The Advent of ARDF
Within
ASA, Tom Davis’s death and the general inadequacy of the PRD-1 sparked
the search for a better tactical D/F alternative. The French had used
airborne radio direction finding with some success in their Indochina
war, but initially ARDF was not high on the list of possible solutions
for the U.S. Army. When improvements of existing ground-based D/F
designs failed to pan out, ASA called upon the Electronic Warfare
Laboratory (EWL) at Fort Monmouth, New Jersey. Engineers there had done
some preliminary work on an ARDF system, including experiments with a
Sikorsky H-19 helicopter. When these, too, proved unsuccessful, the
team turned to the U-6 “Beaver”, a rugged single-engine airplane built
by DeHavilland Canada for use in the bush country. The Beaver dated from
the Korean war; over the years the army had procured several hundred as
general purpose utility aircraft.
The RU-6
In
March, 1962, the EWL engineers departed for Vietnam to field test what
would later be designated the AN/ARD-15 ARDF system. Dipole antennas,
spaced about 40 feet apart, were mounted on the leading edges of the
Beaver’s wing, in effect turning the entire airframe into a large
“H-Adcock” antenna array. The ARD-15 operated on the “aural null”
principle; when the signal of interest could no longer be heard (the
null), the target was either directly off the nose or tail of the
aircraft. In practice, keeping the target off the nose proved to be a
non-issue. For proper functionality, however, the wings of the aircraft
had to be kept level while chasing the signal. This necessitated
“kicking rudder” to fishtail the aircraft from side to side while the
operator determined the null and notified the pilot, who then noted the
heading of the aircraft.
All
this was relatively straightforward, if perhaps uncomfortable for the
operator who was whipsawed to and fro as the pilot attempted to find the
null. The tougher problem with ARDF is to determine the precise
location of the aircraft at the instant the line of bearing to the
target is taken. In the Beaver, this was accomplished by banking the
aircraft, taking a look out the side window, and marking the
corresponding spot on a map in the co-pilot’s lap! The drill was
repeated until either the target ceased to transmit or enough LOBs had
been taken to triangulate a fix. From the tactical standpoint, the fact
that the aircraft had to repeatedly turn directly towards the target was
certain to raise questions in the minds of enemy radio operators.
| Flight Detachment, 3d RRU
In late March, 1962, after some
controlled field tests in the Saigon area using captured enemy
equipment, ARDF went operational in the form of the “Vietnam Flight
Detachment” of the 3d RRU. The detachment was made up of pilots detailed
from other units and intercept operators seconded from WHITEBIRCH or
other ASA functions. Over the next few weeks, the crews of the
Beavers fixed six major targets associated with the Viet Cong command
structure. Within four months, the crews of the Beavers had flown 162
missions, during which 23 transmitters were fixed, locating an estimated
16 enemy headquarters.
Despite
these results, Washington’s counterinsurgency plan was having little
effect in the hinterlands and Viet Cong terrorism was on the rise. To
the typical American advisor, who by disposition and
training thought in terms of conventional tactics, the problem boiled
down to one thing—the communist guerillas could be whipped if they would
only stand and fight!
|
 |
Debacle at Ap Bac
In
the waning days of 1962, ARDF provided just such a set-up when the U-6
Beavers of the 3d RRU fixed a Viet Cong transmitter in a “liberated”
village about forty miles southwest of Saigon. Collateral intelligence
also indicated that the radio served a major enemy headquarters, guarded
only by a reinforced company. Here was an opportunity tailor made for
American tactics and technology. U.S. Army helicopters, escorted by
“Huey” gunships, would lift an ARVN battalion into a landing zone north
of the village, while M-113 armored personnel carriers moved a
mechanized company in from the west. A local civil guard force would
block from the south. The VC would have to fight or, if they attempted
to flee to the east, be decimated by artillery and air strikes.
The
ARDF fix may have been accurate; the rest of the intelligence was not.
The enemy force was considerably larger than reported and was arrayed
mostly to the south and east of the expected location, closer to the
village of Ap Bac. Nonetheless, the South Vietnamese outnumbered the VC
by at least 4 to 1 as the battle began in the first hours of daylight on
2 January 1963. One part of the operation went as anticipated—the enemy
stood his ground. Almost nothing else went right. In action lasting
well into the night, the South Vietnamese suffered around 60 killed and
more than 100 wounded. Five U.S. helicopters had been shot down, 3
Americans killed, and another 6 wounded.
From
a combat standpoint, the battle at Ap Bac was simply one of thousands
of fire fights in the Vietnam war. The South Vietnamese had not
performed well, but the Viet Cong were gone, and had likely suffered
greater casualties. But interpretation of the battle results sparked
open hostility between the news media and MACV leadership. For months
American correspondents had painted a considerably bleaker picture of
the war than the consistently rosy official reports being funneled to
Washington. The debacle at Ap Bac seemingly reinforced the reporters’
contention that not only was the war going badly, the American people
were not being told the truth about it.
|
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__________
The source for the majority of this article is Robert J. Hanyok, Spartans in Darkness: American SIGINT and the Indochina War, 1945-1975, pages 129-131. A digital (pdf) version may be downloaded from NSA's website at
https://www.nsa.gov/public_info/_files/cryptologic_histories/spartans_in_darkness.pdf
Joe Martin5 July 2015
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