Download pdf - Tong Quan GPS

GPS the global positioning system A. L THUYT V GPS:I- Gii

thiu v GPS:

1.1 Tng quan v GPS (Global Postioning System):Thng thng GPS bao gm mt chm sao 24 v tinh.Chm sao ny c hon thnh vo thng 7,1993,c xem nh l nng lc hot ng ban u(initial operational capability (IOC)).Tuy nhin cng b chnh thc l vo ngy 8,thng 12,1993. m bo h thng v tinh ny bao ph khp ton b tri t mt cch lin t c,nhng v tinh ny c sp xp sao cho mi 4 v tinh c t trong mi 6 mt phng qu o(Hnh1).Vi s b tr ny,khong t 4 n 10 v tinh s lun hin hu ti bt c ni no trn th nh v. gii,nu gc ngng (elevation angle) l 10 . Ta ch cn duy nht 4 v tinh cung cp s khong 0.01),vi mt nghing khong 55 so vi xch o.Na trc ln ca qu o Qu o v tinh GPS th gn nh l hnh trn(mt hnh elipse vi tm sai cc i GPS l khong 26560 km( cao v tinh khong 20200 km bn trn b mt tri t).Chu k qu o v tinh tng ng khong 12 gi thin vn (sidereal hour;23:56:4.1).H thng GPS c tuyn b chnh thc l t c mt kh nng hot ng y (full operational capability (FOC)) vo ngy 17,thng 7,n m 1995,m bo kh nng hot ng thc t ca ti thiu 24 v tinh GPS ,khng dng vo th nghi m.Thc t ,khi m GPS t c FOC ca n,chm sao v tinh thng ln hn 24 v tinh.

1 dhspkt-hcm-2009

GPS the global positioning system

Hnh 1: Chm sao GPS S-band(10cm-radar short-band):1.55 -5.2 Ghz. L-band (20cm-radar long-band):950Mhz 1450 Mhz

1.2 Nhng phn on GPS:GPS bao gm 3 phn on: phn on khng gian,phn on iu khin ,phn on ngi s dng(Hnh 2). Phn on khng gian chnh l chm sao 24 v tinh. Mi v tinh GPS pht i tn hiu, bao gm nhng thnh phn sau: hai sng sin ( thnh ph n sng mang), hai chu i d liu s,v mt thng ip iu hng. D liu s v thng ip iu hng kt hp vi sng mang bng cch iu ch nh phn biphase( Hnh 3). Sng mang v chui d liu s ch yu c s dng xc nh khong cch t my thu ca ngui s dng n nhng v tinh GPS. Thng ip iu hng bao gm ta ca v tinh,ta ny biu din di dng hm bin i theo thi gian v mt s thng tin cn thit khc.Tn hiu pht c iu khin bi nhng ng h nguyn t( atomic clocks) c chnh xc cao onboard trn nh ng v tinh.

2 dhspkt-hcm-2009


Hnh 2: Nhng phn on GPS

Hnh 3: m ha tn hi u dng phng php biphase Phn on iu khin ca h thng GPS bao gm mt mng li rng khp nhng trm theo di (tracking station), vi mt trm iu khin chnh (MCS-master control station) nh v Colorado Springs, Colorado, the United States. Nhim v ban u ca phn on iu khin l theo di du vt ca nhng v tinh GPS m nh v v tin

3 dhspkt-hcm-2009

GPS the global positioning systemon v tr v tinh, tnh trng h thng, hot ng ca ng h nguyn t, d liu kh quyn, nin gim v tinh (the satellite almanac) (tn hiu ny cha nhng thng tin v v tr ca v tinh trn qu o v c lu vo b nh ca my thu,khi v tinh di chuyn th cc thng tin ny c ng lin tc c cp nht vo my thu cng v i qua cc tn hiu m n gi i),v mt s s quan tm khc.Thng tin sau c ng gi v upload ln nhng v tinh GPS thng qua ng link bng S. Phn on ngi s dng bao gm dn thng v qun i.Vi mt b thu GPS kt ni vi mt antenna GPS, mt ngi s dng c th thu c tn hiu GPS, tn hi u ny sau c s dng xc inh v tr ca anh y hoc c y d ang sng bt c ni no trn th gii. Hin ti, GPS sn dng vi tt c ngi s dng khp ni trn th gii vi mc chi ph khng tr c tip (no direct charge).

1.3 Nhng th h v tinh GPS:

Hnh 4: Nhng th h v tinh GPS hi n hnh Ban u, chm sao v tinh GPS c xy dng vi mt chui 11 v tinh, gi l nhng v tinh Khi I. V tinh u tin trong chui ny (v trong h thng GPS) bt u hot ng vo ngy 22 thng 2 nm 1978, ci cui cng bt u hot ng vo ngy 9 thng 10 nm 1985. Nhng v tinh Khi I c xy dng ch yu dng vo th nghim.

4 dhspkt-hcm-2009


Gc nghing mt phng qu o ca nhng v tinh ny, so vi ng xch o, l 63 , sau c sa i trong nhng th h v tinh theo sau. Mc d thi gian sng trong hn 10 nm. V tinh Khi I cui cng c rt ra khi h thng dch v vo ngy 18 thit k ca nhng v tinh Khi I l 4.5 nm, mt vi ci duy tr trong dch v nhiu

thng 11 nm 1995. Th h v tinh th hai gi l nhng v tinh Khi II/IIA. Khi IIA l mt phin bn cao hn Khi II, vi s tng ln trong kh nng lu tr d liu thng ip iu hng t 14 ngy i vi Khi II n 180 ngy i vi khi IIA. iu ny c ngha rng nhng v tinh Khi II v Khi IIA c th thc hin chc nng mt cch lin t c ,m khng cn c s h tr t mt t, trong nhng khong thi gian t 14 n 180 ngy, tng ng ln lt vi hai h thng. Tng cng 28 v tinh Khi II/IIA c thi hnh trong khong thi gian t thng hai, 1989 n thng 11 nm 1997.Trong s nhng v tinh ny, 23 ci vn cn ang trong dch v. Khng ging nh v tinh Khi I, mt phng qu o ca nhng v tinh Khi II/IIA c gc nghing l 55 so vi mt phng xch o. Thi gian sng

thit k cho nhng v tinh Khi II/IIA l 7.5 nm, nhng thc t thng vt qu con s ny. m bo an ninh quc gia,mt vi tnh nng bo mt bao gm: kh nng c th la chn(SA-selective availability) v chng la o(antispoofing) c thm vo cu trc tn hi u ca nhng v tinh Khi II/IIA. Th h mi ca nhng v tinh GPS, c bit nh l Khi IIR, hin ti ang c thi hnh. Nhng v tinh b sung ny c kh nng tng thch tr li i vi nhng v tinh Khi II/IIA, iu ny c ngha l ngi s dng hon ton c th nm bt c nhng thay i trong Khi IIR. Khi IIR bao gm 21 v tinh vi thi gian sng c thit k l 10 nm. Thm vo , nh vo chnh xc cao hn nh mong i, nhng v tinh Khi IIR c kh nng hot ng mt cch c lp trong khong thi gian t nht l 180 ngy m khng cn s iu chnh t mt t hoc l xy ra thoi ha chnh xc. Kh nng nh v t tr ca th h v tinh ny t c mt phn nh vo cc v tinh ny c kh nng sp xp ln nhau (mutual satellite ranging capabilities.). V i s h tr ca

5 dhspkt-hcm-2009

GPS the global positioning systemephemeris v d liu clock c upload ln nh k trong khong thi gian 210 ngy bi phn on iu khin mt t nhm h tr nh v t tr. Hu ht nhng tnh nng ny c thm vo 12 v tinh cui cng ca Khi IIR, nm trong chng trnh hin i ha GPS, c tin hnh vo u nm 2003. Vo thng 7 nm 2001, su v tinh Khi IIR hot ng thnh cng. Theo sau Khi IIR l khi IIF (flow-on), bao gm 33 v tinh. Khong thi gian sng ca mi v tinh l 15 nm. Nhng v tinh khi IIF c nhng kh nng mi nh vo chng trnh hin i ha GPS, ci thin mt cch n tng tnh chnh xc trong ch nh v GPS t tr.

1.4 Chm sao GPS hin hnh:Chm sao GPS hin hnh cha 5 v tinh Khi II, 18 v tinh Khi IIA, v su v tinh Khi IIR. Nh vy, tng s v tinh GPS trong chm sao hin hnh l 29, vt qu chm sao-24 v tinh thng thng 5 v tinh.Tt c nhng v tinh Khi I khng cn hot ng na. Nhng v tinh GPS c t trong su mt phng qu o, nhng mt phng qu o ny c dn nhn t A n F. Khi m hin ti, h thng c nhiu v tinh hn chm sao 24-v tinh thng thng, mt mt phng qu o c th cha 4 hoc l 5 v tinh. Tt c mt phng qu o u c 5 v tinh, ngoi tr mt phng qu o C- ch c 4 v tinh. Nhng v tinh c th c nhn dng bi nhiu h thng khc nhau. H thng nhn dng v tinh ph bin nht trong cng ng ngi s dng GPS l SVN (Space V ehicle Number) v PRN (Pseudo Random Noise) (vd: SVN-48/PRN-07 l v tinh th su ca Khi IIR-M gm 31 v tinh). Khi v tinh II/IIA c trang b vi bn ng h nguyn t onboard: hai cesium (Cs) v hai rubidium (Rb). ng h cesium c s dng lm ng h thi gian s cp iu khin tn hiu GPS. Tuy nhin, nhng v tinh khi IIR ch s dng ng h rubidium.

