测绘工程专业英语：Section 5 Electronic Distance Measurement Systems

Section5ElectronicDistanceMeasurementSystemsEssentiallytheinstrumentsmentionedinthischapterconsistofatransmitter,setupatoneendofthelengthtobemeasured,sendingoutacontinuouswave,tothereceiverattheotherend.Thiswave,termedthecarrierwave,isthenmodulatedandthelengthdeterminedasexplainedlater.ChoiceoffrequencyThisisafundamentalprobleminEDMsystems.Theelectromagneticspectrumiscontinuousfromvisiblelightwithfrequenciesoftheorderof1014HZ,correspondingtowavelengthsoftheorderof10-6m,tolongradiowaveswithfrequenciesof104or105Hz,correspondingtowavelengthsoftheorderof104m.Therelationshipbetweenfrequencyandwavelengthisshownin

Itisconvenienttodividetheinstrumentsincurrentuseintothreedistinctcategoriesdependingonthefrequencyofthecarriersignal:(1)lowfrequencyradiosystemswithcarrierfrequenciesoftheorderof105to106Hz(wavelengthsoftheorderof103or102(2)microwaveradiosystemswithcarrierfrequenciesoftheorderof1010Hz(wavelengthsoftheorderof10-2m(3)visibleandinfra-redlightsystemswithcarrierfrequenciesoftheorderof1014Hz(wavelengthsoftheorderof10-6mGenerallyspeakingitisfoundthatthelowerfrequencysignalsprovidegreaterrangebutrequirelargertransmitters,andbeingaffectedbytheatmospherearethereforelessaccurateforEDMpurposesthanthoseofhigherfrequency.However,formarineandairnavigationandformuchhydrographicworklongrangeisvital,accuracyrequirementsarecomparativelylowandpermanentorsemi-permanenttransmittersareappropriate.Allthesefactorspointtotheuseoflowfrequencysignalsandindeedmostposition-fixingsystemsoperateinthelowormediumfrequencyrange.Intheserangeswavelengthsareoftheorderof102or103Forpracticalfieldinstrumentsforengineeringandlandsurveyingthehigherfrequenciesaremostusefulastheinstrumentscanbemadesmallandtransportableandthepropagationthroughtheatmosphereismorestable.However,atthesefrequenciesitismoredifficulttomeasurethephasedifferences,asdiscussedlater,andthewavelengthsaresosmallthatitisimpracticaltousedirectlythewavesthemselvesforthemeasurements.Thesolutionadoptedistomodulatethehighfrequencycarrierwithalowerfrequencywaveandtousethemodulatedwaveformeasurementpurposes.Modulationisaprocesswherebycertaincharacteristicsofthecarrierwavearevariedorselectedinaccordancewithanothersignal.Thecarriersignaldoesnothavetobeatapreciselydeterminedfrequencybutitmustbeproducedefficientlyandinsuchaformthatitcanbemodulatedeasily.Themodulationsignal,beingtheoneusedfortheactualmeasurementhastobeatanaccuratelycontrolledfrequency.Itisoftenproducedbyacrystal-controlledoscillatorwhichmaybehousedinathermo-statically-controlledoventoenhancethestabilityofthesignal:awarming-uptimeshouldbeallowedforthemostpreciseresults.Someinstrumentsuseanamplitudemodulation,whilstothersuseafrequencymodulation,thedifferencesbeingindicatedinFig.2,butthedifferenceisunimportantfromthepointofviewoftheoperator.Inamplitudemodulationtheamplitudeofthecarrierwaveisvariedaboveandbelowitsunmodulatedvaluebyanamountproportionaltotheamplitudeofthemodulationsignalandthefrequencyofthatsignal.Theamplitudeofthecarrierremainsconstantinfrequencymodulationbutnowitsfrequencyiscontinuouslyvariedbyanamountproportionaltotheinstantaneousamplitudeofthemodulatingsignalandatthefrequencyofthatsignal.

