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with a very long history [1, 2, 3] 1 For this reason, it is difficult to understand the current operations of time and frequency measurements without some
Chapter 17: Fundamentals of Time and Frequency
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And by counting events or cycles per second, we can measure frequency Time interval and frequency can now be measured with less uncertainty and more
Very Long-term Frequency Stability: Estimation using a Special
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evaluate frequency stability at longer averaging times than given by the definition of Avar, and (2) it has the highest number of equivalent degrees of
Time-Frequency Analysis of Time-Varying Signals and Non
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The ambiguity or doppler-lag function is the Fourier transform in both variables of the Wigner distribution The ambiguity function has some nice properties,
The history of time and frequency from antiquity to the present day
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as measured by clocks, and frequency, as realized by some device, were difficult to exploit the ability of the clock to measure long time intervals
First detection of frequency-dependent, time-variable dispersion
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tens of AUs) but steep density variations in the interstellar electron content We find that long-term trends in DM variability equally affect DMs measured at
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13 mar 1998 Other time intervals will be analyzed with a longer window, to gain better frequency resolution The method is implemented using a set of
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DATEDUE
WOTI.921teILL
- , - ____
GAYLORD'HINTEDINU.S.A.
NationalBureauofStandard
TIMEANDFREQUENCY:
TheoryandFundamentals
ByronE.Blair,Editor
TimeandFrequencyDivision
InstituteforBasicStandards
\J ,$,NationalBureauofStandards
Boulder,Colorado-80302
JUL101974
U.S.DEPARTMENTOFCOMMERCE,FrederickB.Dent,Secretary NATIONALBUREAUOFSTANDARDS,RichardW.Roberts,Director
IssuedMay1974
LibraryofCongressCatalogNumber:73-600299
NationalBureauofStandardsMonograph140
Nat.Bur.Stand.(U.S.),Monogr.140,470pages(May1974)
CODEN:NBSMA6
^6-19^ ForsalebytheSuperintendentofDocuments.U.S.GovernmentPrintingOffice.Washington,D.C.20402 (OrderbySDCatalogNo.C13.44:140).Price$8.65
StockNumber0303-01202
ABSTRACT
ThisisatutorialMonographdescribingvariousaspectsoftimeandfrequency(T/F).Included arechaptersrelatingtoelementalconceptsofprecisetimeandfrequency;basicprinciplesofquartz oscillatorsandatomicfrequencystandards;historicalreview,recentprogress,andcurrentstatus ofatomicfrequencystandards;promisingareasfordevelopingfutureprimaryfrequencystandards; relevanceoffrequencystandardstootherareasofmetrologyincludingaunifiedstandardconcept; statisticsofT/FdataanalysiscoupledwiththetheoryandconstructionoftheNBSatomictime scale;anoverviewofT/Fdisseminationtechniques;andthestandardsofT/FintheUSA.The Monographaddressesboththespecialistinthefieldaswellasthosedesiringbasicinformation abouttimeandfrequency-TheauthorstracethedevelopmentandscopeofT/Ftechnology,its improvementoverperiodsofdecades,itsstatustoday,anditspossibleuse,applications,and developmentindaystocome. Keywords:Accuracy;Allanvariance;atomicfrequencystandards;atomictimescales;AT(NBS); BIH;buffergases;CCIR;clockensembles;clocks;crystalaging;Csfrequencystandard;dissemi- nationtechniques;figureofmerit;flickernoise;frequencydomain;frequencystability;frequency standards;frequency/timemetrology;hydrogenmaser;leapseconds;Loran-C;magneticreso- nance;masers;NBS-III;NBS-5;NBS/USNOtimecoordination;Omega;opticalpumping;pre- cision;quartzcrystaloscillators;radioT/Fdissemination;Rbfrequencystandards;satelliteT/F dissemination;short-termstability;SIUnits;TAI;televisionT/Fdissemination;thalliumbeam standards;time;timedispersion;timedomain;time/frequencystatistics;timescalealgorithm;time scales;"unifiedstandard";URSI;USAstandardtimezones;UTC(NBS);UTC(USNO).
PreparationofthisdocumentwassupportedinpartbytheDirectorateofCommunicationsEngineering,COMSEC/DataSystemsDivision,AirForceCommunications
Service.
in
FOREWORD
ItisindeedfittingthatatimeandfrequencyMonographshouldappearnow.Theyear1973 marksthe50thAnniversaryofthetransmissionofstandardradiofrequenciesonaregularly announcedschedulefromWWVatWashington,DC;the25thAnniversaryoftimeandfrequency broadcastsfromWWVHinHawaii;andthe25thAnniversaryofthefirstlaboratoryoperationofan atomicclock(attheNationalBureauofStandards,Washington,DC).Thatthefieldoftimeand frequencyisslowlyemergingasaspecialtytechnologyisnotsurprisingintoday'sworldoftime- referencedmeasurementsandsystemssuchasthoserequiredfornavigationandcommunication. Itisparadoxicalthattime,relatedcenturiesagosolelytothedailyapparentmovementofthesun, waterclocks,mechanicalinstruments,andthelike,shouldbeatthecenterofthiscomplexfield. Nevertheless,itisbecomingapreciseandimportantscientificdiscipline,incombinationwith elementsofphysics,chemistry,quantummechanics,electronics,radiopropagation,andstatistics toformatomicfrequencygenerators,timescales,andvariedtechniquesofdissemination. Theurgentdemandsofscientificendeavors,requiringprecisionmeasurementsinthe1950's, ledtheNationalBureauofStandards(NBS)topublishHandbook77 - athreevolumepublication ofPrecisionMeasurementandCalibration;oneofthesevolumes - ElectricityandElectronics containedasmalltimeandfrequencysection.Withtheadvanceinthe1960'sofscientificmeth- odology,standardsofmeasures,andcomplexexperimentaltechniques,NBSpublishedanew seriesofPrecisionMeasurementandCalibrationinsomeelevenvolumes - VolumeVofthis SpecialPublication300wasdevotedentirelytopapersonFrequencyandTimepublishedinthe
1960's.ThepresentMonographresultsfromrecommendationsofrecentTimeandFrequency
DivisionAdvisoryPanelstopublishcurrentinformationthatwouldbe"basic,understandable,and practical."ThePanelswereconcernedthatthegainsandutilityoftimeandfrequencytechniques bemadereadilyavailabletobothnewandexperiencedworkersinthefieldinatutorialandcompre- hensiblemanner.ThathasbeentheobjectiveoftheauthorsofthisMonograph. TheInstituteforBasicStandards(IBS),oneoffourInstitutesoftheNBS,isgiventhefollowing responsibility - it"shallprovidethecentralbasiswithintheUnitedStatesofacompleteandcon- sistentsystemofphysicalmeasurement;coordinatethatsystemwithmeasurementsystemsofother nations;andfurnishessentialservicesleadingtoaccurateanduniformphysicalmeasurements throughouttheNation'sscientificcommunity,industry,andcommerce."1Assuch,IBSisatthe centeroftheNationalMeasurementSystemwithaprimarygoalofcompatibilityamongstandards; i.e.,everyuserinameasurementsystemexpectstoobtainthesamevalueofameasurementbased onthesamereference,butindependentofthemeans.ApersondesiringtimemaylistentoWWVby radio,dialthetelephonetimenumber,orrecordasatellitetimesignal.Thedegreeofaccuracymay varybuttheinformationisrelatedtoacommonsourceandiscompatible. Ishouldemphasizethatthemajoreffortofthisworkistodescribebasicprinciplesandprac- tices;itisoflittleusetomaintainandupdatemeasurementstandardswithoutpublicizingtheir construction,theoryofoperation,capabilities,andmannerofuse.Wesincerelyhopethatthis Monographwillbecomeavaluablesourcebookofinformationtothepracticing"timeandfre- quency"specialistaswellastothosenewinthefieldseekinganunderstandingofcompatibility amongNationalandInternationaltimeandfrequencystandardsandtimescales,telecommunica- tion/navigationsystems,andvarieddisseminationtechniques.Itisequallyimportantthatthe materialmightstimulatenewideasandapplications,whichofthemselveswouldbroadenthedepth andscopeoftimeandfrequencytechnology.
E.Ambler,Director
InstituteforBasicStandards
NationalBureauofStandards
May8,1973
1U.S.DepartmentofCommerce,DepartmentalOrganizationalOrder30-2B,NBSMissionStatements,June12,1972(seeann.ll.A-chap.11).
