1 Introduction

The Engineering Test Satellite VIII (ETS-VIII) [1] has been designed to inherit the satel-lite technologies accumulated in the develop-ment of the Engineering Test Satellite VI(ETS-VI), the Communications and Broad-casting Engineering Test Satellite (COMETS),the Data Relay Test Satellite (DRTS), and oth-ers. Its purpose will be to further technologi-cal development and provide a platform forthe experiments described below, contributingto the establishment of the advanced commonfundamental technologies and satellite com-munications technologies required for futurespace activities. A diagram of the ETS-VIII inorbit is shown in Fig.1, and its main specifica-tions are provided in Table 1.

- A three-ton-class geostationary satellite fea-turing the world’s most advanced bus tech-nology, designed to support a variety of cut-ting-edge space applications

- World’s largest and most advanced largedeployable reflector (LDR; maximum outerdiameter: 19 m×17 m)

- Mobile communications system for geosta-tionary satellites capable of voice and datacommunications with portable terminals

- Mobile satellite digital multimedia broad-casting enabling transmission of voice andimages of high quality comparable to that ofcompact disc (CD) media

- Essential positioning technologies includinghighly precise time reference devices

YONEZAWA Katsuo and HOMMA Masanori 13

3 Development of Satellite System

3-1 Overview of ETS-VIII Satellite

YONEZAWA Katsuo and HOMMA Masanori

Engineering Test Satellite VIII (ETS-VIII) has purposes of establishing 3-ton-class geosta-tionary satellite bus technology in the world class, equipped with large deployable reflec-tors, answering to the emerging demands in mobile communications and global positioningnecessary for the early 21-st century, and conducting the relevant experiments on orbit.ETS-VIII satellite bus incorporates NASDA-developed technologies such as reduced-weightmodular body structures, 7.5 kw-class high power supply in 100 volts, multipurpose 1553Bdata bus and CCSDS-based packet transmission, attitude stabilization for the satellite withlarge flexible structures, unified satellite controller of 64-bit MPU with reprogramming capa-bility.

Keywords ETS-VIII, 3-ton-class geostationary satellite bus, Large deployable reflector

Diagram of the Engineering Test Satel-lite (ETS-VIII) in orbit

Fig.1

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The National Space Development Agencyof Japan [NASDA; consolidated as of October1, 2003 (Heisei 15) with the Institute of Spaceand Astronautical Science (ISAS) and theNational Aerospace Laboratory of Japan(NAL) to form the Japan Aerospace Explo-ration Agency (JAXA)] is developing theETS-VIII together with the CommunicationsResearch Laboratory (CRL), the Nippon Tele-graph and Telephone Corporation (NTT), andthe Advanced Space CommunicationsResearch Laboratory (ASC). NASDA’s geo-stationary satellite bus carries a payload thatincludes NASDA’s large deployable reflector(the first of its kind to be deployed, a 400-Whigh-output phased array feeder section (ASC,CRL, NTT), a transponder section (ASC), anon-board switchboard section for mobile com-munications (ASC, CRL), a highly precisetime reference device (NASDA, CRL), and afeeder link device (NASDA), among otheritems. Fig.2 shows the system configurationof the ETS-VIII in detail. Fig.3 shows a sceneof the assembly of the ETS-VIII flight model(August 2003).

Throughout the system design of the ETS-VIII, a redundant configuration is adopted foreach item of equipment. In terms of thoseitems that cannot feature a redundant configu-

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

System configuration of ETS-VIIIFig.2

Assembly of ETS-VIII flight model(August 2003)

Fig.3

Main specifications of the Engineer-ing Test Satellite (ETS-VIII)

Table 1

ration in terms of their functions (referred toas “single-point” components, including theantenna, apogee engine, power supply busline, etc.), each such single-point componentis provided with an alternate function so thatoverall system functioning may be maintainedin case of a fault.

A flight model of the ETS-VIII has alreadybeen manufactured and assembled, and hasbeen undergoing integrated system testingsince this summer (2003), with developmentto be complete next spring (2004). Fig.4shows the development schedule for the ETS-VIII.

The ETS-VIII is to be launched by the H-

IIA204 rocket from the Tanegashima SpaceCenter at the end of 2004. Fig.5 shows thesequence of system operations for the ETS-VIII.

