Federal Systems

Division

TRW Space &

Technology Group

(IAS1-CJB-174904) SATELLITE VCICE BROADCAST

SYSTEM STUDY. VCIUHE I: EXECUTIVE SUMMARY

Contractor Report, 11 Apr. 1S£4 - 11 Jul.

1985 (TRW, Inc., Eedondo Eeach, Calif.)

RrV

N86-24678

Unclas

p HC A03/af CSCL 17B G3/32 43076

Satellite Voice Broadcast

System Study

Volume 1 - Executive Summary

July 1985

Prepared for

NASA Lewis Research Center
1. Report No.

CR 174904

2. Government Accession No 3 Recipient's Catalog No

4 Title and Subtitle

SATELLITE VOICE BROADCAST SYSTEM STUDY

VOLUME 1 - EXECUTIVE SUMMARY

S. Report Date

July 1985

6. Performing Organization Code

7 Author(s)

Dr. Michael Horstein

8. Performing Organization Report No

10 Work Unit No.

9 Performing Organization Name and Address

TRW Inc.

One Space Park

Redondo Beach, CA 90278

11. Contract or Grant No

NAS3-24232

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington D.C., 20546

13. Type of Report and Period Covered

Contractor Report

4-11-84 through 7-11-85

14. Sponsoring Agency Code

15. Supplementary Notes

Contract Monitor Charles E. Provencher, Jr.

NASA Lewis Research Center

Cleveland, Ohio

16. Abstract . -

This study investigates.the feasibility of providing Voice of America (VOA) broadcasts by satellite relay, rather than via terrestrial relay stations. Satellite

voice broadcast systems are described for three different frequency bands: HF

(26 MHz), VHF (68 MHz), and L-band (1.5 GHz). The geographical areas of interest

•at HF and L-band include all major land masses worldwide with the exception of the

th-S-., Canada, and Australia. Geostationary satellite configurations are considered

for both frequency bands. In addition, a system of subsynchronous, circular

satellites with an orbit period of 8 hours is developed for the HF band. VHF

broadcasts,-wWch-a-re~oonfiwd"to~ the So'vtetrtlnionv are provided by a system of

Molniya satellites.

•Satellites intended for HF or, VHF broadcasting are extremely large and heavy. Satellite designs presented here are limited in size and weight to the capability of

the STS/Centaur launch vehicle combination. Even so, at HF it would take 47 geostationary satellites or 20 satellites in '8-hour orbits to fully satisfy the voicechannel requirements of the broadcast schedule provided by VOA. On the other hand,

three Molniya satellites suffice for the geographically restricted schedule at VHF.

-i

At L-band, only four geostationary satellites are needed to meet the requirements

of the complete broadcast schedule. .Moreover, these satellites are comparable in

size and weight to current satellites designed for direct broadcast of video

program material.

17 Key Words (Suggested by Authors))

Voice of America

Satellite voice broadcasting

Large space structures

Phased-array antennas

Hiqh-oower transmitters

19. Security Classif (of this report)

Unclassified

18 Distribution Statement

Unclassified - unlimited

20 Security Classif. (of this page) 21. No. of pages 22. Price*

Unclassified 42

*For sale by the National Technical Information Service, Springfield. Virginia 22161
Federal Systems

Division

TRW Space &

Technology Group

Satellite Voice Broadcast

System Study

Volume 1 - Executive Summary

July 1985

Prepared for

NASA Lewis Research Center
Federal Systems

Division

TRW Space &

Technology Group

Satellite Voice Broadcast

System Study

Volume 1 - Executive Summary

July 1985

Contract No. NAS3-24232

TRW Space

Technology Group

One Space Park

Redondo Beach, CA 90278

Prepared for:

NASA

Lewis Research Center

21000 Brookpark Road

Cleveland, Ohio 44135

Approved:

M/Horstein

Project Manager

:. Sharpies

Manager, Systems Engineering
1. INTRODUCTION

The Voice of America (VGA), a division of the U.S. Information Agency

(USIA), provides voice broadcasts to most areas of the world. These

include, on a daily basis, more than 30 hours of English broadcasts and

more than 100 hours of foreign language broadcasts. The dominant broadcast

type is news, followed by features, news related material, music, and

editorials. News sources include the wire services, domestic and foreign

news bureaus, foreign broadcasts, and periodicals.

