The Tactical Communi-cations Satellite

RICHARD D. BRANDES

Hughes Aircraft CompanvSpace Systems Division

El Segundo. Calif.

Abstract

Early in 1969 the U. S. Air Force placed into synchronous

orbit the largest communications satellite built to date. This

vehicle, the tactical communications satellite (TACSAT),

together with a variety of ground terminals, is designed to

test experimentally and develop tactical communications

concepts for all military services. This paper describes the

spacecraft design focusing on the communications re-

peater. Measured performance characteristics affecting

communications utilization of the spacecraft are presented.

Manuscript received October 2'7 1969: revised November 14. 1969.

This paper- was presented at EASCON 69, Washington. D. C.. Octo-ber27-2 9. 1969.

Summary

In January 1967. the Air Force initiated the develop-ment of a tactical communications satellite (TACSAT)contracting for that spacecraft with the Hughes AircraftCompany. The spacecraft program was part of a largerexperimental tactical communications program involv-ing development, test. and experimentation with spaceand surface satellite communications hardware. The ob-jectives of this program were to develop operational con-cepts for satellite tactical communications, to demon-strate and test these concepts. and to evaluate them forfuture operational systems.The TACSAT spacecraft was formally delivered to the

Air Force in December of 1968 and successfully launchedinto synchronous orbit on February 9. 1969. The space-craft has been in operation since that time supporting ini-tial checkouts of tactical communications ground hard-ware and supporting special operations. notably theApollo launches. This paper describes the key designfeatures of the spacecraft emphasizing the communica-tions repeater characteristics.

Satellite Description

The external design of the satellite is depicted in Fig. 1.Fig. 2 shows a cutaway perspective. The satellite is astabilized gyrostat spacecraft weighing about 1600pounds in orbit. It consists basically of a large spinningcylinder supporting solar cell arrays and a despun plat-form mounting SHF and UHF communications equip-ment. The size of the spinning solar array (approximately11 1 inches in diameter by 132 inches in length) is dictatedby the requirement to supply almost 1 kW of power forspacecraft operations, primarily the high effective iso-tropic radiated power (EIRP) repeaters. The cylinderconsists of a forward and aft solar panel separated by abellyband in which are mounted sensors, control jets,and access plugs. These panels are constructed of fiber-glass on aluminum honeycomb and serve both as solarcell substrates and as load paths for the booster launchloads.Mounted within the cylinder is a cone shaped structure

supporting the inner shaft of a 6-inch diameter bearingassembly. This spinning cone also provides a mountingplatform for batteries, auxiliary telemetry, tracking andcommand equipment, despin control electronics, thehydrogen peroxide propulsion system, and the nitrogenspinup system.The despun platform mounted to the housing of the

bearing assembly provides approximately 60 square feetof mounting surface for the communications electronicsand telemetry equipment. This platform is maintainedearth-oriented by a despin control system using earthsensors as references. Power and necessary signals whichmust be transported across the bearing interface arecarried on a slipring assembly.

IEEE JIRAN'SACTIONS ON' 4EROSPACEI AND) EILEC RONIC SYSTEMS VOL. AES-6. NTo. 4 JlUL 1970

NUTATION DAMPERSHFHORN-C; /;SLIP RING ASSEMBLY

BEARING ASSEMPLYDESPUN PLATFORM

NITROGEN SPIN-UP

ELECTRONIC EQUIPMENT

DESPiN BEARINGSUPPORT ASSY

SOLAR PANEL-FWD

EARTH SENSOR

HYDROGEN PEROXIDE /REACTION CONTROLSUBSYSTEM

-RADIAL JETS (2)

---AXIAL JETS (2)

- SOLAR PANEL-AFT

7 BOOSTER ADAPTER

__s

Fig. 1. TACSAT external design.

The platform also mounts an antenna mast structureto which are attached the several antennas associatedwith the spacecraft. The largest of these is a UHF an-

tenna array consisting of five bifilar helices. The fourouter helices, approximately 96 inches in length, are

hinged for folding within the fairing constraints of thebooster during the launch phase and are deployed once

the spacecraft is in orbit. The middle helix, approxi-mately 72 inches long, is fixed. While this UHF antennaserves for both transmit and receive, separate horns are

provided at SHF frequency for these functions. All com-munications' antennas are designed for pencil beam op-

eration with coverage intended to include the entire visi-

ble earth surface from synchronous altitude, including anallowance for attitude control errors.

