Gamma Ray Observatory (GRO) Prelaunch Mission Operations Rep

8/7/2019 Gamma Ray Observatory (GRO) Prelaunch Mission Operations Rep

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nnsnNational Aeronautics andSpace AdministrationWashington, D.C.20546

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TO: DistributionFROM: S/Associate Administrator for Space Science andApplicationsSUBJECT: Gamma Ray Observatory (GRO) Prelaunch MissionOperations Report (MOR)Enclosed for your information is the GRO Prelaunch MOR whichcovers the GRO deployment mission. This Observatory will belaunched on the STS-37 mission in April.The GRO is the second of four Great Observatories to be launched(the first was the Hubble Space Telescope in April 1990). Eachof these complementary Observatories will view the universe in adifferent region of the electromagnetic spectrum. Together, theGreat Observatories will enable the international sciencecommunity to study astronomical objects in ways never beforepossible.This Observatory is the first spacecraft with a complement oflarge sophisticated instruments designed to study a broad rangeof gamma ray energies. It will be placed in a 450 km circularorbit by the Space Shuttle Atlantis. After about 5 weeks ofactivation, checkout, and calibration, the spacecraft wili begina year-long all sky survey of the gamma ray universe. In lateryears it will view in greater detail intriguing objectsidentified by the survey.This MOR (a) describes the objectives of the GRO mission; (b)provides a discussion of the mission deployment; (c) provides abrief description of the Observatory and its scientificinstruments; and (d) outlines the ground operations elementsrequired to support its launch, deployment, activation, andscience operations.

L. A. FiskEnclosure

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National Aeronautics andSpace Administration

. Mission Operations ReportOFFICE OF SPACE SCIENCE AND APPLJCATIONS REPORTNO. E-S-458-31 -91-01

GAMMA RAY OBSERVATORY


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TABLE OF CONTENTSForeword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Gamma Ray Observatory Program Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Mission Objectives ................................................................................. 4Scientific Objectives ..............................................................................Introduction ................................................................................ 2Objective Outline ..........................................................................Observing Plan ........................................................................... 6Science nstruments .............................................................................. 11General .................................................................................. .llThe Burst and Transient Source Experiment (BATSE) .............................The Oriented Scintillation Spectrometer Experiment (OSSE). ..................... i:,

The Imaging Compton Telescope (COMPTEL) ...................................... 20The Energetic Gamma-Ray Experiment Telescope EGRET) ..................... .23Spacecraft Description ............................................................................ 26Structure and Configuration ............................................................Propulsion Subsystem......................................................... ......... EAttitude Control and Determination System.......................................... 29Communications and Data Handling Subsystem ....................................Electrical Power Distribution Subsystem ................................... ......... i:Thermal Subsystems ................................................................... 35Launch Vehicle .................................................................................... 36StmcturaI Interfaces .....................................................................

Electrical Interfaces ..................................................................... 2Mission Sequence .................................................................................Launch and Pm-ReleaseOperations .................................................. z:Post-ReleaseActivations ................................................................ 44Routine Operations...................................................................... 44Mission Options and Termination ...................................................... 44Mission Support ..................................................................................Ground Systems and Support Elements .............................................. ;;Operational Data Flow ...................................................................Data Management ....................................................................... ztGuest Investigator Program ............................................................ 58Mission Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60GRO Program Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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Figure 1.Figure 2.Figure 3.Figure 4.Figure 5.Figure 6.Figure 7.Figure 8.Figure 9.Figure 10.Figure 11.Figure 12.Figure 13.Figure 14.Figure 15.Figure 16.Figure 17.Figure 18.Figure 19.Figure 20.Figure 21.Figure 22.Figure 23.Figure 24.Figure 25.Figure 26:

LIST OF FIGURESGRO Configuration (top view) ................................................... 8GRO Configuration (bottom view) .............................................. 9Illustrative Full Sky Observing Plan ........................................... 10GRO Science Instruments.. ...................................................... 12BATSE Detector Module ........................................................ .17OSSE Assembly ................................................................... 19COMPTEL Detector Assembly ................................................ .21EGRET Detector Assembly .................................................... .24GRO Structural Elements ........................................................ 27GRO Propulsion Feed System Schematic ...................................... 28ACAD BlockDiagram.. ......................................................... .30C&DH Subsystem Block Diagram.. ........................................... .32Modular Power System .......................................................... 34AESE Location in Payload Bay ................................................ .37GRO Early Mission Timeline (Nominal) ....................................... 39In-Bay Functional Checkout ................................................... .40Deployment/Release Events.. ................................................... .41GRO Unbuthing and AppendageDeployment................................ 42GRO Battery Charging and ReleaseAttitude .................................. 43Spacemft Activation Timeline ................................................. .45Instrument Activation ............................................................. 46Typical Instrument Activation Timeline ....................................... .47Tiieline and TDRS Coverage or 180 Attitude Maneuver ................ .48Delta-V Maneuvers ................................................................ 49GRO Mission: Launch to Termination Stages .............................. .50GRO Mission Options ........................................................... 51

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LIST OF FIGURES (continued)Figure 27. Ground Systems and Flight Operations . . . . * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Figure 28. Command and Telemetry Communication Links . 55Figure 29. GRO ScienceData Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56Figure 30. GRO Telemeuy Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Figure 31. GRO Data Archive and Distribution System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Figure 32. GRO ScienceFYogramSupport Organization . . . . . ..*............. ............ 59

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LIST OF TABLESTable 1. Summary of GRO Instrument Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Table 2. GRO Instrument Teams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

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FOREWORDMISSION OPERATIONS REPORTS are published expressly for the use of NASA seniormanagement, as required by the Administrator in NASA Management Instruction NM18610.3D, dated May 13,1982. The purpose of these reports is to provide NASA seniormanagement with timely, complete, and definitive information on flight mission plans, andto establish official mission objectives which provide the basis for assessmentof missionaccomplishment.Reports are prepared and issued for each flight project just prior to launch. Followinglaunch, updating reports for each mission are issued to keep management currentlyinformed of definitive mission results as provided in NASA Management InstructionHQMI 86lO.lB.These reports are sometimes highly technical and are for personnel having program/projectmanagement responsibilities. The Public Affairs Division publishes a comprehensiveseries of reports on NASA flight missions which are available for dissemination to thenews media.

Published and Distributed by theOFFICE OF HEADQUARTERS OPERATIONSNASA HEADQUARTERS


