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A System Design of an Optical Wireless Communication System for Cubesats

Permanent Link: http://ufdc.ufl.edu/UFE0042657/00001

Material Information

Title: A System Design of an Optical Wireless Communication System for Cubesats
Physical Description: 1 online resource (78 p.)
Language: english
Creator: Alluru, Seshupriya
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: cubesat, optical, small
Electrical and Computer Engineering -- Dissertations, Academic -- UF
Genre: Electrical and Computer Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Optical wireless communications provide a promising, high bandwidth alternative to radio communications, where high performance links are desired. For large satellites (say, wet mass > 1000kg), laser cross links have been successfully established since 2001 by various space agencies in Europe and Japan. Thus far, the cross-links have been able to achieve data rates in Gbps range for distances greater than 10,000km. Such gains would be monumental improvement for communications in small satellite domain , where the typical communication payload uses radio antenna that achieve an average data rate of 10kbps. The thesis looks at exploring the components of an Intersatellite Laser Link System for large satellites that is responsible for establishing cross links as a means of communication and the feasibility of a similar system to achieve long distance crosslinks for the CubeSat. A brief study of the laser crosslink system of the large satellites is provided. Then, the parameters and requirements of the subsystems are discussed and determined for the CubeSat frame. An analysis of the contribution to the weight of the CubeSat and power consumption requirements are performed with respect to the CubeSat specifications. A link budget analysis was carried out through simulations for the optical system and it was seen that distances upto 200km were achievable with error free communications.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Seshupriya Alluru.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: McNair, Janise Y.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042657:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042657/00001

Material Information

Title: A System Design of an Optical Wireless Communication System for Cubesats
Physical Description: 1 online resource (78 p.)
Language: english
Creator: Alluru, Seshupriya
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2010

Subjects

Subjects / Keywords: cubesat, optical, small
Electrical and Computer Engineering -- Dissertations, Academic -- UF
Genre: Electrical and Computer Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Optical wireless communications provide a promising, high bandwidth alternative to radio communications, where high performance links are desired. For large satellites (say, wet mass > 1000kg), laser cross links have been successfully established since 2001 by various space agencies in Europe and Japan. Thus far, the cross-links have been able to achieve data rates in Gbps range for distances greater than 10,000km. Such gains would be monumental improvement for communications in small satellite domain , where the typical communication payload uses radio antenna that achieve an average data rate of 10kbps. The thesis looks at exploring the components of an Intersatellite Laser Link System for large satellites that is responsible for establishing cross links as a means of communication and the feasibility of a similar system to achieve long distance crosslinks for the CubeSat. A brief study of the laser crosslink system of the large satellites is provided. Then, the parameters and requirements of the subsystems are discussed and determined for the CubeSat frame. An analysis of the contribution to the weight of the CubeSat and power consumption requirements are performed with respect to the CubeSat specifications. A link budget analysis was carried out through simulations for the optical system and it was seen that distances upto 200km were achievable with error free communications.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Seshupriya Alluru.
Thesis: Thesis (M.S.)--University of Florida, 2010.
Local: Adviser: McNair, Janise Y.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2010
System ID: UFE0042657:00001


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ASYSTEMDESIGNOFANOPTICALWIRELESSCOMMUNICATIONSYSTEMFORCUBESATSBySESHUPRIYAREDDYALLURUATHESISPRESENTEDTOTHEGRADUATESCHOOLOFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHEREQUIREMENTSFORTHEDEGREEOFMASTEROFSCIENCEUNIVERSITYOFFLORIDA2010

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c2010SeshupriyaReddyAlluru 2

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IdedicatethisthesistomyMumandArunaakka. 3

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ACKNOWLEDGMENTS IwouldliketothankDr.McNairforconsentingtobemyadvisorandforherconstantsupport,encouragementandguidancethroughthecompletionofthisthesis.IwouldliketothankDr.AnnGordonRossandDr.FitzCoyforagreeingtobeonmysupervisorycommittee.IwouldliketothankmyfriendsFaridaandPrasanthiforbeingsuchwonderfulandcheerfulroommmates.Finally,Iwouldliketothankmybrotherandparents.. 4

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TABLEOFCONTENTS page ACKNOWLEDGMENTS .................................. 4 LISTOFTABLES ...................................... 7 LISTOFFIGURES ..................................... 8 ABSTRACT ......................................... 10 CHAPTER 1INTRODUCTION ................................... 11 1.1Introduction ................................... 11 1.2HistoryofOpticalCrosslinks ......................... 13 1.2.1Semi-ConductorInter-SatelliteLinkExperiment(SILEX) ...... 13 1.2.2AdvancedRelayandTechnologyMissionSatellite(ARTEMIS)andOpticalGroundStation(OGS) ................... 14 1.2.3ARTEMISandSatellitePourl'ObservationdelaTerre(SPOT)-4 .. 15 1.2.4ARTEMISandOpticalInter-orbitCommunicationEngineeringTestSatellite(OICETS) ............................ 15 1.2.5ARTEMISandLiaisonOptiqueLaserAroporte(LOLA) ....... 16 1.2.6NearFieldInfraredExperiment(NFIRE)andTerraSAR ....... 17 1.2.7AlphaSAT ................................ 17 1.2.8SmallSatelliteLaserCommunications ................ 17 1.3MissionScenarios ............................... 18 1.4IntersatellieteLaserLinkComponents .................... 19 2OPTICALSOURCEANDDETECTOR ....................... 23 2.1OpticalSource ................................. 23 2.1.1ChoiceofLaserDiode ......................... 24 2.1.2Operation ................................ 25 2.1.3OpticalRequirements ......................... 25 2.1.4OpticalDesign ............................. 25 2.1.5ElectricalandMechanicalRequirements ............... 27 2.1.6ModulationSchemes .......................... 28 2.1.6.1OnandOffKeying ...................... 29 2.1.6.2Pulsepositionmodulation .................. 29 2.1.6.3FrequencyShiftKeying ................... 31 2.2OpticalDetector ................................ 31 3POINTING,TRACKINGANDACCQUISITIONASSEMBLY(PAT) ........ 35 3.1Pointing,AcquisitionandTrackingStrategies ................ 36 3.1.1AcquisitionStrategy .......................... 36 5

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3.1.2TrackingandPointingStrategy .................... 36 3.2PATAlgorithm .................................. 38 3.3ElectroMechanicalDesignChoice ...................... 38 4TRANSMITTERANDRECEIVER ......................... 42 4.1CommonAstronomicalTelescopes ...................... 42 4.1.1Off-AxisNewtonian ........................... 42 4.1.2Cassegrain ............................... 43 4.1.3Off-AxisGregorian ........................... 43 4.1.4CassegrainwithRefractiveElements ................. 44 4.1.5Schmidt-Cassegrain .......................... 45 4.1.6Maksutov-Cassegrain ......................... 45 4.2TelescopeCharacteristics ........................... 46 4.2.1Variationoff/# .............................. 46 4.2.2ResistancetoJamming ........................ 47 4.2.3DiffractionLimitedTelescopes ..................... 48 4.2.4TelescopeMaterials .......................... 48 5ANALYSISOFTHESYSTEMDESIGN ...................... 50 6LINKBUDGETANALYSIS .............................. 52 6.1OptiWaveSoftware ............................... 52 6.2Linkbudgetequation .............................. 52 6.2.1TransmitterPower ........................... 55 6.2.2Transmittergain ............................. 57 6.2.3TransmitterLoss ............................ 58 6.2.4FreeSpaceloss ............................. 58 6.2.5Receivergain .............................. 58 6.2.6ReceiverLoss .............................. 58 6.2.7OpticalLinkBudgetParameters .................... 59 6.2.8RadioFrequencyLinkBudgetParameters .............. 60 6.3Results ..................................... 60 7CONCLUSIONS ................................... 64 APPENDIX:A ....................................... 65 REFERENCES ....................................... 73 BIOGRAPHICALSKETCH ................................ 78 6

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LISTOFTABLES Table page 1-1Typicalcommunicationcharacteristicsofsmallsatellites ............. 13 2-1Laserdiodewavelengths[ 1 ],[ 2 ] ........................... 24 2-2Laserdiodeparametersummary[ 3 ] ........................ 29 2-3Operatingwavelengthsofopticaldetectors .................... 33 3-1PATparameters ................................... 40 4-1Telescopeparametersummary[ 1 ] ......................... 47 4-2Telescopematerialdensities[ 4 ] ........................... 49 4-3Telescopeparameters ................................ 49 5-1OpticalPayloadParametersSummary ...................... 51 6-1Componentsofthecommunicationsystem .................... 54 6-2Parametersofequations ............................... 57 6-3Parametersofrateequations ............................ 57 6-4Cubesatopticallinkparameters .......................... 59 6-5Cubesatradiofrequencyparameters ....................... 60 7

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LISTOFFIGURES Figure page 1-1SILEXproject[ 5 ]. ................................... 18 1-2Missionscenario ................................... 19 1-3ISLblockdiagram .................................. 21 1-4ISLblockdiagram .................................. 22 2-1Collimatordesign ................................... 26 2-2OOKmodulation ................................... 30 2-3ISLblockdiagram .................................. 34 3-1PATassembly ..................................... 37 3-2PATalgorithm ..................................... 39 3-3TopviewcrosssectionofPATsystem. ....................... 40 3-4SideviewofcrosssectionofPATsystem ..................... 40 3-5ISLblockdiagram .................................. 41 4-1Off-AxisNewtonian .................................. 43 4-2Cassegrain ...................................... 44 4-3Off-AxisGregorian .................................. 44 4-4Cassegrainwithrefractiveelements ........................ 45 4-5Schmidt-Cassegrain ................................. 45 4-6Maksutov-Cassegrain ................................ 46 6-1Communicationsystem ............................... 53 6-2Transmittedlasersignal ............................... 59 6-3IntersatellitecommunicationdistancewithRFlinks ................ 60 6-4Qfactoroftheopticallinkvslinkdistance ..................... 61 6-5Receivedpowervslinkdistance .......................... 61 6-6BERat150km .................................... 62 6-7BERat10Km ..................................... 62 8

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A-1Withoutaberration .................................. 65 A-2Aberration ....................................... 65 A-3Aberration ....................................... 66 A-4Aberration ....................................... 66 A-5Astigmatism ...................................... 67 A-6Beamradius ..................................... 69 A-7Beamdivergence ................................... 69 A-8Numericalaperture .................................. 70 A-9Pointaheadangle .................................. 71 9

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AbstractofThesisPresentedtotheGraduateSchooloftheUniversityofFloridainPartialFulllmentoftheRequirementsfortheDegreeofMasterofScienceASYSTEMDESIGNOFANOPTICALWIRELESSCOMMUNICATIONSYSTEMFORCUBESATSBySeshupriyaReddyAlluruDecember2010Chair:Janise.Y.McNairMajor:ElectricalandComputerEngineeringOpticalwirelesscommunicationsprovideapromising,highbandwidthalternativetoradiocommunications,wherehighperformancelinksaredesired.Forlargesatellites(say,wetmass>1000kg),lasercrosslinkshavebeensuccessfullyestablishedsince2001byvariousspaceagenciesinEuropeandJapan.Thusfar,thecross-linkshavebeenabletoachievedataratesinGbpsrangefordistancesgreaterthan10,000km.Suchgainswouldbemonumentalimprovementforcommunicationsinsmallsatellitedomain,wherethetypicalcommunicationpayloadusesradioantennathatachieveanaveragedatarateof10kbps.ThethesislooksatexploringthecomponentsofanIntersatelliteLaserLinkSystemforlargesatellitesthatisresponsibleforestablishingcrosslinksasameansofcommunicationandthefeasibilityofasimilarsystemtoachievelongdistancecrosslinksfortheCubeSat.Abriefstudyofthelasercrosslinksystemofthelargesatellitesisprovided.Then,theparametersandrequirementsofthesubsystemsarediscussedanddeterminedfortheCubeSatframe.AnanalysisofthecontributiontotheweightoftheCubeSatandpowerconsumptionrequirementsareperformedwithrespecttotheCubeSatspecications.Alinkbudgetanalysiswascarriedoutthroughsimulationsfortheopticalsystemanditwasseenthatdistancesupto200kmwereachievablewitherrorfreecommunications. 10

