1 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Chng 1:T VN 1.1 - t vn v tnh hnh nghin cu hin nay: H thng nh v ton cu GPS (Global Positioning System) l h thng xc nh v tr da vo cc v tinh nhn to gm 24 v tinh ch o v mt vi v tinh d tr c phn b quanh trit trn 6 quo gn trn vi ng knh 20138 kmvcgcnghing550sovimtphngxcho.mbobaophkhp mi ni trn tri t, cc v tinh GPS c sp xp sao cho mi qu o c t nht 4 v tinh phn b u.V mt hnh hc, bt k im no trn tri t cng thy c t 4 n 10 v tinh trong chm sao v tinh GPS. Cc v tinh GPS bay vng quanh trithailntrongmtngy(michukquomtkhong11gi58pht) theo mt qu o rt chnh xc v pht tn hiu c thng tin xung tri t. Cc my thu GPS nhn thng tin ny v bng php tnh lng gic tnh c v tr my thu ca ngi dng. TnhiumccvtinhGPSphtxungmythucangidnggm2 sngsin(cncgiltnssngmang),2mCA(CoarseAcquisition)vP (Precision) [mi v tinh s pht mt m khc nhau] v bn tin nh v. Ta ca cc v tinh GPS nm trong bn tin nh v. Khong cch t my thu ca ngi dng ntngvtinhGPScxcnhdavotnssngmangvmCAhayP tng ng ca v tinh (xc nh khong thi gian t lc pht tn hiu ti v tinh n lcnhn c tnhiu GPS timy thuca ngidng,da vo vn tc nh sng ta s tnh c qung ng t my thu ca ngi dng n v tinh pht ra mCAhayPtngng).Khibitctacavtinhvkhongccht my thu ca ngi dng n v tinh , da vo hnh hc ta ch cn 3 v tinh l xc nh c ta my thu ca ngi dng. Doxungclocktinipht(vtinh)vnithu(mythucangidng) khng ng b nn c sai s thigian tr, dn n sai skhong cch t my thu ca ngi dng n v tinh, do v tr my thu ca ngi dng cng sai theo. V 2 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit vy m h s sai s ng h c a ra v y l l do ti sao phi cn n v tinh th 4. Ngoi ra cn c nhiu ngun sai s khc nh sai s ch nh SA (Selective Availability)nhmngnngavictnhvchnhxcthigianthccacc thu bao tri php, sai s qu o v tinh, sai s do truyn dn a ng, sai s tr khi tn hiu truyn qua tng in ly v tng i lu, sai s do my thu. Bn cnh cc ngun sai s trn, vn truyn dn trong mi trng mt ngi s dng cao rt d dn n hin tng fading v can nhiu ti b thu ca ngi dng. Vn t ra l tm nhng gii php hn ch ngun can nhiu, fading v hng lot cc ngun sai s khc. Ta khng th can thip vo v tinh gim ngun sai s ti c. Ngun sai s do c kch hot SA, sai s qu o v tinh, sai s ng h v tinh s c loi b hon ton bng k thut DGPS (Differential GPS). Ngun sai s tr khi tn hiu truyn qua tng in ly v tng i lu cng c hn ch bng kthut DGPS. Ngun sai sdo truyn dn ang, can nhiu v vn truyn dn ti u cng c hn ch bi anten thng minh. Nhim v n mn hc l nghin cu tm hiu v h thng GPS; ng thi tm gii php m bo vic truyn sng, truyn dn ti u v hn ch nh hng ca sai s. n mn hc cng tm hiu cc gii php v k thut DGPS,gii thut Unconstrained LMS Beamforming bng hp trong b to bp s dng tn hiu tham khocttitrmthamkho(hoctibthucangisdng)nhmbo m truyn dn ti u gia trm tham kho v cc b thu ca ngi s dng. 1.2 - Mc ch, i tng v phm vi nghin cu: Trong phm vi n mn hc s c gng tm hiu v h thng nh v ton cu GPS, k thut DGPS, thut ton LMS v anten thng minh. 1.3 - B cc ca ti: n mn hc c chia lm 5 chng chnh bao gm: Chng 1: t vn . 3 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit - t vn v nu ra mc ch cng nh ngha ca ti. Chng 2: C s l thuyt. - H thng nh v ton cu GPS. - K thut DGPS. - Thut ton LMS. - Anten thng minh. - B to bp thch nghi LMS bng hp. - Li ch ca anten thng minh. Chng 3:ng dng anten thng minh trong h thng GPS v DGPS. Chng 4: Kt qu m phng. - Kt qu m phng gii thut LMS. - Kt qu m phng gii thut LMS trong khng gian 2D. - Kt qu m phng gii thut Unconstrained LMS Beamforming bng hp. - Kt qu m phng gii thut Unconstrained LMS Beamforming bng hp trong b to bp s dng tn hiu tham kho cho dy anten ULA c N=20 phn t trong khng gian 2D. Chng 5: Kt lun v hng pht trin ti. - Chng ny a ra mt s kt lun v hng pht trin ca n. 1.4 - ngha ca ti: titmhiuvnghincuvhthngnhvtoncuGPS,kthut DGPS, thut ton LMS v anten thng minh. 4 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Chng 2:C S L THUYT 2.1- Tng quan v h thng nh v ton cu GPS: 2.1.1- Gii thiu h thng GPS: H thng nh v ton cu GPS (Global Positioning System) l h thng xc nh v tr da vo cc v tinh nhn to. c thit k v qun l t u nhng nm 1970biBQucPhngHoaK,banuGPSchdnhchoccmcchqun s, nhng t nm 1980 chnh ph Hoa K cho php phc v cho c mc ch qun s ln dn s. H thng GPS hot ng t nm 1993 cung cp thng tin nh v lin tc 24 gi mt ngy mi ni trn th gii di bt k iu kin thi tit no. Cc vtinhGPSbayvngquanhtrithailntrongmtngy(michukquo mtkhong11gi58pht)theomtquortchnhxcvphttnhiuc thngtinxungtrit.CcbthuGPSnhnthngtinnyvbngphptnh lng gic tnh c chnh xc v tr ca mnh. Hnh 2.1:Chm sao v tinh GPS H thng GPS gm mt chm sao 24 v tinh ch o v mt vi v tinh d trcphnbtrn6quogntrnvingknh20138kmvcgc nghing 55o so vi mt phng xch o. m bo bao ph khp mi ni trn tri t, cc v tinh GPS c sp xp sao cho mi qu o c 4 v tinh phn b u. V mt hnh hc, bt k im no trn tri t cng thy c t 4 n 10 v tinh trong chm sao v tinh GPS. 5 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Ccvtinhcnuibngnnglngmttrivcccngunpind phng duy tr hot ng khi chy khut vo vng khng c nh sng. Cc tn la nh gn mi qu v tinh gi chng bay ng qu o nh. 2.1.2- Cc mng ca h thng GPS: HthngGPScchiathnh3mngchnh:mngkhnggian(space segment),mngiukhin(controlsegment)vmngngisdng(user segment) Hnh 2.2: Cc mng ca h thng GPS. Mng khng gian lchm sao 24v tinh,mi v tinh pht tn hiu gm 2 sng sin (c gi l cc tn s sng mang), 2 m s v bn tin nh v. Dng cc sngmangvmxcnhkhongcchtbthuGPScangisdngn cc v tinh GPS. Bn tin nh v cha ta ca cc v tinh l hm thi gian. Cc tnhiuphttvtinhciukhinbinghintcchnhxccao trn v tinh. 6 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit MngiukhingmmttrmiukhinchMCS(MasterControl Station), 5 trm monitor v 3 trm iu khin mt t. Trm MCS t gn Colorado Springs(Colorado,M)cnhimvgimsttonbhthngGPS.5trm monitor Colorado Springs, Hawaii, Kwajalein, Diego Garcia v Ascension Island c trang b bng cc b thu GPS cht lng cao v b to dao ng cesium nhm mc ch theo vt (tracking) lin tc tt c cc v tinh GPS. Ring 3 trm monitor Kwajalein,DiegoGarciavAscensionIslandctrangbbngccantennamt t ti thng tin ln cc v tinh GPS. Tt c cc trm monitor v trm iu khin mt t c iu khin t xa bi MCS.Nhim v chnh ca mng iu khin l xc nh v d bo cc d liu nh v v