空间大地测量学

空间大地测量学主讲：李征航教授助教：刘万科博士武汉大学测绘学院卫星应用工程研究所2008年09月空间大地测量学1.原子钟（AtomicClock）2.甚长基线干涉测量（VLBI）3.激光测卫(SLR）4.卫星测高(SatelliteAltimetry)5.多普勒技术(DopplerTechnique）6.卫星跟踪卫星(SST)内容提要Part1.AtomicClock•TheNationalPhysicsLaboratoryinEnglanddevelopedthefirstaccuratecaesiumatomicclockin1955•In1967theInternationalBureauofWeightsandMeasures(BIPM)adoptedtheatomicdefinitionforanSIsecond•DefinitionofAtomicSecond:地面状态的铯133原子对应于两个超精细能级跃迁9192631770个辐射周期的持续时间。•科学家当前正在研制更高精度的原子钟：1secondin10billionyearsAtomicFountains(原子喷泉钟)•15fountainsinoperationatSYRTE,PTB,NIST,USNO,PennSt,INRIM,NPL,ON,JPL.6withaccuracyat1×10-15.Morethan10underconstruction……ABriefHistoryofAtomicClocksatNIST1945--IsidorRabi,aphysicsprofessoratColumbiaUniversity,suggestsaclockcouldbemadefromatechniquehedevelopedinthe1930'scalledatomicbeammagneticresonance.1949--UsingRabi’stechnique,NIST(NationalInstituteofStandardsandTechnology)announcestheworld’sfirstatomicclockusingtheammoniamoleculeasthesourceofvibrations.1952--NISTcompletesthefirstaccuratemeasurementofthefrequencyofthecesiumclockresonance.TheapparatusforthismeasurementisnamedNBS-1.NBS-11954--NBS-1ismovedtoNIST’snewlaboratoriesinBoulder,Colorado.1955--TheNationalPhysicalLaboratoryinEnglandbuildsthefirstcesium-beam(铯原子束)clockusedasacalibrationsource.1958--Commercialcesiumclocksbecomeavailable,costing$20,000each.1959--NBS-1goesintoregularserviceasNIST'sprimaryfrequencystandard.1960--NBS-2isinauguratedinBoulder;itcanrunforlongperiodsunattendedandisusedtocalibratesecondarystandards.NBS-21963--ThesearchforaclockwithimprovedaccuracyandstabilityresultsinNBS-3.NBS-31967--The13thGeneralConferenceonWeightsandMeasuresdefinesthesecondonthebasisofvibrationsofthecesiumatom;theworld’stimekeepingsystemnolongerhasanastronomicalbasis.1968--NBS-4,theworld’smoststablecesiumclock,iscompleted.Thisclockwasusedintothe1990saspartoftheNISTtimesystem.1972--NBS-5,anadvancedcesiumbeamdevice,iscompletedandservesastheprimarystandardNBS-51975--NBS-6beginsoperation;anoutgrowthofNBS-5,itisoneoftheworld’smostaccurateatomicclocks,neithergainingnorlosingonesecondin300,000years.1993--NIST-7comesonline;eventually,itachievesanuncertaintyof5x10-15,or20timesmoreaccuratethanNBS-6.1999----NIST-F1beginsoperationwithanuncertaintyof1.7x10-15,oraccuracytoaboutonesecondin20millionyears,makingitoneofthemostaccurateclocksevermade(adistinctionsharedwithsimilarstandardsinFranceandGermany).