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GAMMA RAY OBSERVATORY PROGRAM OVERVIEWThe NASA Astroph)sics Program is an endeavor o understand he origin and fate of theuniverse, to understand the birth and evolution of the large variety of objects in theuniverse, from the most benign to the most violent, and to probe the fundamental laws ofphysics by examining their behavior under extreme physical conditions. These goals arepursued by means of observationsacross he entire electromagnetic spectrum, and throughtheoretical nterpretation of radiations and fields associatedwith astrophysicalsystems.Astrophysics orbital flight programs are structuredunder one of two operational objectives:(1) the establishment of long durationGreat Observatories for viewing the universe infour major wavelength regions of the electromagnetic spectrum (radio/infrared/submilli-meter, visible/ultraviolet, X-ray, and gamma ray), and (2) obtaining crucial bridging andsupporting measurementsvia missions with directed objectives of intermediate or smallscopeconductedwithin the Explorer and Spacelabprograms. Under (1) in this context, theGamma Ray Observatory (GRO) is one of NASAs four Great Observatories. The otherthree are the Hubble SpaceTelescope (HST) for the visible and ultraviolet portion of thespectrum, the Advanced X-ray Astrophysics Facility (AXAF) for the X-ray band, and theSpace nfrared TelescopeFacility (SIRTF) for infrared wavelengths.GROs specific mission is to study the sources and astrophysical processes hat producethe highest energy electromagnetic radiation from the cosmos. The fundamental physicalprocesses hat are known to produce gamma radiation in the universe include nuclearreactions, electron bremsstrahlung, matter-antimatter annihilation, elementary particleproduction and decay, Compton scattering, synchrotron radiation. GRO will address avariety of questions relevant to understanding he universe, such as: the formation of theelements; the structure and dynamics of the Galaxy; the nature of pulsars; the existence ofblack holes; the possible existenceof large amounts of antimatter, energetic and explosivephenomena occurring in galactic nuclei; the origin of the cosmic diffuse background;particle acceleration n the Sun, starsand stellar systems;processesn supernovae;and theorigin and evolution of the universe itself. This is achieved through use of fourindependent and complementary instruments developed by different Principal InvestigatorTeams. These nstruments cover different, but overlapping, energy bands anging from 10keV to 30 GeV and are much larger and much more sensitive than any gamma-rayinstruments previously flown in space.CR0 is a NASA cooperative program. The Federal Republic of Germany, with co-investigator support from The Netherlands, the European Space Agency, the UnitedKingdom, and the United States, has principal investigator responsibility for one of thefour instruments. The Federal Republic of Germany is also furnishing hardware elementsand co-investigator support for a second nstrument.GRO was originally conceived, designed, and developed as a Principal Investigator classmission in recognition of the complex nature of the instruments required. A plan to extendits mission lifetime led to its inclusion in NASAs Great ObservatoryProgram and the needfor a vigorous Guest Investigator program to maximize scientific productivity. Themission is structured n four phases: Phase 1 will last 15 months, during which the GROInstrument Teams will carry out an all-sky survey and other high priority observations;Phase 2 will last one year, with 30 percent of the observing time allocated to GuestInvestigators; Phase 3 will also last one year, with 50 percent of the observing timeallocated to Guest Investigators; Phase4 allocations have not been determined at this time.The data analysis associatedwith each nstrument requires a high degreeof familiarity withthe instrument, requiring the Guest Investigators, in the early phases of the program, to

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work directly and closely with one or more of the GRO Instrument Team scientists or theInstrument Specialists n the GRO Science Support Center. In later phasesof the mission,Guest Investigators may propose o work with less dependence n the Instrument Team.The GRO spacecraft s a 3-axis stabilized, free-flying spacecraft with a gross weight of15900 kg of which approximately 6000 kg is scientific payload. It is capable of pointing atany celestial target for long periods of time (days to weeks). Celestial pointing is to bemaintained with an accuracy of 0.5 degrees. Attitude determinations will provideknowledge of the pointing direction to an accuracy of 2 arcminutes. Absolute timing is tobe accurate to 0.1 millisecond. GRO has an onboard propulsion system withapproximately 1900 kg of monopropellant hydrazine. This system s to be used routinelyfor attitude control and for maintaining the desired orbital altitude. Sufficient fuel is to beretained for a controlled reentry. Primary electrical power of 4000 watts is provided bytwo silicon solar cell arrays. Approximately 2000 watts are available for the Observatory,with nickel-cadmium (NiCd) batteriessupplying power during eclipse periods.GRO is a dedicated SpaceShuttle mission and requires approximately one-half the volumeof the shuttle payload bay. The shuttle is to place the Observatory directly into the desiredcircular orbit at an altitude of 450 km at 28.5 inclination. If orbit injection occurs at alower altitude, GROs onboard propulsion system will be used to achieve the initialoperational orbit at 450 km. GRO will subsequentlybe maintained at an altitude between450 and 440 km until the propellant is reduced o the level required for a controlled reentryinto the ocean. This range of altitudes was selected because at lower altitudes theObservatory would encounterexcessivedrag and at higher altitudes it would be exposed ohigher levels of background radiation. If radiation levels near 450 km are found to belower than anticipated, a higher maximum altitude could be selected o extend mission life.An expectedorbital lifetime of about 10 years has been calculated assumingdeployment ofthe GRO in the 450 km circular orbit is accomplished.Nominal GRO timelines for launch and prelaunch operations specify selected functionalcheckouts n the Orbiter bay, beginning 21 hours after lift-off. Removal from the bay withthe Remote Manipulator System (RMS) and release rom the RMS will take place on FlightDay 3. Solar arrays and the high gain antennaare deployed, and batteriesare taken throughone charging cycle, while GRO is attached o the RMS. The shuttle will move away fromGRO following release. Completion of spacecraftactivations and checkout of the attitudecontrol system will be completed on Flight Day 5. Science nstrument activation begins onFlight Day 5, and the sciencemission is expected o begin begin, on Day 19.NASAs Tracking and Data Relay Satellite System (TDRSS) and CommunicationsNetwork (NASCOM) will link GRO to the GRO Payload Operations Control Center(POCC) in the Multisatellite Operations Control Center (MSOCC) at the Goddard SpaceFlight Center (GSFC). Real time data are to be transmitted at 32 kbps and the onboard taperecorder dumps the data at 512 kbps every other orbit. GROs onboard Communicationsand Data Handling System (C&DH) is basedon a standard NASA module that has beenmodified to provide a packetized telemetry data system, which will permit rapid datacapture and delivery, via NASCOM, to geographically distributed users. Sciencedata andthe required auxiliary data (e.g., attitude, orbit parameters,mode IDs, etc.) are merged andput into telemetry packets onboard the spacecraft. This eliminates the need for mergingsciencedata and auxiliary data on the ground prior to delivery to the science nvestigators.Each of the GRO Principal Investigator Instrument Teams has establisheddata processingfacilities with associated oftware for analyzing data from its instrument. The GRO ScienceSupport Center at GSFC will coordinate data access, istribution, and archiving activities.


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MISSION OBJECTIVES

Primary Objective:l The ultimate success f the Gamma Ray Observatory Mission will be udged on thebasis of obtaining two years of gamma-ray measurements (10 KeV to 30 GeV)covering close to the entire celestial sphere, with instruments that provide an orderof magnitude greater sensitivity and accuracy than previously-flown gamma-raymissions.

SecondaryObjectives:l Provide a uniform full-sky gamma-ray survey using the wide-field imaginginstruments, and make selected high-priority observations using the narrowaperture and independently-oriented nstruments.l Provide Guest Investigators 30 percent of the observation time at the completion ofphase (first 15 months) and 50 percent of the observation ime at the completion ofphase I (an additional 12 months).