VGA's technical headquarters are in Washington, DC. VOA programming,

which originates in Washington, is transmitted to listeners via relay

stations located in the U.S. and in a number of foreign countries, as well

as through leased facilities. Most transmissions to U.S. and foreign relay

stations are at high frequency (HF), although in some cases satellite links

are now used.

Many of the relay station transmission facilities are quite old and/or

of lower power than desired. Consequently, VOA has embarked on a $1 billion

modernization program to replace antennas at high priority sites, upgrade

transmitters at existing relay stations, and begin construction of several

new projects.

In conjunction with the modernization effort, VOA is considering the

potential application of new technologies. To this end, the USIA has

funded NASA to contract with two satellite manufacturers to investigate the

role that satellite direct broadcasting might play in VGA's future operations. This volume summarizes findings of a 1-year study conducted by TRW

in answer to this question.

To be of value, broadcasting must take place at frequencies within the

bandwidth of receivers in the hands of the populace. The most widely

received shortwave bands are between 6 and 11 megahertz. A considerably

smaller percentage of radios can receive frequencies between 20 and 26

megahertz. However, to be reasonably assured that satellite transmissions

will penetrate the ionosphere, they should be above 20 megahertz. An

additional reason for desiring a higher broadcast frequency is to reduce

R5-059-85
the size of the satellite antenna, as the antenna diameter needed to produce a given beamwidth varies inversely with frequency.

The array of frequency bands considered in this study is indicated in

Figure 1. Band 1 comprises four RF subbands for which direct broadcasting

allocations exist. Because of the questionable nature of ionospheric penetration at the lower subbands, Band 1 system designs are described for the

top subband.

Band 2, which lies in the VHF band, was considered in this study only

for broadcasts to the Soviet Union, in which there exists a significant

population of suitably tunable receivers. Because of the inverse relationship between antenna size and frequency, Band 2 systems are assumed to

operate at 68 megahertz.

Band 3 receivers are virtually nonexistent today. However, because of

the long-range objectives of this study and the attractiveness of the

higher frequency bands for direct broadcasting, the characteristics of Band

3 broadcast systems were investigated as well.

The three frequency bands under the Band 4 heading are currently .

allocated to direct broadcasting in Regions 1, 2, and 3 as defined by the

International Telecommunications Union. Following the initial phase of

this study, TRW was directed by NASA to pursue system concepts only for

Bands 1, 2, and 3.

Band 1 transmissions employ double-sideband amplitude modulation

(DSB-AM), with a maximum baseband frequency of 5 kilohertz. Broadcasts in

Bands 2 and 3 use frequency modulation (FM), with a maximum baseband frequency of 15 kilohertz and a maximum deviation of 75 kilohertz. The RF

bandwidths of the DSB-AM and FM transmissions are 10 and 250 kilohertz,

respectively.

The present study is confined to developing satellite voice broadcast

system concepts for the different frequency bands. A subsequent contract

will be awarded by VOA to develop projections of receiver populations

worldwide. The results of the latter study will be evaluated by VOA in

conjunction with the system concepts developed under the present pair of

contracts to assess the attractiveness of those system concepts.

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Once this assessment has been made, it will be possible to evaluate

the merits of satellite broadcast systems as a complement to, or substitute

for, terrestrial broadcast facilities. This comparison will be facilitated

by life-cycle costs developed for each of the satellite systems as part of

the present study.

The feasibility of satellite voice broadcasting depends on the

required electric field strength. The baseline system requirement is 300

yV/m in Band 1 and 250 uV/m in Band 2. In Band 3, the signal strength

requirement is stated differently — namely, to produce a demodulated

signal-to-noise ratio of 49 dB with an indoor receiver and an outdoor

antenna. The field strength requirements in Bands 1 and 2 were

subsequently relaxed in considering a number of system variations.

Following a series of tradeoffs and analyses performed in the initial

study phase, four system concepts were selected for further investigation.

They are enumerated by frequency band and orbit below. The inclination of

the 8-hour orbit is 28.5 degrees (equal to the latitude of Cape Canaveral),

to maximize the satellite weight in orbit.

