通信及基础工程6

Unit7TextAABriefIntroductiontoSatelliteCommunicationsABriefIntroductiontoSatelliteCommunicationsAcommunicationssatelliteisanorbitingartificialearthsatellitethatreceivesacommunicationssignalfromatransmittinggroundstation,amplifiesandpossiblyprocessesit,thentransmitsitbacktotheearthforreceptionbyoneormorereceivinggroundstations.Communicationsinformationneitheroriginatesnorterminatesatthesatelliteitself.Thesatelliteisanactivetransmissionrelay,similarinfunctiontorelaytowersusedinterrestrialmicrowavecommunications[1].TextTextWordsNotesDiscussionHomeThecommercialsatellitecommunicationsindustryhasitsbeginningsinthemid-1960s,andinlessthan50yearshasprogressedfromanalternativeexotictechnologytoamainstreamtransmissiontechnology,whichispervasiveinallelementsoftheglobaltelecommunicationsinfrastructure.Today’scommunicationssatellitesofferextensivecapabilitiesinapplicationsinvolvingdata,voice,andvideo,withservicesprovidedtofixed,broadcast,mobile,personalcommunications,andprivatenetworksusers.TextTextWordsNotesDiscussionHomeThecommunicationssatelliteisacriticalelementintheoveralltelecommunicationsinfrastructure,asrepresentedbyFigure7-1,whichhighlights,bytheshadedarea,thecommunicationssatellitecomponentasrelatedtothetransmissionofinformation.Electronicinformationintheformofvoice,data,video,imaging,etc.,isgeneratedinauserenvironmentonorneartheearth’ssurface.Theinformation’sfirstnodeisoftenaterrestrialinterface,whichthendirectstheinformationtoasatelliteuplink,whichgeneratesanRF(radiofrequency)[2]radiowavethatpropagatesthroughtheairlinktoanorbitingsatellite(orsatellites).(tobe

continued)TextWordsNotesDiscussionHome(continued)Theinformationbearingradiowaveisamplifiedandpossiblyprocessedatthesatellite,thenreformattedandtransmittedbacktoareceivinggroundstationthroughasecondRFradiowavepropagatingthroughtheairlink.Mobileusers,indicatedbythevehicleandhandheldphoneonthefigure,generallybypasstheterrestrialinterfaceonlyfordirectmobile-to-mobilecommunications.TextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeSomebasiccommunicationssatellitesystemdefinitionsandparametersareintroducedinthisarticle.Figure7-11.SatelliteCommunicationsSegmentsWebeginwiththecommunicationssatelliteportionofthecommunicationsinfrastructure,shownbytheshadedovalinFigure7-1.Thesatellitecommunicationsportionisbrokendownintotwoareasorsegments:thespacesegmentandtheground(orearth)segment.TextTextWordsNotesDiscussionHome(1)SpaceSegmentTheelementsofthespacesegmentofacommunicationssatellitesystemareshownonFigure7-2.Thespacesegmentincludesthesatellite(orsatellites)inorbitinthesystem,andthegroundstationthatprovidestheoperationalcontrolofthesatellite(s)inorbit.ThegroundstationisvariouslyreferredtoastheTracking,Telemetry,Command(TT&C)ortheTracking,Telemetry,CommandandMonitoring(TTC&M)station.TheTTC&Mstationprovidesessentialspacecraftmanagementandcontrolfunctionstokeepthesatelliteoperatingsafelyinorbit.TheTTC&Mlinksbetweenthespacecraftandthegroundareusuallyseparatefromtheusercommunicationslinks.TTC&Mlinksmayoperateinthesamefrequencybandsorinotherbands.TTC&Mismostoftenaccomplishedthroughaseparateearthterminalfacilityspecificallydesignedforthecomplexoperationsrequiredtomaintainaspacecraftinorbit.