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CHAPTER1INTRODUCTION 1.1IntroductionACubeSatisaminiaturisedsatellite(10x10x10cm,weighing1kg)whichoffersallthestandardfunctionsofanormalsatellite(attitudedeterminationandcontrol,uplinkanddownlinktelecommunications,powersubsystemincludingabatteryandbody-mountedsolarpanels,on-boarddatahandlingandstoragebyaCPU,pluseitheratechnologypackageorasmallsensororcamera).Theycanevenhavedeployablesolarpanels,antennasorbooms.Limitedorbitcontrolusingmicropropulsion,S-bandinsteadofVHF/UHFandwirelessdatatransferinsidetheCubeSatarenowbeginningtobeused.IttakesabouttwoyearstodevelopaCubeSatfromtheprovisionoffundinguntillaunch.ThehardwarecostofaCubeSatisintherange100,000USD.Uptonow,about40CubeSatshavebeensuccessfullylaunched,worldwideanestimated70-100CubeSatsarebeingreadiedforlaunchinthenextfewyears.AsingleCubeSatissimplytoosmalltoalsocarrysensorsforsignicantscienticresearch.Hence,fortheuniversitiesthemainobjectiveofdeveloping,launchingandoperatingaCubeSatiseducational.However,whencombiningalargenumberofCubeSatswithidenticalsensorsintoanetwork,inadditiontotheeducationalvalue,fundamentalscienticquestionscanbeaddressedwhichareinaccessibleotherwise.NetworksofCubeSatshavebeenunderdiscussionintheCubeSatcommunityforseveralyears,butsofarnouniversity,institutionorspaceagencyhastakentheinitiativetosetupandcoordinatesuchapowerfulnetwork.CubeSatreliabilityisnotamajorconcernbecausethenetworkcanstillfullyachieveitsmissionobjectivesevenifafewCubeSatsfail.SuchanetworkofsatellitesisbeingbuiltanddeployedbytheQB50programinvolvingalistofspaceagenciessuchasNationalAeronauticalSpaceAgency(NASA),EuropeanSpaceAgency(ESA),(InnovativeSolutionsinSpace)ISISanduniversitiessuchasCaliforniaPolytechnicSateUniversity.Useofradiofrequency 11

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communicationsforapplicationsthatrequirelowdatavolumesisaconvenientandstandardapproach,whiledataintensiveapplicationswouldbenetfromthehighdatarateavailabilityofopticalcommunications.Lasers,withtheirhighdirectionalityandlowpowerrequirements,promisetobeabetterwaytoimplementlongdistancehighdataratespacecommunications.Forverylongdistances,becauseofapolynomialrelationshipbetweenpoweranddistance,multi-hopcommunicationismuchmorepowerefcientthanlongrange,directsourcetosinkcommunication.Forexample,thegreaterpowerefciencycomesfromtheabilitytoemitahighlydirectionalbeam,radiatingverylittlepowerinunintendeddirections.Inaddition,atypicalcubesatelliteRFantennahasapowerratingof500mW,whilethetypicalpowerratingforalasertransmitterusedinlargesatellitesisabout100mW.Lasersoperateatfrequenciesmuchhigherthanradiomicrowavefrequencies.Hence,theyhavethepotentialtoprovidehigherdatarates.Forexample,successfultestshavebeenconductedamonglargesatellitestoachieveafree-spaceopticallinkoperationatspeedsofupto40Gbps,whichissufcientforhighresolutionimageorvideooranyotherdataintensiveoperations.Also,LongrangeRFcommunicationlinks,becauseoftheirweaksignalstrengths,aresusceptibletojammingandthusarepronetoDOS(DenialofService)attacks.Byusingmulti-hopcommunications,muchbettersignalstrengthscanbeachieved,whichmeansmorerobustnessagainstDOSattacks.Inter-satellitelasercommunicationisthekeytopowerefciency,securityandmoreimportantlyfordeploymentofanydistributedsystemlikeaCubeSatcluster.Inaddition,becauseoftheirhighdirectionality,lasercommunicationsareveryrobustagainstjammingandelectromagneticinterference.Apreliminarysurveyofthetraditionaluseofsmallsatelliteshasshownthatinter-satellitecommunicationshavenotbeenahighpriority.Inmanycases,thecommunicationsintenthasbeensimplytobeabletopingthesmallsatellite,toreceiveshortcontrolcommandsfromagroundstation,andtotransferverysmallamountsof 12

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databacktothegroundstationintermittently.Thus,mostcongurationsusesimpleantennasforcommunication.Typicalcharacteristicsaresummarizedinthetable 1-1 Table1-1. Typicalcommunicationcharacteristicsofsmallsatellites CharacteristicMinimumMaximumTypical DataRate1200bps(e.g,CanX1,Cute1)38.4kbps(e.g.,AeroCube2)9600bpsPower350mW(e.g.,Cute-1.7)2W(e.g.,AeroCube)500mWTotaldownload320KB(e.g,CP4)6.77MB(e.g,CSTB1)0.5-5MB 1.2HistoryofOpticalCrosslinksLargesatellitelasercommunicationswasbegunintheearly2000s,withtheEuropeanSpaceAgencysefforttolaunchitsAdvancedRelayandTechnologyMissionSatellite(ARTEMIS)(inJuly2001)anddemonstratelasercommunicationsabilityusingtheSILEX(Semi-ConductorInterSatelliteLinkExperiment)opticalcommunicationssystem. 1.2.1Semi-ConductorInter-SatelliteLinkExperiment(SILEX)Almostthirtyyearsago,insummer1977,EuropeanSpaceAgency(ESA)placedatechnologicalresearchcontractfortheassessmentofmodulatorsforhighdata-ratelaserlinksinspace.ThismarkedthebeginningofalongandsustainedESAinvolvementinspaceopticalcommunications.RecognizingthepotentialperformanceedgeofopticalcommunicationsoverRFtechnologiesintermsofsize,weightandpower,ESAplacedduringthepasttwodecadesalargenumberofstudycontractsandpreparatoryhardwaredevelopments,conductedundervariousESAR&DandSupportTechnologyPrograms.Inmid1980's,ESAtookanambitiousstepbyembarkingontheSILEXprogram,todemonstrateapreoperationalopticallinkinspace.TheSILEXPhaseAandBstudieswereconductedaround1985,followedbytechnologybread-boardingandpredevelopmentofthemaincriticalelements,whichweretestedon 13

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theso-calledSystemTestBed,toverifythefeasibilityofSILEX.AdetaileddesignphasewascarriedoutinparallelwiththeSystemTestBedactivitiesuptoJuly1989. 1.2.2AdvancedRelayandTechnologyMissionSatellite(ARTEMIS)andOpticalGroundStation(OGS)In1993developmentsofasatelliteindependentlasercommunicationterminalcheck-outfacilitybeganinTenerife,Spain.Thefacility,calledopticalgroundstation(OGS),becameoperationalbyendof2000.Alsointheearly2000s,theESAexperimentwithARTEMISbegan.ARTEMIScarriespayloadsforthedemonstrationandpromotionofadvancedtechnologiesandservices,inparticulardatarelay,landmobilecommunicationsandnavigation.Thepre-shipmentreviewofARTEMIStookplaceinESAattheendof1999,andthelaunchofARTEMISwasinitiallyscheduledforFebruary2000ontheJapaneselauncherH2A.ProblemswiththeJapaneselauncher,however,madeitnecessarytolookforanalternativelaunchoptioninordertoavoidfurtherdelays.Eventually,ARTEMISwaslaunchedon12July2001onAriane5,but(duetounderperformanceofthethirdstageoftheAriane510launcher)thesatellitewasleftinafartoolowellipticalorbit(about17500kmx590kminsteadof36000kmx860km).Within10days,andbyusingmostofitson-boardpropellantforatotalof8motorrings,ARTEMISwasbroughtintoacircular,howevernon-geostationary,orbitwith31000kmaltitude,a0.8degreesinclinationandanorbitalperiodof20hours.InordertocheckoutasearlyaspossiblethehealthoftheSILEXpayloadonARTEMIS,rsttestswithESAsopticalgroundstationonTenerifewereperformedon15November2001at01:00UTC.Pointing,acquisitionandtrackingofthesatelliteweredemonstratedandtwolinksessionsof20minuteseachwereperformed.TheOGS/ARTEMISspacetogroundlinkstatisticscount,137sessionsofwhich25failedwithanaccumulatedlinkdurationof2days,10hoursand37minutes.Eventually,withhelpofitsionthrustersinitiallyforeseenforNorth/SouthstationkeepingARTEMISwasspiralledouttoitsgeostationary(GEO)orbitalpositionof21.5o 14

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East.Themaneuverlastedfrombeginningof2002untilFebruary1st,2003duringwhichnosatelliteoperationswerepossible,becausethethrustdirectionrequiredaspacecraftattitudechangeawayfromnominalNadirpointing. 1.2.3ARTEMISandSatellitePourl'ObservationdelaTerre(SPOT)-4Asmentionedabove,theSILEXPhaseAandBstudieswereconductedaround1985,withadetaileddesignphasecarriedoutinparallelwiththeSystemTestBedactivitiesuptoJuly1989.Atthattime,SPOT-4,aFrenchspaceagency(CNES)satellitewasbeingdeveloped,andSILEXphaseC/DwithanopticalterminalwasintegratedintoSPOT-4.Thiswasanimportantdecision,asitmadeasuitablepartnersatelliteavailablefortheESAdata-relaysatelliteproject.In1998,SPOT-4waslaunched.AftertheinitiallaunchofARTEMIS,specically,inNovember2001,SPOT-4operatingat832km,andARTEMIS,operatingat31,000km,exchangedtherst-evertransmissionofanimagebylaserlinkfromonesatellitetoanothersatellite.ThelinkingwasperformedusingSILEXbetweentheOpaleterminalonARTEMISandthePastelterminalontheSPOT4satellite.Theterminalsexchangedhigh-denitionimagerydataat50megabitspersecond.ARTEMISsubsequentlybeamedthedataatitsleisuretothereceivingstationoperatedbySpotImageatToulouse,usingaconventional20GHzradiolink.AfterARTEMISreacheditsnalorbitaldestination,theSPOT-4/ARTEMISinter-satellitelinkstatisticssinceMarch2003count1327sessions,ofwhich57failed,withanaccumulatedlinkdurationof10days,18hoursand30minutes. 1.2.4ARTEMISandOpticalInter-orbitCommunicationEngineeringTestSatel-lite(OICETS)TheJapaneseSpaceExplorationAgency(JAXA)conductedtherstconceptualdesignandfeasibilitystudyofKirari,alsoknownastheOpticalInter-orbitCommunicationEngineeringTestSatellite(OICETS),in1992.In1993JAXAandESAagreedonacooperationtoperformopticalcommunicationexperimentsbetweenOICETSandARTEMIS.ThepreliminarydesignofOICETS(anditslasercommunicationterminal 15