.~GRO Program Manager

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Charles J. Pellerin, Jr.Director, Astrophysics Division

GRO Program Scientist

Lenmud A. FiskAssociate Administrator forSpaceScienceand Applications

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SCIENTIFIC OBJECTIVESIntroductionGamma-ray astronomy, the study of the highest energy electromagnetic radiation from thecosmos, provides the most direct measurementsof the presence and dynamic effects ofenergetic charged particles, element synthesis, and particle acceleration both within andoutside the Galaxy. The scientific questions that GRO is uniquely designed to addressinclude: In what settings and by what processes are charged particles accelerated torelativistic speeds Is the universemade up of entirely of matter, or could there be balancingregions of antimatter ? How were the elementscreated hat make up todays universe? Whatis the nature of quasarsand what is the sourceof their incredible power? What happens n asupernovaexplosion? What causes he sporadic ntense bursts of radiation called gammaray bursts? What is the origin of the cosmic gamma-ray background radiation? Becausegamma rays have a very low interaction cross-sectionand no electrical charge, they have avery high penetrating power and can reach the solar system, and the top of the Earthsatmosphere, rom essentially any part of the Galaxy or Universe with their paths unaffectedby magnetic fields. On the other hand, the atmosphere s opaque o gamma rays; hence, heobservations must be made from space. The gamma-ray photons observed n space haveretained the detailed imprint of spectral,directional, and temporal features mposed at theirbirth, even f they were born deep n regions opaque o visible light and X rays, or at timesfar back in the evolutionary history of the Universe.The full range of gamma rays coversmany decadesof energy, from the merger with X raysnear 20 keV to energies n excessof 100 Gev. A variety of physical processes re nvolvedin the production of gamma rays and many of theseprocesses re not detectable n any otherpart of the electromagneticspectrum. Thus, the study of gamma rays opens a new window,not only to the Universe, but to a whole new realm of astrophysical processes. Some of thephysical processeswhich can be uniquely examined using gamma rays are: (a) nuclearinteractions of energetic nuclei observable through their gamma-ray products, (b)electromagnetic processescharacteristic of energetic electrons as observed through theirsynchrotron, bremsstrahlung, and Compton emissions, (c) gamma-ray line-producingprocesses uch as radioactive decay, cyclotron processes, osmic-ray interactions, etc., (d)matter-antimatter annihilations such as baryons-antibaryonsand electron-positron, and (e)poorly understood relativistic processes n supergravity (e.g., quasar, black hole, galacticnuclei) systems.GRO will be the worlds first satellite to provide an all-sky mapping of the cosmos atgamma-ray energies. Of the 30 or so discrete gamma-ray sourcesobserved o date, only afraction have been identified with objects observedat other wavelengths. This is a result oflimited sensitivity and angular resolution n previous gamma-ray nstruments. GRO, with itsunprecedented dvance n sensitivity (over a factor of 10 beyond previous nstruments), willallow detailed ocalization and study of thesesourcesand will increase he number of knownsources to several hundred. The most energetic objects in the universe, includingsupernovae, neutron stars, black holes, cores of galaxies, and quasars, are known, orpredicted to be intense sources of gamma ray emissions. Locations and opticalidentifications, basedon gamma ray observations,are needed or a thorough understandingof theseobjects. In addition, spectralmeasurementswill address he formation of elementsby nucleosynthesis n supernovae nd novae.Time variability studies will address he natureof neutron stars,pulsars and the energy production processesn the active galaxies.Gamma-ray emissions are also observed n the form of diffuse glows in the plane of theGalaxy, extended sources in the Galaxy, and relatively uniform diffuse extragalacticbackground radiations. Some unknown fraction of the galactic diffuse glows comes from

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emissions rom unresolveddiscrete sources; he remainder may be attributed to cosmic-rayinteractions with other matter in the interstellar medium. Extended galactic sourcesare, inparticular, believed to be principally caused by cosmic-ray interactions with variousdistributions of gas and dust in interstellar space,which means that gamma rays can be auseful tool for exploring the structureand dynamics of the Milky Way.Celestial gamma rays have also been observed in the form of intense bursts, implyingtransient sources, asting only a fraction of a second o about 100 seconds. They were firstobserved by Vela spacecraft in 1967 and roughly 100 bursts per year have since beenrecorded. Although intensely studied, subject o the unpredictability of their occumenceanddifficulties imposed by their short duration, they are still not well understood. A galacticorigin related to neutron stars s favored by the most recent theories. Transient gamma-rayemissions of a different nature are also produced during solar flares. GRO is particularlywell instrumented or burst studiesand ncludes modes hat will provide unique informationfor studying flare accelerationmechanisms.Objective OutlineBased on the scientific rationale for the GRO program and the recommendations of theCommittee on Space Astronomy and Astrophysics of the National Academy of SciencesSpace Science Board, the GRO Science Working Team adopted the following set ofscientific objectives:l A study of discrete objects such as black holes, neutron stars,and objects emitting onlyat gamma-ray energies.l A search for gamma-ray line emissions indicative of sites of nucleosynthesis (thefundamental process n nature or building up the heavy elements) and other gamma-raylines emitted in astrophysicalprocesses.l The exploration of the Galaxy in gamma rays in order to study the origin and dynamicpressureeffects of the cosmic-ray gas and the structural features revealed through theinteraction of the cosmic rays with the interstellar medium.l A study of the nature of other galaxies as seenat gamma-ray wavelengths, wirh specialemphasison radio galaxies, Seyfert galaxies and Quasi Stellar Objects.l A search or cosmological effects, through observationsof the diffuse gamma radiation,and for possible primordial black hole emission.l Observations of gamma-ray bursts, their luminosity distribution, their spectral andtemporal characteristics nd their spatial distribution.l Map the distribution of diffuse gamma-ray ine emission (positron annihilation at 0.5 11MeV and 26Al decay at 1.809MeV) to determine ts origin.Observing PlanFour complementary nstruments,described n a following section, are required to fulfill theprogram objectives. Their optimum energy rangesand prime functions are noted below:BATSE: The Burst and Transient Source Experiment will monitor the entireunoccultedsky for transienteventsand burstsand it will provide burst triggersignals o other instruments, BATSE is optimized for the 60 keV to 600 keV

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range with secondary, ower sensitivity, spectroscopicdetectorscovering therange rom 10 keV to 100 MeV.OSSE: The Oriented Scintillation Spectrometer Experiment will observe line andcontinuous gamma-ray sources n the 0.1 to 10 MeV range and solar gammarays and neutronsabove 10 MeV.COMPTEL: The Imaging Compton Telescope will perform a very sensitive celestialsurvey in the 1 to 30 MeV range with a wide field of view, good angularresolution, and low background.EGRET: The EnergeticGamma-RayExperiment Telescopewill search or diffuse anddiscretegamma-raysources rom 30 MeV to 30 GeV and t will measure heirintensity, energy spectrum,position, and time variations.The objective of the first phase (15 months) of the mission is to provide a nearly uniformfull-sky survey for the two wide-field imaging instruments, COMPTEL and EGRET, whileat the same time providing for high priority observations by the narrow aperture andindependently-oriented OSSE experiment. The BATSE instrument has continuoussensitivity to the entire unocculted sky and will observe bursts, solar flares, and othertransients.The all-sky survey is to be achieved with a series of thirty-three two-week (nominal)exposuresby the COMPTEL and EGRET instruments which are co-aligned with the Z-axisviewing direction (see Figures 1 and 2). The sky-survey viewing plan is constrained byspacecraft thermal and electrical considerations, based on the Suns location, and by therequirement that likely discrete sourcesof gamma radiation be observable by the OSSEexperiment. The OSSE detectors can point over a range of 180 about the Y-axis; thispermits OSSE to observesources n the X-Z plane. Hence, for each Z-axis pointing of thespacecraft,an X-axis orientation is chosen o provide secondary argets or OSSE. Figure 3is a Galactic map of the different Z-axis pointing directions to illustrate the degree ofuniformity provided by a preliminary plan for a June 4, 1990 aunch. The ellipses representan approximation to the field of view of one of the wide-field instruments at the one-halfsensitivity level and the numbers indicate the sequence of d ifferent Z-axis pointingdirections. The actual pointing directions and the sequencewill dependon the actual launchdate.Deviations from the all-sky survey plan are to be considered f an unusual astrophysicalevent happens o occur during the survey phase. This could extend the 15 months requiredto complete the survey.Observing plans for subsequent haseswill be basedon priority observationsselectedby thePrincipal Investigator Teams, he findings from Phase1, and the priorities assigned o GuestInvestigator proposals requesting observing time. During Phase 2, lasting one year, 30percent of the total observing ime is to be allocated o Guest nvestigatorsproposing OSSE,COMPTEL, and EGRET investigations. During Phase3, also lasting one year, 50 percentof the total observing time on OSSE, COMPTEL, and EGRET will be allocated to GuestInvestigators. Observing allocations for Phase4, the remainder of the mission, are to bedetermined at a later date. BATSE will continue to monitor for gamma ray bursts and othertransient sources.

