•

VOLUME

EXCLUDED FROM

DECLASSIFICATION LAW E.O. 12958

REFER_

TO•• 1 405 
of .2

ADVANCED WEAPON SYSTEMS STUDY

PART III - ADVANCED'UILITARY SATELLITES

VOLUME II

TR-59-0000-00604

1 March 1959

WOMATE

CAA LtV Or

AT

 I R1250710

S 0 000

;N roc. stIVGIFIERA0ErnA

Work Completed 10 September 1958

SPACE TECHNOLOGY 

LABORATORIES P. O. Box 95001 Los Angeles 45, California

01* kommelssion

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rehlitisd 14

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.--TrAi.177.7NTiAt

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TR. 59-0000-00604 _mom* Page II-ii

CONTENTS

CHAPTER 1 - 'INTRODUCTION AND SUMMARY

CHAPTER 2 - COMMUNICATION RELAY SERVICE

Section

2.1 DISCUSSION OF. PRESENT COMMUNICATION

CAPABILITIES 

Page

1

5

Z. Z TYPES OF COMMUNICATION SERVICES AND

RECOMMENDED ORBITS

Z. Z. 1 High-Density Service 

2.2.2 Advantages of •24-Hour Orbits for High-Density

Service 13

2. Z. 3 Low-Density Polar Service 17

2.2.4 Advantages' of •24-Hour Orbits for Polar Service 20

Z. 2.•5 Other Low-Density Service 23

2.2.6 Summary 25

2.3. TRANSMISSION CONSIDERATIONS .. • • • • • • 26

2.3.1 Basic Relationships ... . . .•. . ... . .. ... 27

2.3. 2 Antenna Gain and Aperture 28

2.3.3 Signal-to-Noise Ratio Requirements 29

2.3.4 Desired Bandwidth . . 30

2.3.5 Noise Level COnsiderations 33.

2.3.6. Power Requirements ... • . • • . . . 34

2.4 INSTRUMENTATION CONSIDERATIONS . .. • • . w 38

Z. 4.1 Characteristics of Possible Amplifiers ... • • • 38

2.4.2 Traveling-Waye-Tuba Amplifiers. . • . . . • • • • 38

2.4.3 Klystron Amplifiers 38

2.4.4 Backward-Wave Amplifiers • 39

Z. 4.5 Molecular Amplifiers Wagers) ' 39

2.4.6 Summary . .. ... . . . . . . .. •. . . . .. 41
Tit-59-.0000-006Q4

Page

CONTENTS (Continued)

Section Page

2. 4 POSSIBLE SATELLITE.BORNE. RELAY SYSTEMS 41

2. 5. 1 Block Diagram of ZI.Relay System 41

2. 5. 2 Block Diagram of Intercontinental Relay

System • 45

2. 5. 3 Block Diagram of Low-Density Relay

System 45

2. 5. 4 Reliability and Lifetime Considerations 47

2.6 HIGHER CAPABILITY GLOBAL COMMUNICATION RELAY

SYSTEMS 48

2.6. 1 Requirements for Communication Relay Systems

of Higher Capabilities 48

2.7 INTERIM COMMUNICATION SYSTEM FOR A 24-HOUR

SATELLITE 411.0000••••4 51

2. 7. 1 Reasons for and Types of Interim Communication

Systetxts .51

2. 7. 2 Transmitter Power Requirments 54

2. 7. 3 Detailed Description of Possible S

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TR-59-0000-00604

Page II-iv

CONTENTS (Continued)

• Section ///

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CHAPTER 5 - INFRARED EARLY WARNING METHODS FROM

24-HOUR SATELLITES

ANTENNA DESIGN CONSIDERATIONS FOR 24-HOUR

SATELLITES 

B NOISE CONSIDERATIONS 106

AMPLIFIER CHARACTERISTICS 119

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Tit-59-0000-00604

Page II-v

ILLUSTRATIONS

Page 

2-1 Areas Originating Great Majority of Communications

of Military Interest 10

2-2 Earth as Seen from Satellite in 24-Hour Orbit at Latitude

300W 11

2-3 Earth as Seen from Satellite in 24-Hour Orbit at Latitude

1700W 1Z

2-4 Maximum Latitude of Continuous Coverage as a Function

of Satellite Altitude for N Eqiatorial Satellites 15

2-'5 Auroral Absorption Region of Northern Hemisphere 19

2-6 Area Continuously with Line-of-Sight of 24-Hour Polar

Satellite When Above 300N Latitude 22

2-7 Continuous Coverage Areas for Low-Density Polar

Relay Satellites 

2-8 Block Diagram of Possible Satellite-Borne Relay System

for ZIService 

2-9 Block Diagram of Possible Satellite-Borne Relay System

for Intercontinental Service 

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TR...59...0000-00604

Page

ILLUSTRATIONS (Continued)

use.

Beamwidths of a Single Polystyrene Dielectric Rod

Page,

A-1

Antenna of Varying Length 103

A-2 Approdmate Antenna Beamvrldth as a Function

of Aperture Dimension 104

B-1 Noise Levels from Various Sources, Half-Wave Dipole

Receiving Antenna 108

B-2 Noise Levels from Various Sources, 15-db Receiving

Antenna Directed at Noise Source 110

Br3 Noise Levels for Various Sources, 30.-db Gain Receiving

Antenna 111

131.4 Illustration of Definitions of Celestial Coordinates . . . . • • 112

13-5 Mustration of Variation of Celestial Coordinates' of Ecliptic 114

B-6 Mercator Projection of Heavens Showing Some of the More

Prominent Stars Between December 160 Degrees, and the

Two Most Intense Radio Stars 115

B-7 Declination of Antenna Beams to or from Satellite in

24-Hour Orbit and Any Point Within 500 Miles of

Earth's • Surface 116

C-1 T WT Amplifier Chart 121

C-2 Klystron Amplifier Chart 123

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TR.- 59 - 0000-00604

Page II-vii

TABLE

Table Page

2-1 Typical Characteristics of Components Suitable

for..ZL Relay 

2-2 Weight and Power Requirements for Initial. Global

Communication Relay Systems of Various Capabilities.. . . 49

2-3 Power Requirement for FM Multiplex Systems . . . . . . . 56

2-4 Estimated Weight and Power. Requirement' of Interim

FM Multiplex System Components . . ., ***** ••.., 58

2-5 teary of Weight. and Power Requirements of Interim

Communication Relay Systems . . ... . . . .....•.. . . 60

Z-6 Estimated Weight and Power Requirements of Interim

UHF Voice .Relay Components . . . .

///

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Parameters of 8-Foot-Square Array, No Cavity

Backing 105

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TR-59-0000-00604

Page II-1

CHAPTER 1

. INTRODUCTION AND SUMMARY

Chapters Z to 6 of this volume describe the results of an investigation

of the feasibility of various applications of high-altitude satellites. Applica-

.

lions analyzed in detail in the course of this study include (1) communications

. .

relay; (2) relay of electronic or photographic reconnaissance data from lowaltitude satellites; (3) early warning of ICBM attack utilising infrared

detection; (4) interception of communication signals for communications

intelligence purposes; and (5) jamming of enemy radar and communications

systems. The investigation of the feasibility of other applications, including

bombing, direct electronic or photographic reconnaissance, naliigation,, and

guidance, was not included in the present study because it appeared that other

techniques for accomplishing the same objectives were more effective or much

simpler and cheaper.

The use of a high-altitude satellite for communication relay purposes

offers the possibility of eliminating the deficiencies of present long-range

communication techniques. This investigation indicates that satellites in

24-hour orbits are best suited for this application. Furthermore, since the

great majority of communications will occur between a relatively limited •

number of points on the earth's. surface, and also since a less specialized

system (which attempts to provide relay service between points anywhere

within line-of-sight of the relay) is necessarily inefficient in its power

utilization for high-density traffic, a system ofmaximum efficiency requires

separate provision of high-traffic and low-traffic-density service. Thus, it

has been determined that an initial global communication system providing

primary coverage of both high- and low-traffic-density areas of the earth

can be obtained with not more than eight 24-hour satellite., five of which

are situated in equatorial orbits and the remaining three in polar orbits.

Furthermore, if booster payload capabilities should prove sufficient, two of

these satellites could be eliminated by combining appropriate low- and highdensity relays into single satellites. If the desired system components are

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TR-59-0000-00604

Page 11-2

-COWDEN-Ilk

available at the time of development of this system, the equipment required

for an initial global communication relay system would weigh from 75 to

100 pounds and would require a total power of about 1700 watts. Equipment

for a global communication system of greater capabilities would entail an

• increase in weight and power requirements.

The study also includes an investigation of the feasibility of a number

of interim communication relay systems which would not provide a global

communication capability, but which nevertheless present a great improvement over present communication techniques. Although the weight and

equipment required for these interim systems is not substantially less than

for equipment of the global system, power requirements are only about

one-eighth those of the global system equipment, thus permitting the use of

a smaller, More immediately available power supply and less severe weight

restrictions on the other components because of weight saved in the vehicle

power supply. Moreover, the resultant narrowing of bandwidth simplifies

the ground stations such that presently available equipment, properly

modified, will suffice, whereas the global system requires a. sizeable

development program in ground multiplexing equipment if the full bandwidth capability is to be utilized.

Investigation was made of the feasibility of using a satellite in .a 24-hour

equatorial orbit to relay electronic or photographic data obtained by a lowaltitude satellite. This relay would eliminate the necessity for storing such

data in the low•altitude satellite, and would thus simplify the problem of data

recovery. It has been found that one 24-hour satellite will permit direct

relay of such data to the North American continent, while two such satellites

will permit direct relay to the United States proper. Weight and power

requirements for the high-altitude reconnaissance data relay would be similar

to those given above for the initial global communication relay equipment.

This application would permit a substantial increase in the over-all capabilities of a low-altitude reconnaissance system.

The feasibility of an ICBM early warning system utilizing detection of

ICBM rocket flames by an infrared detection system in a 24-hour equatorial , satellite has been investigated. This system has been compared' with the

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MEN

TONFICaTfri

SECRET TR.-59-0000-00604

Page

low-altitUde (1000 miles) infrared early warning system presently planned as .

a part of the 11710 program. In contradistinction to the low-altitude system,

the 24-hour satellite would search for rocket firings above the horizon, using

.•

a space background, since from this altitude it does not presently appear

feasible to effect a detection against the infrared background emanating from

the earth and cloud cover. The 24-hour satellites provide the advantage that

continuous rather than statistical coverage of the USSR is provided. Moreover, for simile detection capability (coverage of the. USSR above 55° N

latitude; seven detections with a 14-db signal-to-noise ratio), only seven

satellites are required in comparison with 20 for the low-altitude system.

'Balanced against this advantage is the relative .complexity and difficulty of

establishing these satellites in orbit.

The derivation of maximum communication intelligence from intercepted

communication .signals requires continuous reception of such signals. Thus,

the use of a 24-hour -satellite for this purpose, because its position is stationary with respect to the earth's surface, would permit such continuous

reception .instead of the intermittent interception usually obtained by present

techniques. However, although sufficient sensitivity can be provided to

intercept. many signals, directional discrimination is so poor that a high

probability exists of mutual interference. If the probability of mutual interfere ►ce is made sufficiently small by decreasing the system sensitivity,.

the number of signals which can be intercepted becomes extremely small.

Thus, the feasibility of a communication interception .system cannot be

determined on technical grounds :but rather on the basis of the value attributed

to whatever information can be received under conditions when mutual interference is'Very probably and directional information is lacking.

• • The use of a 24-hour satellite for jamming enemy communications and

radar systems presents the possibility of jamming from an unexpected source

in a previously unknown location. However, although a single frequency

could be jammed, the jamming of an entire band in which the operating frequency of a communications or radar system might be located requires power

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TR-59-0000-00604

Page 11-4

which is one to two orders of magnitude above that which seems feasible in

the relatively near future. Therefore, it may be concluded that jamming is

not a feasible application of a high-altitude satellite, except possibly for any

minor advantage which might be derived from jamming of a narrow frequency

band..

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. 11t-594000-00604 •

Page 11-5

COMMUNICATION RELAY SERVICE

2.1 DISCUSSION-OF PRESENT COMMUNICATION CAPABILITIES

The deficiencies of present-day communication methods become

increasingly important as the distanceover which communication must be effected

increases. For purposes of this:discussion, long-range communication will be

defined as communication Over ranges exceeding 300 or 400 miles. However,

if the communication stations are in line-of-sight of each other, distance plays

a negligible role, and a satellite-borne. communications relay system presents

a unique opportunity for overcoming the deficiencies of present long-range

communications systems since it Can relay byline-of-sight between two or

more widely spaced. ground stations.

Cominunication .to such long ranges is presently effected by means of

radio traDAInglialcete and transmissions over wire-line.circuits. However,

. . circuits require a direct physical connection between both ends of

a circuit. Since such direct connections are difficult to establish for certain

classes of service and are relatively easy to break in most other 'circumstances, wire line circuits have a relatively limited usefulness outside of the

Zone of lnterior M.14.1*(the United,States proper). Even in the ZI• and in other

applications. .where direct connections can.be established, such as between

continents: . wire4ine circuits suffer . from the fact that each. line requires. a

large nuMberlOt repeater stations. to attain, a long-range capability. In addition,

altheugh.110.10.1t4tirtransailestOn'performance is Possible,. in : theory, if

repeater spacing, io very.shert.and if very heavy:Cable is used, cost and speed

of Installation:OOmioine .with. cast of Maintenancei s and. over-all systemcomplexity . . • . . , • 

to limit irnprictite available wire-line . circuits to relatively low bandwidths

(a few tens of kilocycles to a few megacycles are standard). Thus wire-line

circuits tend to require a multiplicity of expensive, cumbersome .equipment

to provide a long-range, large-bandwidth capability eien'when the direct

physical connection between stations can be' established.

Radio communication over long ranges can be established by using

transmissions in the low-frequency or very-low-frequency ranges (below

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TR-59-0000-00604

Page 11-6

300 kilocycles), by standard high-frequency. transmissions in the 3 in

30-megacycle range, or by utilizing ionospheric or meteoric scatter mechanisms in the VHF range above 30. megacycles. Medium frequenciee (300

kilocycles to •3 megacycles) may also be used for long-range communication

at night, but extremely high ionospheric absorption restricts such transmissions

to short ranges the daytime. For this reason, the medium-frequency range

will not be considered further in the following discussion. Long-range communication may be effected also by utilising a series of short-range links such

as the microwave relay links •hich.are in common use in the ZI•and in certain

areas overseas. However, this technique also requires a multiplicity of equip-

. Meat which tends to decrease the over-all system reliability as well an to

increase the problem of system maintenance and of course, its over -all cost.

The attenuation of 'signals transmitted in the low • and very-low-frequency

bands is quite small, and diurnal and seasonal variations are relatively small

in comperison with those observed , et high frequencies. *Thus, it would seem

thit,propagation at these -frequeicieewouldprovide an efficient mechanism for.