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calledLUCE)wasnishedin1994andtheightmodelreadyin2001.JAXAvalidatedtheperformanceoftheengineeringmodelofitsLUCEterminalinaspacetogroundlinkwithARTEMISfromESAsOGSinSeptember2003.OICETSwaslaunchedonAugust23rd,2005intoacircularsunsynchronous610kmorbitandtherstlasercommunicationexperimentswithARTEMISwereperformedonDecember9th,2005.UnlikeSPOT-4,OICETScanalsoreceivedataanditdemonstratedtheworldsrstbidirectionalopticalinter-satellitecommunicationlink.Allsubsequentlinkshavebeensuccessful,withveryshortacquisitiontimesandexcellenttrackingperformances.Theselinkswereconstantlybeingmeasuredfortheapproximately15hoursofexperiments.TheOICETS/ARTEMISinter-satellitelinkstatisticscounts83sessionsofwhich2failed,withanaccumulatedlinkdurationof14hoursand21minutes.TheDLRInstituteforCommunicationandNavigationperformed8space-groundlessnesscommunicationexperimentswithOICETS. 1.2.5ARTEMISandLiaisonOptiqueLaserAroporte(LOLA)LOLA(LiaisonOptiqueLaserAroporte)wasaFrenchnationaldemonstratorprogramtoachieveanopticallinkbetweenanairbornecarrierrepresentativeofthefuturemedium-andhigh-altitudeunmannedareavehicle(UAVs)(MALEandHALE)andARTEMIS.Thepurposeoftheexperimentwastocharacterizethepropagationoflightbeamsintheatmosphereandtovalidatethesystemperformancecapabilitiesofthelink.ThelinkwastobeusedforsecuretransmissionofinformationreceivedfromUAVsbyoperationcentresafewthousandkilometresaway,withinaboutonesecondandatveryhighdatarates.Thishighcapacitydatastreamcouldconsiderablyreducethetimeneedtotransmitinformationfromthetheatreofoperation,wouldimprovecontrolofinformationandwouldbringsignicantoperationaladvantages.InDecember2006,ARTEMISsuccessfullyrelayedopticallaserlinksfromMystre20,equippedwiththeairbornelaseropticallinkLOLA.Atthattime,theairbornelaserlinks,establishedoveradistanceof40,000kmduringtwoightsataltitudesof6000 16

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and10,000meters,representedaworldrst.Therelaywassetupthroughsixtwo-wayopticallinksbetweenLOLAonMystre20andtheSILEXlaserlinkpayloadonboardARTEMISinitsgeostationaryorbitalposition. 1.2.6NearFieldInfraredExperiment(NFIRE)andTerraSARThelatestlasercommunicationterminaltobelaunchedistheSOLACOSsystem,whichwasdevelopedbytheGermanSpaceAgencytoyonTerraSAR,aGermanfundedEarthobservationsatelliteusingsyntheticapertureradarinX-band.SolidStateLaserCommunicationsinSpace(SOLACOS),manufacturedbyTesatSpacecom,Germany,andbasedoncoherentbinaryphaseshiftkeyingmodulationat1064nm,wasoriginallyintendedasapossiblereturnchannelforSILEX,usingadatarateof5.6Gbpsoverlinkdistanceupto10,000km.However,itspartnerterminalforalasercommunicationdemonstrationbetweenLEOsatelliteswaslaunchedonboardtheAmericanNFIREsatellite.In2008,NFIREandTerraSARdemonstratedthefastestinspace-bornecommunicationschanneltodate.OperatinginLowEarthOrbit,a5.5-Gbpsinter-satellitebidirectionalopticalcommunicationlinkwassuccessfullytestedbetweenNFIREandTerraSAR-Xatarangeofabout5,000km. 1.2.7AlphaSATFinally,theAlphaSatspacecraftisasuccessortoARTEMISandisdevelopedincooperationbetweenESAandtheFrenchSpaceAgency(CNES).80%ofAlphaSatspayloadcapacitywasawardedtoInmarsatGlobalLtdtoextendthecapabilitiesofitsBroadGlobalAreaNetworkServiceswhichcurrentlyconsistsofawiderangeofhighdaterateapplicationstoanewlineofuserterminalsforaeronautical,landandmaritimemarkets.Figure 1.2.7 givesarepresentationoftheopticalcrosslinksdiscussed. 1.2.8SmallSatelliteLaserCommunicationsThestateofsmallsatellitelasercommunicationsislargelybasedonlaboratoryresearch,withsystemsbeingplanned.(NationalInstituteofInformationandCommunicationTechnology(NICT)isengagedinjointresearchwithMitsubishiHeavyIndustriesto 17

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Figure1-1. SILEXproject[ 5 ]. launchasmallsatellite(approx.150kg),referredtoasSmartSatintoanellipticalorbitforpreliminaryvericationofopticalinter-satellitecommunications.AlsoatNICT,theSpaceCommunicationsGroupisdevelopinganon-mechanical,compactopticalterminalequippedwithatwo-dimensionallaserarray.Thenon-mechanicaltransceiverisproposedtofacilitateareductioninthesizeoftheopticalcommunicationssystem. 1.3MissionScenariosSmallsatellitesarecurrentlyplayinganimportantroleintheeldofremotesensing.Opticalcommunicationscouldplayanimportantroleinfacilitatingthedevelopmentofdataintensiveremotesensingapplications.[ 6 ]givesadetailedsurveyoftheabouttheuseoftheofsmallsatellitesinvariousremotesensingapplications.Asanexample,SatellitessuchastheBiSpectralInfraRedDetection(BIRD)experimentalsmallsatellite(2001-2004)havebeenlaunchedfordetectionofhoteventslikeforestresandvolcaniceruptions.Anotherimportantapplicationofremotesensingistostudytheeffectofenvironmentalchangeonthepatternsofhumanhealthanddisease.Useofsmallsatelliteimagingcanbecarriedouttostudytheeffectsofenvironmentalparametersthatinuencespatialandtemporalpattersofvectorbornediseases.Adedicatedremotesensingsurveillancesystemcanidentifyhighriskareaswhereinpreventivemeasurescanbedeployedattheearliest.Formalaria,anexamplerelevant 18

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Figure1-2. Missionscenario tomanyothervector-bornediseases,keyenvironmentalparametersincludethespatialandtemporalpatternsofvegetationtypeandcondition,landuse,standingwater,andhumansettlements.Remotesensingsatellitescanprovidevaluableinputsondisastermanagementbothforanaccuratepredictionandforarapidassessmentofthelocationandextentofdamage.Parametersofremotesensingsatellitesincludespectralcoverage,spectralresolution,revisittimeandspatialresolution.Currentspaceborneinfrastructureprovidesenvironmentalmonitoringexcellentlybutnotdisastermitigationduetolowrepetitionandbadspatialresolution.Anidealrepetitionrateof0.5-1hourandresolutionof10m-1kmisrequiredfordisastermitigation.Clustersofsmallsatellitescansolvethisproblemasthecosttodevelopandlaunchseveralofthemisnotprohibitive.Inordertoincreasetheamountofdatatransferapossiblemissionscenarioisshowiningure 1-2 wherein,datacollectedbythesmallsatellitesistransmittedtoalargersatelliteintheGEOorbitusingopticallinksandthelargersatellitetransmitsthedatatothegroundstation.ThisscenarionotonlyprovidesseamlesscoverageoftheareadesiredandhighresolutionimagesbutalsolargeamountofdataforanalysisasthedatatransferratesofopticallinkscangouptoseveralMbps. 1.4IntersatellieteLaserLinkComponentsTheIntersatellitelaserlinksystem(ISL)oflargesatellites,includesseveralcomponentstofacilitatetheestablishmentoflongdistancetodeepspacecrosslinks. 19

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However,allthecomponentsarenotrequiredtoachievethesamepurposeforcrosslinksforCubeSats.TheblockdiagramoftheISLisshowningure 1.4 foraCubeSat.Asstatedby[ 7 ],a3UCubeSatcanhaveamaximumweightof3kgsandtheavailabilityofpowerdependentonthenumberofsolarpanelsandhencecanisassumedtovaryfrom3-6Watts.Eachofthesubsystemwillbedescribedindetailwithemphasisontheoperation,requirementstobemetandnallysummarizingtheparametersforeachfortheCubeSat.Thethesisisorganizedinthefollowingmanner.Chapter2describestheOpticalSourceandDetector,Chapter3thePointingAcquisitionandTrackingSystem,Chapter4theOpticalTransmitterandReceiver,Chapter5analysestheproposedsystem,Chapter6providesthelinkbudgetanalysisandcomparisonwithRadioFrequencyandChapter7concludesthework. 20

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Figure1-3. ISLblockdiagram 21

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Figure1-4. ISLblockdiagram 22

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CHAPTER2OPTICALSOURCEANDDETECTORThesubsystemsdiscussedinthischapteraretheopticalsourceandopticaldetectoremphasizedingure 1.4 .IntherstsectionthevariouscomponentsandtherequirementsoftheopticalsourcearediscussedandthecharacteristicsofthesourcethatcanbeinstalledontheCubeSatissummarized.Thesecondsectiondiscussesthevariousopticaldetectors,theirpropertiesandtheirrelevancetotheCubeSat. 2.1OpticalSourceTheopticalsourceisthedevicethatproducesthesignalforcommunication.SuchadevicethatcanproduceanopticalsignalistheLaser.Severalclassicationoflasersexistaccordingtothedifferenttypesoflasingmediumthatproducesthesignal.LasersareclassiedintoGas,Solidstate,DyeandFreeelectronlasers.Gaslasersarefurtherclassiedintochemicalandexcimerlasers.SolidstatelasersarefurtherclassiedintoFibrehosted,photoniccrystalandSemiconductorLasers.Intheinitialstagesofcrosslinkdesign,Gaslaserswereprimarilyusedastheopticalsource.Gaslasersrequirekilowattsofpowertobeturnedatthesametimeproducingahighpoweredoutputsignal.Buttheirbulkinessandthedifcultyinproducingacompactdesignmadethemunsuitabletobeusedasanopticalsource.Also,sincethelasingmediumwasgasorachemicalsubstance,theefciencyofthemediumtoproduceaconstantoutputpowersignaldegradesovertime.TheinventionoftheSolidstatelasers,whicharecompactlasers,thedesignoftheopticalsourcebecamemoresimpler.Whilethesolidstatelasersproduceahighpoweredoutputsignal,theyneedtobepumpedbyanothersemiconductorlaserandcanprovidekilowattsofoutputpowerandhenceareveryusefultoestablishdeepspaceopticallinkswithlinkdistancesupto40,000kms.Thoughthepowerconsumedisrelativelylessthanthegaslasers,theystillrequiredlargeAmpsofcurrenttobeturnedon.Thenextcategoryoflasersthatweresuccessfullyexperimentedwithandhavebeenusedtoestablishmultiplecrosslinks 23

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aretheSemiconductorLasers.Drivenbyacurrentsource,theyarethemostcompactofallthelasersusedasanopticalsource.SeveralcategoriesofsemiconductorlaserdiodesexistsuchastheVerticalSurfaceCavityEmittingLaser,QuantumCascadeLaser,DistributedFeedbacklasersandFabryPerotLasers.Alltheselasersuseasemiconductormediumasthelasingmediumbuttheprocessoflasingdiffersfromonelasertoanother.Foradetaileddiscussionlasersthereaderisreferredto[ 1 8 9 ].Fromthepreviousdiscussion,itcanbeseenthatthelargesatellitehaveanumberofoptionswhenitcomestomakingachoicefortheopticalsource.Thesamedoesn'tapplytothecaseoftheCubeSatastheoptionsarelimitedonlytothedifferenttypesofsemiconductorlaserdiodes.Thefollowingsectionsdescribetherequirementstobemetbytheopticalsourceandthemodulationschemesthatcanbeused. 2.1.1ChoiceofLaserDiodeAlaserdiodeischosendependingontherequiredoutputpowerandwavelength.Theaverageoutputpowerisdependentthedistanceatwhichtheintersatellitelinksneedstobeestablishedandtheavailabledrivecurrent.ForCubeSats,drivecurrentsvaluesof>10mAandlessthan1Aforthepayloadsareavailable.Suchsmallcurrentsaresufcienttodrivethelasertoproduceopticaloutputpowersintherangeof10mW-25mW.SincebothCO2andsolidstatelaserscannotbeusedduetotheirvolume,weightandpowerconstraints[ 10 ],theonlychoiceofdiodesthatcanmeetpayloadrequirementsoftheCubesatsarethesemiconductorlaserdiodessuchasDistributedFeedbacklaserdiode(DFB),VerticalSurfaceCavityEmittingLaserdiodes(VSCEL)andQCLSomeofthecommonlaserdiodesandtheiroperatingwavelengtharetabulatedintable 2-1 Table2-1. Laserdiodewavelengths[ 1 ],[ 2 ] TypeofLaserDiodeWavelength(um) DiodePumpedNd:YAG(YAG)0.503-1.064IndiumGalliumArsenidePhosphide(InGaAsP)1.3-1.5AluminiumGalliumArsenide(AlGaAs)0.8VSCEL0.75-1.050 24

