MastVibration Damping System Themastvibration damping requirements weredrivenbyseveral coupled factors, specifically: a)the metrology system's mastdynamic motiontracking capability, b)SRTMinstrument pointing requirements, c)attitude control disturbance torques associated, andd)theOrbiter dynamic disturbance environment induced bytheattitude control system. SRTMutilized ametrology system toprovide relative outboard antenna position andattitude knowledge duringradarinterferometer operation. Thismetrology system consisted oftwosubsystems. Thesubsystem, whichdrovethedamper design, consisted oftheAstros Target Tracker (ATT),andthreeOptical TargetAssemblies (OTA's). TheATTwasessentially astar tracker thathadbeenrefocused to60-m.EachOTAcontained anLED,whichwaspointed towards the ATT,andactedasapsuedo-star thattheATTwasabletotrack.Thecombination oftheATTandOTA's provided anaccurate estimate forfiveoftheoutboard antenna's sixrigidbodydegrees offreedom, the ATTandOTAconstellation doesnotaccurately measure rangetotheoutboard antenna. ALeicarange findersupplemented theATTbydirectly measuring thedistance toaretroreflector arraymounted onthe outboard antenna. Withrespect toinstrument pointing, thenominal attitude duringdataacquisition wasto: 1)flytheOrbiter withitstailpointed alongthevelocity vector, 2)rotatetheOrbiter aboutitsroll-axis such thatthemastwas45degrees fromthelocalvertical, and3)radarradiating surfaces oriented towardthe ground. TheOrbiter reaction controlsystem wasusedtomaintain SRTMpointing withina0.1degattitude deadband, and0.01deg/sec attitude ratedeadband. Giventheattitude requirement andcombined SRTM/Orbiter massproperties, thegravitygradient torquewasthedominant disturbance torquetothe Shuttle reaction control system. Specifically, thegravity gradient torquetended torotatethecombined Orbiter andmastsystem suchthatthemastlongitudinal axiswasoriented alongthelocalvertical. The Orbiter's DigitalAuto-Pilot wasconfigured suchthatthe9-kg(24-1b)Vernier Reaction Control System jets wereusedforattitude controlduringradaroperations. Basedontheattitude control requirements, and configuration versusthedisturbance torqueapplied tothesystem, thereaction controlsystem generated positive rollcommands thatresulted injetfiringstocounter thegravitygradient torque.Asaconsequence ofthesejetfiringstransient vibrations inthemastweregenerated. Masttipmotionwasnotaconcern to theoperation oftheradarasaninterferometer provided thattheknowledge ofthetipmotion was acquired. Hence, theATTandLED'swereaddedtotrackthemastmotion. Thecapability oftheATTto acquire andtrackthemotionoftheLED'sdefined amaximum rateofmastmotionthatcouldbetracked, thislimitwasdefined tobe6cm/sec (2.36in/sec)atthetipofthemast.Additionally, inorderfortheATT toacquire theLEDthemasttiprateswererequired tobelessthan6mm/sec (0.24in/sec)fortenpercent ofthetimeduringdataacquisition. Therefore amastvibration damping system wasimplemented to enabletheATTtoacquire andtrackthemastmotion giventhevibration environment generated bythe attitude controlsystem. Vibration Damping Requirements SRTMimplemented amastvibration damping subsystem inordertomeetmasttipdynamic motion requirements. Earlyintheproject design phase, apreliminary coupled Orbiter anddeployed mast structural dynamic mathmodelwascreated. Thismathmodel wasemployed inattitude control simulations toprovide: a)propellant consumption estimates, b)masttipdynamic motionestimates, andc) mastdamping requirements. Giventhatthefinaldamping system implementation wasnotdetermined, the preliminary mathmodelwasa"modal" model, i.e.,thetruecomplex modalbehavior associated with discrete viscous damping elements wasapproximated. Theresultsoftheseearlysimulations showed that themastdamping mechanisms should bedesigned toachieve "high",i.e.,greater than10%,damping ratiosinthedeployed mast'sfirstorthogonal bending vibration modepairandthefirsttorsional vibration mode.Theserequirements werelaterconfirmed viasimulation ofafinalcoupled modelwhichincluded the complex modalbehavior associated withthediscrete damping elements. Vibration Damping System Concept Conceptually, theapproach employed towards thedesignofthemastdamping system wastoconcentrate sufficient modalstrainenergyatthemastinterface totheinboard antenna suchthatonlyafewdiscrete damping elements arerequired todampthemast.Inpractise, whatthismeans isthat,thestructural elements whichconnect themasttothetoinboard antenna structure aresoftened, i.e.,theirstiffness is reduced, suchthatapproximately halfthemodalstrainofthedeployed system's firstmodesofvibration is concentrated attheseelements. 85