测绘工程专业英语：Section 6 Determination of True Bearing by Gyro-thodolite

Section6DeterminationofTrueBearingbyGyro-thodoliteThegyroscopehasbeenusedinnavigationasanorth-seekingdeviceforaconsiderableperiodoftimeandcertainmanufacturersofsurveyinginstrumentsarenowproducingunitswhichallowthedirectestablishmentofthemeridianbytheodolitewithouttheneedforcalculationsbasedonastronomicalobservations.ThedevelopmentofthegyroscopefortheprecisetransferofbearingsundergroundinminingsurveyingdatesfromaboutthebeginningoftheFirstWorldWar,butaninstrumentcapableofregisteringbearingstowithinoneminuteofarcdidnotappearuntilaftertheendoftheSecondWorldWar.Eventhenitwasofconsiderableweight,butadvancesinthedesignofgyroscopesforairborneinertialnavigationsystemshaveallowedmuchmorecompactandportableunits.ForinstancetheWildGAK1,describedlater,hasatotalmassofabout13.6kg,anditsaccuracyisofthesameorderasthatobtainedinsunobservations.Inthisinstrumentarotororspinnerisdrivenatspeedsinexcessof20000rev/min,theaxisofspinbeinginahorizontal-plane.Figure1showstheearthrotatingaboutitsaxiswithanangularvelocityofω;ataplaceinlatitudeφanangularvelocityofωcosφobtainsaboutthemeridian.Thespinnerofagyroscopesetupatthispointwilltrytomaintainitsinitialspatialpositionprovidedthatitsangularmomentum,Iω,islarge,buttherotationoftheearthitselfpullsthespinneroutofthisplane.Thereisaconsequentreactionintheformofarotation,orprecession,aboutthevertical(oroutput)axisofthespinnerwhichholdsuntilthespinaxisliesinthemeridianattheplace.Figure2(a)showsthethreeaxesofthespinnerandFig.2(b)showsthespinaxisatanangleofatothemeridian.Theearth'srotationcausesaninterferencetorque(ME)equaltolωωcosφ,andaconsequentprecessionalcoupleofMEsinaisinducedwhichforcesthespinnerintothemeridianwithanangularrateofωcosφsina.Whenthespinnerissuspended(asitisintheGAK1attachment)itwillbeseenthatthemaximumprecessionalcoupleoccurswhenφiszero,i.e.attheequator,andthatthegyrowillfloatfreelyatthepoleswherecosφiszero.Theprecessinggyrodoesnotalignimmediatelyonthemeridianbutoscillatesaboutit.ThiscanbeplacedoninstrumentssuchasWildTIA,T16orT2theodoliteswhichmustbesuitablymodified,anditisthenconnectedtoacontrolunitorconverter.Theoscillatingsystem(Fig.3(a)),containingthespinnerandopticalsystemwithanindexmarkiscarriedbyatapesuspendedverticallywithinthe'chimney'ofthesupportingcaseandfastenedatthetop.Theconvertercontainsanickel-cadmiumbatteryandanelectronicunit;thebatteryisconnectedbyleadstoasix-wayadaptoronthesupportingcase,andiscontrolledbyaselectorswitch.Afurtherswitchindicating'run'and'brake'isprovidedwithtwoindicators,oneofwhich,'measure',showsagreencolourwhenthespinnerisrevolvingatitsworkingspeed.Whensettingup,thetheodolitetelescopeshouldberoughlypointedtowardsnorthbymeansof,say,prismaticcompassortubularcompassobservations,andthegyroattachmentcanthenbefastenedonthetheodolitewiththetelescopeintheface-leftposition.Thecablefromtheconverterunitisattachedandtheobserverchecksthatthegyroisclampedbymeansofadeviceunderneaththesupportingcase.Thebatteryswitchissetto'on'andwhentheotherswitchissetto'run'the'wait'indicatorremainsredforashortperiodoftimebeforethegreencolorshowsat'measure',indicatingthatthespinnerisnowrunningatitscorrectoperationalspeedandthatitcanbeundamped.Asthespinneroscillatesaboutthemeridianthegyroindexmarkcanbeseenmovingacrossanauxiliaryscaleinanobservationtube,andwhenthespinaxisandthelineofsightofthetheodoliteareinthesameverticalplanethatmarkshouldbecentredinthemiddleofthescale,whichisalsomarkedbyaV-shapedindex(Fig.3(b)).Thusifthemid-positionoftheoscillationsisestablished,thetelescopelineofsightatthestationisorientedtowardsthenorth,andthepurposeofallobservationsistodeterminethatposition.Acalibrationconstant(E),whichisthehorizontalanglebetweenthedirectionofthelineofsightinthemid-positionofoscillationandthemeridianatastation,musthoweverbedeterminedforeachinstrumentatfrequentintervalsbycomparingthebearing,AZ,determinedbyastronomicalobservationswiththat,AG,determinedbythegyrotheodolite,i.e.E=AZ-AG.Thecalibrationconstantsofgyrotheodolitescanshowasystematicdriftcausedbyunrequiredtorqueonthegimbalsystemwhich,inthecaseofthesuspendedspinner,isthetapeandthebearingofthespinaxis.Chrzanowskihassuggestedinanarticle'Newtechniquesinmineorientationsurveys',CanadianSurveyor24(I)1970,thatchecksbemadeimmediatelybeforeandaftertheundergroundworkandonthesameday.Theformofoscillationissinusoidalandinthemiddleofanoscillationtheindexmarkismovingatmaximumspeedintherelevantfieldofviewbutitslowsdownquitemarkedlyastheturningpointsareapproachedandforaveryshortperiodisstationaryatsuchpoints.IfthegyroindexmarkhasbeenkeptcentredwithintheV-shapedindexbyusingthehorizontalmotionofthetheodolitealidade,thecorrespondinghorizontalcirclereadingofthetheodolitecanbeobtainedforthisturningpoint.Variousmethodsoforientationareavailableincludingquickmethodsforpre-orientationbeforetheadoptionofamoreprecisemethod.Inoneofthequickmethodsthelineofsightshouldbesetwithin15°oftheestimatedmeridianandbyfollowingupthegyroindexmark,withtheV-shapedindexincoincidence,horizontalcirclereadingsareobtainedfortwosuccessiveturningpointsonoppositesidesofthemeridian.Themeanhorizontalcirclereadinggivestheapproximatemeridianandthelineofsightofthetelescopecanthereforebesoestablished.Itisessentialthatthetwoindexmarksbekeptincoincidencesothatthesuspensiontapeisheldfreefromtorsion.Someinitialpracticeisrequiredforthisandtofacilitatetheworksomeinstrumentsareprovidedwithawider-rangehorizontalslow-motionscrew.Anaccuracyoftheorderof±3'isquotedbyMessrsWildforthismethod.Themethodsdevisedforthepreciselocationofthecenterofoscillation,canbeclassifiedintotrackingandnon-trackingmethods.Oneoftheformergroupisanextensionoftheabovequickmethodinthatseveralsuccessiveturningpointsareobserved,ensuringthatthecoincidencepreviouslymentionedismaintained.Thecorrespondinghorizontalcirclereadingshavetobeobtainedveryquicklysincethegyroindexmark,relatingtothespinaxis,isonlystationaryforaveryshortperiodwhenitalsohastobecentredaccuratelyonthezerodivisionofthescale.Knowingthecirclereadingsforpointingsonthereferringobject(M,meanofreadingsontwofaces)andthemeridian(N),thetwoarerelatedtogivethegeographicalazimuthas(M-N+E).Fourtosixturningpointsareobservedforhighestaccuracy.NewWordsandExpressionsbearing：象限角，方位[角]，方向角；矿层走向；轴承，支承gyroscope：陀螺仪，回转装置navigation：导航north-seekingdevice：指北设备meridian：子午线，经线，子午圈astronomicalobservation：天文观测inertialnavigationsystem：惯性导航系统sunobserv