tinh. Cc trm monitor theo vt lin tc cc v tinh GPS v pht cc gim st thu thp c n MCS x l. Kt qu x l gm cc v tr v tinh l hm thi gian, tnh trng h thng, cc thng s ng h v tinh,dliupsut,ninlchvtinh,...DliunhvtinycMCSgi n1trongcctrmiukhinmtttrmiukhinmtttilnccv tinh GPS qua di bng tn S. Mng ngisdngbao gm tt c ccmythu GPS qun s vdn s. KhibthuGPScktniviantennaGPS,ccmythuGPSstnhtonv bit c v tr ca mnh trong khng gian 3 chiu. C 2 loi dch v m B Quc Phng M a vo cc ng dng dn s l dch v nh v c bn SPS (Standard Positioning Service) v dch v nh v chnh xc PPS (PrecisePositioningService).TrongdchvSPScchnhxctithiu khong 100m cho chiu ngang v 156m cho chiu cao, cn dch v PPS c chnh xc khong 10m cho chiu ngang v 15m cho chiu cao. Dch v SPS c chnh xc thp hn dch v PPS l do tn hiu GPS s dng dch v SPS c trn thm mt s sai s gi l sai s SA (Selective Available). Sau ngy 1/5/2000, B Quc Phng M quyt nh khng s dng sai s SA na v chnh xc ca 2 dch SPS v PPS khng cn cch nhau qu xa. 7 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit 2.1.3- Cc th h v tinh ca h thng GPS: *Th h v tinh th nht: gm 11 v tinh Block I c phng ln u tin vongy22/12/1978vlncuicngvongy9/10/1985vimcchthc nghim.Mtphngquocanhngvtinhnynghin63osovimtphng xcho.Dtuithcaccvtinhnycthitk4.5nm,nhngmtsv tinh hot ng hn 10 nm. V tinh Block I cui cng c rt ra khi qu ovo ngy 18/11/1995. *Thhvtinhthhai:gm28vtinhBlockIIhayBlockIIAc phngvoquotthng2/1989nthng11/1997(BlockIIAlversionci tin t Block II).nnm 2002 cn23 vtinh trong qu o.Mtphng quo ca nhng v tinh ny nghing 55o so vi mt phng xch o.Tui th thit k ca cc v tinh ny l 7.5 nm nhng hu ht u hot ng vt qu thi hn. *Thhvtinhmi:gm6vtinhBLOCKIIRcphngvoqu o vo thng 7/2001.Cc v tinh Block IIRc thitk tng thch vicc v tinhBlockII/IIAvctuiththitkl10nm.Ngoichnhxccaohn mong i, cc v tinh Block IIR cn c kh nng t hot ng t nht 180 ngy m khngcnshiuchnhmtthaykhngbgimchnhxc.Stnhv ca th h v tinh nyc c l nh vo kh nng sp xp ca vtinh vi nhau. Ngoi ra, lch thin vn d bo trc v d liu clock i vi chu k 210 ngy c ti ln v tinh bi mng iu khin nm mt t h tr cho vic t nh v.Nm 2003 a vo quo thm 12 vtinhBlock IIR thng qua chng trnhimi GPS. * Th h v tinh tip theo Block IIR l Block IIF gm 33 v tinh. Tui th ca Block IIF c thit k 15 nm.Cc v tinh nyc kh nngmi quachng trnhimiGPSnhmcithinchnhxctnhvGPS.VtinhBlockIIF u tin c phng vo nm 2005. ChmsaovtinhGPSvothng7/2001ctngcng29vtinhgm5vtinh BlockII,18vtinhBlockIIAv6vtinhBlockIIR(vtqu5vtinhsovi 8 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit chm sao v tinh thng thng l 24). C 4 ng h in t trang b trn cc v tinh BlockII/IIA:2nghintloicesium(Cs)v2nghintloi rubidium(Rb).nghcesiumlngunnhgichnhiukhintnhiu GPS.Cc v tinh Block IIR ch dng ng h loi rubidium.Cc th h v tinh GPS c minh ha hnh 2.3. Hnh 2.3: Cc th h v tinh GPS. Hin nay trn th gii c ba h thng v tinh dn ng. GPS v GLONASS ang hot ng, GALILEO hon thnh vo nm 2008. C ba h thng nh v ton cu ngynaycgitnchungl Hthngvtinhdnngtoncu (GNSS, Global NavigationSatellite System). Phn nys tm lcmt s thng tin v ba h thng v tinh nhn to: GPS, GLONASS v GALILEO. 2.1.1.1- GPS: Tn gi GPS (Global Positioning System) dng ch h thng nh v ton cudoBqucphngMthitkviuhnh.BQucphngMthnggi GPSlNAVSTARGPS(Navigation Signal Timing and RangingGlobal Positioning System). Mi ngi u c th s dng GPS min ph. V tinh u tin caGPScphngvothng2nm1978,vtinhgnynhtlvtinhGPS IIR-M1 c phng vo thng 12 nm 2005 (Wikipedia, 2006). GPS bao gm 24 v tinh (tnh n nm 1994), c b sung thnh 28 v tinh (vo nm 2000), chuyn ng trong 6mt phng qu o (nghing 55 so vi mt phngxch o) xung quanhtritvibnknh26.560km(Yasuda,2001).Haynicchkhccao 9 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit trungbnhcavtinhGPSsovimttvokhong20.200km(Wikipedia, 2006). 2.1.1.2 - GLONASS: HthngGLONASS(GlobalOrbitingNavigationSatelliteSystem,H thngvtinhdnngquotoncu,tingNga : ;Global'nayaNavigatsionnaya Sputnikovaya Sistema) do Lin bang S vit (c) thit k v iu hnh. V tinh u tincaGLONASScLinXalnquongy12thng10nm1982, vo ngy 24 thng 9 nm1993 h chnh thc c a vo s dng. Ngy nay h thngGLONASSvncCnghoNgatiptcduytrhotng.Hthng GLONASS bao gm 30 v tinh chuyn ng trong ba mt phng qu o (nghing 64.8sovimtphngxcho)xungquanhtritvibnknh25.510km (Yasuda, 2001). 2.1.1.3 - GALILEO: C hai h thng GPS v GLONASS c s dng chnh cho mc ch qun s.i vi nhng ngi s dng dn s c th c sai s ln nu nh c quan iu hnh GPS v GLONASS kch hot b phn gy sai s ch nh, v d nh SA ca GPS. Do vy Lin hp u Chu (EU) ln k hoch thit k v iu hnh mt h thngnhvvtinhmimangtnGALILEO,mangtnnhthinvnhc GALILEO,vimcchsdngdns.Vicnghincudnhthng GALILEO c bt u trin khai thchin t nm 1999 do 4 quc gia Chu u Php,c,ItaliavAnhQuc.GiaionutrinkhaichngtrnhGALILEO bt u nm 2003 v hon thnh v a vo s dng trong nm 2010 (chm hn so vi thi gian d nh ban u 2 nm) (Wikipedia, 2006).GALILEO c thit k gm 30 v tinh chuyn ng trong 3 mt phng qu o (nghing 56 so vi mt phng xch o) xung quanh tri t vi bn knh 29.980 km (Yasuda, 2001). 10 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Hnh 2.4 - Nh thin vn hc Galileo Galilei (1564-1642) Hng mcGPSGLONASSGALILEO S v tinh28 (tnh n 2000)3030 Smtphng qu o 6MEO3MEO3MEO nghing MPQ 55o64.8o56o Bnknhqu o 26.560 km25.510 km29.980 km Chu k11 gi 58 pht 2 giy11 gi 15 pht 40 giy14gi21pht36 giy Tnssng mang L1: 1575.42 MHz L2: 1227.60 MHz L5: 1176.45 MHz G1:1602+Kx0.5625 MHz G2:1246+Kx0.5625 MHz K = 7~24 G2 = G1x7/9 E1: 1589.742 MHz E2: 1561.098 MHz E5: 1202.025 MHz E6: 1278.75 MHz C1: 5019.86 MHz Phng trnhCDMAFDMACDMA Dng m sChui M di m s1023 bit 2.35x1014

511 bit 5110000

N/A 11 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Tcms (C/A L1, P L1, L2) 1.023 Mcps 10.23 Mcps

0.511 Mcps 5.11 Mcps

E1, E2: 2.046 Mcps E5:10.23/1.023 Mcps E6: 20.46 Mcps Thi gian chunUTC (USNO)UTC (Nga)UTC Sai s ch nhSA ( b 2000)Khng cKhng c Thng ip dn ng (navigation messages) EphemerisYu t qu oVtr,tcvgiatc ba chiu - AlmanacYu t qu oYu t qu o- Tctruynd liu L1: BPSK: 50 bps L2: BPSK: 25 bps L5: QPSK: 50 bps

