1. The TechSat21 programwas initiatedby the Air ForceResearchLaboratoryto developanddemonstrate the key enablingtechnologiesfor distributedsatellite systems.To help focustheresearchin theTechSat21 programandprovideabasisof comparisonfor theidea of a virtual satellite,a referencemissionwas selected. Thismissionwaschosentobeonethatstressesmanyof the operationalandhardwaretechnologiesrequiredfor a distributedsatellitesystemandis alsoof interestto theAir Force.A missionandsystemdesigndeveloped by theAir ForceSpaceSensorStudyTeam(1995)and extendedby the SpaceBasedRadarIntegratedProduct Team (Ref. 11) was selectedfor this purpose.This spacesystemis designedto augmentandenhancethe Joint Surveillance Target Attack Radar System (JSTARS)by providing theaterdetectionandtracking of slow moving groundvehicles,or GroundMoving Target Indication (GMTI). The spacesystemis also requiredto performradarimagery(SyntheticAperture Radar,or SAR) andprovide GMT1 and SAR datato CONUSandthetheaterin atimely manner.A summary of the mission requirementsis provided in Table 1 (Refs.10,11).
2. This basicdeignwas evaluatedby the Aerospace Corporation'sConceptualDesign Center(CDC). The CDC provides a system level concurrent design capabilityusingsatellitesubsystemexpertsandsystems engineers.The CDC producesa systemlevel design (launch, ground and space segments),with detailed estimatesof the characteristicsof each elementand subsystemandcapturesthe interactionsbetweenthem. In addition,theCDC computesthelife cyclecostof the system.Two designswere developed(Ref. 12), one basedonexistingtechnology(freezedate1996)andone basedon advancedtechnology(freezedate2003-5).A description of these satellite designsis provided in Figure 1. The satellite design based on existing technologyis estimatedto be 12,500kg andrequiresa Titan IV for launch.The life cycle costis estimatedto be$26.8B(1997dollars).
3. .The advancedtechnology design exploits new developmentsin spacepower generationand storage, phasedarrayantennas(Ref. 13), antennastructures, space-capablerad-hardprocessors,electricpropulsion, and advancedelectronicspackaging which will be ready for insertion in the 2003-5 timeframe. This satellitedesignis estimatedto be4,400kg andrequires aDeltaII for launch.Thelife cyclecostof this system isestimatedtobe$14.9B(1997dollars).
4. To effectively suppressclutter and resolve the ambiguitiesassociatedwith small antennason these satellites,one needsadditional sensedinformation.A techniquehas been devised in which each satellite would transmita signalorthogonalto the others,while receivingand coherentlydetectingthe returnedsignal from everysatellites'transmission,includingits own as depicted in Figure 2. These N*(N-1) independent measurements,whereN is the numberof satellitesin the cluster,provide the capability to synthesizevery narrowbeams,producinggoodspatialresolutiononthe surface.When combinedwith fine resolutionin range and Doppler for the MT1 mode, this allows,moving targetsto beseparatedfrom thegroundclutter.Thekey isto suppresstheclutterthatappearsin thesamerange-Doppler cell as the target using the inherent spatial resolution of the cluster. Space-Time Adaptive Processing(STAP)isonesuchprocesswhich optimally combinesthe N*(N-1) receivedsignals,eachsampled with a differentsetof time delays.Intelligentchoiceof the aperturepositionsand delayscan substantiallyaid the process.For instance,for GMT1 applicationsa formation following linear configuration and a time samplingwhich effectively arreststhe relative motion of thesatelliteclusterrelativeto theground(Displaced Phase Center Antenna or DPCA processing) is effective.Moregeneralsatellitegeometriescanproduce effective beampatternswhich, with appropriatetime delays in the STAP, produce superior clutter suppression.