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SCIENCE INSTRUMENTSGeneralThe four GRO instruments representan integrated complement whose common goal is thedetailed study of the celestial gamma-ray spectrum. Collectively, the selected nstrumentsspan the photon spectrum from 10 keV to 30 GeV, almost 6 decades in energy. Eachinstrument has been chosen such that its sensitivity regions overlap those of its nearestneighbors, thus allowing for complete continuity of spectral measurements and cross-calibration of each of the instruments (see Figure 4). Without exception, each nstrumentrepresentsa major step forward in experimental capability. The photon sensitivity over theentire 6-decade ange has been mproved by more than an order of magnitude over the bestpreviously flown instruments. Significant improvements in angular resolution have alsobeen made. Salient instrument characteristicsare summarized n Table 1.The principal Investigator(s) and Co-Investigators hat constitute each Instrument Team arelisted in Table 2, together with their respective nstitutions and major responsibilities.The Burst and Transient Source Experiment (BATSE)(a) BATSE Instrument ObjectivesThe BATSE is designed for continuous monitoring of a large fraction of the sky so thatgamma-ray bursts and other transient sourcescan be detected, ocated and studied. Thereare at least three different classes of gamma-ray bursts which have distinct spectral andtemporal characteristics. These three classes may indicate vastly different energyproduction mechanisms for gamma-ray bursts or even different types of emitting objects.Many models of gamma-ray bursters have been developed by theorists. In recent years,most models contain a neutron star as the central object. Among the events which arespeculated to trigger a gamma-ray burst are: a starquake n the neutron star, a compactobject impacting the surface of a neutron star, or a thermonuclear explosion of materialaccumulatedon the neutron star surface.One of the unique features of gamma-ray bursts is their extreme variability on shorttimescales. The BATSE detector array will be able to observe variations on timescales asshort as several microseconds. Observations of fast time variations will allow models ofemission mechanisms and source geometry to be studied in detail. Spectral variations ontimescales as short as 0.1 millisecond will also be observed. Such measurements willallow the testing of candidate emission mechanisms. For example, models based onradioactive decay, inverse Compton emission, and synchrotron emission, followingsudden injection of relativistic electrons into a strong magnetic field, would each predictdifferent spectral variations during a burst.Of prime importance in the study of gamma-ray bursts s the location of the burst with highaccuracy so that optical and/or radio counterparts may be found and studied. The bestmethod to obtain arcminute class locations is through an interplanetary network of burstdetectors. By recording the time of arrival of a burst at widely separated ocations, theburst location is accurately determined by triangulation methods. The GRO burst detectorsare expected to be a key element of this network due to their high sensitivity and accuratetiming capabilities. The detectors on interplanetary spacecraft are, of necessity, small andrelatively insensitive. As a result, only the strongest bursts can be located by the existingnetwork. Numerous additional bursts, too weak to be detectable by the interplanetarynetwork, will be detectable with the large-areadetectors aboard GRO. Their positions may

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Table 2. GRO Instrument TeamsBATSE

InstitutionMarshall Space Fhght Center

PersonnelG. 1. Fishman, PIC. A. Meegan, Co-IT. A. Pamell, Co-IR. B. Wilson, Co-I

ResponsibilitiesExperiment management, ardware,softsvare, nd data analysis

University of California.San Diego (UCSD)Goddard SpaceFlight Center

J. L. Matteson, Co-IB. J. Teegarden,Co-IT. L. CIine, Co-I

Flight hardware components,anddata anaIysisData analysis

University of Alabama,Huntsville (UAH) W. S. Paciesos, Co-I Test and calibration, data analysis,and mission operations

OSSEInstitution

Naval ResearchLaboratory (NRL)Personnel

J. D. Kurfess, PIW. N. Johnson, Co-IG. H. Share, Cc+1R. L. Kinzer, Co-IM. S. Shickman, Co-I

ResponsibilitiesProgram management,groundsupport equipment, flight software,instrument, scintillator, andphototube subcontracts; calibmtion,integration, mission operations, anddata analysis

Northwestern University (N-U) M. Ulmer, Co-I Sciencedata processingand dataZllldpiSRoyal Aerospace Establishment,Farnborough, EnglandClemson University (CU)

C. Dyer, Co-ID. D. Clayton, Co-I

Detectorbackgroundmodel and dataanalysisData analysis and theoreticalinterpretations

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Table 2. GRO Instrument Teams (continued)

COMPTELInstitution Personnel

Max-Planck-Institute for V. Schonfelder,PIExtraterrestrial Physics, G. Lichti, Co-IGarching, West Germany R. Diehl, Co-I

ResponsibilitiesProgram management, owerdetector, veto domes, nstrumentintegration and calibration, and dataanalysis

Laboratory for SpaceResearch,Leiden, Netherlands B. N. Swanenburg,Co-IW. Hermsen,Co-IH. Aarts, Co-IAnalog electronics, low voltagepower supply, and data analysis

University o f New Hampshire J. A. Lockwood, Co-IW. R. W ebber, Co-IJ. Ryan, Co-IUpper detectorassembly, ront-end-electronics, and data analysis

EuropeanSpaceAgency (ESA)ESTEC,Noordwijk, Netherlands K. Bennett, Co-IB. G. Taylor, Co-I Digital electronics, inflightcalibration sources,ground supportequipment, and data analysis

EGRETInstitution

GoddardSpaceFlight CenterPersonnel

C. E. Fichtel, Co-PIR. C. Hartman, Co-ID. A. Kniffen, Co-ID. J. Thompson, Co-ID. L . Bertsch, Co-I

ResponsibilitiesProgrammanagement, pacecraftinterface, spark chambers, hermalanalysis, data analysis, mechanicalstructure, time-of-flight electronics,digital data handling electronicsinstrument integration software, andsoftware system

Stanford University

Max-Planck-Institute forExtraterrestrial Physics,Garching, West Germany

R. Hofstadter, Co-PI*E. B. Hughes, Co-I*P. Nolan, Co-IK. Pinkau, Co-PIH. A, Mayer-Hasselwander, o-IH. Rothermel, Co-IM. Sommer, Co-IG. Kanbach, Co-1

Lower crystal, packaging, andelectronics; data analysis: energycalibration and calibration facilityAn ticoincidencedomeandelectronics, data analysis, pressurevessel,and calibration fixture

Grumman AerospaceCorp.