BPSK: 50 bpsQBSK E1, E2, C: 300 bps E5: 330 bps E6: 2500 bps Chu k d liu12 pht 30 giy2 pht 30 giy- nh dng d liu30 bit / t100 bit / string- Dliuhiuchnh in t CKhng c- Bng 1: So snh mt s thng s k thut ca ba h thng v tinh dn ng ton cu GPS. 2.1.4- Cu trc tn hiu GPS: Mi v tinh GPS pht mt tn hiu radio cao tn gm hai tn s sng mang c iu ch bi hai m s v mt bn in dn ng. Hai tn s sng mang ny c pht tn s 1575.42 MHz (gi l sng mang L1) v 1227.60 MHz (gi l sng mang L2). Theo , bc sng tng ng l 19 cm v 24.4 cm; kt qu ny c rt ra t quan h gia tn s sng mang v vn tc nh sng trong khng gian. Vic s dng hai tn s trn cho php sa mt li nghim trng ca GPS l tr tng in ly. Tt c cc v tinh GPS u pht cng tn s sng mang L1 v L2. Tuy nhin, m iu ch l khc nhau cho cc v tinh, vic ny lm gim thiu s can nhiu tn hiu. 12 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Hnh 2.4: Minh ha (a) Sng sin ; (b) m s Haim GPS gi lm thu th ( m C/A) vm thu chnh xc(mP). Mi m cha mt lung s nh phn 0 v 1 gi l cc bit hay cc chip. Cc m ny c gi chung lm PRNv chng ging nh tn hiu ngu nhin. Nhng thc t, cc m ny c pht nh s dng mt thut ton. M C/A ch c i ch vo song mangL1,trongkhi,mPciuchvocsongmangL1vL2.Siu ch ny cgi liu ch lng pha, dopha ca sngmang c dch i 1800 khi gi tr ca m thay i t 0 n 1 hoc t 1 n 0. + M C/A: M C/A c s dng trong cc ng dng dn s v ch c truyn i trn dy bng tn L1. Chui m C/A lp li theo chu k 1ms ca 1 on gm 1023 chip, xut hin vi tn s 1.023MHz. Bc sng ca m C/A khong 300m. Mi v tinh cgn1mC/Aringbitvduynhtnhmchophpccbthucangi dngnhnbittnhiuthunhnclcavtinhno.Hnh2.5minhhacu trcmC/AtrnsngmangL1.Dliunhv50bitmigiy,mibit(20ms) gm 20 chu k m C/A (cn gi l 20 epoch m C/A ), mi chu k m C/A (1ms) l 1 chui 1023 chips, mi chip (0.9775s) gm 1540 chu k sng mang L1 c tn s 1575.42Mhz. Trong epoch ca m C/A chnh l cnh ln ca chip u tin trong mi on 1023 chips. + M P: M P c s dng trong cc ng dng qun s v c truyn i trn c 2 bng tn L1 v L2. Chui m P lp li theo chu k 1 tun ca 1 on gm 6.19x1012 chip,xuthinvitns10.23MHz.BcsngcamPkhong30m.Hnh2.6 13 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit minh ha cu trc m P trn sng mang L1. D liu nh v 50 bit mi giy, mi bit (20ms) l 1on gm 204600 chips,mi chip gm 154 chu k sng mang L1 c tn s 1575.42Mhz. Hnh 2.5: Minh ha cu trc m C/Atrn sng mang L1 (Theo trang 31 [3]). Hnh 2.6: Minh ha cu trc m P trn sng mang L1 14 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit + Cu trc ca bn tin nh v: Thngtintrongbntinnhvccutrckhung(frame)nhminhha hnh2.7.Bntinhonhocha25khung,mikhungcha1500bits.Mikhung c chia nh thnh 5 khung con. Mi khung con cha 300 bits gm 10 t (word), mi t cha 30 bits v bit c trng s ln nht c pht u tin. V vy, tc 50 bit/s th cn 6 giy pht 1 khung con 300 bits v cn 30 giy pht 1 khung 1500 bits hnh 2.7. pht ton b bn tin nh v 25 khung th cn 750 giy hay 12.5 pht. Mi khung con khi u bng 30 bits TLM (telemetry word), trong 8 bits u tin l cc bit preamble b thu xc nh khung con, 22 bit cn li cha cc bit parity v bn tin telemetry m n ch c gi tr i vi thu bao c ng k. T th 2 ca mi khung l HOW (hand-over word) gm 29 bit Z-count c nhim v m cc epoch c sinh ra bi thanh ghi X1 (c 1.5s xut hin 1 ln) ca b pht mPtrongvtinh.19bitctrngsthpnhtcaZ-countcgilTOW (time-of-week) cho bit s epoch X1 xut hin khi bt u tun hin ti. S bt u tun hin ti xut hin epoch X1 vo lc na m ca ti th by hoc sng ch nht. Con s ca TOW tng t zero lc u tun n 403199 v sau quay li zero utuntiptheo.SmzerocaTOWlunxyraukhungcon1ca khung u tin. V b thu c th dng cc bit preamble ca TLM xc nh chnh xc thi im bt u ca mi khung con v y chnh l phng php xc nh thiimphtcabtkbphnnotrongtnhiuGPS.10bitctrngsln nht ca Z-count c gi l WN (weeknumber) cha s tun GPS.WN khng l bphncaHOWnhngxuthinthayvo10bitutincatth3trong khungcon1.3bitcaHOWcdngnhndngkhungconnotrongs5 khung con ang c pht.Con s ca TOW c tnh t HOW trong khung con 5 dngnhndngkhungnotrongs25khungangcpht(tngngs trang t 1 n 25). 15 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Hnh 2.7: Minh ha cu trc bn tin nh v (Theo trang 34 [3]). + Khung con 1 chad liu hiu chnh ng hGPS i vi vtinh dng cc h s a thc nh ngha s thay di hiu chnh theo thi gian. Thi gian c nh ngha bi cc ng h trong v tinh c gi l SV time (space vehicle time), cn thi gian sau khi hiu chnh c ng dng c gi l GPS time.Thi gian tham kho t0c c dng lm mc thi gian tnh sai s ng h v tinh.Thi gian tr trong tng in ly TGD c dng hiu chnh cc sai s tr khi truyn qua tng in ly. Thng tin IODC (issue of date, clock) cho bit s tp hp d liu ng h pht cnh bo cho ngi dng thay i cc thng s ng h. + Khung con 2 v 3 cha d liu qu o xc nh v tr v vn tc chnh xccavtinh.Khnggingnhdliuninlch,dliunyrtchnhxc.Cc thnhphncadliuquovthuttontnhvtrvtinhthamkhobng 3.1 v bng 3.2 trang 36 v 37 [3].Thng tin IODE (issue of date, ephemeris) c bonngidngkhiccthngsquothayi.Milnccthngsmi c mng iu khin ca h thng GPS ti ln v tinh GPS th con s IODE thay i theo. 16 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit + Khung con 4 ca c 25 khung u cha nin lch ca cc v tinh vim PRN(pseudorandom)nhs25vcaohn,cbitnnhccbntinc bit, cc s hng hiu chnh trong tng in ly, cc h s chuyn i t GPS time sang UTC time. + Khung con 5 ca c 25 khung u cha nin lch ca cc v tinh vi m PRN nh s t 1 n 24. Lu mi v tinh u pht 25 khung, d liu nin lch ca tt c cc v tinh u c tng v tinh pht i. Khng ging nh d liu qu o, d liu nin lch c gi tr cho cc chu k di lu (nhiu thng) nhng t b sai snhiu.Ngoiradliuchatrongbntinnhvlsaiskhongcchngi dng URE (user range error), n c tnh sai s khong cch do cc sai s qu o v tinh, cc sai s nh gi, sai s ch nh SA (selective availability) v cc c cho bit trng thi hot ng ca cc v tinh. 2.1.5- Cc sai s trong h thng GPS: okhongcchgivphasngmangbnhhngbinhiuloinhiu ngu nhin v sai lch (li h thng). Nhng li ny c th c phn loi thnh li bt ngun t v tinh,li bt ngun tmy thu, li dotruyn sng(khc xtng kh quyn). Hnh 2.8 cho thy cc li v sai lch. Cc li bt ngun t v tinh bao gm lch thin vn, hay li qu o, li ng h v tinh v nh hng ca kh nng chn lc. Li chn lc c thc hin do ch ca b quc phng M lm gim chnhxcGPSclpvldoanninh.Tuynhin,vicnychmdtvo ngy1/5/2000.Libtnguntmythubaogmlinghmythu,lia ng,nhiumythuvsbinthintmphacaanten.Litruyntnhiubao gm trca tn hiu GPS khi n truyn qua tng in ly v tng i lu ca bu kh quyn.Trnthct,chtrongmitrngchnkhng(khnggiantdo)tnhiu