6 dhspkt-hcm-2009

GPS the global positioning systemChm sao v tinh GPS 7/2001:

1.5 Nhng v tr iu khin (control sites):Phn on iu khin ca GPS bao gm mt trm iu khin chnh (MCS), mng rng khp nhng trm gim st, v nhng trm iu khin mt t(Hnh 5)

7 dhspkt-hcm-2009


Hnh 5: Nhng v tr iu khin C 5 trm gim st, nh v ti Colorado Springs(vi MSC), Hawaii, Kwajalein, Diego Garcia, v o Ascension. V tr ( hoc ta ) ca nhng trm gim st ny c xc nh chnh xc. Mi trm gim st c trang b vi nhng b thu GPS cht lng cao v mt b dao ng cesium nhm mc ch theo du vt lin tc tt c nhng v tinh GPS trong tm nhn. Ngoi ra, ba trm gim st (Kwajalein, Diego Garcia, v Ascension Island) cn c trang b vi nhng antenna mt t upload thng tin ti nhng v tinh GPS. Tt c nhng trm gim st v nh ng trm iu khin mt t khng c duy tr hot ng lin tc,v c MCS iu khin t xa.

8 dhspkt-hcm-2009

GPS the global positioning systemNhng quan st GPS thu thp c t nhng trm gim st c pht ti MCS x l. Kt qu x l bao gm d liu iu hng v tinh, v tr v tinh nh l mt hm ca thi gian, tham s ng h v tinh, d liu kh quyn, nin gim v tinh (satellite almanac), v nhng thng tin c n thit khc. D liu iu hng ti mi ny c gi ti mt trong nhng trm iu khin mt t upload ln nhng v tinh GPS thng qua ng link bng S. Ngoi ra, gim st tnh ton v n ca h thng GPS l mt trong nhng nhim v ca MCS. Trng thi ca v tinh c thit lp vi iu kin khng tt (unhealthy) bi MCS sut qu trnh ho t ng hoc ngng hot ng ca v tinh. Tnh trng v tinh xut hin nh mt phn ca thng ip iu hng v tinh, xc nh trn mt nn gn vi thi gian thc (near real-time basis). Trng thi cn hot ng hoc l ngng hot ng ca v tinh c lit k trong danh m c, sau , trng thi ny c tng thut trong mt thng ip gi l bn tin bo trc vi ngi s dng Navstar (Notice Advisory to Navstar Users (NANU)), bn tin ny sn dng vi cng chng thng qua, v d, Trung tm iu hng bo v bi bin M (the U.S. Coast Guard Navigation Center).

1.6 GPS - mt vi khi nim c bn:Khi nim pha sau GPS kh l n gin. Nu nh khong cch t mt im trn Tri t (mt b thu GPS ) ti ba v tinh c xc nh cng vi thng tin v v tr v tinh, vy th v tr ca im (hoc b thu) c th c m t bng cch p dng mt cch n gin nhng khi nim ca s ct b(resection). Tt c ch c vy. Nhng chng ta xc nh khong cch t v tnh ti im quan st cng nh v tr ca v tinh nh th no? Nh cp trc , mi v tinh GPS pht lin t c mt tn hiu v tuyn to thnh tng th t hai sng mang, hai m v mt thng ip iu hng. Khi b thu GPS v tr ON, n s thu ly tn hiu thng qua antenna b thu. Mt khi b thu nhn c tn

9 dhspkt-hcm-2009

GPS the global positioning systemhiu GPS, n s x l nh vo nhng phn mm tch hp bn trong. Kt qu x l bao gm cc khong cch ti nhng v tinh GPS (cn gi l tm gi -the pseudoranges) v ta v tinh c xc nh thng qua thng ip iu hng. Theo l thuyt ch duy nht cn 3 khong cch n 3 v tinh m c theo du vt mt cch ng thi. Trong trng hp ny, b thu s c nh v ti ch giao nhau ca ba hnh cu, mi hnh cu ny c mt bn knh tng ng vi khong cch v tinh-b thu v tm l v tinh (Hnh 6). Tuy nhin, thc t phi cn n 4 v tinh m t lch gia ng h b thu v ng h v tinh.

Hnh 6: Nguyn tc c bn nh v GPS Cho n tn gn y, chnh xc khi miu t vi phng thc ny, c gii hn 100 m theo tiu chun chnh xc ngang, 156m theo tiu chu n chnh xc dc, v 340 ns i vi thnh phn thi gian, kh nng xy ra l 95%. Mc chnh xc thp ny lin quan ti nh hng ca SA (Selective Availability), mt k thut c s dng lm suy gim mt cch c ch tm tnh chnh xc trong ch nh v thi gian thc t tr (the autonomous real-time positioning accuracy) v i nhng ngi s dng khng c cp php. Vi quyt nh ca Tng thng M v vic loi b k thut ny, chnh xc theo tiu chun ngang c trng i ci thin khong 22m (kh nng c th xy ra l 95 %). Xa hn na, ci thin tnh chnh xc trong nh v GPS, mt k thut gi l phng

10 dhspkt-hcm-2009

GPS the global positioning systemphp vi sai c s dng, trong s dng hai b thu theo du vt ng thi cng mt v tinh. Trong trng hp ny, c th t c mc chnh xc nh v t di mt centimet ti vi met. Li ch khc ca GPS l kh nng m t vn tc ca ngi s dng, m c th c xc nh bi vi phng php. Phng php c s dng rng ri nht l nh gi tn s Doppler ca tn hiu GPS thu c. Bit rng dch Doppler c xem nh l chuyn ng tng i gia b thu v v tinh. Ngoi ra, GPS cn c th c s dng m t thuc tnh ca nhng b phn cng (body), nh l my bay hoc l tu bin. T thuc tnh y c ngha l s nh hng hoc phng hng ca mt thn th cng, m c th c miu t bi ba gc xoay ca ba trc ca mt thn th cng cng vi s lu tm n h thng tham chiu (reference system). Thuc tnh ny c miu t bng cch trang b phn thn ti thiu l 3 b thu GPS (hoc mt b thu c bit) kt ni vi ba antenna, m c sp xp thnh mt ng khng thng. D liu c tp hp ti b thu sau c x l thu c thuc tnh ca phn thn cng ny.

1.7 Dch v nh v GPS:Nh ni trc y, GPS ban u c pht trin nh mt h thng qun i, nhng sau sn dng i vi tt c mi ngi dn. Tuy nhin gi li th ca qun i, U.S.DoD cung cp hai ch nh v v o thi gian (timing) GPS: dch v nh v chnh xc (the Precise Positioning Service (PPS)) v dch v nh v tiu chun (the Standard Positioning Service (SPS)). PPS l dch v nh v v o thi gian t tr chnh xc nht. N s dng mt trong nhng m pht GPS gi l m P(Y), m ch c th c truy nhp bi nhng ngi c cp php. Nhng ngi ny bao gm lc lng qun i M. Tnh chnh xc c mong ch trong ch nh v ny l 16m theo tiu chun chnh xc ngang v 23m theo tiu chun chnh xc dc (95 % kh nng c th xy ra).

11 dhspkt-hcm-2009

GPS the global positioning systemTuy nhin, SPSth t chnh xc h n PPS. N s dng m pht th hai gi l m C/A, cung cp min ph cho tt c mi ngi trn ton th gii, c ngi c cp php v ngi khng c cp php. Ban u SPS cung cp mc chnh xc nh v l 100m theo tiu chun chnh xc n gang v 156m theo tiu chun chnh xc dc (kh nng c th xy ra l 95%). iu ny t c trong iu kin c SA. Sau khi loi b SA, tnh chnh xc nh v t tr SPS trong hin ti c th so snh c vi PPS.

II - Nguyn l hot ng c bn ca GPS:2.1 Hot ng c bn:Cc v tinh GPS bay vng quanh Tri t hai ln trong mt ngy theo mt qu o rt chnh xc v pht tn hi u mang thng tin xu ng Tri t. Cc my thu GPS nhn thng tin ny v bng php tnh l ng gic tnh c chnh xc v tr ca ngi dng. V bn cht, my thu GPS so snh th i gian tn hi u c pht i t v tinh vi thi gian nhn c tn hi u ti b thu. Sai l ch v thi gian cho bit my thu GPS cch v tinh bao xa. Ri vi cc khong cch o c t b thu n v tinh, my thu c th tnh c v tr ca ngi dng v hin th ln bn in t ca my. My thu GPS phi kha c vi tn hi u ca t nht ba qu v tinh tnh ra v tr hai chiu (kinh v v ) v theo di c chuyn ng ca v tinh. Vi bn hay nhiu hn s lng v tinh hi n din trong tm nhn, my thu c th tnh c v tr ba chiu (kinh , v v cao). Mt khi v tr ngi dng tnh c th my thu GPS c th tnh thm cc thng tin khc, nh tc , hng chuyn ng, bm st di chuyn, khong hnh trnh, kho ng cch ti im n, thi gian Mt Tri mc, ln v nhi u thng tin khc na.