The topmost antenna is for telemetry, tracking, andcommand operation. This antenna is a biconical hornbeamwidth of approximately 30 degrees with a sym-

metrical about the longitudinal (spin) axis of the space-

craft.Thermal control of the spacecraft is entirely passive.

The spinning and despun equipment compartments are

maintained at bulk temperatures of 55 to 85°F dependingon equipment operating status and time of year. Princi-pal thermal control surfaces are an aluminized teflonthermal barrier mounted at the bellyband to prevent ex-

cessive cooling of the spinning compartment and a sun

BRANDES: TACTICAL COMMUNICATIONS SATELLITE

Fig. 2. TACSAT configuration.

shield mounted over the forward end of the spacecraft.The shield reflects sunlight away and serves as a radiatingsurface for the several hundred watts dissipated by theelectronic equipment on the despun platform.Apart from the communications repeater which will

be discussed in detail later, the principal electronic sub-systems are the telemetry and command (T&C), thepower, and the despin control subsystems.The T&C subsystem is all PCM and SGLS compat-

ible. Functions of decoding commands and encodingtelemetry are performed on both the spinning and despunportions of the spacecraft to minimize the number of sig-nal channels across the spinning interface.The power control electronics include shunt limiters to

control voltage excursion on the power busses by dis-sipating unused solar panel power, current sensors, andbattery charge and discharge controllers. Battery capac-ity is over 20 ampere-hours and is designed to support allfunctions other than communications during solareclipse.The despin control subsystem consists of earth sensors

for references, servo control electronics to maintain de-sired platform pointing direction, and a bearing andpower transfer assembly. The control electronics pro-vide both precision pointing of the platform and activenutation damping through the dynamic coupling of themass-unbalanced platform. The bearing housing andshaft are machined from beryllium and mount the 6-inchdiameter angular contact ball bearings on 10-inch cen-ters. Wet lubrication techniques are employed in allbearings. The slipring assembly used for power transferuses dry lubrication at the brush/slipring interface.

It is appropriate at this point to reflect briefly on acomparison of the initial spacecraft concept with theultimate design. The intent of the program was to pro-vide experimental hardware for testing tactical satellitecommunications and, hence, to be conservative in thespacecraft development approach. There was disciplinedplanning in the procurement and the design of the space-craft to minimize new spacecraft developments and to

433

TABLE

TACSAT Program Development Features

ConfigurationSize: 9 foot diameter, 25 foot heightPrime power: 980 watts

StabilizationGyrostat implementationBearing and slipring technology

ComplexityRepeater: Crosscoupled UHF/SHF repeaters, 8 operating modes25 000 parts including 3000 integrated circuits

Component developments20-watt SHF TWTFiberglas/honeycomb structural panelsBeryllium bearing assembly and UHF antenna

Program development approachStructural qualification analyses and model test, no vibrationNo spacecraft engineering modelFly qualification spacecraft.

use space-proven technology wherever possible.In several subsystem areas, demonstrated approaches

of lesser performance were selected rather than newer de-velopments with superior capabilities but, as yet, un-proven in space. For example, an existing space-qualifiednitrogen spinup system and hydrogen peroxide systemswere selected instead of next generation hydrazine hard-ware. A pendulum liquid damper similar to that used inprevious spacecraft and a conventional regulated buspower system design were employed over more advancedconcepts.

Nonetheless, from an overall point of view, the re-quirements of a very high powered, very large spacecraftcould not be satisfied without major advances in space-craft technology. Table I lists some of the more signifi-cant development features of the spacecraft. The highpower requirements together with the large spacecraft'ssize are obvious features. Of perhaps greater significance,this is the first spacecraft ever stabilized with gyrostattechnology which frees the designer from moment ofinertia design constraints. The complexity in terms ofparts count and the intricacies of the repeater design werealso advanced developments. Specific components whichrepresented major technological challenges included sig-nificant use of beryllium within the structure, develop-ment of a new 20 watt TWT, and development of load-bearing solar panels to support the spacecraft duringlaunch. There were program development featuresapart from technology which also were quite unusual,not the least of which was flying the qualification space-craft because funding limitations precluded the completedevelopment of the flight model.