2 ETS-VIII bus system

The payload mass required for the ETS-VIII to accomplish its mission is as high asapproximately 1.2 tons, involving power con-sumption (including the bus) of about 7 kW.The payload mass of the two-ton-class geosta-tionary ETS-VI and COMETS satellites wasapproximately 600 kg and the generatedpower was about 4 to 5 kW; since the ETS-

YONEZAWA Katsuo and HOMMA Masanori 15

Development schedule of the ETS-VIIIFig.4

Sequence of system operations of the ETS-VIIIFig.5

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VIII requires greater capacity, we begandevelopment of a three-ton-class geostationarysatellite bus for the ETS-VIII. In addition, inthe overseas, demands for large capacity andadvanced communications and broadcastingfunctions have increased. As a result, overallenlargement, increased power supply, longerservice life, and increased payload ratio are

required, leading to the promotion of thethree-ton-class geostationary satellite bus.Accordingly for the ETS-VIII a three-ton-class geostationary satellite has been designedto take flexibility and expandability (in termsof mass, electric power, etc.) into account.Fig.6 shows a functional system diagram ofthe ETS-VIII bus equipment, while Fig.7

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

ETS-VIII functional system diagram (bus equipment)Fig.6

ETS-VIII functional system diagram (on-board experimental equipment)Fig.7

shows a functional system diagram of theETS-VIII on-board experimental equipment.The main technologies developed in connec-tion with the ETS-VIII three-ton-class largesatellite bus are described below.

- Supports a three-ton-class bus. Improvementin payload capacity, with a payload weightratio (an index of satellite bus technology) of40% (traditionally 30%).

- Large lightweight modular body structureresulting in a shorter development schedule(the assembly realized through modulariza-tion)

- Power supply may be increased thanks toadvanced heat-exhaust technology (enlarge-ment of effective radiating surface by anorth-south connecting heat pipe)

- Data-handling technology enablingimproved system generality, expandability,and operability (system is generalized with a1553B data bus, and international compati-bility is ensured through CCSDS-compliantpacket transmission), as well as improvedautomation and a reduction in the number ofcomponents

- Highly stable, highly precise attitude controlfor a satellite having large flexible structures(two sheets of large deployable reflectorsand two paddle wings consisting of largesolar cells)

- Integration of attitude control, data process-ing, and satellite management functions intoa satellite controller (equipped with a 64-bitMPU)

- Addition of a fault-tolerant function and a re-programming function that allows re-writingof loaded software from the ground

Focusing on these main technologicaldevelopments, the subsystem design of theETS-VIII satellite bus is described below.

2.1 Telemetry command systemAs a data transmission system for teleme-

try commands (i.e., a multiplexing system),the ETS-VIII employs a packet transmissionsystem compliant with CCSDS (Consultative

Committee for Space Data System) recom-mendations, using USB (United S-Band) tech-nology. The features of this system are as fol-lows.

- Compatibility with future international stan-dards, evidenced by its adoption by NASAand ESA

- Flexible output formats and output frequen-cies

- Sophisticated error correction allowing trans-mission of large amounts of data, images,software, etc.

- Permits interface adjustment with onboardsatellite subsystem

The CCSDS packet system inherited theachievements of the ETS-VII, and is furtherminiaturized and sophisticated in the ETS-VIII. An on-board CCSDS processing devicehas been realized in an LSI. As data bus sys-tems, the ETS-VIII employs the MIL-STD-1553B data bus, a virtually international mul-tipurpose standard interface, and featuresexpandability beyond the conventional systemof establishing connections via a remote inter-face unit (RIU). The adoption of the 1553Brenders it possible to establish a direct connec-tion between the on-board equipment and thedata bus and provides an interface for teleme-try commands with variable-length packets,which increases the degree of freedom in datatransmission from on-board equipment.

2.2 Power supply systemThe power supply system for the ETS-VIII

employs a 100-V regulated power supply bus.Although NASDA satellites after the ETS-VIused 50-V brute supply busses, a 100-V regu-lated power supply bus was selected for theETS-VIII based on the anticipated requiredpower supply (more than 6 kW) and therequirement that the system be upgradeable toprovide higher power (10 to 15 kW), a featureconsidered necessary for future satellite appli-cations. Among the various advantages of the100-V regulated power supply bus, the powerefficiency of the overall system is increased

YONEZAWA Katsuo and HOMMA Masanori 17

18

through a reduction in power transfer loss. Interms of the ETS-VIII, the difference betweena 100-V regulated power supply bus and a 50-V brute supply bus translates into a power dif-ference of 400 to 500 W. In addition, the 100-V regulated power supply bus provides thefollowing further advantages.

