资源描述:
《gis专业英语global positioning system-chapter 3》由会员上传分享,免费在线阅读,更多相关内容在工程资料-天天文库。
1、Lesson1GlobalPositioningSystemGlobalPositioningSystemisanetworkofsatellitesthatcontinuouslytransmitcodedinformation,andmakesitpossibletopreciselyidentifylocationsonearthbymeasuringdistancefromthesatellites.Asstatedinthedefinitionabove,GPSstandsforGlobalPositioningSystem,andreferstoagroupofU.S.Depa
2、rtmentofDefensesatellitesconstantlycirclingtheearth.ThesatellitetransmitverylowpowerradiosignalsallowinganyonewithaGPSreceivertodeterminetheirlocationonearth.Thisremarkablesystemwasnotcheaptobuild,costingtheU.S.billionsofdollars.Ongoingmaintenance,includingthelaunchofreplacementsatellites,addstoth
3、ecostofthesystem.Amazingly,GPSactuallypredatestheintroductionofthecomputer.Itsdesignersmaynotforeseenadaywhenwewouldbecarryingsmallportablereceivers,weighinglessthanapound,thatwouldnotonlytelluswhereweareinpositioncoordinates(latitude/longitude),butwouldevendisplayourlocationonanelectronicmapalong
4、withcities,streetsandmore.GPSisoneofhistory’smostexcitingandrevolutionarydevelopments,andnewusesforitareconstantlybeingdiscovered.ButbeforewelearnmoreaboutGPS,it’simportanttounderstandabitmoreaboutnavigation.NavigationSinceprehistorictimes,peoplehavebeentryingtofigureoutareliablewaytotellwherethey
5、areandtohelpguidethemtowheretheyaregoing.Cavemenprobablyusedstonesandtwigstomarkatrailwhentheysetouthuntingforfood.Theearliestmarinersfollowedthecoastcloselytokeepfromgettinglost.Whennavigatorsfirstsailedintotheopenocean,theydiscoveredtheycouldcharttheircoursebyfollowingthestars.TheancientPhoenici
6、ansusedtheNorthStartojourneyfromEgyptandCrete.AccordingtoHomer,thegoddessAthenatoldOdysseusto“keeptheGreatBearonhisleft”duringhistravelsfromCalypso’sIsland.Unfortunately,thestarsareonlyvisibleatnight—andonlyonclearnights.Thenextmajordevelopmentsinthequestfortheperfectmethodofnavigationwerethemagne
7、ticcompassandthesextant.Theneedleofacompassalwayspointsnorth,soitisalwayspossibletoknowinwhatdirectionyouaregoing.Thesextantusesadjustablemirrorstomeasuretheexactangleofthestars,moon,andsunabovethehorizon.However