2.2 Hot ng ca GPS c th b nh hng bi cc yu t sau:

12 dhspkt-hcm-2009

GPS the global positioning system- Khi cc v tinh qu gn nhau, chng s khin vic xc nh v tr chnh xc tr nn kh khn hn. - V tn hi u radio i t v tinh xuyn qua tng in ly v tng i lu, tc cn thit tn hiu truyn ti thit b nhn s b chm i. H thng GPS c d phng iu bng cch tnh thm kho ng thi gian chm tr trung bnh, nh ng cng khng c hon ton chnh xc. - Chng ngi ln nh cc dy ni hay cc to nh cao tng cng lm cho thng tin b sai lch. - Gia thit b nhn (nht l ca ngi dng c nhn) v i v tinh (c th khng hon ton trng khp v mt thi gian, v cc v tinh i khi chy lch khi qu o.

III - chnh xc ca GPS:Cc my thu GPS ngy nay cc k chnh xc, nh vo thit k nhiu knh hot ng song song ca chng. Cc my thu 12 knh song song (c a Garmin) nhanh chng bm st cc qu v tinh khi v a mi c bt in ln v chng duy tr ch c chn lin h ny, thm ch trong tn l r m rp hoc trong thnh ph vi cc to nh cao tng. Tnh trng nht nh ca kh quyn v cc ngun gy sai s khc c th nh hng ti chnh xc ca my thu GPS. Cc my thu GPS c chnh xc trung bnh trong vng 15 mt. Cc my thu mi hn vi kh nng W AAS (Wide Area Augmentation System) c th tng chnh xc trung bnh t i di 3 mt. Khng cn thm thi t b hay mt ph c c li im ca W AAS. Ngi dng cng c th c chnh xc t t hn vi GPS Vi sai (Differential GPS, DGPS) sa li cc tn hiu GPS c chnh xc trong khong 3 n 5 mt. Cc Phng v B bin M vn hnh d ch v sa li ny. H thng bao gm mt mng cc i thu tn hi u GPS v pht tn hi u sa li bng cc my pht hiu chnh. thu c tn hi u sa li, ngi dng phi c my thu tn hi u vi sai bao gm c n-ten dng cng vi my thu GPS ca h

13 dhspkt-hcm-2009

GPS the global positioning systemIV - Tn hiu GPS:Cc v tinh GPS pht hai tn hi u v tuyn cng sut thp di L1 v L2. (di L l phn sng cc ngn ca ph in t tri rng t 0,39 ti 1,55 GHz). GPS dn s dng tn s L1= 1575.42MHz trong di UHF. Tn hiu truyn trc th, c ngha l chng s xuyn qua my, thu tinh v nh a nhng khng qua phn ln cc i tng cng nh ni v nh. L1 cha hai m "gi ngu nhin" (pseudo random), l m Protected (P) v m Coarse/Acquisition (C/A). Mi mt v tinh c mt m truyn dn nht nh, cho php my thu GPS nhn dng c v tinh thng qua tn hi u. Mc ch ca cc m tn hi u ny l tnh ton kho ng cch t v tinh n my thu GPS. Tn hi u GPS cha ba thnh phn thng tin khc nhau m gi ngu nhin, d liu thin vn v d liu lch. M gi ngu nhin n gin ch l m nh danh xc nh c qu v tinh no v ang pht thng tin no. C th nhn s hiu ca cc qu v tinh trn trang v tinh ca my thu Garmin bit n nhn c tn hiu ca qu no. D liu thin vn cho my thu GPS bit qu v tinh u trn qu o mi thi im trong ngy. Mi qu v tinh pht d liu thin vn ch ra thng tin qu o ca v tinh v cc v tinh khc trong h thng. D liu lch c pht u n bi mi qu v tinh, cha thng tin quan tr ng v trng thi ca v tinh (tt hay khng tt), ngy gi hin ti. Phn ny ca tn hi u l ct li c th tin hnh nh v.

V - Chi tit v GPS: c th nh v hoc tm v tr ca ngi s dng vi GPS th i hi cn phi hiu v cu trc tn hi u GPS v phng php o c c thc hin. Hn th na, khi

14 dhspkt-hcm-2009

GPS the global positioning systemtn hiu GPS c thu thng qua b thu GPS, hi u v kh nng v nhng gii hn ca cc dng b thu GPS khc nhau l iu cn thit. Ngoi ra, khi o lng GPS, ging nh tt c nhng s o lng khc, u c li, m c th c loi b hoc gim thiu bng cch kt hp nhiu s quan st GPS khc nhau.

5.1 Cu trc tn hiu GPS:Mi v tinh GPS u pht i tn hi u v tuyn c to thnh t hai tn s sng mang (hoc l sng sin); hai thnh ph n sng mang ny dng iu ch hai b m v mt thng ip iu hng (Hnh 7). Hai tn s sng mang c pht ti 1,575.42 Mhz (L1) v 1,227.60 Mhz (L2). Bc sng tng ng ln lt l ~ 19 cm v ~24.4 cm, xc nh c kt qu ny l t mi quan h gia tn s sng mang v tc ca nh sng trong khng gian. L i ch t hai tn s sng mang cho php iu chnh mt li GPS nghim trng, gi l tr hon t ng in ly. Tt c nhng v tinh GPS u pht cng m t cp tn s sng mang L1 v L2. Tuy nhin, iu ch m th khc nhau i vi tng v tinh, do ti thiu ha mt cch ng k can nhiu v tinh. o L1 = 154 10.23 = 1575.42 Mhz o L2 = 120 10.23 = 1227.60 Mhz

15 dhspkt-hcm-2009

GPS the global positioning systemHnh 7: (a) sng sin (b) m s Hai b m GPS c gi l Coarse Acquisition (hoc l m C/A) v Precision (hoc l m P). Mi m bao gm mt chui nhng bit nh phn, 0 v 1. Thng th ng nhng m ny gi l m PRN bi v b ngoi chng trng nh l nhng tn hi u ngu nhin. Nh ng s tht, nhng m ny c pht ra da vo cng thc ton hc. Hin ti m C/A c iu ch ch duy nht trn sng mang L1, trong khi m P c iu ch trn c sng mang L1 v L2. S iu ch ny c gi l iu ch Biphase v pha sng mang b dch i 180 khi m gi tr m thay i t 0 sang 1 hoc t 1 sang 0.

M C/A l mt chui bao gm 1,023 bit nh phn m l p li chnh n trong m i

millisecond.iu ny c ngha rng tc bit ca m C/A l 1.023 Mbps.Nh vy khong thi gian ca mt bit xp x khong 1ms.Mi v tinh c gn vi ch mt m C/A duy nht ,iu ny lm cho b thu GPS c th nhn dng c v tinh d dng.o lng m C/A t chnh xc h n khi so snh m t cch tng i vi s o lng m P .Tuy nhin ,n th t ph c tp hn v sn dng vi tt c mi ngi. M P l mt chui rt di bao gm nhiu bit nh phn m l p li chnh n sau 266 ngy. Ngoi ra, tc ca n th nhanh h n 10 ln so vi m C/A (tc ca n l 10.23 Mbps). Thc hin php nhn th i gian m m P cn lp li chnh n, 266 ngy, vi tc ca n l 10.23Mbps, cho ra kt qu: chiu di ca chui m P l 2.35 10 bit. Mdi-266-ngy ny c chia thnh 38 on; m mi on di 1 tun. Trong , 32 on c gn n nhng v tinh GPS khc nhau. l, mi v tinh pht i ch duy nht mt on-di-1-tun ca m P , m c khi pht vo thi im giao nhau gia th by v ch nht. Su on cn li c d tr dng trong nhng vic khc. Mt v tinh GPS c nhn dng bi mt phn on-1-tun duy nht c gn cho n trong m P. V d: v tinh GPS vi ID l PRN 20 c xc nh l v tinh GPS c gn phn on1-tun th 20 ca m P . Ban u, m P c thit k v nhng mc ch qun s. N lun

16 dhspkt-hcm-2009

GPS the global positioning systemsn dng vi tt c mi ngi s dng cho n ngy 31 thng 1 n m 1994. Ti thi im , m P c vit li bng cch kt hp vi m W. Kt qu l s ra i ca m Y; m ny c cng tc bit vi m P . M mi ny cn c gi l m AS (AntiSpoofing). o Chipping rate for C/A is 1.023 Mhz o Chipping rate for P(Y) is 10.23 Mhz

Tn hiu iu hng GPS l mt chui d liu c thm vo c hai sng mang L1 v L2 bng phng php iu ch biphase nh phn vi tc pht l 50Kbps. N bao gm 25 frame, mi frame gm 1,500 bit, tng cng l 37,500 bit. Do , truyn dn hon ton thng ip iu hng phi mt ht 750 giy hoc l 12.5 pht. Thng ip iu hng bao gm nhng thng tin nh : ta ca v tinh GPS di dng hm ca thi gian, tnh trng v tinh, thng tin hi u chnh ng h v tinh, nin gim v tinh v d liu kh quyn. Mi v tinh truyn dn thng ip iu hng ca chnh n cng v i thng tin ca nhng v tinh khc, bao g m v tr gn chnh xc c a v tinh v tnh tr ng hot ng. Biu m ca GPS v pha ca sng mang l:

17 dhspkt-hcm-2009


Hot ng khi phc modul ca m GPS:

Di tn ph tn hiu ca GPS: Ch rng m C/A c cho bn di vi mc nhiu thp, tn hi u a m trong b thu bi on m c tnh ton bn trong c p cho cc k t.

