测控技术与仪器专业英语阅读翻译.docx

测控技术与仪器专业英语阅读翻译.docx

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测控技术与仪器专业英语阅读翻译

测控技术与仪器专业英语阅读翻译

篇一：

测控技术与仪器专业英语课后阅读翻译（1,5~10）

第1章课后

UnderwaterAcousticSignal

Intheoperationofasonarsystemtheoperatorisrepeatedlyfacedwiththeproblemofdetectingasignalwhichisobscuredbynoise.Thissignalmaybeanechoresultingfromatransmittedsignaloverwhichtheoperatorhassomecontrol,oritmayhaveitsorigininsomeexternalsource.Thesetwomodesofoperationariseinradarsurveillanceandindisciplinesfortechniquesandforillustrationsofthebasicprinciples.

Sincetherearemanywaysinwhichonecanthinkaboutsignaldetection,itisdesirabletodefineatermtodenotespecialcases.Theworddetectionwillbeusedwhenthequestiontobeansweredis,?

Areoneormoresignalspresent?

?

whenthesystemisdesignedtoprovideanansweredtothisquestion,eitherdeterministicorprobabilistic,onespeaksofhypothesistesting.Thecaseofasinglesignaloccurssooftenthatmanysystemaredesignedtoprovideonlytwoanswers,?

Yes,asignalispresent,?

or?

No,thereisnosignal.?

Onecanmaketheproblemmorecomplicatedbyendeavoringtoclassifythesignalintocategories.Decisionsofthislatterkindwillbereferredtoastarget

classification.

Normallyapieceofdetectionequipmentisdesignedtooperateinafixedmodeandtheparameterssuchasintegratingtimeofrectifiercircuitsorpersistenceoftheoscilloscopetubeforvisualdetectioncannotbechangedreadily.Therewillalwaysbesomeuncertainsignals,whichtheobserverwillbehesitanttorejectoraccept.Inthesecasestheoperatormighthavethefeelingthatiftheintegratingtimeofthedetectororthepersistenceoftheoscilloscopetubewerelonger,hecouldreachadecisionabouttheexistenceofthesignal.Wald（1950）hasformulatedthisintuitivefeelingintoatheoryofdetection.Whenoneisabletovarydeliberatelytheintervaloverwhichonestoresdatainthe

receptionsysteminordertoachieveacertainlevelofcertainty,onespeaksofsequentialdetection.Frequentlyitisdesirabletodeterminenotonlythepresenceorabsenceofthesignalbutalsooneormoreparametersassociatedwiththesignal.Theparametersofinterestcanvarywidelyfromasimplequantitysuchastimeofarrivalortargetbearingtotherecoveryofthecompletewaveform.Whenasystemisdesignedtorecoveroneormoreparametersassociatedwiththesignal,onespeaksofsignalextraction.

Thewordsignalwasnotdefinedanditwasassumedthatthereaderhadanintuitivefellingfortheword.Someelaborationmaybeinordersincethedefinitionofsignalsubjectiveanddependsontheapplication.Onemaysaythat?

signal?

iswhatonewantstoobserveandnoiseisanythingthatobscurestheobservation.Thus,atunafishermanwhoissearchingtheoceanwiththeaidofsonarequipmentwillbeoverjoyedwithsoundsthatareimpairingtheperformanceofanearbysonarsystemengagedintrackingasubmarine.Quiteliterally,oneman?

ssignalisanotherman?

snoise.

Signalscomeinallshapesandforms.Inactivesonarsystemonemayusesimplesinusoidalsignalsoffixeddurationandmodulationsthereof.Thereareimpulsivesignalssuchasthosemadewith

explosionsorthumpers.Attheotherextremeonemaymakeuseofpseudorandomsignals.Inpassivesystems,thesignalswhosedetectionissoughtmaybenoiseintheconventionalmeaningoftheword;noiseproducedbypropellersorunderwaterswimmers,forexample.Itshouldbeevidentthatoneofourproblemwillbetheformulationofmathematicaltechniquesthatcanbeusedtodescribethesignal.Althoughthesourceinanactivesonarsearchsystemmaybedesignedtotransmitasignalknownshape,thereisnoguaranteethatthereturnsignalwhosedetectionissoughtwillbesimilar.Infact,therearemanyfactorstochangethesignal.Theamplitudelossassociatedwithinversespherical

spreadingismostunfortunateforthedetectionsystemnutitdoesnotentailanydistortionofthewaveshape.（Incidentally,wherethewavecanbeapproximatedlocallyasaplanewave.）Theacousticmediumhasanattenuationfactor,whichdependsonthefrequency.Thisproducesaslightdistortionofthewaveshapeandacorrespondingchangeintheenergyspectrumofthepulse.Themajorchangesinthewaveformresultfromacousticboundariesandinhomogeneitiesinthemedium.

Whenechoesareproducedbyextendedtargetssuchassubmarines,therearetwodistinctwaysinwhichechostructureisaffected.First,thereistheinterferencebetweenreflectionsfromthedifferentleadstoatargetstrengththatfluctuatesrapidlywithchangesintheaspect.Secondly,thereisthe

elongationofthecompositeechoduetothedistributionofreflectingfeaturesalongthesubmarines.Thismeansthatthedurationofthecompositeechoisdependentinasimplemannerontheaspectangle.IfTisthedurationofth