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2.1.2OperationThechosenlaserdiodeisturnedonbythebiascurrent.Inordertoobtainamodulatedoutput;themodulatingcurrentissuperimposedonthebiascurrent.Theoutputofthelaserdiodeisthenpassedthroughacollimatorwhichproducesabeamofthedesiredquality.Thedesignofthelaserdiodetransmitterpackagemustmeetthespeciedvolumeandweightconstraintsofthepayload. 2.1.3OpticalRequirementsBeamQuality:Theparameterofmajorinterestfromthepointofviewoftheopticaldesignisthebeamqualitywhichspecieshowtightlyorhowdivergentisthebeamofthelaserdiodetransmitterpackage.Agoodopticallinkdesignensuresthatthebeamdivergenceisasminimumaspossible.Thisparameterisdeterminedbyboththelasersourceandcollimationoptics.MinimizingWavefrontError:TheinherentastigmatismofthelaserdiodehastobecorrectedbydedicatedmeanslikecylindricallensestoachievethespecieddemandinggureofWavefrontError(WFE)[ 3 ],[ 1 ].Higherorderdeviationfromthelaserdiodewavefrontfromtheidealplanewavecannotbecompensated.Agoodcorrectionofaberrationsforthecollimatoritselfisrequiredtocopewiththespecication.LinkEstablishment:Foratransceiversystem,thestabilityofthebeamdirectionisrelatedtotherequirementonco-alignmentbetweenthetransmitterandreceivedbeamintheopticalterminal.Thealignmentcannotbemadeduringcommunicationandhencehastobemadebeforethecommunicationstarts.Theopticaldevicesthatarethepartofthesystemhavetomaintainastablealignmentoftheopticalsignalforatypicalcommunicationperiodsayfor24hrs. 2.1.4OpticalDesignCollimatorspecications:Theopticaldesignconsistsofdeterminingthespecicationsofthecollimatorwhichincludethefocallengthandnumericalaperturebasedonthebeamdiameterandthee2divergenceangle.Anexampleofacollimator 25

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assemblyisshowningure3.Theopticaldesignofthecollimatorisverydemandingduetothecombinationonvariousrequirementssuchaschromaticaberrations,opticaleldandwavefronterror.Fortheexamplecollimatorassemblyshowningure 2-1 ,therstsixlensesareusedforobtainingthedesiredbeamwidthandhavetobemanufacturedwiththickness+/-0.05mm.Tolerancevaluesforairgaps+/-1um.Thecollimatorlensesshouldbefullyachromatizedforthewavelengthspecied[ 3 ]. Figure2-1. Collimatordesign Compensationofaberrations:Aberrationswhichareproducedduetothedeviationofthelasersourcefromtheprincipleaxisofthecollimatorhavetobecompensatedbyprovidingforthelateraldisplacementofthelenses.Efciencyofthelaserdiode:Iftheefciencyofthelaserdiodedrops,thedrivecurrenthastobeincreased,therebycausinganincreaseinthedissipatedheatandthetemperaturegradientintheheatsinkofthelaserdiode.Thechangeinthetemperaturegradientcausesalateraldisplacementofthelaserdiodebeamwaistandthedisplacementhastobecompensatedbytheterminalpointingassemblyatthesamemaintainingthebeamquality.Astigmatism:Laserdiodeastigmatismhastobecorrectedbyplacingapairofcylindricalplanoconvexlensesinfrontorbehindthecollimator.Thelenseshaveto 26

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berotatedtocompensatetheamountofastigmatismpresent.Ingure4,thelensesmarkedas'c'areusedforcorrectingtheastigmatism. 2.1.5ElectricalandMechanicalRequirementsRiseTime:Therisetimeofthepulseisdeterminedfromthedataratetobeachieved.Foradatarateof120Mbps,usingQPM,apulsewidthof4nswithrisetimesofof1nshavebeenachieved.FrequencyResponse:ThefrequencyresponseofatransmitterisafunctionofthedrivecurrentspeedandtheimpedancematchingbetweenthelaserdriveroutputandtheLaserDiodeTransmitterPackage(LDTP)input.ThelaserdriveroutputandtheLDTPinputhastobeproperlymatchedastheresistanceatlaserdiodeinputissmallerthantheoutputofthelaserdriver.Iftheyarenotproperlymatched,reectionsofthedriversignalcanoccur.Also,theinherentinductanceofthelaserdiodepackagehastobeminimizedOperatingtemperature:Theoperatingtemperatureofalaserdiodecanvaryanywherefrom20degreeCelsius-100degreeCelsius.Thetemperatureatwhichthelasermeetsthepowerbudgetrequirementsmustbechosenandmaintained.MechanicalDesignTheMechanicaldesignhastotakeintoconsiderationthefollowingimportantaspects: Accommodationofthecollimatorandthecylindricallensesforcorrectingastigmatism. Smalllensdiametersresultinginsmalldimensionsofthehousing. Positioningofthelaserdioderelativetothecollimatingoptics.Thematerialforthelensshouldbechosensoastohaveahighcoefcientofthermalexpansion.Eachopticalcomponenthastohaveitsownmountastheyhavetobeindividuallycentered.Alaserdiodesupportisusedformountingthelaserdioderelativetothecollimatingoptics. 27

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ElectricalDesignTheelectricaldesignhastotakeintoconsiderationthefollowingimportantaspects:Protectionagainstnegativecurrent:Inordertoprovideprotectionagainstnegativecurrentamonitorphotodiode(MPD)hastobeincludedinthelaserdiodepackage.ASchottkydiodeneedstobeconnectedinparalleltothelaserdiodetoprotectitfromnegativecurrentsurgeandathermistorhastobepositionednearthelaserdiodebaseplateinordertomeasurethetemperatureofthelaserdiodewithahighaccuracy.ImpedanceMatching:Ashighspeedmodulationisrequired,thetwolinesformodulationcurrentandformonitordiodecurrenthavebeenmatchedtotherequiredimpedance.Thedrivelineshouldhavethesameimpedanceastheresistanceofthelaserdiode,whichisaround2Ohm.Sincethisverylowvaluecannotberealizedandacompromiseof10Ohmhasbeenchosen.Thelaserdriver,whichismatchedtothe10Ohmline,hastoabsorbthereectionsinducedbythelaserdiode[ 3 ],[ 1 ].Themonitorcurrentlinehasstandard50Ohmimpedance.Theabovementionedimpedancescanchangedependingontheresistanceofferedbythedifferenttypesofdiodes.TheparametersofaLaserDiodeTransmitterPackageoriginallydesignedforlargesatellitesthatmeetthevolume,weightandpowerconstraintforthesmallsatellitesisoutlinedinthetable 2-2 2.1.6ModulationSchemesThecommunicationprocessingelectronicsdeterminethetypeofmodulationthathastobeappliedtothelaser.Conversely,itcontrolsthemodulatorwhichinturnmodulatesthelaserbasedontheinputfromtheelectronics.DataratesachievedwithanISLingeostationaryorbitwithaseparationof40,000kilometersareabout360-500Mbits/sec.ThecommonandsimplestmodulationtechniquesareOnandOffkeying(OOK),Q-aryPPM(QuaternaryPulsePositionmodulation)andFrequencyShiftKeying(FSK). 28

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Table2-2. Laserdiodeparametersummary[ 3 ] ParameterRange ClearAperture9mmWavelength0.8um-1.064umTransmittance95.00%Stabilityofbeamdirection5uradWavefrontCurvatureRadius250mWavefrontError1/20wavesSpectrumWidthundermodulation4nmPolarizationPurity1/100AverageOutputOpticalPower30mW-400mWPeakOpticalpower(DC=25%)DependsonthediodespecsRiseTimeinPulsedOperationDependsondatarateFor120Mbps,pulsewidthis4nsandrisetimeis1ns.OperationalTemperatureRange20-30degreesDimensions53x50x57mmWeight180g 2.1.6.1OnandOffKeyingAmplitudeShiftkeyingisaformofmodulationthatrepresentsdigitaldataasvariationsintheamplitudeofacarrierwave.Theamplitudeofananalogcarriersignalvariesinaccordancewiththebitstream,keepingfrequencyandphaseconstant.OnandOffKeying(OOK)canbeconsideredtobeaspecialcaseofASKwhereinthebinarybit'1'isrepresentedbythepresenceofacarrierandbinary'0'isrepresentedbytheabsenceofacarriersignal.AvariantofOnandOffkeyingistheNon-ReturntoZeroschemewherin,binary'O'isrepresentedbyalowpowersignalratherthantheabsenceofthesignal.Advantages:ThecircuitryforOOKisrelativelyverysimpleandinexpensive.Disadvantages:Itisverysensitivetoatmosphericnoise,distortionsandpropagationconditions.ForLaserdiodes,binary1isrepresentedbyashortpulseoflightforaspecicdurationandbinary0isrepresentedbyabsenceoflightforaspecicduration.AnexampleofOOKisshowningure 2-2 2.1.6.2PulsepositionmodulationPulse-positionmodulation(PPM)isaformofsignalmodulationinwhichMmessagebitsareencodedbytransmittingasinglepulseinoneofthe2Mpossibletime-shifts. 29

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Figure2-2. OOKmodulation ThisisrepeatedeveryTseconds,suchthatthetransmittedbitrateisM/Tbitspersecond.Operation:Itisoftenimplementeddifferentiallyasdifferentialpulse-positionmodulation,wherebyeachpulsepositionisencodedrelativetotheprevious,suchthatthereceivermustonlymeasurethedifferenceinthearrivaltimeofsuccessivepulses.Itispossibletolimitthepropagationoferrorstoadjacentsymbols,sothatanerrorinmeasuringthedifferentialdelayofonepulsewillaffectonlytwosymbols,insteadofaffectingallsuccessivemeasurements.Advantages:OneoftheprincipaladvantagesofPPMisthatitisanM-arymodulationtechniquethatcanbeimplementednon-coherently,suchthatthereceiverdoesnotneedtouseaphaselockedloop(PLL)totrackthephaseofthecarrier.Thismakesitasuitablecandidateforopticalcommunicationssystems,wherecoherentphasemodulationanddetectionaredifcultandextremelyexpensive.Disadvantages:Akeydifcultyofimplementingthistechniqueisthatthereceivermustbeproperlysynchronizedtoalignthelocalclockwiththebeginningofeachsymbol.Asidefromtheissuesregardingreceiversynchronization,thekeydisadvantageofPPMisthatitisinherentlysensitivetomultipathinterferencethatarisesinchannelswithfrequency-selectivefading,wherebythereceiver'ssignalcontainsoneormoreechoesofeachtransmittedpulse.Sincetheinformationisencodedinthetimeofarrival(eitherdifferentially,orrelativetoacommonclock,thepresenceofoneormoreechoes 30

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canmakeitextremelydifcult,ifnotimpossible,toaccuratelydeterminethecorrectpulsepositioncorrespondingtothetransmittedpulse. 2.1.6.3FrequencyShiftKeyingFrequency-shiftkeying(FSK)isamodulationschemeinwhichdigitalinformationistransmittedthroughdiscretefrequencychangesofacarrierwave.ThesimplestFSKisbinaryFSK(BFSK).BFSKusestwodiscretefrequenciestotransmitbinary(0sand1s)information.Withthisscheme,theiscalledthemarkfrequencyandtheiscalledthespacefrequency.InOpticalcommunications,avariationofFSKknownasM-FSKisusedwherein2ormorefrequenciesareusedtobinarydata.Disadvantage:ThelaserdiodeneedstobetunabletoawiderangeoffrequenciesforFSKtobeusedandthiswouldcausethelaserdiodetodeviatefromitscentralwavelengthovertime.Tosummarize,onoffkeyingandNonReturntoZerocanbeimplementedwithsimplecircuitrywhencomparedwithPPMandFSKwithpenaltyofsmallerdataratewhencomparedtoFSKandPPM.AlsoPPMandFSKrequirestringentclocksynchronizationtechniquessincecoherentdemodulationandnotdirectdetectionistheprimarymethodtodemodulatethereceivedsignal.HencethecandidatemodulationschemeisONOFFkeyingorNRZ. 2.2OpticalDetectorTheopticaldetectoristhereceivingelementthatconvertstheopticalsignalintoanelectronicsignal.Theelectronicsignalisfurthersenttothedemodulatingelectronicsthatretrievetheoriginaltransmittedsignal.ThefourimportantopticaldetectorsarethePINdiode,APD,CCDandCMOSdetectors.Thedetectorsarecharacterizedtwomainparametersnamelyquantumefciencyandresponsivityataspecicwavelength.Quantumefciencyofadetectoristheabilitytogenerateelectronsforeveryincomingphotonandresponsivityisamountofelectriccurrentgeneratedperwattofincidentopticalpower.Detectorsoperateinareversebiasedmodeandhencehavehigh 31