PhasedifferenceanddistancesTherearetwooptionsavailableintheuseofmicrowavesfordistancemeasurement,eitherpulsetransittimesorphasechangesbeingmeasured.Intheformercasetheunmodulatedcarrierisusuallyapulsetrainorseriesofpulses,notthecontinuouswavereferredtopreviously.Whenresolutionofdistanceto0.01morbetterisrequiredthelattersystemisusuallyadopted,beingbasedontherelationshipbetweenthetransmittedandreceivedsignal.Consideratransmittersendingoutanoscillatingsignalataconstantfrequency,f,toareceivertouchingit.Werethetwotouching,thenthetransmittedsignalandreceivedsignalwouldbeinphase,butasthereceivermovesawayfromthetransmitterthereceivedsignalwilllagbehindthetransmittedsignalduetothetimeoftravelofthatsignal.Thustherewillbeaphasedifferencebetweenthesignals,andifthedifferenceinphasebetweenthesignalsatthetransmitterandthereceiverismeasuredthedistancebetweenthemcanbededuced.Whenthatdistanceisequaltothewavelengththephasedifferencewillbe2πandthesignalswillbeinphase,asinfacttheywillbeeachtimethedistanceapartisanintegralwavelength.Thereforewithinanunknowndistance,d,allthatonemeasurementofphasedifferencewillgiveistheresidualpartofdoverandaboveanintegralnumberofcompletedwavelengths.Itisnotpossibletocompareinstantaneouslythephaseofthesignalsatatransmitteranddistantreceiver.ThereforeEDMsystemsadoptthetechniqueofeitherretransmittingthesignalbacktothetransmitter(microwaves)orreflectingthesignalbacktothetransmitter(electro-optical)andmakingthephasecomparisonthere.Thusitisalwaysadoublepathwhichismeasured.Thefundamentalequationwhichrelatesslopedistancetophasedelayorphasedifferencemaybewrittenaswhered=doubledistance,i.e.totaltravelofwaveλ=modulationwavelength=V0/μfn=numberofcompletewavelengthswithindφ=phasedifferencebetweentheoutgoingandincomingsignalsa=anadditiveconstantrelatedtogeometricalandelectricaleccentricitiesμ=refractiveindexf=frequencyGenerallynisunknownanddcanbefoundbyrepeatingthemeasurementsofphasedifferenceatfrequenciesdifferingfroma'fine'measuringfrequency.Incertaininstrumentsthisprocessiscarriedoutautomaticallywhereasinotherstheprocessiscarriedoutbytheoperators.InFig3twomodulationfrequenciesareshownwiththephasedelaysthenarisingduetothedistancedoftravel.Thewavelengthsaresuchthatfivegivenbyonefrequencyoccupythesamelengthasfourgivenbytheother(lower)frequency.Thisparticularlength(200m)isalsocoveredbyonewholewavelengthwhosefrequencyisequaltothedifferenceofthetwomodulationfrequencies.Moreoverthedifferenceinphasebetweenthetwoisalwaysequaltothephaseofthewavegivenbythedifferencefrequency.Thus,measuringthetwophasedelaysconsequenttotravellingoveradoubledistanceupto200misequivalent,onsubtraction,tomeasuringthephasewhichwouldhavebeengivenhadthedifferencefrequencybeenapplied.Aphasedifferenceof2πappliestoadistanceof200m,andsoifaphasedifferenceof1.54πbededucedonsubtraction,adoubledistanceof1.54π×200/2πmisinvolved,i.e.154m.Naturallywhenadistanceinexcessof200misbeingmeasured,thisparticulardifferencefrequencygivesanunknownnumberofwholelengthsof200mplusapartlengthof200m.Thusasetofdifferentfrequencieshastobeapplied,whichwhenrelatedtothebasic'fine'frequencyallowsstagesofdoubledistancesuchas2000m,20000mand200Inthemeasurementillustratedthephasedifferencewiththelowerwavelengthwas1.70πandsincethiscorrespondsto1.70π×40/2π,i.e.34.0m,thedistancemeasuredmustthereforehavebeen(n×40+34.0)wherenisaninteger.Themeasurementwiththesecondwavelengthwasmadeprimarilytoenablentobeidentified.Thefiguresintheillustrationhavebeenarrangedtobeexact,andsothedistancecalculatedusingthedifferencefrequencyworksoutexactlyas154m,i.e.(3×40+34).Inpracticetheremaybesmallinaccuraciesinthemeasurementsandtheresultfromthedifferencefrequencymaynottallyexactlywiththefirstmeasurement,e.g.ifthephaseinthesecondmeasurementhadbeen0.15πtheresultwouldhavebeen1.55π×200/2π,i.e.155mbutthedistancewasknowntobe(n×40+34.0)andtheresultiscloseenoughtoenablentobeidentifiedas3givingthedistanceas154.0masbefore.ThisisanimportantpointwhichshouldbewellunderstoodasitisthiswhichisthereasonfortheuniqueaccuracyofEDM.Ifthewavelengthiscorrectandtheintegralnumberofwavelengthsiscorrectlyidentifiedthenthemajorpartofthedistanceisdeterminedwithouterror;theonlypartofthedistancewhichismeasuredistheresidualpartoverandaboveanintegralnumberofwavelengths.EffectivewavelengthIthasbeenmentionedabovethatthelengthofthedoublepathisalwaysmeasuredbyEDMsystems.TomeasureadistanceDwithasignalofwavelengthλandadoubledistanced=2Dwithasignalofwavelength2λgivesanidenticalresultinrespectofbothnumbersofcompletewavelengthsandresidualphasedifference.Insteadofdeterminingtheactualdoubledistancetravelledbythesignalandthendividingitbytwo,itisconvenienttouseaneffectivewavelengthofhalfthetruewavelengthandthencetocalculatethesingledistancedirectly.ByusingeffectivewavelengthsofλA=20mandλE=25mthesamesingledistanceof77mwouldhavebeenderived.InpracticetoachievehighprecisionλAiskeptcomparativelyshort,10mbeingacommonvalue.Precisionisdeterminedalsobytheaccuracyofmeasurementofφ.Someinstrumentscanresolvetoonly1%phaseothersto0.1%orbetter.Phasemeasurementsmaybeachievedinvariousways,i.e.(1)byaresolverwhichconsistsofastatorandrotor,thelatterbeingplacedintoanangularpositionwithrespecttotheformercorrespondingtothephasedifferencebetweenthetransmittedandreturnedsignals,(2)byavariablelightpath,(3)byadigitalsysteminwhichthetransmittedwavewhenpassingthroughzerovoltageactivatesacounterwhichcountspulsesofaselectedfrequencyuntilstoppedbythereturnedwave.ItisrecommendedthatreferencebemadetoElectromagneticDistanceMeasurement,secondedition(GranadaTechnicalBooks)byC.D.Burnsideformoredetailedinformationonthisandotheraspectsofthesubject.Inmoderninstrumentsthephasedifferencebetweentheoutgoingandincomingsignalsisnotmeasuredattheoperatingfrequenciesbutistransformedtoacorrespondingdifferenceatmuchlowerfrequency.Thisgreatlyimprovestheaccuracyofmeasurementofphasedifferencesuchthatresolutiontoonethousandthpartofacyclecanbereadilyobtained.NewWordsandExpressionsElectronicDistanceMeasurement：电子