18 dhspkt-hcm-2009

GPS the global positioning systemVic tnh ton ph cng sut c xut pht t s bin i Fourier ca mt sng vung vi rng l 2pi v bin bng 1. Hm trong DSP l Sinc:

sin(x) 1 sin c(x) = = x 2Power = P(t) = y2(t)

e

ix

5.2 Hin i ha GPS:Cu trc tn hi u GPS hin hnh c thit k vo nm 1970, cch y khong 39 nm. Trong nhng nm sp ti, Hy vng chm sao GPS c thm s kt hp ca nhng v tinh Khi IIR, Khi IIF v c kh nng l nhng v tinh Kh i III. p ng nhng i hi

19 dhspkt-hcm-2009

GPS the global positioning systemtng lai, cc nh ch to GPS nghin cu mt vi phng n iu chnh c th nhm sa i mt cch thch h p cu trc tn hi u v kin trc h thng ca chm sao GPS tng lai. Chng trnh i mi ny nhm n vic cung cp d tha tn hi u (signal redundancy) v ci thin tnh chnh xc nh v, tn hi u lun sn dng v tch h p h thng. Chng trnh hi n i ha bao gm: B sung m C/A trn sng mang L2 v m qun s mi (m M) trn c hai sng mang L1 v L2. iu ch m C/A trn c hai tn s sng mang L1 v L2 cho php ngi s dng vi ch duy nht mt b thu GPS c th thu c tn hiu chnh xc mc d vn chu nh hng ca tng in ly. Sau khi loi b SA v s lng v tinh y cng vi nhng tnh n ng mi, hy vng rng chnh xc theo tiu chun ngang ca h thng GPS s t c khong 8.5m (95% th i gian) hoc tt hn. iu ch m C/A vo sng mang L2, mc d ci thin chnh xc ca h thng GPS, nhng vn cha chc chn s dng trong nh ng ng dng hng khng dn d ng v nhng ng dng ny i hi m bo an ton tuyt i. Nguyn nhn ch yu l do ngun gy can nhiu rt ln t nhng rada mt t hot ng gn di L2. Nh vy, p ng nhng yu cu ca ngi s dng hng khng, tn hi u dn dng th ba c tn s 1,176.45 Mhz (gi l L5) s c thm vo 12 v tinh u tin ca khi IIRF cng vi m C/A trn L2 v m M trn L1 v L2. So v i L1 v L2, L5 c nng lng cao hn. Ngoi ra, L5 c mt bng thng qung b rng (ti thiu l 20Mhz), tc bit cao hn (10.23Mhz), do cung cp chnh xc cao hn trong iu kin nhi u v a ng. M mi s di hn m C/A hin ti, do gim c s t can nhiu h thng thng qua vic ci thin nhng c tnh tng quan cho v t ng (auto- and cross-correlation). Cui cng, xt n thng ip iu hng qung b ca L5, d mang d liu nhiu hn hoc t hn so vi nhng knh L1 v L2, s c mt cu trc hon ton khc bi t v hiu qu hn. B sung nhng kh nng ny s ci thin mt cch ng k tnh chnh xc trong nh v ca h thng GPS t tr.Tng t, nhng ngi s dng ch nh v RTK

20 dhspkt-hcm-2009

GPS the global positioning system(Real-Time Kinematic) c th phn tch c ngay lp tc nhng tham s nguyn ambiguity lc u. Hin i ha GPS bao gm lun c vic nghin cu nhng v tinh th h Khi III k tip, d kin hon thnh vo n m 2030. Cui cng, nhng phng tin iu khin ti mt t ca h thng GPS cng s c nng cp ln nh mt phn ca chng trnh hi n i ha GPS. Cng vi s nng cp ny, chnh xc theo tiu chu n ngang ca h thng GPS c ch i s l 6 m (95% th i gian) hoc tt hn.

5.3 Nhng dng b thu GPS:Vo nm 1980, ch duy nht mt b thu GPS thng mi sn dng trn th trng vi mc gi vi trm ngn $ U.S. Tuy nhin iu ny thay i mt cch ng k, khi m trn th trng ngy ny, c hn 500 b thu GPS khc nhau. Ga tr nhng b thu hi n hnh thay i t $100 cho nhng b thu cm tay n gin v khong $15,000 i vi nhng b thu phc tp, cht lng cao dng trong tr c a hc. Ga s tip tc gim trong tng lai khi m cng ngh sn xut b thu tr nn tin ti n hn. Mt b thu GPS i hi mt anten gn km vi n, gn bn trong hoc bn ngoi. Anten nhn tn hi u v tinh n, chuyn i nng lng ca tn hi u thnh dng in, v sau , b thu s x l tn hiu thu c. Nhng b thu GPS thng mi c th c chia thnh 4 lo i, xt n kh nng thu ca tng loi. l: single-frequency code receivers; single-frequency carrier-smoothed code receivers; single-frequency code and carrier receivers; dual-frequency receiver. Nhng b thu n tn ch duy nht truy cp tn s L1, trong khi nh ng b thu song tn truy cp c tn s L1 v L2 (chn hnh b thu). Ngoi ra, cc b thu GPS cn c xp loi da vo s lng knh m n c th thu c, thay i t 1 n 12 knh. Mt b thu GPS tt s c chc nng a knh, vi mi knh dnh quan st lin t c ring mt v

21 dhspkt-hcm-2009

GPS the global positioning systemtinh. Hin ti, hu ht nhng b thu GPS u c t 9 n 12 knh c lp (hoc song song). Cc c im khc nh:gi c, d s dng, tiu th nng lng, kch thc v trng lng,kh nng lu tr d liu bn trong v m rng bn ngoi, kh nng giao tip s c xem xt khi la chn mua mt b thu GPS. Dng b thu GPS u tin, singlefrequency code receivers, khi tin hnh o c tm gi (pseudoranges) ch s dng duy nht m C/A. Khng m t o c no khc sn dng. y l dng b thu r nht v ng thi cng t chnh xc nh t, hu nh c s dng v mc ch gii tr. Dng b thu th hai, single-frequency carrier-smoothed code receivers, cng ch dng duy nht m C/A o c tm gi nhng kt hp thm tn s sng mang c phn tch cao hn bn trong my x l s liu o c c, do tm gi m my o s c chnh xc cao hn. Nhng s liu m dng b thu th ba o c c bao gm: tm gi dng m C/A di dng th, kt qu o c pha-sng mang L1, v thng ip iu hng. Ngoi ra, dng b thu ny cng c kh nng thc hin cc chc nng ging vi nhng dng b thu kia. B thu song tn l dng b thu t tin nht v tinh vi nh t. Trc khi kch hot AS, b thu ny c kh nng xut ra tt c nhng thnh ph n tn hi u GPS (sng mang L1 v L2, m C/A, m P trn c L1 v L2, v thng ip iu hng). Tuy nhin, sau khi kch hot AS, m P c m ha thnh m Y . Do , b thu khng th xut ra ngoi m P hoc l sng mang L2 s dng k thut thu tn hi u truyn thng. gii quyt vn ny, nhng nh s n xut b thu GPS pht minh ra nh ng k thut khng i hi thng tin ca m Y . Hin ti, hu ht nhng b thu s dng hai k thut ph bin bao gm k thut theo di du vt Z (Z-tracking) v k thut tng quan cho (cross-correlation). C hai k thut ny khi phc li c sng mang L2 mt cch y nht nhng cng tn hiu b suy gim nhiu. suy gim ny trong k thut tng quan cao hn so vi k thut Z.