In short, TACSAT represents a prototype vehicle forthe next generation of communication satellites in addi-tion to being a test vehicle for tactical communications.This foundation has been exploited in the Intelsat IV andadvanced military spacecraft. Thus the development of

FOLLOWING SEPARATIONSPIN SPEED : 55.2 RPM

ATTITUDE ERROR '2 3

ANTENNABORESITE

MOTORS

SEPTSN-U ~~EARTH g

JETS ' fTIANSTAGE

SEPARATION SPIN-UP( 3 FPS) ( 54 RPM) NOMJNAL

Fig. 3. Separation/acquisition sequence.

EARTH ACQUISITION( 5 MIN)

this spacecraft technology by itself represents a signifi-cant accomplishment of the tactical communicationsprogram.

Spacecraft Operations

The spacecraft was injected into synchronous equa-torial orbit by the Titan IIIC on February 9, 1969. Afterreorientation by the Transtage so that the spin axis wasnormal to the orbital plane, the spacecraft was separatedby an explosive bolt/spring separation system. Separa-tion sensors initiated spinup by activating the cold gasnitrogen spinup system. At the same time, structural tiesbetween rotor and platform were disengaged and thedespin system activated to despin the platform relativeto the rotor. The spacecraft automatically stabilized, ac-quired the earth, and oriented the despun platform to-ward the center of the earth, using earth sensor data asdepicted in Fig. 3. UHF antennas were deployed byground command on the following day and repeatercheckout commenced.

After injection and acquisition, the spacecraft was ma-neuvered from the injection point to the current syn-chronous equatorial station at a longitude in the westernUnited States. Propulsion capability is adequate to main-tain station, change station, and perform necessary atti-tude corrections.

Accurate maintenance of the spacecraft platformpointing toward the earth suborbital point is necessary tomaximize EIRP for the earth coverage antennas. Overallpointing capability is approximately 0.1 degree rms.As indicated in Fig. 4, east-west errors are primarily

associated with the platform servo errors which, in turn,are dominated by earth sensor noise. North-south er-rors are primarily associated with spin axis attitude driftsderiving from solar radiation torques. While the pointingprecision achieved could clearly be degraded somewhat

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS JULY 1970434

MAX N-S POINTINGERROR U0.25- (3e) \

-1

SPIN AXIS -

ERROR ELLIPSE

l

PLATFORM BORESITE

STABLE

FLAT DISC T ,

UNSTABLE

SLENDER ROD1/IT

1, - SPIN MOMENT OF INERTIA

IT ' TRANSVERSE MOMENT OF INERTIA

=,- SPIN RATE MOMENT OP INERTIA

CONDITIONALLYSTABLE

DESPUNPLATFORM

MOTOR

ROTOR

DUAL SPINCAN BE STABLE FOR AREITRARY MASSPROPERTIES

STABILITY DEPENDANT ON .$, DAMPINGDISTRIBUTION, AND DESPIN CONTROLTRANSFER CHARACTERISTICS

Fig. 4. Pointing errors in steady operation.

without significantly affecting communications perfor-mance, the high accuracy maintained does not excessivelyburden spacecraft operations; attitude corrections havetypically been required only every 3 to 6 weeks sincelaunch.

Attitude correction maneuvers are performed in amanner similar to other spin stabilized synchronoussatellites. On-board earth sensors and sun sensors sensethe angle between spacecraft spin axis and their respec-tive references. This data is transmitted via telemetry andthe communication beacons to ground stations of theAir Force satellite control facility and other communica-tions stations. Ground computation and smoothing ofthe data are performed to determine inertial attitude ofthe spacecraft. Attitude correction commands are com-puted and transmitted to the spacecraft for execution bythe on-board propulsion system.

Orbital correction maneuvers are accomplished in asimilar manner. Ground tracking of the telemetry carrieris used to determine ephemeris quantities and requiredmaneuvers. Appropriate commands are then trans-mitted to the spacecraft for execution.