GPS mi truyn vi tc nh sng. Cng vi nh hng ca nhiu, v tr GPS tnh ton c cn b nh hng bi v tr hnh hc ca cc v tinh GPS c quan st bi my thu. Cc v tinh cng ta rng trn bu tri th chnh xc t c cng cao. Mt sli v sai lch c th c giihn hoc gim bt thng qua s kt 17 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit hp cc quan st GPS mt cch thch hp. V d, kt hp quan st L1 v L2 vi chnh xc cao c th loi b nh hng ca tng in ly. Cc sai s trong h thng GPS c chia thnh 3 ngun sai s chnh l sai s do v tinh, sai s khi truyn tn hiu v sai s do b thu ca ngi s dng. 2.1.5.1 - Sai s do v tinh: Vtrvtinhlhmcathigian,vtrnychatrongbntindnng qung b ca v tinh, c d on t cc quan st GPS trc ti cc trm iu khinmtt.Vicmhnhhacclctcngvovtinhthngskhng chnh xc hon ton, iu ny gy nn cc li d on v tr ca v tinh, gi l li lch thin vn. Bnh thng, li lch thin vn thng nm trong khong t 2m n 5m, v c th ln n 50m khi sdng kh nng chn lc. Li khong cch do skt hp ca li lch thin vn v li ng h v tinh nm mc 2.3m. Mt li lch thin vn cho mt v tinh c th noc nhn bit bi tt c ngi dng trn thgii.Tuynhin,nhngngidngkhcnhauquanstcngmtvtinh nhng gc khc nhau, do khong v v tr o c chu nhng nh hngkhc nhau ca li lch thin vn. iu ny c ngha l vic kt hp o c ca haimy thungthicngbmmtvtinhkhngthloibhontonlilchthin vn. Nhng vi nhng ngi sdng gn nhau c thnhn bit c hu ht li khongcch do li lch thin vn bng cch ly sai khc ca cc kt qu quan st ny. Trong nh v tng i, ngi ta s dng quy tc ngn tay ci nh gi th nh hng ca li lch thin vni vi phn giing ranh gii: ling ranh gii / di ng ranh gii = li v tr v tinh / phm vi v tinh. iu ny c nghalnulivtrvtinhl5mvchiudingranhgiil10km,thli ng ranh gii do li lch thin vn xp x khong 2.5mm. 2.1.5.2 - Sai s ch nh SA (selective availability): GPSbanucthitknhvclpthigianthcvdnng bngmythumC/A dng cho dn s vi chnh xcthphnmy thum P dngchoChngI.CchthngthngtinGISvGNSSquns.Nhngtht 18 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit ng ngc nhin l chnh xc ca c hai loi my thu trn gn nh l ging nhau. m bo an ninh quc gia, b quc phng M thc hin kh nng la chn trn cc v tinh GPS Block II ngn chnnh v c lp thi gian thc i vi nhng ngi dng khng c php. SA chnh thc hot ng vo ngy 25/3/1990. SAarahailoili.Liutingillidelta,lktqucaviclmbin ngnghvtinh,yllichungchomingidng.Lithhailli epsilon, y l li qu o bin i chm c thm vo. Khi SA c bt ln, cc liphngngangvphngthngngthngthngcthlnn100mv 156m tng ng, vi xc sut 95%. Hnh 2.9 cho thy v tr theo phng ngang ca mt trmmy thu GPS theo thi gian, chyu donh hng ca SA. Ging nhli khong cch do li lch thin vn, li khong cch do li epsilon hu nh c th nhn ra gia hai ngi dnggn nhau. V vy, sdng GPS vi sai (DGPS) s loi b nh hng ca li epsilon. Trn thc t, DGPS cungcp chnh xc cao hn my thu m P ng c lp do s loi b hoc gim bt cc li chung, bao gm c SA . Hnh 2.8:S thay i v tr ca my thu GPS ng yn khi c SA Theonhngnghincumrng,chnhphMdngSAvongy 1/5/2000,vicnycithinrtnhiuchnhxccaGPSclp.Vivic dngSA,chnhxcthngthngtheophngngangvngtheothtl 22m v 33m (95% thi gian). Hnh 2.10 ch ra cc li GPS sau khi dng SA. Vic loi b SA m ra mt cnh ca mi cho s pht trin nhanh ca th trng GPS (vddnngchoccphngtingiaothng).McdloibSAsgyra 19 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit nhhngnhnDGPS,nhngnslmgimchiphcitvvnhnhh thng DGPS. iu ny chyu do gim tc truyn dn yu cu . Hnh 2.9: S thay i v tr ca my thu GPS ng yn sau khi chm dt SA 2.1.5.3 - Sai s qu o v tinh (ephemeris error): Sai s qu o v tinh lsai s gia qu o d on v qu o thc ca v tinh(thngkhong2mn5m,vcthlnn50mnuckchhotSA). Thng tin v qu o d on ca v tinh c m ha v a vo d liu nh v truynchoccbthucangisdng.Davothngtinny,bthuca ngisdngsdonvtrcavtinh.VimnglithuthpdliuGPS trn ton cu,cc hng dch v GPS nh IGS International GPS Service, NGS US National Geodetic Survey, GSD Geodetic Survey Division, NRCan Natural Resources Canada chnh xc ca 2 qu o ng nh hn 10cm. D liu v qu o ng c pht n cc b thu phi mt 12 gi cho thng tin qu o cc nhanh ca IGS (ultra rapid orbit) v 12 ngy cho thng tin qu o chnh xc nht ca IGS (the most precise IGS orbit). 2.1.5.4 - Sai s ng h ca v tinh (satellite clock error): Mi v tinh GPS (Block II v Block IIA) cha 4 ng h nguyn t gm 2 cesium v 2 rubidium.Cc v tinh th h mi hn (Block IIR) ch cha cc ng h rubidium.nghcesiumcchntnghptnsvnhthichotnhiu GPS. Mt trong cc ng h ny, chyu l ng h xezi i vi cc v tinh Block IIvBlockIIA,cchncungcptnsvnhthiphttnhiuGPS. 20 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Nhng ng h khc l cc ng h d phng .Mc d cc ng h v tinh GPS c chnhxccao,nhngchngkhngphilhonhotuyti.nnhca chng vo khong 1/1013 n 2/10 13trong khong thi gian l mt ngy. C ngha lsaiscaccnghvtinhvokhong8.64n17.28nsmtngy.Sais khongcchtngngt2.59mn5.18m,ktqunycthtnhraddng bngcchnhnsaisnghvivntcnhsng(299729458m/s).nghxezicxuhng hot ng tt hn qua thi gian di so vi ng h rubi. Thc t,nnhcanghxeziqua10ngyhocluhnlnnviphn1014. Hotngcanghvtinhcgimstbihthngiukhinmtt. Lng dch i c tnh ton v truyn i theo dng ba h s ca mt a thc bc hai . Sai s ng hv tinh gy thm cc sai s chooc GPS. Cc li nynh hng chungn mi ngi dng quan st cng mt v tinh v c th loi bbng vic ly sai khc gia cc my thu ny. p dng vic sa sai ng h v tinh trong bntindnngnycngcthsacha saisnghvtinh.Tuynhin,b quamtlivinssdnnsaiskhongcchkhongvim(sais1nstng ngvisaiskhongcchl30cm).Ngcli,ccmythuGPSsdngcc nghtinhthrtin,chnhxcnhhnnhiusoviccnghvtinh. CcnghvtinhGPStuycchnhxccaonhngvnbsais8.64ns17.28nsmingytngng2.59m 5.18m(sais1nstngngtmsais 30cm). 