Performance Measurement Test Performance testsontheMODIS engineering model(EM)andlifetestunit(LTU)scanners weremadeto determine thechanges inperformance overtheexpected on-orbit lifeoftheflightunit.TheEMandLTU scanners testedwereequivalent unitstothePFM(inservice) andFM-1scanner, whichisscheduled tofly shortly ontheEOS-PM (Aqua) program. Asaminimum, thescanner drivesystems mustoperate continuously forfiveyearsinaconstant velocity modeforEarthscanning. Thescanner drivesystem utilizedaphase-lock loopcontrolsystem formaintaining scanrateatthedesired valueduringoperation. Scanmotordragtorqueandphaseerrorsignalperformance frombothtestunitswasmeasured, recorded andevaluated. TheEMscanner hadseenafewhundred thousand rotation cycleswhereas theLTUhad completed morethanonemission lifetime andabout58millioncyclesinvacuum testing. Bothscanners testedhadtheirspinaxisvertical withanequivalent inertiadiskattached torepresent the scanner mirroratthetopendofthedrivesystem shaft.Thehardware usedtorecordthedragtorqueand phaseerrorsignalswasadataacquisition system running LabVIEW TMsoftware andalldatawasrecorded at250samples persecond. Figures 14through 19document resultsobtained fromthetestsperformed andarediscussed below. Figure14documented therunning orsteadystatedragtorqueoftheEMscanner, whichhadanaverage valueof67mN-m(9.4oz-in).Duringthistest,theEMunitmaintained therequired rotation rateof20.3 rpmwithphase-lock control. Thebeginning-of-life requirement forthisvaluewas110mN-m(15oz-in), whichwasmet.Thecurrent limitonthePFMandFM-1MODIS unitswasaboutoneampfortheflight condition whichindicated thatthebeginning oflifetorquemarginforthescanner wasgreater than20. Figure15documented themeasured phaseerrorsignaloftheEMscanner underturnONandsteady stateconditions. Whenthescanner wasinitially turnedon,therewasalargeerrorsignalgenerated between thecommanded rateandtheactualrate,shownbytheindicated spikeshortlyafterturn-on. The phase-lock circuitry wasdesigned toreduce theerrorbetween thecommanded rateandtheactualrate overagiventime,therefore theerrorsignalreduced shortlyafterturn-on toavalueofabout1voltpeakto-peak. Thiswasequivalent toaphaseerrorofabout20micro-radians understeadystatescanner rotation. InFigures 16and17eightattempts tobringtheEMscanner outofphaselockbyincreasing theexternal dragtorqueonthedrivesystem wereshown. Duetothelimitation ofthetestsetup,theexternal dragwas increased manually andtheLEDontheEMscanner electrical control boxwasobserved togoout indicating thescanner wasoutofphaselock.Thetwofiguresalsoshowed maximum dragtorquewhere thescanner wentoutofphaselock.Thisdragtorquecompared tothemaximum available torqueofthe scanner motorestablished themarginthatexisted forbeinginphaselockfortheEMscanner unit.When thetransient dragtorque events onFigures 16and17,wereomitted, retrieving andaveraging the maximum dragfromtheeightattempts indicated thatthedragatwhichpointthescanner phaselockwas lostoccurred atabout710mN-m(100oz-in).ThePFMandFM-1unitshadascanner actuator current limitofaboutoneampfromtheirpowersupplies, whichindicated amargin onphaselockofabout200% atthebeginning-of-life. ThismeantthatthePFMandFM-1unitscanners hadaboutthreetimesthe available torquecapacity intheirscanner drivesystems tomaintain scanner phaselock. Figure18documented therunning orsteadystatedragtorqueoftheLTUscanner whichhadanaverage valuemeasured ofabout86mN-m(12.2oz-in)afterrunning morethanonemission lifetime of58million cyclesinvacuum. During thistest,theLTUunitmaintained therequired rotation rateof20.3rpmwith phase-lock control. Themeasurement indicated thatattheendof(onemission) life,thetorquemargin for thescanner wasgreater than20. Figure19documented themeasured phaseerrorsignaloftheLTUscanner underturnONandsteady stateconditions. Similarly aswiththeEMunitsystem, whentheLTUscanner wasinitiallyturnedon,there wasalargeerrorsignalgenerated between thecommanded rateandtheactualrate,shown bythe indicated spikeshortlyafterturn-on. Thephase-lock circuitry wasdesigned toreduce theerrorbetween thecommanded rateandtheactualrateoveragiventime,therefore therewasareduction inerrorsignal shortlyafterturn-on toavalueofabout0.5voltpeak-to-peak. Thiswasequivalent toaphaseerrorof about10micro-radians underLTUscanner steadystaterotation. 10