Gyrostat Stabilization of Spacecraft

An important feature of the spacecraft design is itsgyrostat stabilization. Conventional spinning spacecraftmust always be spun about their maximum moment ofinertia in order to maintain long term stability. If spunabout any other axis, the nonrigid-body aspects of thedesign will slowly cause a nutation angle to develop andgrow until the vehicle tumbles and spins about its maxi-mum moment of inertia. All of the first generation syn-chronous altitude communications satellites which werespin-stabilized were spun about their maximum momentof inertia. This includes the Syncom, ATS, the Intelsatfamilies, and the DSCS Phase 1.

Fig. 5. Gyrostat stabilization requirements.

The desire for higher RF power coupled with boosterfairing dimensional limitations often make it desirable tobuild a vehicle spun around the minimum moment ofinertia axis. This can be accomplished in a gyrostatspacecraft by despinning an appropriately sized nutationdamper. This approach couples conveniently with a de-sire to provide a despun platform oriented toward theearth for mounting of high gain antennas. Insertion of adamper on this platform ensures the stability ofthe space-craft while still providing many of the simplicity featuresassociated with spin stabilization.The requirements to ensure stability are shown in a

simplified fashion in Fig. 5. The existence of energy dis-sipation devices (shown as dashpot/spring/mass systemsin the figure) within the spinning body tend to destabilizethe vehicle unless it is spinning about the maximum mo-ment of inertia. In the gyrostat spacecraft, it is requiredthat these damping forces on the despun platform be ap-propriately greater than the corresponding forces on arotor which tend to destabilize the spacecraft. Principalamong these latter forces are such things as fuel slosh,structural vibration, etc.TACSAT prelaunch computations of the magnitudes

of these destabilizing forces indicated that they were or-ders of magnitude smaller than the stabilizing force ofthe damper placed on the platform and that stability wasthus assured with an adequate margin. It was very dis-concerting, therefore, to discover in the immediate post-launch moments of spacecraft operation that the nuta-tion angle initiated at separation by the tip-off dynamicsdid not decay as expected but maintained a fairly steadylevel at an amplitude ofabout 1 degree. This nutation dis-appeared within a few weeks but has reappeared atintervals up to the present time. As might be expected,the phenomenon precipitated a major study effort.

While it is not the purpose of this paper to discuss thestudy in detail, the conclusions are worth summarizing.

BRANDES: TACTICAL COMMUNICATIONS SATELLITE 435

Reexamination of the destabilizing forces on the rotor,together with component testing, ultimately indicatedthat certain destabilizing forces within the bearing assem-bly were several orders of magnitude greater than hadbeen anticipated and were in fact momparable to thestabilizing contribution of' the nutation damper. In thisstandoffbetween stabilizing and destabilizing forces, thespacecraft is fairly neuti-aX In teri-ms of stability up tonutation anigles of about 1 degree. At this point nonlitneaifeatures within the despjii control system substantiallyincrease the stabilizing clharacteristics of that subsystemand limit any nutation buildup to that level.

These findings aie entirelv consistent with gyvostatstab:ility theory, but do suggest modifications ftor futuredesigns. First. minor changes in the bearing assemblyhave eliminated the destabilizing feature. Second, addi-tional stability margin can be provided through more effi-cient platform dampers. This latter approach is fairlystraightforward with the new dampers being developedfor advanced gyrostat spacecraft; an increase in nutationstabilization by an order of magnitude could be accom-plished on a TACSAT vehicle for a weight penalty of lessthan 10 pounds. In any case. for the beamwidths involvedon the TACSAT vehicle, the I degree nutation that hasbeen observed at certain intervals has no significant effecton the mission.

a team pack using a 3 foot antenna. about 700 channelscan be serviced.The capacities for vocoded voice are depicted in Figs.

8 and 9. At UHF. for most users, capacity is restricted bythe limited bandwidth available. Pure bandwidth limitsand channel limitations where avoidance ol third orderintermodulationi products is desired are both depicted.For- any but the aircraft system, the capacity is not powerlimited. Substantial link margins exist. thereforc. for svs-tern degradations in the tactical environment.

At SHF, the wide bandwidth availability permits operation at spacecraft power limits for the smaller user. Asshown in Fig. 9. for small (e.g. 3 loot terminals) ade-quate excess bandwidth exists for spread spectrummodulation transmission and for selecting intermodula-tion free channels.