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHome(2)GroundSegmentThegroundsegmentofthecommunicationssatellitesystemconsistsoftheearthsurfaceareabasedterminalsthatutilizethecommunicationscapabilitiesoftheSpaceSegment.TTC&Mgroundstationsarenotincludedinthegroundsegment.Thegroundsegmentterminalsconsistofthreebasictypes:•fixed(in-place)terminals;•transportableterminals;•mobileterminals.TextTextWordsNotesDiscussionHomeFixedterminalsaredesignedtoaccessthesatellitewhilefixedin-placeontheground.Theymaybeprovidingdifferenttypesofservices,buttheyaredefinedbythefactthattheyarenotmovingwhilecommunicatingwiththesatellite.Examplesoffixedterminalsaresmallterminalsusedinprivatenetworks(VSATs)[3],orterminalsmountedonresidencebuildingsusedtoreceivebroadcastsatellitesignals.TextWordsNotesDiscussionHomeTransportableterminalsaredesignedtobemovable,butonceonlocationremainfixedduringtransmissionstothesatellite.Examplesofthetransportableterminalaresatellitenewsgathering(SGN)trucks,whichmovetolocations,stopinplace,andthendeployanantennatoestablishlinkstothesatellite.TextTextWordsNotesDiscussionHomeMobileterminalsaredesignedtocommunicatewiththesatellitewhileinmotion.Theyarefurtherdefinedaslandmobile,aeronauticalmobile,ormaritimemobile,dependingontheirlocationsonorneartheearthsurface.TextTextWordsNotesDiscussionHome2.SatelliteLinkParametersThecommunicationssatellitelinkisdefinedbyseveralbasicparameters,someusedintraditionalcommunicationssystemdefinitions,othersuniquetothesatelliteenvironment.Figure7-3summarizestheparametersusedintheevaluationofsatellitecommunicationslinks.Twoonewayfree-spaceorairlinksbetweenEarthStationsAandBareshown.Theportionofthelinkfromtheearthstationtothesatelliteiscalledtheuplink,whiletheportionfromthesatellitetothegroundisthedownlink.Notethateitherstationhasanuplinkandadownlink.Theelectronicsinthesatellitethatreceivestheuplinksignal,amplifiesandpossiblyprocessesthesignal,andthenreformatsandtransmitsthesignalbacktotheground,iscalledthetransponder,designatedbythetriangularamplifiersymbolinthefigure(thepointofthetriangleindicatesthedirectionofsignaltransmission).Twotranspondersarerequiredinthesatelliteforeachtwowaylinkbetweenthetwogroundstationsasshown.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeAchannelisdefinedastheone-waytotallinkfromA-to-S-to-B,ORthelinkfromB-to-S-to-A.Theduplex(two-way)linksA-to-S-to-BANDB-to-S-to-Aestablishacircuitbetweenthetwostations.Ahalf-circuitisdefinedasthetwolinksatoneoftheearthstationsthatisA-to-SANDS-to-A;ORB-to-SANDS-to-B.Thecircuitdesignationsareacarry-overfromstandardtelephonydefinitions,whichareappliedtothesatellitesegmentofthecommunicationsinfrastructure.TextTextWordsNotesDiscussionHome3.SatelliteOrbitsWeintroduceherethesatelliteorbittermsforthefourmostcommonlyusedorbitsinsatellitecommunications,showninFigure7-4.Thebasicorbitaltitude(s)andtheone-waydelaytimesareshownforeachorbit,alongwiththecommonabbreviationdesignations.