Project Report of South Viet Nam Secure Voice Study

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-02820A001200030031-1
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 3
• Document Creation Date: December 9, 2016
• Document Release Date: August 23, 2000
• Sequence Number: 31
• Publication Date: June 27, 1966
• Content Type: MFR

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RDP78-0284001200030031-1
27 June 1966
MEMORANDUM FOR THE RECORD
SUBJECT: Project Report of South Viet Nam Secure Voice Study
1. A study was undertaken in early 1966 to determine the
feasibility of establishing with standard off-the-shelf radio
equipment a Class A secure voice communications network in South
Viet Nam. The proposed network would link eight major cities
(see attachment) in South Viet Nam. The purpose of this report
is to describe certain salient considerations which are involved
in selecting a mode of transmission which would satisfy the
above requirement in the most reliable and economical manner.
Fulfilling the requirement through the use of existing military
facilities is not covered in this report.
2. NSA and our R&D Lab have already proven the feasibility
of using standard commercially available VHF-FM radio equipment
with the KY/8 for secure voice transmission over a limited range
of distances. The actual coverage, of course, is dependent upon
the effective radiated power of the system and the nature of
intervening terrain between terminals. Normally distances of
forty miles are typical for 100 watt base station units and low
gain antennas. Since the VHF-FM mode of transmission is fairly
inexpensive and simple, considerable effort was devoted to
determine if a system consisting of standard mobile base station
type units (transmitter and receiver) could be configured or
arranged to satisfy the basic requirement. A General Electric
80 watt base station unit costs about $1,330 and their 330 watt
base unit costs about $2,550.
3. Distances between adjacent cities (see attachment) vary
between thirty-six miles (shortest) to one hundred and forty-six
miles (longest). The topography between the designated eight
cities was studied in detail and path profiles of the earth's
contour between adjacent cities were made where it was deemed
necessary. Due to the curvature of the earth alone, 200-foot
towers would be required at each end of a forty mile path for a
grazing line-of-site condition. Examination of the profiles
reveals that only two of the possible eight paths are not ob-
structed by mountainous terrain. These two paths which are over
swampland are Can Tho to My Tho and My Tho to Saigon. The extra
propagation loss in the obstructed paths make successful
transmission in the 132-174 mcs band difficult. The highest
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powered transmitter offered by Motorola (375 watts) coupled
into a high gain antenna system (13 db above an isotropic
radiator) will still not be able to offset the high obstruction
losses. Additional antenna gain is available in the 406-470 mcs
band but is offset by an increase in free space path loss and in
obstruction loss and a decrease in available transmitter power.
5X1A5a1
4. The radio transmission group at the
6X1A5a1 acility was given four pertinent pa profiles and
was requested to calculate the reliability for each path based
on standard 150 mcs base station equipped with 1 KW amplifiers
and 19 db antennas (quad array of 13 db yagis). The results of
their study indicate that a reliability better than 99 percent
throughout the year would exist for the Pleiku to Dan Me Thuat
path and the Dan Me Thaut to Dalat path if the forementioned
special equipment were used. However, satisfactory performance
over the remaining paths is not feasible with the specified
equipment limitations. Therefore, in view of the above factors,
transmission in either of the 132-174 or 406-470 mcs band is not
considered a likely means to fully satisfy the original require-
ment.
5. Remaining commercial base station units which would
also be compatible with the KY/8 fall into two frequency bands,
25-54 mcs and 66-88 mcs. The 25-54 mcs band is highly undesir-
able for the following reasons. Prevailing multipath conditions
resulting from erratic ionospheric support present for fre-
quencies in the 25-54 mcs band would severely distort the high
speed digital signal (18.75 kbs). A second serious disadvantage
is that the ambient noise level in the 25-54 mcs band is rather
high ( in the order of 10 microvolts). Propagation in the 66-
88 mcs band would not be greatly different than that in the
132-174 mcs band.
6. Two remaining possible modes of transmission are tropo
scatter or HF radio. The cost (over $150K) and size of a
typical tropo scatter radio system designed to be compatible
with the KY/8 make this mode highly impractical. HF transmission
in the 2-14 mcs band is feasible but would require a complex
layout of equipment. For instance, the KY/8 would be replaced
by a HY/2-vocoder, HN-9A-interface unit, and the KG-13 crypto
unit. The transmitting portion would consist of a high speed
data modem, an exciter, a linear amplifier (1 KW minimum) and
an extremely high radiation angle transmitting antenna.
Diversity reception and automatic frequency control receiving
units are mandatory for HF hi-speed data circuits. Since the
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-3-
PNA-6A antenna is not effective for distances under 500 miles,
two receiving half wave dipoles are necessary unless frequency
diversity is to be employed. The equipment cost for one HF
secure voice circuit would be about $120K. In addition to the
high cost and equipment complexity reliable HF propagation
for paths between 50 to 500 miles is notoriously more difficult
to maintain than for longer paths.
7. In conclusion it is evident that there is no simple
and clear method for satisfying the original requirement. The
VHF-FM/KY-8 mode for transmission is relatively simple and
inexpensive but can not be expected to provide reliable service
over most of the obstructed paths. A tropo scatter system would
be impractical due to its high cost and large size. Although
the HF mode of transmission can probably satisfy the require-
ment the resulting system would be rather large, complex, and
expensive. Based on the findings of this report no attractive
solution is available. In view of the preceding statement, it
is recommended that a conference be held to determine a future
course of action. Possibly the use of installed military
facilities should be considered.
25X1A9a
Attachment:
Proposed Secure Voice Network
South Viet Nam
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