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Big ASA 335th Radio Research Company
Army's Radio Research Units
9th Infantry

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The enemy's jungle cover was no match for the finding capabilities of the Army's Radio Research Units
by William E. LeGro
From: Marc Robinson
Robin6491 AT

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 two-bladed 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 pa