IV
PREFACE
"NationalBureauofStandardsTime...ThisisradiostationWWV,FortCollins,Colorado broadcastingoninternationallyallocatedstandardcarrierfrequenciesof2.5,5,10,15,20and25 MegaHertz,providingtimeofday,standardtimeinterval,andotherrelatedinformation..." Behindsuchperiodicradioannouncementslieasubstantiveandstate-of-the-arttechnology whichhasadvancedtremendouslythelastseveraldecades.ItistheobjectiveofthisMonograph togivecomprehensivepicturesofvariousaspectsoftimeandfrequencystandardsintermsofthe past,presentandfuture.Assuch,wetrustitwillserveasatutorialreferencebookbyprovidinga historicalbackground,thepresentcapabilities,andthefuturepotentialofthepreciseandaccurate time-keepingtechnology. Thesubjectmatterispresentedin11majorchaptersofreprinted,updated,andnewmaterial authoredbystaffmembersoftheNBSTimeandFrequencyDivisionandseveralotheragencies, suchastheU.S.NavalObservatoryandtheU.S.ArmyElectronicsCommand.Chapter1describes basicconceptsoftime,frequency,andtimescalesincludingtheInternationalAtomicTime(TAI) scale.Followingbasictimeaspectsarechaptersdescribingcrystaloscillators,fundamentalprin- ciplesofatomicfrequencystandards,andboththehistoricaldevelopmentandrecentprogressin realizingvarioustypesofatomicfrequencystandards.Chapter5detailsimprovementsincesium beamstandardsattheNationalBureauofStandards(NBS)includingabriefdescriptionofthenew NBS-5;thisfrequencystandardshowsatentativepotentialaccuracyof2partsin1013whichis equivalenttoalossofasecondin~160,000years.(Needlesstosay,aclockwillnotrunsucha lengthoftime.However,thecurrentstateofthearttodayrequires6fxsinayearwhichisthesame relativeaccuracy.) Nextaretwochapterswhichprojectone'sthinkingtofuturepossibilities;oneofthesede- scribesareasofpromisefordevelopingtomorrow'sprimaryfrequencystandardswhilethesecond relatesfrequencystandardstoareasofmetrologywhichmightincludeaunifiedstandardfor frequency,time,andlength.Chapter8presentsbasicprinciplesofstatisticsusefulforanalyzing timeandfrequencydataandfordescribingthequalityofsuchmeasurements.Thisisfollowedby achapterwhichdepictsindetailboththetheoryandformationoftheNBSatomictimescaleAT (NBS).AT(NBS)isoneofseveninputstotheTAIscalemaintainedbytheBureauInternational d'Heure(BIH)inParis,France. Ifalaboratorymaintainsastate-of-the-artfrequencystandardandtimescale,suchstandards areofsmallvalueunlesstheycanbemadeavailablereadilytodistantusers.Chapter10describes variousdisseminationtechniquesforbridgingthegapbetweenafrequencystandardandvaried classesoftimeandfrequencyusers.TheMonographconcludeswithChapter11,ajointpaperof theNBSandUSNO,whichdepictsthestandardsoftimeandfrequencyintheUSA;thechapter detailsandcomparesthetimekeepingmissionsandresponsibilitiesofbothorganizations.By mutualagreement,timeatbothnationallaboratoriesismaintainedwithinabout±5/asofeach other.Chapter11includesabriefdescriptionofinternationalagenciesinvolvedwithtimekeeping responsibilitiesaswellastheUniformTimeActof1966oftheUSAgivenintheU.S.Code. Therehasbeenanearnestattempttothoroughlydocumenteachchapterwithacomplete referencelisting.Inmanyinstancesaselectedbibliographyalsohasbeenincludedtoaidareader seekingadditionalinformationinthegivensubjectareas.Wehaveattemptedtobeconsistentand completeinlistingthereferences;journaltitleabbreviationsarethosegiveninthe1961Chemical Abstracts - ListsofPublicationsorthe1966RevisedandEnlargedWordAbbreviationListfor USAZIZ39.5-1963 - AmericanStandardforPeriodicalTitleAbbreviations.Manyofthebiblio- graphiclistingscanbeseenatpublicoruniversitylibraries.NBSTechnicalNotesandpublications withaUSGPOnotationmaybepurchasedfrom: V
SuperintendentofDocuments
U.S.GovernmentPrintingOffice
Washington,DC20402.
ReferencesshowinganADorNaccessionnumberandaNTISnotationareavailableatreproduc- tioncostsfromtheNationalTechnicalInformationServiceasfollows:
NationalTechnicalInformationService
U.S.DepartmentofCommerce
5285PortRoyalRoad
Springfield,VA22150.
Limitedreprintsofresearcharticlesandreportsareavailableusuallyfromindividualauthors. Wehavetriedtomaintainconsistencyintheuseofsymbolsortheirequivalentsinallchapters; definitionsaregivenintheGlossaryofSymbols - AnnexAofChapter8.Toaidreadersunfamiliar withlettersymbolsinthetimeandfrequencyfield,wehaveaddedanabbreviationfistingwhich followsthesubjectindex. Arapidlyincreasingnumberoftechnologiesanddisciplinesareemployingtimeandfrequency techniques.Includedareareassuchasnationaldefense,telecommunications,publicsafety, electricutilities,navigation,highspeeddata-computingnetworks,etc.Therearealsoindications thatalargenumberofsystemsindependentlygenerateanddisseminatetheirowntimeandfre- quencyinformation,largelythroughuseoftheradiospectrum.Wewouldpointoutthattheradio spectrummightbeconservedtoalargeextentifbothcommunicationandnavigationsystemswere designedtoincludesynchronizationpulseformatsconvenientforT/Fdisseminationwithoutcom- promisingprimarysystemconcerns.Bythesametoken,asystemdesignerbettercanaccommodate afrequencydisseminationfunctionthroughreferencingsystemfrequenciestorecognizednational frequencystandardsandchoosingconvenientsubsystemfrequencieswherefeasible. Theeditorwishestogratefullyacknowledgethecooperationandpatienceofthemanyauthors whohavecontributedtothisMonograph.ConsiderablehelpandsuggestionsweregivenbyDr. YardleyBeers,Mr.RogerEaston,Mr.DonaldHammond,Dr.RichardKlepsynski,Dr.Allan Mungall,Dr.DavidWait,Dr.BernardWieder,andmanystaffmembersoftheTimeandFrequency Division.CreditalsomustbegivenbothtotheNBSVisualInformationGroupforthemanydis- tinctiveillustrationsinthisbookandtotheNOAALibrarypersonnelforhelpinlocatingand verifyingmanyreferencecitations.LastbutnotleastspecialthanksaredueMrs.SharonErickson forherdiligenceandeffortinprocessingthefinalcopyofthispublication. TheauthorstrustthattheMonographwillprovehelpfulintheunderstandingoftheintriguing andeverchangingfieldoftimeandfrequency;theywouldwelcomesuggestions,commentsand/or questionsaboutthesubjectmaterial. Timemoveson;theWWVbroadcastproclaims"atthetonehours,minutes
GreenwichMeanTime."
ByronE.Blair,Editor
NationalBureauofStandards
Boulder,Colorado80302
April23.1973
VI
Contents
Page
Abstract'Ill
ForewordIV
ErnestAmbler,Director,InstituteforBasicStandards,NBS,Washington,DC20234
PrefaceV
ByronE.Blair,TimeandFrequencyDivision,NBS,Boulder,CO80302CHAPTER1.BASICCONCEPTSOFPRECISETIMEANDFREQUENCY1
JamesA.Barnes,TimeandFrequencyDivision,NBS,Boulder,CO80302
Introduction3
Clocksandtimekeeping3
Basicconceptsoftime4
Timescales4
InternationalAtomicTime(TAI)Scale10
Theconceptsoffrequencyandtimeinterval10
Usesoftimescales11
Conclusions13
References13
Bibliography14
AnnexA - DefinitionsofthesecondandTAI15
AnnexB - Standardfrequencyandtimebroadcastagreements29 AnnexC - Resultsof6thSessionofConsultativeCommitteeforDefinitionoftheSecond (CCDS)37CHAPTER2.PARTA-STATEOFTHEART-QUARTZCRYSTALUNITSANDOSCILLATORS41 EduardA.Gerber,ElectronicsComponentsLaboratory,U.S.ArmyElectronicsCommand,Ft. Monmouth,NJ07703,andRogerA.Sykes,BellTelephoneLaboratories,Inc.,Allentown,PA 18103
Introduction43
Quartzcrystalunits44
Frequencystabilityasafunctionof
Temperature48
Time(aging)48
Stress,vibrationandacceleration50
Drivelevel50
Nucleareffects51
Quartzcrystalcontrolledoscillators52
Conclusions55
PARTB-PROGRESSINTHEDEVELOPMENTOFQUARTZCRYSTALUNITSANDOSCIL-LATORSSINCE196657 EduardA.Gerber,ElectronicsTechnologyandDevicesLaboratory,U.S.ArmyElectronics Command,Ft.Monmouth,NJ07703,andRogerA.Sykes,formerlywithBellTelephoneLabora- tories,Inc.,Allentown,PA18103
Introduction59
Quartzcrystalunits59
Frequencystabilityasafunctionof
Temperature60
Time(aging)61
Stress,vibrationandacceleration61
Drivelevel61
Quartzcrystalcontrolledoscillators62
Conclusionsandpossiblefuturedevelopments62
References63CHAPTER3.THEPHYSICALBASISOFATOMICFREQUENCYSTANDARDS65 AllanS.Risley,TimeandFrequencyDivision.NBS.Boulder.CO80302
Introduction67
Atomicenergylevels67
Theinteractionbetweenatomsandelectromagneticradiation71
Choosingasuitableatom73
Thethreemajorexamplesofatomicfrequencystandards78
Summary83
References84
Bibliography84CHAPTER4.PARTA-AHISTORICALREVIEWOFATOMICFREQUENCYSTANDARDS85 RogerE.Beehler,FrequencyandTimeDivision,Hewlett-PackardCompany,PaloAlto,CA94304 (nowwithTimeandFrequencyDivision,NBS,Boulder,CO80302.)