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operatingvoltagesintherangeof15-30V.WhilethePINdiodeshavearesponsivityintherangeof0.5-0.7A/WandlowoperatingvoltagesandareusedforshortlinkdistancestheAPDshavearesponsivityintherange0f20-80A/Wandhigheroperatingvoltagesandareusedforlonglinkdistances.Aswithpinphotodiodes,siliconAPDscanbeusedinthewavelengthrangeof300nmtoapproximately1050nm.For800nmto1700nm,InGaAsdiodesareavailable.Chargecoupleddevices(CCDs)andCMOSfocalplanearrayscouldbeconsideredasreceiveelements,too.Bothareavailableinone-dimensionalandtwo-dimensionalcongurationswithuptoafewthousandpixelsinonedimension.Themajordrawbackofthistechnologyisthelimitedpixelreadoutfrequencyofsome10MHz.Assumingalinesensorwith100pixelsandapixelclockfrequencyof10MHz,amaximumdatarateof100kbit/sandanangularresolutionontheorderof1/100oftheeld-of-viewcouldbeachieved.Anotherdisadvantageisthelargeamountofreadoutelectronicsrequired(withitsextrapowerconsumption)[ 11 ].Usingopticalpreamplicationcanimprovetheperformanceofreceivers.Whendirectdetectionwithoutpreamplicationisused,theonlysourcesofnoisearebackgroundnoise,detectordarknoiseandelectronicampliernoise[ 12 ].Theelectronicampliernoiseisnegligiblewhencomparedwiththeothertwonoises.Withopticalpreamplication,severalnoisesourcessuchasSignalShotnoise,AmpliedStimulatedEmission,SpontaneousBeatNoiseetcareprevalent.Opticalpreampliersifrequiredareavailableasbreampliersorassemiconductoropticalampliers.Useoftheseampliersagainaddstotheweightandpowerconsumptionofthesystemandhenceshouldbeincorporatedintothedesignonlywhenabsolutelynecessary.Toprovidebackgroundsuppressionandchannelselection(thelatterinWDMAsystemsonly),anopticalbandpasslterhastobeusedatthereceiverinput.Ingeneral,wideeld-of-view,narrowbandwidth,andlowinsertionlossarecontraryrequirementsandacompromisewillhavetobefoundamongthem.Thinlminterferenceltersareavailablewithbandwidthsdownto1nm,theirtransmissionvariesfrom60%for 32

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Table2-3. Operatingwavelengthsofopticaldetectors MaterialWavelength(nm) Silicon190-1100Germanium400-1700IndiumgalliumArsenide800-2600LeadIIsulphide<1000-3500 thenarrowtypestoaround90%forlterswithseveral10nmbandwidth.ThecenterwavelengthofadielectricFabry-Perotltershiftswithangleofincidencebysome1to2nm/,thuslimitingthepassbandforwide-eld-of-viewapplications.Duetotheirminimalangulardependenceandlowtransmissionloss,absorptionltersbasedonGaAsareanattractivealternativetointerferencelters.Table 2-3 givestheoperatingwavelengthsofsomeofthedetectors. 33

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Figure2-3. ISLblockdiagram 34

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CHAPTER3POINTING,TRACKINGANDACCQUISITIONASSEMBLY(PAT)ThePointing,AcquisitionandTrackingsubsystemconsistsinacquiringandtrackingthecounterterminalincominglaserbeamaswellasinpointingthetransmitterterminalsoutgoingbeamwithanaccuracywhichenablesdatatransmissionbetweentwosatellites.Theoperationsthatareneededtobecarriedoutbythesubsystemconsistof[ 13 ],[ 14 ],[ 15 ]: Acquisitionphasewhichhastocompensatefortheinitialbeampointingerrorduetospatialacquisitionerrors,mainlyephemeriserrorandspacecraftlocationpredictionerrors. Trackingphasewhereinoncethebeamisacquired,ithastotrackoutlocalangulardisturbancestransmittedfromthehostplatformandthedynamicelementsofthepayloadwithsubmicroradianaccuracy. Pointingphasewhereintheterminal'sopticalheadispointedtowardstheoppositesatelliteaftercompensationforrelativeplatformmotionsandnitetransittimeoflight.Forlargesatellites,duetothelessstringentspaceandpowerconstraints,eachofthethreetasksareperformedbyseparatesystems.Acquisitionisperformedbytheacquisitionsystem,thetrackingphasecarriedoutthecoarsepointingsystemandthepointingphasecarriedoutbythenepointingsystem.EachsysteminturnitscontrolledbyadifferentelectronicsystemsuchastheAcquisitionProcessingElectronics,CoarsePointingElectronicsandtheFinePointingElectronics.Fordetaileddiscussiononthesesystems,thereaderisreferredto[ 3 13 16 20 ].Theselargesystemsareverynecessarytoestablishlongdistanceanddeepspaceopticallinksandhencecannotbedoneawaywithforlargesatellites.SinceweareawareoftheconstraintsoftheCubeSat,wecanworkwiththeassumptionthatasinglesystemwouldsufcetosatisfythePATneedsoftheCubeSat. 35

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InordertocarryoutthePATtasksfortheCubeSat,thePATsystemshouldbeabletoorienttheopticalelementdirectingtheopticalsignalalongitsxandyaxis.Therearetwopossibledevicestocarryoutthistask. Anelectromechanicaldevicethatcanprovidetorquetothegimbaledopticaldevice. AMEMSmirrorthatcanbeusedtodirecttheopticalsignalintherequireddirection.WhiletheMEMSmirrorscanbesubjecttoradiationwearandtear,themorerobustoptionistogofortheelectromechanicaldevicewhichisthesubjectoffurtherdiscussion. 3.1Pointing,AcquisitionandTrackingStrategies 3.1.1AcquisitionStrategyToestablishalinktostartcommunicationbetweentwosatellitesS1andS2,thesatelliteS1mustsendoutabeaconsignal.Thedivergenceofthebeaconsignalislimitedtosay700urad.Theconeofuncertaintycouldbelimitedto8000urad.ThesatelliteS2scanstheconeofuncertaintytillitsterminalisilluminatedbythelaserbeam.Onceilluminated,itmustdetectthedirectionoftheincominglight,correctingitsLineofSight,starttrackingandemitsitscommunicationbeamtowardsthetransmittingsatellite.OncethesatelliteS1receivesthecommunicationbeamfromS2,itstopssendingitsbeacon,startstrackingandcorrectsitslineofsightandsendsitscommunicationbeam[ 9 ],[ 13 ].Thetwosatellitesarenowinmutualclosedlooptracking. 3.1.2TrackingandPointingStrategyTheextremelyhighpointingaccuracyof1microradismetbyusingtheincominglightfromthecounterterminalasareferencetothepointingactuators.Thetwoterminalsarethusinaco-operativeclosedtrackingloopduringcommunication.Thetrackinganglecorrespondstowherethetrackedterminalwaswhenthelightwasemittedwhereastheidealpointingdirectioncorrespondstowherethepointedterminalwillbewhenthelightarrives,i.e.thepointinganglemustbeoffsetwithrespecttothe 36

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Figure3-1. PATassembly trackingreferencewiththesocalledpointaheadangle(duetorelativetransversesatellitevelocityandthenitevelocityoflight)[ 14 ].ThePATassemblyisshowningure 3-1 providesdeectionoftheincomingandoutcominglaserbeamsaroundtwoorthogonalaxesandthusperformsthefollowingoperations: Inthescanningmode,itsweepsthebeaconoverawideangularrange. Intheacquisitionmodeitprovidesafastdeviationangleoverawiderangetorecentertheincomingbeamonthetrackingsensorassensedbytheacquisitionsensor. Inthetrackingmodeitcontrolsoftheangularpositionoftheincomingbeamassensedbythetrackingsensorwithhighbandwidthandaccuracy.ThedesignspecicationsofthePATassemblydetailedasfollows:PositionalAccuracy:Thedistancebetweenthesatellitesmaybeseveralthousandsofkilometres.Toestablishalinkbetweentwosatellitesthatisinordertopointthelaserbeamontothemirroroftheothersatellite,requiresapositionalaccuracyof1urad.Thedeviceshouldalsobeabletomovethemirrorby+/-2degreestoprovidecoarsepointingaroundtheneighbouringsatellite. 37

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Bandwidth:Thedevicemusthavesufcientbandwidthtorejectsatellitevibrationsanddisturbances.Forsmallsignaldisturbancesontheorderof0.01degrees,thisbandwidthshouldbeabout200Hz.Largerdisturbancesofabout1degreeneedtoberejectedbelow1Hz[ 15 ].Actuatingthemirror:Twotypesofdevicesareusuallyconsideredforactuationofthebeam-steeringmirror:voicecoilsandpiezoelectricactuators.Theformeroffersapotentiallylargestrokeforlittlepowerinput,butissomewhatlimitedinbandwidthduetoitslowforceoutputandconsequentlylowstiffness.Piezoelectricactuatorsbycontrastofferhighbandwidthandstiffness,butprovideverysmallpositionoutputevenatrelativelyhighvoltages.Theidealdeviceshouldcombinethebenetsofbothtechnologies.Whileperformingalltheabovedescribedfunctionsthepowerconsumptionofthedeviceshouldbeverylow[ 13 ],[ 15 ] 3.2PATAlgorithmAnexampleacquisitionalgorithmisshowningure 3-2 ,wherethetransmitterbeamiswidenedsuchthatitilluminatesthereceiverfromanypositionwithintheareaofuncertainty.Atthebeginningoftheacquisitionprocess,thereceiveantennapointsatthecenteroftheareaofuncertaintyThenitstartsthespatialsearchbysequentiallyscanningtheuncertaintyareaalongaspiraltrack.Whenthetransmitterisfound,thereceiverswitchesintotrackingmode,whereaspatialtrackingloopisclosedbyevaluatingthesignalfromapositionsensitivedetectorandusingthisinformationtocontrolthealignmentoftheopticalantenna.Thetransmitterbeamdivergenceisnotreducedduringthistrackingmode[ 14 ], 3.3ElectroMechanicalDesignChoiceAdesignexampleofaPATsystemthatmeetsthe3UCubeSatspecicationsisbrieysummarizedin[ 15 ].Itisessentiallyacongurationoffouridenticalelectromagneticcircuits,positionedat90degreesfromeachotheraroundthecircumferenceofacircleasshowningures 3-3 and 3-4 .Foursmallpermanentmagnetsaremountedtoa 38

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Figure3-2. PATalgorithm movingmirrorplatform,whilefourcorrespondingcoilsandcoilcoresresidewithinaxedhousing.Consequently,mostofthecomponentsarestationary.Eachcircuitmagneticallypullsonthemirrorplatform.However,byincreasingpositivecurrentinonecoilamagneticeldisgeneratedwhichopposesthatofthepermanentmagnetdirectlyinfrontofit.Simultaneouslyincreasingnegativecurrentinthecoil180degreesawayaddstothemagneticeldinthatcircuit.Inthisway,adifferentialforceisproducedwhichtiltsthemirrorplatformaboutitscenter.Similaroperationoftheothertwocoilsproducesmotionalongtheorthogonalaxis,sothatthemirrorcanbepositionedanywherewithinanopticalcone.TheplatformismountedonaexibleBeCudiaphragmthatprovidesalinearrestoringtorqueinanydirectionagainstwhichtheelectromagnetictorquecanreact,yieldingpositionalcontrol.Also,byvaryingthetotalcurrentinallfourcoilssimultaneously,theplatformcanbepositionedlinearlyalongitscenterline,providinganadditionaldegreeoffreedomifdesired.TheparametersofthedescribedPATsystemaretabulatedintable 3-1 39