By way of comparison, it is interesting to determinethe number of conventional telephone channels thatcould be supported by a spacecraft producing the ElRPof TACSAT but using a large ground station of the com-mercial communications class (85 foot antennas with lownoise receivers, high powered transmitters, etc.) and ig-noring ancillary limitations such as bandwidth, multi-plexing capability, etc. For these simplifying assump-tions, it is relatively easy to show that TACSAT would becapable of supporting over 20 000 high quality telephonechannels. This is indicative of the step advance made intechnology with the building and launching ofTACSAT.

Repeater Communications Performance

The simplified block diagram in Fig. 6 illustrates thebasic operational features of the communications re-peater. Functionally, the complete subsystem consists ofa UHF frequency translating repeater and a SHF fre-quency translating repeater. each capable of operatingwith selectable bandwidths extending from 50 kHz to10 MHz. Additional communication flexibility is pro-vided by crossover modes of operation in which an in-coming UHF carrier signal may be retransmitted at SHFand, conversely, incoming SHF carriers may be retrans-mitted at UHF. Biphase digital modulation beacon sig-nals are also associated with each of the repeaters. Themodulating signal is a pseudorandom binary bit streamwhich may be used at the ground stations for synchroni-zation or timing functions. The frequency and bandwidthoptions of the repeater are indicated in Fig. 7. Principalperformance parameters are depicted in Table II.

Systems Communication Capability

The communications system capacity of the TACSATspacecraft is geared to the use of small mobile tacticalstations. Capacities for even the smallest of these users issignificant. For aircraft with 0 dB antennas, several hun-dred teletype channels are available at UHF. At SHF, for

Repeater Detailed Description

UHF RepeaterThe repeater antenna system consists of five helical

antennas phased and matched to provide at earth's limbabout 15 dB transmit gain and about 12 dB receive gain.The antennas are designed to generate and receive righthand circularly polarized waves.The UHF receiver processes the received RF signal

from the UHF antenna, which varies in level from 1-50to- 105 dBW. This signal is filtered by the UHF diplexerto prevent interference by the repeater's own transmitterwhich is feeding the same antenna line. The diplexer alsoprovides rejection of external signals at the image fre-quency of the receiver wide-band channel. The filteredRF signal is amplified in a wide-band transistor preampli-fier and split into two channels, one for narrow-band andthe other for wide-band operation.The wide-band channel is converted in frequency to

an IF signal by a 400 MHz local oscillator signal from thereference generator. The IF center frequency, 92.5 MHz.corresponds to a received frequency of 307.5 MHz. Thissignal is amplified and sent to the IF processor at a levelbetween- 106 and -61 dBW. The receiver noise power.in a 1O MHz bandwidth, is at a level of -84.9 dBW atthis point or 21.1 dB above the minimum signal. The IF

IEEE TRANSACTIONS ON AEROSPA(CE AND ELECTRONIC SYSTEMS JULY 197043'6

Fig. 6. Communications repeater block diagram.

Fig. 7. Communications frequency/bandwidth options.TRANSMIT RECEIVE

UHF 100o Hz v 50 kHz 100 kHz_ 50 kHzBEACON kzzj

425 kHz 425 kHz

2.54. 1 37

249.6 303.4 307.5MHz

SHF

7298.5 v

7257.5 7982. 5MHz

processor filters, amplifies, and limits the signal androutes it to the SHF transmitter in the crosscoupledmodes. The wide-band 10 MHz UHF receiver signal isused only for crossover operation.The narrow-band channel from the UHF receiver pre-

amplifier is filtered to reject the image signals and is con-verted to IF by a 320 MHz local oscillator signal from thereference generator. The IF frequency of 16.6 MHz cor-responds to an RF input at 303.4 MHz. The IF signal isamplified and fed to the IF processor at a level between-92 and -47 dBW. The receiver noise power in a 425kHz bandwidth is -83.9 dBW, or 8.1 dB above the mini-mum signal level.