Introduction87
Developmentofbasictechniques87
Applicationsofbasictechniquestothedevelopmentofspecifictypesofatomicfrequency standards93
Conclusions99
References99
VII PagePARTB-RECENTPROGRESSONATOMICFREQUENCYSTANDARDS101 RogerE.Beehler,TimeandFrequencyDivision,NBS.Boulder.CO80302
Introduction103
Laboratorycesiumstandards103
Hydrogenstandards105
Commercialcesiumstandards107
Commercialrubidiumstandards108
References108CHAPTER5.PARTA-IMPROVEMENTSINATOMICCESIUMBEAMFREQUENCYSTANDARDSATTHENATIONALBUREAUOFSTANDARDSIll
DavidJ.Glaze,TimeandFrequencyDivision,NBS.Boulder,CO80302
Introduction113
ModificationstotheNBS-IIISystem113
AccuracycapabilityofNBS-III(1969)115
Someimmediategoals116
Conclusions116
References117PARTB-THENEWPRIMARY'CESIUMBEAMFREOUENCYSTANDARD:NBS-5.... '. '. '.119 DavidJ.Glaze,HelmutHellwig,StephenJarvis,Jr.,ArthurE.Wainwright,andDavidW.Allan.
TimeandFrequencyDivision.NBS,Boulder,CO80302
Introduction121TheNBS-5system121
Preliminaryexperimentalresults(statusApril1973)122 References124CHAPTER6.AREASOFPROMISEFORTHEDEVELOPMENTOFFUTUREPRIMARYFREQUENCYSTANDARDS125 HelmutHellwig,TimeandFrequencyDivision,NBS,Boulder,CO80302
Introduction127
Effectsonfrequency:particleinterrogation127
Effectsonfrequency:particleconfinement129
Effectsonfrequency:particlesandparticlepreparation130 Existingconceptsforquantumelectronicfrequencystandards131
Frequencystabilityforonesecondaveraging133
Accuracycapability133
Conclusions134
References135CHAPTER7.ACCURATEFREQUENCYMEASUREMENTS:RELEVANCETOSOMEOTHERAREASOFMETROLOGY137 HelmutHellwigandDonaldHalford,TimeandFrequencyDivision,NBS,Boulder,CO80302
Introduction139
Accuratefrequencymeasurements:principlesandmethods140
Surveyofaccuratefrequencymeasurements142
Significanceandimpactofaccuratefrequencymeasurements144
Summary145
References146
Selectedbibliography:futuretrendsinaccuratefrequency/timemetrology148 CHAPTER8.STATISTICSOFTIMEANDFREQUENCYDATAANALYSIS151 DavidW.Allan,JohnH.Shoaf,andDonaldHalford,TimeandFrequencyDivision,NBS,Boulder,CO80302
Introduction153
Definitionsandfundamentals153
Characterizationoffrequencystability154
Requirementsforameasureoffrequencystability155
Conceptsoffrequencystability156
Thedefinitionofmeasuresoffrequencystability(SecondMonerttype)156
Translationsamongfrequencystabilitymeasures159
Examplesofapplicationsofpreviouslydevelopedmeasures161 Measurementtechniquesforfrequencystability162Summaryoffrequencystabilitymeasures165 Practicalapplicationsoffrequencystabilityspecificationandmeasurements165 Terminologyforspecificationoffrequencystability165
Comparisonofmeasurementtechniques167
OperationalsystemsformeasurementoffrequencystabilityatNBS(highfrequencyregion)168 OperationalsystemsformeasurementoffrequencystabilityatNBS(microwaveregion)171
Conclusions/summary175
References175
AnnexA - Glossaryofsymbols176
AnnexB - Translationofdatafromfrequencydomaintotimedomainusingtheconversionchart (table8.1)178 AnnexC - Spectraldensities:frequencydomainmeasuresofstability178 VIII Pane
AnnexD - Asamplecalculationofscript=Sf180
AnnexE - AsamplecalculationofAllanVariance,o-^(t)181 AnnexF - Computingcounterprogramusinganefficientoverlappingestimatorfor(o-'^(/V=2,
T,r,fh))il2182
AnnexG - Selectedfrequencystabilityreferences:bibliography182 AnnexH - Detailedprocedureforcalibratingmicrowave(frequencystability)measurement system187 AnnexI - Measurementprocedurefordeterminingfrequencystabilityinthemicrowaveregion...188 AnnexJ - Tablesofbiasfunction,B\andB>.forvariancesbasedonfinitesamplesofprocesses withpowerlawspectraldensities190
Table8.1 - Stabilitymeasureconversionchart166
CHAPTER9.THENATIONALBUREAUOFSTANDARDSATOMICTIMESCALE:GENERATION,
STABILITY,ACCURACY,ANDACCESSIBILITY205
DavidW.Allan,JamesE.GrayandHowardE.Machlan,TimeandFrequencyDivision,NBS,
Boulder.CO80302
Introduction.-207
Basictimeandfrequencyconsiderations208
Clockmodeling208
TheAT(NBS)timescalesystem214
TheUTC(NBS)coordinatedscale220
UTC(NBS)accessibility221
TheInternationalAtomicTimeScale(TAI)223
Conclusions223
References224
AnnexA - Optimumfiltersforvariousnoiseprocesses(Eq(9.19))225 AnnexB - TimedispersionwithrecursivefilterappliedtoflickernoiseFM226 AnnexC - Mini-computerprogramforfirstordertimescalealgorithm227
AnnexD - Selectedbibliographyontimescalesandtheirformation230CHAPTER10.TIMEANDFREQUENCYDISSEMINATION:ANOVERVIEWOFPRINCIPLESANDTECHNIQUES233
ByronE.Blair,TimeandFrequencyDivision,NBS,Boulder,CO80302
Introduction235
Disseminationconcepts235
Radiodisseminationoftimeandfrequency239
Disseminationoftimeandfrequencyviaportableclock290TFDviaothermeans294
T/Fuserandsystemevaluation299
Conclusions302
References303.
AnnexA - Characteristicsofradiofrequencybands4through10310a AnnexB - Characteristicsofstandardfrequencyandtimesignalsinallocatedbands311 AnnexC - Characteristicsofstabilizedfrequencyandtime-signalemissionsoutsideallocated frequencyassignaments312 AnnexD - Characteristicsoffrequencystabilizednavigationsystemsusefulfortime/frequency comparisons313CHAPTER11.THESTANDARDSOFTIMEANDFREQUENCYINTHEUSA315 JamesA.Barnes,TimeandFrequencyDivision,NBS,Boulder,CO80302,andGemotM.R. Winkler,TimeServiceDivision,USNO,Washington,DC20390
Introduction317
Timescales - termsofreference317
Timeandfrequency(T&F)activitiesoftheNationalBureauofStandardsandtheU.S.Naval
Observatory318
Internationalorganizationsinvolvedinstandardtimeandfrequency323
Thelegaldefinitionof"standardtime"325
Summary325
References325
AnnexA - NBSenablinglegislation329
AnnexB - USNOauthorizingdocuments347
AnnexC - NBStimeandfrequencyresponsibilities359
AnnexD - USNOprecisetimeandtimeinterval(PTTI)responsibilities363
AnnexE - NBS/USNOtimecoordination/services377
AnnexF - Treatyofthemeter385
AnnexG - Legaldocumentsconcerning"standardtime"389
SubjectIndex415
TimeandFrequencyAbbreviationListing458
CHAPTER1
BASICCONCEPTSOFPRECISETIMEANDFREQUENCY*
JamesA.Barnest
Contents
Page
1.1.Introduction3
1.2.ClocksandTimekeeping3
1.3.BasicConceptsofTime4
1.4.TimeScales4
1.4.1.UniversalTime(UTO)5
1.4.2.UniversalTime1(UT1)6
1.4.3.UniversalTime2(UT2)6
1.4.4.EphemerisTime(ET)6
1.4.5.AtomicTime(AT)7
1.4.6.CoordinatedUniversalTime(UTC)Priorto19728
1.4.7.TheNewUTCSystem8
1.4.8.ComparisonsofTimeScales9
1.4.8.1.ReliabilityandRedundancy9
1.5.InternationalAtomicTime(TAI)Scale10
1.6.TheConceptsofFrequencyandTimeInterval10
1.6.1.TimeIntervalandTimeScales11
1.7.UsesofTimeScales11
1.7.1.TimeScalesforSystemsSynchronizationUses11
1.7.2.TimeScalesforCelestialNavigationandAstronomicalUses11
1.8.Conclusions13
1.9.References13
1.10.Bibliography14
Annexl.A.DefinitionoftheSecondandTAI15
Annexl.B.StandardFrequencyandTimeBroadcastAgreements29 Annexl.C.Resultsof6thSessionofConsultativeCommitteeforDefinitionoftheSecond (CCDS)37 •ManuscriptreceivedJanuary9,1973.
tTimeandFrequencyDivision,InstituteforBasicStandards,NationalBureauofStandards,Boulder,Colorado80302.
1 "Whatistime?Theshadowonthedial,thestrikingoftheclock,the runningofthesand,dayandnight,summerandwinter,months,years, centuries - thesearebutarbitraryandoutwardsigns,themeasureof time,nottimeitself..."
Longfellow,
Hyperion,Bk.ii,Ch.6.