*Deceased

A. Favale, Co-IE. Schneid, Co-I Calibration, instrument integration,and data analysis

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be determined to within a few degrees by GRO alone, depending on their intensity, bycomparing the responsesof individual BATSE detectors pointed in different directions.The detection by GRO of hundreds of gamma-ray bursts (anticipated within a two yearperiod) will allow relatively accurate determination of the distribution of these sources nthe Galaxy, and possibly the identification of some bursts with extragalactic objects. Forthe first time, these burst locations will become available within days of their occurrence.The coarse ocation of bursts will also enable deep, wide-field, optical photographs to betaken of those regions.Another objective of the BATSE is to provide the GRO with a full-sky monitor of long-lived strong sources. The other experiments on the GRO have limited fields of view andare constrained in their pointing capability. The BATSE will be able to detect and locatestrong transient sources and outbursts of known sources rom all regions of the sky. TheBATSE instrument can also study the stronger pulsing hard x-ray and gamma-ray sourcesfrom all parts of the sky anytime during the mission.(b) BATSE InstrumentationThe instrument consists of eight wide-field detector modules, with one module at each ofthe eight comers of the GRO spacecraft to permit maximum unobstructed, continuousviewing of the sky. All modules have dentical configurations as shown in Figure 5. Eachmodule contains two detectors: a large-areadetector, optimized for broad energy coverageand directional response,and a spectroscopydetector, optimized for broad energy coverageand good energy resolution.The main detector is a disk of sodium iodide (NaI) scintillation crystal, 20 inches indiameter and l/2-inch thick. A light collector housing on each detector couples thescintillation light into three, 5inch diameter photomultiplier tubes. An anticoincidenceshield on the front side is used to reduce the background due to charged particles, and athin lead and tin shield inside the light housing reduces the amount of background andscattered adiation entering the backside. The spectroscopydetector s a 5inch diameter, 3-inch thick, NaI scintillation crystal. Signals from the detectors are processed n the detectorelectronics unit and routed to the central electronics unit for digital processing. The centralelectronics unit is a microprocessor-based data system that processes and stores largeamounts of data from all 16 detectors n various formats. It takes one orbit to read out thedata from a burst. At other times, background and calibration data are recorded along withdata used to study long-lived transients and pulsing sources. Of particular importance forobtaining calibration data are the observationsof solar flares and the emission from hard x-ray sources such as that in the Crab nebula. Other long-lived hard x-ray and gamma-raysourcescan be observedas they are occulted by the earth each orbit.It is expected that the BATSE will discover from 100 to 400 bursts per year. Some ofthese bursts will be in the field of view of other experiments, in which case time profiles,spectra, and locations can be compared. An onboard burst trigger signal from BATSE willbe sent to the other GRO instruments to allow other GRO detectors to gather data on theburst. A solar flare trigger signal is provided to the OSSE and COMFIEL instruments.The Oriented Scintillation Spectrometer Experiment (OSSE)(a) OSSE Instrument ObjectivesThe OSSE has been designed to undertake comprehensive observations of astrophysicalsources in the 100 KeV to 10 MeV range. Secondary capabilities for gamma-ray and

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neutron observations above 10 MeV have also been included, principally for solar flarestudies. The use of NaI(Tl) scintillation detectors provides a large area and highsensitivity, optimized for line and continuum measurements n the 0.1 to 10 MeV region.The highest sensitivity is maintained by implementation of an observational technique thatminimizes susceptibility to background variations due to orbital geomagnetic atitude effectsand spallation-produced radioactivity. This technique uses a single axis offset pointingcapability to modulate a celestial sourcescontribution to the observedgamma-ray flux on atime scale (typically 2 minutes) that is short compared to the background variations so thatit can be extracted from the background. The offset pointing capability also enables theexperiment to undertake observations of selected sources, such as transient objects, atlocations on the sky other than in the direction of the spacecraft Z-axis. The single axisnature of the offset pointing capability means that only a limited portion of the sky isavailable for any given spacecraftorientation. For example, this capability will be used tomonitor solar flare activity without impacting the planned observational program of theother GRO experiments. It can also be used to observe secondary sources during orbitalperiods of earth occultation of the primary source.(b) OSSE InstrumentationFour identical detector systems are used to obtain the operational flexibility to meet thevariety of objectives previously outlined. As shown in Figure 6, each of the detectors ismounted in a single axis orientation control system which provides offset pointing over arange of 192 degrees. The detectors are generally operated in co-axial pairs. While onedetector of a pair is observing the source, the other detector can be offset to monitor thebackground After a programmable time interval, the detectorswill interchangeobservationdirections by opposite rotations.The primary element of each detector system is the NaI portion of a 33 cm diameterNaI(Tl)-CsI(Na) phoswich. Pulse-shape discrimination is used to distinguish energydispositions in the NaI primary element from those in the CsI portion of the phoswich. ANaI annular shield assembly, together with the CsI portion of the phoswich, forms theactive anticoincidence shield for background rejection. The angular responseof a detectoris defined by a tungsten alloy passive collimator also located within the NaI annular shield.The passive collimator provides the 3.8 x 11.4 rectangular field of view throughout the0.1 - 10 MeV energy range. Each phoswich is viewed by seven photomultiplier tubes(PMTs), providing an energy resolution of 8% at 0.661 MeV. Active gain stabilization isused to maintain this energy resolution (Delta E/E of 20% in the central part of the energyrange) by individually adjusting the gain of each of the sevenPMTs.The experiments data acquisition and control system incorporates varied modes ofoperation depending on the type of information desired during a particular observation.Diagnostic capabilities and redundancy are important features of the system; the ability toreconfigure the experiment in the event of a failure has also been ncluded. This versatilityis achieved through use of redundant programmable microprocessors.Energy spectra n the 1OOKeV 10 MeV energy range are processedby two pulse-heightanalyzers for each detector. Individual spectra are accumulated and transmitted over aninterval between 2 and 32 secondsdepending on the instrument operating mode. Typicaloperating modes will use intervals of 4 or 8 seconds. Energy losses >lO MeV areprocessedby a third pulse-height analyzer for each detector and also read out on a 4-secondtime scale.


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An alternate data mode provides the time resolution necessary for the analysis of fastpulsars. In this mode, spectral resolution is sacrificed to obtain time resolutions of up to0.125 msec. High time resolution studies of gamma-ray bursts are made using the activeshield elements of each of the detectors. The rates rom all of the elements are summed andsampled by a burst detection circuit which will store 4096 samples of the summed shieldrates at up to 4 msec time resolution. Crude directionality of the burst can be obtained bycomparison of the relative responses f the individual shield elements.The Imaging Compton Telescope (COMPTEL)(a) COMPTEL Instrument ObjectivesCOMPTEL has been designed to perform a very sensitive survey in the energy range from1 to 30 MeV. It combines a wide field of view (about 1 steradian) with good angularresolution. The lack of reliable results from previous investigations in this energy rangereflects the extreme observational difficulties involved. A basic problem with thesemeasurements has been the background generated n the instruments. Therefore, a greateffort has been made to minimi ze the background n COMPTEL so that the detection limitsshall be determined only by counting statistics.The three essential properties of the instrument: (1) imaging capability, (2) broad aperture,and (3) low background characteristics, reflect the best available techniques to achieve theprincipal scientific objectives. These are the study of:

l galactic and extragalacticpoint sourcesl diffuse emission from the Galaxy. cosmic diffuse fluxl broadened ine emission

