do for a month without success.
Since the middle of June, two platoons of the 1st Engineer Battalion (Combat) had been improving and lengthening the runway in the Minh Thanh plantation. The job was nearing completion and General Depuy would have to recover the heavy equipment— dozers, graders, rollers, loaders and dump trucks—soon. So he passed the word to the province headquarters, and to the Republic of Vietnam 5th Division, in whose area the 1st Division was operating, that he would be sending a troop of cavalry to Minh Thanh on July 9 to escort the engineer equipment to An Loc. He knew that the Viet Cong would learn of this plan through their agents in both headquarters.
Bill Depuy gave the responsibility for the trap at Minh Thanh to the 1st Brigade, commanded by Colonel Sidney B. Berry, whose plan called for the employment of four infantry battalions, about half of the cavalry squadron, three batteries of 105mm howitzers, and one battery each of 155mm and 8-inch howitzers from the division’s general support battalion. Troops B and C, 1st Squadron, 4th Cavalry, were the bait. The infantry battalions were positioned to close in after the trap was sprung.
The Viet Cong took the bait, for they had been badly misled by their intelligence. They had not discovered the infantry battalions the 1st Infantry Division had waiting for them in the plantation, and they had not foreseen the significance of all those artillery batteries with their tubes pointed toward Minh Thanh. About all they were able to do was keep track of the cavalry column. As the troopers waited for their marching orders beside the road south of An Loc, they watched a Vietnamese on a motorbike pass their column and disappear in the direction of Minh Thanh. They had seen the same man pass their column just before the ambush at Srok Dong on June 30. No one in the column doubted that heavy action was imminent.
Late in the morning of July 9, the squadron began to move southwest along the Minh Thanh road. B Troop was leading, with C Troop following behind at a 1,000-meter interval. At three minutes past eleven, the advance guard reported four VC crossing the road. The Division 02 saw them from his helicopter above the column. Seven minutes later the VC fired their first shot, a 75mm rocket into the first armored cavalry vehicle. The road blazed with fire and the artillery covered the woods alongside with showers of exploding shell. By noon it was all but over on the road, although the rifle battalions sweeping the jungles continued skirmishing with escaping enemy squads until late in the afternoon.
By the time the battlefield had been cleaned up a few days later, almost 300 dead enemy soldiers had been found. They were from all three battalions or the Viet Cong 272nd Regiment. Significantly, all units of the 9th VC Division avoided combat for the next 3½ months.
The Minh Thanh road battle was but one of many such engagements during the Vietnam War where ARDF made a critical difference. Such technical intelligence had been available in earlier wars, but it had been so closely controlled by higher headquarters that its tactical battlefield value was almost nil. But in Vietnam the ASA personnel— the guardians of this highly secret collection system—emerged from their cloistered compounds to deal directly with the infantry commanders and respond to their information requirements. They were unknown warriors, but they were warriors nonetheless.