Thischapterdescribessomeelementsoftimekeepingandgivesbasicconceptsoftimeand frequencysuchasdate,timeinterval,simultaneityandsynchronization.Thischapterdetailscharacter- isticsofnumeroustimescalesincludingastronomical,atomic,andcompromisesofboth.Theuni- versaltimescalesarebasedontheapparentmotionofthesuninthesky.whileatomictimescales arebasedontheperiodicfluctuationsofaradiosignalinresonancewithacertainspeciesofatoms. ThechapterincludesadescriptionoftheUTCtimescalebothbeforeandafterJanuary1,1972,and delineatesrequirementsofanInternationalAtomicTimescale. Keywords:Atomictime;clocks;ephemeristime;frequency;navigation/time;TAIscale;time;time interval;timescales;universaltime;UTCsystem. 2
1.1.INTRODUCTION
Themeasurementoftimeisabranchofscience
withaverylonghistory[1,2,3].
1Forthisreason,it
isdifficulttounderstandthecurrentoperationsof timeandfrequencymeasurementswithoutsome background.Thischapterpresentsabriefhistory ofthescientificandengineeringaspectsoftimeand frequency.Thediscussioncommenceswithabasic considerationofclocksandconceptsinvolvedin timemeasurements.Timescalesaredescribedand delineatedaseitherastronomical,atomic,orcom- promisesthereof.Universaltimescalesarebased ontheapparentmotionofthesuninthesky,while atomictimescalesarerelatedtoperiodicfluctua- tionsofaradiosignalinresonancewithacertain speciesofatoms.Thischapterincludesadescrip- tionoftheUTCsystemwhichcommencedJan- uary1,1972.Characteristicsandrequirementsof anInternationalAtomicTime(TAI)2scaleindicate thattheatomictimescaleoftheBureauInter- nationaldel'Heure(BIH)isalogicalchoice.How- ever,theTAIscalewillnotreplacesomeofthe needsforastronomicaltimescaleswhichareneces- saryforearthposition.Thereaderisreferredto
Chapter9fordetailsofconstructingandmaintain-
inganatomictimescale. 10 100
1.)PRIMARYDATAFROMWARD[41
,
MARRIS0N[5],&FRASER[12]
.
2.)INTENTISTOSHOWBENCH-MARKSOFPROGRESS.VARIOUSCLOCKTYPESCANSHOWIMPR0VE0ACCURACIES.
3.)PRIORTO1000A.D.VARIEDTIMEKEEPERSWEREUSED
SUCHASMECHANICAL
WATERCLOCKS,N0CTURNIALS,SUNDIALS,TIMECANDLES
,
ETC.,OFLITTLEKNOWN
ACCURACY.
CESIUMCESIUMATOMIC|ATOMIC(IMPR0VED"(NBS-5EST)
NBSIII)I
BAROMETRIC
COMPENSATION
(ROBINSON)
PENDULUM(HUYGEN•IMPROVEDESCAPEMENT
(GRAHAM )
CROSS-BEAT
ESCAPEMENT
(BURGI )
VERGESF0LI0T
BALANCECLOCKS
14001600
CALEN0ARYEARS,A.D.
Fig.1.1.Progressintimekeepingaccuracy.
1.2.CLOCKSANDTIMEKEEPING
Inearlytimes,thelocationofthesuninthesky
wastheonlyreliableindicationofthetimeofday.
Ofcourse,whenthesunwasnotvisible,onewas
unabletoknowthetimewithmuchprecision.Peo- pledevelopeddevices(calledclocks)tointerpolate betweencheckswiththesun.Thesunwassortof a"masterclock"thatcouldbereadwiththeaidofa sundial.Anordinaryclock,then,wasadeviceused tointerpolatebetweencheckswiththesun.Thedif- ferentclockdevicesformaninterestingbranchof history;wewillnotreviewthemtoanyextenthere excepttopointouttheirgaininaccuracyovera periodofyearsasshowninfigure1.1andtorefer thereadertotheworksofWard,Marrison,andHood [4,5,6].(Timekeepinghasshownnearly10orders ofmagnitudeimprovementwithinthelast6cen- turieswithabout6ordersoccurringwithin70years ofthe20thcentury.)Thus,aclockcouldbea"pri- maryclock"likethepositionofthesuninthesky,or itcouldbeasecondaryclockandonlyinterpolate betweencheckswiththeprimaryclockortime standard.Historically,somepeoplehaveusedthe word"clock"withtheconnotationofasecondary timereferencebuttodaythisusagewouldbetoo restrictive.
1Figuresinbracketsindicatetheliteraturereferencesattheendofthischapter.
2The6thSessionoftheConsultativeCommitteefortheDefintionoftheSecond
recommendsthatInternationalAtomicTimebedesignedbyTAIinalllanguages(July
1972).
Whenonethinksofaclock,itiscustomaryto
thinkofsomekindofpendulumorbalancewheel andagroupofgearsandaclockface.Eachtimethe pendulumcompletesaswing,thehandsoftheclock aremovedapreciseamount.Ineffect,thegearsand handsoftheclock"count"thenumberofswingsof thependulum.Thefaceoftheclock,ofcourse,is notmarkedoffinthenumberofswingsofthepen- dulumbutratherinhours,minutes,andseconds.
Oneannoyingcharacteristicofpendulum-type
clocksisthatnotwoclockseverkeepexactlythe sametime.Thisisonereasonforlookingforamore stable"pendulum"forclocks.Inthepast,themost stable"pendulums"werefoundinastronomy.Here oneobtainsasignificantadvantagebecauseonly oneuniverseexists - atleastforobservationalpur- poses,andtimedefinedbythismeansisavailableto anyone - atleastinprinciple.Thus,onecanobtain averyreliabletimescalewhichhasthepropertyof universalaccessibility.Inthischapter,timescale isusedtorefertoaconceptuallydistinctmethodof assigningdatestoevents.
Inaveryrealsense,thependulumofordinary,
present-dayelectricclocksistheelectriccurrent suppliedbythepowercompany.IntheUnited
Statesthepowerutilitiesgenerallysynchronizetheir
generatorstotheNationalBureauofStandards (NBS)lowfrequencybroadcast,WWVB[7].Thus, therightnumberofpendulumswingsoccureach day.Sinceallelectricclockswhicharepoweredby thesamesourcehave,ineffect,thesamependulum, theseclocksdonotgainorlosetimerelativetoeach other;i.e.,theyrunatthesamerate.Indeed,they willremainfairlyclosetothetimeasbroadcastbyWWVB(±5seconds)andwillmaintainthesame timedifferencewithrespecttoeachother(±1milli- second)overlongperiodsoftime.
Ithasbeenknownforsometimethatatomshave
characteristicresonancesor,inaloosesense,"char- acteristicvibrations."Thepossibilitytherefore existsofusingthe"vibrationsofatoms"aspendu- lumsforclocks.Thestudyofthese"vibrations"has normallybeenconfinedtothefieldsofmicrowave andopticalspectroscopy.Presently,microwaveres- onances(vibrations)ofatomsarethemostpre- ciselydeterminedandreproduciblephysical phenomenathatmanhasencountered.Thereis ampleevidencetoshowthataclockwhichuses "vibratingatoms"asapendulumwillgeneratea timescalemoreuniformthanevenitsastronomical counterparts[8,9,10].
Butduetointrinsicerrorsinanyactualclocksys-
tem,onemayfindhimselfbackinthepositionof havingclockswhichdriftrelativetoothersimilar clocks.Ofcourse,therateofdriftismuchsmaller foratomicclocksthantheoldpendulumclocks,but nonethelessrealandimportant.Ifatallpossible,one wouldliketogaintheattributeofuniversalaccessi- bilityforatomictimealso.Thiscanbeaccomplished onlybycoordinationbetweenlaboratoriesgenerat- ingatomictime.Bothnationalandinternational coordinationareinorder.
1.3.BASICCONCEPTSOFTIME
Onecanusetheword"time"inthesenseofdate.
(By"date"wemeanadesignatedmarkorpointona timescale.)Onecanalsoconsidertheconceptof timeintervalor"length"oftimebetweentwo events.Thedifferencebetweentheseconceptsof dateandtimeintervalisimportantandhasoften beenconfusedinthesingleword"time".Thissec- tionexploressomebasicideasinherentinthevarious connotationsoftime.
Thedateofaneventonanearth-basedtimescale
isobtainedfromthenumberofcycles(andfractions ofcycles)oftheapparentsuncountedfromsome agreed-uponorigin.Similarly,atomictimescalesare obtainedbycountingthecyclesofasignalinreso- nancewithcertainkindsofatoms.(Severalatomic timescales[9]havechosenthe"zeropoint"atzero hoursJanuary1,1958(UT-2),butthisisnotuni- versalamongallatomictimescalesinexistence today.)Oneofthemajordifferencesbetweenthese twomethodsisthatthecyclesofatomicclocksare much,muchshorterthanthedailycyclesofthe apparentsun.Thus,theatomicclockrequiresmore sophisticateddevicestocountcyclesthanare requiredtocountsolardays.Theimportanceof thisdifferenceisamatteroftechnologicalcon- venienceandisnotveryprofound.Oftechnological significancearethefactsthatatomicclockscanbe readwithmuchgreatereaseandwithmanythou- sandsoftimestheprecisionoftheearthclock.In additionthereadingofanatomicclockcanbepre- dictedwithalmost100,000timesbetteraccuracy thantheearthclock.
