測(cè)控技術(shù)與儀器專業(yè)英語翻譯

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UnderwaterAcousticSignal

Intheoperationofasonarsystemtheoperatorisrepeatedlyfacedwiththeproblemofdetectingasignalwhichisobscuredbynoise.Thissignalmaybeanechoresultingfromatransmittedsignaloverwhichtheoperatorhassomecontrol,oritmayhaveitsorigininsomeexternalsource.Thesetwomodesofoperationariseinradarsurveillanceandindisciplinesfortechniquesandforillustrationsofthebasicprinciples.

Sincetherearemanywaysinwhichonecanthinkaboutsignaldetection,itisdesirabletodefineatermtodenotespecialcases.Theworddetectionwillbeusedwhenthequestiontobeansweredis,‘Areoneormoresignalspresent?’whenthesystemisdesignedtoprovideanansweredtothisquestion,eitherdeterministicorprobabilistic,onespeaksofhypothesistesting.Thecaseofasinglesignaloccurssooftenthatmanysystemaredesignedtoprovideonlytwoanswers,‘Yes,asignalispresent,’or‘No,thereisnosignal.’Onecanmaketheproblemmorecomplicatedbyendeavoringtoclassifythesignalintocategories.Decisionsofthislatterkindwillbereferredtoastargetclassification.

Normallyapieceofdetectionequipmentisdesignedtooperateinafixedmodeandthe

parameterssuchasintegratingtimeofrectifiercircuitsorpersistenceoftheoscilloscopetubeforvisualdetectioncannotbechangedreadily.Therewillalwaysbesomeuncertainsignals,whichtheobserverwillbehesitanttorejectoraccept.Inthesecasestheoperatormighthavethefeelingthatiftheintegratingtimeofthedetectororthepersistenceoftheoscilloscopetubewerelonger,hecouldreachadecisionabouttheexistenceofthesignal.Wald(1950)hasformulatedthis

intuitivefeelingintoatheoryofdetection.Whenoneisabletovarydeliberatelytheintervaloverwhichonestoresdatainthereceptionsysteminordertoachieveacertainlevelofcertainty,onespeaksofsequentialdetection.

Frequentlyitisdesirabletodeterminenotonlythepresenceorabsenceofthesignalbutalsooneormoreparametersassociatedwiththesignal.Theparametersofinterestcanvarywidelyfromasimplequantitysuchastimeofarrivalortargetbearingtotherecoveryofthecomplete

waveform.Whenasystemisdesignedtorecoveroneormoreparametersassociatedwiththesignal,onespeaksofsignalextraction.

Thewordsignalwasnotdefinedanditwasassumedthatthereaderhadanintuitivefellingfortheword.Someelaborationmaybeinordersincethedefinitionofsignalsubjectiveanddependsontheapplication.Onemaysaythat‘signal’iswhatonewantstoobserveandnoiseisanythingthatobscurestheobservation.Thus,atunafishermanwhoissearchingtheoceanwiththeaidofsonarequipmentwillbeoverjoyedwithsoundsthatareimpairingtheperformanceofanearbysonarsystemengagedintrackingasubmarine.Quiteliterally,oneman’ssignalisanotherman’snoise.

Signalscomeinallshapesandforms.Inactivesonarsystemonemayusesimplesinusoidalsignalsoffixeddurationandmodulationsthereof.Thereareimpulsivesignalssuchasthosemadewithexplosionsorthumpers.Attheotherextremeonemaymakeuseofpseudorandom

signals.Inpassivesystems,thesignalswhosedetectionissoughtmaybenoiseintheconventionalmeaningoftheword;noiseproducedbypropellersorunderwaterswimmers,forexample.It

shouldbeevidentthatoneofourproblemwillbetheformulationofmathematicaltechniquesthatcanbeusedtodescribethesignal.

Althoughthesourceinanactivesonarsearchsystemmaybedesignedtotransmitasignalknown

shape,thereisnoguaranteethatthereturnsignalwhosedetectionissoughtwillbesimilar.Infact,therearemanyfactorstochangethesignal.Theamplitudelossassociatedwithinversesphericalspreadingismostunfortunateforthedetectionsystemnutitdoesnotentailanydistortionofthewaveshape.(Incidentally,wherethewavecanbeapproximatedlocallyasaplanewave.)Theacousticmediumhasanattenuationfactor,whichdependsonthefrequency.Thisproducesa

slightdistortionofthewaveshapeandacorrespondingchangeintheenergyspectrumofthepulse.Themajorchangesinthewaveformresultfromacousticboundariesandinhomogeneitiesinthemedium.