2.1.5.5 - Sai s do truyn dn a ng (multipath error): Phntpanglmtngunlinghimtrngchoc ocphasng mang v khong cch gi. Li a ng xut hin khi tn hiu GPS n anten my thutheonhiungkhcnhau.Nhngngnycthlngtnhiunhn thng v tn hiu phn x t cc vt th xung quanh anten my thu. Phn tp a ng lm mo tn hiu gc qua s giao thoa ca tn hiu phn x anten GPS. N nh hng n c o c pha sng mang v khong cch gi; tuynhin,nh hngi viokhongcchgiln hnnhiu.ln caphn 21 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit tpang psonghasngmangcthttigitrlnnhtl1/4chuk (khong 4.8 cm i vipha songmang L1). Khong cch gi a ng theo l thuyt c th lnn vi chc mt i vi om C/A. Tuy nhin, vi nhng tin b mi trong cng ngh my thu, khong cch gi a ng thc t gim i rtnhiu. V d v nhng cng ngh ny l b tng quan Strobe (Ashtech, Inc.) v ( NovAtel, Inc.). Vi nhng cngngh lm gim phn tp a ng ny, li phn tp a ng ca khong cch gi c th gim xung n vi mt, thm ch trong c mi trng phn x mnh. Hnh 2.10: nh hng ca phn tp a ng Trong cng mt mi trng, c th nh gi li a ng bng cch s dng tng quan qua cc ngy ca phn d d on. y l do cu trc hnh hc v tinh vt phn x - anten lp li hng ngy. Tuy nhin, li a ng trong cc php o khongcchginhnhaucthxcnhcnusdngquanst2tns. Ngitavnchatmramtmhnhangtngquttt,chyuldocu trchnhhcvtinhvtphnx-antenlunthayi.Tuynhin,cnhiu tng lm gim bt nh hng ca phn tp a dng. Phng n d dng nht l chn v tr quan st khng c vt hn x gn anten thu. Mt phng php khc gimnhhngcaphntpanglsdngmtantencvngchm( vng chm l mt mt phng c nhiu vng kim loi ng tm lm suy hao tn hiu 22 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit phn x). V tn hiu GPS c s dng phn cc trn quay phi trong khi tn hiu phnxquaytri,lmgimnhhngcaphntpangcngcths dng anten vi mt b phn cc phi hp cho tn hiu QPS ( quay phi). Tuy nhin, nhcimcaphngphpnyltnhiuphntpangcngcthtr thnh phn cc quay phi nu phn x hai ln. Hnh 2.11:Nhiu a ng Khiokhongcchnmivtinh,tagisrngtnhiuvtinhc truyn thng t v tinh n anten ca my thu. Nhng trong thc t ngoi tn hiu trc tip ny antenmythu cn nhn c cc tn hiu phn x n t mt t v cc vt th gn anten qua nhiu ng gin tip khc nhau, xen nhiu vo tn hiu trc tip, gy ra sai lch v thi im n ca tn hiu thc s.Nungtruyngintipdihnngksovingtruyntrctip (ln hn 10m) hai mu tn hiu trn tch ri nhau th nh hng gy ra bi nhiu a ng v c bn c th c khc phc bi cc k thut x l tn hiu. 23 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit 2.1.5.6 - Sai s do tng in ly (ionospheric error): Hnh 2.12:Sai s do tng in ly Tng in ly c cao t 50km 1000kmso vi mt t l ni xy ra s ion ha cht kh (do s tc ng ca cc tia bc x mt tri nh tia cc tm v tia X) v c mt in t thay i theo cao. Tng in ly c chia thnh nhiu lp con tytheomtintnhsau:lpD(50km 90km);lpE(90km 140km);lp F1(140km 210km);lpF2(210km 1000km).TronglpF2cmtint cao nht. cao v dy cc lp thay i theo thi gian do nh hng thayi bc x mt tri v t trng tri t (v d nh lp F1 bin mt vo ban m). Tnginlylmtmitrngphntn(dispersivemedium)nnnlm gp khc (i hng) hay thay i tc ca tn hiu GPS. Sai s do lm gp khc tnhiukhitruyntnhiuqua1lpconskhngngknugcngng (elevation angle) ca b thu ln hn 50, nhng sai s do tc tn hiu b thay i lunlmtsaisngk.Tnginlylmtngtctruynphasngmang nhng lm gim tc m PRN v d liu nh v vi cng mt bin . Ni cch khc, tng in ly lm ko di thi gian truyn m v d liu nh v ng thi rt 24 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit ngn thi gian truyn pha sng mang. iu ny c ngha l khong cch t v tinh n b thu ca ngi s dng trong php o pha ca sng mang s ngn hn so vi khongcchthct,cnkhongcchtvtinhnbthuquaphpomsdi hn so vi khong cch thc t. Tc ng ca tng in ly n tn hiu GPS ph thuc vo mt electron t do TEC (Total Electron Content) dc theo ng truyn tn hiu GPS. Mt in t t do khng c nh m thay i theo tng thi im trong ngy (TEC c gi tr lnnhtlc2gichiuv10gititheogiaphngvthpnhtlcna m), thay i theo tng ma trong nm (ma ng ln hn ma h), thay i theo chu k tun hon 11 nm ca h mt tri (hot ng bc x ca mt tri t cc i theochuk11nm(gnnhtlnm2001)vTECcngtgitrcci,thay i theo vtr a l trn tri t (mt s v tr c bit trn tri t c s bin i bt thng ca TEC nh vng cn xch o, vng cc quang v vng cc). TcngcatnginlyntnhiuGPScnphthucvotnstn hiu. Ta bit l tng in ly to ra 1 khong delay khi tn hiu truyn qua n. Tn s cng thp th tr cng nhiu (sng mang L2 tr nhiu hn sng mang L1 do tn s sng mang L2 thp hn tn s sng mang L1).Tr trong tng in ly t 5m 15m vcthn150mdiphngxmttriddivogiatra.Ngoiviclm thay i vn tc truyn sng, tng in li cn lm suy hao cng tn hiu (gim i khong 0.2dB). Nhiu phng php v thut ton c nghin cu v p dng nhm xc nh v loi b sai s gy ra do tng in ly, gp phn ci thin cht lng ca bi ton nh v.Trong s , phng php thiu qu cao nht l phng php kt hp 2 tn s (dual frequency) dng trong k thut DGPS (Differential GPS). 2.1.5.7 - Sai s do tng i lu (tropospheric error): Tngilultngkhquyntrungtnh(khngcstnticacc electron t do) nm t mt t ln n cao 50km. Tng i lu l 1 mi trng khng phn tn do c tn s sng v tuyn di 15GHz. Tng i lu lm gp khc 25 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit tnhiuGPSkhintruynquavlmkodithigiantruyntnhiu,do khong cch thu c qua tn hiu s di hn so vi khong cch thc gia v tinh v b thu. y l tc ng chnh ca tng i lu ln tn hiu GPS. Khc vi tng in ly, khong thi gian tr m tng i lu to ra khng ph thucvotnstnhiu.Do,khngthdngphngphpkthp2tns trong k thut DGPS loi tr sai s gy ra bi tng i lu. Ngoira,khongthigiantrcngphthucvogcngngbthuca ngi s dng khi hng v v tinh. Tn hiu i t v tinh c gc ngng thp khi i qua tng i lu s c gc ngng cao hn, gc ngng cng ln th khong thi gian tr cng nh. Ngi ta c tnh sai s khong cch trong tm khong 2m khi v tinh vng thin nh (zenith) ca b thu, khong 9m khi v tinh vng gc ngng 150 v khong 25m vng gc ngng 50. ng thi, khong thi gian tr ny cng ph thuc vo nhit , p sut v mcakhngkhdcngtruyntnhiukhitruynquatngilu.Da vo c im ny, ngi ta phn khong thi gian tr gy ra do tng i lu thnh 2 thnh phn: thnh phn kh (dry delay) v thnh phn t (wet delay). Thnh phn kh chim khong 90% tng sai s gy ra. Tuy nhin, n li d dng c xc nh vichnhxccaobngccthutton.Thnhphntthphthucvohi nc dc theo ng truyn tn hiu, ch yu tp trung cao t mt t ln n 15km.Khcvithnhphnkh,thnhphntrtkhxcnh.Ngitacng dngccthngsotkhtngnhpsutkhngkh,nhit,m, tnh ton thnh phn t, nhng chnh xc cng khng cao. 2.1.5.8 - Sai s do b thu ca ngi s dng: Lsaisgyradonhiuxuthintibthucangisdngtcc ngun nhiu nh : anten, cc b khuch i tn hiu trong b thu, cc dy dn hoc domchintbntrongbthuvcnhngngunthuphttnhiukhcxung quanhvtrtbthu.Mcsaisgyradonhiubthutythuccng 26 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit ca nhiu so vi cng tn hiu GPS thu c ti b thu ca ngi s dng v thng c biu din bng t s tn hiu trn nhiu SNR (signal to noise ratio). Sai s b thu trong php o m vo khong 0.25m n 0.5m, cn trong php o pha th b hn, vo khong vi mm. Ngi s dng b thu khng c cch no loi b sai s ny, nhng h c th kimtrachtlngcabthutrckhisdngchng,cbittrongccng dng i hi chnh xc cao. Ngi ta thng dng 1 trong 2 phng