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Listening for the enemy historically has been a mission demanding concentration, infinite patience, and quiet. Lose the quiet" in favor of "tolerance of incessant noise while packed into a small plane." and you have the radio direction-finding mission in Vietnam

I was the "new guy" in what could have been considered the most elite unit of the Army to serve in Vietnam. At the time it was simply called the 3rd Radio Research Unit (RRU) Air Section. It was January of 1966. In order to arrive at my position as the newest member of the team, I had volunteered for the Army, was selected for the Army Security Agency (ASA), and had volunteered for a four-year first enlistment (verses my two-year draft obligation, or the standard three-year enlistment for all others). I had completed a tour of duty in Southeast Asia, had volunteered for my current tour in South Vietnam and for flight duty, and most important of all, my application for transfer to the Air Section had been vetted and approved by my peers.

The 3rd RRU Air Section was a non-TO&E [Table of Organization & Equipment] unit, somewhere between company and battalion size, with nearly 30 aircraft. It had detachments in each of the four Corps Tactical Zones in Vietnam. We were part of the ASA's 53rd Special Operations Command, which was operating in Vietnam under the cover name of 3rd RRU, and our particular mission was to develop methods of short-range radio direction finding using small aircraft as platforms. The previously accepted method of short-range direction finding using jeep-mounted PRD-1 [Portable Radio Direction-Finder Model 1] units had proven to be unworkable, and dangerous in the fluid environment of a guerilla war. Furthermore, the concept of using radio direction-finding specialists on aircraft and simply tying them into the existing DF net hadn't worked either. We then started using Morse-intercept operators in the aircraft and a free-roaming technique. Each aircraft was assigned an area of interest to patrol, and although the operator had a list of priority targets, he had discretion to work any target of interest within the patrol area. This is where I came into the picture.

Airborne radio direction-finding (ARDF) equipment had evolved since Herb Hovey arrived in country in 1962 carrying the mission gear to configure the first three DeHavilland RU-6A Beaver aircraft as checked baggage. The RU-6As were supplemented by twin-engine Beechcraft RU-8D Seminoles. There was also one Beechcraft RU-8F Queen Air carrying a search operator and a DE operator and one DeHavilland RCV-2 Caribou configured for both radio intercept and radio direction finding. Several of the RU-8s had Ryan Doppler navigational systems installed that had been tested and approved for ARDF. The Collins 51S1 receivers fitted in our aircraft were similar to the R390A receivers that I had previously used. The "switchology" for the mission gear was simple and logical: Longwire antenna for search, dipole antennas for direction finding, and the switching necessary to change between antennas and feed the signal to the pilot via the intercom. We were developing and refining operating techniques as we went along, so my lesso ns on operator techniques consisted of discussing what has previously worked for the other operators. (Three months later I would be flying with a major from Arlington Hall in the right seat, who was observing my operating techniques and writing the first ARDF operators manual.)

In flight, the operator searched a known frequency spectrum on the radio until he located a likely signal. He then determined if it was an enemy transmitter, tried to identify the target, attempted to copy enough traffic to prove identification, determined if the target would be transmitting long enough to fix, and determined if the target was close enough to work by switching between longwire and directional antennas. After meeting all of the parameters necessary to qualify the target, the operator switched the top radio to the directional antennas, tuned the bottom radio (connected to the longwire antenna) to the same signal, and fed the signal from the top radio over the intercom. Then it was showtime! The pilot began a gentle turn; simultaneously the co-pilot looked over the side of the aircraft and identified the exact location of the aircraft using a tactical map. As the aircraft turned, the signal faded when the nose (or tail) was pointed at the source of the signal (the aural null). The pilot then rol led the wings level and skidded the aircraft back and forth across the "null" while watching the compass. When he had determined the null -- usually less than a five-degree spread -- he called, "Mark," and the copilot then marked the exact position of the aircraft on an acetate-covered map with a grease pencil. The pilot called the compass headings for the null to the co-pilot, and immediately began a turn 90 degrees to the null. The landmarks in the area usually determined the direction of this turn. The maneuver resulted in a "shot." If you flew on the new heading for a half-minute or so and then repeated the process, you had a "cut" (I cut = 2 shots), which was not report-worthy but was useful for determining the general area where the target was located. A third shot qualified as a "fix" but was not considered accurate. A fourth time got a good fix, worthy of reporting (I good fix = 4 shots). This was what we got paid to do.