•

transfer of. inforMation, Unfortunately, however, the propagation of atrCowpheric noise from its paints of origin to the vicinity of a receiver 'is equally

efficient, %so that noir • levels in these bandeari extremely high and the required

trausteitted poWere are , correspondingly large. • .FurthermOr.a. ;because of the

larg•'. wavelengths of such transmissions, it is alinost:impossible .in. practice

to• construct, an antenna •of-sugicient• length to be more:than a very small frac!-

*ion aWavelengtit, • Thus,- .and VLF antennas are notOriously. inefficient

as compered with antennas for;higher -frequencies and therefore :the•power

retaiand.to• obtain any zetscinithle bandwidth..aanahiliti mike* ‘LF g4134 VLF

cotrailettair impractical: •Yrinally, even if !the. povier were LP or

VLF •antenna. having any reasOneble ef4cigoneffr alio has' 164th.'it:hiSh* athat the

*vex 41.rsfooteitt is limited to a bandwidth. of 'a law:tene. of etalak.per second,

certainly''not.the .typo •bandwidth: capability desitedior a tarsi long-range

coiturtuUlCittion system.

Under.normal conditions, signals transmitted in the high-frequency bans

wall propagate to • long ranges with quite high efficiency. Unfortunately, highfrequency transmissions are subject to severe disturbances caused by solar

. .

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11111Es% TR-59-0000-00604

Page II-7

flares and various other solar emissions. Most of theabisorption experienced

by a high-frequency wave reflected fromthe ionosphere occurs in the D-region

of the ionosphere, which is located below the point at which the wave is reflected.

During a solar .flare, the ultraviolet radiation emitted by the sun causes an

abruptincrease in the ionisation of the ionosphere D-region, with a corres- .

pondingly sharp, and very large, increase in the absorption of high-frequency

transmissions. This effect is observed over all of the sunlit hemisphere of

the earth, and the only •way to maintain communication during such a fadeout

is to transmit with greatly increased power. Experimental data indicate,

however, that an increase in . transmitted power of as much as 80 to 90 db

may occasionally be required, a completely impractical value when normal

transmitter powers are of the order of kilowatts.

Additional high-frequency absorption phenomena are observed in the.

polar regions of the earth. The sun is continually emitting . charged particles,

and those incident upon the atmosphere of the earth.are guided by the earth's

magnetic field to the . polar regions. Here these particles cause an increase

in the ionisation of the lower: ionosphere, resulting in a consequent increase in

the absorption of high-frequency transmission. During a solar flare, the rate

of emission of charged particles from the sun is vastly increased, and a

corresponding increase in.absorption or "polar blackout" is observed on highfrequency transmissions. Such polar blackoits have been observed to continue

as long as several days and, as with fadeout caused by solar flares, can be

. overcome only by transmitting with very much increased power.

Finally, the high-frequency range is used throughout the world for

normal long-range transmission, with the result that the spectrum in the 3-

to 30-megacycle frequency range is extremely crowded. Thus, it would not

be practical to expect that bandwidths of the order of megacycles could be

made available for services such as would be•possible with a satellite-borne

communication relay. system.

The last long-range propagation.mechanism to be considered in this

section is that of VHF ionospheric and meteoric scatter. It has been found

recently that radio waves in the lower VHF range will be scattered due to

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TR.-59-0000-00604

Page 11-8

the turbulent structure of the lower part of the ionosphere. Also, avery large

number of meteors impinge upon the atmosphere of the earth eachday, which,

although not visible to the . naked eye, produce trails of ionization which are

capable of scattering radiation in.the lower VHF range. Because of the.altitude

. • at which such 'scattering takes place, however, the maximum range over which

such propagation can be. effected is limited to•about 1 .200 to 1400 miles.

Although longer ranges can be obtained with Such. scatter links by Using

several links in series, such .a multipliCity o

•

 f equipment has the same • .

disadvantages that were discussed above in connection with microwave relay

links. Also, 'because of the inefficiency . of the scattering process, this type

of propagation requires extremely high transmitter power, high gain, and

carefully oriented transmitting and receiving.antennas..Finally,. these scattering rnechanismifare euCii that bandviidths no higher than a few kilocyCles can

be realised even if special modilation..techniques are used. Thuc io•no-

. .

spheric and meteoric scatter proVide a liMited-range communication capability

 .. at the expense of low efficiency•lind limited bandwidth, and do not seem to

. warrant more than-passing consideration s a possibility for providing the type a

of long7Tange cOannnulicatiOn service that could be rendered by a satellite-borne

• relay system.

It ill "evideit.from .the .feregoing discussion: that all present-day communicatiokmetbods Syffer

.. 

• from one or .liiore deficiencies which render them far

less that:v:ideal. fei traismisiion of Information over long ranges. : In fact, a

relatively !malt nu*lboi of submarine coaxial cables and high-frequency radio

channels, with sharply limited over-aif .bandWidth.capabilitiei, are the 'only

means existing today of obtaining.I.ong-range intercontinental communications.

On the other hand, the use. of a satellite.borne communication relay system

to effect long-range transmission of information..seems to hold . great promise

for eliminating the deficiencies in present techniques. The investigation of

the feasibility of such .a satellite-borne communication relay system has

formed a major part of the present study, and such a system is discussed in

detail in the following sections. _GthittuzzetijAL__

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TR-59-0000-00604

Page 11-9

2.2 TYPES OF.• COMMUNICATION SERVICES AND RECOMMENDED ORBITS

1.2.1 High-Density Service 

'The natural tendency, when considering the service to be provided by a

satellite-borne communication relay system, is. to require that communication

bs.possible between.any two points, within the directline-of-sight of the _relay.

However, careful. consideration shows that sucha procedure results in.extremely

inefficient use of the power radiated by the satellite. A large proportion of the

area of the earth: which.will be seen by any satellite is ocean, an.area in which

the relative density of Communication sources may be expected to be fairly low.

On the other hand, the majority of the sources of communication will be located

on the land areas of the earth--in fact, upon a relatively small fraction of the

land areas of the earth —and the relative density' of these communication

sources will be extremely high. Thus, efficient use of the capabilities of

satellite-borne communication relay systems can be obtained by providing

high-traffic-density lor large-bandwidth) capabilities between 'restricted portions of the surface of the eartheand relativelylow-traffic-density capabilities

to the remainder.

This conclusion is graphically illustrated by Figures. 2-1, 2-2, and

2-.3. Figure 2.•.1 is a Mercator projection of the earth .with . those areas from

• which .originate almost all of the communication signals of importance to

U.S. . Military forces indicated by heavy shading. Figures. 2-2 and 2-3 are

pictarea of a dlabatakoa.tr9111 a d4otenoe equivalent to the radius of a 24-hour

equatorial satellite orbit and aligned. with the meridians corresponding to

longitudes 170°'W and 30°W, respectively. (The reasons for recommending

such orbits.; are . discussed 111 . .14 later paragriph.) Again, those sources of

the great majority of communication signals of interest to this gcri-ernment

are 'indicated by' black. outline s. •

Frorn.these „Spree, it. may be seen' that. two types of high-density

traffic exist which must be handled •by a vommunication relay system.. First,

a great amount. of traffic will exist between various points in the ZI which

otherwise would have to be handledin its entirety by some presently available

long-range communication method. For example, communication would be

imi

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TR-59-0000-00604

Page II-11

Figure 2-2. Earth as Seen from Satellite in 24-Hour Orbit

at Latitude 300W (coverage areas outlined).
TR-59-0000-00604

Page II-12

-CONFIDENTIAL

Figure 2-3. Earth as Seen from Satellite in 24-11our Orbit

at Latitude 170°W (coverage areas outlined).

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TR-59-0000-00604

Pige II-13

desired between Washington and Omaha, between Washington and Colorado

Springs, between Washington and various points on the West Coast, and so on.

Furthermore, because of the extremely high-density traffic which may be

expected for this type of service, it seems reasonable to allocate the complete

capacity (if necessary) of a relay system carried by a 24-hour satellite to such

ZI communications. The second type of high-density traffic, which may be

expected from an inspection of Figures 2-1, 2-2, and 2-3, is traffic between

North America and Asia and between North America-Greenland and Europe. As

shown in Figures 2-2 and 2-3, all of the areas of interest indicated inFigur e 2-1

can be seen from one or the other of two satellites positioned in 24-hour equatorial orbits above longitudes 170°W and 30°W, and, as outlined in Figures 2-2

and 2-3, only relatively small parts of the visible portion of the earth would

have to receive radiated power from the satellite in order to provide the desired

coverage. Of course, these are nominal coVerage areas and may.be modified

to fit specific requirements.

2.2.2 Advantages of 24-Hour Orbits * for High-Density Service 

It will be noted that the foregoing discussion assumes the use of satellites

in 24-hour equatorial orbits. It might be argued that, by utilizing orbits of

smaller radius, either smaller boosters can be used or larger payloads can be

placed in orbit. It is necessary to weigh such possible advantages against the

system complexity brought about by not using the 24-hour system. However,

as shown in VoluMe 2, Chapter 1, the payload which can be put into an orbit

is relatively insensitive to orbit altitude for orbits having a radius in excess of

10,000 miles. Therefore, if there is to be any substantial increase in payload

capability (or reduction in booster size), it must be for satellites having

altitudes of less than 10,000 miles.

The primary justification for recommending 24-hour equatorial satellites for ZI and intercontinental communication relay lies in two basic

considerations. First, the use of 24-hour equatorial satellite orbits permits • • •

*

Actually the period of one revolution is. one sidereal day or 23 hours,

56 minutes, 4.09 seconds which is the period of rotation of the earth,

rather than the 24-hour interval between successive passages of the

sun over a given meridian.

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TR-59-6000-00604

Page 11-14

the desired coverage for high-density traffic with three satellites, whereas the

use of any other type of orbit cannot decrease this number and will, in general,

require a substantially greater number if an appreciable reduction in satellite

altitude is to be effected. Figure 2-4 illustrates the particular case where the

satellites are evenly spaced in an equatorial orbit. This figure shows the

maximum latitude of a ground station at whiCh continuous contact with at least

.. .

one of the satellites can be provided, as a function of the altitude of the orbit,

and with the number of satellites in the orbit being used as a parameter. As

can be seen from the figure, for example, continuous coverage of a latitude of

65° (Alaska; Iceland, and Southern Greenland), cannot be provided by any

number of satellites having altitudes less than 5500 miles, while even at an

• altitude of 10;000 miles, four satellites are still necessary.

The minimum number of nonequatorial satellites required to provide

continuous contact with the areas to be covered is extremely difficult to deter -

mine as a function of satellite altitude be:Cause of the very large number of

schemes which might be considered. •However,, regardless of. the disposition

of the satellites and the inclination of their orbits, it is quite generally true

that no fewer than three low-altitude.satellites, and in most cases a substantially larger number, are required for continuous coverage of the area of the

earth within the latitudes of interest.

It should be noted that if low-altitude satellites are used and if the

sending and receiving stations are far apart, then, even if both sending and

receiving stations are each within line-of-sight of a satellite, both will not

be within the line-of-sight of the same satellite. This will require relaying

the message from satellite to satellite, possibly using intermediate ground

stations to effect the message transfer. Direct satellite-to-satellite transmission involves either the use of • much larger satellite• antennas or else

(assuming fixed power) the loss ofbandwidth. The use of intermediate

ground stations permits transmission with large :bandwidths because large

antennas can be used at the ground stations; however, such a system suffers

from the cost and reliability penaltie s of requiring additional repeater stations

all with tracking antennas. Furthermore, because of the rotation of the

CONFIDENTIAL

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TR-59-0000-00604

Page 11-16

earth, the satellites will cover not only the high-density communications

area but will spend much of their time over ocean areas where the necessary

relay stations cannot be provided.

Clearly, to minimize the system complexity and also to allow the full

bandwidth capability, it is more satisfactory to have the same satellite within

line-of-sight of both the sending and receiving stations. However, this can

be done only if the satellite is above some minimum altitude. As an example,

if one wishes to transmit from the West Coast of the United States to Japan, a

satellite at the midpoint of the Great Circle. arc connecting these two locations

must be at an altitude of about 900 miles to be within the line-of-sight of both

ground stations. If the satellite is to retain contact with both sending and

receiving stations for a significant segment of its orbit, or if it is to maintain

contact when it is not on the Great Circle connecting the two stations, then the

. altitude must be much higher. • For example, in order to maintain contact with

both San Francisco and Tokyo for an arc of 30° in longitude (which would

require a total of 12 such satellites), an equatorial satellite must be at an

altitude of over 7000 miles above the surface of the earth. Furthermore, it

may be shown that for San Francisco-Tokyo communication no fewer than

"five equatorial satellites are required for any altitude, even at infinity, with

the single exception of the 24-hour equatorial satellite (in which case only one

is required).

The second basic consideration which argues for the use of a 24-hour.

equatorial satellite is that only such orbits provide satellites stationary with

respect to the surface of the earth. Thus, the necessity for antenna tracking

is eliminated both on the ground and in the satellite. This permits the use of

large antennas both in the satellite and on the ground, and consequently yields

an increased communication relay capability with the same amount of

transmitted power. Moreover, having fixed antennas permits a substantial

simplification of the instrumentation .of the system since there is no acquisition problem and no scanning program is required. Since reliability is of

paramount importance in a communication relay satellite where maintenance

will be impossible, such a simplification in satellite equipment is extremely

valuable.

eft

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TR-59-0000-00604

Page 11-17

It should be noted that a high-altitude satellite need not require any

more power than a low-altitude satellite, providing the cross-sectional area

at the earth of the illuminating beam is equal in the two cases. The essential

point is that a given area, namely the high-traffiofdensity area,.must be covered

and, since the area covered is independent of the satellite height, there need be

no power loss due to altitude. If any high-altitude satellite other than a 24-

hour satellite is used and if a narrow beam is used to illuminate only the highdensity regions and thereby save power, then the satellite antenna must be

rotated so that it continues to illuminate the area of interest. This not only

leads to additional instrumentation complexities in the satellite but complicates

the problem of attitude stabilisation because of the reaction torques resulting

from the antenna rotation.

The discussion in' he foregoing paragraphs indicates that, if the

desired ZI and intercontinental communication relay capabilities can be

provided by satellites positioned in 24-hour equatorial orbiti, such capabilities will be obtained with a system requiring no antenna tracking and the

number of satellites required will be at a minimum. The only question

remaining is whether or not it is feasible to launch the necessary payloads

into 24-hour orbits using boosters which will have early availability. The

weights required for the satellite-borne communication system are examined

in Sections 2.6 through 2.8 of this chapter. In •Volume 2, Chapter 5, they are

combined with the other system weights (power supply, guidance, etc. ) and

compared with the payload capabilities of current boosters. It is there found

that even the Thor booster with some modification can place a useful payload

into a 24-hour orbit. This feasibility, coupled with the many desirable aspects

from the communications system standpoint of 24-hour equatorial orbits,

argues very strongly for their use to provide high-traffic-density communication.

2.2,3 Low-Density Polar Service 

Since the traffic density from portions of the surface of the earth,

other than.those specifically designated in Figures 2-1 through 2-3, is •

expected to be low, a much smaller. bandwidth capability will be required of

a system providing such service than in the case discussed above. This

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NM* TR.-594000-00604

Page II-18

advantage is offset by the fact that the transmitted power from the satellite

must be radiated to a substantially larger area. In addition, communication

must be established with aircraft (as well as ships and ground-based stations),

so. the dimensions (and consequently gain) of the aircraft-borne receiver

antennas are substantially less than those which can be used by a ground-based

installation.