IntheU.S.literatureonnavigation,satellitetrack-
ing,andgeodesy,theword"epoch"issometimes usedinasimilarmannertotheword"date."How- ever,dictionarydefinitionsofepochshowgra- dationsofmeaningssuchastimeduration,time instant,aparticulartimereferencepoint,aswellas ageologicalperiodoftime.Thus,epochoftensimul- taneouslyembodiesconceptsofbothdateanddura- tion.Becauseofsuchconsiderableambiguityinthe word"epoch,"3itsuseisdiscouragedinpreference totheword"date,"theprecisemeaningofwhichis neitherambiguousnorinconflictwithother,more popularusage.Thus,thedateofaneventmightbe:
30June1970,14h,35m,37.278954s,UTC,forexam-
ple,whereh,m,sdenotehours,minutes,andsec- onds.(ThedesignationUTC,meaningUniversal
TimeCoordinated,willbediscussedlater.)Onthe
otherhand,"date"shouldnotbeinterchangedindis- criminatelywith"epoch"or"time."
Anotheraspectoftimeisthatofsimultaneity;i.e.,
coincidenceintimeoftwoevents.Forexample,we mightsynchronizeclocksuponthearrivalofport- ableclocksatalaboratory.Hereweintroducean additionalterm,synchronization,whichimpliesthat thetwoclocksaremadetohavethesamereadingin someframeofreference.Notethattheclocksneed notbesynchronizedtoanabsolutetimescale.Asan example,twopeoplewhowishtocommunicatewith eachothermightnotbecriticallyinterestedinthe date,theyjustwanttobesynchronizedastowhen theyusetheircommunicationsequipment.Many sophisticatedelectronicnavigationsystems(and proposedcollisionavoidancesystems)donotdepend onaccuratedatesbuttheydodependuponvery accuratetimesynchronization.Evenordinarytele- visionreceiversrequireaccuratetimesynchroniza- tion.Wethusseesomeofthecomplexitiesinvolved inconceptsoftimeandhowvariedcombinationsof timeaspectsareembodiedinandinfluencevarious time-relatedactivities.
1.4.TIMESCALES
Asystemofassigningdatestoeventsiscalleda
timescale.Theapparentmotionofthesuninthe
3Thissuggestionawaitsdefinitiverecommendationsorstatementoftermsbyparties
suchastheCCIRStudyGroup7,InterimWorkingParty7/2on"FormsofExpression forUseoftheStandard-FrequencyandTimeSignalService";InternationalCouncilof ScientificUnions;andtheIEEEStandardsCommittees,amongothers. 4 skyconstitutesoneofthemostfamiliartimescales butiscertainlynottheonlytimescale.Notethatto completelyspecifyadateusingthemotionofthesun asatimescale,onemustcountdays(i.e.,makea calendar)fromsomeinitiallyagreed-uponbeginning.
Inaddition(dependingonaccuracyneeds)one
measuresthefractionsofaday(i.e.,"timeofday") inhours,minutes,seconds,andmaybeevenfrac- tionsofseconds.Thatis,onecountscycles(and evenfractionsofcycles)ofthesun'sdailyapparent motionaroundtheearth.
Therearebothastronomicaltimescales[11]and
atomictimescales[9]whichcanprovideabasisfor precisesynchronization.Asensibleuseofthe unqualifiedword"time"istheusewhichembodies allofthesevariousaspectsoftimescales,time measurement,andeventimeinterval(orduration).
Thisistotallyconsistentwiththedictionarydefini-
tionoftheword.Thus,thestudyofsynchronization wouldbeproperlysaidtobelongtothebroader studyoftimeingeneral.Thus,itisnotonlymislead- ingbutwrongtosaythat"time"isonlydetermined byastronomicalmeans.Indeed,therearemany classesoftime - astronomicaltime,biologicaltime, andatomictime,tonameafew[12].
Timederivedfromtheapparentpositionofthe
sunintheskyiscalledapparentsolartime.Asun- dialcanindicatethefractionsofcycles(i.e.,time ofday)directly[13J.Calendars,liketheGregorian
Calendar,aidincountingthedaysandnamingthem.
Copernicusgaveustheideathattheearthspins
onitsaxisandtravelsaroundthesuninanearlycir- cularorbit.Thisorbitisnotexactlycircular,how- ever,and,infact,theearthtravelsfasterwhen nearerthesun(perihelion)thanwhenfurtherfrom thesun(aphelion).Thedetailsoftheearth'sorbit andKepler'slawof"equalareas"allowsonetosee thatapparentsolartimecannotbeauniformtime [14].Thereisalsoaneffectduetotheinclinationof theearth'saxistotheplaneofitsorbit(ecliptic plane).Apictorialdiagramofthesun-earth-moon relationshipsisshowninfigure1.2.
1.4.1.UniversalTime(UTO)
Itispossibletocalculatetheseorbitalandincli-
nationeffectsandcorrectapparentsolartimeto obtainamoreuniformtime - commonlycalledmean solartime.Thiscorrectionfromapparentsolartime MAR21
VERNALEQUINOX
APHELION
JUL5,1966
PATH
PERIHELION
JAN3,1966
LUNARORBIT
NODALLINE
PERIOD=27.32DAYS
INCLINATIONOFEARTH'S
EQUATORTOITSORBIT
(23.45°)"20°TO26°
INCLINATIONOFLUNAR
ORBITTOEARTH'SEQUATOR
18.06°TO28.1°
5°INCLINATIONOFLUNAR
EQUATORTOITSORBIT
PERIGEE=3.64x1
0 8m -EARTH-MOON
APSIDELINE"
APOGEE=4.06x10m
Fig.1.2.Sun,earth,moonrelationships.(CourtesyofA.D.Watt) 5 tomeansolartimeiscalledtheEquationofTime andcanbefoundengravedonmanypresent-day sundials[13].
Ifoneconsidersadistantstarinsteadofourstar
- thesun - tomeasurethelengthoftheday,thenthe earth'sellipticorbitbecomesunimportantandcan beneglected.Thiskindoftimeistheastronomer's siderealtimeandisgenericallyequivalenttomean solartimesincebotharebased,ultimately,onthe spinoftheearthonitsaxis - thesecondofsidereal timebeingjustenoughdifferenttogiveasidereal yearonemore"day"thanthatofasolaryear.In actualpractice,astronomersusuallyobserveside- realtimeandcorrectittogetmeansolartime.Uni- versalTime(UTO)isequivalenttomeansolartime asdeterminedattheGreenwichMeridian,some- timescalledGreenwichMeanTime(GMT).
Time,ofcourse,isessentialtonavigationindeter-
mininglongitude[15].Ineffect,anavigatorusinga sextantmeasurestheanglebetweensomedistant starandthenavigator'szenithasshowninfigure1.3.
Itisapparentthatforagivenstarthereisalocusof
pointswiththesameangle.Bysightingonanother star,adifferentlocusispossibleandobviouslythe positionofthenavigatorisatoneoftheintersections ofthetwoloci.(Athirdsightingcanremovethe ambiguity.)Thepositionofthisintersectiononthe earthobviouslydependsontheearth'srotational position.Itisimportanttoemphasizethatcelestial navigationisbasicallyconnectedtoearthposition andonlytotimebecausetheearthalsodefinesa usefultimescale. LOCAL
Fig.1.3.Principleofcelestialnavigation.
1.4.2.UniversalTime1(UT1)
Inorderforthenavigatortousethestarsfornavi-
gation,hemusthaveameansofknowingtheearth's position(i.e.,thedateontheUTscale).Thus,clocks andsextantstogetherbecamethemeansbywhich navigatorscoulddeterminetheirlocations.With navigationprovidingabigmarketfortimeandfor goodclocks,betterclocksweredeveloped,and thesebegantorevealadiscrepancyinUniversal
Timemeasuredatdifferentlocations.Thecauseof
thiswastracedtothefactthattheearthwobbleson itsaxis;thelocationofthepoleasitintersectsthe earth'ssurfaceisplottedfor1964-69infigure1.4.
Ineffect,oneseesthelocationofthepolewandering
overarangeofabout15meters.Bycomparingastro- nomicalmeasurementsmadeatvariousobserva- toriesspreadovertheworld,onecancorrectfor thiseffectandobtainamoreuniformtime - denoted
UT1[16].
Fig.1.4.Pathofearth'spole1964-1969.
1.4.3.UniversalTime2(UT2)
Withtheimprovementofclocks - bothpendulum
andquartzcrystal - itwasdiscoveredseveralyears agothatUT1hadperiodicfluctuations(ofunknown origin)withperiodsofone-halfyearandoneyear.
Thenaturalresponsewastoremovethesefluctua-
tionsandobtainanevenmoreuniformtime - UT2.
Thus,thereexistsawholefamilyofUniversalTimes
basedonthespinoftheearthonitsaxisandvarious otherrefinementsasdiagrammedinfigure1.5.In thishistoricalprogression,onenotesthatUT1isthe truenavigator'sscalerelatedtotheearth'sangular position.UT2isasmoothedtimeanddoesnotreflect thereal,periodicvariationsintheearth'sangular position.