System Band Orbit

1 1 Geostationary

2 1 8-hour circular

3 2 Molniya

4 3 Geostationary

Each of the four satellite systems is designed to satisfy the channel

requirements of a broadcast schedule provided by VOA. This schedule specifies the number of voice channels to be provided to each of 15 geographical zones at 15-minute intervals throughout the day. The broadcast zones

are defined in Figure 2. (System 3 deals only with the four zones that

constitute the Soviet Union.) A compressed version of the broadcast

schedule, in which the channel requirements are allowed to change only at

1/2-hour intervals, is shown in Figure 3.

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2. SATELLITE SYSTEM DESIGN

Because of the large satellite power requirements in Bands 1 and 2,

the antenna beamwidth is a critical design parameter. It is found that a

3-degree beamwidth, as viewed from geostationary orbit, represents a good

compromise between the joint desires to: 1) avoid radiating power outside

zone boundaries, and 2) minimize antenna size. For orbits other than

geostationary, the beamwidth is modified to illuminate the same size area

as from geostationary orbit. The power requirements in any frequency band

are thereby rendered invariant to the choice of orbit.

The satellite RF power requirements depend on the number of channels

transmitted to each beam area. It is assumed that the same number of

channels are transmitted to each area of a particular zone (but not necessarily by a single satellite). Because it is not necessary that every

channel be transmitted throughout an entire zone, the number of channels

per beam area will sometimes be less than the corresponding entry in Figure

3. The transmitter power assigned to Zone i can be written as C-jN-jP^g,

where Ci is the number of channels required in each beam area of Zone i, NIis the number of beams needed to cover Zone i, and P^g is the transmitter

power assigned to each channel in a given beam (i.e., the transmitter power

per channel-beam). Thus, the transmission requirements of a zone are

characterized by its C x N product (i.e., the number of channel-beams

associated with that zone).

The channel-beam demand of the broadcast schedule, as it applies to

Systems 1,2, and 4, is shown in Figure 4. Broadcast service is measured

in units of channel-beam-hours (CBH). The total daily service called for

by the schedule in Figure 4 is 894 CBH.

The channel-beam demand for System 3 is given in Figure 5. Differences

between corresponding entries in Figures 4 and 5 result from different numbers of beams being required for zonal coverage from geostationary orbit

and Molniya apogee.

The number of satellites required to meet the demands of the broadcast

schedule depends on the channel-beam capacity of an individual satellite.

Satellite capacity, in turn, depends on the lift capability to low earth

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orbit (LEO) of the Space Transportation System (STS). The latter is taken

to be 65,000 pounds; this capability should be available by the end of the

decade.

The resulting satellite capacity values are shown in Figure 6.

Differences in satellite capacity between Systems 1 and 2 result from the

disparity in STS/Centaur payload capability. On the other hand, the payload difference between 8-hour and Molniya orbits is minor. The difference

in satellite capacity between Systems 2 and 3 results, instead, from the

difference in RF power requirements.

2.1 Baseline System Designs

For the geostationary satellite systems, three satellite locations are

needed to provide broadcasting to all 15 zones: 65° west, 30° east, and

115° east longitude Each zone is assigned to one of the three locations.

The associated channel-beam requirements at each location are then summed

for each 1/2-hour of the day. The maximum sum for any 1/2-hour period at

each location determines the satellite requirements at that location.

In the case of the Band 1 geostationary system, the number of satellites needed at each location is found by dividing the maximum channel-beam

demand by two, which is the satellite capacity. The resulting numbers of

satellites are: 11 at 65° west, 22 at 30° east, and 14 at 115° east. The

total number of satellites is therefore 47.

By contrast with the Band 1 and 2 requirements, the required RF power

per channel-beam in Band 3 is only 70 watts. Moreover, a 16-ft antenna

suffices to produce a 3-degree beam from geostationary orbit. As a result,

a single satellite could provide all required broadcasting from each of the

orbit locations. However, the maximum channel-beam demand is considerably

greater at 30° east than at either of the other two locations. It is more

efficient, therefore, to design a satellite to handle the maximum requirement at either 65° west or 115° east (which is 27 channel-beams) and to

deploy two such satellites at 30° east. Thus, four satellites are required

in all. Each satellite weighs about 4000 pounds.