From the discussion in Section 2.1 of the deficiencies in presently avail-

.

able long-range communication methods, it may be inferred that a much greater

problem exists in communicating with stations or signal sources in the polar

regions than with stations located in temperate or tropical regions of the earth.

But communication to and from points in the polar regions is extremely impor-•

tantlo the military forces of the United States. An inspection of a globe will

show immediately that any attack upon this country probably would be carried

out across the polar regions, as would any retaliatory attack which the. United

States might dispatch. For this reason a•large number of installations are

maintained by this country in the polar regions of Canada, Alaska, and Greenland,

and a large amount of training, reconnaissance, and other similar activities*.s

carried out on and over these areas. Thus, while the importance of establish-

•ing communication to points on or above the ocean areas of the earth should not

be minimized, it would seem from the above considerations that communication

relay service to the polar regions should be given high priority.

The polar regions to which communication relay service should be

established may be determined from Figure 2-5. This figure shows, on a

polar stereographic projection of the NorthernHemisphere, the auroral absorption region of the northern latitudes. Although auroral effects can extend beyond ,

this region, its low-latitude limit does provide a reasonably accurate boundary

for the polar areas where the most severe effects limiting normal long-range

communication methods may be expected to occur. Therefore, the area

bounded by the low-latitude limit of the shaded region shown in Figure 2-5 will

be defined as the minimum area for which .a communication relay capability

must be established by a polar communication relay system. However, it

would be desirable to have this capability include a capability for effecting

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TR-59-0000-00604

Page 11-19

-CONRDENTIAtFigure 2-5. Auroral Absorption Region of Northern Hemispher e.

-CONRDENNt

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TR-59-0000-00604

Page 11-20

direct recall from the United Slates proper of missions launched over the

polar regions. For this reason, the orbits recommended inthe following paragraphs for a polar communication relay system have been determined so that

this desired capability can be provided.

Communication relay service to the se polar areas must be e stabli shed

by using satellites in nonequatorial orbits, since no satellite at any reasonable

altitude in an equatorial orbit can include latitude 90°N withiii its line-ofsight. Such nonequatorial orbits, unfortunately, do not permit establishment

of a satellite stationary above a given point on the surface of the earth. Thus,

more than one satellite will be required to provide the desired coverage even

though only one satellite may be used for relay purposes at any given time.

2.2.4 Advantages of 24-Hour Orbits for Polar Service 

Although. many different classes of orbits may be postulated which

would provide the desired polar communication relay capability, 24-hour

polar orbits (in which the orbital plane contains the axis of the earth) possess certain advantages. The first advantage, of course, is the fact that

the satellite will pass over the same point on the surface of the earth each

time it passes a given point in its orbit. This substantially reduces the

antenna, tracking problems which have been introduced unavoidably by the

necessity of using nonequatorial orbits. In a system having mobile terminals such as aircraft, ships, etc., this becomes an important factor.

A second advantage of 24-hour polar orbits for a polar communication relay system arises from the fact that the path of the satellite relative

to the surface of the earth is what may be termed a figure-eight path. A

24-hour satellite crossing the equator in a northerly direction will cross

the North Pole after the earth has rotated by 90 degrees and will cross the

equator again (going •south) after the earth has rotated 180 degrees. Since

the satellite crosses the same point on the equator both in its northbound

and southbound path, the resultant path relative to the surface of the earth

is essentially a figure eight, as stated above.

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TR-59-0000-00604

Page 11-21

• The portion of such a path lying inthe Northern Hemisphere is shown

in Figure Z-6, where the point at which the orbit crosses the equator has been

chosen as 60°W longitude. If three satellites are established in 24-hour polar

orbits such that the path of each satellite relative to the surface of the earth

coincides with the path shown in Figure 2-6, but with their equatorial crossing

time 8 hours apart (this requires the orbital planes to be .separated by 1200),

considerations of syMmetry will show that at least one satellite is at or above

30°N latitude at. all times. As shown also in Figure 2-6, the area continuously

within line-of-sight of such a satellite when it is above 30°N latitude includes

both the desired polar areas and the entire United States. Furthermore, inspec-

.

tion Will. show thatthe point at which the satellite crosses the equator may vary

anywhe.re from about '600 to 90°W longtitude without altering this conclusion.

Thus, the desired polar communication relay service could be established

between any point in the United States and the desired polar coverage areas

through the use of three satellites properly e stablished in 24-hour polar orbits.

Finally, it may be shown that three satellites are the minimum required

to provide continuous relay service between desired polar areas and the United

States, regardless of the type of orbit chosen. For example, four 24-hour.

satellites would be required if their orbital planes coincide instead of being •

separated as in the foregoing discussion, and any substantial decrease in satellite altitude would necessitate the use of an even greater. number. Similarly,

considerations of geometrical symmetry show that the use of nonpolar orbits

There is an alternative which appears feasible at first sight, the use of

24-hour satellites in a highly elliptic orbit with apogee at the North Pole

and perigee at the South Pole. This results in the satellite spending

twelve hours within an •angle of 14.4 degrees from the North Pole. By

having two such satellites 12 hours out of phase, the communications

between the polar regions of the earth and the United States can presumably be accomplished. However, the difficulty with this scheme is

that the. oblateness of the earth causes a rotation of the line of apsides,

that is the major axis rotates within the plane of motion so that an orbit

which originally had its apogee at the North Pole would later have its

apogee at lower latitudes. After the apogee had moved a sufficient

distance from the North Pole, the satellites would no longer spend.

12 hours in the immediate vicinity . of the pole; moreover, the diurnal

tracking problem would not-repeat from day to day. This is not compatible with the simple antenna systems which are required for aircraft

installation.

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TR-59-0000-00604

Page 11-22

Soviet

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Figure 2-6. Area Cuntin.muly with Line-of-Sight of 24-Hour Polar

Satellitejv Latitude.

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sta

Area of continuous coverage

Region If high auroral absorption

under normal conditions

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Tltr59 0000-00604

Page 11-23

cannot reduce the required number of satellites below three, regardless of

the type of orbit or orbital altitude chosen. Thus, the use of 24-hour polar

satellites, in orbits chosen to ensure that each satellite traces the same path

relative to the surface of the earth, permits obtaining the desired polar coverage with a minimum number of satellites and with the advantage of a substantial reduction in the antenna tracking problems. For these reasons, the

feasibility of a polar communication relay system using such orbits, as determined in the course of this investigation 'and described in later sections of this

report, has caused such orbits to be recommended for this purpose.

2.2.5 Other Low-Density Service 

The final communication relay capability to be considered in detail in

this report is that of providing low-traffic-density communication relay service to and from the ocean areas and less important land areas of the earth.

One of the most important communication requirements of the Strategic Air

Command today is the requirement for a recall capability to aircraft over

any portion of the earth. Means for satisfying this requirement do not exist

today, but certainly could be provided by satellite-borne communication relay

facilities.

The polar orbits discussed in the preceding section, and recommended

for use in . providing polar communication relay service, possess as additional

important property in that they can provide communication relay coverage of

much of the ocean and less important land areas of.the earth. Both shaded and

cross-hatched, areas outlined in Figure 2.-7 are areas in the Northern Hemisphere

continously within line-of-sight of at least one of the. three 24-hour polar satellites positioned in the polar orbits as previously described. (The continuouscoverage area in the Southern Hemisphere will be symmetric about the equator

with that shown for the Northern Hemisphere.) These polar satellites can

provide continuous coverage of the entire Atlantic Ocean with an exception of

the relatively small segment off the extreme southwest tip of Africa, and in

addition can provide continuous coverage of a fairly substantial portion of the

Pacific Ocean as •well.

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TR-59-0000-00604

Page 11-24

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Polar Relay Satellites.
TR-59-0000-00604

Page 11-25

The cross-hatched areas in Figure 2-7 represent those portions of the

continuous-coverage area in the Northern Hemisphere for which continuous relay

service cannot be provided from the central United States using the recommended polar satellites. However, such .satellites will permit continuous and

direct relay service between, the central United States and any part of the

shaded area. including the North Atlantic Ocean (except the Gulf of Guinea) as

well as much of Eur• ope, North Africa, and Siberia. • In addition, service.to

much of the South Atlantic and South America (not shown in the figure) is

provided.

The polar, orbits do not cover large portions of the Pacific Ocean. But,

as may be seen from an inspection of Figure 2-3, continuous coverage of this

area may be obtained from a single .equatorial satellite positioned above about

latitude 170°W, and such.a satellite can provide continuous relay service from

'any point in this area to the central United States.

The combination of three polar satellites and the low-density traffic

satellite over the. Pacific would provide communications between aircraft (or

ships) in the polar regions or over the NorthAtlantic and Pacific Oceans. An

extension of coverage to the Asian and African continents and the Indian Ocean

could be provided by a 24-hour equatorial satellite over about 65°E longitude,

but the satellite could not communicate directly with the United States. It

could, however, communicate with either a European base (say London) or

Japan (or the Philippines). for relay to the United. States via the high-density

intercontinental satellite links. This relayed information would use only a

very small fraction of the channel capacity of the high-density link.

2.2.6 Sunimary 

The geometrical considerations discussed in this and the foregoing

section indicate that primary coverage of both high-and low-traffic-density

areas of the earth could be obtained with the following 24-hour satellites:

High-Density Traffic 

ZI to Europe

ZI to Asia

ZI Internal

Equatorial, Longitude =

Equatorial, Longitude =

Equatorial, Longitude

306,1

170°W

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TR-59-0000-00604

Page 11-Z6

Low-Density Traffic to Aircraft. etc.

Polar Region, Siberia, North America, North Atlantic,

Europe, North Africa, and poitions of North said South

Pacific Ocean, South Atlantic Ocean, and South America

3 Polar Satellites crossing equator at 60°W

Pacific Area 1 Equatorial, Longitude 170°W

Asia, Africa and

Indian Ocean 1 Equatorial, Longitude =65°E

It should be noted that, on a purely geographical basis, the 24-hour

satellites used for high-density intercontinental relay service could be used

also to provide low-density service to the Pacific Ocean .and Atlantic Ocean

areas. Furthermore, an equatorial satellite placed at 40°W could perform

both the ZI service and partial ZI-to-Europe service. Therefore, if payload

capabilities of the satellite vehicle permit multiple functions to be carried

out, then fewer satellites will be required. This possibility will be discussed

in somewhat more detail in.a later section.

The foregoing discussion of areas of coverage has neglected the

increase in coverage which can be attained by use of the refraction of radio

waves in the atmosphere. The effect of refraction for a normal atmosphere

is to increase the coverage by about 1 degree, or 60 nautical miles. Since

•

this is a relatively small amount compared to the coverage of interest, and.

since it will vary with variations in the refractive index of the atmosphere,

this effect has been omitted in all.foregoing and subsequent calculations.

2.3 TRANSMISSION` CONSIDERATIONS

Having determined recommended satellite orbits and positions from

the geometrical considerations of the foregoing sectiOns, the power requireMeats of a communication relay system utilizing such satellites must be

established. The factors which determine such power requirements are

discussed in the following sections, and required transmitter powers are

obtained for each type of communication relay service discussed above.

EMI

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'CONFIDENTIAL_

=

TR-59-0000-00604

Page 11-27

2.3.1 Basic Relationships 

The power at the input terminals of a receiver at a distance It from

the transmitter, is given by the expression

p

Pt Gt it

4

Ar nr •

wR

(2.1)

where Pt is the transmitter power, Cst is the gain of the transmitting antenna

in the direction of the receiver, and lit is the radiation efficiency of the transmitting antenna. Ar is the receiving aperture (or effective receiving area) of

the receiving antenna and q r 

is the efficiency of this antenna. In any practical

communications system, the received power Pr must exceed the noise by a

factor equal to the signal-to-noise ratio, establishing the requirement that

Pr = (S/N)N. Substituting this expression into Equation (2.1), we obtain an

expression for the required transmitter power; Pt, given by

.4 x.R2 (SiN) N 

t •Gt % Aril r

(2.2).

An alternative; and possibly more familar, relation is obtained by

expressing the receiving aperture in terms of the, antenna gain and operating

wavelength,

This yields the equation,

(

4w R)2 IS/MN 

k Gt Grlr

•

in which the fdctor (4s.zy X)2

 corresponds to the free-space transmission loss

between isotropic antennas. Equation (2.3) shows complete Symmetry between

sending and rec.eiving:antennas (the well-known reciprocity relationship)..

Assuming that each.antenna is used both for transmitting and receiving, and

that the noise levels at ground and Satellite receivers are eqUal, the power

.

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l

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• (2.3)
TRA9-0000-00604

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requirements will be equal for ground and satellite transmitters. Since power

is more readily available at the ground station, the more critical link will be

from satellite to ground. Consequently, in the subsequent treatment, we shall

be concerned primarily with evaluating the power required for transmission

from the satellite.

Equation (2.2) shows that the required transmitter power for any given

transmission range is directly proportional to the noise level (which depends

upon the bandwidth) and to the required signal-to-noise ratio, and is inversely

proportional to the transmission gain, receiving aperture, and efficiency product of the antennas. In general, the efficiency of an antenna, being virtually

independent of size and operating frequency, is determined once the type of •

antenna is chosen. Furthermore, the coverage reqUirementestablisb.ed in the

preceding paragraphs for a satellite-borne communication relay system will

determine, in turn, the transmission gains of the satellite antennas. Consequently, assuming signal-to-noise ratio requirements and noise characteristics

have been established, the transmitter power required to effect such coverage

will be determined only by the receiving aperture of the ground antennas. These

system parameters—antenna gain and aperture, signal-to-noise requirements,

and noise characteristicit —are discussed in the following sections. The results

are then used to determine the transmitter power required for the particular

communication relay systems considered in this investigation.

2.3.2 Antenna Gain and Aperture

As stated in the preceding paragraph, coverage requirements determine

the maximum gain which can be achieved with the satellite antennas. However,

since the gains of an antenna is proportional to. the product of the antenna's area,

or receiving aperture,. and the square of the operating frequency, the dimensions

of the antennas satisfying these gain requirements will decrease with increasing

operating frequency. Therefor e, since the dimensions of the satellite antennas

may be expected to be limited to approximately the order of the physical dimensions of the satellite itself, the foregoing'considerations suggest the use of the

highest possible operating frequency to minimize antenna packaging problems.

This is particularlitrue of those narrow-beam antennas which would be necessary to minimize jamming (see Section 2.9) and (for interim systems) to provide

iy

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-GONFTENTIAL

TR-59-0000-00604

Page 11-29

point-to-point communication relay service. Of course,use of higher operating

frequencies must depend on the availability of the necessary components, and

the fict that amplifier tubes are not available at frequencies above 12 lanc.

(see Appendix C) has led to the use of 10 'talc in subsequent calculations of

power requirements.

It is clear from the discussion of Equation (2.2) of the preceding section

that the receiving aperture or equivalent area is the only parameter of the

ground antennas affecting the powei requirements for transmission from the

satellite to the ground. Because of the theorem of reciprocity, therefore, we

may conclude that their aperture or size is the only paiameter of these ground

antennas affecting the power requirements for the reverse transmission, i. e. ,

from the ground to the satellite. Nevertheless, the antenna gain or bearnwidth

of the ground antennas is a consideration in determining the accuracy, with •

which the satellite must be established in its orbit and subsequently tracked by

the ground antennas. Therefore, at the high frequencies required to limit

satellite antenna sizes, the extremely large (narrow-beam) ground antennas

which might otherwise prove feasible will have their dimensions limited by the

severe tracking.and satellite orbit control problems which such narrow beams

would introduce.