5.4 Nhng h thng thi gian:

22 dhspkt-hcm-2009

GPS the global positioning systemThi gian th c hin mt vai tr quan trng trong nh v vi GPS. Tn hiu GPS c iu khin bi nhng thit b o thi gian chnh xc - ng h nguyn t. Ngoi ra, o lng khong cch v tinh - b thu u da trn c ng h v tinh v ng h b thu.Ngoi ra, GPS cng l mt h thng o lng thi gian, c s dng ng b ha thi gian. Nhiu h thng thi gian c s dng trn ton th gii vi nhiu mc ch khc nhau. Trong s , Coordinated Universal Time (UTC) v GPS Time l hai h thng thi gian quan tr ng nht i vi ngi s dng GPS. UTC l mt h thng da trn International Atomic Time(TAI). TAI l thang o thi gian (time scale) chnh th c, c tnh ton da trn nhng thang o thi gian (time scale) c lp m c pht ra bi nhng ng h nguyn t t ti cc phng th nghi m thi gian khc nhau trn ton th gii. Tuy nhin, cng vic trc a v iu hng i hi mt h thng thi gian c mi lin h vi hot ng xoay trn ca Tri t (khng phi l ng h nguyn t). Phng php gii quyt: iu chnh thng xuyn thang o thi gian (time scale) UTC bng cch tng ln 1s, gi l giy nhun (leap seconds), gi n sai lch ch 0.9 giy i vi mt thang o thi gian (time scale) khc g i l Universal Time 1(UT1), vi UT1 l thang o thi gian quc t c mi lin h vi hot ng xoay trn ca tri t. Nhng leap-second thnh thong c a vo ngy 30/6 hoc l vo ngy 31/12. K t thng 7 nm 2001, leap second c a vo ln cui vo ngy 1, thng 1 n m 1999, to ra sai s gia TAI v UTC chnh xc n 32 giy (TAI gi tr cao hn UTC). GPS Time l thang o thi gian (time scale) c s dng tham kho, hoc l gn km thi gian vi tn hi u GPS. Thang o ny c tnh ton da trn nhng thang o thi gian m c pht ra bi nhng ng h nguyn t ti nhng trm gim st v trn nhng v tinh GPS onboard. Khng c leap second no c a vo GPS Time, do GPS Time l mt thang o thi gian (time scale) lin t c. GPS Time c thit lp trng vi UTC vo ngy 6/1/1980. Tuy nhin, lin quan n nhng leap second c a UTC, GPS Time nhanh hn UTC 13 giy vo ngy 1/1/1999. Thng tin sai s gia UTC

23 dhspkt-hcm-2009

GPS the global positioning systemv GPS Time gn km trong thng ip iu hng GPS. C ng h b thu v ng h v tinh GPS u sai lch so vi GPS Time, nguyn nhn t li ca c ng h b thu v ng h v tinh.

5.5 o c tm gi:

Hnh 8: o lng tm gi Tm gi (pseudorange) l khong cch gia b thu GPS v v tinh GPS (chnh xc hn l gia anten ca b thu v anten ca v tinh). Khong cch t b thu n v tinh l mt thng s cn thit tnh ton v tr. M P hoc m C/A c th c s dng o lng tm gi(Hnh 8). Th tc xc nh tm gi GPS c th c m t nh sau. Chng ta hy t m thi tha nhn rng ng h b thu v ng h v tinh hon ton ng b vi nhau. Khi m m PRN c pht ra t v tinh, b thu cng s pht ra m t bn sao ging ht m . Sau mt khong thi gian, t ng ng vi khong thi gian tin hi u truyn i trong khng gian, m pht s c thu bi b thu. Bng cch so snh m pht v b n sao ca n, b thu c th tnh ton c thi gian m tn hi u truyn i tnh t lc pht. Thc hin php nhn tn hi u vi tc ca nh sng (299,729,458 m/s) cho ra k t qu l khong cch t v tinh n b thu.

24 dhspkt-hcm-2009

GPS the global positioning systemTuy nhin, gia ng h b thu v ng h v tinh khng h ng b. Trong thc t, khong cch gia b thu v b pht o c b sai lch bi li ng b gia ng h b thu v v tinh, cng v i nhng li khc na. V vy thng s o c gi l tm gi (pseudorange) ch khng phi l mt tm thc s. GPS c thit k m tm o bi m dn dng C/A phi km chnh xc h n khi dng m qun s P . Bi v phn gii ca m C/A l 300m, nh hn 10 ln so vi m P . Nhng vi s ci tin trong cng ngh thu, chnh xc t c gn nh l tng t i vi c hai m.

5.6 o c pha-sng mang:Mt cch khc o c khong cch v tinh-b thu l s dng k thut o lng pha-sng mang. B thu v v tinh pht ra cng m t tn hi u sng mang trong cng th i im. B thu so snh tn hiu sng mang b tr hon do v tinh pht v i sng mang ca chnh b thu pht ra. Khong cch c tnh bng cch cng s bc sng y ca sng mang vi dch pha (o c ti b thu,

25 dhspkt-hcm-2009

GPS the global positioning systemkhong ~1% chiu di bc sng) (Hnh 10). Kt qu o c chnh xc h n khi o c dng m. Nguyn nhn b i v bc sng ca sng mang-i vi L1 l 12 cm, cn L2 l 24 cm - nh hn rt nhiu so vi rng bit ca m. Hnh 9: o c pha sng mang

Tuy nhin, c mt vn . Nhng sng mang ny u l nhng sng sin thun ty, v vy tt c chu k u ging nhau. Do , b thu GPS khng c cch no c th phn bit chu k ny vi chu k kia. Ni cch khc, b thu khng th no xc nh c s lng bc sng truyn gia v tinh v b thu. V vy, s lng bc sng y cn phi xc nh gi l tham s ambiguity. May mn l b thu c kh nng pht hi n nhng s thay i pha. Do tham s ambiguity duy tr khng i ton thi gian,mi n l khng xy ra mt mt tn hiu.

Hnh 10: full: s bc sng y ca song mang; final portion: dch pha

26 dhspkt-hcm-2009

GPS the global positioning systemR rng rng, nu vn tham s ambiguity c gii quyt, khong cch chnh xc c th o c, kt qu l s nh v chnh xc. nh v chnh xc c th t c thng qua nhng k thut nh v tng i trong thi gian th c hoc trong ch postprocessing. Khng may l, t c iu ny, i hi phi c hai b thu GPS theo di lin tc cng mt v tinh hi n hu trong tm nhn.

5.7 Trt chu k (cycle slip):Trt chu k l s gin on hoc mt c nhy no bn trong o lng phasng mang, gy ra bi s mt mt tn hi u tm thi (Hnh 11). Mt mt tn hi u lin quan n nhng chng ngi vt m cn tr tn hiu v tinh GPS nh l: nhng ta nh, cy, cu v nhiu i tng khc. Ch yu l do tn hi u GPS yu v s can nhi u tn hi u. Can nhi u tn hiu radio, tnh tr ng ri lon tng in ly mnh li t, v tr khng cao ca b thu cng gy ra s mt mt tn hi u. Trt chu k cng c th b gy ra bi nhng trc trc ca chnh b thu. Trt chu k c th xy ra trong mt thi gian ngn hoc c th duy tr trong vi pht, thm ch c th hn. Trt chu k c th nh hng n mt tn hi u v tinh hoc nhiu hn. Trt tn hi u phi c nhn dng hoc c iu chnh m trnh gy l i ln n nhng ta nh v c tnh ton. C vi ph ng php thc hin iu ny. Kim tra quan st sai phn b ba (triple difference observable), phng php ny c th c nh hnh b ng cch kt hp nhng quan st GPS khc nhau theo m t cch no , y l phng php ph bin nht trong thc t. Mt trt tn hi u (cycle slip) s ch duy nht nh hng n mt sai phn b ba (triple difference) v do n s ch xut hin nh mt gai trong chui d liu sai phn b ba. Trong mt vi trng hp, nh l xy ra nhng hot ng mnh lit trong tng in ly, tht kh d ra v sa cha mt cch chnh xc tr t tn hi u (cycle slip) s dng phng php quan st sai phn b ba (triple

27 dhspkt-hcm-2009

GPS the global positioning systemdifference observable). Kim tra li d tha iu chnh (adjustment residuals) c th c ch trong vic d ra bt k trt chu k cn li no.

Hnh 11: Trt chu k GPS Mt phng php gi l kim tra baseline mc khng (zero baseline test) c s dng d ra trt tn hi u m nguyn nhn gy ra l do tr c trc b thu. i vi phng php ny, hai b thu c kt ni n mt anten thng qua m t b chia tch tn hi u (splitter). Trt chu k c th c d bng cch kim tra d tha iu chnh (adjustment residuals).

5.8 Kt hp tuyn tnh nhng s quan st GPS:

28 dhspkt-hcm-2009

GPS the global positioning systemo lng GPS b lm cho sai lch i bi nhng li (errors and biases), m th t kh c th a ra mt m hnh y v chnh xc. Nh ng li ny gii hn tnh chnh xc nh v ca nhng b thu GPS c lp. May mn l nhng b thu gn ging nhau c cng dng li. Nh vy, i vi nhng b thu ny, mt s lng ln li GPS c th b loi b mt cch n gin, bng cch kt hp nhng thng tin m cc b thu ny quan st c. V nguyn tc, c ba nhm li tn hi u GPS: li lin quan n b thu, li lin quan n v tinh, l i kh quyn. S liu o c c t hai b thu GPS theo du vt ng thi cng mt v tinh s c cng dng li: li lin quan n v tinh v li lin quan n kh quyn. Thu hp khong cch gia hai b thu li th nhng li ny cng tng ng nhau hn. Do , nu chng ta tm ki m nhng thng s khc nhau t s liu o c c ca hai b thu,nhng dng li ny s c gim thiu mt cch ng k. Thc t l, li xung ng h v tinh b loi b mt cch ng k khi p dng phng php kt hp tuyn tnh ny. Phng php kt hp tuyn tnh ny cn c gi l s sai bit n gia nhng b thu (between-receiver single difference) (Hnh 12).