TABLE 11

Communication Performance Parameters

UHFCarrier power (16 power amplifiers)Beacon power (16 power amplifiers)Transmit antenna gain (peak)Beacon antenna gain (peak)Receiver noise figureReceive antenna gain (peak)Frequency stability:

Initial setting3 year drift

Phase linearity error (500 kHz)SHF

Carrier powerBeacon powerTransmit antenna gain (peak)Beacon antenna gain (peak)Receiver noise figureReceive antenna gain (peak)Frequency stability:

Initial setting3 year drift

Phase linearity error (10 MHz)

23.6 dBW8.0 dBW

17.1 dB17.8 dB3.7 dB

17.6 dB

<1 X 10- 7

<I x 10-611 degrees

14.6 dBW0.2 dBW19.3 dB19.3 dB6.9 dB

18.4 dB

<I X 10-7<lx 10-616 degrees

In the IF processor, the signal is split into several pathscorresponding to different modes of operation. The sig-nals are filtered according to mode requirements, furtheramplified, limited in amplitude, and fed to a mode selec-tion switch which routes the desired channel output tothe UHF transmitter at a level of -32 dBW.

BRANDES: TACTICAL COMMUNICATIONS SATELLITE 437

IF-5I000

VOCODED VOICE - 2400bps10 MHz BANDWIDTHDC ODD VOICIE - 2400

'51 kHz Q-ANVIDTH

Lz

zz

uj

cs

-5

zLz

z

50

10

GROUND TERMINAL ANTENNA GAIN, dB

Fig. 8. UHF communication capability.

The IF signal sent to the UHF transmitter is up-con-verted to the transmitter output frequency of249.6 MHz,filtered to select the desired sideband, and passed throughanother limiter. The limiter removes amplitude modula-tion present on the dual channel signals. The limited sig-nal is summed with the UHF beacon signal from thereference generator. The composite signal is amplified toa sufficient level to drive 16 power amplifiers. It is thensplit 16 ways in a hybrid power divider and connected tothe power amplifiers, each of which requires 40 mW.Each of the power amplifiers raises the signal level to18.5 watts. For maximum transmitter power. all 16power amplifiers are used, and their outputs are com-bined in a coaxial power summer. The summer output ispassed through the beacon image filter to reduce the levelof the beacon image generated in the power amplifier by20 dB.The filter output connects to the diplexer which rejects

transmitter noise in the receiver band. The final outputlevel with all amplifiers on is 23.6 dBW.

SHF RepeaterThe SHF repeater operates on the received microwave

signal from the SHF receive horn antenna. This signal,which varies in level between -150 and- 85 dBW, isfirst filtered to reduce the transmitter signal that hasleaked into the receive antenna to a tolerable level. A

BANDVVIDTH JMI

EARTH STATION TEMPERATURE= 1000° K

VB.E. R. = 10 5

3RD ORDER IM LIMIT

TEAM PACK

jI , .. _-

6 9

ANTENNA DIAMETER, FEET

12 15

Fig. 9 SHF communication capability.

tunnel diode RF amplifier is used to provide a 7.0 dBnoise figure for the repeater. The tunnel diode amplifieroutput is filtered to reject noise generated at the imagefrequency and is mixed down to IF. The local oscillatorsignal is derived by a X19 multiplier operating on a425 MHz reference signal from the reference generator.The IF output, a single channel at 92.5 MHz, is amplifiedand sent to the IF processor.

In the IF processor, the signal is split into two paths,one of which is directly amplified in a 10 MHz channel,limited, and fed to a SHF mode selection switch. On theother path, a second frequency conversion occurs to ob-tain a 16.6 MHz IF to permit realization of narrow-bandchannel modes. This signal is split into three paths, fil-tered, and limited. One path provides a 425 kHz channelto the UHF transmitter. The other two provide 50 kHzand 1 MHz channels, respectively, to a mode selectionswitch, which also has a 425 kHz input channel from theUHF repeater. This switch selects the desired channeland feeds it to a mixer which reconverts the signal backup to 92.5 MHz to be compatible with the SHF trans-mitter input.

In the SHF transmitter, the IF signal is limited andfurther amplified to provide a high level drive signal to apair of redundant high level mixers. The IF signal is up-converted to 7257.5 MHz by a local oscillator signal anddiplexed with a SHF beacon signal.