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Table3-1. PATparameters ParameterRange MaximumDeectionrange+/-35000microradiansBandwidth200Hzfordisturbancesof0.01degrees1Hzfordisturbancesof1degreeDimensionLessthan3inchesinlength,breadth,heightanddiameter.Weight400gms-250gmsPower1.5wattsintrackingmode5-16wattsinacquisitionmode Figure3-3. TopviewcrosssectionofPATsystem. Figure3-4. SideviewofcrosssectionofPATsystem 40

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Figure3-5. ISLblockdiagram 41

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CHAPTER4TRANSMITTERANDRECEIVERTheopticaltransmitteristhemostimportantcomponentofthelasercommunicationsystemasitperformsthefollowingtwomajorfunctions.First,itisusedasalaunchpadfortheopticalsignalandindoingsomustpreservetheopticalqualityofthesignalSecond,itactsasameanstofocustheincomingsignalontotheimagingopticswhichinturnrelaythesignaltotheopticaldetector.Largersystemsuseasingleinstrumenttotransmitaswellasreceivethesignal,whilesmallersystemsmakeuseofdifferentinstrumentsfortransmissionandreceptionofthesignal.Thetelescopesusedinthelargesatellitescanbeusedtoconstructthetransmitterandreceiverforthesmallsatellitestoo.However,thespecicationshavetobetailoredtomeetthedesignconstraintsofthesmallsatellites.Inthischapter,weexplorethevariouscongurationsoftelescopesalongwiththeirmeritsanddemerits.Thecandidatetelescopeisselectedandthespecicationsareoutlinedforthesmallsatellite.Thetelescopesforthelasercommunicationsystem(LCS)systemarederivedfromthevariouscommonastronomicaltelescopesandtheircharacteristicsaresummarizedasfollows[ 21 ]. 4.1CommonAstronomicalTelescopes 4.1.1Off-AxisNewtonianTheoffaxisnewtoniantelescopeconsistsofaoff-axisparaboloidalmirrorcutoutofalargerparaboloidalparentmirrorasshowningure.Theoff-axischaracteristicproducesatelescopethatisunobstructedasneitherthetransmittingorreceivingsignalisblockedandhencetheinstrumentdoesn'tplayaroleindegradingthequalityofthesignal.Adeectingmirrorisemployedtodirectthesignaltowardsthetelescopesfortransmissionorawayfromthetelescopeswhereitcanbeconvenientlyhandledbytheimagingoptics.Sincetheoffaxismirrorisinfactapartofthelargerparaboloid, 42

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Figure4-1. Off-AxisNewtonian reducingthefocallengthofthemirrorcannotbecarriedoutwithoutdegradingoverallopticalimagequalityasveryshortfocalmirrorsareverydifculttomanufacture. 4.1.2CassegrainThecassegraincongurationconsistsofaparaboloidalprimarymirrorandahyperboloidalsecondarymirror.Thecongurationisverycompactduetothefoldingemployedandthelengthofthetelescopecanbeasmuchasone-thirdthefocallengthofthetelescope.Sincethecongurationconsistsoftwomirrors,theplacementofthesecondarywithrespecttotheprimaryisverycriticalandtolerancesoftheorderoffractionsofathousandthsofanincharecommon.Thefactthatthedesignconsistsoftwomirrorsproducesaeldofviewofhalfadegreeandwavefrontqualitiesoflambda/16rmshavebeenproducedinthepast.Thesecondarydiameteris0.2to0.25oftheprimarydiameter. 4.1.3Off-AxisGregorianTheoff-axisGregoriantelescopeisasshowningure 4-3 .Ascanbeseen,theopticalsignalisfocusedbeforebeingincidentonthesecondarymirror.Thepointoffocuscanbeusedtoinsertaeldstopwhichdeterminestheeldofview(FOV)ofthetelescope.AlsoaLyotstopcanbeinsertedatthepointasshowntorejectstraylightreectedfromtubes,bafesoredgesoftheprimarymirror.Theoff-axisGregoriantelescopethuscombinestheunobscuredcharacteristicofthecassengraintelescope 43

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Figure4-2. Cassegrain Figure4-3. Off-AxisGregorian andtheGregorianstraylightrejectioncapabilitythusmakingitagoodcandidateforacrosslinktelescope. 4.1.4CassegrainwithRefractiveElementsInordertoincreasetheFOVofastronomicaltelescopes,refractiveelementsareaddedtotheCassegraintelescopeasshowninthegure.Telescopeswithrefractiveelementsarecalledcatadioptric,i.etheycombinereective(catoptric)elementswithrefractive(dioptric)elements.Currently,telescopesmakeuseofdoubletandtripletlensestoincreasetheFOV. 44

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Figure4-4. Cassegrainwithrefractiveelements Figure4-5. Schmidt-Cassegrain 4.1.5Schmidt-CassegrainThistelescopesisacombinationofaSchmidtandCassegraindesign.TheschmidtdesignmakesuseofanasphericplatetocorrectsphericalabberationsandwhencombinedwithaCassegrainalargeateldtelescope.Aeldofgreaterthan1degreeisavailablewithdiffractionlimitedperformance. 4.1.6Maksutov-CassegrainTheMaksutov-Cassegrainisawideeldofviewcatadioptrictelescope.ItproducesFOVof10'sofdegreesbutwithadisadvantageofasphericalfocalplane.Aat-eldtelescopewithalessambitiousFOVcanbeproducedwithaCassegraintwomirrorconguration,aeldsimilartotheSchmidt-Cassegrain.Forlargerapertures,the 45

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Figure4-6. Maksutov-Cassegrain meniscuscorrectorisheavierforthesamesizetelescopethanacorrespondingSchmidtplate.Withproperdesignparametersassignedduringthedesignprocess,thesecondarycanbeplacedonthesphericalcorrectorplatebythesimpleexpedientofreectivitycoatingthecenteroftheplateasillustrated.Theradiusofthesecondaryisthesameasthebacksideofthemeniscuscorrector.Thesetelescopesareofextremelyhighqualityofabout(/40)rms. 4.2TelescopeCharacteristicsThefollowingsectiondescribessomeoftheimportanttelescopecharacteristicsandtheirinuenceontheconstructionofthetelescopeforaLCSterminal.Asummaryofthecharacteristicsofthetelescopesdescribedistabulatedin 4-1 4.2.1Variationoff/#ThepositionoftheimageformationplaysanimportantroleindecidingonthetypeoftelescopethatistobeusedinaLCSterminal.TheCassegraintelescopeanditsvariantsproduceimagesthatarebehindtheprimaryandhencemakesitconvenienttohandletheincomingsignal.TheCassegrainf/#orfnumberisthetheratioofthefocallengthtotheapertureofthetelescopeandtypicalnumbersfortheCassegraintelescoperangefromf/8tof/15andtheprimaryf/#mayrangefromf/8tof/1.5.Theshorterprimariesarefasterandmoredifculttomanufacturewhileatthesametime 46

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resultinginashortertelescopeandthelongerprimariesareslowerresultinginalongertelescope. Table4-1. Telescopeparametersummary[ 1 ] FigureNumberCentralObscurationFOVRelativelengthInternalFieldStopRelativeWeightTube 4-1 NoUpto1=4oLongNoLightOpen 4-2 YesAbout12oShortNoMediumOpen 4-3 NoAbout1=2oLongYesMediumOpen 4-4 Yes>1oShortNoLightMediumOpen 4-5 Yes>1oShorttoMediumNoMediumtoHeavyClosed 4-6 Yes>1oMediumNoHeavyClosed 4.2.2ResistancetoJammingCrosslinksemployedinmilitaryoperationsmustbejamresistant.Jammingistheprocessofinterferingwithasignalbytransmittinganothersignalwithsimilarpropertiessuchasmodulation,rightwavelengthandpower.Thejammingcantakeplacebyinterferingwiththeacquisition,trackingorcommunicationprocesses.Theabilityofthecrosslinksystemtoresistjammingiscalledoff-axisrejectionandthemostusefulvaluetoquantifythispropertyiscalledpointsourcerejectionorPSR.PSRistheratiooftheenergyincidentonthereceivertotheenergyenteringthetelescopeattheangleofinterest.ThereareseveraltechniquesavailabletoincreasethePSR.Someofthemsummarizedbelow: Toshieldtheprimaryatlargeroff-axisangles. Useofinternalbafesurfacesofverylowreectanceontheorderofafewpercent. Theprimarymirrorhastobesmoothlypolished. 47

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Areimaginggroupoflensesisusedfollowingthetelescopewhichpermitstheincorporationofaeldstoporlyotstop. Thedesignofthetelescopeisveryimportant.Sometelescopesaredesignedforoff-axisrejection.Suchanexampleistheoff-axisGregoriansincethereisnosecondaryornospider,andthepresenceofaeldstophastheeffectofeliminatingextraneousoff-axislight. ThesmallFOV,thebettertherejectionofstraylight.Forexample,asystemwitha100uradFOVhas100timesbetterstraylightrejectionthanasystem100mradFOV. 4.2.3DiffractionLimitedTelescopesThetermdiffractionlimitedisgenerallyassociatedwiththeRayleighquarterwavecriterion,whichmeansthatanytelescopeoranyopticalelementalsothatexhibitsawavefronterrorequaltoorlessthan00.075(lambda/13.3)waversRMScanbeconsideredtobediffractionlimited.Itisreferencedtothewavelengthofinterest.Toachievediffractionlimitedperformanceinatelescopeafterithasbeeninstalledinsatellite,launchedandon-orbitforyearswhileundertheinuenceoftemperatureswings,thermalgradients,materialinstabilitiesisnoeasytask.Thedesignmustbeexact,thematerialschosenmustbesuitableforthetask,themanufactureandalignmentmustbeperfectandnallytheenvironmentinwhichthetelescopesmustoperateneedtobewellunderstoodandcontrolledtoappropriatelimits.Thetelescopesformsthetransmitbeamandanyerrsinthegurewilldegradethebeamquality,thusweakeningthereceivedsignalathecompanionterminal.Forsometelescopes,awindowacrosstheaperturemaybedesirableforrejectionofsunlightbyuseofanappropriatesolarrejectioncoating.Thedirectimpingementofthesunontheprimarycancauseseveredistortionwithresultingtransmitsignalloss.Resistancetoalaserweaponthreatmayalsobeincreased. 4.2.4TelescopeMaterialsTelescopelensesonceconstructedarehousedinacylindricaltubemadefromonethematerialslistedintable 4-2 .Theotherimportantpropertiesofmaterialssuchas 48

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Table4-2. Telescopematerialdensities[ 4 ] MaterialDensitygm=cm2 Beryllium1.85Aluminum2.70ULE2.20Glass2.53FusedSilica2.02Zerodur2.53Invar368.03Titanium4.43SXA(Al)2.91GraphiteEpoxy1.78 Table4-3. Telescopeparameters ParameterValue Aperture10cmWeight1.85gm/cm21kg-2.4kgapproxPrimarymirrorfocallength(f/4)40cmPrimarymirrorfocallength(f/1.5)15cmSecondarymirrordiameter2.5cmFieldofview1=2degree ExpansionCoefcient,ThermalConduction,ElasticModulus,MicroStressAnalysis,SpecicHeatarenotdiscussedastheydon'tcontributetotheweightofthetelescopeandarebeyondthescopeofthiswork.Sincedensityisthemaincontributortotheweightofthetelescope,thematerialwiththelightestdensityisconsideredtobeusedfortheconstructionofthetelescope.Onstudyingtheastronomicaltelescopeswithrespecttotheircontributiontotheweightofthepayload,theircharacteristicsandthematerialsusedforconstruction,theCassegraintelescopewasfoundtoprovidetheleastweightandhencewasnalizedasthecandidateopticaltransmitter.UsingthespecicationsoftheCassegraintelescope,theopticaltransmitterandreceiverforthesmallsatellitewouldhavethefollowingspecicationsaslistedin 4-3 basedonthepreviousdiscussion. 49