As can be seen from Figures 5 and 6, the satellite capacity in Molniya

orbit (12 channel-beams) exceeds the maximum broadcast requirement -in any

1/2-hour period (10 channel-beams). Moreover, a single satellite can be

10

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R5-059-85
placed in Molniya orbit so as to maintain continuous visibility of the four

Soviet zones for nine hours. It follows that three such satellites, each

broadcasting for eight hours per day, can fully satisfy the Band 2 broadcast requirements. The eight-hour broadcast interval for each satellite is

centered about apogee passage. Because of the slow angular motion of the

satellite near apogee, transmission requirements during the broadcast period may be assumed constant and equal to those obtaining at apogee.

System design for subsynchronous orbits is considerably more difficult

than for either geostationary or Molniya orbit. The reason is that satellite coverage of the various zones is time varying. Orbit periods of 6, 8,

and 12 hours were considered. The 8-hour orbit was selected, primarily

because it provides a better balance between satellite coverage and

capacity.

Because the broadcast schedule is divided into 1/2-hour intervals, the

following rules are observed:

a) Broadcasts must begin on the hour or 1/2-hour

b) The minimum continuous broadcast period is 1 hour

c) Broadcast periods are constrained to be multiples of 1/2 hour

Additionally, it is assumed that satellite/zone assignments are not varied

during any 1/2-hour period. .

With these ground rules, it is found that 20 satellites in 8-hour

orbit are required to provide at least single-satellite coverage of the 15

zones during broadcast periods. However, this minimum-coverage constellation does not include sufficient satellite capacity to satisfy the channelbeam requirements throughout the day. Of the daily total of 894 CBH specified in Figure 4, only 735 CBH can be provided.

Feeder links for satellite broadcasts should operate in real time, if

at all possible. The alternative (which does not apply to stationary

satellites) is to record the program material when the satellite is in view

of a feeder-link station, for subsequent broadcast when the satellite is in

view of the target area. In order of preference, feeder-link station

locations include:

12

R5-059-85
1) U.S. (CONUS plus Alaska and Hawaii)

2) U.S. territories

3) Friendly host countries.

If none of these possibilities exists, real-time transmission can still be

accomplished by satellite relay.

For the two geostationary satellite systems, each of the three orbit

locations is visible from at least one of the desired station locations.

Therefore, real-time feeder links can be established in all cases. For the

Molniya system, each satellite is visible from CONUS throughout its 8-hour

broadcast period. Consequently, real-time feeder links are possible in

this case as well.

Two different ground tracks are involved in the 20-satellite, 8-hour

system. For one ground track, real-time feeder links can be established

for all but 1 hour of the day. For the other ground track, there is a

2 1/2-hour period during which the satellite is not in view of any of the

desired station locations. For these intervals, either satellite relay or

storage of program material is necessary.

2.2 System Variations

A number of system variations were examined, based on reductions in

coverage, number of broadcast channels, or required field strength. Of

these, two sets of variations are of particular interest.

The first variation involves transmission of a single voice channel to

each of the 15 zones during prime listening hours. The latter are defined

to comprise two hours in early morning and two hours in early evening. A

broadcast schedule that accomplishes this objective is shown in Figure 7.

The entries, which represent the channel-beam requirements in each 1/2-hour

period, are equal to the number of beams needed for zonal coverage. The

schedule in Figure 7 minimizes the maximum number of channel-beams required

of any satellite.

For a geostationary system designed to operate in Band 1, four

satellites are needed at each of the previously selected orbit locations to

satisfy the broadcast requirements of Figure 7. In all , 12 satellites are

13

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14
required. For a Band 3 system, a single satellite with a capacity of seven

channel-beams is needed at each orbit location.

For an 8-hour-orbit system, the orbit inclination is increased to 37

degrees. This reduces the satellite capacity from six to five channelbeams, but it also reduces the number of satellites needed for complete

24-hour zonal coverage from 20 to 16. Some gaps in northern coverage

result from use of a smaller number of satellites. However, a 12-satellite

constellation can provide 180 CBH of broadcasting daily, compared with 192

CBH provided by a 16-satellite constellation and 196 CBH called for by the

schedule in Figure 7. Therefore, a 12-satellite constellation is the

preferred choice.