Design considerations for 24-hour satellite antennas are discussed in

detail in Appendix A. It is concluded that polyrod antenna arrays are the most

suitable type of antenna for a communication .relay satellite in all cases where

size limitations prevent the use of the larger, but more efficiently fed, parabolic dish antenna. Since antenna size is also an important factor for the

aircraft installations of the low-traffic-density systems, such arrays seem

most suitable for this application as well. The total weight of the largest array

considered (16 rods k/2 in diameter and 10). in length) should not weigh more

than 10 to 20 pounds at S-band or a pound or so at X-band.

2.3.3 Signal-to-Noise Ratio Requirements

Signal-to-noise ratio requirements must be established on the basis of

the quality of service as well as the medium Over which the information is to be

transmitted. For example, the signal-tO-noise ratio required of a beyond the

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TR-59-0000-00604

Page L1-30

•

-eft/34E1VA

horizon radio link, whether it be a scatter link or a normal pheric-

,

reflection link, must be determined using the statistics of signal fading. To

obtain the desired reliability with.such links, the average signal-to-noise

ratio must be substantially higher than the ininirnurn allowable signal-tonoise ratio (about 15 db). On the other hand, with line-of-sight radio links

or with wire-line systems, the absence of fading permits a substantially

better quality of service and a minimum allowable signal-to-noise ratio of

20 to 25 db is common.

A communication relay system utilizing satellites in appropriate orbits

is a.completely new type of service", and, in view of the difficulties of establishing

and maintaining such a service, it is imp' ortant to be .conservative in the design.

Therefore, if•it is feasible from the standpoint of satellite weight to establish

an extremely high-quality service (one with a high signal-to-noise ratio), we

feel that this should be done. (It should be noted that a high signal-to-noise

ratio will also'reduce susceptibility to: enemy jamming, although jamprord

operation will also require the use of one of the several techniques for. spreading

the signal over the total available bandwidth to deny the enemy an effective spotjamming capability.) For these reasons the calculations carried outin the course

of this study haye utilized a peak signal-to-noise ratio of 30 db, with a minimum

allowable peak signal-to-noise ratio of 20 db established for cases where the

power and satellite weight) required to provide acceptable bandwidths would be

excessive with a 30-db signal-to-noise ratio. The 30-db figure will permit

transmission of high-quality video and voice . information, while providing an

error rate for normal digital signals (such•.as teletype) which is so small as to

be negligible. For the low-density relays, which are the only cases where the

minimum allowable signal-to-noise ratio must be accepted in the initial global

system, the limited bandwidth obtainable even under these ' conditions prevents

the transmission. of video information. Nevertheless, the error rate for digital

 . • .

signals will still be 'comparable. to that obtained:With wire-line transmissions.

2.3.4 Desired. Bandwidth '

A satellite-borne military communication relay system would be so

.

useful that any bandwidth capability provided would be completely Utilized:

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•
TR-59-0000-00604

Page 11-31

However, limitations on the weight of the payload will cause a corresponding.

limitation in the bandwidth capabilities. Therefore, it is evident, at least for

the early stages of the satellite program, that the number of users of such

facilities and the information transmitted must be restricted.

A reasonable estimate of the bandwidth capabilities which an initial global

communication relay system should provide may be obtained by considering the

types of transmissions the system might be required to handle. In general, the

signals might consist of video transmissions (used in a quite general sense and

including facsimile), voice transmissions, and teletype transmission, with datalink transmissions possibly being included under certain circumstances. The

. . •

bandwidth of a video signal, as estimated from the bandwidth requirements for

a television. signal, will be taken as about 4.5 megacycles, since the remainder

of the 6-megacycle band normally allocated for a television channel is used for

sound, chromatic information (in.color TV), and guard band. The bandwidth

required for a 'voice signal may vary, depending upon the quality desired. How-

.

ever, it is felt that toll-quality speech should be a design objective for this

system, so a 4-kilocycle bandwidth requirement will be assumed for voice

signals. Finally, teletype and data-link transmissions normally occur in digi-

. tal form, and bandwidth •requirements may be determined according to accepted

practice. The bandwidth of typical teletype circuits may vary between about

30 and 300 cycles per second, with the latter value coming into more and more

common.use as the necessary equipment is developed. Thus, '13 or more teletype signals, depending upon their pulse transmission rate, maybe multiplexed

into the bandwidth . required for each voice signal. SiMilarly, although data-link

requirements may vary widely, a value of 20 kilocycles will be assumed as

typical of a relatively high-speed link. Thus, 66 or more teletype signals, up

to 5 voice signals, or some appropriate combination'of these, may be

multiplexed into the bandwidth required for a single data-link transmission.

From these probable bandwidth requirements for the various types of

possible signals, the bandwidth requirements for an initial satellite-barna

global communication syitem may be estimated. Viten.a high-traffic density

capability is required, an over-all bandwidth of a. few megacycles is obviously

not sufficient since it would not permit more than one video'channel to be

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TR-59-0000r00604

Page 11-32

provided. Onthe other hand, a bandwidth of the order of hundreds of megacycles

or more would provide bandwidth for some tens of video channels, a capability

which, although useful, undoubtedly exceeds that required of the high-trafficdensity relay in an initial global system. Thus, a .reasonable estimate for this •

type of service may be taken to be some tens of megacycles; for purposes of

calculation, a figure of 50 megacycles total, or 25 megacycles each way

between the terminals of an intercontinental relay, has been chosen. This

50-megacycle system capability would provide, for example, sufficient bandwidth for 5 video channels and 625 voice channelcin each direction for a twoway system. Teletype or data7link channels could be substituted for the voice

channels in the ratios given in the foregoing paragraph.

In the case where low-traffic-density service must be provided, the

requirement for transmission of video data may be. eliminated as unnecessary

for a minimal system. Furthermore, because of the difference in class of

service, it may be expected that a low-traffic-density system need not provide

more than 5 to 10 per cent of the capability for voice, teletype, or data-link

signals required of the high-traffic-density relays. Therefore, the bandwidth

requirement for an initial low-traffic-density global communication relay system

will. be taken as 100 kilocycles, a value which provides a capacity. either for

5 high-speed, data-link channels, 25 voice channel*, or over 300 teletype

channels, or for an appropriate combination of these types of signals as desired.

It should be emphasized that, although the bandwidth figures chosen

above are considered reasonable design objectives for a fully operational initial

global communication system, it may prove desirable for certain reasons to

obtain increased bandwidth by reducing system signal-to-noise ratio, or conversely, it may be desirable to exchange bandwidth capability for higher

signal-to-noise ratio. Alternatively, by reducing bandwidth one could effect

a reduction in power requirements and thus in the weight of the power supply

and cooling systems; the weight allowance could then be used to provide reliability by providing equipment redundancy. Thus, these bandwidth figures are

only nominal figures to be used in the following sections to provide estimates

of power requirements for an initial global system. In a later section, the

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TR-59-0000-00604

Page 11-33

-CONFIDENTIAL

objectives will be compromised to provide interim systems with reduced

bandwidth capabilities (although still a great improvement over all present

techniques for long-range communication), and thereby permit their early

achievement.

Z.3.5 . Noise Level Considerations 

Appendix B presents a general discussion of the various types of radio

noise together with .a preliminary evaluation of their relative importance for

the various types of satellite-borne relay and reconnaissance systems investigated in• his study. In the case of the communication relay system considered

in this section, these results, 'summarised below, show that set noise will.

limit reception under all conditions except for short intervals during the vernal

and autumnal equinoxes, when solar noise can become important once each day

for the Z1 and intercontinental relays.

At an operating frequency of 10 bac, the maximum cosmic noise which

can be received from the galactic plane, even on high-gain antennas, is less

than set noise. Furthermore, the cosmic noise from discrete radio stars, when

received by aircraft-borne antennas with the relatively small gains appropriate

bathe polar relay, system (the only case where the important radio stars are

within the beam of the receiving antenna), is also less than set noise at 10 kmc.

Thus, cosmic noisels not important in limiting reception in this frequency

range. Even with solar noise, the antenna gaini will not be sufficient to cause

set noise to be exceeded except for the ground antenna used in the high-trafficdensity systems. The narrow beamwidth of this high-gain ground antenna,

however, limits the time interval during which solar noise exceeds set noise

to, at most, 3 minutes each day for at most, 3 days at each equinox, since

only during such.a period could the sun fall within the beam of any given

ground antenna. Therefore, since the interruptions in relay service which,

. • may be caused by solar noise represent such.a negligible fraction of the•totali

time available for transmission, and since, moreover, such interruptions can!

be predicted with very high accuracy, set noise will be considered the factor

limiting reception for both the low-traffic-density and high-traffic-density

communication relay systems investigated in this study.

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TR-59-0000-00604

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-CONFIDENNt--

2.3.6 Power Requirements*

The results of the foregoing sections may now be used, together with

Equation (2.2) of Section 2.3.1, to determine, the transmitter power required

for the various satellite-borne communication relay services investigated in

the course of this study. The discussion in Section 2.3.1 of the effect of the

operating frequency upon the required transmitter power indicated that the

highest possible operating frequency , should be used. This conclusion,

coupled with the fact that the necessary amplifier' tubes are not available at

frequencies above about 12 lcmc (see Appendix C), has led to the use of an

operating frequency of 10 lcmc . in the calculations of required transmitter

power for initial global communication relay service. Furthermore, the

calculations use a noise level based upon an absolute temperature of 300° Kelvin

for the input impedance of the ground-station receiving tube, and a noise

figure of 7 db for the receiver itself. This 7-db noise figure represents

a value which is commerical)* available today at 7 kmc and which may

reasonably be expected to be achieved at the slightly higher X-band frequencies

by the time this system could be operational.

a. Zone of the Interior Relay. System 

The United States covers about 60 degrees of longitude and about

25 degrees of latitude. It may be shown that, , for a satellite positioned in a

24-hour orbit above about 950W lcingitude, the vertical angle subtended by the

United States will be a little less than 4 degrees, while the horizontal angle

will be about 10 degrees. At 10 kmc this would require.anantenna array having

dimensions of only about 5 inches by 10 inches by 1 foot or so in depth, and

would provide a gain of about 30 db with about aS0per cent radiation efficiency.

The power requirements calculated in this chapter are based .upon.the assumption that relatively unsophisticated modulation techniques are used, so that transmission bandwidth and information bandwidth canbe considered essentially

equivalent. It is recognized that sophisticated modulation techniques present

attractive possibilities for improving system performance, but evaluation of

their relative merits requires detailed knowledge of such paraMeters as the

number of transmitting stations served by a given relay, the information bandwidth required for each station, allowable intermodulation.and crosstalk, and

so on, Hence, full evaluation of such techniques must await the detailed system

design study which wouldlogically follow the feasibility study reported herein.

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TR-59-0000-00604

Page .11-35

The size of the antenna which may be used on the round is limited

by the accuracy. with.which the vehicle can be tracked by the ground antenna

servo system. It is also limited by the dimensibnal accuracy (in terms of

fractional wavelengths) to which the antenna.can be constructed and afterwards

maintained under conditions of loading by wind, snow, ice, etc. Maintenance

of the necessary tolerances for use at 10 kmc is well within the present state

of the art for a 60-foot dish, which will provide a bearnwidth of 0.1 degree.

This beantwidth is quite compatible with.the.tracldng..accuracy obtainable with

such a system, and will provide a gain of about 63 db. Because of the simplicity of feeding such.a Alish, its radiation efficiency. may be expected to be over

90 per cent. In. Chapter 2 of Volume it is shown that the satellite may be

held to within approximately 1 degree of its nominal position.over a lifetime of

6 months to a year. • Consequently, the tracking referred to need • be only over

an-angle of about 1 degree,_ and this may be accomplished without significant

loss of radiation. efficiency by moving the feed rather than the dish.

Using these results and requiring a 30-db signal-to-noise ratio,

we obtain a required transmitted power for ZI communication relay. service of

5.0 watts per megacycle, or a total of 250 watts for a 50-megacycle bandwidth.

It should be noted that, due to the reciprocity theorem, this figure holds . •

regardless of whether transmission takes place to or from the 'satellite. How-

.

ever, since the peak power limitation is not as serious for the ground-based

equipment as for the satellite-borne relay, it seems advantageous to transmit

at a substantially higher power level from the ground and. thus decrease the

required amplification in .the satellite. This results in both a weight saving

and an increase in the difficulty, experienced by an enemy trying to jam the

system.

It should be noted that the horizontal angle subtended by the United

States at the 24-hour satellite could be reduced by positioning the satellite over

about 50°W longitude instead of over 95 9W longitude as suggested above. This

would permit a decrease in the diameter of the ground antenna to about 30 feet

or, alternatively, a decrease in required transmitter power to about 125 watts

for a 50-megacycle bandwidth. However, it will be shown in the following section that the desired intercontinental relay capability can be provided only if

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TR-59-0000-00604

Page 11-36

both a t 404,,„ .transmitter power of about 250 watts and 60-foot-dish d antennas.

are used. Thus, the possibility of modifying the ZI requirements by using the

alternate location mentioned for the ZI relay satellite will be .considered to be

an option- whose desirability would be determined more by operational

considerations than by basic technical feasibility.

b. Intercontinental Relay. System 

The coverage areas illustrated in Figure 2-2 for the relay between

the eastern North America and Europe require antennas having beamsvidths of

about 2 degrees by 9 degrees. This may be attained with .11.11.antenna array

having dimensions about 6 inches by 2 feet by 1 foot or so in depth.which would

provide a gain of about 33 db. Assuming as above that the ground.antennas are

60-foot dishes or their equivalent, and again requiring, a signal-to-noise ratio

of 30 db, we obtain :a required transmitter power of about 125 watt, for transmission either to North America or to Europe of a signal having a 50r-megacycle

bandwidth. However, since the desired initial global communication relay

service will require the simultaneous transmission to both America and

Europe, the total required transmitter power for the "American-European" .

intercontinental relay becomes about 250 watts. This requirement will be

discussed in more detail under Section 2.4.2.

The coverage areas shown in Figure 2-3 for the intercontinental

relay from Asia to western North America require two satellite antennas having

beamwidths which•are essentially the same as those required for the AmericanEur opean relay, plus a third antenna to provide coverage for the Hawaiian Islands.

A beamwidth of 1' degree will yield the Hawaiian coverage without unnecessarily

complicating the attitude stabilisation problem of the satellite, end may be

obtained with an.antenna array having dimensions of about 4 feet by 4 feet by

1 foot in depth. Requiring a 30-db signal-to-noise ratio as above, the transmitter power necessary to provide this additional coverage for the Hawaiian

Islands is found to be about 10 watts. Thus, the total required transmitter

power for the intercontinental relay between North America and Asia becomes

about 260 watts for a bandwidth of 50 megacycles.

