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 Intheoperationofasonarsystemtheoperatorisrepeatedlyfacedwiththeproble mofdetectingasignalwhichisobscuredbynoise.Thissignalmaybeanechoresu ltingfromatransmittedsignaloverwhichtheoperatorhassomecontrol,oritmay haveitsorigininsomeexternalsource.Thesetwomodesofoperationariseinrad arsurveillanceandindisciplinesfortechniquesandforillustrationsofthebasicp rinciples. sincetherearemanywaysinwhichonecanthinkaboutsignaldetection,itisdesir abletodefineatermtodenotespecialcases.Theworddetectionwillbeusedwhen thequestiontobeansweredis,?Areoneormoresignalspresent??whenthesyste misdesignedtoprovideanansweredtothisquestion,eitherdeterministicorprob abilistic,onespeaksofhypothesistesting.Thecaseofasinglesignaloccurssooft enthatmanysystemaredesignedtoprovideonlytwoanswers,?Yes,asignalispr esent,?or?no,thereisnosignal.?onecanmaketheproblemmorecomplicatedby endeavoringtoclassifythesignalintocategories.Decisionsofthislatterkindwil lbereferredtoastarget

classification.

normallyapieceofdetectionequipmentisdesignedtooperateinafixedmodean dtheparameterssuchasintegratingtimeofrectifiercircuitsorpersistenceoftheo scilloscopetubeforvisualdetectioncannotbechangedreadily.Therewillalway sbesomeuncertainsignals,whichtheobserverwillbehesitanttorejectoraccept. Inthesecasestheoperatormighthavethefeelingthatiftheintegratingtimeofthe detectororthepersistenceoftheoscilloscopetubewerelonger,hecouldreachad ecisionabouttheexistenceofthesignal.wald(1950)hasformulatedthisintuitiv efeelingintoatheoryofdetection.whenoneisabletovarydeliberatelytheinterv aloverwhichonestoresdatainthe receptionsysteminordertoachieveacertainlevelofcertainty,onespeaksofsequ entialdetection.Frequentlyitisdesirabletodeterminenotonlythepresenceorab senceofthesignalbutalsooneormoreparametersassociatedwiththesignal.The parametersofinterestcanvarywidelyfromasimplequantitysuchastimeofarriv alortargetbearingtotherecoveryofthecompletewaveform.whenasystemisde signedtorecoveroneormoreparametersassociatedwiththesignal,onespeakso fsignalextraction. Thewordsignalwasnotdefinedanditwasassumedthatthereaderhadanintuitiv efellingfortheword.someelaborationmaybeinordersincethedefinitionofsign alsubjectiveanddependsontheapplication.onemaysaythat?signal?iswhaton ewantstoobserveandnoiseisanythingthatobscurestheobservation.Thus,atun afishermanwhoissearchingtheoceanwiththeaidofsonarequipmentwillbeove rjoyedwithsoundsthatareimpairingtheperformanceofanearbysonarsysteme

ngagedintrackingasubmarine.Quiteliterally,oneman?ssignalisanotherman? snoise.

signalscomeinallshapesandforms.Inactivesonarsystemonemayusesimplesi nusoidalsignalsoffixeddurationandmodulationsthereof.Thereareimpulsive signalssuchasthosemadewith

explosionsorthumpers.Attheotherextremeonemaymakeuseofpseudorando msignals.Inpassivesystems,thesignalswhosedetectionissoughtmaybenoisei ntheconventionalmeaningoftheword;noiseproducedbypropellersorunderw aterswimmers,forexample.Itshouldbeevidentthatoneofourproblemwillbeth eformulationofmathematicaltechniquesthatcanbeusedtodescribethesignal. Althoughthesourceinanactivesonarsearchsystemmaybedesignedtotransmit asignalknownshape,thereisnoguaranteethatthereturnsignalwhosedetectioni ssoughtwillbesimilar.Infact,therearemanyfactorstochangethesignal.Theam plitudelossassociatedwithinversespherical spreadingismostunfortunateforthedetectionsystemnutitdoesnotentailanydi stortionofthewaveshape.(Incidentally,wherethewavecanbeapproximatedlo callyasaplanewave.)Theacousticmediumhasanattenuationfactor,whichdep endsonthefrequency.Thisproducesaslightdistortionofthewaveshapeandaco rrespondingchangeintheenergyspectrumofthepulse.Themajorchangesinthe waveformresultfromacousticboundariesandinhomogeneitiesinthemedium. whenechoesareproducedbyextendedtargetssuchassubmarines,therearetwo distinctwaysinwhichechostructureisaffected.First,thereistheinterferencebe