php kim tralkimtrakhngkhongcch(zerobaseline)vkimtrakhongcchngn (short baseline). Phng php zero baseline l phng php kim tra trong ngi ta dng 1antenthutnhiuGPSrichotnhiunyquabchiacngn2hay nhiubthuGPStngaycnhnhau.Vcng1antenthuvcng1vtrnn ccgitrkhongcchtvtinhnccbthuphilnhnhau,cnnucs khcbitnhauthskhcbitchnhlsaisgyradobthucangis dng. Phng php short baseline th dng 2 h thng (mi h thng gm 1 anten v 1 b thu) t cch nhau khong 2m thu tn hiu GPS v theo di lin tc trong 2 ngy. Vi cch b tr ny th s khc bit gia 2 kt qu o t khong cch t v tinh n 2 b thu chnh l sai s ca h thng thu (sai s ca b thu v c anten thu) cngvisaisdotruyndnang.Nhngdosaistruyndnangl nh nhau trong mi ngy nn khi tr kt qu theo di mi ngy cho nhau th ta s loi b c sai s truyn dn a ng v do ch cn li sai s ca h thng thu. -Tm li:Cc sai s trn c minh ha hnh 2.11 trong trng hp nh v m C/A v m P. 27 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Hnh 2.13: Minh ha cc sai s ca h thng GPS Do c s bo mt cao v c dng trong qun s, b mt quc gia cho nn nh v m P s gy t sai s hn m C/A. 2.1.6- Bin i tm pha anten : MythuGPSthutnhiutvtinhsauchuyninnglngny thnhdngin,dnginnycxlbimythuGPS.immtitn hiuGPScthuvgiltmphacaanten.Thngthng,tmphakhng trng vi tm vt l (hnh hc) ca anten. N thay i ty theo cao v phng v ca v tinh cng nh cng ca tn hiu. Kt qu l gy thm sai s khong cch cho php o. Mc ca sai s gy ra bi tm tm pha thay i ty theo loi anten, thnglkhongvicm.Tuynhin,rtkhmhnhsbinthincatmpha anten,vvycnphichkhichnloianten.ivinhngngranhgii ngn vi cng loi anten mi u, sai s tm pha c th loi b nu cc anten ny cng nh hng . Pha trn cc loi anten khc nhau hay s dng nh hng khc nhau s khng th loi b c sai s ny. Domc nh hng nh, sai s tm 28 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit pha anten c th b qua trong hu ht cc ng dng thc t. Cn ch ra rng li tm pha c th khc nhau i vi cc sng mang L1 v L2. iu ny nh hng n chnh xc ca vic kt hp tuyn tnh t do tng in ly, c bit khi quan st ng ranhgiingn.ivinhngngranhgiingn,saistngquanlntheo khongcchvloibnhngsaikhc.Vvy,sdngntncththch hp hn i vi nhng ng ranh gii trong kiu tnh. 2.1.7- Cch tnh ta b thu da theo hnh hc: Vtrca1imbtktrongkhnggiancthbitcbngccho khong cch t im n mt vi im bit trong khng gian. + a cng thc tnh v tr b thu ca ngi s dng da vo 3 v tinh (hnh 2.12):Gi s ta bit trc ta ca 3 v tinh l S1(x1, y1, z1), S2(x2, y2, z2), S3(x3, y3, z3) v ta o c khong cch t S1, S2, S3 n user l1,2, 3. Ta cn xc nh v tr b thu ca ngi s dng l U(xu, yu, zu). zxyU(xu,yu,zu) OS1(x1,y1,z1)S2(x2,y2,z2) S3(x3,y3,z3) 1 23 Hnh 2.14: Minh ha v tr b thu ca ngi s dng v v tinh. Ta c: 1 = ) ( ) ( ) (1112 2 2z zy yx xuuu + +2 = ) ( ) ( ) (2222 2 2z zy yx xuuu + +(2.1) 3 =) ( ) ( ) (3332 2 2z zy yx xuuu + +29 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Hphngtrnh(2.1)gm3phngtrnhv3ns.Vnguyntc,nu bit c ta ca 3 v tinh l x1, y1, z1, x2, y2, z2, x3, y3, z3 v cc khong cch t b thu ca ngi s dng n 3 v tinh l1, 2,3 th ta s tnh c ta b thu ca ngi s dng l xu, yu, zu. + o cc khong cch1,2, 3 t b thu ca ngi s dng n 3 v tinh: GisvtinhthiphttnhiuthiimtSi,bthucangisdng nhn c tn hiu thi im tr hn l tu, th thi gian tn hiu i t v tinh th i n b thu ca ngi s dng l : tSi - tu. V tn hiu truyn vi vn tc bng vn tc canhsnglc=2.99792458*108m/s,nnkhongcchtbthucangis dng n v tinh th i l: iT = c(tu - tSi )(2.2) yiTlkhongcchtbthucangisdngnvtinhthi (cobibpseudorange).Trnthct,xungclocktiniphtvtinhv xung clock ti b thu ca ngi s dng l khng ng b. Thi im ng v tinh th i pht tn hiu n b thu ca ngi s dng l: tSi = tSi +Abi(2.3) Thi im ng b thu ca ngi s dng nhn c tn hiu ca v tinh th i l: tu = tu + but(2.4) T (2.2), (2.3) v (2.4) ta c khong cch t b thu ca ngi s dng n v tinh th i khi c sai s thi gian tr l: iT = c(tu - tSi) = c(tu - tSi) - c( Abi - but) =iT - c( Abi - but)(2.5) Bn cnh sai s ng h xung clock, cn c cc sai s khc nh sai s nh v v tinhADi; sai s tr trong tng i luATi; sai s tr trong tng in lyAIi; saisnhiukhiobthu i;siuchnhthigiantngiAi.Do, khong cchi t b thu ca ngi s dng n v tinh th i c vit li nh sau: i =iT - c( Abi - but) +ADi + c( ATi +AIi + i +Ai)(2.6) 30 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Mt trong cc sai s trn c th c hiu chnh. V d sai s tr trong tng i lu c th c lp m hnh, sai s tr trong tng in ly c th c hiu chnh trong b thu dng 2 tn s. Cc sai s trn s gy ra s mt chnh xc khi xc nh v tr b thu ca ngi s dng. Tuy nhin, sai s ng h trn b thu ca ngi s dng khng th c hiu chnh qua thng tin thu nhn c. Sai s ny vn cn l ns.Vvy,taphicngvovphiccphngtrnh(2.1)chohssais nghbthucangisdnglbu.Hphngtrnh(2.1)trthnhh3 phng trnh c 4 n s xu, yu, zu, bu . Do ta cn c thm v tinh th t c h (2.7) gm 4 phng trnh c 4 n s nh sau: 1 = ) ( ) ( ) (1112 2 2z zy yx xuuu + ++ bu 2 = ) ( ) ( ) (2222 2 2z zy yx xuuu + ++ bu 3 = ) ( ) ( ) (3332 2 2z zy yx xuuu + ++ bu(2.7) 4 = ) ( ) ( ) (4442 2 2z zy yx xuuu + ++ bu Mt cch tng qut, ta c cng thc tnh khong cch t v tinh th i n b thu ca ngi s dng l: i = ) ( ) ( ) (2 2 2z zy yx xu iu iu i + ++ bu(2.8) -Tm li: + Ta cn 4 v tinh tnh ta b thu ca ngi s dng nhm gim sai s ng h ca v tinh v sai s ng h trn b thu ca ngi s dng. + Cc ngun sai s trong h thng GPS gm c 3 ngun sai s chnh l sai s do v tinh, sai s khi truyn tn hiu v b thu ca ngi s dng. Sai s do v tinh gm c sai s ch nh SA, sai s qu o v tinh v sai s ng h v tinh. Sai s khi truyn tn hiu gm c sai s truyn dn a ng, sai s do tng in ly v sai s 31 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit dotngilu.Tibthucangisdngcccsaisdonghbthu, nhiu xut hin ti b thu ca ngi s dng t cc ngun nhiu nhanten, cc b khuchitnhiutrongbthu,ccngunthuphttnhiukhcxungquanhv trt b thu. + nh v GPS dng mt b thu ch dng trong trng hp cn chnh xc thp. + loi tr hoc gim cc sai s ph bin khi dng SA, sai s qu o v ng h v tinh, ta dng k thut DGPS v n cho chnh xc cao hn so vi dng 1 b thu c m P. + Vn truyn dn trong mi trng mt ngi s dng cao d b hin tng fadingvcannhiu.Cthgimhintngfadingvcannhiutibthuca ngi s dng bng cch dng b lc Kalman. + Sai s truyn dna ng vvn truyn dn ti u tib thu ca ngi s dng cng c hn ch bi antenna thng minh. 2.1.8- Gii thiu v b thu tn hiu GPS: B thu tn hiu GPS theo cng ngh s (Digital) c