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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