In the second set of variations, the field strength requirement is

reduced to 150 wV/m in both Band 1 and Band 2. This represents a 6 dB

power reduction in Band 1 and a 4.4 dB power reduction in Band 2. These

power reductions permit a decrease in number of satellites and/or satellite

complexity.

For the geostationary Band 1 system, the fourfold reduction in field

strength can be translated into a similar increase in satellite capacity,

to eight channel-beams. As a result, 11 satellites suffice to (nearly)

satisfy the requirements of the broadcast schedule in Figure 4.

The baseline Molniya satellite capacity of 12 channel-beams (at a

field strength of 250 vV/m) completely satisfied the demands of the broadcast schedule for the four Soviet zones. Nothing would be accomplished,

therefore, if the reduced field strength were used to increase the satellite capacity. Instead, the satellite size and weight are reduced, with an

individual satellite still capable of providing the full broadcast service.

If the antenna diameter is halved, for example, the satellite beams

are expanded to 5.4 degrees. Each of the four Soviet zones can be covered

by a single beam of this size. With a minor adjustment to the broadcast

schedule, the maximum broadcast requirement in any 1/2-hour period

(expressed in terms of a 5.4-degree beamwidth) is three channel-beams.

On the other hand, the satellite capacity based on the full STS lift

capability is nine channel-beams. It is possible, therefore, to downsize

the satellite by using less than the full STS lift capability. A satellite

15

R5-059-85
capable of supporting three channel-beams requires only 40,000 pounds of

lift capability, as compared with the full STS capability of 65,000 pounds.

Three such satellites, spaced by eight hours along a common ground track,

fully satisfy the demands of the broadcast schedule.

For a subsynchronous satellite system, the most straightforward way to

take advantage of the field-strength reduction is to increase the orbit

inclination from the 28-degree value used in the 8-hour baseline system.

As mentioned earlier, 16 satellites can provide 24-hour coverage of all 15

zones at an inclination of 37 degrees, while a reduction in the number of

satellites to 12 introduces some gaps in zonal coverage. Nevertheless,

because of the 24-channel-beam satellite capacity at the reduced field

strength, 12 satellites can provide 864 CBH of broadcasting daily (out of

894 demanded).

A second option is to reduce the satellite antenna size through a

doubling of the equivalent geostationary beamwidth to 6 degrees. Because a

single beam will now radiate power well outside the boundaries of many zones,

the effective capacity of the satellite is reduced by this approach. However, 12 satellites can still provide 845 CBH daily (expressed in terms of an

equivalent geostationary beamwidth of 3 degrees).

A final system alternative is to increase the orbit altitude to a

value corresponding to (for example) a 12-hour period, while maintaining a

3-degree beamwidth. The wider coverage from 12-hour orbit permits a significant reduction in the number of satellites needed for 24-hour coverage of

all zones. Because of the increased satellite capacity that accompanies a

field strength reduction to 150 uV/m, an 8-satellite constellation can

provide as many as 851 CBH daily.

The salient features of the system designs for a 150 pV/m field

strength requirement are summarized in Figure 8.

One final system variation concerns the use of single-sideband amplitude modulation (SSB-AM), rather than DSB-AM, in Band 1. An SSB-AM signal

format requires 7.8 dB less transmitter power than DSB-AM, based on a

100-percent modulated double-sideband signal. This decrease in transmitter

power is equivalent to a field strength requirement of 122 pV/m. The

satellite requirements are therefore somewhat less than those for a field

16

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strength of 150 yV/m. It is found that seven geostationary satellites can

provide a daily total of 882 CBH. For an 8-hour-orbit system, 12 satellites are again needed for reasonably complete zonal coverage. Each of the

satellites can be made lighter than for the 150 uV/m field strength

requirement, while providing the'same amount of broadcast service.

3. LIFE CYCLE COSTS

To develop life cycle costs (LCC) for the various systems, a nominal

20-year program span was adopted, together with a satellite life of 7

years. Schedules depicting the satellite development period and the cumulative launch profile for the four baseline systems are shown in Figure 9.