s tng qut nh sau: Hnh 2.15: S tng qut ca b thu tn hiu GPS. 32 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit BthutnhiuGPScchcnngthunhntnhiutccvtinhGPSv phn bit tn hiu no thuc v tinh no ri gii m tn hiu v tinh xc nh v tr, tc v thi im truyn tn hiu ca cc v tinh. Tng trn trong s hnh 2.13 l phn cng hon ton gm anten thu, cc b lc thng di BPF (Band Pass Filter) gim can nhiu nm ngoi di bng tn m khng lm suy gim tn hiu GPS, cc b khuch i Amp (Amplification), cc b trn (Mixer) v b chuyn i ADC (Analog to Digital Converter). thun li chovicxltnhiustngdi,likhuychicatngRF(Radio Frequency) trong tng trn ca b thu GPS khong 35dB55dB. Tng trn c chc nngthunhnsngvtinh,chuynithnhtnhiutrungtnIF(Intermediate Frequency) ri s ha. Tngdixldliucshabngphnmmgm2giaion. Giaion1thchintchmvsngmang,xcnhvtr,tcvthigian truyn tn hiu ca cc v tinh; ng thi cng xc nh gi tr php o m, php o pha v dch chuyn Doppler ca tng v tinh. Giai on 2 da vo cc thut ton thc hin tnh ton v c tnh v tr, tc , thi gian ca b thu. Phngphpomchnhlphngphpokhongcchgisdngm C/A hoc P da vo thi im pht tn hiu (ti v tinh), thi im thu tn hiu (ti b thu) v vn tc truyn sng (chnh l vn tc nh sng). Phng php o pha chnh l phng php o khong cch gia v tinh v b thu da vo vic o pha ca sng mang tn hiu GPS (L1 v L2).Phng php ny c th hin qua cng thc: = N + /T (2.9) Vilbcsng(L1=19.04cmvL2=24.44cm),Nlsnguynln chu k sng mang gia v tinh v b thu (phn chn),/T l phn l ca 1 chu k sng mang. 33 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Hot ng ca b thu GPS: RF chain ADCUser positionSatellite positionsEphemeris & pseudorangeSubframe identifyTracking AcquisitionAntennaSoftwareHardware Hnh 2.16 : Minh ha h thng hn v hot ng ca b thu tn hiu GPS. Tn hiu GPS c thu vo dy anten s qua dy RF khuch i ln bin ngvtnscchuynthnhtnsngramongmun.Sautnhiu c chuyn t tng t sang s nh b chuyn i ADC. Saukhicsha,tnhiuscxltipbngphnmm.Chng trnhnhnbitvthuthpdliuvtinh(Acquisition)cthchin.Chng trnh Tracking c dng tm s dch pha ca d liu nh v. Cc khung con (Subframes) v d liu nh v c th c c t s dch pha ca d liu nh v. D liu qu o v cc khong cch gi (Ephemeris & pseudorange) c th c c t d liu nh v. D liu qu o c dng c c cc v tr v tinh (Satellite positions) .Cui cng, v tr ca ngi s dng (User position) c th c tnh da vo cc v tr ca v tinh v cc khong cch gi. nhn bit 1 v tinh i, b thu phi t to ra nhng m ging ht vi m ca v tinh i (m replica) ri dch chuyn chng theo thi gian so snh chng vi dy mcatnhiuvtinhangthunhnc.Nu2dymtrngkhtnhauthv tinh pht tn hiu ny chnh l v tinh i. Vic so trng dy m ny khng mt nhiu thigianivimC/AnhngmtnhiuthigianivimP.khixcnh c v tinh i no ang nm trong tm quan st ca mnh, b thu s tin hnh thu 34 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit nhn d liu t v tinh ny, tch b nhiu v tip tc lp li qu trnh nhn bit v thu thp ny. Trong cc b thu truyn thng, 2 khi Acquisition v Tracking c thc hin bng phn cng. -Tm li: n s ng dng thut ton LMS giai on 2 ca phn mm x l d liu nm trong tng di s tng qut ca b thu tn hiu GPS hnh 2.13. 2.2 - Gii thiu v k thut DGPS: DGPS (Differential GPS) l k thut gim sai s cc v tr trong h thng GPSbngcchdngdliuphtbthucatrmthamchiutivtrbit. Dng ph bin nhtca kthut DGPS l xc nh ccnh hnggp chung ca cc sai s qu o v sai s ng h v tinh trong bn tin nh v (bao gm cc nh hngcaSA)titrmthamchiuvphtccsaiskhongcch(pseudorange corrections)nbthucangidngthigianthc(realtime).Bthudng cc sai s ny xc nh v tr ca n. Kt qu: + Cc ngun sai s b loi b hon ton l sai s SA, sai s qu o v tinh, sai s ng h v tinh. +Ccngunsaisdotrtrongtnginlyvtngilucthchnch nhng cc sai s ny tng theo khong cch. + Cc ngun sai s hon ton khng b loi b l cc sai s truyn dn a ng v cc sai s do my thu. K thut DGPS cdng khiyu cumc chnh xc vimt. Mt b thu ttitrmthamchiucnhctabit,bthucnlicangidng cha bit ta c th ng yn hoc di chuyn m ta cn tnh ta nh minh ha hnh 2.15. 35 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Phng php ny da trn thc t l sai s khong cch o t v tinh n 2 bthubngnhaukhihaibthukhngcchnhauquvitrmkm.Phnmm c lp trnh trong b thu tham chiu s da vo ta ca b thu tham chiu bit v ta nhn c t bn tin nh v ca cc v tinh tnh khong cch t b thu tham chiu n cc v tinh, ri a ra chnh lch gia khong cch tnh v khong cch m m oi c c cc sai s khong cchA i (hay cc hiu chnh DGPS). Cc sai s ny c pht n b thu ca ngi s dng. Sau bthucangisdngsdngccsaisA inyhiuchnhcckhong cch o ti . Sau cng, cc khong cch hiu chnh c dng tnh ta ti b thu ca ngi s dng. nh v GPS sai phn ci thin c chnh xc cao t vi centimeter n vi metre, trong khi nh v GPS dng 1 b thu c chnh xc theo phng ngang ca cc b thu dn dng c m C/A l 100m (c kch hot SA) v 22m (khng kch hot SA) (trang 69[1]). nh v GPS dng 1 b thu ch c dng trong trng hp cn chnh xc thp, cn nh v GPS sai phn c dng cho cc ng dng c chnh xc cao nh kho st v v bn , GIS (Geographical Information Systems). Hnh 2.17: Minh ha m hnh GPS sai phn. C2mhnhDGPSlDGPSccb(LADGPSLocalAreaDGPS)vDGPS din rng (Wide Area DGPS). 36 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit + DGPS cc b (LADGPS): l m hnh DGPS s dng 1 b thu mc (base receiver)tti1trmthamkho(referencestation)duynht.Bthumcny nhn tn hiu t cc v tin GPS v da vo ta bit trc ca n a ra cc saiskhongcchnhmhiuchnhchoccbthucangisdngcchtrm thamkhokhngquvitrmkm.Victruyndliuhiuchnhtbthumc n cc b thu ca ngi s dng c thc hin trn sng v tuyn da trn chun RTCM(Radio Technical Commission for Maritime Services). M hnh DGPS cc bchochnhxcvim(phpom)vvicm(phpopha)trongbiton nh v. Khong cch t cc b thu ca ngi s dng n b thu mc cng ngn th chnh xc cng cao. Hiu qu ca h thng cn ph thuc vo cng sut pht tn hiu ti trm tham kho, kh nng nhn tn hiu ti b thu ca ngi s dng, c tnhngtruyn(mcnhiu,vtcn,).MhnhDGPSccbcminh ha hnh 2.16. Hnh 2.18: Minh ha m hnh DGPS cc b. +DGPSdinrng(WADGPS):lmhnhDGPSsdngnhiutrm thamkhonhmhiuchnhccsaiskhongcchchoccbthucangis dng vt qu khong cch cho php ca m hnh DGPS cc b. Trc tin cc sai s hiu chnh t cc trm tham kho c gi n 1 trm trung tm (master station) qua cc ng truyn mt t nh h thng cp quang. Trm trung tm s phn tch 37 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit v kt hp d liu thu c xc nh cc sai s hiu chnh cho tng v