1.4.4.EphemerisTime(ET)
Atthispointitisdesirabletogobackintime -
neartheturnofthecentury - andtracesomeother astronomicalstudies.Inthelatter19thcentury, 6
UNIVERSALTIMEFAMILY
APPARENTSOLARTIME
CORRECTEDBY
EQUATIONOFTIME|MEANSOLARTIMET
[UTO]
CORRECTEDFOR
MIGRATIONOFPOIES
At-005secfUT1
CORRECTEDFOR
KNOWNPERIODICITY
AtO.05secmUT2
Fig.1.5.Universaltimefamilyinterrelationships.
SimonNewcombecompiledasetoftables,basedon
Newtonianmechanics,whichpredictedtheposi-
tionsofthesun,themoon,andsomeplanetsforthe future.Atableofthissortiscalledanephemeris.It wasdiscoveredthatthepredictedpositionsgrad- uallydepartedfromtheobservedpositionsina fashiontoosignificanttobeexplainedeitherby observationalerrorsorapproximationsinthetheory-
Itwasnoted,however,thatifthetimeweresome-
howinerror,allthetablesagreedwell.Atthispoint itwascorrectlydeterminedthattherotationalrate oftheearthwasnotconstant.Thiswaslatercon- firmedwithquartzclocksandatomicclocks[17,18,
19].Theastronomers'naturalresponsetothiswas,
ineffect,touseNewcombe'stablesforthesunin reversetodeterminetime - actuallywhatiscalled
EphemerisTime.EphemerisTimeisdeterminedby
theorbitalmotionoftheearthaboutthesun(notby rotationoftheearthaboutitsownaxis)andshould notbeaffectedbysuchthingsascoremantleslip- pageorothergeometricalchangesintheshapeof theearth. •100
170018001900
CALENDARYEARS,A.D.
2000
Fig.1.6.TrendofUT-ETforoverthreecenturies(datafrom
Brouwer[20]).
ThevariationsinUTscalesorearthrotationrates
havebeenstudiedextensivelybyBrouwerandmany others[20,21,22].Inthischapter,weneedonly pointoutthegeneralnatureandsizeofthevaria- tionswhichhavebeenobserved.Brouwer'sstudy coveredalongperiodoftime;thecurvesshownin figure1.6summarizemuchofhisdataandanalysis.
Theyreflecttherandombehavior[23JofUT,marked
onoccasionbysuddenerraticchangessuchasseen in1963[24J.Theabscissasinfigure1.6arepropor- tionaltotheastronomicaltimescale,ET.Atpresent
ETcanbeconsidereduniformwithrespecttoAT
[25Jandisagoodcomparisonscaletobeusedin detectinglong-term(gross)propertiesofatime scale.ItisofvaluetorecognizethatBrouwerdeter- minedthattherandomprocesseswhichaffectthe rotationoftheearthonitsaxiscausedthermsfluc- tuationsinUniversalTimetoincreaseast312 ,fort greaterthanoneyear.Forperiodsoftheorderofa yearorlessitappearsthatthevariationsintheUT2 timescalecausethermsfluctuationstoincreaseas thefirstpoweroft(flickernoisefrequencymodula- tion)[26J.Thecoefficientofthislineartermisabout
2X10~9oralmostafactorof105worsethansome
cesiumclocks.Thepresentmeansofdetermination ofETarenotadequatelyprecisetoallowdefinitive statementsaboutpossiblesystematicvariationsofET[11].
1.4.5.AtomicTime(AT)
Aswaspointedoutpreviously,thedateofan
eventrelativetotheUniversalTimeScaleis obtainedfromthenumberofcycles(andfractionsof cycles)oftheapparentsuncountedfromsome agreed-uponorigin.(Dependingontheneed,one mayhavetoapplycorrectionstoobtainUTO,UT1,or
UT2.)Similarly,atomictimescalesareobtainedby
countingthecyclesofasignalinresonancewith certainkindsofatoms.
Inthelatterpartofthe1940's,HaroldLyonsat
theNationalBureauofStandardsannouncedthe firstAtomicClock[27].Duringthe1950'sseveral laboratoriesbeganatomictimescales[28.29.30].
TheBureauInternationaldel'Heure(BIH)hasbeen
maintainingatomictimeforsomeyearsnow,and thistimescalereceivedtheofficialrecognitionas
InternationalAtomicTime(TAI)oftheGeneralCon-
ferenceofWeightsandMeasures(CGPM)inOcto- ber1971[31](seeann.l.A).Beginning1January
1972,thisatomictimescalehasbeenbroadcast
(withsomemodifications)bymostcountries(see "ThenewUTCsystem"insec.1.4.7).
Inreview,wehavediscussedthreebroadclasses
oftimescalesasillustratedinfigure1.7.TheUni- versalTimefamilyisdependentontheearth'sspin onitsaxis;EphemerisTimedependsontheorbital motionoftheearthaboutthesun;andAtomicTime, whichdependsonafundamentalpropertyofatoms, isveryuniformandprecise.Becauseofthe"slow" 7
UNIVERSALTIME4
(UTO,UT1,UT2)T
MEASUREDTO3msIN1DAY
EPHEMERISTIME^
(ET)V
MEASUREDTO50msIN9YEARS
ATOMICTIME&MEASUREDTO<0.1MsIN1minT(DIFFUSIONRATESOF0.1Fs/day
FORENSEMBLESOFCLOCKS]
Fig.1.7.Classesoftimescaleswithtypicalaccuracies.
merit[33J.UTCwassupposedtoagreewithUT2to withinViosecond(V20secondbefore1963).On occasionitwasnecessarytoresettheUTCclockby
Viosecond(V20secondbefore1963)inordertostay
withinthespecifiedtolerancesasshowninfig- ure1.8.Also,byinternationalagreement[33J.the offsetsinclockratewereconstrainedtobeaninte- gralmultipleof5partsperbillion(1partperbillion before1963).Inaddition,afewstations(e.g.,WWVB)broadcastaSteppedAtomicTime(SAT) signalwhichwasderiveddirectlyfromanatomic clock(norateoffset)butwhichwasresetperiodic- ally(moreoftenthanUTC)tomaintainSATwithin aboutViosecondofUT2[34J. orbitalmotionoftheearth(1cycleperyear),meas- urementuncertaintieslimittherealizationofaccu- rateephemeristimetoabout0.05secondfora
9-yearaverage,whileUTcanbedeterminedtoa
fewthousandthsofasecondinaday,andATtoa fewbillionthsofasecondinaminuteorless.
1.4.6.CoordinatedUniversalTime(UTC)
Priorto1972
From1960through1971manybroadcasttimesig-
nals(e.g.,MSF,WWV,CHU)werebasedonatime scalecalledCoordinatedUniversalTime(UTC)[32].
TherateofaUTCclockwascontrolledbyatomic
clockstobeasuniformaspossiblefor1year,but thisratecouldbechangedatthefirstofacalendar year.TheyearlyratewaschosenbytheBIH. Table1.1liststhefractionaloffsetsinrateoftheUTCscalerelativetoapureatomictimescale.
Table1.1.FrequencyOffsetsofUTCfrom1960to1972
Offsetrateof
YearUTCinparts
per1010
1960-150
1961-150
1962-130
1963-130
1964-150
1965-150
1966-300
1967-300
1968-300
1969-300
1970-300
1971...-300
1972^future0
TheminussignimpliesthattheUTCclockranslow
(inrate)relativetoatomictime.Theoffsetinclock ratewaschosentokeeptheUTCclockinreason- ableagreementwithUT2.However,onecouldnot exactlypredicttheearth'srotationalrateanddis- crepancieswouldaccrue.Byinternationalagree-
COORDINATEDUNIVERSALTIME(UTC)
[PRIORfO1972) YEARS
UTC-AT
UI2-AI
ff =n»(50»lO"") it=1/10sec Fig.1.8.RelationshipbetweenUTCandUT2timescales - prior to1972.
1.4.7.TheNewUTCSystem
ThefactsthattheclockrateofUTChavebeen
offset(seetable1.1)fromthecorrect(atomic)rate andthatthisoffsetchangedfromtimetotimeneces- sitatedactualchangesinequipmentandofteninter- ruptedsophisticatedsystems.Astheneedsfor reliablesynchronizationhaveincreased,theoldUTCsystembecametoocumbersome.Anewcom- promisesystemwasneededtoaccountbetterforthe ever-growingneedsofprecisetimesynchronization.AnewUTCsystemwasadoptedbytheInter- nationalRadioConsultativeCommittee(CCIR)in
GenevainFebruary1971[35,36]andbecameeffec-
tive1January1972(seeann.LB).Inthisnewsys- temallclocksrunatthecorrectrate(zerooffset).
Thisleavesusinapositionofhavingtheclockrate
notexactlycommensuratewiththelengthoftheday.
Thissituationisnotunique.Thelengthoftheyear
isnotanintegralmultipleoftheday.Thisisthe originof"leapyear."Inthiscase,yearswhichare divisibleby4haveanextraday - February29 - unlesstheyarealsodivisibleby100,andthenonly iftheyarenotdivisibleby400.Thus,theyears1968,
1972,'1976,and2000areleapyears.Theyear2100
8 willnotbealeapyear.Bythismeansourcalendar doesnotgetoutofstepwiththeseasons.