- Reduced system mass and improved systemreliability through a 50% reduction in thenumber of harnesses

- Reduced solar-cell panel area (a reductionequivalent to 70% of a single panel) and pad-dle mass

- Mitigation of attitude disturbance, through areduction in loaded propellant attributable tothe reduced solar-cell panel area

- High degree of freedom in the selection ofbattery stages; future expandability to higherpower

On the other hand, the change in voltagefrom 50 V to 100 V entails special considera-tion in design to discharge measures and theselection of parts. To address these points, weintend to design and implement ample dis-charge measures and are verifying selectedparts through component experiments. A 100-AH NiH2 battery has been adopted, developedbased on the 50-AH nickel hydrogen (NiH2)battery aboard the DRTS.

2.3 Solar-cell paddle systemSolar-cell paddles consist two wings, north

and south, each measuring 13.5 m×2.5 m,comprised of a total of four panels, whichrotate to follow the sun, generating 7,500 W ofpower (predicted for summer solstice afterthree years of operation). For these solar-cellpaddles, a lightweight rigid system is adopted;this system will be less affected by theincrease in mass accompanying future widerpaddles (to accommodate increased powersupply) and is also relatively cost-competitive.This system uses a rigid aluminum honey-comb panel featuring a highly elastic carbonfiber reinforced plastic (CFRP) surface, offer-ing reduced thickness and high stiffness;

therefore, a comparatively rough material canbe used for the cells of the honeycomb core,leading to a lighter paddle. A highly efficientSi solar cell (Si-NRS/BSF) was selected, tak-ing weight reduction, miniaturization, and costinto consideration. As a concrete measureagainst dielectric breakdown accompanyingthe voltage increase to 100 V, a number ofmeasures were implemented in the design ofthe solar cell, including reduction in the poten-tial difference between adjacent cell arraysand embedding of insulation in the gapsbetween adjacent cell arrays.

2.4 Attitude control systemThe ETS-VIII employs an attitude control

system developed based on the COMETS andDRTS systems [2]. Such a system stabilizesattitude using a gyroscope as a reference; itupdates roll angle and pitch angle using anEarth sensor, and adjusts yaw angle using afine-tuned Sun sensor. Disturbance by solarradiation pressure acting upon the largedeployable reflector of the ETS-VIII isapproximately thirty times that seen in con-ventional satellites. Therefore, the zero-momentum method is adopted for the attitudestabilization system; four large-capacity 50-Nms reaction wheels absorb the disturbance ina four-skew arrangement, and the absorbeddisturbance is unloaded (removed) by thrusterjet. However, in preparation for potential mal-functioning of the on-board gyroscope, thesystem is capable of control by the bias-momentum method, through estimation of theexternal disturbance or other means. Since itis difficult to conduct an overall structural testof the large deployable reflector on theground, it is anticipated that a certain degreeof indeterminacy will be included in the speci-fications of the flexible structures to be usedfor attitude control. Therefore, the ETS-VIIIis designed to mitigate calculation delaythrough high-speed operations of the satellitecontroller (SC), to permit the control band-width to be set as wide as possible, to stabilizephase, and to render the SC adaptable to varia-tions in the specifications. Moreover, the sys-

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

tem identifies the mode frequencies of theflexible structures in orbit (a total of 13accelerometers are mounted on the satellite toacquire the necessary data), allowing the con-trol system to be tuned using the re-program-ming function of the SC.

2.5 Modular body structure The ETS-VIII modular body structure is

based on a central cylinder composed of anantenna tower (the “rooftop”) carrying anantenna power supply, a payload module(upper stage) carrying onboard experimentalequipment, a bus module (middle stage) carry-ing bus equipment, and a propulsion module(lower stage) carrying a subsystem of thepropulsion system. In the central cylinder, thefuel tank and oxidizer tank are arranged verti-cally. The construction of the modular bodystructure system is shown in Fig.8. The mod-ular body structure will support the ETS-VIII’s launching mass of 5.8 tons and is capa-ble of supporting up to 6 tons without alter-ation of the design. Moreover, its payloadcapacity is 40%, as compared to the 30% fig-ure for the ETS-VI and COMETS two-ton-class satellites. The mass of the modular body

structure is less than 340 kg, due to the weightreduction provided through the use of CFRP,and to reductions in the number of materials,parts, joints, and the like. Since the payloadmodule and the bus module are partitioned byweb-panels on the north and south planes, theinteriors of these modules can be easilyaccessed by removing the access panels on theeast and west sides. Moreover, since the mod-ular satellite body can be mounted on a dual-axis rotational dolly, accessibility to internalon-board equipment and external on-boardequipment is ensured—the entire modularsatellite body can be made to rotate. Further-more, assembly, disassembly, and testing canbe executed in parallel for each module, sothat work can be performed efficiently in ashort period of time.