I reported to the air section and flew my first mission on the same day. Our aircraft were parked on the flight line only a couple of hundred yards from our operations building, and we simply gathered up our gear and weapons and walked to the aircraft. Considering the nature of the job, it all seemed quite casual. On that first day, I was flying in a RU-8D with an experienced operator acting as co-pilot and instructor. Although the U-8 seems to be a large hulking beast from the outside, the operator's area is less than spacious. The pilots shared a bench seat designed for three and had a bit of room, but the operator was in the second row, wedged between the skin of the aircraft and the mission gear. One of the things that I learned is how to sit absolutely immobile for four hours. Being a "southpaw." I also learned to be a bit of a contortionist. With a pencil in my left hand, a clipboard balanced on my knees, and the switches and knobs that control the mission gear also on my left; I had to cross my right hand over to operate the mission gear. In order to save weight, the interior of the aircraft was stripped of everything unnecessary including soundproofing. There was a mechanical inverter behind the operator's seat that emitted a high-pitched whine, up to five radios (two mission gear, one to talk to the ASA direct support units [DSUs], and two aircraft radios) chirping and squawking through the headset, and 480 cubic inches of non-muffled, supercharged Lycoming engine exhausting within a few feet of either ear. It was loud!

Our area of responsibility, Ill Corps, was further divided into regular patrol areas. On that first day, my assigned patrol area was the Western Arc, which ran from the end of Runway 27 at Ton Son Nhut to the Cambodian border. It was mostly open grassland and rice paddies, broken up by the occasional hedgerow. The open nature of the country made it unpopular for large concentrations of VC, and it was considered a quiet area -- just the place to break in a new guy. Nevertheless, the Army of the Republic of (South) Vietnam (ARVN) was opening a large sweep operation in the area, and it proved to be rather busy. The whole mission was conducted to a backdrop of artillery fire prepping landing zones. A-1 Skyraiders were dropping bombs that blew great chunks of mud way up in the air and then returned to strafe with their 20mm cannon. Helicopters swooped in to drop off troops, then raced back out of the area; all the while artillery strikes bloomed about. Just trying to negotiate the traffic in the area and make sur e that we avoided the gun target line of the artillery working the area was a full-time job.

When we had taken a few shots and it appeared that we had a useable fix, we moved on and began searching for another target. The co-pilot then called the location of the shots (using UTM grid coordinates) and the compass bearings back to me, and I wrote them down on a yellow legal pad along with identifying data. We repeated the process of locating enemy transmitters three times in four hours. After the mission was flown, I took the plotted fixes to Operations, where I plotted them on a large-scale map using an onionskin overlay. Then I turned in the fixes along with identifying data to be included in various reports that were be sent out overnight. The end user received the intelligence the following day.

At the end of that first day, as a 20-year-old enlisted man, I was amazed and somewhat intimidated by the level of responsibility I had assumed. Within a few months, we were plotting our final fix in the aircraft and calling the results to DSUs, who were attached to the headquarters of the ground units that we were supporting. A year later, we tested the feasibility of calling selected fixes directly to supporting arms and controlling artillery or air strikes on the target. By the end of the war, DSUs were relaying fixes to inbound B-52 bombers. JED

Richard McCarthy enlisted in the Army Security Agency in October 1963 and served in Southeast Asia from August 1964 through August 1967. Major Assignments were 5th Radio Research Unit (Bangkok, Thailand) and 3rd Radio Research Unit Air Section (Saigon, Vietnam). After leaving the service with the rank of Sp/5, E-5, he earned his commercial pilots license. Currently, Richard is "mostly retired" and living on a small ranch in central Texas with his wife.
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Author:McCarthy, Richard
Publication:Journal of Electronic Defense
Date:Jun 1, 2002
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