Whenechoesareproducedbyextendedtargetssuchassubmarines,therearetwodistinctwaysinwhichechostructureisaffected.First,thereistheinterferencebetweenreflectionsfromthedifferentleadstoatargetstrengththatfluctuatesrapidlywithchangesintheaspect.Secondly,thereistheelongationofthecompositeechoduetothedistributionofreflectingfeaturesalongthesubmarines.Thismeansthatthedurationofthecompositeechoisdependentinasimplemannerontheaspectangle.IfTisthedurationoftheechofromapointscatterer,andListhelengthofthesubmarine,thedurationofthereturnedechowillbeT=(2L/c)cosA,whereAistheacuteanglebetweenthemajoraxisofthesubmarineandthelinejoiningthesourceandthesubmarine.Cisthevelocityofsoundinthewater.Ofcourse,LcosAmustbereplacedbythebeamwidthofthesubmarinewhenAisnear.

AfinalsourceofpulsedistortionistheDopplershiftsproducedbytherelativemotionsbetweenthesource,andthetarget(ordetectorinpassivelistening)mayeachhaveadifferentvelocityrelativetothebottom,thevarietyofeffectsmaybequitelarge.

水下聲波信號(hào)

在聲納操作過程中，操作員常常需要對(duì)受噪聲干擾的信號(hào)進(jìn)行檢波。干擾信號(hào)可能來自操作員發(fā)出信號(hào)的反射波或者外部聲源的信號(hào)。這兩種類型的干擾對(duì)主動(dòng)聲納和被動(dòng)聲納都會(huì)造成很大影響。類似的狀況在雷達(dá)監(jiān)測(cè)、工程類和圖像類專業(yè)的基本原理都會(huì)涉及到。當(dāng)你想到信號(hào)檢測(cè)時(shí)有多種方法，那么定義一個(gè)術(shù)語來表示特別狀況便是可行的。當(dāng)問題的答案是“當(dāng)前有一個(gè)還是一個(gè)多個(gè)信號(hào)？”時(shí)，檢波一詞將被使用。一個(gè)系統(tǒng)被設(shè)計(jì)來為這種問題供應(yīng)答案--無論是必定性還是偶然性，這就需要談及假設(shè)檢驗(yàn)；當(dāng)一個(gè)信號(hào)反復(fù)消失的狀況下，很多系統(tǒng)只被設(shè)計(jì)供應(yīng)兩個(gè)答案：“是的，當(dāng)前有一個(gè)信號(hào)”或“不，當(dāng)前沒有信號(hào)”。力圖將信號(hào)分類會(huì)使問題簡(jiǎn)單化，因?yàn)楹笳叩慕Y(jié)論將涉及到目標(biāo)分類。一般來說，一種檢波儀器只被設(shè)計(jì)在固定的類型和參數(shù)下工作，不簡(jiǎn)單被轉(zhuǎn)變，例如時(shí)間積分檢波電路和光學(xué)檢測(cè)的輝光示波管。當(dāng)消失不明信號(hào)時(shí)，觀看者在拒絕或接收信號(hào)方面有所遲疑。在這種狀況下，操作員會(huì)有種感覺假如檢波電路或者示波管能夠延長時(shí)間那么他就能下結(jié)論該信號(hào)是否存在。沃爾德（1950）在他的檢波理論系統(tǒng)闡述了這種直覺。假如（一個(gè)檢測(cè)檢測(cè)方法）能夠主動(dòng)去轉(zhuǎn)變時(shí)間間隔并在接收系統(tǒng)里儲(chǔ)存數(shù)據(jù)以便達(dá)到確定的某一水平，這就是挨次檢測(cè)。

一般不僅能夠確定信號(hào)存在與否，而且還能確定一個(gè)或多個(gè)與信號(hào)關(guān)聯(lián)的參數(shù)。在還原完整波形時(shí)我們所感愛好的參數(shù)在各簡(jiǎn)潔重量間有很大差別，例如信號(hào)的到達(dá)時(shí)間和相位。

當(dāng)一個(gè)系統(tǒng)被設(shè)計(jì)來提取一個(gè)或多個(gè)信號(hào)參數(shù)時(shí)，這就是信號(hào)抽取。

信號(hào)一詞并沒有明確的定義，只是在讀者對(duì)它有直觀了解時(shí)的一種假設(shè)。有些較為詳

細(xì)的解釋為了對(duì)信號(hào)一詞進(jìn)定義可能導(dǎo)致是比較主觀的或者狹隘與所應(yīng)用的條件。或許你會(huì)說信號(hào)就是你想觀看到的而噪聲就是對(duì)觀看者產(chǎn)生干擾的信號(hào)。但是，一個(gè)漁民在用聲納設(shè)備搜尋海洋時(shí)，四周用來追蹤潛艇的聲納干擾導(dǎo)致的信號(hào)削減經(jīng)常會(huì)使他欣喜若狂。毫不夸張地說，一個(gè)人的