Withthisasanexample,itispossibletokeepthe
clocksinapproximatestepwiththesunbytheinfre- quentaddition(ordeletion)ofasecond - calleda "leapsecond."Thus,theremaybespecialsituations wherea"minute"contains61(or59)seconds insteadoftheconventional60seconds.Thisshould notoccurmoreoftenthanaboutonceayear.By internationalagreement,UTCwillbemaintained withinabout0.7secondofthenavigators'timescale,
UT1.Theintroductionofleapsecondsallowsa
goodclocktokeepapproximatestepwiththesun.
Becauseofthevariationsintherateofrotationof
theearth,however,theoccurrencesoftheleap secondsarenotpredictableindetail.
1.4.8.ComparisonsofTimeScales
Itisofvalueincomparingtimescalestoconsider
foursignificantattributesofsometimescales: a.accuracyandprecision, b.reliability, c.universalaccessibility, d.extension.
Intheareasofaccuracyandprecision,atomic
timescaleshaveaclearadvantageovertheirastro- nomicalcounterpart.Atomicclocksmaybeableto makeareasonableapproachtothereliabilityand accessibilityofastronomicalclocks,however,astro- nomicaltimescalesarebasedona"single"clock whichisavailabletoeveryone(i.e.,onlyonesolar systemisavailableforstudy).Also,manyatomic clockscanshowdisagreements,animpossibility withonlyoneclock.Theextensionofdatestopast events(indeed,remote,pastevents)isafeature whichatomicclockswillneverpossess.Theirutility forfutureneeds,however,isquiteanothermatter.
Theneedsofthegeneralscientificcommunityand,
inparticular,thetelecommunicationsindustriesare makingevergreaterdemandsonaccurateandpre- cisetimingcoveringlongertimeintervals.Often theseneedscannotbemetbyastronomicaltime.
However,thecontinuedmotionofthesolarsystem
givesareliabilitytoastronomicaltimescaleswhich atomicclockshavenotyetattained.
Onecanimaginesynchronizingaclockwithahy-
potheticallyidealtimescale.Sometimeafterthis synchronizationourconfidenceintheclockreading hasdeteriorated.Figure1.9showstheresultsof somestatisticalstudieswhichindicatetheprobable errorsofsomeimportantclocksaftersynchroniza- tion.Therearereallytwothingsofsignificanceto noteinfigure1.9:First,AtomicTime(stateofthe art,1964)isabout10,000timesmoreuniformthan
UniversalTime,andsecond,measurementuncer-
taintytotallylimitsanyknowledgeofstatistical fluctuationsinEphemerisTime.
PROBABLECLOCKERRORS
(NON-UNIFORMITY) Fig.1.9.Probableerrorsof3clocktypesaftersynchronization.
1.4.8.1.ReliabilityandRedundancy
Inthepast,reliableoperationofatomicfrequency
standardshasbeenasignificantproblem.Presently, however,commercialunitswithaMeanTime
BetweenFailure(MTBF)exceeding1yeararenot
uncommon[37].Finiteatomsourcelifetimepre- ventsunlimitedoperationwithoutinterruption, however.
ItistruethataMTBFexceeding1yearreflects
significantengineeringaccomplishments,butthisis farfromcomparabletothehighreliabilityofastro- nomicaltime.Theobvioussolutionistointroduce redundancyintheclocksystem.Onecanusesev- eralatomicclocksinthesytemandthisshould certainlybethebestapproachinthesenseof accuracyandreliability - itisexpensive,however.
Supposethesynthesizer-countersubsystemofa
clocksystemshouldjumpasmallamountandcause adiscontinuityinitsindicatedtime.Itispossible thatsuchatransientmalfunctioncouldoccurwith nooutwardlyapparentsignsofmalfunctionofthe apparatus.Itisalsoapparentthatifonlytwoclocks areavailableforintercomparison,itisimpossibleto decidewhichclocksufferedthetransientmalfunc- tion.Thus,threeclocks(notnecessarilyallatomic) constituteanabsoluteminimumforreliableopera- tion.Ifoneormoreofthesehasanextendedprob- abledowntime(e.g.,whiletheatomsourceis replenished)then4or5clocksbecomeamorework- ableminimum. 9
Itshouldbenotedherethatonecouldassemblea
largegroupofclocksintoonesystemandthesystemMTBFcalculatedfromthe'individualMTBF's mightextendintogeologictimeintervals.Thissys- temMTBFisundoubtedlyover-optimisticdueto neglectofthepossibilitiesofcatastrophesoropera- torerrors.Nonetheless,withvariousatomicclocks spreadovertheearth,itshouldbepossibletomain- tainanatomictimescalewithareliabilitythatcould satisfyalmostanyfuturedemand.
1.5.INTERNATIONALATOMICTIME(TAI)SCALE
InrecentyearstheGeneralConferenceofWeights
andMeasures(CGPM)hasbeenencouragedto adoptanInternationalAtomicTime(TAI)scale[38] (seeann.l.A).Forsuchascaletobeofvaluethe followingattributesarerequired: a.Itmustprovidegreateraccuracyandconven- iencethantheastronomicalcounterparts. b.Itmustbehighlyreliablewithalmostno chanceofafailureoftheclocksystem.(This canbeaccomplishedbyusingmanyclocksdis- persedovertheworldbutwhichcanbeinter- comparedaccurately.) c.Theatomictimescalemustbereadilyavailable everywhere.
Indeed,allofthesepointsappeartobemorethan
adequatelycoveredbytheatomictimescaleofthe
BIH.InOctober1971theCGPMendorsedtheBIH
atomictimescaleastheInternationalAtomicTime scale[31](seeann.l.A)definedasfollows: "InternationalAtomicTimeisthetimereferencecoordi- nateestablishedbytheBureauInternationaldel'Heureon thebasisofthereadingsofatomicclocksfunctioningin variousestablishmentsinaccordancewiththedefinitionof thesecond,theS.I.Unit(InternationalSystemofUnits)of time."
TheAtomicTime(AT)scalesmaintainedinthe
U.S.(bybothNBSandUSNO)constitute~37Ya
percentofthestablereferenceinformationusedin maintainingastableTAIscalebytheBIH[39].The questionoftheaccuracyofrateoftheTAIscaleis notnowcompletelyspecified[40].Thereisaques- tionofformalaveragingproceduresforcorrecting
TAI.AspecialmeetingoftheConsultativeCommit-
teefortheDefinitionoftheSecond(CCDS)washeld inParis,France,July1972,toconsiderthestatusof atomicfrequencystandardsandimprovedrealiza- tionoftheTAI,amongotherpertinentquestions. Recommendationsofthiscommittee(6thSessionofCCDS)aregiveninAnnexl.C.
Dr.Guinot,DirectoroftheBIH,recommendeda
newmethodofcalculationofTAI.Byusingindivid- ualclockdatainplaceoflocaltimescalesfromvar- iouslaboratories,improvedweightingofclockdata isanticipated[40].Someadvantagesoftheindivid- ualclockprocedurehepointedoutareasfollows: "1.Mostofthelocaltimescalesarebasedonasmallnum- berofclocks.IrregularitiesofTAIareduetochangesof frequenciesofcertainTA(i)whichcannotbeseenatthelocal levelwhenthenumberofstandardsineffectiveusehastobe lessthan3.Thishappensfrequently.Inaglobaltreatment, suchirregularitieswouldbevisible.
2.Isolatedstandardscouldbeemployed.Forexample,at
least12cesiumstandardsconvenientlyavailableandcom- paredtotheLoranCpulsesorTVpulsesareavailablein
Europe,notincludingthoseattheLoranCstations.
3.Thetreatmentofallthestandardswouldbeunified,
describedindetail,andunderstoodbyall.Atpresent,itis practicallyimpossibletounderstandhowTAIiscalculated sinceitisnecessarytounderstandthemethodsofeach participatinglaboratory,methodswhicharenotalways published.
4.ThedirectcalculationofTAIwouldallowacomplete
freedomtothelaboratoriesinordertoestablishtheTA(i), accordingtotheircriteria,withoutwhichtheyhavetobe preoccupiedwiththecriteriaadoptedbyTAI."
AtentativescheduleforworkoftheBIHfor
improvingTAIisgiveninAnnexl.C.Acomplete descriptionoftheconstructionofalocalatomic timescaleistreatedbyD.Allanetal.inchapter9.
TheUTCscalesoftheUSAarecoordinatedwith
UTC(BIH)towithinatoleranceofabout±10fxs.
AlloftheUTCscalesaresupposedtoagreewith
UTC(BIH)to±1millisecondbyInternational
RadioConsultativeCommittee(CCIR)agree-
ment[36].ForthosedesiringaccurateUTinforma- tion,correctionsareencodedintostandardtime broadcasts[36].Yet,evenwiththeexistenceofan
InternationalAtomicTimescale,onemustrecog-
nizethattherewillbecontinuedneedforthe astronomicaltimescales.Apersondoingcelestial navigation,forexample,mustknowearthposition (UT1).
1.6.THECONCEPTSOFFRE-QUENCYANDTIMEINTERVAL
Thefourindependentbaseunitsofmeasure-
mentcurrentlyusedinsciencearelength,mass, time,andtemperature.Itistruethat,exceptfor fieldsofsciencesuchascosmology,geology,and astronomy,timeintervalisthemostimportant concept,and(astronomical)dateisofmuchless importancetotherestofscience.Thisistruebe- causethe"basiclaws"ofphysicsaredifferential innatureandonlyinvolvesmalltimeintervals.In essence,physical"laws"donotdependuponwhen (i.e.,thedate)theyareapplied.