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHome(1)GeosynchronousOrbit(GSOorGEO)TheGSOorbitisbyfarthemostpopularorbitusedforcommunicationssatellites.AGSOsatelliteislocatedinacircularorbitintheequatorialplane,atanominaldistanceof36000kmatastablepoint,whichmaintainsthesatelliteatafixedlocationinthesky.Thisisatremendousadvantageforsatellitecommunications,becausethepointingdirectionremainsfixedinspaceandthegroundantennadoesnotneedtotrackamovingsatellite.AdisadvantageoftheGSOisthelongdelaytimeof∼260ms,whichcanaffectnetworksynchronizationorimpactvoicecommunications.TextTextWordsNotesDiscussionHome(2)LowEarthOrbit(LEO)Thesecondmostcommonorbitisthelowearthorbit(LEO),whichisacircularorbitnominally160to640kmabovetheearth.Thedelayislow,∼10ms,howeverthesatellitemovesacrossthesky,andthegroundstationmustactivelytrackthesatellitetomaintaincommunications.TextTextWordsNotesDiscussionHome(3)MediumEarthOrbit(MEO)TheMEOissimilartotheLEO,howeverthesatelliteisinahighercircularorbit–1600to4200km.ItisapopularorbitfornavigationsatellitessuchastheGPSconstellation.TextTextWordsNotesDiscussionHome(4)HighEarthOrbit(HEO)TheHEOistheonlynon-circularorbitofthefourtypes.Itoperateswithanellipticalorbit,withamaximumaltitude(apogee)similartotheGSO,andaminimumaltitude(perigee)similartotheLEO.TextTextWordsNotesDiscussionHomeSatelliteorbitsthatarenotsynchronous,suchastheLEO,MEO,orHEO,areoftenreferredtoasnon-geosynchronousorbit(NGSO)satellites.TextTextWordsNotesDiscussionHome4.FrequencyBandDesignationsThefrequencyofoperationisperhapsthemajordeterminingfactorinthedesignandperformanceofasatellitecommunicationslink.Thewavelengthofthefreespacepathsignalistheprincipalparameterthatdeterminestheinteractioneffectsoftheatmosphere,andtheresultinglinkpathdegradations.Also,thesatellitesystemsdesignermustoperatewithintheconstraintsofinternationalanddomesticregulationsrelatedtochoiceofoperatingfreespacepathfrequency.TextTextWordsNotesDiscussionHomeTwodifferentmethodsofdesignationhavecomeintocommonusetodefineradiofrequencybands.Letterbanddesignations,derivedfromradarapplicationsinthe1940s,dividethespectrumfrom1to300GHzintoeightbandswithnominalfrequencyranges,asshownonFigure7-5.TheK-bandisfurtherbrokendownintoKU-band(K-lower)andKA-band(K-above).TextTextWordsNotesDiscussionHomeTheboundariesofthebandsarenotalwaysfollowed,andoftensomeoverlapisobserved.Forexample,somereferencesconsiderC-bandas3.7–6.5GHzandKu-bandas10.9–12.5GHz.Thebandsabove40GHzhaveseenseveralletterdesignationsused,includingQ-band,W-band,U-band,andW-band.Theambiguityinletterbanddesignationssuggeststhattheyshouldbeusedwithcaution–particularlywhenthespecificfrequencyisanimportantconsideration.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeAseconddesignationdividesthespectrumfrom3Hzto300GHzintobandsbasedondecadestepsofnominalwavelength,asshowninFigure7-6.Thisdesignationislessambiguousthantheletterdesignation,mostsatellitecommunicationslinksoperatewithinonlythreeorfourofthebands,VHFthroughEHF,withthevastmajorityofsystemsintheSHFband.