APMDesignConcept Atrade-off wasperformed atthebeginning ofthedevelopment phaseinordertoselectthemost appropriate designconcept inviewofthegivensetofrequirements. Basedonthekeyrequirements such as: •flexibledesignallowing toaccommodate complete Antennas andAntenna Dishesaswell •geometric constraints (lowoverallheight) •possibility toroutflexibleKa(orKu)bandwaveguides alongorthroughthecentreoftheAPM •highantenna masstobesupported •highstiffness •simpleandreliabledesign •lowrecurring costs •minimum possible mass atwo-axis gimbalsystem(cardanframe)waschosenasabaseline. Inordertoavoidstaticoverdetermination inthelaunchpositionandtoavoidthatthegimbalactuators havetotakedynamic loads fromtheantenna, theAPMactuator outputisdecoupled fromthegimbalI/Fduringlaunch. Bythis concept theactuator canbesizedasalightweight unitofhighoutputprecision. Thegimbalactuators are designed aslinearactuators andduringmissiontheactuator outputtransforms itslinearmotionintoa rotational oneatthegimballevel.Analmostconstant preloadactingtotheactuator I/Fisprovided by meansofaspringpackage integrated tothegimbal. Thegimbalsystemarranges bothrotationaxesin onehorizontal planesothataverycompact andstiffconfiguration isachieved. Thegimbalframe provides acentralopening thatcanbeusedtoaccommodate asetofflexiblewaveguides, ifrequired by theapplication. ThechosenAPMconcept allowsadjustment toaninitialoperational pointing positionoftheAntenna alreadyonground. Afterreleaseofthetwoseparate HoldDownandRelease Mechanisms (HRMs), the spring-loaded gimbalcontacts theactuator outputshaft,thusproviding thepre-adjusted initialpointing position. Allfurtherpointingactivities willthenbeperformed bycommanding theactuators toany arbitrary position. MajorSub-units andImportant APMComponents TheoverallAPMlayoutconsisting ofthefollowing mainassemblies andsub-assemblies isshowninthe nextfigure(Figure2).Acorresponding partslistisdepicted inTable4.SincetheHRM'shavenodirect interface totheAPM,theseitemsaredescribed inaseparate sectionofthispaper. GimbalAssembly GimbalUnit TheGimbalUnitformsthecentralloadcarrying partandfunctional unitoftheAPMGimbalAssembly. Its taskistoprovidetherequired rotational degrees offreedom abouttwoaxes,toallowtheattachment of theantenna, andtotakethelaunchloads.TheGimbalUnitcomprises thetwoBaseBrackets connecting theAPMtothesatellite, thetwoFrameBrackets formingtheinterface totheantenna, andtheCardan Frameinterconnecting theBaseBrackets andtheFrameBrackets viathecorresponding Bearing Units. Thecardanframeismanufactured outoftitaniumforstiffness reasons. Inordertocompensate for thermaldisturbances, eachgimbalaxisisdesigned withafixedandafloatingpairofbearings. The floatingbearingpairofeachaxisisequipped withadoublemembrane element manufactured outof BeCualloy. Atthebeginning ofthedevelopment phase,simpleGimbalBrackets manufactured outofaluminum and equipped withpairsofpretoaded hybridballbearings inanXarrangement, wet-lubricated TiCballs,were considered asabaseline. Inalatestageofthedesign,itwasfoundthatthetemperature regime specified atthebeginning ofthedevelopment hadtobesignificantly changed onbasisoftheoutcome of thethermalanalysis results. Basedontheseresults,thequalification temperatures forthemechanism 289