GPS是怎样工作的-简单易懂的介绍

HowGPSWorksOutline•WhatisGPS•HowGPSworks•HowGPScodeswork•WhyIuseGPSformyresearchHowdoyouusethesesatellitestocalculateyourposition?TheGlobalPositioningSystemisaconstellationof31satellitesthatisusedtocalculateyourposition.Insteadofsatellites,letsusetransmittersontheground.GrandJunctionsendsasignaltoRadon’sGPS.Whatkindofsignal?GrandJunctionTransmitterRadoninBoulderitputsthetimeonthesignal.Forthistowork,we’llneedforboththetransmitterandRadontohaveclocks.GPSWhenRadon’sGPSreceivergetsthesignal,hecomparesthetimeonthesignalwiththetimeonhisclock.So,aGPSsignaltellsyouhowfaryouarefromthetransmitter.TimeDifference(inseconds)*2.99792458108meters/second=Distance(inmeters)IfthedistancefromtheGPStransmitteris250miles,thatmeansyouaresomewhereonacircleofradius250miles.Nowadda2ndtransmitterinFt.Collins.AndathirdtransmitterinPuebloRadonisattheintersectionofthe3circlesThisonlyworksif:•Youknowwherethetransmittersare.•GPSsignalsalsotransmitthesatellitelocations.•Everyonehasgoodclocks.•TheGPSsatelliteshaveverygoodclocks.AGPSusercanusea4thsignaltopiggy-backontotheGPSsatelliteclocks.•Andyoucantellthetransmittersapart.•Thesignalsaremadeinawaysothatyoucantellwhichtransmittersentthem.•Forrealproblems,weusetheintersectionofthreespheres,notthreecircles.IntersectingSpheresButonly1pointisontheEarthWhenGPSreceivesasignal•Itcomparesthatsignalwithalltheknowncodes(therearecurrently37).•Thereceiverdetermineswhichsatelliteitis.•Itdecodesthetiminginformation,multipliesbythespeedoflighttofindtheradiusofthesphere.•Onceithasdonethatfor3satellites,itcandeterminethelocation.HowdoGPSsignalssendallthisinformation?•Theyusecodes!Binarycodes.•Eachsatellitehasadifferentcode.000010001010011100001110010010001000010001010110001111011100101011011001111010110010110010100110011111101100111100100110011010001110001001000101100010110111000000110110010001000101101000101001000000011111000110001011111011111100110111001011011110001111110101001010000101010011100001101001110110001111011111000011111111110100100100100110011101010111110000100010110100111111000010011011110011100011011010110110101000010110100101000101001000111001110001010010111010111010101000001011011100110110011010000000000011101110110001101101010101101100011100011001100111110101111100111010101000001111110010010100000011101000111101101001011011000001001001001100001101100001111011101110001101110110100111001000110101010000110110100101110010111111111011000111000000110111000110000001000000001000001101010001010111101100011101101000110010101111100111101000000011011110011001110101111000001111011001000100101011100000000100001010101001111101100111011011111100101111000100110101Forexample,herearethefirst1000numbersofthecodeforsatellite1100111101110100010011011111111101111001011011110110011011111010101000111110110001100010011001101000010000010111111100001000011010110101110101101001100000110100001100010101011001000100100000110000011110000111010000011100100111011000000010110011110001001010101111101010011110010111110110010100010111000010011100001111101110101110101101100111100100110101110010001101101111101100110101110000111010111000110001111000001000111011011100010000011010011001001110000100010111000100100011011111000111010101001100000000110011110011101010000100100011100101010100111001011011111001111111001101001110110011101100101001010011001010111011100111000110111100110000010100011110011011110110011110100110111010011100110101010110100000101110001110001110101100011110001001010011101010110000110001000110010101110011000011111000001111100010010001101000101000101001001000110000110010000011000110001010000110110010110100110011000101101110011110010001010010100011110011101100001111101100101Thisisthecodeforsatellite6Strategy•FirstweneedtolearnhowGPScreatesthesecodes•Thenweneedtocomeupwithawaytoquicklytellthecodesapart.Howdoyoucreatecodes?•Youusebinaryadditionrules.•0+0=0•1+0=1•0+1=1•1+1=10(butonlyusethelastbit,0)•GPSuses“shiftregisters.”•Themoreshiftregistersyouhave,themorecomplicatedyoucanmakeyourcode.Register1Register2Register3Code111-Startwithall1’sinyourshiftregistersAddRegister1andRegister3Theanswer0goesintoRegister1andeverythingshiftstotheright.Hereisanexamplewith3shiftregistersForthisexample,1+1=10==0ResultinginRegister1Register2Register3Code111-0111Next0+1=1Register1Register2Register3Code111-01111011After2N-1steps(Nisthenumberofregisters),thecoderepeatsRegister1Register2Register3Code111-0111101101010010100111001110For3shiftregisters,thecoderepeatsafter7steps.RealGPS•Uses10shiftregisters.•Theyadddifferentregisterstoproducethecodesfordifferentsatellites.•Satellite1uses2and6.•Satellite2uses3and7,andsoon.•A10-shiftregistercoderepeatsafter210-1,or1023.Howdoyoucomparecodes?100111101110100010011011111111101000010001010011100001110010010001Everytimethenumbersagree,add1.Everytimethenumbersdisagree,subtract1.Thisexample:2differentsatellites10011110111010001001101111111111100001000101001110000111001001000114agree11disagreeTotalscore:3Perfectagreementwouldbe35011000101010110010001001000001100000111100001100010101011001000100100000110000