COMPTEL also provides the possibility of detecting linear polarization from stronggamma-ray sources and, like the other GRO instruments, will provide gamma-ray burstcapabilities. Energy spectra of solar and cosmic gamma-ray bursts can be measured withgood resolution between 0.1 and 10 MeV. Bursts that are within the field-of-view of thetelescope can be located to better than 1 degree accuracy. In addition, COMPTEL has thecapability to measureneutrons from solar flares, if the sun s in the field-of-view.(b) COMPTEL InstrumentationCOMPTEL (see Figure 7) uses two detector arrays in series for detection by a two stepprocess. In the upper one (Dl) a liquid scintillator, NE 213 A, is used. In the lower one(D2) NaI(Tl) crystals are used. A gamma-ray s first Compton-scatteredby interacting withan electron in Dl and the location of the interaction and the energy lost to the recoilingelectron are measured. The scattered gamma-ray is then absorbed n D2 and the locationand energy of the absorbedphoton is measured. The energy of the incident gamma-ray isthe sum of the two measured energies. The locations of the interactions in Dl and D2define a circle in the sky where the gamma ray originated. After numerous photons havebeen recorded and sorted, the direction of the source can be determined by comparingwhere the paths of the circles intersect. The l-sigma angular resolution is 1.3 at 2.75MeV, slightly greater below, and less above, this energy. The energy resolution is about8% at 1 MeV and less than 6.5% above 4 MeV.As shown in Figure 7, the two detectors are separated by a distance of 1.5 m. Eachdetector is entirely surrounded by a thin anticoincidence shield (consisting of two plastic

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ANTICOINCJDENCE (AC! DOME VT01 MOOULES(NE 213A)

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scintillator domes) which provide the basic nfwtion necessary o recognize (and therebydisregard the effects of) charged particles. Between both detectors there are two smallplastic scintillation detectors containing weak Cobalt 60 sources; hese are used as gamma-ray calibration sources.The upper detector (D 1) consists of 7 cylindrical modules of liquid scintillator NE 2 13 A.Each module is approximately 28 cm in diameter and 8.5 cm thick and is viewed radially by8 photomultiplier tubes. The total area of Dl is 4310 sq cm. The lower detector (D2)consists of 14 cylindrical NaX(Tl) blocks, 28 cm in diameter and 7.5 cm thick, which aremounted on a baseplate with a diameter of about 1.5 m. Each NaI block is seen frombelow by seven, 7.5 cm photomultipliers. The total geometrical area of D2 is 8620 sq cm.Each anticoincidence shield consists of two 1.5 cm thick domes of plastic scintillator NE110. A dome is viewed by 24 photomultipliers. Except for the front-end electronics of thephotomultipliers, all main electronics are mounted on a platform outside the detectorassembly.Each calibration source consists of a cylindrical piece of Cobalt 60-doped plasticscintillator, 3 mm thick and 1.2 cm in diameter, which is seen by two 1.9 cmphotomultipliers.A gamma ray is electronically identified by a delayed coincidence between the Dl and D2detectors, combined with the absenceof a veto from all charged particle shields and fromthe calibration detectors. The quantities measured or each event are:

l the energy of the Compton electron n Dl (energy >50 keV)l the location of the collision in Dll the pulse shapeof the scintillation pulse in Dll the energy loss in D2 (energy >500 keV)l the location of the interaction in D2l the time-of-flight of the scatteredgamma ray from Dl to D2l the absolute ime of the event

The pulse shapemeasurementsand the time-of-flight measurementsare performed in orderto reject background events.A gamma-ray event is initially selectedaccording to coarsecriteria establishedon the pulseheights of signals from Dl and D2, time of flight, the absence of a veto signal from allanticoincidence domes, and the provision that no preceding nteraction has deposited a largeamount of energy in the triggered cell (overloads).satisfied, a series of actions is initiated as follows: If the initial event selection logic is(1) pulse height analysis of the 15photomultipliers of the identified modules of Dl and D2, plus the sum of the eight Dl andseven D2 PMTs; (2) time-to-digital conversion of the time-of-flight and pulse-shapediscriminator circuit outputs; (3) the triggering of the digital electronics; and (4) thesampling of the event time. In addition to this normal double-scattering measuring mode,two of the NaI crystals of the telescope will be used to measure the energy spectra ofcosmic gamma-ray bursts and solar flares.The digital outputs of the pulse height analyses and the time-of-flight conversions arefurther checked against upper and lower limits, pulse shape, and ratio .of energy deposits,which are stored n the final gamma event selection ogic via serial telecommands. The dataare stored n the event buffer, with those events satisfying the final selection criteria havingfirst priority and those failing one or more criteria having secondpriority for transmissionthrough the telemetry. Calibration data are transmitted regularly either as events or asspectra.

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Estimates of the orbital trigger rate and the need to transmit an adequate sample ofcalibration events have led to an event message ate of 48 events per 2.048 seconds. Forone event, 16 x 16 bit words are required. COMPTELs data rate is thus 6125 bits/set.The Energetic Gamma-Ray Experiment Telescope (EGRET)(a) EGRET Instrument ObjectivesThe principal objective of the EGRET experiment is high sensitivity observations of thegamma-ray sky in the energy range from 20 MeV to 30,000 MeV (30 GeV). It will havethe best angular resolution of any of the GRO instruments and will provide good energyresolution.The high energy telescopewill be able to recognize point sources hat are nearly two ordersof magnitude fainter than the Crab nebula. For strong sources the position should bedetermined to about 10 arcminutes, and the strongest high energy sources should beidentified with a positional accuracy of 5 arcminutes. Spectra should be measurable overthe entire energy range for the stronger sources.For the diffuse galactic plane emission, the spectrum will be measuredwith high accuracy,and spatial variations in the spectrum should be measurable on a scale of a few degrees.Features which subtend more than about 0.5 will be resolvable as extended sources. Thediffuse radiation away from the galactic plane will be separable into galactic andextragalactic components on a scale of about 5. The extragalactic component will bestudied for spatial variations in intensity and spectrum.(b) EGRET InstrumentationThe telescope s shown SchematicaIly n Figure 8. The directional telescopeconsistsof twolevels of a four by four scintillator array with selectedelements of each array in a time-of-flight coincidence. The upper spark chamber assembly consists of 28 spark chambermodules interleaved with 27 tantalum plates, each with a 0.02 radiation length (which is afunction of the energy of the radiation and the type of material) in which the gamma-raymay convert into an electron pair; the initial direction of the electrons may be determinedfrom their tracks in the spark chamber. The lower spark chamber assembly (between thetwo time-of-flight scintillator planes), allows the electron trajectories to be followed,provides further information on the division of energy between the electrons, and permitsseeing the separation of the two particles for very high energy gamma rays. The energy ofa gamma-ray will usually be determined from measurementsmade in the Total AbsorptionShower Counter (TASC), a 76 cm x 76 cm NaI(Tl) scintillator crystal eight radiationlengths thick located below the lower time-of-flight scintillatar plane.A gamma-ray entering the telescopewithin the acceptance ngle has a known probability ofconverting into an electron-positron pair in one of the thin metal foils between the sparkchambers n the upper portion of the telescope. If at least one of the electrons s detected bythe directional time-of-flight coincidence system as a downward moving particle, and thereis no signal in the large anticoincidence scintillator surrounding the upper portion of thetelescope, he track imaging system is triggered, providing a digital picture of the gamrna-ray event, and the analysis of the energy signal from the NaI(Tl) crystal is initiated.Incident charged particles are rejected by the anticoincidence dome. Low energy backwardmoving charged particles which do not reach the anticoincidence dome are rejected by thetime-of-flight measurement. Events other than the desired gamma-rays, such as thosegamma-rays nteracting in the thin pressure essel nside the anticoincidence scintillator, arerejected n the subsequent ata analysis.