29 dhspkt-hcm-2009

GPS the global positioning systemHnh 12: Mt vi phng php kt hp tuyn tnh GPS Tng t, khi ti n hnh o c hai ln s dng cng mt b thu GPS quan st hai v tinh khc nhau, s liu o c c s c cng li ng h b thu. Do , tm kim s sai bit gia hai thng s o c ny s c th loi b c li ng h b thu. S sai bit ny gi l s sai bit n gia nhng v tinh (between-satellite single difference). Khi hai b thu quan st hai v tinh mt cch ng thi, kt qu thu c s l hai sai bit n gia nhng b thu (between-receiver single difference). Ly hai s liu ny tr cho nhau, kt qu thu c gi l s sai bit i (double difference). Phng php ny loi b gn nh hon ton li ng h ca b thu v v tinh.Nhng li khc cng c gim thiu mt cch ng k.Ngoi ra, phng php ny gip duy tr bn cht s nguyn ca cc tham s ambiguity Do , n c s dng trong ch nh v GPS chnh xc da trn pha-sng mang. Mt phng php kt hp tuyn tnh quan tr ng khc gi l sai phn b ba (triple difference), y l kt qu t vic tnh hi u s gia hai quan st sai bi t i (double-difference) qua hai khong thi gian. Tham s ambiguity duy tr khng i min l khng c mt trt chu k no xy ra (cycle slips). Nh vy, khi s dng phng php sai phn b ba (triple difference), tham s ambiguity s b trit tiu. Tuy nhin, nu c mt trt tn hi u (cycle slip) trong d liu, n s nh hng ch duy nht n mt s quan st sai phn b ba (triple-difference), v do s xut hin nh l mt gai trong chui d liu sai phn b ba (triple -difference). V l do ny m phng php kt hp tuyn tnh hai sai phn b ba (triple -difference) c s dng d ra nhng trt tn hiu (cycle slip). Tt c cc phng php kt hp tuyn tnh ny c th c s dng ch vi d liu n tn, c th l d liu quan st pha-sng mang ho c d liu quan st tm gi (pseudoranges). Nu d liu song tn sn dng,ta c th s dng nhng phng php kt hp tuyn tnh khc. M t trong nhng phng php kt hp tuyn tnh l ph ng php

30 dhspkt-hcm-2009

GPS the global positioning systemkt hp tuyn tnh t do-tng in ly (ionosphere-free linear combination). Kt hp nhng quan st t do-tng in ly (ionosphere-free) vi nhng thng s L1 v L2 c th kh c hiu ng ca tng in ly. Ngoi ra, nhng s liu thu c t o c pha ca sng mang L1 v L2 c th kt hp vi mt tham s gi l quan st ng rng (widelane), y l tn hiu nhn t o vi bc sng khong 86 cm. Tn hi u vi bc sng di ny c s dng trong phn tch nh ng tham s ambiguity.

5.9 Nhng dng li ca GPS:C tm gi GPS v nhng o c pha-sng mang u b nh hng bi cc li (errors and biases). Nhng li ny c th c phn lp da vo ngun gc pht sinh t i v tinh, t i b thu, ngun gc pht sinh lin quan n qu trnh truy n tn hi u (nh l khc x ca kh quyn) (figure). Nhng li ngun gc t v tinh bao gm ephermeris hoc cn gi l li qu o (v v tinh c th i chch qu o cht t nn trm iu khin mt t s lu gi cc thng tin v qu o , cao, v tr v tc ca v tinh. Nhng thng tin ny t v tinh pht xung trung tm iu khin, y n s c x l v gi ln v tinh hiu chnh, thng tin chnh xc sau khi c x l thng theo tn xut 4-6 h mt ln - c gi l ephermeris s c gi theo tn hi u v tinh n my thu), nhng li ng h v tinh, v nh hng ca li nhn t o SA. SA c c tnh a vo bi U.S, DoD nhm lm suy gim tnh chnh xc c a h thng GPS t tr v nhng l do bo mt. Tuy nhin, SA b loi b vo ngy 1/5/2000. Nhng li c ngun gc ti b thu bao gm: li ng h b thu, li a ng, nhi u b thu, v nhng thay i pha trung tm c a anten. Li truyn tn hiu bao gm tr hon tn hi u GPS khi i qua tng in ly v tng i lu ca bu kh quyn. Thc t, ch duy nht trong mi trng chn khng (khng gian t do), tn hiu GPS mi di chuyn hoc truyn dn c vi tc nh sng.

31 dhspkt-hcm-2009

GPS the global positioning systemNgoi ra, chnh xc nh v ca GPS khi tnh ton cn b nh hng bi v tr hnh hc ca nhng v tinh GPS c quan st bi b thu. Nhng v tinh GPS cng tr i rng ra trn b u tri th chnh xc t c cng cao. Mt vi li ny c th c lm gim i hoc b kh b bng cch thng qua phng php kt hp tuyn tnh nh ng quan st GPS. V d nh, kt hp nhng quan st L1 v L2 s loi b c gn nh hon ton nh hng ca tng in ly. M hnh ton hc ca nhng li ny hon ton c th xc nh c.

5.9.1 Li ephermeris GPS:V tr ca v tinh l m t hm theo th i gian, hin din trong thng ip iu hng v tinh qung b, c th c d on trc t nhng s quan st GPS trc ti nhng trm iu khin mt t. in hnh, d liu GPS xp chng trong m i khong thi

gian 4h c s dng bi nhng h thng iu khin chc nng d bo trc nhng thnh phn qu o v tinh mi trong mi

32 dhspkt-hcm-2009

GPS the global positioning systemHnh 13: Nhng dng li GPS chu k l 1 gi. Ni chung, m hnh cc l c tc ng ln v tinh thng l khng hon ho, gy ra mt vi li i vi nhng v tr v tinh c xc nh, gi l li ephermeris. Thng thng, nhng li ephemeris vo khong t 2m n 5m, v c th l 50m di nh hng ca SA. Nhng li khong cch t s kt hp ca li ephemeris v li ng h v tinh c gi tr l 2.3m [mc 1: vi l lch tiu chun] (Hnh 13). Li ephemeris ca mt v tinh l l i in hnh i vi tt c mi ngi s dng GPS trn ton th gii. Tuy nhin, nhng ngi s dng khc nhau th y cng mt v tinh di nhng gc nhn khc nhau, nh hng ca li ephemeris i vi o lng khong cch v v tr tnh ton th khc nhau. Do , kt hp (ly sai phn) nhng o c khc nhau ca hai b thu theo du vt ng thi ng thi cng mt v tinh khng th no loi b hon ton li ephemeris. Tuy nhin, vi nhng ngi s dng h thng GPS gn nhau v khng gian,hu nh chc chn s c mt li khong cch in hnh, lin quan n li ephemeris,m c th c loi b d dng thng qua vi c ly sai phn nhng quan st.

5.9.2 Selective availability:Ban u, GPS c thit k m nh v v iu hng t tr, thi gian thc vi nhng b thu m C/A s t chnh xc h n i vi nhng b thu m qun s -m P . Nhng xt trn thc t, c hai b thu u chnh xc nh nhau. m bo an ninh qu c gia U.S.DoD kch hot SA trn nhng v tinh GPS Khi II nhm loi b chnh xc trong nh v t tr thi gian thc. SA chnh thc hot ng vo ngy 25/3/1990 (Hnh 14). SA to ra hai dng li. Dng u tin gi l li delta, nguyn nhn xu t pht t nhng tc ng i vi ng h v tinh, y l li ph bin i vi tt c ngi s dng trn ton th gii. Dng th hai gi l li epsilon, hay cn gi l li qu o thay i chm. Vi SA, nhng li ngang v dc c th ln ti 100m v 156m, vi 95 % kh nng

33 dhspkt-hcm-2009

GPS the global positioning systemc th xy ra. Tng t nh li khong cch lin quan n li ephemeris, li khong cch lin quan n li epsilon hu nh l in hnh i vi nhng ngi s dng GPS c khong cch gn nhau v khng gian. Do , s dng h thng GPS sai phn (DGPS) s loi b c nh hng ca li epsilon. Xt trong th c t DPGS cung cp chnh xc tt hn b thu m P c lp, lin quan n kh nng loi tr hoc gim thiu nhng li thng thng, bao gm c SA. Chnh ph M loi b SA vo ngy 1/5/2000, kt qu l chnh xc ca h thng GPS c ci thin nhiu. chnh xc theo tiu chu n ngang v d c ln lt l 22m v 33m (95 % kh nng c th xy ra) (Hnh 15).

34 dhspkt-hcm-2009


Hnh14: Nhng thay i v tr nh v khi c SA

Hnh 14: Nhng thay i v tr nh v khi c SA

35 dhspkt-hcm-2009

GPS the global positioning systemHnh 14: Nhng thay i v tr nh v khi c SA

Hnh 14: Nhng thay i v tr nh v khi c SA

36 dhspkt-hcm-2009

GPS the global positioning systemHnh 15:nhng thay i v tr nh v sau khi loi b SA Mc d s kin loi b SA khng nh hng nhiu n chnh xc ca DGPS, nhng cng gip ch trong vi c gim chi ph ci t v iu hnh h thng DGPS. Nguyn nhn ch yu bi s suy gim i vi tc truyn dn theo yu cu.