Two complete sets of satellites (i.e., twice the fleet quantity) are

required in each case. In addition, a 10-percent spare-satellite contingent (not shown) has been assumed. If the broadcast service provided is

taken proportional to the number of satellites on orbit, each system provides the equivalent of 14 full years of service over the program span.

In addition to the satellite costs, the LCC include launch costs of

$100 million for a full STS load and $58 million for a Centaur-class upper

stage. Additionally, there are earth stations for satellite control and

feeder-link transmission. There are two such stations in System 3, four in

Systems 1 and 4, and seven in System 2. The cost per station is taken as

$10 million. Finally, there are operations and maintenance costs of $12.5

mi 11 ion/station/year. The LCC for each baseline system is given in Figure

10.

To provide a measure of system effectiveness, the LCC are normalized

to the broadcast service provided. The latter is computed by multiplying

the number of CBH provided daily by 5110, which is the number of days in 14

years. The entries in the last column of Figure 10 are the normalized LCC

for the four baseline systems.

The high normalized cost of the geostationary system is attributable

to the small satellite capacity of two channel-beams. By contrast, the

capacity of a satellite in 8-hour orbit is six channel-beams. The daily

number of CBH per satellite is approximately the same for the Molniya and

8-hour systems. However, the 8-hour system benefits from considerably more

18

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"learning" and also has the nonrecurring satellite cost spread over a

larger number of satellites. Hence, the smaller LCC/CBH for the 8-hour

system.

. System 4 has by far the smallest LCC/CBH. The satellites in this system

are not very different in size from some current commercial satellites.

Operations and maintenance costs, which are minor for the other three systems, are estimated to approach 50 percent of the System 4 LCC.

Measures of system cost other than the normalization of LCC with

respect to CBH may be of interest. For example, the system cost per year

of (full) operation is found by dividing the LCC by 14. In round numbers,

the annual costs of Systems 2 and 4, which provide comparable amounts of

programming (735 versus 894 CBH daily), are $1 billion and $140 million,

respectively.

Life cycle costs for the system variations are presented in Figure 11.

The effectiveness of the Band 1 geostationary, prime-time system is comparable to that of the 8-hour system because both require the same number

of satellites. The relative loss of effectiveness of the 8-hour system for

prime-time broadcasting is attributable to underutilization of satellite

capacity, particularly over the mid-latitude zones. The LCC/CBH of the

Band 3 geostationary, prime-time system is four times that of the baseline

system, because the service provided is five times smaller while the

life-cycle number of satellites is only reduced from nine to seven.

The number of satellites in the 8-hour system designed for a field

strength of 150 p.V/m is determined by coverage requirements, as is the case

for the 8-hour prime-time system. The LCC/CBH is much smaller for the

former system because of the greater broadcast service provided. The small

LCC/CBH for the geostationary system has the same explanation. The LCC/CBH

for the 12-hour system is only slightly lower than for the 8-hour system,

despite the decrease in number of satellites, because the satellites are

larger and more complex. Finally, the Molniya system LCC/CBH is only

modestly smaller than for the baseline system, because the number of

satellites is unchanged.

The LCC/CBH for SSB-AM in Band 1 are $1300 for the geostationary

system and $1880 for a system of 8-hour satellites.

21

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4. SATELLITE CONFIGURATIONS AND TECHNOLOGY

The baseline system satellite configurations are largely determined by

two technology selections: antenna type and primary power source. The

antenna choice is between a parabolic reflector and a phased array. The

narrow beamwidths required from geostationary and Molniya orbit necessitate

the use of a reflector type of antenna. The particular reflector chosen is

the cable-catenary. A 10-ft model of the reflector is shown in Figure 12.

At subsynchronous altitudes, the phased array is preferred because of its

greater flexibility in generating multiple, steerable beams. The phased

array also reduces considerably the power required from any single

transmitter.

Three types of primary power source were considered: solar panels,

nuclear reactor, and solar dynamic power conversion. Based on a weight

analysis, it was concluded that only solar panels lead to an acceptable

electrical power subsystem weight for the required power levels. A significant factor in this analysis is the absence of a requirement for eclipse

operation.