喷泉原子钟内部构造图VideoDemonstrationofHowaCesiumFountainWorks（喷泉钟的动画演示，请用鼠标点击上述画面）NIST-F1CesiumFountainAtomicClockThePrimaryTimeandFrequencyStandardfortheUnitedStatesTheuncertaintyofNIST-F1iscontinuallyimproving.In2000theuncertaintywasabout1x10-15,butasofthesummerof2005,theuncertaintyhasbeenreducedtoabout5x10-16,whichmeansitwouldneithergainnorloseasecondinmorethan60millionyears!ItisnowapproximatelytentimesmoreaccuratethanNIST-7,acesiumbeamatomicclockthatservedastheUnitedState'sprimarytimeandfrequencystandardfrom1993-1999.GalileoatomicclocksRubidiumclockHydrogenmaserclockGalileosatellites:rubidiumatomicfrequencystandardsandpassivehydrogenmasers.Thestabilityoftherubidiumclockissogoodthatitwouldloseonlythreesecondsinonemillionyears,whilethepassivehydrogenmaserisevenmorestableanditwouldloseonlyonesecondinthreemillionyears.OfficeofNavalResearch---'matchbox'atomicclock•onesecondevery10,000years•Ultra-miniatureRubidium(Rb)AtomicClock,40cm3NISTChip-ScaleAtomicClock•OnAug.30,2004•aboutthesizeofagrainofrice(1.5millimetersonasideand4millimetershigh),consumelessthan75thousandthsofawatt(enablingtheclocktobeoperatedonbatteries)andarestabletoonepartin10-10,equivalenttogainingorlosingjustonesecondevery300years.•thephysicspackagewillbeintegratedwithanexternaloscillatorandcontrolcircuitryintoafinishedclockabout1cm3insize.Part2.VLBI-VeryLongBaselineInterferometry河外射电源（河外类星体）射电望远镜射电望远镜射电望远镜是一种能接收和处理来自太空的无线电信号的装置，由巨大的抛物面天线，高精度的原子钟，数据接收和处理设备等组成。灵敏度是指射电望远镜“最低可测”的能量值，此值越低灵敏度越高。为提高灵敏度常用的办法有降低接收机本身的固有噪声、增大天线接收面积、延长观测积分时间等。1)射电干涉测量分辨率指区分两个彼此靠近射电源的能力，分辨率越高就能将越近的两个射电源分开。利用射电望远镜进行观测时其角分辨率可用下列公式来估算：式中为角分辨率，为射电望远镜所接收的无线电信号的波长，通常为13cm和3.6cm,为射电望远镜接收天线的口径。DD(2-1)那么，怎样提高射电望远镜的分辨率呢？对单天线射电望远镜来说，天线的直径越大分辨率越高。但是天线的直径难于作得很大，目前单天线的最大直径小于300米，对于波长较长的射电波段分辨率仍然很低，因此就提出了使用两架射电望远镜构成的射电干涉仪。对射电干涉仪来说，两个天线的最大间距越大分辨率越高。另外，在天线的直径或者两天线的间距一定时，接收的无线电波长越短分辨率越高。阿雷西博(Arecibo)天文台，波多黎各（西印度群岛），USA直径：305m、51米深、1974年建成占地大约20英亩，40000块铝制面板组成，900吨的接收平台射线频率:50MHz(6m)~10,000MHz(3cm).TheAreciboObservatoryispartoftheNationalAstronomyandIonosphereCenter(NAIC),anationalresearchcenteroperatedbyCornellUniversityunderacooperativeagreementwiththeNationalScienceFoundation(NSF).AreciboObservatory,NationalAstronomyandIonosphereCenter世界上最大的钢结构的射电望远镜，直径100米，实际尺寸100×110mGreenBankTelescope,NationalRadioAstronomyObservatory,WestVirginia,USANationalRadioAstronomyObservator年2月，日本空间科学研究所成功地发射了一颗VLBI空间观测研究卫星（VSOP），它可从东京分辨出悉尼的1颗米粒大小的东西,能在揭开黑洞结构等发挥重要的作用。2)联线干涉测量为较大幅度的提高角分辨率，有人提出了联线干涉测量的方法（见右图）。通过此方法我们就组成了一台虚拟的口径为D的大射电望远镜。此时D即为两台射电望远镜的距离。VLA：VeryLargeArraySocor