3 1966 Tropospheric Scatter Communications Past, Present, and Future
3 1966 Tropospheric Scatter Communications Past, Present, and Future
3 1966 Tropospheric Scatter Communications Past, Present, and Future
In the 11 years since the first practicable operational this method of communication is now generally under-
tropo systems were developed, there has been a stood to embrace a radio system that permits communica-
tremendous growth in their number and complexity. tion over the distances indicated, with excellent relia-
This article describes the principles and applications bility and good information capacity, using relatively high
of tropospheric scatter communications and discusses transmitted power, frequency modulation, and highly
current development trends. Included is a table sensitive receiving apparatus. The name "tropospheric
listing practically all of the tropo systems installed scatter communication" is now usually referred to in the
around the world, with pertinent technical data, engineering vocabulary simply as "tropo."
together with maps showing the geographic location Figure 1 is a conventional sketch of a tropo span. This
of most systems. The maps are keyed to the table for graphic representation is merely symbolic, and indicates
easy reference. in a general way that the circuit utilizes high power and
large directional antennas, may be duplexed, can sur-
Tropospheric scatter communication may be defined as mount ground obstacles of considerable magnitude, and
a method or system of transmitting, within the tropo- will span a relatively large distance over the earth's sur-
sphere, microwaves in the UHF or SHF bands to effect face.
radio communication between two points on the earth's
surface separated by moderate distances of from 70 to
600 miles. Such a span or hop may be augmented by (Above) Three-branchtroporelaystationinSpain,forming
other spans in tandem to permit end-to-end or through part of a large U.S. Air Force defense communications sys-
circuits up to many thousands of miles. More specifically, tem in Western Europe and the Mediterranean.
h
i --
~~~~~~~~~~~~~~~~~~~Low-cost,
high-performanice
wide-band system: replaces
tO~~~~ W ~~costly right-of-way mainten-
ance of coaxial or multiple
Line of sight cable or overhead wiring.
(LOS) -_ _
~~-~--
<~- Diffraction mode is very
Dilanefsurfaction 3 -;specialized form of UHF
( lanurface) used only rarely where
rugged terrain prevents use
.--i of direct LOS and permits
A ~~
:
~-~2 S~~~-- 3 W
,r
_
,' longer path with obstacle gain.
~~~ 14k'
N(~~~~~i6 ~~~~~~~~~~~Only
- practical wide-band,
reliable ground-based method
of achieving 7d- to 600-mile hop
where unsuitable intervening
territory prevents use of
Go o=btacl Scatter cttranl LOS or diffraction modes.
Fig. 2. Principal ground-to-ground communication modes atmosphere, ay- bending occurs and is accompanied by
utilizing microwave (70 MHz to 20 GHz) region of radio
spectrum; characteristically wide-band service (100 kHz partial reflections This hypothesis has been advanced by
to 20 MHz). Carroll and others 9-12
A third approach, long a subject of speculation by
many, has been formalized through a derivation by Bul-
lington.rst3 This concept is based primarily on the average
value and the standard deviation of the index of refrac-
these apparent anomalies as resulting from atmospheric tion at the earth's surface, both of which decrease ex-
turbulence. This turbulence is thought to produce "blobs" ponentially with height. The result is a quantitative ex-
of atmosphere whose refractive indexes are sharply dif- planation of tropospheric radio propagation, derived
ferent from those of the surrounding atmosphere. When without the use of arbitrary numerical factors.
irradiated by a microwave signal, these blobs reradiate Regardless of the true mechanism, much is known con-
the signal, scattering it in all directions. Some of this cerning the characteristics of the microwave energy field
scattering is in the forward direction, which produces the propagated beyond the horizon. It has been learned
field at a receiving location. The entire process is thought through observation of a large mass of empirical data
to occur in the region between the stratosphere and the collected from operational tropo links.
surface of the earth-that is, the troposphere. This First, it is known that the average amplitude of the
hypothesis forms the basis for the National Bureau of field propagated beyond the horizon is greatly attenu-
Standards methods8 for predicting the strength and time ated with respect to the transmitted field. The amount of
availability of beyond-the-horizon fields. Although the attenuation can be calculated as a function of the angular
hypothesis itself remains unproved, the method has been distance between the transmitting and receiving sites.
widely accepted; the hypothesis combined with massive Angular distance is a parameter that takes into account
empirical information ordinarily affords a usable solu- curvature of the earth, terrain configuration, and clina-
tion to the problem of predicting performance. tology. For all but the most unusual circumstances, it is
Another promising hypothesis explains these fields as closely related to the linear distance between the two
being the products of mode propagation based on partial sites. This relationship is the basis for the general state-
reflections. The gravitational stratification of the atmos- ments that appear later regarding the traffic capacity vs.
phere, where the lowest layer is the densest and each distance capabilities of the tropo scatter medium.
succeeding layer is less dense, with sharp boundaries be- Second, the amplitude of the received field varies sub-
tween the layers, is thought to be responsible. When an stantially with time over a given path. The National
electromagnetic wave is propagated through such an Bureau of Standards has gathered and analyzed a mass
R FfRFI F lK
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equipment
Optional
tHpower amplifiers of l kW or greater
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tension mode, the device is highly useful in maintaining
communications on marginal circuits, particularly when
propagation conditions are unfavorable. Since thermal
noise usually controls system noise under these condi-
tions, increased equipment noise is of no consequence.