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Gamma-ray bursts will be recorded n both the anticoincidence dome and the large NaI(Tl)crystal, where an energy spectrum measurement will also be made in the 0.6 to 140 MeVinterval. After each burst trigger signal from BATSE, the spectra are recorded in four timeintervals, which are preset between 0.125 and 16 seconds. Following the recordinginterval, the system is unable to recognize any additional BATSE triggers during the 35minute readout period.

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SPACECRAFT DESCRIPTIONStructure and ConfigurationFigures 1 and 2, top and bottom views, schematically illustrate the configuration of theassembled spacecraft. With approximately 6000 kg of scientific payload, 8000 kg ofspacecraft weight and 1900 kg of propellent, the total weight of GRO is about 15900 kg,GRO occupies about one-half the volume of the shuttle payload bay prior to deployment.Developed within the constraints and requirements of the National Space TransportationSystem (NSTS), the GRO structure accommodates the scientific instruments with anarrangement hat provides unobstructed ields of view and supports all subsystemelementsof the spacecraft. Figure 9 provides a break-away illustration of the platform, instrumentadapters, and secondary structures. Three appendagestructures - the high-gain antennaboom and the two solar arrays - are deployed while GRO is attached o the NSTS RemoteManipulator System. RMS).Propulsion SubsystemThe GRO propulsion subsystem provides impulses for orbit altitude changes, orbitmaintenance,attitude control, and controlled reentry. As schematically illustrated in F igure10, the propulsion subsystemconsists of the following major subassemblies:

Four diaphragm propellant tanks capable of supplying up to a total of 1905 kg ofhydrazineAn orbit Adjust Thruster Module (OATM) consisting of four 100 pound thrustersand solation valvesFour Dual Thruster Modules (DTMs) each consisting of two 5 pound thrustersfor attitude controlTwo Propellant Distribution Modules (PDMs) containing the feed system filters,propellant pressure ransducers,and latching isolation va lvesTwo propeUant/pressurant ill and Drain Modules (FDMs)One On-Orbit Refueling Module (OORM) containing the JSC-supplied refuelingcoupler

The system operates n a blowdown mode over a pressure range of 2.76 x 10 to 5.79 x106 dynes per square centimeter (400 to 84 psia). The propellant is high-puritymonopropellant hydrazine and the pressurant s nitrogen. Crossover valves permit centerof gravity managementcapability by controlli.ng the quantity of propellant in each ank.The four Grbit Adjust Thrusters (OATS) will be fired simultaneously to provide AV (changein velocity) impulses for orbit altitude changesas needed o maintain operational altitudes orfor controlled reentry. The OATS are off-modulated to provide control about the roll andpitch axes during AV burns. The five pound Attitude Control Thrusters (ACTS) are cantedfrom the Z-axis and fired in pairs to provide the necessarycontrol torque about any of thethree Observatory axes n the event of a failure in the primary or secondaryattitude controlmodes.

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Attitude Control and Determination SubsystemThe primary purpose of the Attitude Control and Determination (ACAD) subsystem s topoint the GRO instruments to selected celestial gamma-ray sourcesand to supply attitudeinformation for data processing. The ACAD subsystem s a three-axis system made up ofmany NASA standard components, together with other flight-proven hardware. A blockdiagram of the ACAD subsystem is shown in Figure 11. The sensors are shown on theleft, the processing and distribution electronics n the middle, and the actuatorson the right.The primary sensorsare the Fixed-Head Star Trackers (FHST) and the Inertial ReferenceUnit (IRU). Other sensors nclude the Three-Axis Magnetometer (TAM), Fine Sun SensorAssembly (FSSA), and Coarse Sun Sensor Assembly (CSSA). The prime actuators arethe four skewed Reaction Wheel Assemblies (RWAs) that are unloaded by the MagneticTorquer Assembly (MTA) or the ACTS in a backup mode. The Attitude ControlElectronics (ACE) processesdata from the sensorsand sends t to the observatorys On-Board Computer (OBC) that is part of the Command and Data Handling (C&DH)subsystem. The primary control algorithms are located in the OBC. In the event of anOBC failure, the Control Processor Electronics (CPE) in the ACE processes he attitudedata and drives the actuators. The ACE also acts as the interface to the C&DH system forcommands and telemetry.The Solar Array Drive Assembly (SADA) and the High-Gain Antenna Drive (HGAD) arealso considered part of the ACAD system. The HGAD control signals are derived from theOBC that uses he Observatory attitude information and ground-supplied Observatory andTracking and Data Relay Satellite (TARS) ephemeris data to compute the antenna positionangles. The solar-array drive is nominally positioned at a fixed orientation every 14 daysby ground commands. In the event of a failure, the solar-array drive uses CSSA data todrive the arrays to an index position to provide a sun-tracking mode.The primary and backup modes of operation for the ACAD system are defined below.

The standby mode is used during deployment from the shuttle. Sensors and processingelectronics are on, but all actuatorsare disabled.The normal pointing mode provides the capability to point the GRO to an accuracy of 0.50degree per axis in any orientation required for the science mission. The FHST and IRUprovide attitude information to the OBC. The IRU gyros provide inertial rate data and theFHSTs perform star measurements for attitude and gyro drift updating. Torques forstabilization are provided by any three of the four reaction wheels. Three MTAs are drivenfor wheel momentum unloading. Earth magnetic field measurementsare provided by theTAM.(cl NormalThe normal maneuver mode allows holding attitude or maneuvering to any requiredorientation up to 180 in less than 1 hour. Control torques are provided by the ReactionWheel Assemblies (RWAs). The only difference from the normal pointing mode is that themomentum unloading and gyro updating functions are disabled.W ThrusterThe thruster maneuver mode provides the capability to maheuver he Observatory hrough a180 turn in inertial space n less than 30 minutes using the attitude control thrusters. Thethruster pairs are fired sequentially to provide the required control torques about each axis.