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TR-59-0000-00604

Page 11-37

c. Polar Relay System 

The coverage capabilities aho y/aid: Figure 2-7 for low-traffic-density

communication relay service are based upon the utilization of the entire line-ofsight coverage which maybe obtained from satellites in the recommended polar

orbits. The total angular line-of-sight region subtends an.angle of 16 degrees

at the 24-hour satellite so antennas must have 16-degree beamwidths. Since

the diameter of the necessary parabolic dish is only about 5 inches, a size

which does not cause any packaging problems, the high .radiation efficiency of

such dishes dictates-their use for these particular antennas. The attitude

stabilization of the satellite ensures that it. will. always maintain the same

orientation with respect to the earth, so fixed . antennas may be used to obtain

the desired coverage. On the other hand, the ground antennas will be required

to track the relay, satellite. In addition, the mobile stations with which.coznmunications are to be established will include aircraft, so such "ground

antennas" will be limited in their maximum dimensions. A 15- by 1.5- by

13-inch antenna array, whose dimensions seem quite, reasonable for an aircraft installation, will provide an antenna besunwidth of about 3 degrees at

10 lane. Such .an antenna beam should not be too narrow to .permit.• tracking

from aircraft, and will provide an antenna gain of aboUt 33 db.

Thus, we obtain a required transmitter power of 2.2 kilowatts to

provide a 30-db signal-to-noise ratio for the 100-kilocycle bandwidth assumed

in Section 2.3.4 for the low-traffic-density relay of an initial global communication relay system. It should be noted, however, that this required transmitter

power decreases to a value of 220 . watts if the signal-to-noise ratio is decreased

to the 20-db minimum value recommended in Section' 2.3.3.. If higher transmitter powers are available than the 220-watt: value thus obtained for this •

system, the use of the lower signal-to-noieeratio permits a substantial increase

in the bandwidth capability of the system. This possibility of trading signal-tonoise ratio for band3idth capability will be discUssed in .more detail later as a

part of the over-all systems problem of providing the optimum communication

relay capability consistent with the useful payload which can be established in a

satellite orbit.

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TR-59-0000-00604

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d. Equatorial Relay for Low-Traffic-Density Areas 

As in the case of the polar relay satellites, coverage of all areas

within line-of-sight is desired from any equatorial satellite providing lowtraffic-density service to the Pacific and (possibly) Indian Ocean and environs.

Thus, the problem of providing such coverage is identical to that discussed

above, and it may be concluded that the required transmitted power for an

equatorial low-traffic-density relay is essentially equal to that obtained is the

preceding section for polar relays.

2.4 INSTRUMENTATION CONSIDERATIONS

2.4.1 Characteristics of Poslible Amplifiers,

There are three types of amplifiers which operate in the frequency

' range above S-band (about 3 lurid) and which have any appreciable bandwidth,

namely, traveling-wave-tube amplifiers, ldystron amplifiers, and backward-

.

wave amplifiers. The characteristics and availability of these amplifiers are

discussed in some detail•in Appendix C and are summarized below.

2.4.2 Traveling-Wave-Tube Amplifiers 

Traveling-wave-tube amplifiers are inherently broad-band devices,

usually operating over an octave band. Even the so-called narrow-band

traveling-wave tubes, designed for good noise figure and gain characteristics,

have bandwidths of the order • of 10 per cent of the operating frequency. A wide

selection of types of C.W traveling-wave-tube amplifiers is available today in

most power ranges up to ZOO watts, including low-power receiving tubes with

excellent noise figures. Most of these tubes require solenoids to focus their

electron beam, and thus are rather heavy , and.require a substantial amount of

power. However, many periodically focused tubes are now under development

• and by about mid-1959 these tubes should be much lighter.

2.4.3 Klystron Amplifiers 

Klystron amplifiers are basically high-Q, cavity-tuned devices with

bandwidths limited to a few megacycles. Although .a somewhat larger bandwidth can be attained at the cost of a substantially reduded gain, the noise

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TR.-59-0000-00604

Page 11-39

figure of klystron amplifiers is high, so that they c AP/aiti. nrntly be used as

receiving tubes in the low-power ranges. In general, klystron amplifiers are

somewhat lighter than presently. available traveling-wave-tube amplifiers of

comparable output power levels, but the number of these amplifiers from

which.a selectioncan be made is limited. Nevertheless, CW klystron amplifiers

are available with output powers up to 2 kilowatts or so, and, since attaining

2 kilowatts with traveling-wave tubes requires many tubes operated in parallel,

the klystron amplifiers offer attractive possibilities for systems using 2 kilowatts

and whose bandwidths do not exceed klystron capabilities.

2.4.4 Backward-Wave Amplifiers 

A backward-wave amplifier can amplify a somewhat wider band thane.

klystron amplifier and, in addition, has the extremely rapid voltage tuning

ability of a backward-wave oscillator. In general, a given amplifier will operate

over approximately. an octave band, with bandwidths ranging from about 0.1 per

cent at- the lower edge of the band to about 1 per cent at the high edgeof the

band. Since tuning over this band can be effected in the order of microseconds,

this type of amplifier can eliminate the need for a backward-wave local oscillator and mixer combination normallyneeded:When sucltrapidtuning changes are

required in a receiver. 'Unfortunately, however, backward-wave amplifiers

are a very new development in the 'field of traveling-wave structures. As of

the date of this report, only two types of backward-wave amplifiers are commercially available, and although a number of companies are investigating

them, the resultant tubes are mostly in the experimental rather than the later

developmental stages. Therefore, the use of backward-wave amplifiers will

not be considered further in the present feasibility study. However, it is

recommended that these devices be evaluated for possible use when they

become available in a more useful variety of tube types.

2.4.5 Molecular Amplifiers (Masers) 

Recent investigations have .disclosed that electromagnetic signals can

be amplified through interaction with beams of excited molecules. Although

initial experiments have utilised gas molecules, theoretical investigations

indicate that similar effects can be obtained with liquids and even solids, In

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TR-59-0000-00604

Page 11-40

many cases, molecular amplifiers operate at extreme mperatures,

and because noise figures of less than 1 db are obtainable, receivers utilizing

such amplifiers are capable of yielding extremely high sensitivity in comparison

with more standard techniques. Unfortunately, due to the fact that such

molecular amplification utilizes extremely high-Q molecular resonance

phenomena, the bandwidths obtainable with molecular amplifiers providing

reasonable gain have not exceeded about 100 kc or so. Wider bandwidths should.

be possible in molecular amplifiers utilizing solids, but even. so, the obtainable

bandwidths are not expected to exceed a few megacycles.

Because of the limited bandwidth, molecular amplifiers are primarily

of interest here for possible application in low-density relay systems. A

second possibility exists that the use of such amplifiers could reduce power

requirements sufficiently to render practical a high-density system composed

of a multiplicity of low-power, relatively narrow-band relay links• operatingin

parallel. There is, of course, no necessity for carrying the molecular amplifier in the satellite since the. "ground" transmitters are capable of transmitting

relatively high power. They would, however, find their principal use in the

"ground" receiver which would permit the use of smaller-powered transmitters

in the.satellite. However, even for ground use, the development of molecular

amplifiers is still in the early, predominantly laboratory stages. Furthermore,

the size and weight of a molecular amplifier, with its associated cooling equip-

. ment, renders such devices impractical at the present time for the aircraft

receivers which are used in the low-density communications system. Because

of these two reasons, molecular amplifiers have not been considered further in

this report for possible application in an initial global communication relay

system which would become operational in a time period of 4 to 5 years. However, because of the importance attached to the possibility of reducing power

requirements in the satellite-borne communication relay system, it is strongly

recommended that the development of molecular amplifiers be encouraged to

the maximum possible extent and that the possible application of suchamplifiers

in systems considered in this report be re-evaluated at the earliest time when

such devices can be considered as available for use in a practical operational

system.

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TR-59-0000-00604 Page 11-41

2.4.6 Summary

Traveling-wave tubes are the only amplifiers available which can provide

wide bandwidths (of the order of some tens of megacycles or more). Furthermore, the dbtainable traveling-wave receiving tubes have such low-noi s e figures

in .comparison with klystron amplifiers that their use as systeminput amplifiers

is necessary regardless of the system bandwidth. Klystron amplifiers can be

used only as the final power amplifier in a narrow-band system, and the alKailable tubes from which a selection can be made is sharply limited. Hence, only

traveling-wave-tube amplifiers can be used in the high-density relay, and must

be used also as the low-power amplifiers in the low-density relays. The choice

between traveling-wave-tube or klystron amplifiers as a final power amplifier

for the low-density relay will be dictated by tube availability and the desire to

minimize total system power requirements.

2.5 POSSIBLE SATELLITE-BORNE RELAY SYSTEMS

2,5.1 Block Diagram of ZI -Relay System

A functional block diagram of a possible satellite-borne communication

relay, system for ZI relay service is shown in Figure 2-8. Signals are received

on.a plane-polarized antenna and passed through a low-pass filter to the input

of the .first traveling-wave-tube amplifier. The low-pass filter is designed to

eliminate feedback of signals, retransmitted by the relay, which will be at a

higher frequency than any of the received signals. The first traveling-wavetube amplifier provides a gain of about 50 db and, operating well below

saturation, determines the over-all noise figure of the airborne system. The

amplified output of the first traveling-wave-tube amplifier is mixed with a

variable frequency local-oscillator signal, and the resulting output is filtered

in a single ,sideband filter to remove the unwanted frequency components resulting

from the mixing process.

The output signal from the single-side band filter is amplified in the

final two traveling-wave-tube amplifiers, and is then retransmitted from a

plane-polarized transmitting antenna whose plane of polarization is orthogonal

to that of the receiving antenna. This use of cross-polarized antennas for

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Page 11-42 •

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Relay System for Z1 Service.

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Page 11-43

reception and transmission is suggested to decrease the amount of filtering .

required to obtain the necessary isolation between received and transrintted

signals. The input power to the system will be at a level of about -40 dbm,

and since the transmitted power level is about 250 watts, or +54 dbm, some

110 db or so of isolation is necessary if any input to the first traveling-wavetube amplifier at the retransmission frequency is to be substantially below the

the level of the desired signal. (This extremely high isolation would not be

needed for a system in which jannxdng need not be considered, but is required

if the power needed to jam the relay by merely overloading the input amplifier

is to be maximized.) The use of cross-polarized.antennas could provide an

initial isolation of from 20 to 30 db, thereby substantially reducing the filtering

problem which otherwise might be present.

The input signal level of -40 dbm quoted above is 50 db above the noise

in .a 50-megacycle bandwidth, assuming a 7-db noise figure for the • systemu

The fact that. this level is 20 db above that necessary to provide the 30-db

signal-to-noise ratio required of the system permits the elimination of the

first traveling-wave-tube amplifier from the system, since the 20-db gain

which may be expected of such low-level receiving tubes at X-band is thus no

long required. In addition, the use of these higher powers in the system require

the use of correspondingly higher powers by a jammer. Furthermore, with

this input level a traveling-wave-tube amplifier with•a 27-db noise figure will

provide the same system operation as would a low-level tube, having a 7-db

noise figure, with an input level 20 db lower. Of course, the higher input

level requires the use of an additional 20 db of power in the ground-based system, but peak power limitations • are not as severe ..on the ground as in the

satellite-borne system. •

Table 2-1 lists the. pertinent characteristics of traveling-wave-tube

amplifiers which seem suitable for use in the satellite-borne system, as well

as the estimated characteristics of other components required for the relay.

Also given are estimates of the weights and power requirements of periodically

focused traveling-wave-tube amplifiers which would be comparable to those

listed. In cases where exact valuei are not available, an edbeated gtieiS has

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indicated in parentheses. These results indicate quite graphically the reduction

in weight and power requirements whichmay be effected when periodically

focused traveling-wave tubes become available for use, since the estimated

weight and power requirement totals with.sUch tubes are some 60 pounds less

and almost 1000 watts less than would be required with tubes available today.

2.5.2 Block Diagram of Intercontinental Relay System 

A functional block.diagram of a possible satellite?-borne communication

relay system for intercontinental relay service is shown in Figure 2-9. This

system is identical in its basic operation to that described in the above paragraph for ZI relay service, the-only modifications being those required to

permit the use • of two antennas (three, if coverage of Hawaii is provided). In

this case the receiving antennas feed two identical low-pass filters and the

outputs of these filters are then combined in a directional coupler before being

applied to the input of the first traveling-wave-tube amplifier. The output of

the final traveling-wave-tube. amplifier is fed in parallel to two identical transmitting antennas. Since this will cause a decrease of 3 db in the signal-to-noise

ratio of the signal received' on the ground, the desired 30db 84(nel-to-noise ratio

may be obtained by increasing the transmitted power from the value of about

125 watts .calculated in.Section2.3.6b for a total bandwidth of 50 megacycles to

a value of about Z50 watts. Thia latter figureisthe same.as the 250-watt power

requirement for the ZI relay as calculated in Section 2.3.6a.

Thus, the estimated weight and power requirements for the intercontinental communication relay will be the same as those given above for the ZI

relay with the exception of the additional weight necessary for a second antenna

and its feeds,,a second low-pass filter, a,nd a diredtional coupler.. The total weight

of these components is estimated to be about 7 pounds so the total estimated

weight and power requirements for the intercontinental communication relay

become about 1:60 pounds and 2720 watts, respectively, Using present-daY

tubes, and 100 pounds and 1760 watts if periodicallyfocused tubes are available. .

2.5.3 Block Diagram of Low-Density Relay System 

The functional block diagram' of the satellite-borne system designed to

provide polar and ocean-area communication relay service is identical to that

shown in Figure 2-8 for the ZI communication relay with the exception that the

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Page 11-47

.€-ONFIDENTim.

Output traveling-wave tube is replaced by a klystron amplifier. For the

low-density relay system, the 200-watt Sperry SAS-28 (weighing just over

5 pounds and requiring a total input power of about 1550 watts) or its equivalent is

suggested, since the site of the antenna required for the desired coverage does

not preclude operation at S-band frequencies. Furthermore, the 30-db gain of

this tube would permit a tube ouches the 500-milliwatt Sperry STS-75 traveling-

. wave tube to be used for both of the remaining amplifier stages, resulting in

weight and power savings of about 5 pounds and either 310 or 70 watts, respee-

.

tivelyroyer the totals giiten in Table 2-1 for the 'first two travelingrwave tubes.

Thus, the total estimated weight and power requirements for the low-traffic-

.

density relay system become about 103 pounds and 2045 watts, respectively,

using present-day tubes, and about 73 pounds and 1725 watts if periodically

:focused tubes are available.

. 2.5.4 Reliability and Lifetime Considerations 

Since the global communication relay system discussed in the preceding

sections must operate unattended, the reliability which.may,be attributed to such

operation is an important factor in determining the feasibility of the over-all

system. For this reason, the simplest possible relay systems have been

described above, and the number of components comprising such a system

should ' be held to an absolute minimum. Because the traveling-wave-tube

amplifiers suggested for use in this system are a relatively new type of tube, •

data regarding expected tube lifetimes have not been available in sufficient

quantities to permit reliable statistical evaluation. However, with careful

design and construction techniques there is no reason why the life of such

tubes should not equal or exceed that which maybe expected from more common

types of electron tubes. Recent measurements have indicated that, at least for

lower powered tubes, useful lifetimes well in . excess of 500Q hours are not

unusual. It is expected that similar lifetimes may be obtained with high-power

traveling-wave-tube and klystron amplifiers, particularly by the time this

system would become operational. Furthermore, system lifetime could be

substantially prolonged through the incorporation of spares, with fail-safe

detection circuitry to detect unit failure and switch a . replacement into the

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Page 11-48

circuit. Hence, it is expected that the useful lifetime of the electronic portion

of the global communication relay system should be at least 6 months or so.