5.9.3 Li ng h b thu v ng h v tinh:Mi v tinh GPS Khi II v Khi IIA u c 4 ng h nguyn t: 2 ng h cesium v 2 ng h rubidium. Nhng th h v tinh m i hn, Khi IIR, ch c duy nht ng h loi rubidium. Mt trong nhng ng h ny, ban u l ng h cesium i vi c Khi II v Khi IIA, c la chn cung cp tn s v nhng yu cu v thi gian nhm pht ra tn hi u GPS. Nhng ci cn li ch l d phng. Nhng ng h v tinh GPS, mc d c chnh xc cao, nh ng li khng hon ho. n nh trong kho ng t hoc qua mi chu k mt ngy. Do , li ng

h v tinh vo khong 8.64 n 17.28ns mi ngy. Li khong cch tng ng vo khong 2.59 m n 5.18 m, ta c th d dng tnh ton b ng cch thc hin php nhn l i ng h vi tc nh sang (~299,729,458m/s). So snh v i ng h rubidium, nh ng ng h cesium c khuynh hng hot ng tt hn qua mi chu k thi gian di hn. Trong thc t, n nh ca ng h cesium qua mi chu k 10 ngy hoc hn tng ln nhiu. Hot ng ca ng h v tinh c gim st bi trm iu khin mt t. Ga tr sai lch c tnh ton, r i pht i nh mt phn ca thng ip iu hng di dng ba h s ca mt a thc bc hai. Li ng h v tinh khi n cho o c GPS sai s li cng sai s hn. Nhng li ny ph bin i vi tt c ngi s dng GPS ang quan st cng m t v tinh v c th c loi b thng qua phng php ly sai phn nhng quan st b thu. iu chnh li gi tr ng h v tinh trong thng ip iu hng cng chnh l iu chnh li li ng h v tinh. Tuy nhin, bin php iu chnh ny gy ra sai s khong vi nano giy,

37 dhspkt-hcm-2009

GPS the global positioning systemtng ng vi li tm trong khong vi met (li mt nano giy tng ng vi li tm khong 30 cm). Tri li, nhng b thu GPS ch s dng ng h thch anh,c chnh xc th p hn nhiu so vi ng h nguyn t. Nh vy, li ng h b thu s ln hn nhi u so vi li ng h v tinh. Tuy nhin, c th loi b li ny thng qua tnh sai phn gi a nhng v tinh khc nhau, ho c c th xem y nh l mt tham s v danh no trong tin trnh nh gi. Nh ng ng h ngoi chnh xc h n (thng l cesium hoc l rubidium) c s dng trong m t vi ng dng thay th cho nhng ng h thch anh n i. Mc d ng h nguyn t ngoi c kh nng hot ng chnh xc h n nhiu, nhng chng li rt mc tin, tr gi n vi ngn dollar cho ng h loi rubidium, $20,000 i vi ng h cesium.

5.9.4 Li a ng:

38 dhspkt-hcm-2009


Hnh 16: Hiu ng a ng Li a ng l ngun li c bn cho c o c pha-sng mang v o c tm gi. Li a ng xy ra khi tn hi u GPS n ti anten my thu thng qua nhi u ng khc nhau. Nhng ng ny c th l ng trc tip ca tn hi u ti hoc ng tn hi u phn x t nhng vt th xung quanh anten (Hnh 16).L i a ng lm mo dng tn hiu GPS gc t can nhiu ca nhng tn hi u phn x ti anten thu. L i a ng nh hng i vi c pha-sng mang v t m gi; tuy nhin, nh hng trong o lng tm gi ln hn trong o lng pha-sng mang. nh hng ca li a ng i vi o lng phasng mang c th t n gi tr cc i l mt phn t chu k (khong 4.8cm i vi sng mang L1). Li a ng nh hng n tm gi, v mt l thuyt, c th t n vi chc met i vi m C/A. Tuy nhin, vi nhng k thut mi, tin tin trong

39 dhspkt-hcm-2009

GPS the global positioning systemcng ngh b thu, li a ng nh hng n tm gi c gim i mt cch ng k. V d nh l cng ngh Strobe corrector (Ashtech,Inc) v MEDLL (NovAtel,Inc). Vi nhng k thut ny, nh hng ca li a ng trong o c tm gi gim ti vi met, ngay c trong mi trng c mt phn x cao. Trong cng iu kin,s hin din ca li a ng c th c xc nhn bng cch s dng php tnh t ng quan ngy-ngy nh ng d tha nh gi. Nguyn nhn bi v khi hnh h c gm v tinh-i tng phn x-anten lp li mi mt ngy thin vn (sidereal day). Tuy nhin, li a ng trong o c tm gi v nh hnh c th c nhn dng nu hin din nhng quan st song tn. Ni chung, mt m hnh l i a ng tt vn cha sn dng, nguyn nhn ch yu bi v khi hnh h c v tinh-i tng phn x-anten lun thay i. Tuy nhin, c vi phng php gim bt nh hng ca li a ng. Mt phng n n gin l la chn mt v tr quan st m khng c i tng phn x trong nhng vng ph cn xung quanh anten thu. M t phng n khc l s dng anten chock ring (m t thit b chock ring l mt mt phng t, trn c vi vnh ai kim loi ng tm, c tc dng lm suy hao tn hi u phn x). Khi m tn hi u GPS c phn cc trn theo cnh tay phi, trong khi ,tn hiu phn x th phn cc trn theo cnh tay tri, t c th gim thiu hiu ng ca li a ng bng cch s dng mt anten c phn c c thch hp vi tn hi u GPS (phn cc trn theo cnh tay phi). Tuy nhin, k thut ny c iu bt tin l khi tn hi u phn x hai ln, do tn hiu phn x chuyn t phn cc cnh tay tri sang phn c c cnh tay phi.

5.9.5 Li pha trung tm ca anten:Mt anten GPS thu tn hi u v tinh n v sau chuyn i nng lng ca tn hiu thnh dng in. im m ti tn hiu GPS c thu gi l trung tm pha anten. Ni chung, trung tm pha anten khng trng v i trung tm hnh h c ca anten. Trung tm pha ca anten thay i ph thuc vo gc nng (elevation ) v gc phng v (azimuth) ca v tinh GPS cng nh l cng ca tn hi u thu (Hnh 17).

40 dhspkt-hcm-2009


Hnh 17: Gc phng v ln ca li ph thuc vo dng anten, in hnh th ng vo khong vi centimet. Tuy nhin, tht kh c th m hnh ha nh ng thay i trung tm pha c a anten, do ch c th lu tm n vic chn la dng anten. i vi nhng baseline ngn, cng mt dng anten t i mi u cui, li pha trung tm c th loi b nu nhng anten ny hng theo cng mt hng. S dng nhng anten khc nhau ho c l s dng nhng nh hng khc nhau s khng loi b c li ny. a s l li ny b b qua trong hu ht nhng ng dng GPS. Nhng li pha trung tm pha c th khc nhau i vi nhng s quan st pha-sng mang L1 v L2. iu ny nh hng n chnh xc trong ph ng php kt hp tuyn tnh t do tng in ly,c bit khi quan st nh ng baseline ngn. i vi baseline ngn, nhng li ny c tnh tng quan cao vi nhau qua kho ng cch v s b loi b thng qua sai phn. Do , s dng mt tn s duy nht l thch h p hn c i vi nhng baseline ngn trong ch tnh.

5.9.6 Tr hon tng in ly:Ti phn cao nht ca bu kh quyn Tri t, bc x tia cc tm v tia X n t mt tri tng tc vi nhng phn t v nguyn t khng kh .Kt qu l s ion ha

41 dhspkt-hcm-2009

GPS the global positioning systemkhng kh: hnh thnh m t s lng ln nhng electron mang in tch m t do v nhng nguyn t, phn t mang in tich dng t do. Mt vng nh vy gi l tng in ly. Tng in ly m rng t mt cao xp x khong 50km n khong 1,000km hoc hn. Ni chung, gi i hn bn trn ca tng in ly khng c xc nh r rng. Mt electron bn trong tng in ly khng phi l hng s, n thay i theo cao. Do , tng in ly c chia ra thnh nhi u tng con, da vo mt electron. Nhng tng ny c t tn ln lt l D (50-90 km), E (90-140 km), F1 (140-210 km), v F2 (210-1,000 km), vi F2 l tng c mt electron cc i. cao v dy ca nhng tng ny thay i theo thi gian, ph thuc vo bc x ca mt tri v vng t trng ca Tri t. V d nh: Tng F1 bin mt sut m, v d nhn thy vo ma h hn l vo ma ng. Cu hi t ra: Tng in ly nh hng nh th no n nhng s o lng GPS. Tng in ly l mi trng phn tn, n b cong tn hi u GPS v thay i tc ca tn hi u GPS khi tn hi u GPS i xuyn qua nhng tng in ly khc nhau tin n b thu GPS. B cong ng dn tn hi u GPS gy ra li khng ng k, c bit l trong tr ng hp dn, t gy ra nhng li khong cch th cn phi xem xt n. Tng in ly s tng tc truyn dn ca pha sng mang vt qua tc nh sng, ng thi n s lm chm li m PRN (cng nh l thng ip iu hng) vi cng mt lng. Nh vy, khong cch gia b thu v v tinh s l qu ngn nu o bng pha-sng mang, ho c l qu di nu o bng m PRN, khi so snh v i khong cch thc s. Tr hon tng in ly tng ng vi mt s lng electron t do nm dc theo ng truyn ca tn hi u GPS, s lng electron ny gi l TEC (total electron content). Tuy nhin, TEC ph thuc vo mt vi thng s sau: 1) thi gian trong ngy (m t electron t mc cc i vo khong u bui tra v t mc cc tiu vo khong na m tnh theo gi a phng) ; gc ngng ca v tinh ln hn 5 . Nhng nu tng in ly nh hng n tc truyn

