Introduction Inthespaceindustry, itiscommon, duringon-ground testing toprovide a"zeroGenvironment" for gravity-sensitive hardware. TheSGANT isanorbitreplaceable communication andtracking antenna. It provides theKu-band signaltransmitting andreceiving capability between ISSandTDRSS. Inorbit,itis mounted onarigidmastonthespacestation withtheSpace-to-Ground Transmitter andReceiverController (SGTRC) mounted closetothebaseofthemast.TheSGANT isusedforcommunication with theTDRSS satellites whichareingeo-synchronous orbit.Thespacestation onwhichtheSGANT is mounted isina90-minute LowEarthOrbitandmaintains aconstant attitude withrespect totheground below.Thespacestationtherefore performs onecomplete rotation withrespect toTDRSS aboutevery90 minutes. Thisrotation isimparted tothebaseoftheSGANT whichmusttherefore besteered to compensate. Tosimulate thesteering operation andverifytheautotracking function oftheantenna, an on-ground tracking testisdefinitely amust. Theon-ground groupleveltracking testisachallenge duetotheunbalanced antenna mechanical design andtoughspecification requirement. Thespecification requirements aresummarized below(Ref.1). 1.Balance and/oroffloadtheSGANT toallowtwo-axis operation ina1Genvironment. 2.LettheSGANT moveoverarangeofatleast+5°ineachaxisatanyangular speedupto 3°/second, andatanyangular acceleration upto5°/second 2.Theadditional torqueimposed not exceeding 10.17N-m (7.5ft-lbf)and27.12N-m(20ft-lbf)maximum, toeitherofthegimbalaxes, respectively. 3.Limitthestatictorqueoneachgimbalaxistolessthan10.17N-m (7.5ft-lbf)whentheSGANT is stationary atthecenterofthis+5°angular rangeineachaxis,toprevent unwanted rotation when theSGANT isturnedoff. 4.Provide +5°motionfortwoSGANT positions, namely EL=0°/XEL=0 °,andEL=90°/XEL=0 °. 5.Provide afacilitybywhichtheelevation axisoftheSGANT canberotated toits90°position using itsownpowerwithout imposing anadditional torquerequirement ofmorethan27.12N-m(20ft-lbf) totheelevation gimbalaxis. 6.Thefixtureshallbesufficiently lightandrigidforthefirstresonant frequency around eitherofthe SGANT rotation axestoexceed 2.5Hz. 7.Avoidapplying loadstotheSGANT duringanyphaseoftheground test,whichcorresponds to factorsofsafetylessthan100%. 8.Bedynamically simple. 9.Beattached totheSGANT atanexisting attachpoint. 10.Definetheexactanglesinvolved forsoftware limiting. DuringthePreliminary Design Review, acounter balancing weightmechanism wasproposed. However, oncethedetailed design andanalysis started, problems wereencountered. Thebalance weight mechanism creates alotofunnecessary loadandfriction ontheflightgimbals. Someoftheload requirements areverydifficulty tomeet.Asaresult,thebalancing weight mechanism approach was finallyabandoned. Initsplace,manyalternative proposals wereconsidered, themostsuccessful one beingthespringmotoroffloading testmechanism. Todesign theoffloading mechanism, thefollowing stepsdefined hereinweretaken.Ultimately, theprojectwassuccessful. Simplification oftheAntenna Structure toaMassModel Simplifying theoverallSGANT antenna structure toamassmodeleliminated muchunnecessary analysis work duringthedesignphase.AllSGANT antenna components weresimplified tomasspointsand,usingaweightless bar,allmasspointswereconnected together toformthemodel.Theprocess ofsimplifying theSGANT antenna NASA/C P--2002-211506 340