tinh GPS. CcsaishiuchnhnyctruynlnvtinhatnhGEO(geostationary satellite)vsaucphtngcxungccbthucangisdnghiu chnh. Mi b thu ca ngi s dng s nhn duy nht 1 b hiu chnh DGPS tng ng vi v tr ca n. M hnh DGPS m rng c minh ha hnh 2.17. H thng WADGPS u tin c xy dng vi quy m ln nht l h thng WAAS (Wide Area Augmentation System) ca M. Cc h thng WADGPS khc nhEGNOS(EuropeanGeostationaryNavigationOverlayService)chuu, MSAS(MultifunctionalTransportationSatellite-basedAugmentationSystem) Nht v GAGAN (GPS And GEO Augmented Navigation) n . Hnh 2.19: Minh ha m hnh DGPS m rng. Tm li: + K thut DGPS ci thin c chnh xc cao (sai s vi cm n vi m), loi tr hon ton cc ngun sai s do SA, sai s qu o, sai s ng h v tinh. Hn ch cc s gim khong cch do cc ngun sai s tr trong tng in ly v tng i lu. 38 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit + K thut DGPS khng loi tr c cc ngun sais dohin tng fading, can nhiu, sai s truyn dn a ng v vn truyn dn ti u ti b thu. + Lun vn a ra thut ton LMS m phng v thy c ng dng ca anten thng minh. + ng thi lun vn cng a ra gii php antenna thng minh nhm khc phc sai s do truyn dn a ng v ci thin vn truyn dn ti u ti b thu. 2.3 - Thuttan LMS: 2.3.1-Thut ton LMS: (Theo trang 95-98[5]) Bng cch tng phc tp x l tn hiu dy anten, ta c th t c cc cithinhiusutngksovidngcchthngbpchuyni.Trongdy anten thch nghi hnh 2.24, vector trng s wk,i c iu chnh hoc c thch nghi cc i ha cht lung tn hiu m n c gi tr i vi b demodulator ca tn hiu k ti thi im i. Hnh 2.20: Cu trc dy anten thch nghi (Theo trang 95[5]). Lu:dictrngchoctnhhaymhnhtnhiumongiivi ngi s dng th i ti ng ra ca dy anten . 39 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit Trong cc k thut to bp ti u, vector trng s c xc nh sao cho cc tiu hahmmc tiu (cost function).c bit, hmmc tiu nylin hpnghch o vi cht lng tn hiu ti ng ra dy anten. hmmc tiu cc tiu th cht lng tn hiu s t cc i ti ng ra ca dy anten. Hai k thut ph bin nht c ng dng rng ri vo cc h thng thng tin l tiu chun sai s bnh phng trungbnhtithiuMMSE(MinimumMeanSquareError)vbnhphngnh nht LS (Least Square). Trong 2 k thut ny, bnh phng sai phn gia ng ra dy anten z(t) = wkHui(t) v c tnh ca tn hiu mong i dk(t) c to ra i vi thu bao th k c ti thiu ha bng cch tm vector trng s thch nghi wk. Cc gii php MMSE c a ra di dng cc trung bnh v to ra vector trng s n wk ti u cho hu ht cc thc t c th trong mi trng dng. y l gii php c dng trong l thuyt v b lc Wiener kinh in. Trong gii php ny, hm mc tiu c ti thiu l: J(wk) = E[wkHui - dk,i2 ](2.45) Vi dk,i = dk (iTs ) v ui = u(iTs),Ts l chu k ly mu. Hmmctiulgitrmongicasaisbnhphnggiangrady anten ca tn hiu th k v loi tn hiu mong i ti thi im th i. Ta c th vit li (2.45): J(wk) = wkHE[uiuiH]wk - E[dk,iuiH]wk - wkHE[uidk,i*] + E[dk,idk,i*](2.46) Tngqut,tacctiuhmvectorbngcchxcnhvtrmdc (gradient)cahmtnzero.dcca1hmvectorphccnhngha nh sau: Vf(w) = ((((((

c ccc) (...) (10w fw faaM + ((((((

c ccc) (...) (10w fw fbbM(2.47) y wi = ai + jbi . Dng nh ngha ny ta tm c: 40 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit V(wHAw) = 2Aw(2.48) V(wHc) = 2c (2.49) V(cHw) = 0(2.50) Do ta c th vit : VJ(wk) = 2E[uiuiH]wk -2E[uidk,i*] = 2Rwk 2P(2.51) y R l ma trn tng quan ca vector d liu: R = E[uiuiH](2.52) V P l tng quan cho gia vector d liu v tn hiu mong i: P = E[uidk,i*](2.53) t dc ca hm mc tiu bng zero, ta tm ra rng gii php wk lm cho hm mc tiu J(wk) t cc tiu l: wk = R-1P(2.54) Gii php (2.54) l vector trng s dy anten ti u theo ngha MMSE. giiphngtrnh(2.54),cckthutthchnghithngcdngvi phng php lp nhm cung cp vector trng s c cp nht wk,i sau mi ln tnh ton. c bit, cc gii thut ny c phc tp mi bc thp hn gii php trc tip trong phng trnh (2.54), v c th di theo (track) cc knh khng dng. Mt gii php ti u lm ti thiu hm mc tiu l: wk,i+1 = wk,i - 21 VJ(wk,i)(2.55) y l h s hi t v n iu khin tc thch nghi.Cc loi gii thut nhphngtrnh(2.55)cgilcckthutdcngunhin(Stochastic Gradient) bi v chng dng dc ca hm sai s bnh phng trung bnh cp nht vector trng s. Thay (2.51) vo (2.55) ta c: wk,i+1 = wk,i - (Rwk,i P) = wk,i - ( E[uiuiH]wk,i E[uidk,i*]) (2.56) Nu ton t k vng c b t phung trnh (2.56) th ta c: 41 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit wk,i+1 = wk,i - ui(uiHwk,i dk,i*) = wk,i - uiei,k* Haywk,i+1 = wk,i - uiei,k*(2.57) Phng trnh (2.57) c gi l gii thut sai s bnh phung trung bnh ti thiu LMS (Least Mean Square) v n l 1 k thut c s dng rng ri cho cc dy anten thch nghi cp nht theo vng lp. Vi:ei,k = wk,iHui dk,i(2.58) ei,klsaistcthigiangradyantenvpngmongi,ei,k*llinhp phc ca ei,k. Mt gii php khc cho vic lc thch nghi l cc tiu ha sai s gia ng ra cadyantenvpngmongitrnsmuthigianhuhnthaychovic tnh trung bnh tng.iu nydnn cc gii php bnh phng ti thiu LS. Ta nh ngha hm mc tiu LS: J(wk) ==10,2Pmm kHkmd uw(2.59) Ton t gradient nh nh ngha (2.47) thng thu c l: J(wk) = 2 =10Pmw u u kPnHn m=10 - 2 =10Pmd un kPnm ,*10= (2.60) Ta nh ngha ma trn d liu l:A = [u0u1 uP-1 ](2.61) V vector tn hiu mong mun l:dk = [dk,0 dk,1 dk,P-1]T(2.62) Gii php rng buc hm dc LS bng zero l:wk = (AHA)-1AHdk(2.63)M hnh tng qut ca h thng anten thng minh dng tn hiu tham kho c dng nh hnh 2.25 (theo hnh 2.8 mc 2.5 chng 2 [6]). 42 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit W0*W1*WN-1*++ Gii thut c tnh trng s......y(t)= WHUe(t)=r(t)-y(t) r(t)(Ng ra)(Tn hiu sai s) (Tn hiu tham kho)-+u0(t)u1(t)uN-1(t)(Anten 0)(Anten 1)(Anten N-1)(Cc trng s ) Hnh 2.21: M hnh tng qut ca h thng anten thng minh dng tn hiu tham kho. YucucagiithutUnconstrainedLMSltmravectortrngstiuW sao cho cng sut thu c ti ng ra ca dy anten Pr trong cng thc (2.35) l nh nht (tc E[Pr] min vi E l k vng ton hc) m khng tha mn iu kin rng buc v li nh hng ca h thng. Vetor trng s c cp nht theo cng thc (3.2.1) mc 3.2 chng 3 [6] l: W(n+1) = W(n)- g(W(n)) (2.64) +yW(n+1)lvectortrngscntnhbclpthn+1,W(n)lvector trng s c bc lp th n. +(step size) l h s hi t v n iu khin c tnh hi t ca gii thut nh nhanh v tip cn (fast and close) khi cc trng s c tnh t n gi tr trng stiu.Theomc3.2.3chng3[6],ngitachngminhcrnggii 43 SVTH:Minh Tun-Hong HuyGVHD:Ths Nguyn Tun Kit thut LMS s n nh v vector trng s s hi t v gi tr ti u khitha mn cc iu kin sau: 0