2.6 Thermal control systemA time-proven combination active/passive

thermal control system is employed to controlthe on-board equipment to ensure that a mar-gin of 15℃ or more is ensured with respect toallowable temperatures throughout the entiremission period. The thermal control system ofthe ETS-VIII features a north-south connect-ing heat pipe panel of CFRP surface for anEarth panel of the payload module. Thisresults in high-capacity heat exhaust, which inturn increases power supply in future satel-lites, reduces limitations on the placement ofheat-generating equipment, and allows foreffective use of the Earth panel. The systemprovides automated thermal control using anon-board computer: temperature is read fromsensors mounted on the modular satellitebody, and the heater is automatically turned onand off aboard the satellite. This reduces theamount of required ground operations, permitsthermal control to be reset in orbit, andenables fine temperature control in operation).The north-south connecting heat pipe isembedded in the Earth panel, allowing forequalization of heat between areas exposed tosunlight and those in shade, or between areaswith more heat-generating equipment andthose with less, through thermal connection of

YONEZAWA Katsuo and HOMMA Masanori 19

Construction of ETS-VIII modular bodystructure

Fig.8

20

the south and north planes. Overall heatexhaust is improved as a result.

Furthermore, plating the payload modulewith CFRP surfaces (featuring improved ther-mal conductivity) provides greater strength,higher rigidity, and lower thermally-induced-distortion, leading to more stable orbital align-ment than available using conventional alu-minum surface. The ETS-VIII also carries adeployable radiator for use in orbital experi-ments, and can deploy radiating paddles inspace to verify its own operating characteris-tics and heat-exhaust capacity.

2.7 Propulsion systemThe propulsion system of the ETS-VIII is

made up of a 500-N dual-liquid apogee engine(AKE) for geostationary orbit launch, 20-Ndual-liquid thrusters for attitude control andeast-west orbit control, and 20-mN xenon (Xe)ion engine devices for north-south orbital con-trol. The four separate firings of the apogeeengine together transfer the satellite into ageostationary orbit. The number of thrustersfor attitude control and east-west orbital con-trol has been reduced from the 20, mounted ineach of the ETS-VI and COMETS satellites,to a total of 12 thrusters. The apogee engineand the thrusters for attitude control and east-west orbital control share a fuel tank and anoxidizer tank. For east-west orbital control,two thrusters are activated simultaneously.

For north-south orbital control, the two ionengines are activated in an ascending node andin a descending node, respectively. The ionengine has been improved to provide greaterservice life, a simplified structure, and easieroperation, based on the achievements of theETS-VI and COMETS. The thruster mount-ing positions have been changed from theeast/west arrangement seen on the ETS-VIand COMETS satellites to a north/south orien-tation on the anti-Earth plane. Additionally,simultaneous dual-thruster operation has beenchanged to separate single-thruster operation(to reduce peak power), and a gimbal mecha-nism has been added to enable adjustment ofthe propulsion vector.

2.8 Integrated designThe ETS-VIII integrates the following

three tasks using the satellite controller (SC)through time-division processing: data pro-cessing in the telemetry command system,high-speed calculation in the attitude controlsystem, and satellite management (e.g., heatercontrol and battery management). The advan-tages of integration include enriched FDIR(Fault Detection, Isolation and Reconfigura-tion) and enhanced re-programming functions,in addition to a reduced number of onboardcomponents and increased flexibility in satel-lite operations. A 64-bit MPU newly designedby NASDA for space applications is used inthe computer of the SC.