4EEE TRANSACTIONS ON AEROSPACE AND ELECTROINIC SYS-TEMS JULY 1970

50

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438

The diplexed output of one of the redundant mixers isselected by a latching ferrite RF switch and fed into thetraveling-wave tube input switch. This switch is a Fara-day rotator switch which splits the input power and feedsit to any two of three output ports, depending upon thestate of the dc current being passed through its controlcoil. Logic circuitry in the transmitter provides the propercontrol current and turns on the corresponding pair oftraveling-wave tube amplifiers upon receipt of commandsignals. The communications repeater subsystem con-tains three 20 watt (nominal) traveling-wave tube ampli-fiers, of which any pair is selected by ground commands.The outputs of the two operating amplifiers are recom-bined in another traveling-wave tube switch connected ina reverse manner as compared to the input switch. Theswitch output signal is passed through band rejection fil-ters to reduce the second harmonic of the signal and thetransmitter noise level in the receiver band. The filteredsignal is next passed through a power monitor circuitwhich provides telemetry indication of the power outputof the transmitter. The SHF transmitter output, which isfed to the SHF transmit antenna, is at a level of 14.6 dBW.

Reference/Beacon Code Generator

The various local oscillators and beacon signals arederived from a single master oscillator in the referencegenerator to provide coherency in all UHF and SHF fre-quencies. A pair of redundant, temperature controlled,highly stable 5 MHz crystal oscillators are used. The out-put signals are derived by frequency division in highspeed integrated flip-flop dividers and by frequency mul-tiplication in various types of transistor, step recovery,and varactor multipliers. The beacon signals are biphasemodulated with a coded sequence.'

UHF Transmitter

The transmitter characteristics constitute one of themore crucial aspects of the UHF repeater design. As de-picted in Fig. 6, the UHF transmitter consists of 16 par-allel transistor amplifiers whose individual outputs aresummed together prior to transmission through thediplexer and antenna. The key to this transmitter is thepower summer. The summer is designed such that anynumber ofpower amplifiers can theoretically be operatedat a given time. The dc power availability in the space-craft permits nominally 13 amplifiers to be on. The re-maining power amplifiers can be used for peak periods ofoperation where inore power is available and for redun-dancy. The power summer has a characteristic thatswitching in or out a power amplifier does not theo-retically affect the load seen by the remaining amplifiers.The circuit diagram for this summer is depicted in

Fig. 10. The outpuits of all the amplifiers are connected toa transmission line summing network, as shown in the

OUTPUT50Q

RFNPUJT

RFINPUT

RFINPUT

Fig. 1 0. Power summer circuit diagram.

figure. The amplifier output connects to one end of aquarter-wavelength transmission line. The other ends ofthe 16 lines are tied together to form the summing pointfor the RF power. Isolating resistors are connected fromeach input point to a common junction. RF power re-flected from the junction of the transmission line to theother input ports is 180 degrees out of phase with the RFvoltage derived through the resistors as a result of itshaving passed through two quarter-wave lines. Thus,ideally, no RF power is produced at the other portsbecause of input at one of the input ports, and isolation isachieved. VSWRs of less than 2: 1 at the amplifier portsexist for 9 through 16 amplifiers operating.The design of the UHF transmitter permits the space-

craft UHF ERP to be adjusted as a function of seasonand time in orbit to provide maximum utilization of thedc power. Fig. 11 indicates how the number ofpower am-plifiers varies over time. The dc power availability isaffected by sun angle variations over the year and by ra-diation-induced degradation of the solar panel.The efficiency of the transmitter is optimized for power

availability which corresponds to 12 to 14 amplifiersoperating. Fig. 12 indicates the relative efficiency of thesystem as the number of amplifiers is varied. Efficiency isreferenced to the antenna input port and includes alllosses in the power summer, filter, diplexer, and cabling.The variations are caused by power summer residual mis-match and represent additional losses of 0.1 dB for 16amplifiers operating and about 0.5 dB with 9.

UHF Antenna Characteristics

The UHF antenna design presented significant chal-lenges. Basic requirements were to obtain the maximumgain possible over a 19 degree coverage area correspond-ing to the visible earth as seen from synchronous altitude.These requirements had to be met within the constraintsof size and weight permitted by the booster and fairing.The principal technical factor affecting the design selec-

BRANDES: TACTICAL COMMUNICATIONS SATELLITE 439

Li

Aj_TFj Mj

A.MPLI FIRS

Fig. 1 1 UUHF carrier power.NFig12OUHFtraFnsm Pit:fTcIey. -i

Fig. 1 2. UHF transmitter efficiency.