Furthermore, as design techniques improve, this expected system lifetime may

well exceed a year or more in later models of the system.

2.6 HIGHER CAPABILITY GLOBAL COMMUNICATION RELAY SYSTEMS

2.6.1 Requirements for Communication Relay Systems of Higher Capabilities

The communication relay systems described in the foregoing sections

were designed to provide an initial global communication relay capability. As

such, the estimated weight and power requirements obtained above represent

more or less of a lower bound for satellite-borne global communication relay

systems. However, if the various techniques described in Volume 2 of this

report for providing the ascent vehicle with an increased payload capability

prove successful, the additional weight and power which may be allocated to

the relay system itself would permit a substantial increase in the system

capabilities. The following paragraphs describe briefly the progression in

which system capabilities might be.increased, together with an estimate of

the necessary incremental weight and power requirements. (The results of

this section are summarized, for convenience in Table Z-2.)

The first step to be taken, if an increased satellite payload should be

available, would be to increase the over-all system reliability. This would

be accomplished by providing a spare for each unit of the system, together

with circuits to sense unit failures and switch the proper spare into the

circuit. In this manner, a sort of elementary fail-safe operation is added to

the system, and its useful life could be substantially prolonged. For the

traffic-density systems, the estimated weight and power requirements for

this additional reliability would be about 180pounds and 10 watts, respectively,

using present-day tubes, and 120 pounds and 10 watts if periodically focused

tubes are available. For the low-traffic-density systems, these estimated.

additional weight and power requirements become about 130 pounds and

10 watts using present-day tubes, and about 100 pounds and 10 watts using

periodically focused tubes.

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The capability of the high-density satellites could be further increased

by adding a low-density coverage system. This would eliminate the necessity

for a separate satellite to provide low-density coverage of the Pacific Ocean

areas. Including spares and fail-safe circuitry, the additional increase in

weight and power requirements for such coverage would be 233 pounds and

2055 watts using present-day tubes,and173pounds and 1735 watts if periodically focused traveling-wave tubes are available. The capability of the highdensity satellites could then be further increased by doubling the capacity of

the low-density equipment. This could be accomplished by replacing the

SAS-28 output amplifier by the 500-watt Sperry. SAC-33 klystron amplifier,

with the necessary drive power being supplied from a tube such as the Geisler.

G210 or Sylvania TW621. The input traveling-wave-tube amplifier would be

replaced by a C-band tube such as the Sperry STC-67. Again including spares

and fail-safe circuits, the estimated additional weight and power requirements

for this increased capacity would be about 76 pounds and 947 watts using presentday tubes, and 56 pounds and 813 watts if periodically focused tubes can be

used.

The next increase in the capability. of the low-density systems could be

to double* their bandwidth capabilities as described above. As before, the

estimated increased in weight and power requirements for this system, including

spares for all units and the necessary failure detection and switching circuits, is

76 pounds and 947 watts, respectively, if present-day tubes are used, and

56 pounds and 813 watts if periodically focused tubes are available.

Finally, the capacity of the low-density system may be increased to a

value eight times that of the original system by using the Varian VA-806 klystron

amplifier as an output tube. The high gain of this tube permits the use of a

Sperry STX-76 traveling-wave-tube amplifier for both the first two amplifier stages. Assuming the provision of spares for each unit of the system, and

fail-safe circuitry, as before, the increased capability of this system over the

double capacity low-density system described above requires additional weight

and power of 74 pounds and 3263 watts if present-day tubes are used, and

74 pounds and 3397 watts using periodically focused tubes.

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The results given above illustrate clearly the large increases in system

capabilities which can be obtained if sufficient satellite payload capabilities can

be provided. Of course, further increases in system capabilities could be

achieved using additional and/or higher poilered components, in the manner

used to obtain the above results, with consequent increases in weightfandpower

requirements of a magnitude comparable to those shown in Table 2-2. However,

it is probable that a limit exists beyond which point any additional payload capa-

.bilities which could be provided would be utilised for other purposes. A deter-

. mination.of this "point of diminishing returns," so to speak, for communication

relay. systems is far beyond the scope of the feasibility study reported herein,

and no attempt will be made even to estimate such a -"maximum required -

capability.,

2.7 INTERIM COMMUNICATION SYSTEM FOR A 24-HOUR SATELLITE

2.7.1 Reasons for and Types of Interim Communication Systems 

• Achieving the initial global communication systein described in the

preceding sections would require an•appreciable developmental Program even •

if it were developed gradually. Standard modulatiOnand.multip/exing techniqties

would have to be modified to meet the requirements of this system, and new

techniques would be necessary in certain areas. For example, power require-

. meats are such that a nuclear power supply would be desirable, which .would

necessitate.considerable development in a new technology and .so on, Thus, a

realistic -estimate of the time required for developing an initial global communication system is around 4 or 5 years. However, other communication

requirements exist.today which can be satisfied by a satellite-borne relay

system. Hence, „ale reasonable to investigate the feasibility of various interim

communication relay systems which, although not possessing the full capabi- ,

lities of the initial global system described above, nevertheless. represent a

eubatantial improiement over presently available communication techniques.

If an initial global system is scheduled for completion in 4 or 5 years,

the development of an interim system might be accomplished in 2 years. Thus,

maximum .possible utilization 'should be Made *of existing techniques

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TR-59-0000-00604

Page II- 52

and equipment, particularly for any ground equipment . where multiplexing may

be necessary. Furthermore, the time required for developing and establishing

such .a system in orbit will be strongly affected by the power requirements of

the satellite-borne equipment, since these will determine whether or not a

nuclear power supply (and the attendant refinement of a new technology) is

necessary. Hence, an interim communication relay . system should also have

substantially lower power requirementS (akd if possible should weigh

appreciably less) than the initial global communication system. Moreover, if

weights can be kept down, particularly the significant weight of the power

supply, then somewhat less refined rocketry may be used for establishing

the orbit and more weight can be allotted to the guidance system used to

establish. the interim satellite in orbit. Two general classes of systems

which meet these requirements have been investigated: (1) those systemi

providing point-to-point or, at most, restricted-area-to-restricted-area

relay of multiplexed signals having a total information bandwidth of no more

than a few megacycles; and (2) those systems providing relay service between

any two points within a large area with a bandwidth of at most a few kilocycles.

For point-to-point systems, FM transmission of frequency-divisionmultiplexed signals seems the most likely technique which would permit

development of the necessary equitanent within approximately t years. Frequency

• modulation tends to reduce any effects of system nonlinearitiess, and use can be

made of presently available frequency-multiplex equipment (except possibly the

equipment necessary to multiplex a number of subcha.nnel signals into the final

wide-band signal). Accordingly, three such systems have been'investigated

which provide a reasonable range of capability and from which the requirements

for this class of systems as a whole may be estimated. The systems investigated include: (1) point-to-point simplex (one way at a time, but reversible)

transmission of FM-multiplexed signals with.a nominal information bandwidth

of 5 megacycles;

*

 (2) point-to-point duplex (simultaneous two-way) transmission

It should be noted that simplex transmission of multiplexed signals is far:

from satisfactory, particularly for voice and teletype signals, since all baseband signals must be interrupted whenever the direction of transmission is

reversed. Consequently, elaborate storage is required to obtain even assem-

. blance of normal duplex operation. Simplex systems are investigated here

primarily to provide a comparison to the requirements for bill duplex operation.

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of FM-multiplexed signals of the same nominal bandwidth; and '(3) restricted

area-to-restricted are simplex transmission with a nominal total bandwidth

of 2.5 megacycles.

The second class of interim systems, those providing service between

points within a large area, should permit communication between mobile

stations, including aircraft. Therefore, relaying voice transmissions is

desirable, although a high-speed teletype capability might be quite acceptable.

However, because of the large number of potential user s of such a relay service,

and because of the limited time available for system development, the 'use of

existing equipment, instead of the manufacture of new equipment, becomes even

more mandatory in this case than in the case of the point-to-point relays.. For

this reason, the one possibility investigated of this second class of systems

utilizes transmission in the 200- to 400-megacycle UHF•communication band,

with.a bandwidth nominally chosen at 3 ldlocycles. Dipole antennas are chosen

to eliminate the necessity for antenna tracking, with the result that the UHF

equipment now existing for 'aircraft communications may be used in its entirety,

and new development is necessary only for the satellite-borne equipment.

It should be noted that each of the systems chosen for investigation

satisfies, to a greater or lesser extent, an existing communication requirement

which is not fulfilled using present techniques. Nevertheless, a useful generality

has been retained in that the particular points to which zommunication is to be

established, and types of transmissions themselves, have not been specified.

For example, a point-to-point relay could be established • viithin the continental

limits of the United States. , or perhaps from the United States to Europe to Asia,

and so on,• and a similar flexibility exists with the UHF voice relay. The

frequency-multiplexed signal may consist of a large number of voice signals or,

with .suitable restrictions, 'one or two video signals may be substituted. Similarly, the UHF voice relay might be utilized instead for a.number of multiplexed

teletype signals if such a capability were desired. Thus, a flexibility of appli-

.

cation . exists whith.Perrhits . conZhisions regaiding the power and 'weight

requirements (and, consequently, feasibility) for a large number of possible

interim systems to be derived from the results of this investigation.

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.TR-59-0000-00604

Page 11-54

2.7.2. Transmitter Power Requirements

a. FM Multiplex Systems 

The transmitter power requirements for the interim FM multiplex

systems may. be established most easily by deterzning the modifications in

coverage, bandwidth capability, and system signal-to-noise ratio (and resultant

power requirement changes) :Which must be made in the appropriate relay of

the initial global communication system. The reasons for choosing 10 kmc as

an operating frequency for the initial global communication relays apply also

to the interim relay systems, so the system noise levels used previously for

operation in this frequency range apply as well.

It will be assumed that the FM multiplex systems will utilize a

deviation ratio of five, where the deviation ratio may be defined for a sinewave modulating signal as the ratio of the frequency deviation at the peak of

the input signal to the frequency of the input signal. For example, if the

intelligence to be transmitted consists' of'a sine wave with a frequency of

5 megacycles, the carrier signal will swing 25 megacycles on each side of

its center frequency for a deviation ratio of five. But operation with a frequencymodulated system requires a certain threshold signal-to-noise ratio if the final

output signal-to-noise ratio is not to be degraded. This threshold signal-to-noise

ratio, measured at the wideband input to the discriminator, is a function of the

deviation ratio, and about 13 db is found to be quite adequate for the assumed

deviation ratio of five. Thus, since the transmission bandwidth of such an FM

signal is some ten times the information bandwidth, provision of a transmissionband signal-to-noise ratio of 13 db requires an equivalent information7band

threshold signal-to-noise ratio of 23 db. (This equivalent information-band

signal-to-noise ratio is necessary since all previous calculations of required

power have been on the basis of information bandwidth.) It should be noted that

operation at threshold is quite satisfactory with multiplexed speech signals,

since the nominal 4-kilocycle base band for this case yields very good final

signal-to-noise ratios when the FM signal detected under these threshold conditions is demultiplexed. However, the base band for a signal such as a video

signal is of the same order of magnitude as the 5-megacycle information

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TR-59-0000-00604

Page 11-55

bandwidth assumed above, and a sufficiently high final signal-to-noise ratio

will not be attained unless detection of the FM signal occurs at several db

above threshold to obtain additional FM noise-suppression gain.

Additional modifications necessary for the point-to-point systems

include a decrease in the bandwidth of the satellite-borne antennas to about

1 , degree since wider coverage than this is not required and the necessary

(4-foot-square) antenna arrays can be packaged to meet the size limitations

imposed upon the system while being launched into orbit. Also, the diameter

of the ground antenna assumed for the high-density relays in the initial global

system should be decreased to about 30 feet for an interim system, since this

will somewhat simplify the problem of meeting the dimensional tolerances

necessary for operation at 10 kmc. Finally, the duplex system will be derated

by 6 db instead of the 3 db which would normally be expected in an attempt

to compensate for (unknown) effects of system nonlinearities.

The adjustnients in the 250-watt power requirement, obtained in

Section 2.3.6b for the intercontinental relay of the initial global system, are

summarised in Table 2-3 for each of the FM multiplex systems. It will be

noted that a bandwidth of only Z.5 megacycles is used for the last case, namely,

the restricted-area-to-restricted-area simplex system. This reduction is

necessitated by the fact that no klystron or traveling-wave-tube amplifier is

presently available or in late developmental stages with precisely the saturation power output required for a 5-megacycle operation. The reduction to

2.5 megacycles permits the use of a tube at its maximum capability, and thus

provides a more efficient system.

b. UHF Voice System 

The required transmitter power for the UHF voice system can be

calculated as easily from basic transmission relationships (see Equation (2.2),

Section 2.•.1) as by modification of the required powers calculated for the lowdensity relay in the interim global communication system. Assuming operation

at 300 megacycles, dipole receiving and transmitting antennas with high

efficiencies, and a receiver noise figure of about 5 db, the transmitter power

required to attain a 15-db signal-to-noise ratio in a 3-kilocycle voice band is

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TR-59-0000-00604

Page 11-56

+23

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Table 2-3. Power Requirement for FM Multiplex Systems.

Power • Required

Description Reduction Power

. System of Modification (db) (watts)

Decrease signal-to-noise

ratio for FM operation

at threshold • +

Decrease total bandwidth

to 5 mc +10

Decrease ground

antenna diameter to 30 ft - 6

Decrease beamwidth of

satellite antennas to 1 deg +12

Total

De cr ease signal-to -noise

ratio for FM operation at

threshold

Decrease total bandwidth

to 5 MC (2.5 me each way) +10

Decrease ground antenna

diameter to 30 ft • - 6

Decrease beamwidth of

satellite antennas to 1 deg +12

Derate system 6 db for

duplexing - 6

Total +17

Decrease signal-to-noise

for FM operation at

threshold + 7

Decrease total bandwidth

to 2.5 mc +13

Decrease ground antenna

diameter to 30 ft - 6

Total +14

FM Multiplex

point-to-point,

simplex transmission

FM Multiplex pointto-point, duplex

transmission

FM Multiplex,

restricted area-torestricted area,

simplex transmission

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TR-59-0000-00604

Page 11-57

found to be about 135 watts. Assuming AM voice transmission, this trans-.

initter power is an average value which will yield the 15-db minimum average

signal-to-noise ratio necessary for reliable reception of voice information.

2.7.3 Detailed Description of Possible Systems 

A block.diagram of the FM multiplex systems would be identical in all

essential factors with the block diagram of the initial global communication

system's intercontinental relay shown in Figure 2-9. Similarly, the operation

will be identical with that described in.Section 2.5.1. As in the intercontinental

relay of the initial global system, traveling-wave-tube amplifiers would probably be used for all amplifies stages in the FM multiplex systems. Inthiis case,

however, TWT amplifiers are used, not to provide the necessary transmission

bandwidth but because a greater selection of TWT's is available than klystron

amplifiers (see Figures C-1 and C-2), and the ability to choose a final output

tube which most closely fits the system requirements permits attaining maid-

.

mum over-all system efficiency and minimum total power requirements.

The weight and power requirements for the various components of the

FM multiplex systems are summarised in Table 2-4. It will be noted that

periodically focused tubes are available tdday which can completely satisfy

the amplifier requirements of the point-to-point simplex system, so the

estimated total weight and power requirements fbr this system become about

80 pounds and about 174 watts, independent of any future tube developments.

