receivestheoff-nadirsurfaceclutter,whichwillthenbesubtractedfromtheprimarydatainthegroundreprocessing. TheMARSISradaroperatesatfrequencies uptoabout6MHz. Therequiredmechanical configuration forthedeployedantennashowninFigure1isadipole transmit/receive antennadeployedintheorbitvectorwithatotaltip-to-tiplengthof40meters,plusa7metermonopolereceive-only antennainthenadirdirection.TraditionaldesignssuchasaSTEMantenna wereconsidered fortheapplication butwouldnotmeetthecombination ofrequirements forstiffness, weight(-8kgmassbudget),thermalstabilityandcleanliness (nodebrisgeneration). TheFFT,despiteits earlystageofdevelopment, mettheserequirements anditssimplicitygavetheexperiment teamsufficient confidence inasuccessful outcometoproceed.TheprogramstartdatewasinFebruary2000.After overcoming someminordesignproblemsthequalification wascompletedinOctoberof2001.Atthetime ofwritingthispaper,theflighthardwareisassembled andreadyforacceptance test. TheantennastructureconsistsoflightweightS-Glass/Kevlar compositetubes.Thedipolesare38mmin diameterandthemonopoleis20mmindiameter.Withthesedimensions thetubesaresufficientlystiffto achievethedeployedfrequencyrequirement of0.05Hz. Thetubesarefoldedandthencompressed forstowage.Theyarefoldedatpointsalongthelengthwhere cutoutsinthesideofthetubepreventasingularityinthematerial.Thisallowstheextremelyflexible compositetofoldinamannersimilartoa"carpenter's tapehinge".The20-meterdipolesfoldtoalength of1.53metersandthesmallermonopoleisfoldedto1.3meters.Whenfolded,thetubesarecompressed intoastowageboxforlaunch.ThestowageboxisaNomexHoneycomb graphite-skinned construction withthreedoorsthatenclosethecompressed elements. ThestowagemethodisshowninFigure2andisshowninprocessinFigure3.Thethreetubesare stowedinboxesinlayersandreleasedsequentially sothatthereisnodangerofentanglement duringthe process.Thetubesarestowedbyfoldingthematdefinedpointsalongtheirlengthwherecutouts,as showninFigure4,relievethestresssingularities ateachsideandallowtherestofthetubetoflexinto thefoldedshape.Thedipolesandthemonopolearecompressed from38mmto19mmandfrom20mm to10mm,respectively, andconsequently haveconsiderable stowedstrainenergy.Deployment ofthe tubesisinitiatedsequentially bysmallpyrotechnic devicesthatreleasethedoorlatches.Oncetriggered, thetubespushthedoorsopenveryrapidlyonreleaseanddeployouttotheirrequiredconfiguration by releasingtheenergyofstowageandthestrainenergyinthehinges.(Thisdoorenergyisabsorbedby innovative,progressive frictiondampers). Thestowedtubesemergerapidlyfromtheirstowageallheadinginthesamedirection.Therootmounting turnsthetwodipolesthrough90degreesintotheflightvectorwhilethemonopolefiresstraightoutinthe nadirdirectionandstaysthere.Thetubesreachtheirfullextensioninlessthantwosecondsbecauseof theinitialreleaseofstowedenergybutthedipoletubestakeanother30secondstorotateintoposition. Thedipolehingeshaveaverylowtorqueof0.2N-mandthereforeacceleratethefairlylargeinertia slowly.Onreachingfulldeployment, thetubesoscillatebeforecompletelystraightening, thoughthetubes' construction significantly dampensthisoscillation, asexplainedbelow.Thedipoleelementdeployment sequenceisshowninFigure5. Thecompositetubesarenotconductive persealthoughtheyareIndiumTinOxide(ITO)coatedto preventstaticbuildup.Theactualantennastructureisapairofstranded22-gauge,silver-plated copper wiresthatrunalongthelengthofthetube.Thesewiresareinterconnected ladder-style alongtheirlength toprovideredundancy. Inaddition,theyareslightlyrippledalongthelengthtopreventdifferentialthermal motionbetweenthewiresandthenearzerocoefficientthermalexpansion(CTE)tubes.Thewiresare connected tothefeedandsensingelectronics, whicharemountedclosetotheantennarootsand beneaththestowageboxasshowninFigure6. Asdiscussedbelow,oneofthemostcontroversial aspectsoftheantennaprojecthasbeenthetest program.Thestructuraltubesoftheantenna(thin-wallS-Glass/Kevlar composite,38-mmdiameterforthe dipolesand20-mmforthemonopole) areunabletosustaintheirownweightinEarth'sgravity. Additionally, thedeployment kinematics showninFigure5preventtheuseofconventional deployment NASA/CP--2002-211506 184