With the antenna and primary power source specified, it is possible to

configure satellites for the various baseline systems. A satellite concept

for the Band 2 Molniya system is shown in Figure 13. The Band 1 geostationary satellite concept is similar except for the dimensions. Specifically, the antenna diameter for the geostationary satellite is 267

meters. The solar panels are placed outboard of the antenna to avoid both

shadowing of the panels and blockage of the reflected RF signals.

The antenna feed geometry is shown in Figure 14. Each feed element

generates a distinct 3-degree beam from geostationary orbit or a 2.7-degree

beam from Molniya orbit. The feed element is the crossed dipole, which

consists of two orthogonal, unequal-length dipole arms. The relative

dipole lengths are adjusted for phase quadrature, to obtain circular polarization, which is needed to combat the effects of Faraday rotation. The

bandwidth of the crossed-dipole element is typically 3 percent, which is

adequate for the present application. The dipole arms are designed to fold

during launch.

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The satellite concept for the 8-hour-orbit system is shown in Figure

15. The 8 by 8 element array, which is 80 meters on a side, produces a

beamwidth of 6 degrees. Element spacing is one wavelength. The element

selected is the short-backfire dipole (Figure 16). This element is short,

mechanically rigid, self-supporting, and has a "flat-top" gain pattern.

The latter feature is important in minimizing scan loss. Circular polarization is conveniently obtained by unequal-length dipoles. The antenna is

very compact and lightweight, and can be designed to fold during launch.

By contrast with the Band 1 and 2 satellite designs, the Band 3

geostationary satellite (Figure 17) is comparable in weight to present-day

(video) direct-broadcast satellites. The 16-ft antenna is required to

produce a 3-degree beamwidth at 1.5 GHz. Individual transmitters must

support as many as three separate voice channels. The transmitter output

power in this multicarrier mode is 210 watts, which is three times the

per-carrier requirement of 70 watts.

Besides the antenna development (whether cable-catenary or

phased-array) for the Band 1 and 2 satellite designs, other critical

technologies include the electrical power subsystem and the transmitters

(Figure 18). Silicon solar cells of 2-mil thickness have been assumed

throughout. Cells of this thickness have been developed in 2 x 4 cm size.

From a cost standpoint, larger cells (e.g., 3x6 cm) of 2-mil thickness

are needed for an array generating on the order of 100 kW. To avoid excessive distribution losses, a 200-V distribution system is needed. This

level of distribution voltage is likely to be developed for Space Station.

There is also a need for lightweight, flexible cabling material, particularly with the large dimensions of the Band 1 geostationary satellite.

High-power transmitter development is crucial to Band 1 and 2 operation. Individual power amplifier outputs are about 10 kW for the Band 1

geostationary satellites and the Band 2 Molniya satellites, and about 1 kW

for the 8-hour Band 1 system. The high reliability and high efficiency of

MOS-FET devices makes them the preferred power-amplifier technology,

provided the indicated power levels can be achieved.

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It is anticipated that the emphasis by the Japanese government on

solid-state devices for commercial broadcasting will propel MOS-FET development to the power levels needed for Bands 1 and 2.

The evolutionary expectations for solid-state devices in Bands 1 and 2

do not apply to Band 3. There is no apparent commercial impetus for development of MOS-FETs or other devices more efficient than current bipolar

transistors. Unless the low (30-percent) efficiency of these transistors

or the lower reliability of TWTs is acceptable, technology enhancement will

be necessary to achieve the required Band 3 transmitter power levels.

The need for technology development in the area of attitude control is

uncertain at this point. This question should be resolved by further

analysis. Pending this determination, altitude control has been labeled as

a critical technology in Figure 18.

5. SINGLE-CHANNEL SATELLITE BROADCAST SYSTEMS

The original set of broadcast requirements (as embodied in the

broadcast schedule of Figure 4) led to Band 1 satellite systems comprising

many satellites of extremely large physical dimensions, with very large

costs. As a result, TRW was directed to perform an additional task to

determine the broadcast capability that can be derived from a single

satellite with sufficient power to broadcast just one voice channel. Three

values of field strength- 300 vV/m (the baseline value), 150 uV/m, and 50

uV/m — were to be considered.