Fig. 13 UHF antenna patterns. (A) Transmit (B) Receive.

(A)NORT H

(B)NORTH

tion was the wide separation between transmit and re-

ceive frequencies amounting to almost 25 percent. Forthis reason, a helical array was selected for its broadbandcharacteristics. Even so. considerable problems were ob-tained in attempting to keep the receive gain high over a

19 degree sector and resulted in a fairly unusual antennacharacteristic. Range testing using both four element andfive element helical arrays indicated that slightly betterperformance. e.g., 0.9 to 0.3 dB, could be obtained witha four element array at the transmit frequency. However,the beam associated with this array was too narrow toprovide adequate gain at receive frequency over the en-

tire 19 degree sector The approach finally decided uponwas to utilize a fifth element in the center of the array

whose diameter and pitch were optimrized for the receive

frequency. This tended to broaden the beam at receive

sufficient to keep the gain at required values. The result-ing antenna patterns at receive and transmit are indicatedin Fig. 13. It is interesting to note that the peak gain is

higher at receive than at transmit, but falls off more

rapidly so that, over the required coverage sector, theminimum gain is about 2 dB lower.

SHF Transmitter

The SHF transmitter output power is formed by com-

bining the outputs of any two of the complement of threeTWTs carried on board the spacecraft. The output of

IEEE TRANSACTIONS O)N AEROSPACE AND ELECTRONIC SYSTEMS JUIX 1970

-I1l10°

SOUT H

440.

19.3 dB (ZERO AZIMUTH GAIN)

90

80/\

70

60

16.0 dB AT -9.5A

40 / | \15.2 dB AT +9.5-GAIN FIGU RES INCLUDE

3 / e WAVEGUIDE LOSS

20

10 '< 1.1 l,

18 12 6 0 6 12 18 1816 14 12 10 8 6 4 -22 0 2 4 6 8 10 12 14 16 18

BORESIGHT ANGLE, DEGREES

(A)Fig. 14. SHF antenna patterns. (A) Transmit. (B) Receive.

each TWT is slightly over 20 watts, so that the combinedRF output power is 40 watts. After combining losses inthe switch and filtering, the net output ofthe system at theantenna ports is reduced to 30 watts. The efficiency of thesystem as measured at the tube is 24 percent and overallto the antenna ports is 17 percent.

SHF Antenna Characteristics

Separate receive and transmit earth coverage horns are

used at SHF in order to avoid any diplexing problemsand to minimize losses. These horns are fin loaded forbalance and they produce typical patterns as shown inFig. 14.

BORESIGHT ANGLE, DEGREES

(B)

In-Orbit Testing

Since its launch, TACSAT has been used for com-

patibility testing between ground terminals and thespacecraft and for system network tests. At the writing ofthis paper, there is limited multiple access test experienceavailable. One interesting test in this area is the actualuse of the TACSAT for operational support of Apollorecovery operations. In this activity, a network of termi-nals are interconnected through TACSAT, including theApollo recovery aircraft, the recovery carrier, and theirground terminals. This operation is apparently quitesatisfactory and indicates the usefulness of a highpowered spacecraft to link together a number of mobileearth stations.

Richard D. Brandes was bom in Cleveland, Ohio, on December 19, 1935. He receivedthe B.S. degree from Massachusetts Institute of Technology, Cambridge, in 1957,and the M.S. degree in engineering from the University of California at Los Angeles.He joined the Hughes Aircraft Company in 1957 where he worked in missile

guidance and radar analyses. He subsequently performed system analyses and con-

ceptual design for a number of advanced space and missile programs. He was asso-ciated with the tactical communications satellite (TACSAT) program at Hughesfrom the preliminary design phase through launch. He served as Manager of SystemsEngineering and Analyses and later as Assistant Program Manager. He is currentlywith the Space Systems Division, El Segundo, Calif.

Mr. Brandes is a member of the American Institute of Aeronautics and Astro-nautics, Sigma Xi, Tau Beta Pi, and Eta Kappa Nu.

BRANDES: TACTICAL COMMUNICATIONS SATELLITE

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III(718.4Bg AllMUTH GAIN)

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441