Theoriginaltranslation stagedriveassemblyhadtwoissuesthatweredeemedunacceptable during functionaltesting.Oftheseonestemmedfromtorquemarginsandtheotherwascluetoactuationtime. Theoriginal14mN-m(2in-oz)steppermotorwith100:1harmonicdrivegearreducerwaseasilystalled andwasthoughtthatitmightnotbeabletodrivethestagealongtherailsunderlessthanideal circumstances. Evenwiththehighgearreductionthroughtheharmonicdriveandthetorquemultiplication oftheleadscrewmostofthemotor'soutputtorquewasconsumedintheoperationoftheharmonicdrive. Thehighgearreductionalsocausedthetimetoactuatethegratingthroughitslimitstobeunacceptably long-45minutesfortencentimeters oftravel.Thereasonsforusingtheoriginaldrivemechanism was duetothecostconstraints andthatfactthatthismotorwasavailableasasparefromasimilarmotor usedonanotherpayload.Thesolutiontotheproblemsmentionedwastoreplacethestepper/harmonic driveassembly withaRapidsyn 282mN-m(40in-oz)Motordirectly coupled totheleadscrew.Thismotor wasreadilyavailable asitwasalsotheflightspareforHITSgimbaldrive.Evenwiththeremoval ofthe harmonic drivethenewmotorwasstillabletoprovide thenecessary accuracy duetothesmallstepsize of1.8degrees perstepfurtherreduced bythe10threadperinchleadscrew. Vibration testsoftheGDMexposed another problem intherotation stagesupport rails.Intheoriginal design, therotation stagewasconnected totherailsbysemi-encircling linearbearing. Duringvibration testing incertain orientations therotation stageexhibited excessive rattling. Thiswasdueinparttothe looserthanexpected tolerances inthebearing andalsofromthebearing notbeingabletofullyencircle theshafttheyrodeon.Theamount ofplayinthesystem leading totherattlewasonlyapparent inthe vibration tests.Thisleadtotheconclusion thatduringtheteststhelinearbearings mayhavebeenslightly expanding andthusallowing therotation stagetoliftofffromtherailsbyasmallamount ononecycleof thevibration andthenbackagainontheothercycle.Tosolvetheproblem, theshaft-mounted railswere replaced withcylindrical shaftsofslightly greater thickness supported onlyattheends.Thesemiencircling linearbearings werereplaced byfullyencircling linearbearings. Originally thisideaof supporting onlytheendsoftheshaftwasconsidered butconcerns aboutvibrations caused itbe dismissed. Itwasthought thattherewouldbeexcessive shaking enough todamage theGDMifthe rotation drivemoved totheunsupported middleoftheguideshaftduringlaunch. Without thecenter supports acagingsystem hadtobedeveloped toholdtherotation stageatasupported endoftheguide railduringlaunch. Costreduction wasadriverintheGDMconcept ofoperations bothduringlaunchandonorbit.Toreduce thecomplexity andcostofthesystem thecagingscheme wasbuiltintotheGDM.Ratherthanuseamore traditional paraffin actuator basedsystem theGDMutilized theextrarotation of5°beyond therequired 25°.Torestrain thegrating duringlaunchthegrating wasdriventothelimitfarthest fromtheballscrew driveassembly. Twostainless steelpinsintheGDMhousing werethenextended andthetranslation stagewasbackdrivenagainst them.Afterthepinwasengaged thegrating wasthendrivenalongthe translation stageagaintothepointwherethegrating wouldlockinplace.Afterlaunchthegrating was movedinthereverse sequence toandthepinswoulddropdownanduncage thegrating. Grating Rotation DriveOn-Orbit Performance During operation theGDMwouldhavetobothrotateandtranslate toenable HITStoacquire the wavelengths bandsofinterest aswellasfocustheimage. Ratherthanuseaclosedloopcontrolsystem requiring theuseofencoders orresolvers theGDMreliedonanopenloopsystem. Beforeeachactuation theGDMrotation andtranslation stages weredriventoonelimitthatwasdesignated asthestarting position. Thestarting position wasalsodefined through theuseofmechanical microswitches andoptical switches asfiducials. Fromthefiducials, stepswerecounted astheyweresenttothestepper motors. Thisscheme hassofarproven tobeeffective inthatanyerrorsthathavearisenwouldnothavebeen prevented byamoredetailed knowledge position. TheerrorthatdoesaffecttheimagequalityofHITSisduetocogging intherotation stagestepper motor. Mostoftenthepositions thatareentered intotheGDMnormally coincide withthemotorrotation increments. However, atcertain positions themotortendsto"slip"tothenextincrement orstep.This difference resultsinanangular errorontheorderofatenthofadegree. Theangular errorthenresultsin ashiftoftheimagealongthedetector ontheorderofonemillimeter. Fortunately, thiserroriseasily corrected bytranslating theimagebacktothecorrect location duringanalysis ontheground. NASA/CP--2002-211506 219