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CHAPTER5ANALYSISOFTHESYSTEMDESIGNOpticalcrosslinkscanbeestablishedoverdistancesassmallas10mandaslargeas1000'sofkm.Thedesignparametersofthesubsystemsweredeterminedinordertoachievecommunicationoverlongdistances.Hence,forlongdistancecrosslinkseachoneofthesubsytemplaysavitalroleandcannotberemovedifaccuratecrosslinkdesignisanecessity.Theweight,volumeandEPSrequirementsoftheproposedsystemaresummarizedintable 5-1 .Inregardtotheweightofsystem,ascanbededucedfromthetable,thecontributiontotheweightofthepayloadfromtheopticalsource,PATanddetectorisveryminimal.Alargepartofthecontributiontotheweightcomesfromthetransmitter/receivertelescopes.Thetransmitterandreceivertelescopesareanessentialdevicetoachievelongdistancecrosslinks.Thereforecarefulconsiderationofthematerialstomanufacturetelescopesshouldbecarriedoutcarefully.Inregardtothepowerconsumptionofthesystem,thePATsystemconsumesthemaximumamountofpowerandinthecaseofthedetector,voltagesintherangeof15-30Varehardtobegeneratedgiventheverylowpowergenerationof3WbytheCubeSat.HencethoughthesystemsuccessfullymeetsthevolumeandweightspecicationsandcanbecompactlypackedinsidetheCubeSat,itdoesntsatisfythepowerrequirements.Also,itnotpossibletoduplexlinkduethespaceconstraintsasaduplexlinkisnecessaryforthesuccessfulacquisition,tracking,pointingandestablishmentofthecrosslink.Itcanbeseenthatlowweight,oneprimaryreasonforwhichopticalcommunicationsarepreferredoverRFforlongrangecrosslinksisaslightlydifcultparametertoachievefortheCubeSat.However,aviablealternativeistodeploythesystemaon6UCubeSatasthissystemwouldnotonlymeetthevolumeandweightconstraintsofthe6UCubeSatbutalsothepowerrequirements.However,ifshortrangecrosslinksabout1kmaredesired,thenthesubsytemssuchasthePATandtelescopescanberemovedfromthesystemdesign.Itisthen 50

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Table5-1. OpticalPayloadParametersSummary Subsystem%WeightVolumeEPSRequirements Source953x50x57mm10-50mAPAT20L=4.35cmH=3.95cm1.5W-15WTx/Rx50L=15cm,H=10cmNopowerconsumptionDetectorNegligibleNegligible15V-30V assumedthatthetransmitterandreceiverareperfectlyalignedandthereisnoneedforpointing,acquisitonandtrackingofthereceiverterminalforlinkestablishment.Ifhowever,pointing,acquisitionandtrackingisrequried,anapproachintheformofaatmirrortiltablealongoneortwoaxesinfrontoftheterminalapertureisattractiveduetotheavailabilityoflightweightelectrostaticallytiltablemirrorsbasedonMEMStechnology.Thissilicon-basedtechnologyallowstomanufactureelectricallydrivenmirrorswithdiametersbetween300and2000mwithswitchingtimesaslowasafewmsandangularmovementsbetween3and20.(Suchasystemcan,ofcourse,alsoprovidecontinuouspointingofnarrowbeamsintwoorthogonaldirections.)Theoutputlaserpowerhastobesufcienttotraversethelinkdistancewithoutbeingcorruptedtoaundetectablelevelbythebackgroundnoise.Atreceiverend,theaperturesizeofthephotodiodewhichisabout5mmissufcienttoreceivethetransmittedsignal. 51

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CHAPTER6LINKBUDGETANALYSISAlinkbudgetisatooltohelpthecommunicationsystemdesignertodesignandanalyseanycommunicationlinkwherethemediumoftransmissioncanbewirelineorwirelessandthesignalfortransmissionofdataberadiofrequencyoroptical.ThischapterdescribestheopticalcommunicationlinkdesignfortheCubeSatusingtheparametersofsubsytemsthenalizedinchapter5wherenecessary.ThesoftwareusedforthedesignandanalysisofthelinkistheOptiSystemsoftware. 6.1OptiWaveSoftwareOptiSystemisaninnovative,rapidlyevolving,andpowerfulsoftwaredesigntoolthatenablesuserstoplan,test,andsimulatealmosteverytypeofopticallinkinthetransmissionlayerofabroadspectrumofopticalnetworksfromLAN,SAN,MANtoultra-long-haul.Itofferstransmissionlayeropticalcommunicationsystemdesignandplanningfromcomponenttosystemlevel,andvisuallypresentsanalysisandscenarios.AsimplexcommunicationsystemismodelledusingOptiSystem.Aduplexcommunicationsystemisnotconsideredforthesimulationanalysisasthesystemdesignwouldn'tmeettheconstraintsoftheCubeSat.Thefollowingcomponentsfromthesoftwarewereusedtomodelthesystemaretabulatedintable 6-1 6.2LinkbudgetequationInthissectionweoutlinethelinkbudgetequationsandexplaineachtermindetail.TheelectricalparametervaluesfortheRFCubeSatsystemandproposedOpticalSystemarelistedandthecorrespondingresultsanalyzed. Pr=PtGtLtLRGrLr(6)where,Pt=PowertransmittedpowerGt=Effectivetransmitantennagain 52

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Figure6-1. Communicationsystem 53

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Table6-1. Componentsofthecommunicationsystem ComponentDescription GeneratesthesequenceofBits Opticalsource ModulatestheopticalSource AvalanchePhotodetector Filtersoutunwantedfrequencies Demodulatesthereceivedsignal 54

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Table 6-4 .Continued ComponentDescription Analyzesthebiterrorrate MediumofTransmissionoftheOpticalSignal Visualizethesignal Lt=EfciencylossassociatedwiththetransmitterLR=FreespacerangelossGt=ReceiveantennagainLr=Efciencylossassociatedwiththereceiver 6.2.1TransmitterPowerThetransmitsourcepowerisadirectentryintotheopticallinkequation.Itisthemeasuredsignalpowerattheoutputofthelaser.Usually,thetransmitpowerisspeciedinwatts,ifthedesignerisconsistenttheaveragepowercanbeusedandspeciedinmilliwatts.Incasethelaserishermeticallysealed,thepowercomingoutofthehermetic 55

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sealneedstobeconsidered.Thepowerattheoutputofthelaserisgivenby Pr(f)=HT(f).Id(f)(6)whereHTisthetransferfunctionofthelaserandId(f)istheinjectedcurrentinA.HTisgivenby HT(f)=HT(0).HT(f)(6)MultimodeFabryPerotLaserThetransferfunctionofamultimodefabryperotlaserisexpressedas[ 22 ],[ 23 ]: HT=h.c .q.int.ext.Id)]TJ /F7 11.955 Tf 11.95 0 Td[(Ith Id(6)where ext=lnh1 Rli .l+lnh1 Rli(6) int=nr nr+r(6) HT(f)=f2o f2o)]TJ /F5 11.955 Tf 11.95 0 Td[(4.2.f2+j2f(6)where f2o=(Io)]TJ /F7 11.955 Tf 11.95 0 Td[(Ith) spphIth(6) =Io spIth(6)QuantumCascadeLaserTheQuantumCascadelaserandDistributedFeedbackLasersaremodelledusingthefollowingrateequations[ 22 ],[ 23 ]: dN dt=I q.Vact)]TJ /F7 11.955 Tf 11.96 0 Td[(go(N)]TJ /F7 11.955 Tf 11.95 0 Td[(No)(1)]TJ /F3 11.955 Tf 11.95 0 Td[(".S))]TJ /F7 11.955 Tf 13.52 8.09 Td[(N n+Ne n(6) dS dt=go(N)]TJ /F7 11.955 Tf 11.95 0 Td[(No)(1)]TJ /F3 11.955 Tf 11.96 0 Td[(".S)S+N n)]TJ /F7 11.955 Tf 14.24 8.08 Td[(S p(6) 56

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Table6-2. Parametersofequations ParameterDescription IdInjectedcurrentIthThresholdcurrentRlMirrorReectivityLosscoecientlCavitylongitudinaldimension(m)IoPolarizationcurrent(A)spCarrierrecombinationlifetime(s)Dumpingfrequency(Hz)foResonantfrequency(Hz) S Pf=po Vacthc=(6) Table6-3. Parametersofrateequations ParameterDescription NActiveregioncarrierdensityqElectronchargeVactActiveregionvolumegoGainCoefcientNoOpticaltransparencydensity"FenonmenologicalgainsaturationtermSPhotonDensitynCarrierlifetimeNeEquilibriumcarrierdensityOpticalConnementFactorSpontaneousemissioncouplingfactorpPhotonlifetimeoLasingwavelengthDifferentialquantumefciencyperfacethPlank'sconstantcVelocityofLight 6.2.2TransmittergainThegainofthetransmittingantennaisgivenby Gt=4A (6) 57

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where A=a2 4(6)wherea=apertureofthetransmittingantenna. 6.2.3TransmitterLossTransmitterlossisameasuredlossthatiscausedbyaberrationsduetoimperfectioninthemanufacturingprocessandmechanicalstressesontheopticsresultinanon-perfectopticalwavefront.Eachopticalsurfacethattheraytraversescausesanopticallossthatismultiplicative.Thismultiplicativelosscausesadegradationofthetransmitpower.Inouranalysis,thislossisconsideredtobenegligible. 6.2.4FreeSpacelossThefreespacelossisgivenby LR= 4R2(6)whereR=Distanceoverwhichthelinkhastobeestablished. 6.2.5ReceivergainThegainofthereceivingantennaisgivenby Gr=(4A)=(6)where A=(a2)=4(6)wherea=apertureofreceivingantenna. 6.2.6ReceiverLossThelossatthereceiverissimilartothelossthatoccursatthetransmitterbutarisesfromtheopticalelementsatthereceivingterminal.Inouranalysis,thislossisconsideredtobenegligible. 58

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6.2.7OpticalLinkBudgetParametersTheparametersforsimulationoftheopticalsystemaredetailedin 6-4 andthetransmittedlasersignalshowningure 4-3 .Theparametersofimportanceincludethegainofthetransmittingantenna,powerofthetransmitingantenna,laserdiodedrivecurrent,wavelengthofoperation,rangeloss,lossatthetransmitter,datarate,gainofthereceiverandbiterrorrate. Table6-4. Cubesatopticallinkparameters ParameterValue Gt10.36(dB)Pt-20(dBW)LaserDiodeDriveCurrent10-20(mA)1550(nm)LRVariableLt0(dB)DataRate120(Mbps)Gr10.36(dB)BitErrorRate0 Figure6-2. Transmittedlasersignal 59

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6.2.8RadioFrequencyLinkBudgetParametersTheradiofrequencylinkbudgetparametersaretabulatedin 6-5 from[ 24 ]forcomparison.Theseexperimentalparametersarechosenduethefrequencyofoperationbeingnotasfarawayfromtheterahertzrangewhencomparedtotheexistingfrequencyspecicationsof430MHzfortheCubeSat.ThelinkdistancesarecalculatedusingthestandardRFlinkbudgetequations. Table6-5. Cubesatradiofrequencyparameters ParameterValue EIRP-21.9(dBW)Gt10.36(dB)Pt-32.26(dBW)Frequency5.85109(Hz)LRPolarizationLoss-0.3(dB)SystemNoise135(Kelvin)DataRate1200(bps)Gr10.36Eb=No9.6(dB)BitErrorRate10)]TJ /F5 11.955 Tf 7.08 -4.34 Td[(5 6.3Results Figure6-3. IntersatellitecommunicationdistancewithRFlinks 60