In Fig. 8 it can be seen that predetection combining
is performed at intermediate frequencies. For a number
of years a controversy has existed over the relative merits
of postdetection and predetection combining. A discus-
sion of the detailed technical claims of the oppos-
ing sides is beyond the scope of this article. However,
it is brought to the reader's attention that this contro-
versy still exists, even though years of field experience
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B Texas Towers prototype, PAGE 2 210 15120 80 F 900 72 72 REL LENK KENN 30 10 6 T B
1956, Massachusetts' Expi 130 NRC
C AN TRC-43 (XW-1), 1957, WEST 1 268 F 900 Var WEST WE PROD 28 10 9 N c
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France Expi CFTH
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1958, U.K.-Netherlands" Expi
H South England Tropo, MARC 1 206 4 944 F 900 24 MARC 30 10 D H
1958-59, Englandf Expi
J AN/FRC-53/54, 1959, WEST 1 171 4 104 F 2000 24 72 WEST KENN 28 10 10 D J
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Florida Expi 123
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Carolinas Exp' 93
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Canadal" Def2 228
2 Anchorage-Kodiak, 1955, PAGE 2 190 1 140 65 F 100 6 12 REL WE PAGE 30 1 4 D 2
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Quebec CCo 12
44 Ft. Wadsworth, 1961, ITT 1 45 540 T 2000 12 24 ITT GE AC 15 1 8 D 44
S.E. New York Def2
45 Jasdaf, 1961, NEC 13 1708 61488 91 F 1800 12 NEC 33 100W 45
Japan DeA r 145 48
46 Makpo-Cheju, 1962, MARC 1 98 21176 F 750 12 60 MARC ATE MARC 30 1 10 Q 46
Korea CC'
47 1ndocom, 1962, PHIL 1 260 6240 F 900 24 24 PHIL CRC AC 28 1 8 Q 47
Indonesia Comb4
48 Atlantic Missile Range, PAGE 1 382 9 168 T 900 24 72 REL GE KENN 60 10 2 Q 48
1962, Caribbean Comb'
49 B.C. Telephone Co., 1962, LENK 2 345 16 560 155 F 900 48 252 REL LENK WEBR 60 10 2.5 Q 49
British Columbia-Alaska Comb' 190
50 Colomb Bechar Hammaguir, CFTH 1 71 2 556 F 500 36 CFTH CIT CFTH 30 1 6 D 50
1962, Algeria Def2
51 Philippine Tropo, 1962, BEND 3 425 5100 105 F 900 12 12 REL LENK ASI 28 1 8 Q 51
Philippines Def2 185
52 Back Porch, 1962, PAGE 5 820 59050 106 T 900 72 72 REL LENK ASI 60 10 2 Q 52
South Vietnam DefG2.5 224
53 Taiwan-Philippines, ITT 1 310 3 720 F 450 12 24 ITT KENN 60 10 Q 53
1962, West Pa CifiCk Def2
54 Dew East Nars, 1962, WE 10 2420 203 280 120 F 900 84 84 REL WE Var 30, 60 10 2 Q 54
North Atlantic Comb4 450 120 50
55 U.K.-Spain-Morocco, 1962, PAGE 4 1230 29 520 209 F 900 24 24 REL SIEM BKC 60 10 2 Q 55
Western Europe Def2 515 400 72 RAD 120 50 8
56 Alberta Govt. Telephone, CWES 1 90 2 160 F 5000 24 120 CWES LENK AC 28 5 4.5 Q 56
1963, Alberta, Canada CC'
57 Okinawa-Miyako Jima, PAGE 1 176 2 112 F 2000 12 72 REL GE TAG 30 lOW 8 Q 57
1963, Okinawa Del)
58 Navy Philippine Tropo, USN 1 45 540 F 900 24 24 CRC LENK 30 10 9 Q Ss
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90 Antigua-Tortola, 1965, MARC 1 220 17 600 F 2000 300 REL 1 2.5 Q 90
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91 France-Portugal, 1966, CFTH 1 293 10 548 F 900 36 120 CFTH CEI 90 10 2.5 Q 91
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2 Defense operational traffic modulation
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5 Government ab~le of handling 1 TV channel and 120 grd)
6 Commercial vinge chnesrdtcincmi- m Phase-lock threshold ext.
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7. Advances in further modularization of solid-state 11. Further studies and investigations of the tropo
equipment, including use of integrated circuitry, to propagation mechanism to determine its exact nature and
reduce size and weight and increase accessibility, behavior, and thus afford designers a stronger base for
X. Further development of automatic fault indicator development of improved equipment and perhaps better
and performance monitoring equipment to facilitate bandwidth for increased channel capacity.