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(e) Velhtv ConedMO&The velocity control mode provides the capability to execute velocity correction flings ofdurations selected by external commands. Control torques are developed by off-modulating the OATS and by firing the ACTS. The OATS can provide torques about theroll and pitch axes. The ACTS are used or yaw control.03s . .Poinwn-ReferencedThe Sun-Referenced Pointing Mode (SRPM) provides capabilities to hold attitude or topoint the +X-axis at the Sun within 1.0 degree per axis using RWAs. The processing ofattitude sensor data and control of the actuators is in the Control Processor Electronics(CPE) versus the OBC used in the normal pointing mode. An OBC failure is one way ofgetting into the SRPM. It can also be entered by ground command. Control laws aredesigned to operate with valid gyro data on at least two axes. In the event of bad data oneither the pitch or yaw axis, rate data derived from the fme sun sensorcan be used. In theevent of a roll-axis IRU failure, the system operates on data provided by the fine sunsensor.w SafeThe safe hold mode is a backun to the SRPM. If a failure occurs when in the SRPM. theObservatory will go to the safehold mode. During this operation, the ACAD systemwillbe capable of orienting and maintaining the X-axis within 6 degrees of the sun line.Starting Tom any orientation, this system is capable of reorienting the X-axis towards thesun in 30 minutes. Functions are identical to the SRPM except that the control torques areprovided by the attitude control thrusters.(h) Contingencv Orb-t Mwce ModeThe contingency ordt maintenance mode provides the capability to change the orientationof the Observatory and perform velocity control firings without the OBC. Attitude controltorques are provided by the ACTS.Communications and Data Handling SubsystemThe Communications and Data Handling (C&DH) system, illustrated in Figure 12,provides the observatory s elecommunications and data processing unctions. The C&DHdesign s basedupon the standardNASA module used on the Solar Maximum Mission andLandsats 4 and 5. The C&DH module has been modified for use on GRO by the additionof a Time Transfer Unit (TTU) and the change to a packetized telemetry system. TheC&DH subsystemconsists of the C&DH module, a 60-inch high-gain antenna, wo omni-directional low-gain antennas, and an RF combiner to interface the module with theantennas.The C&DH module includes two second-generationTDRSS transponders or forward andreturn link transmissions to TDRSS., and for command and telemetry transmissions to theshuttle during the in-bay and deployment sequences. Redundant Central Units (CUs) forcommand decoding, telemetry collection, and formatting are provided. Two NASAstandard ape recorders are ncluded for data storage;each has the capability for storing 4.5x 108 bits of data. Other components nclude redundant Pmmodulation Processors PMP)for basebandsignal formatting and mode selection; a NASA Standard SpacecraftComputerNSSC- 1 OBC for attitude control, telemetry processing and monitoring, and commandstorage; and the ITU for correlating spacecraft time to Universal Time, Coordinated(UK), and for distributing time to the four instruments. Timing accuracy s maintained to0.1 ms by the use of the highly stable oscillator that controls the CU. A timing update

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Thermal SubsystemsThe thermal control of subsystemsand instruments is accomplished by appropriate use ofcoatings, blankets, louvers, radiators, and heaters. The instruments are thermally isolatedfrom each other and from the spacecraft structure in order to reduce temperature gradientsand to simplify thermal analysis and testing. The COMPTEL instrument uses variable-conductance heat pipes that transfer heat to a remote radiator in the hot case, whileproviding a near-adiabatic interface in the cold case. The other instruments have passivethermal designs.GRO uses hree groups of heaters,each having redundant thermostats and heater elements.Operational heater circuits are adequate or normal orbital operations in the coldest case.Make-up heaters eplace the power of an instrument or component when it is turned off inorbit. Space Shuttle auxiliary heaters, powered through the AESE, are used to maintaintemperatureswhile GRO is in the shuttle bay.


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LAUNCH VEHICLEGRO has been approved as a dedicated mission on the National Space TransportationSystem (NSTS). To maximize GROs mission lifetime, the Orbiter will releaseGRO into acircular orbit at 450 km altitude and an orbital inclination of 28.5. Prior to deployment,GRO, together with its associatedAirborne Electrical Support Equipment (AESE) system,occupies approximately one-half the volume of the Orbiter payload bay.Structural InterfacesGRO is attached to the Orbiter via four sill and one keel trunnion fittings. The AESEsystem is installed in the bay forward of the GRO as illustrated in Figure 14. A RemoteManipulator System (RMS) grapple fixture is located on GRO (Figure 1) for deploymentoperations and for capture and handling during later missions. For repair and refuelingmissions, GRO has a refueling coupling and a flight support system berthing adapter andtarget. In addition, numerous handrails and foot restraint sockets have been installed tofacilitate Extravehicular Activities (EVAs).Electrical InterfacesThe AESE is used to power GRO auxiliary heaters o keep the Observatory thermally safeafter the payload bay doors are opened, to provide limited alarm telemetry for monitoringcritical parameters, and to power-up GRO for in-bay checkouts. GRO is electricallycoupled to the shuttle via two Standard Umbilical ReleaseSystem (SURS) connectors thatare EVA rematable. The SURS connectors interface the GRO with the AESE, StandardSwitch Panel (SSP), and T-O umbilical. SelectedGRO safety critical data are provided fordisplay on board or at the Mission Control Center (MCC) and Payload Operations ControlCenter (POCC). GRO telemetry and command signals are routed to the ground via theshuttle Payload Interrogator/Payload Data Interleaver (PI/PDI) interface. (Communicationwith GRO is only possible when GRO is turned on for in-bay functional checks and RMSdeployment operations.) Selected GRO telemetry is also displayed on board viadecommutated data from the PDI. These same data are available via hardline wheneverGRO is powered-up (either with the transmitter on or off).For a refueling mission, GRO has an electrical interface installed beside the refuelingcoupling that provides pressure and temperature data on the refueling coupling and theGRO hydra&e propellant tanks.

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MISSION SEQUENCELaunch and Pre-Release OperationsThe timeline in Figure 15 illustrates the sequenceof GRO events prior to beginning thescience mission. GRO is to be unpowered during the Shuttles ascent phase except forpower to keep the star tracker shutters n the closed position. GROs Airborne ElectricalSupport Equipment (AESE) is to be powered-up by the Space Shuttle crew 1.5 hours afterlift-off, immediately prior to the opening of the payload bay doors. The AESE power-upprovides GRO with heater power and very limited alarm telemetry for monitoring criticalparameters.GROs in-bay functional checkout (Figures 15 and 16) on Flight Day-2 is planned to start21 hours after lifr-off and lasts approximately 2 hours. These checks, performed by theOrbiter crew and the flight operations team at GSFC, include: (a) command and telemetrychecks through the shuttle PI/PDI; (b) reaction wheel/attitude control system checks; and(c) direct telemetry and non-operational command checks hrough TDRS via the GRO omniantenna.The power-up and communication aspectsof the checkout also constitute a partial rehearsalfor deployment activities on Flight Day-3.The sequenceof deployment activities, performed by the Space Shuttle crew and the flightoperations team at GSFC on Flight Day-3, is illustrated in F igure 17. Figures 18 and 19,respectively, illustrate GROs position on the Remote Manipulator System @MS) prior toand after deployment of the solar arrays and the high-gain antenna. Contingency plans forappendagedeployment failures require a quick-response EVA for manual deployment bythe astronauts, f needed. The nominal deployment timeline (Figure 17) is designed o meetthe following requirements and safety concerns:

minimum power usageuntil arrays are chargingSpaceShuttle thrusters nhibited during unlatch and deploytemperature control of GRO solar arraysmaintenanceof two-fault tolerance or SpaceShuttle safetydaylight requimd prior to unberthingon-board sequencesor uninterrupted actuationsmaximization of TDRS coverage or appendage eploymentmaximization of battery chargingPrior to release,dual telemetry links (Orbiter and TDRS) are to be established; however, nocommanding via TDRS is planned prior to RMS release. GROs batteries will go througha charging cycle prior to release such that the charge levels at release will allow onerevolution with no solar illumination without exceeding a 67% depth-of-discharge limit.The nominal depth-of-discharge at release is 20% with a redline for release at 35%.Release plans call for a release at sunrise with the +X axis pointed at the sun and withmaximum TDRS coverage. GRO is released n a free drift (standby) mode; the Orbiter willmove away from the GRO after it has been eleased.

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Gamma Ray Observatory (GRO) Prelaunch Mission Operations Rep

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Transcript of Gamma Ray Observatory (GRO) Prelaunch Mission Operations Rep