spartans_in_darkness

EO 1.4. (c)
EO 1.4. (d) "FOP SEORE"Fh'OOMItHH*1
I IBut the latter system registered the
strength of an enemy's transmission and was not
direction finding. So ASA had to go at it from
scratch.
(8//81) Hovey turned to Army engineers at
the U.S. Army's Electronic Command (ECOM)
laboratory at Fort Monmouth, New Jersey, The
ECOM technicians already had candidate equipment,
the Ai"JlARD-IS, which could be fitted on
whatever platform was finally selected. However,
two major problems remained: selection of an
efficient platform and the deconflicting of the sky
and ground waves.
(V) ANIARD-15 directlon Hnqing set in 4 V-8 4i~c~4ft
(8//81) In November, after having traveled to
South Vietnam to study the signals environment
and examine captured VC radio gear, Hovey and
the ECOM engineers began testing aircraft. The
UH-19 (Chickasaw) helicopter was tried first.
However, the rotary blades created too much turbulence
for a steady platform. The chopper also
generated static interference with inboard electronics
that required additional shielding. A second
candidate, the U-6A (Beaver), also known as
the L-20A, proved to be the airframe that could
do the job. Small, with adequate power, and good
pilot visibility, the U-6A also had the advantage of
being available in Vietnam along with the necessary
maintenance support.
(8//81) The next problem was the antenna
configuration. Taking a hint from VHF D/F
receivers, such as the AN/TRD-16, with differentially
connected antennas, the engineers, in an
elegant solution, decided to turn the "problem" of
the plane acting as a huge antenna into an advantage.
By spacing t'NO dipole antennas far enough
from each other on either wing, they created the
ability to discriminate between the incoming HF
ground and skywaves. In effect, the plane was
turned into one large receiving antenna. Instead
of having a direction finding antenna attached to
the plane, such as a loop, the plane itself was now
the DIF antenna.
(8//8I) The resulting plane had an H-Adcock
array of two dipole antennas sticking through the
outer, leading position of the wing. The two
arrays were forty feet apart and connected to an
R-390/URR radio receiver mounted inside the
plane. The rods were coupled in such a way so
that, with the differentially connected dipoles, the
pilot would steer into the signal's null, that is, the
point at which the arriving signal struck both
dipoles simultaneously, and cancelled out each
other. The effect was a drop in the signal's amplitude,
which was detectable aurally by the operator
or pilot listening to the radio. The pilot then
steered the plane along the gyrocompass heading
of the null, using what was known as the "right
angle technique," to locate the median null. While
flying this way, the plane appeared to "fishtail"
along a path. Once a pilot obtained his first bearing,
known as a "line," he needed two more bearings,
referred to as the "cut" and "fix," to locate
the transmitter. This he got by flying to two new
positions and repeating the process of obtaining
another bearing.
Page 130
fOIi SEeRE"FHOOMI~I:J=!!X1