celestial navigation4

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1、celestialnavigationCelestialnavigationfordummies-page5Traditionalmethod:linesofposition(LOPs)andinterceptshttp://www.seamanship.co.uk/deck/navigator/ASNAv/ASNAv%20Site/Text/celnav5.htm(1of4)5/4/20055:01:24PMcelestialnavigationThismethodwasinventedin1875bythe

2、admiralMarcqdeSaint-Hilaire(someothersourcessayY.VillarcauandA.deMagnac).ThetruelineofpositionA,tangenttothecircleofposition,canbemergedintothelineofpositionBbecausetheestimatedpositioneisclosetothetruepositionO.OnthelineofpositionB,theinterceptisthedifferen

3、cebetweenthetrue(observed)altitudeandthecalculatedaltitude:http://www.seamanship.co.uk/deck/navigator/ASNAv/ASNAv%20Site/Text/celnav5.htm(2of4)5/4/20055:01:24PMcelestialnavigationPractically,theprocedureisasfollow:1.findtheestimatedpositionwithanaccuracyof50

4、nauticalmiles(inordertogetafixwith1nauticalmilemaximumerrorduetothemethoditself);2.observewiththesextantastaraltitudeHsatthetimeC(GMT);3.correctthesextantaltitudeHswiththeinstrumentalerror,thedipoftheapparenthorizon,theterrestrialrefraction,theastronomicrefr

5、action,theparallax,thesemi-diameterofthestar(ifneeded)togetHo(observedaltitude);4.computetheazimuthofthestarusingtheestimatedpositionandthedata'softheNauticalAlmanacatthetimeC;5.computethecalculatedaltitudeHc;6.computetheinterceptITC=Ho-Hc;7.plotthelineofbea

6、ring(azimuthofthestar)fromtheestimatedposition;8.plotthelineofpositionperpendiculartothelineofbearing,atadistanceITCfromtheestimatedposition,awayifHc>Ho;9.startoveragainthesteps2-8,atleastonce,togetthedrawingbelow:http://www.seamanship.co.uk/deck/navigator/A

7、SNAv/ASNAv%20Site/Text/celnav5.htm(3of4)5/4/20055:01:24PMcelestialnavigationOistheobservertrue(astronomic)position.Note:ITCisusedinthistextasforeshorteningforInTerCeptandisnotanabbreviation.Don'tconfusetheinterceptITCwithITP-theInterceptTerminalPoint.ITPisth

8、epointthroughwhichthecircleofpositionpasses.TheLOPistangenttothecircleofpositionatthispoint.PreviousASNAvNextGobacktotheASNAvhomepageThisWebSiteandallcontentsarecopyright©1999-2004bySeamanshipIn