Six different orbits were examined. These include geostationary and

Molniya orbits, as well as subsynchronous orbits with 6-, 8-, and 12-hour

periods. The inclination of the 6- and 8-hour orbits is 28.5 degrees,

while that of the 12-hour orbit is 37 degrees. The sixth orbit is referred

to as a "triply-sync" orbit. It is a highly elliptical, sun-synchronous

orbit, with apogee of 7843 kilometers, perigee of 521 kilometers, and

inclination of 116.6 degrees. The orbit period is 3 hours. Apogee is

placed at a latitude of 63.4 degrees, which is the maximum value achieved.

32

R5-059-85
5.1 Satellite Visibility

Visibility for a geostationary satellite is simply described. The

visible region is shown in Figure 19 as a function of latitude and longitudinal offset, for a minimum elevation angle of 20 degrees. Visibility

periods for a satellite in Molniya orbit are immediately evident from a set

of instantaneous visibility contours. Visibility contours valid for either

of two consecutive 12-hour orbits are shown in Figure 20. The only distinction between the two orbit passes is that the longitudinal reference

(i.e., the longitude of apogee) is shifted by 180 degrees.

Satellite visibility for each open contour in Figure 20 (e.g., for t =

4 hr) extends to all points above that contour. Visibility for the closed

contours (e.g., t = 5 hr) is restricted to those points within the contour.

Continuous satellite visibility for the period (t^t,,) requires that the

observer be located above or within all open or closed contours, respectively, for times between t^ and t2< Visibility regions for various

minimum periods are indicated in Figure 20.

The triply-sync orbit offers the possibility of several significant

(from a broadcast standpoint) visibility periods per day. Moreover, these

visibility periods occur at the same local time each day. Figure 21 shows

the period of visibility for three different latitudes as a function of

displacement from the longitude of apogee. Because of the 3-hour orbit

period, a given latitude is crossed, in either the north-south or southnorth direction, at a longitudinal separation of 45 degrees on successive

orbits. Therefore, dividing the longitudinal span over which the visibility period is approximately constant by 45 degrees gives the number of

successive orbits on which this degree of visibility is realized.

Satellite visibility is summarized below for the three values of

observer latitude. For a latitude of 20 degrees, visibility during one

pass in the middle of the sequence will be less than 30 minutes.

Latitude Period of Number

(Degrees) Visibility (Min) of Passes

60 50 5

40 50 4

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Visibility of a satellite in subsynchronous orbit will be described as

a function of observer latitude and longitudinal separation from the satellite ascending node. For a 6-hour orbit, there can be as many as three

separate visibility periods daily; for an 8-hour orbit, two visibility

periods; and for a 12-hour orbit, only one visibility period. Visibility

periods for the three orbits are shown in Figures 22-24, for an observer

located at the longitude of the ascending node. Visibility is considered

only in multiples of 1/2-hour. In extreme cases, therefore, the visibility

periods may be understated by nearly 1 hour.

Because of the wide range of required field strengths and the need for

only a single voice channel, suitable orbital transfer vehicles (OTVs)

range from the Centaur class to the PAM-A. In addition to these two OTVs,

a custom bipropel 1 ant second stage was considered. For each of the 18

orbit/field strength combinations, a representative set of satellite characteristics was determined in conjunction with the choice of an appropriate

OTV. The results are shown in Figure 25.

Since no antenna beamwidth is specified, a tradeoff exists between

antenna aperture and RF power. The approach adopted is to minimize the

antenna aperture. The resulting beamwidth is indicated in each case. For

the geostationary, Molniya, and triply-sync orbits, the aperture refers to

a reflector type of antenna. For the three circular, subsynchronous

orbits, the aperture size is the dimension of a phased array.

The satellite weights in all but three cases reflect the benefits of

advanced technology, circa 1995. For the 6-, 8-, and 12-hour orbits and a

field strength requirement of 50 pV/m, the satellite weights reflect current

technology. In all three cases, the antenna is of modest size. For the 6-

and 8-hour orbits, the 11-meter aperture implies a 2 by 2 phased array.

Because the triply-sync orbit is confined to a radiation belt

encircling the earth, silicon solar cells cannot be used. The three

satellites designed for this orbit use a GaAs concentrator array, which is

relatively impervious to radiation. Because of the considerable weight of

this technology, fully half the satellite weight in each case is attributable to the electrical power subsystem.

37

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