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeIngeneral,thefrequencybanddesignationsareusefulwhengeneralcharacteristicsofsatellitecommunicationssystemsareofinterest.Whenthespecificoperatingcarrierfrequencyorspecificfrequencybandareimportant,however,thebestsolutionistospecifythefrequencydirectly,ratherthanusingfrequencybanddesignations.TextTextWordsNotesDiscussionHomemicrowave[ˈmaɪkrəweɪv]n.微波；微波炉v.用微波炉烹调（或加热）exotic[ɪɡˈzɒtɪk]adj.奇异的，异国风情的pervasive[pəˈveɪsɪv]adj.弥漫的，遍布的reformat[ˌri:ˈfɔ:mæt]vt.重定格式telemetry[təˈlemətri]n.[自]遥测技术；自动测量记录传导antenna[ænˈtenə]n.触须，触角；<美>天线；感觉，直觉WordsWordsTextWordsNotesDiscussionHomeaeronautical[ˌeərəˈnɔ:tɪkl]adj.航空的；航空学的；飞机驾驶员的（等于aeronautic）transponder[trænˈspɒndə(r)]n.异频雷达收发机，（发射机）应答器，转发器equatorial[ˌekwəˈtɔ:riəl]adj.赤道的，近赤道的；中纬线的n.赤道仪WordsTextWordsNotesDiscussionHome[1]MicrowaveCommunication指微波通信，是使用波长在0.1毫米至1米之间的电磁波——微波进行的通信。该波长段电磁波所对应的频率范围是300MHz至3000GHz。与同轴电缆通信、光纤通信和卫星通信等现代通信网传输方式不同的是，微波通信是直接使用微波作为介质进行的通信，不需要固体介质，当两点间直线距离内无障碍时就可以使用微波传送。利用微波进行通信具有容量大、质量好并可传至很远的距离的特点，因此是国家通信网的一种重要通信手段，也普遍适用于各种专用通信网。TextWordsNotesDiscussionNotesHome[2]RF（射频）是RadioFrequency的缩写，表示可以辐射到空间的电磁频率，频率范围从300kHz～300GHz之间。射频就是射频电流，简称RF，它是一种高频交流变化电磁波的简称。每秒变化小于1000次的交流电称为低频电流，大于10000次的称为高频电流，而射频就是这样一种高频电流。射频（300K-300G）是高频（大于10K）的较高频段，微波频段（300M-300G）又是射频的较高频段。TextWordsNotesDiscussionHome[3]VSAT是VerySmallApertureTerminal的缩字，直译为甚小口径卫星终端站。所以也称为卫星小数据站（小站）或个人地球站（PES），这里的“小”字指的是VSAT卫星通信系统中小站设备的天线口径小，通常为0.3-1.4M。VSAT系统具有灵活性强，可靠性高，成本低，使用方便以及小站可直接装在用户端等特点。VSAT系统由一个主站及众多分散设置在各个用户所在地的远端VSAT组成，可不借助任何地面线路，不受地形、距离和地面通信条件限制，主站和VSAT间可直接进行高达2Mbps的数据通信。特别适用于有较大信息量和所辖边远分支机构较多的部门使用。VSAT系统可提供电话、传真、计算机信息等多种通信业务。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.Whatisacommunicationssatellite?2.Describethethreebasictypesofthegroundsegmentterminals.3.Whatarethecommonlyusedsatelliteorbittermsinsatellitecommunications?4.Whichisbyfarthemostpopularorbitusedforcommunicationssatellites?Whataboutitsadvantageanddisadvantage?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.Whatisacommunicationssatellite?Acommunicationssatelliteisanorbitingartificialearthsatellitethatreceivesacommunicationssignalfromatransmittinggroundstation,amplifiesandpossiblyprocessesit,thentransmitsitbacktotheearthforreceptionbyoneormorereceivinggroundstations.Communicationsinformationneitheroriginatesnorterminatesatthesatelliteitself.Thesatelliteisanactivetransmissionrelay,similarinfunctiontorelaytowersusedinterrestrialmicrowavecommunications.TextWordsNotesDiscussionHome2.Describethethreebasictypesofthegroundsegmentterminals.Thegroundsegmentterminalsconsistofthreebasictypes:•fixed(in-place)terminals;•transportableterminals;•mobileterminals.Fixedterminalsaredesignedtoaccessthesatellitewhilefixedin-placeontheground.Theymaybeprovidingdifferenttypesofservices,buttheyaredefinedbythefactthattheyarenotmovingwhilecommunicatingwiththesatellite.Examplesoffixedterminalsaresmallterminalsusedinprivatenetworks(VSATs),orterminalsmountedonresidencebuildingsused