Furthermore, periodicallyfocused tubes are available for the low-power

amplifiers in the other two systems, so only a periodically focused tube with

characteristids similar to those of the Sperry STX-77 need be developed* to

permit utilization of periodically focused tubes for all amplifiers in all three

FM multiplex systems. The total weight and power requirements for both

the point-to-point duplex and restrictid-area-to-restricted-area simplex

systems are estimated to be 101 pounds and 610 watts, respectively, using

presently available tubes, and 86 pounds and 210 watts if all tubes are

periodically focused.

*

Such a tube could probably be developed and produced in the small quantities

required for this system within 2 years of the initiation of such development.

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Page 11-59

the high- and low-density relays for the initial g obal communi- kAs with 

cation system, the desirability of maximizing the reliability of these interim

systems indicates that spare components should be provided throughout the

entire system together with fail-safe detection.and switching circuits. The

additional weight required by this procedure is estimated at about 85 pounds

for the point-to-point simplex system and either 105 or about 90 pounds for

the other two systems, depending upon whether periodically focused tubes are

available. In all three cases, increasing reliability by the above technique

should not add more than.about 10 watts to the total estimated power requirements. For purposes of convenience, these results are summarized in

Table 2-5 along with estimated- weight and power requirements for the UHF

voice relay discussed in the following paragraph.

A block diagram of the UHF voice relay would be identical in all essen-

. dal features with that of the ZIrelityfdr-theinitial global comMuidcaliorisysterns

shown in Figure 2-8. In this case, however, the fact that the operating frequency

is in the UHF communication band permits the use of more commonly known

amplifier tubes and design techniques, including the use of a tetrode power

amplifier such as the 4X250 as a highly reliable output tube. The estimated

weights and power requirements for the UHF voice •system are summarized

in Table 2-6, indicating that this system should not weigh more than about

20 pounds and would require a total power of about 258 watts. If spares are

supplied for all electronic components and fail-safe detection circuits are

added, these estimated weight and power requirements increase by about

33 pounds and 10 watts, respectively. These results are summarized also

in Table 2-5 for comparison with the FM multiplex systems.

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2.10 ALTERNATIVE APPROACHES TO GLOBAL COMMUNICATION

SYSTEMS

The detailed parameters and resultant configurations of the satellite

borne communication systems recommended here depend rather markedly upon

the requirements assumed for the system and upon the equipmeneassu.med to be

available. If any of these requirements should be changed or if other equipment

becomes available, the resulting system could very easily differ in both coarse

as well as fine detail • from that described above. For example, if it is assumed

that a solid-state molecular amplifier will be available for use in the ground

equipment, providing a decrease in effective receiver noise level of 17 to 20 db;

that a signal-to-noise ratio of 20 db instead of 30 db may be sufficient; and that

a bandwidth of 5 me instead of 50 inc would be adequate, these assumptions

would permit the postulation of a system..in which a spinning satellite, stabilized

only with respect to the plane of its orbit, would require no azimuthal transmission directionality (in its orbital plane), and would need only a 16-degree

vertical bandwidth to ensure complete coverage of the earth. The system gains

obtained by the above revised assumptions thus permit the elimination of .

accurate attitude stabilization and still provide sufficient excess gain to allow

decreasing transmitter power requirements to as little as 2 watts, a value

which could possibly yield longer system lifetimes and which would definitely

reduce system weights.

However, as usual, a gain in one respect is not obtained without

increasing losses in others. As discussed in the preceding section, the provision of a jam-resistance capability for high-density relays (which may be

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TRT59-b000-00604

• Page 11-66

considered to include this modified case) would pro • bly necessitate the use

of highly directional antennas to prevent jamming transmitters from falling

within the•main beam. The modified system would be extremely subject to

jamming of the satellite receiver . by a ground jammer, since it has been

postulated that the entire earth will.fall within the main beam of the satellite

antenna. A similar problem exists• with the ground . receiver; lowering the

level of the received signal, by using a molecular amplifier, and decreasing

signal-to-noise ratio, both tend to render the system susceptible to jamming

through antenna side lobes from transmitters in such sources as submarines,

balloons, low-altitude satellites, and so on. Hence, this discussion points

up the fact that, while such moclifications in system parameters to decrease

required transmitter power might be quite desirable for a system intended

for peacetime communication applications, the high reliability (and C01116-•

quent high jam .resistance capability) which is of utmost importance in a

military system almost invariably leads to the conclusion that the highest

possible power level should be used for transmission in order to maximize

the problems facing an enemy jammer.

In addition, the modifications described above would result in a system

of substantially reduced capabilities when compared with the global system

described in the foregoing sections. It may be argued, and quite rightly, that

the reductions in bandwidths and signal-to-noise ratios postulated above still

provide capabilities far in excess of those available at the present time. Nevertheless, it is felt that the importance placed in this -report upon providing high

quality service with bandwidth capabilities is by no means over-emphasized..

All communications experience tends to indicate that the bandwidth capabilities provided by any new system are almost invariably saturated shortly after,

if not even before, the system becomes operational. Similarly, the statements

that a 20-db signal-to-noise ratio is adequate, and that a 30-db signal-to-noise

ratio will provide good quality service, should not be taken to imply that higher

signal-to-noise ratios may not be desired. Signal-to-noise ratios of 40-, 50-,

and even 60-db values obtained on wireline circuits, are by no means excessive

in terms of the quality of communication obtained. Therefore. degradation of

system performance by reducing signal-to-noise ratios from say, 30 to 10 db

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Page 11-67

should not be undertaken without substantial reason. Therefore, it is our

feeling that the general characteristics of the system described before represent an excellent but reasonable system chosen from competing alternatives.

Moreover, since characteristics indicate the present feasibility of a high

quality system, any lowering of the quality of the system would, a fortiori,

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CHAPTER 5

INFRARED EARLY WARNING METHODS FROM 24-HOUR SATELLITES

A brief study of the feasibility of detecting ballistic missiles from the

24-hour satellites using infrared, methods was made. However, because the

study is not sufficiently complete to be included here, it will be issued at a

later date, either in the final version of this volume or•under a separate

cover.

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TR.-59-0000-00604

Page 11-99

APPENDIX A

ANTENNA DESIGN CONSIDERATIONS FOR 24-HOUR SATELLITES

Designing antennas for systems carried in 24-hour satellites requires

careful consideration of a number of closely interrelated factors. Certain

WL over which the antennas must operate, the polarization

of the signals to be received, and the magnitude of the side and back lobes

relative to the main antenna beam. More fundamental considerations,

however, arise from the unique physical surroundings of a satellite, both

during ascent and while in its orbit, which affect its size, weight, .and

over-all construction.

Probably the most obvious restriction placed upon a satellite antenna

is the requirement for minimum weight imposed by the limited payload

capacity of the ascent vehicle. However, since all elements of the satellite

are in essentially a free-fall or weightless condition once the satellite is

established in its orbit, the force• exerted on an antenna under such conditions

are practically zero. Thus, extremely lightweight materials may be used for

its construction if proper packaging techniques are used for protection from

the forces present during the initial ascent. This absence of weight for a

body in free fall introduces new concepts into the design of an airborne

antenna" with respect to size as well as weight, since it now becomes possible.

to consider the use of an antenna of almost unlimited size pioviding, once

again, that it can be packaged to Conform to the dimensional and environmental requirements imposed upon the ascent vehicle. It must be remembered,

however, that large dimensions may be highly detrimental if tracking is

required of the satellite antenna, since an antenna of appreciable size may

have a high moment of inertia, and thus require a higL torque for its rotation,

even though its over-all weight may be quite small. Nevertheless, within

reasonable limits, the size and weight limitations on a satellite aultent►a will

be determined, in the final analysis, by the effectiveness and ingenuity

displayed in packaging the antenna for ascent.

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TR-59-0000-00604

Page 11-100

E 1

The final factor which must be considered in the design of a satellite

antenna is its susceptibility to damage by meteoric impact. The impact

probability on an antenna of any reasonable size by a meteorite capable of

penetrating, say, a millimeter of aluminum is small but not negligible, and

the reliability required of a satellite-borne system dictates careful consideration of this factor.

The design problem for a satellite antenna ma

fe. Ar,./.4,Wee,e

ssary coverage at the operating frequency, although if

tracking antennas are required the size of the aperture thus defined may

become important. Therefore, when these considerations are added to the

tact that freedom of choice of operating frequency exists only in the case of

the relay system, it may be concluded that antenna aperture limitations will

have their most serious effects in reconnaissance and Jamming applications.

The aperture provided by the satellite antennas may be fed by a horn,

by a parabolic segment reflector with.driver arrangement, or by a linear

array of elements. The last arrangement is the most promising of the three

when a requirement for a large aperture antenna precludes the use of the

relatively simple parabolid reflector. However, the wide bandwidths

required for most of the applications of interest in this investigation sharply

restrict the class of antennas which may be used as the elements of an array.

A number of types of feed elements have been investigated during the course

of this study, several of which have properties worthy of discussion here and

which are described further in the following paragraphs.

A number of types of feed elements may be formed by a conductive

coating plated on a flexible dielectric backing, and could be fed in phase by

a Christmas-tree feed structure, also plated on the dielectric backing to

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TR-59-0000-00604

Page I1-101

form the desired antenna array. Because of the bandwidth requirements,

and also because of packaging difficulties, no cavity backing would be provided for the antennas and consequently their beam would be bidirectional.

An array of such antennas mounted upon a thin flexible backing could be

packaged during vehicle take-off and ascent in a form similar to a rolled-up

window shade. When the satellite is established in its orbit,_ compressed

- - - - .

gas, spring tension, or some other similar force would unroll the antenna

array to its full extent with telescoping tubes or some similar mechanism

being used to guide this process. Since . the length of the final satellite

vehicle will probably be from 8 to 10 feet, It is quite conceivable that an

antenna array packaged in the ibove manner could provide a final antenna

aperture some 8 feet square.

Another possible feed element is the dielectric rod or polyrod antenna.

Such an antenna is essentially a traveling-wave structure which radiates

appreciable energy in only one direction, so an antenna array composed of

polyrod radiators would not have the rear lobe present with the plated

element arrays referred to above. The polyrod array would be fed by a

strip-line feed structure incorporated in the base plate. If the polyrods

are to be efficient radiators, however, they must be of appreciable length

in terms of wavelengths at the operating frequency of interest. For this

reason, a relatively rigid base plate would be. required and the maximum

aperture which would be provided would be about one-fourth (4 feet square)

that which seems.possible for a plated-element antenna array. However,

this disadvantage is compensated in part by the fact that a polyrod antenna

array has no rear lobe, as well as by the fact that the relatively limited

beam of the polyrod antennas tends to minimize any side lobes of the array.

Once having chosen a basic feed element, the efficiency of an antenna

array is determined by the number ;of elements constituting the array since

this will determine the over-all mismatch, feed line losses, and similar

factors which establish the final antenna efficiency. Experience has shown

that 16 elements are about the maximum number which can be fed with

reasonable efficiency, and that a net antenna efficiency of about 50 per cent •

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R-59-0000-00604

Page II-102

may be obtained using this number of elements. It should be noted that the

limitation on the number of elements constituting an array results in an

increase in the spacing of these elements, in units of wavelength, as the

operating frequency of an array is. increased. This causes a group pattern

to be generated with side lobes, of the same amplitude as the main beam,

spaced at angles determined by the relation dx sin 9 = n X. Since dx, the

element spacing in wavelengths, does not exceed 4 X for any array considered below, the first such side lobe would occur some 14 degrees off

the main-lobe axis. For a reconnaissance antenna, the fact that the earth

subtends an angle of only 16 degrees at the satellite precludes this side lobe

from intercepting extraneous signals, and the only effect would be a reduction

in over-all gain and (possibly) introduction of extraneous solar noise. In

addition, if a polyrod array is considered, the fact that the unit pattern of

the polyrods are relatively narrow, ranging from 30 degrees fOr a 6X rod

to 22 degrees for a length of 10 X (see Figure A-1), will cause suppression

of the side lobes to quite reasonable amplitudes.

It should be noted that the arrays described above are almost completely

insensitive to any effects of meteoric impact. The only possible event which

could significantly alter the operation of the arrays would be the complete

severance of a portion of the feed structure, and the probability of such an

occurrence is so small as to be'completely negligible.

The approximate beamwidth of an'array of linear elements is shown in

Figure A-2 as a function of the aperture dimension for parametric operating

frequencies. The gain and effective aperture of any given antenna may be

calculated from the results of this figure by standard formulas, proper

account being taken of antenna efficiency and whether or not the rear lobe

of the antenna pattern has been. suppressed. The parameters of an 8-footsquare array of plated element antennas without cavity backing, calculated

in this manner, are given in Table A-1. The parameters of arrays wif.h

dimensions Smaller than 8 feet square may be scaled directly from the results

given in this table. Theoretically, the results for a polyrod array may be

obtained also from Table A-1 merely by increasing gains by 3 db and effective

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TR-59-0000-00604

'Page 11-105

apertures by a factor of two. However, the dimensions of a polyrod antenna

become cumbersome at frequencies below S-band or so, and for this reason

such antennas are not considered below this point.

Table A-1. Parameters of 8-Foot-Square Array, No Cavity Backing.

Frequency

(vac)

Na. of

Elements

Electrical

Spacing

Beamwidth

(degrees)

Assumed Eft' Factor

(db)

Gain*

(db)

Effective

Aperture

(m2)

75 1 -- 72 -1.5 3.5 2:70

150 4 X/4 36 -3 8 1.83

300 4 X/2 18 -3 14 1.83

625 16 X/2 9 -3 20 1.83

1250 16 X 4.5 -3 26 • 1.83

2500 16 2X 2.25 -3 32 1.83

5000 16 4X 1.12 -3 38 1.83

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efficiency factor.

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TR-59-0000-00604

Page II-106

APPENDIX B

NOISE CONSIDERATIONS

Regardless of whether the system considered in this report is used for

communication relay purposes or for the interception of enemy communications,

the minimum signal which can be received with a given reliability will be determined by the noise level of the system. The noise existing in a radio system

arises from several sources, with the contribution from each source being a

function of the operating frequency, the electrical parameters of the system,

and the actual phyisical location and orientation of the system. The various types

of radio noise are discussed in the following paragraphs and their contribution in

limiting signal reception is assessed with respect to the applications of interest.

in this study.

A basic source of radio noise is the thermal noise caused by the thermal

agitation of electrons in resistance elements of the system. For an ideal

receiv,k, the set noise level is determined by the thermalnoise generated in

the resistive component of the inputimpedance. This noise power for the ideal

receiver is given by the expression N = kT(3, where k is Boltzmann's constant, T is the temperature of the input impedance in degrees Kelvin, and j3

is. the effective bandwidth of the receiver. From this expression it may be seen

that; for a giventemperathre„the set noise of an ideal receiver is proportional

only to the bandwidth of the receiver. To a good approximation, this proportionality of noise to bandwidth is. true also for all other types of noise.of importance

in this study. Therefore, the contributions from the various types of noise

may be compared directly if the bandwidth chosen in each case is the same.