Basedontheselawsandextensiveexperimenta-
tion,scientistshavebeenabletodemonstratethat frequencycanbecontrolledandmeasuredwiththe smallestpercentageerrorofanyphysicalquantity.
Thefrequencyofaperiodicphenomenonisthe
numberofcyclesofthisphenomenonperunitof time(i.e.,persecond).Thenameoftheunitoffre- quencyisthehertz(Hz)andisidenticaltoacycle persecond(cps).Sincemostclocksdependupon someperiodicphenomenon(e.g.,apendulum)in orderto"keeptime,"andsinceonecanmakereli- ableelectroniccounterstocountthe"swings"of 10 theperiodicphenomenon,wecanconstructclocks withtimekeepingaccuracy(rateaccuracy)equal totheaccuracyofthefrequencystandard.
Intermsoftheadvancementoftimescales,the
historyofthedefinitionofthesecondcanbe expressedverybriefly.Priorto1956,thesecond wasdefinedasthefraction1/(86,400)ofameansolar day;from1956to1967itwastheephemerissecond dennedasthefraction1/(31556925.9747)ofthe tropicalyearatOOh00m00s31December1899,and since1967,intermsofaresonanceofthecesium atom[41](seeann.LA).Thepresentdefinition ofthesecondstates: "Thesecondisthedurationof9192631770periodsofthe radiationcorrespondingtothetransitionbetweenthetwo hyperfinelevelsofthegroundstateofthecesium - 133 atom".(13thCGPM(1967),Resolution1),[42].
Today'smostpreciseandaccurateclocksincor-
porateacesiumatomicbeamasthe"pendulum"of theclock.
1.6.1.TimeIntervalandTimeScales
Oneshouldnotesourcesofconfusionwhichcan
existinthemeasurementoftimeandintheuseof theword"second."Supposethattwoevents occurredattwodifferentdates.Forexamplethe datesofthesetwoeventswere15December1970,
15h30m00.000000sUTCand15December1970,
16h30m00.000000sUTC.Atfirstthoughtonewould
saythatthetimeintervalbetweenthesetwoevents wasexactly1hour=3600.000000seconds,butthis isnottrue.(Theactualintervalwaslongerby about0.000108seconds[3600secondsX300X10"10 ].
Seetable1.1.)RecallthattheUTCtimescale(like
alltheUTscalesandtheETscale)wasnotdefined inaccordancewiththedefinitionoftheintervalof time,thesecond.Thus,onecannotsimplysubtract thedatesoftwoeventsasassignedbytheUTC scale(oranyUTscaleortheETscale)inorderto obtaintheprecisetimeintervalbetweenthese events.Historically,thereasonbehindthisstateof affairsisthatnavigatorsneedtoknowtheearth's position(i.e.,UT1) - notthedurationofthesecond.
Yet,manyscientistsneedtoknowanexactand
reproducibletimeinterval.Notethatthismight alsobetrueofthenewUTCsystemiftheparticular timeintervalincludedoneormoreleapseconds.
Itisalsoconfusingthatthedatesassignedbythe
UT,ET,andUTCscalesinvolvethesamewordas
theunitoftimeinterval,thesecond.Foraccurate andprecisemeasurements,thisdistinctioncanbe extremelyimportant.
1.7.USESOFTIMESCALES
Thestudyoftimescalescanbedividedintothe
studyoftimescalesusedforsystemssynchroniza- tionandtimescalesusedforcelestialnavigation andastronomy.
1.7.1.TimeScalesforSystemsSynchroniza-
tionUses
Longagopeopleweresimplycontenttoletthe
sungoverntheirfives.Sunriseindicatedtimeto ariseandbeginwork;sunsetsignalledtheday'send.
Withtheadvancementofcivilization,growthof
commerceandcitylifeandtechnologicalgains, communitieswereestablishedwhichinstituted clockssettoagreeroughlywiththeapparent movementofthesun.Thusdevelopedtheideaof localtimeandeachcommunitycouldhaveitsown localtime.Clearly,whenalmostallcommunications andbusinesstransactionsoccurwithinagivencom- munityorlocale,thisisaworkablesolution.With theadventofrailroadsandhencemorerapidcom- munications,this"crazy-quilt"mazeofindividual localtimeshadtoend.Therailroadsaregenerally creditedwithunifyingthevariouslocaltimesinto timezonesinthecontinentalU.S.,resultingina muchmoreworkablenationaltimesystem.In1884 aninternationalconferencerecommendedthatthe meridianofGreenwich,Englandbethestandard referencemeridianforlongitudeandtime[43].
Longitudemeridians,each15°,represent1hour
time-zonedifferences±12hourseastandwestof
Greenwich.Figure1.10showsthestandardtime
zonesoftheworldineffecttoday.
Thisbriefhistoricalsequenceillustratesthat,as
communicationsbecomemorerapidandmorefar- reaching,thegreaterarethedemandsonanall- pervasiveandunifyingconventionofsynchronizing clockswitheachother.Thatis,thisconventionis amatterofconvenienceandthereisnothingsacred orabsoluteaboutwhatourclocksread;it'sjust importantthattheyreadthesametime(orhavea well-definedtimedifferenceasbetweenthetime zones).Inthedayswhentherailroadswerethe primarymeansoftransportationacrosstheNorth
Americancontinent,anaccuracyofafewseconds
oftimewasimportantandsufficient.Nowadays, withtheexistenceofsophisticatedtelecommunica- tionsequipmentcapableofsendingandreceiving severalmillionalphanumericcharacterseach second,therearerealneedsforclocksynchroniza- tionsataccuracylevelsofamillionthofasecond andbetter.
1.7.2.TimeScalesforCelestialNavigation
andAstronomicalUses
Aspointedoutpreviously,timeisessentialfor
celestialnavigation.Ifoneknowswhattimeitis (i.e.,solartime)atsomereferencepoint - saythe
GreenwichMeridian - andalsohislocaltimeas
indicatedbyasundial - onecanfigurehislongitude. 11 oooooooo 12 sincetheearthmakesonecompleterevolution (360°)onitsaxisinabout24hours.Forexample, noonGreenwichMeanTimeis0200HawaiianStand- ardTimeor10hoursdifferent.Thus,onecaneasily calculatethatHawaiiisabout10/24oftheway aroundtheworldfromGreenwich,England - i.e., about150°westofthePrimeMeridian.Ifthis personweretomeasuretheactualpositionofthe sunintheskyusing,say,anavigator'ssextant, thenhecouldgetaratheraccuratedetermination oflocalsolartime.Thekeyproblemisknowledge ofcorrecttimeontheGreenwichMeridian.
Nearly200yearsago,amaninEnglandnamed
Harrisonwasawarded£20,000fordesigningand
buildingachronometerwhichwouldallowthe accuratedeterminationoflongitudewhileatsea (lessthan1minuteerrorafter5monthsatsea[44]).
Untilradiosignalswereavailableintheearly1900's,
navigationatseawastotallydependentupongood clocks.Today,therearemanystandardtimebroad- caststationsintheworldwhichcanprovidetime signalsaccuratetobetterthan1secondofearth time,UT1[45].
Ifastronomicaltimecouldbemeasuredwith
sufficientaccuracyandconvenience,thenastro- nomicaltimecouldbeusedforsystemsynchroniza- tionusesalso.Inactuality,astronomicaltimeis difficulttomeasure,andaccuraciesofafew thousanthsofasecondmayberealizedonlyafter theaveragingofawholeevening'ssightingsbya sophisticatedandwell-equippedobservatory.The accuratedeterminationofUT1involvesmeasure- mentsat,atleast,twoobservatorieswidelysepa- ratedinlongitude.
1.8.CONCLUSIONS
Twoverydifferentusesfortimehavebeendis-
cussed.Thefirstisaconventionwhich,when universallyaccepted,allowsbothrapidandefficient communicationssystemstofunction.Theneeds hereareforextremelypreciseanduniformmeas- urementsoftime.Theseconduseisforcelestial navigationandastronomicalobservations.Here thereisnottheneedforhighlyprecisetime,atleast nottothesamedegreeasthefirstmentioneduse.
Becauseoftheconflictingrequirementsimposed
ontimescalesbythesetwocategoriesoftimescale users,therehasbeenagreatdealofefforttoobtain acompromisetimescalewhichadequatelyreflects therelativeimportanceofthesetwousergroups.
Asonemightwellimagine,withthegrowing
importanceandsophisticationofcommunications systemsandtheimplementationofelectronic navigationsystems(toreplacecelestialnavigation), thetrendinthecompromisetimescaleshasbeen awayfromtimescalesbasedontheearth'srotation (i.e.,astronomicaltimescales)andtowardapure atomictimescale.
Inthiscompromisescale,UTC,onefindshimself
inaratherfamiliarsituation.Thereisnotawhole numberofdaysintheyearandonedoesn'twantthe calendartogetbadlyoutofstepwiththeseasons.
Similarly,thereisnotawholenumberofseconds
inasolardayandonedoesn'twantourclocksto getbadlyoutofstepwiththesun.Thesolution (asnotedabove)isanalogoustotheleapyearwith itsextraday;wehaveanextrasecond - aleap secondwhichmustbeaddedordeletedonoccasion.
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