測控技術(shù)與儀器專業(yè)英語翻譯

本文格式為Word版下載后可任意編輯和復(fù)制第第頁測控技術(shù)與儀器專業(yè)英語翻譯

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

在聲納操作過程中，操作員常常需要對受噪聲干擾的信號進(jìn)行檢波。干擾信號可能來自操作員發(fā)出信號的反射波或者外部聲源的信號。這兩種類型的干擾對主動聲納和被動聲納都會造成很大影響。類似的狀況在雷達(dá)監(jiān)測、工程類和圖像類專業(yè)的基本原理都會涉及到。當(dāng)你想到信號檢測時有多種方法，那么定義一個術(shù)語來表示特別狀況便是可行的。當(dāng)問題的答案是“當(dāng)前有一個還是一個多個信號？”時，檢波一詞將被使用。一個系統(tǒng)被設(shè)計來為這種問題供應(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)一個信號反復(fù)消失的狀況下，很多系統(tǒng)只被設(shè)計供應(yīng)兩個答案：“是的，當(dāng)前有一個信號”或“不，當(dāng)前沒有信號”。力圖將信號分類會使問題簡單化，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)計在固定的類型和參數(shù)下工作，不簡單被轉(zhuǎn)變，例如時間積分檢波電路和光學(xué)檢測的輝光示波管。當(dāng)消失不明信號時，觀看者在拒絕或接收信號方面有所遲疑。在這種狀況下，操作員會有種感覺假如檢波電路或者示波管能夠延長時間那么他就能下結(jié)論該信號是否存在。沃爾德（1950）在他的檢波理論系統(tǒng)闡述了這種直覺。假如（一個檢測檢測方法）能夠主動去轉(zhuǎn)變時間間隔并在接收系統(tǒng)里儲存數(shù)據(jù)以便達(dá)到確定的某一水平，這就是挨次檢測。

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

當(dāng)一個系統(tǒng)被設(shè)計來提取一個或多個信號參數(shù)時，這就是信號抽取。

信號一詞并沒有明確的定義，只是在讀者對它有直觀了解時的一種假設(shè)。有些較為詳

細(xì)的解釋為了對信號一詞進(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。或許你會說信號就是你想觀看到的而噪聲就是對觀看者產(chǎn)生干擾的信號。但是，一個漁民在用聲納設(shè)備搜尋海洋時，四周用來追蹤潛艇的聲納干擾導(dǎo)致的信號削減經(jīng)常會使他欣喜若狂。毫不夸張地說，一個人的