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Figure6-4. Qfactoroftheopticallinkvslinkdistance Figure6-5. Receivedpowervslinkdistance Ingure 6-3 thelinkmarginvs.theintersatellitelinkdistanceareplotted.Thegureshowsasthedistancebetweenthesatellitesincreasesthelinkmargintoachieveabiterrorrateof1E-6decreases.Sinceasignaltonoiseratioof3dBhastobemaintainedtoensurethattheBERdoesnotdropbelowthespeciedvalue,themaximumachievabledistanceforthespeciedparametersis50km.Ingure 6-4 theQfactorvs.theintersatellitelinkdistanceandingure 6-5 thereceivedsignalpowervs.intersatellitelinkdistanceisplotted.TheQfactorisameasureofthesignaltonoiseratioofabinaryopticalsignal.ForQfactorvaluesgreaterthan10anddistanceslessthan200km,theBERisalmost0thusmeetingspecications.Thisresultisfurthersubstantiatedbythegure 6-6 whichshowthe 61

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Figure6-6. BERat150km Figure6-7. BERat10Km BitErrorRatePatternatthereceiver.Thelesserroneousthesignalthewidertheeyeopening.Asthesignalbecomeserroneoustheeyeheightreducesandwhenthesignalisdegradedtosuchanextentastobeunrecognizableatthereceiverend,theeyeheightbecomeszero.Butfordistancesgreaterthan200kmwheretheQfactor 62

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islessthan6andhencetheBERcanvaryfrom1E-9to1E-2.Ascanbeobservedfordistancesgreaterthan200km,theheightoftheeyeisreducedindicatingthattheprobabilityofdetectingthesignalcorrectlyhasreduced.Fordistancesgreaterthan250km,theeyeheightbecomezeroandthesignalcannotbedetectedatall.ThescenarioconsideredhereisahighlyoptimisticscenarioastheBERforthesimulatedsystemissetat0.Throughtheresultsobtainedwehavethusquantiedthemaximumachievableintersatelliteopticallinksdistancesforsmallsatellitesfortheparameterslistedin 6-5 63

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CHAPTER7CONCLUSIONSThethesishasexploredthecomponentsofanIntersatelliteLaserLinkSystemforlargesatellitesthatisresponsibleforestablishingcrosslinksasameansofcommunicationandthefeasibilityofasimilarsystemtoachievelongdistancecrosslinksfortheCubeSat.Abriefstudyofthelasercrosslinksystemofthelargesatelliteswasprovided.Then,theparametersandrequirementsofthesubsystemswerediscussedanddeterminedfortheCubeSatframe.AnanalysisofthecontributiontotheweightoftheCubeSatandpowerconsumptionrequirementsareperformedwithrespecttotheCubeSatspecications.ThenalparameterswerethenpluggedintoacommunicationsystemandcorrespondinglinkbudgetanalysisandcomparisionwiththeRFsystemwerecarriedoutthroughsimulations.WesawthattheopticalsystemdidnotmeetthepowerrequirementsoftheCubeSatbutsatisedtheweightandvolumeconstraintsoftheCubeSat.ThemaximumcrosslinkrangefortheCubeSatwasdeterminedtobe200kmwithaBERof1E-9to1E-2atadaterateof1GbpswhencomparedtotheRadioFrequencysystemwhosemaximumintersatellitedistancewasdeterminedtobe50kmatadatarateof12kbps.Forfuturework,investigationofancompactOpticalSystemforsmallsatelliteswhosewetmassesare>3kgandlessthan10kgcanbecarriedout. 64

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APPENDIX:A ABERRATIONSAclearrepresentationofaberrationscanbeexplainedbyinspectingthegures A-1 A-2 A-3 A-4 FigureA-1. Withoutaberration FigureA-2. Aberration ASTIGMATISMDiodelasersarep-njunctiondevices,whereradiationcreatedbyinjectionofcarriersacrossthejunctionisconnedbyatinyopticalwaveguide.Thiswaveguide, 65

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FigureA-3. Aberration FigureA-4. Aberration withitsplano,partiallyreectingendfacetsperpendiculartothejunction,formstheFabry-Perotcavityofthediodelaser.Twogeneraltypesofopticalwaveguidesmaybefoundindiodelasers.Therstisadielectricslab,orindex-guided,waveguide,inwhichtheopticalenergyisconnedthroughtotalinternalreection.Thisisachievedbysurroundingthegainregionwithlayersofmaterialwithalowerrefractiveindex.Inagain-guidedopticalwaveguide,incontrast,theenergyisconnedbysurroundingtheopticalgainregionwitharegionofopticalloss.Itistheguidingmechanismofthe 66

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FigureA-5. Astigmatism waveguidethatdetermineshowmuchastigmatismwillbepresentattheoutputfacetofadiodelaser.Inindex-guideddiodelasers,index-guidingistheprincipalguidingmechanismbothparallelandperpendiculartothejunctionplane.Intheselasers,thephasefrontoftheopticalwaveisplanarinbothdirections,andthewavefrontemittedfromthelaserwillhaveitswaists,bothperpendicularandparalleltothejunction,locatedattheoutputfacetofthewaveguide.Ingain-guideddiodelasers,gain-guidingistheguidingmechanisminthejunctionplaneandindex-guidingistheguidingmechanismperpendiculartothejunctionplane.Inthistypeoflaser.thewavefrontexitingfromthediodelaseriscylindrical.Thebeamwaistperpendiculartothejunctionplaneisstilllocatedattheoutputfacetofthediodelaser,butthewaistparalleltothejunctionplaneisdisplacedadistance,D,behindthefacet.Thisdistanceisthelongitudinalastigmatisminherentinthediodelaserandmustbeconsideredwhendesigningopticalsystemsincorporatingtheselasers.Astigmatismcanberepresentedasshownin A-5 67

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BEAMRADIUSThebeamradiuswisthedistancefromthebeamaxiswheretheopticalintensitydropsto1=e2(13.5%)ofthevalueonthebeamaxis.Atthisradius,theelectriceldstrengthdropsto1=e(37%)ofthemaximumvalue.Forarbitrary(possiblynotGaussian)beamshapes,severaldifferentdenitionsarecommon.Itispossibletostillusethe1=e2intensitycriterion,orafullwidthathalf-maximum(FWHM),oraradiusincluding86%ofthebeamenergy,etc.Theproblemwiththistypeofdenitionsisessentiallythattheresultdoesnotdependon,e.g.,howquicklytheintensitydecaysinthewingsoftheprole.Toillustratethis, 6-3 showstwointensityproleswhichhavethesameFWHMwidth,althoughthedashedcurveisclearlywiderinameaningfulsense.Inthecaseofcomplicatedintensitypatterns,itisevenmoreobviousthatanFWHMdenitioncannotbeappropriate.Forsuchreasons(andanotherreason,whichisdiscussedbelow),therecommendeddenitionisthatofISOStandard11146,basedonthesecondmomentoftheintensitydistributionI(x,y). wx=2q Rx2I(x,y)dxdy RI(x,y)dxdy(A)Forexample,thebeamradiusinthexdirectioniswherethecoordinatesxandymustbetakentoberelativetothebeamcenter,i.e.,suchthattherstmomentsvanish. BEAMDIVERGENCEThebeamdivergenceofalaserbeamisameasureofhowfastthebeamexpandsfarfromthebeamwaist,i.einthesocalledfareld.Alowbeamdivergenceisimportantforapplicationssuchasfreespacecommunicationsand.Beamswithanapproximatelyconstantbeamradiusoversignicantpropagationdistancesarecalledcollimatedbeams. A-7 visuallyrepresentsthebeamdivergencethateffectslasers. NUMERICALAPERTUREThenumericalapertureofanopticalsystemisadimensionlessparameterthatcharacterizestherangeofanglesthatthesystemcanacceptoremitlight.Thefnumber 68

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FigureA-6. Beamradius FigureA-7. Beamdivergence ofalensshowningureisgivenbyN=f=DwherefisthefocallengthofthelensandDisthediameteroftheentrancepupil.Thenumericalaperture(NA)isgivenby: NA=nsin()=nsin(arctan(D2f))(A)assumingn=1forair.Theapproximationholdswhenthenumericalapertureissmall,anditisnearlyexactevenatlargenumericalaperturesforwell-correctedcameralenses.Fornumericalapertureslessthanabout0.5(f-numbersgreaterthanabout1)thedivergencebetweentheapproximationandthefullexpressionislessthan10%.Beyondthis, 69

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theapproximationbreaksdown.Thenumericalapertureforalaserisdenedslightlydifferently.Laserbeamsspreadoutveryslowlyastheypropagate.Farawayfromthenarrowestpart,thespreadisroughlylinearwithdistance-thelaserbeamformsaconeoflightinthefareld.Thenumericalapertureisgivenby N=nsin()(A)whereistheangleofdivergenceinthefareldpattern.ForaGaussianbeamproledescribedpreviously,theNAisrelatedtoitsminimumspotsizeby NA2 D(A)whereisisthewavelengthoflightinvacuumandDisthediameterofthebeaminthenarrowestspot. A-8 showsthenumericalapertureofalens. FigureA-8. Numericalaperture POINTAHEADANGLEOnecriticalaspectofintersatellitelasercommunicationswithnarrowbeamsresultsfromtheneedtointroduceapointaheadangle.Duetothenitevelocityoflightandrelativeangularvelocityoftwocommunicationterminalsmovinginspace,thetransmitbeammustbedirectedtowardsthereceiver'spositionitwillhavesomelatertime.This 70

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FigureA-9. Pointaheadangle pointaheadangleisgivenby: =vr c(A)whereisthepointaheadangleandvristherelativevelocitycomponentorthogonaltothelineofsightofthesatelliteandcisthevelocityoflight.Pointaheadangleisgenerally40microradforGEO-GEOlinkand70microradforLEO-GEOlink.Thepointaheadangleisgenerallygreaterthanthebeamwidth.Itcanbeintroducedeitherinthereceiveortransmitpathandmustbevariedasvarieswithtime.Sinceitisdifculttodesignacontrolloopforautomaticpointaheadadjustment,todaysconceptsrelyoncalculationofpointaheadanglesbasedonknownephemerisdata.Thepointaheadassemblyhenceisusedtodirectthetransmitorreceivebeamaccordingly. A-9 representsthepointaheadangle. RAYLEIGHWAVELENGTHTheRayleighlength(orRayleighrange)ofalaserbeamisthedistancefromthebeamwaist(inthepropagationdirection)wherethebeamradiusisincreasedbyafactorofthesquarerootof2.Foracircularbeam,thismeansthatthemodeareaisdoubledatthispoint.ForGaussianbeams,theRayleighlengthisdeterminedbythewaistradiuswoandthewavelength: zr=w2c=(A) 71

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wherethewavelengthisthevacuumwavelengthdividedbytherefractiveindexnofthematerialandistheRayleighlength. WAVEFRONTERRORTheextentofimagedeteriorationcausedbywavefrontdeformationsisdeterminedbyitsdeviationfromspherical,averagedovertheentirewavefront.Itisthesocalledroot-mean-square(RMS)wavefronterror,usuallyexpressedinunitsofthewavelengthoflight.Itisasquarerootofthedifferencebetweentheaverageofsquaredwavefrontdeviationsminusthesquareofaveragewavefrontdeviation,orRMS=,withthebracketsindicatingaveragevalue.Forinstance,ifwemeasurewavefrontdeviationsatthreepoints(forsimplicity)as0.5,0.2and0.1,theaverageoftheirsquaredvaluesW2=0.1,whilethesquareoftheiraveragevalueW2=0.071.TheRMSerrorwouldbegivenasRMS==0.17.Thisamountstoastandard,orstatisticaldeviationfromaperfectreferencesphereovertheentirewavefront.Tobemeaningful,theRMSwavefronterrorhastobecalculatedforalargenumberofpointsonthewavefront(oropticalsurface,forthesurfaceRMS). 72

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BIOGRAPHICALSKETCH SeshupriyaAllurureceivedhermaster'sdegreefromtheDepartmentofElectricalandComputerEngineeringinthefallof2010.Shehasabachelor'sdegreefromJawaharlalNehruTechnologicalUniversity,Indiainelectronicsandcommunicationsengineering.Asagraduatestudent,sheworkedinthewirelessandmobilesystemslaboratoryandconductedresearchonsmallsatellitesandwirelesssensornetworks.Herotherresearchinterestsincludeapplicationsofgametheoryandoptimizationinwirelesscommunicationsystems. 78