This comparison is made in this report after assuming a temperature T of

3000 Kelvin for evaluation of set noise.

The quality of a receiver, as far as noise is concerned, is determined

by means of its noise figure. The noise figure, F, of a receiver is defined

by the relation (S/N)0 = (I /F) (S/N)i where (S/N)_o, i is the output signal-tonoise ratio of the actual receiver and input signal-to-noise ratio of an ideal

receiver, respectively. Thus, the noise figure of a receiver is a measure of

the amount by which the actual receiver degrades the signal-to-noise ratio

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Page 11-107

possible with an ideal receiver, or, in other wards, the noise figure indicates the

amount of noise which the actual receiver adds to the ideal receiver noise.

Curve A of Figure B-1 indicates what may be expected from a good receiver

relative to the noise in an ideal receiver. As indicated by the above discussion, the

noise figure of these good receivers is given directly by the values plotted

in this curve.

Cosmic noise is defined as that noise originating from sources-in space

other than the sun. Most of the cosmic noise incident upon the earth originates

in the plane of the galaxy, with the primary source being located in Sagittarius

towards the galactic center. However, a substantial number of point sources or

"radio stars" have been located also, with the principal sources lying in

Cassiopeia and Cygnus.. The cosmic noise level received on a half-wave dipole

antenna from the galactic plane in the direction of the center of the galaxy is

shown, relative to the noise in an ideal receiver, as curve B in Figure B-1.

The noise levels from other parts of the galactic plane are between. .10 and 20 db

below the level given by this curve. Shown also in Figure B-1 is the cosmic

4oise level, relative to ideal receiver noise, received on a half-wave dipole

antenna from Cassiopeia (curve C), which is the moat intense discxete source

of cosmic noise known at'present time.

Another source of radio noise is the sun. The intensity of this noise will

vary depending upon solar activity, with solar noise from a quiet sun, when

there is little or no sun spot or solar flare. activity, being substantially less

than thenoise fromthe sun when it is. "disturbed." Curve D of Figure B-1 shows

the solar noise level from a quiet sun as received on a half-wave dipole antenna,

again with respect to the noise of an ideal receiver. Curve E of Figure B-1 gives

an indication of solar noise level frorn a disturbed sun, but it should be emphasized

that observed values of noise from the disturbed sun may vary by 10 db or so from

the indicated value due to variations in the intensity of the solar disturbances.

The final sources of radio noise are atmospheric noise and man-made

noise. Atmospheric -noise is produced mainly by lightning discharges in

thunderstorms, while man-made noise includes any interference from sources

such as rotating machinery, ignition discharges; etc. Although the frequency

spectrum of atmospheric noise may extend well above the high-frequency band,

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TR-59-0000-00604

Page II-109

the energy in the portion of the spectrum above about 30 megacycles or so is

substantially less than the energy produced by cosmic noise. For a variety of

reasons, which are discussed in more detail in other sections of this report,

frequencies below about 40 megacycles are not useful for the purposes of

interest in this atudy, so atmospheric noise will not be considered further.

Also, since the effect of man-made noise can usually be eliminated or reduced

to negligible proportions by good de sign practice s and adequate shielding, manmade noise will not be considered further in this report.

The curves of Figure B-1 indicate that cosmic noise will limit reception

in a dipole antenna atfrequencies up to about 160 megacycles or so, at which

point the receiver noise takes over as the limiting factor. However, both

cosmic and solar noise originate in sources external to the receiving system,

so these noise levels will vary with the gain, or receiving aperture, of the

receiving antenna. Since the galactic plane is an extended nonuniform source,

the antenna gain normally obtained with radiation from a point source cannot

be realized. For this reason, obtainable antenna gains must be limited to

about 15 db when considering this source. Thus, the cosmic noise received

from the center of the galaxy cannot exceed the values shown in Figure B-1

by more than about 15 db. Also, since the cosmic noise from other parts of

the galactic plane are 10 to 20 db below that from the galactic center, this

maximum gain limits the noise which can be received from the other portions

of the galaxy to maximum values approximately equal to those shown in curve

B of Figure B-1. On the other hand, the sun and the discrete sources of

cosmic noise such as Cassiopeia are point sources, and antenna gains of

50 to 60 db or more can be realized in these cases.

Figures B- Z and B-3 show the various noise levels indicated in Figure B- 1

adjusted for antenna gains of 15 to 30 db, respectively. With a 15-db antenna

gain, Figure B-2 shows that cosmic noise from the galactic plane and receiver

noise are still the limiting factors, but the cross-over frequency has increased

to about 480 megacycles or so. When Higher antenna gains are used, however,

the curves of Figure B-3 (for the case of a 30-db gain) show that the noise from

the disturbed sun replaces the noise from the center of the galactic plane as a

limiting factor. For this particular case, solar noise remains theslimiting

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Page II-11.2

CONFIDENTIAL

Polar Axis

Figure B-4. Illustration of Definitions of Celestial Coordinates.

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TR-59-0000-00604

eONFIDEAMAL

Page 11-113

factor up to almost 1000 megacycles, but if higher antenna gains are available, the

frequency at which set noise replaces solar noise as the limiting factor may

increase to 10 kmc or above.

The final conclusion regarding the type of noise which will limit reception

under any particular conditions cannot be reached without consideration of the

geometry and orientation of the receiving antenna beam. All of the sources of

external radio noise discussed in the above paragraphs are localized to a greater

or lesser extent, so an antenna will not receive the noise from these sources

unless it is pointed toward them. For this reason,: it is useful to digress at this

point so that the celestial coordinates used to define the position of these cosmic

noise sources may be defined. As the earth rotates about its axis, any given

star will. aintain a fixed vertical angle above the equatorial plane. This angle,

indicated as positive when measured northwards from the equator and negative

when measured southwards, is called the declination of the star and is its

first celestial coordinate. The second celestial coordinate, termed the right

ascension, is defined by the angle, measured in the equatorial plane, eastward

from the plane containing the earth's axis and the first point in Aries to

the plane containing the'earthis axis and the star in question. These definitions

of the celestial coordinates are illustrated in Figure B-4.

Since the equatoiial plane. is inclined to the plane of the ecliptic, which is

the plane defined by the earth's orbit about the sun, the celestial coordinates of

the ecliptic (or of the sun) will vary throughout the year. Thus, the sun cannot

be included as one of the so-called "fixed" stars. This variation of the celestial

coordinates of the ecliptic is illustrated in Figure B-5, which shows the earth

in its orbit around the sun, and in Figure B-6 which is a Mercator projection

of the heavens showing some of the more prominent fixed stars as well as the

path of the ecliptic.

The several sources of radio noise may now be considered with rispect.

to their location relative to the orientation of the receiving antennae which will

be used in the various applications of interest in this study. The galactic center

is located at a declination of from about -20 to -30 degrees. As shown in

Figure B-7, any antenna with a relatively narrow beam pointed toward , satellite

in a 24-hour equatorial orbit from any point on the earth or from any satellite

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Page 11-117.

at an altitude of, say, 500 miles or less above the earth, will be pointed at

declinations above -10 degrees. Of course, this statement holds also for an

antenna pointing from satellite to these points within 500 miles of the earth's

surface. Thus, the noise from the gilactic center must be considered for

the applications investigated in this study only in the case of the polar orbits

recommended for low-density communications relay service and in the case

of the equatorial communications interception satellite where narrow antenna

beams cannot be obtained at the lower frequency ranges of interest.

As shown in Yigure B-6, the declinations of the discrete noise sources

located in .Cassiopeia and Cygnus are greater than +40 degrees, and from the

preceding paragraph and Figure B-7 it is evident that noise from these sources

cannot be received by relatively narrow-beam antennas pointed between a

satellite in a 24-hour equatorial orbit and any pOint within 500 miles of the

earth's surface. An antenna oriented for transmission to a satellite in a

polar orbit may intercept these discrete sources of cosmic noise, but if the

antenna has a relatively narrow beam, this will occur, in the worst case,

only once each day and then only fcir a short period whose time of occurrence

can be predicted with very high accuracy.

Finally, reference to Figures: B-1 through B-3 will show that solar noise

can become important only if the antennas have a high gain and Consequently a

narrow beamwidtli. But solar noise caul be received by a narrow-beam antenna

only when the declination of the point at which the antenna is aimed coincides

with the declination of the ecliptic. As shown in Figure B-6, this cannot occur

unless the declination of the antenna beani is between +23 and -23 degrees.

From the discussion in' the preceding paragraphs, it is clear that, in the case

of reception from a 24-hour equatorial satellite, solar noise can be intercepted

only during the relatively. short (and prediCtable) time once each day during the

two periods (of a few days each) throughout the year when the receiving antenna

beam actually points at the .sun.' When receiving signals from polar satellites,

on the other hand, antennas at certain points on the earth's surface may intercept solar noise once each day for nearly a .month at a time. Thus, the possible

effects of such noise on the communication relay application (wherein the problem arises) are evaluated in more detail in the specific discussion of Chapter 2,

Section 2.3.5 of this particular application. Finally, it should be noted that the

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TR-59.0000-00604

.G9NRBENnAL Page 11-118

results shown in Figures B-2 and B-3 indicate that some care might be necessary,

in designing antennas for intelligence applications, to keep the side lobes sufficiently small so the solar noise received through such lobes does not set the

limit to the system performance.

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TR-59-0000-00604

Page 11-119

1

APPENDIX C

AMPLIFIER CHARACTERISTICS

Traveling-Wave-Tube Amplifiers 

Traveling-wave-tube amplifiers are broad-band devices which usually

operate over an octave bandwidth and rarely have bandwidths less than about

10 per cent of the operating frequency. These amplifiers utilize the interaction between a beam of electrons and a slow-wave structure which either

surrounds or is surrounded by the electron beam. As a signal travels along

the slow-wave structure, the interaction of the field with the electron beam

causes the electrons to slow down; the energy thus lost by the electron beam

is transferred to the wave propagating along the slow-wave structure to cause

amplification. Since the electron beam is present continuously, a travelingwave tube acts as a class-A amplifier with maximum power amplification

efficiencies of the order of 10 to 20 per cent. In addition, an axial magnetic

field is required in the tube to prevent defocusing of the electron beam by

mutual repulsion effects. At present, the magnetic focusing field is supplied

in most tubes by a solenoid electromagnet surrounding the tube. These

solenoids may weigh anywhere from about 6 to 60 pounds and will driw a

direct-current power of from 100 to 400 watts, depending upon the output

power of the tube, upon whether the solenoid is designed to obtain minimum

weight, or upon other similar factors.

More recently, periodically focused traveling-wave tubes have been

developed in which permanent magnets are used to provide focusing action

at periodic intervals along the tube. The total.weight of periodically focused

tubes now available runs from about 5 to 12 pounds, depending upon the power

output of the tube, and; since no solenoid is necessary, there is no solenoid

power dissipation. At, the present rate of development, it may be expected

that periodically focused tubes will begin to become available in about

mid-1959 in a variety of types comparable to the variety of solenoid-focused

tubes which are either available or in the late development stages today.

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TR-59-0000-00604

Page 11-120

Although it is probable that some tubes have unintentionally been omit*ld,

the traveling-wave-tube amplifier chart shown in Figure C-1 serves to

indicate that a reasonable amount of choice exists over most of the frequency

range up to 12 kmc and at most power levels up to about 200 watts (for CW

tubes). Higher powers are available in pulsed tubes, but the low duty factor

required with such tubes precludes their use in .the systems investigated

herein.

The noise figure of a traveling-wave tube will depend upon the power

level of the tube, its gain, its operating frequency, and its bandwidth. The

low-power receiving tubes are specially designed for low-noise characteristics

at some expense in bandwidth and gain. Traveling-wave-tube amplifiers are

available today in the 1-milliwatt saturation power range which have noise

figures of 6.5 db at S-band, 8.5 db at C-band (5900-7400 megacycles), and

10 db at X-band. It is probable that tubes with 7-db noise figures will be

available throtighout this range by the time this system could be operational.

Thus, a 7-db noise figure has been used in all calculations of required power

for the communication relay system investigated in this study.

Klystron Amplifiers 

Klystron amplifiers are basically narrow-band, amplifiers with bandwidths limited to a few megacycles by high-Q cavity tuning. It is possible

to increase the operating bandwidth of an operating amplifier to some 20 or

30 megacycles (at 10 kmc) by loading the cavities to decrease their Q, but

the available gain of the amplifier is sharply decreased. The operation of

a klystron amplifier depends upon the velocity modulation of an electron

beam by the input signal. After modulation, the electron beam travels

through a drift space where electron bunching occurs due to the velocity

variations of the individual electrons. The bunched beam then enters a

second cavity, which is tuned to the operating frequency, and induces an

amplified rf voltage. This amplified voltage remodulates the electron beam,

and a cascade amplification effect is produced until the final amplified rf

output is obtained from the final cavity. As with the traveling-wave-tube

amplifier, these tubes act essentially as class-A amplifiers, with power

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Page 11-121

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Page II-122

-CONFIDE-PAL

amplification efficiencies for the higher powered tubes ranging from 15 to

20 per cent.

From the klystron amplifier chart shown in Figure C-2 (again, some

tubes may have been unintentionally omitted), it will be noted that no klystron

amplifiers exist in the range above about 8.5 kmc which can produce the

200-watt power required for the polar-area and ocean-area relays. However,

five klystron amplifiers with output power levels ranging from 200 watts to

2 kilowatts are now available at 5- and C-bands, and at least three more

1-kilowatt tubes at 5-, C-, and X-bands are in the advanced design stages.

O•the tubes currently available, the Sperry S-band SAS-28 (a 200-watt tube)

is the lightest, weighing 5-1/4 pounds. This is about one-third the estimated •

weight of a periodically focused 200-watt CW traveling 7wave-tube amplifier.

Thus, since a decrease in operating frequency from X-band (as used in the

required power calculations) to S-band can be compensated by a corresponding increase in the size of the satellite antenna, and, since the diameter

(about 20 inches) required for the low-density satellite antenna dish at this

frequency is still small, klystron amplifiers merit serious consideration for

the power amplifier stages of the low-density relay in the initial global system.

Relatively strong magnetic focusing of the electron beam is required

in the higher powered klystron amplifiers, and the permanent magnets (or

electromagnets) required to obtain such focusing cause a rapid increase in

weight of such tubes with increasing power. This effect is clearly illustrated

by the. weight and power characteristics of the ldystron amplifiers discussed

in the preceding paragraph: The SAS-28 (200-watt) tube weighs only

5-1/4 pounds; the SAC-33 (500-watt) tube with permanent-magnet focusing

weighs 28 pounds. The electromagnets alone of the Varian 2-kilowatt

tubes weigh from 50 to 170 pounds. It should be noted, however, that the

electromagnets of these last tubes are not designed for airborne applications;

it is estimated that these weights could be reduced by about 25 per cent

without a major development program, and that a complete redesign could

yield a reduction of up to 50 per cent or so.

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Page II-123

Tralp•sacy Moe)

Figure C-Z. Klystron Amplifier Chart.

CONF1PENTI

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TR4-59-0000-00604

Page II-124

Finally, the high noise figures of ldystron amplifiers make such tubes

unattractive for use as input amplifiers where they would determine the overall noise figure of a receiving system. For this reason, the develOpment

of low-power klystron amplifiers for receiving applications has not been

pursued and, as shown in Figure C-2, no such tubes are commerically

available.

IDENTI•

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ACTUAL DISTRIBUTION LIST

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