The MPU periodically diagnoses datafrom sensors, actuators, and other componentsof the attitude control system as part of itsFDIR functions. If a fault is detected, theMPU isolates and assesses the failed part andtakes appropriate actions, such as switching toa redundant system, using the SC’s automaticprocessing functions. Automatic processingof a failure is performed in three phases, cor-responding to the broad division of faults intothe following three levels. In Level 1, the SCis normal, and the failed part is automaticallyre-configured through isolation andswitchover to the redundant system, withoutthe need to recover attitude; in Level 2, the SCis abnormal, and the recovery of attitudebecomes necessary in addition to switchingfrom the failed part to the redundant system;and in Level 3, the failed part presents a faultthat cannot be dealt with by SC switching or afault that causes recovery of attitude to fail, sothe SC immediately moves into sun acquisi-tion mode to secure power and waits for com-mands from the ground. To prepare for a faultthat cannot be resolved by FDIR functions, theSC is provided with a re-programming func-tion. The re-programming program, designedand verified on the ground, is transmittedusing a magnitude command, written in theRAM reprogram area (approx. 128 KB) of theSC, and executed.

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

3 Onboard experimental equip-ment

A functional system diagram of the ETS-VIII onboard experimental equipment isshown in Fig.7.

The large deployable reflector (LDR) is anantenna for S-band mobile communicationsand multicasting experiments, and is com-posed of two antenna planes, each of which iscapable of transmission and reception. Oneantenna plane is composed of 14 hexagonalmodules, and measures 19 m×17 m in outerdimensions; this is the largest antenna of itskind. The satellite is launched into a geosta-tionary orbit with its LDR retracted, and theLDR is then deployed to its final configura-tion. Since this deployment is a critical eventand involves innovative techniques, we intendto develop the antenna and the related technol-ogy empirically, through deployment analysis,launch-environment tests, micro-gravityexperiments, and more throughout all phasesof design, manufacture, and testing [3].

The feeder link communications equip-ment (FLCE) relays (i.e., receives and trans-mits) signals between a ground base stationand mobile communications equipment (an S-band converter, an on-board processor, and apacket switchboard) in the Ka band throughthe FL antenna, enabling experiments inmobile communications and multicasting.

The high accuracy clock (HAC), based ona cesium atomic clock, generates a time refer-ence, enables execution of positioning experi-ments in the S-band and the L-band (using theHAC antenna in combination with GPS data),

and provides the fundamental technology forgeostationary satellite positioning. Moreover,the HAC features an interface for mobile com-munications equipment, and is capable oftransmitting and receiving signals in mobilecommunications and multicasting experi-ments, thus serving as a backup for the S-bandsystem in the event of abnormalities in theLDR.

4 Concluding remarks

To ensure that we will be able to pursuefuture space activities, including those relatedto mobile communications and global posi-tioning, it is indispensable that we develop alarge deployable reflector and at the same timeestablish the technology for a large light-weight bus with a high payload capacity. TheETS-VIII offers increased communicationcapacity, payload ratio, power supply capabili-ty, and service life, based on development of alarge, world-class geostationary satellite busoffering sufficient flexibility to support three-ton-class satellites.

Acknowledgements

We are thankful to those involved in thedevelopment of the ETS-VIII at the NationalSpace Development Agency of Japan, to theMitsubishi Electric Corporation (in charge ofthe system equipment, including the satellitebus), and to NEC TOSHIBA Space Systems,Ltd. (in charge of the mission equipment,including LDR and positioning).

YONEZAWA Katsuo and HOMMA Masanori 21

References1 K. Yonezawa, M. Homma, S. Yoshimoto, S. Hama, T. Ohira, N. Natori, and Y. Tsutsumi, "Outline of Engi-

neering Test Satellite-VIII (ETS-VIII)", ISTS 98-h-01V, 21th International Symposium on Space Technology

and Science, 1998.

2 K. Yonezawa and M. Homma, "Attitude Control on ETS-VIII Mobile Communication Satellite with Large

Deployable Antennas," 21st AIAA/ICSSC, Yokohama, April 2003, Paper # AIAA-2003-2216.

3 H. Homma, S. Hama, H. Kohata, M. Hamamoto, and M. Tanaka, "Experiment Plan of ETS-8 in Orbit: Mobile

Commnications and Navigation," Acta Astroautica 53 ,477-484, 2003.

22 Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

YONEZAWA Katsuo, Ph. D.

Japan Aerospace Exploration Agency

Optimal Systems Control and Estima-tion

HOMMA Masanori, Dr. Eng. Japan Aerospace Exploration Agency Guidance and Control