42 dhspkt-hcm-2009

GPS the global positioning system2) thi gian trong nm (mt electron vo ma ng cao hn ma h); 3) chu k mt tri mi 11 nm (mt electron t n gi tr cc i mi 11 nm,thi im ny tng ng vi mt nh trong ho t ng le sng ca mt tri gi l nh chu k mt tri-solar cycle peak ); 4) v tr a l (mt electron t mc cc tiu ti nhng vng midlatitude (vng ny nm trong khong v v Bc; 23 2622" Nam v 66 3339" Nam ) v khng theo m t quy lut no ti vng xch o v vng a cc tri t). 23 2622" Bc 66 3339"

Tng in ly l mi trng phn tn, m c tr hon m n gy ra ph thuc vo tn s ca tn hi u. Tn s cng thp th tr hon cng l n; v vy nn tr hon t ng in ly i vi sng mang L2 cao hn i vi sng mang L1. Ni chung, tr hon t ng in ly thng vo khong t 5m n 15m, nhng c th vt qu con s 150m trong iu kin mt tri hot ng tch cc, vo gia ngy v gn vi a tng (horizon). Mt electron thay i theo thi gian v v tr. Tuy nhin, mt electron c tnh tng quan cao xt trong nh ng khong cch ngn, do , ly sai phn nhng quan st GPS gia nhng ngi s dng gn nhau v khng gian c th loi b phn ln tr hon tng in ly. Da vo tnh cht phn tn c a tng in ly, tr hon tng in ly c th c m t vi chnh xc cao bng cch kt hp o c tm gi s dng m P trn c L1 v L2. Nhng khng may, m P ch c th c truy nhp duy nht bi ngi dng c cp php. Cng vi s b sung m C/A th hai trn L2, gi i hn ny b loi b. Kt hp o lng pha-sng mang trn c L1 v L2 m t nhng thay i trong tr hon tng in ly. Nhng ngi s dng b thu GPS song tn c th kt hp vi o lng phasng mang L1 v L2, t to ra tn hi u kt hp tuyn tnh t do tng in ly (ionosphere-free linear combination ) nhm loi b tr hon tng in ly. Tuy nhin, phng php ny c nhc im: 1) nhiu quan st cao hn;

43 dhspkt-hcm-2009

GPS the global positioning system2) khng duy tr bn cht s nguyn ca nhng tham s ambiguity. Do , phng php ny khng c ngh i vi nhng baseline ngn. Nhng ngi s dng b thu n tn s khng tn dng c u im nh vo bn cht phn tn ca tng in ly. Tuy nhin, h c th s dng mt trong nhng m hnh t ng in ly c sn, c th gim thiu ti 60% tr hon. M hnh c s dng rng ri nh t l m hnh Klobuchar, nhng h s ca n c pht i nh mt phn ca thng ip iu hng. Mt cch gii quyt khc i vi nhng ngi s dng b thu n tn l s dng nhng thng tin iu chnh t mng thng tin a phng. Nhng thng tin iu chnh nh vy c th c thu vo thi gian thc thng qua nhng ng lin kt thng tin cng ng.

5.10 Gc ta (datum), h thng ta v php chiu bn :Bn c bao gi nhn trn b n v hi rng mnh ang u trn .Hoc mun bit hng i, ng i n ti nhng im nh ni cm tri, bn lngNgi ta dng m t cng c gi l ta xc nh v tr ca chng ta trn b n . Ta l im giao nhau ca hai ng kinh tuyn (longtitude) v v tuyn (latitude). C nhi u phng thc xc nh kinh v v , my thu GPS l mt trong nhng cng c hin i nht.

5.10.1 nh hng: im (waypoints):mc tiu ca GPS l tm ng i t im A n im B.Cc im A,B ny gi l waypoint nh nh ,bi gi xe,sui,ni cm triNu c thng s ta ca nhng im ny, bn c th d dng nhp n vo my thu GPS v tm ng i, hng i ti , d c th bn cha bao gi ti . GPS ch ra cho bn khong cch theo ng chim bay n , hng i ti cc im .

44 dhspkt-hcm-2009

GPS the global positioning system Tuyn ng (route): i khi chng ta cn nhng hng i phc tp hn l ch ni gia im A v B. v d nh i t A n B, chng ta cn i qua nhng im 2,3,4Nh vy, ni cc im ny li 1-2-3-4-5 ta c mt khi ni m mi gi l tuyn ng. C th to ra tuyn ng trn GPS bng cch ni cc im li (Hnh 18).

Hnh 18:route Track log: Bn s dng GPS ghi li chng ng i qua, chng ng ny gi l track log.Track log l t p hp cc im m GPS t ng ghi li (c th t theo ch bao nhiu giy ghi li mt ln) v bn c th s dng tracklog quay li con ng c (track back), khi GPS s da trn tracklog to ra mt route mi quay li im xut pht . Hng bc:c ba loi hng bc: Bc t : Hng bc ca a bn do t trng to nn. Bc tht: Hng bc l im cc bc ni cc kinh tuyn ct nhau. Bc theo bn : Hng bc theo li bn . C nhiu nh dng khc nhau c a ta :

45 dhspkt-hcm-2009

GPS the global positioning system Lat/lon (latitude-longtitude): l nh dng ph thng, nh dng ca n l , pht,.

thng s ny th rt kh tnh ton b ng tay trn bn v phi quy i ra h thp phn ri tnh l i.

giy. 1 = 111

,1 =

= 1.85

,1 =

= 30.83 . Nhng nu nhn

5.10.2 Gc ta ca bn (Map Datum):Bn v biu thng c cc li to ra t im gc gi l gc ta (datum). C nhiu bn m chng ta dng ngy nay c o v t hng chc nm trc. Cng ngh ngy nay cho php chng ta o v c nhng bn cc k chnh xc, tuy vy, GPS cng phi phi hp c vi nhng bn giy c v t xa. Hu nh cc my thu GPS c ti hng chc, thm ch hng tr m gc ta khc nhau. N u khng chn ng gc ta th bn c th c mt thng s hon ton sai l ch. V d: theo h ta 1 th im A c thng s l A (6,9) nhng theo h ta 2 th A(8,14). Nh vy khi ni bn ang h ta x, y no th phi ni chnh xc im gc ca n l theo h no, h 1 hay h 2 (Hnh 19) Cc h (datum) thng dng hin nay l : Hnh 19 VN2000 WGS84 VN2000 c xy dng trn n n ca WGS 84, do vy, vi cc yu cu ph thng vi sai s ti 10-20m, ta c th tm coi hai h ny trng nhau. Ngoi ra cc bn c hn, c th thy cc h ta nh India Thailan, India 1960... khi , nu mun dng thng s ny phi ci t GPS cho ng vi h ta .

46 dhspkt-hcm-2009

GPS the global positioning systemTrn thc t, b mt a hnh ca Tri t khng ng u khin cho vic tnh ton trc a hcv d nh m t v tr ca ngi s dngtr nn kh khn. khc phc vn ny, nhng nh tr c a hc tha nhn mt b mt ton hc phng, gi l b mt tham kho, c lng gn chnh xc hnh d ng khng ng u ca Tri t (ni chnh xc h n l c lng mc nc bin trung bnh c a Tri t). Mt b mt ton hc nh vy gi l mt cu, c s dng rng khp trong ch nh v chnh xc m c thp. i vi nh v chnh xc m c cao nh nh v GPS, b mt ton hc tt nht xp x Tri t v ti cng thi im gip cho vic tnh ton n gin nh c th, l mt hnh

elip gi l biaxial ellipsoid (Hnh 20). M t biaxial ellipsoid tham kh o, hay ni ngn gn l hnh ellipsoid tham kh o, c c bng cch xoay mt hinh elip xung quanh tr c b ca n, trc b. Tng t nh i vi hnh elip, biaxial ellipsoid c th c xc nh bi na trc ln v na trc b (a,b) hoc l (a,f) vi f=1-(b/a). Hnh 20: (a)mi lin h gia b mt vt l ca tri t, geoid v ellipsoid (b) nhng tham s ellipsoidal. Mt hnh ellipsoid tham kh o thch hp gi l geodetic datum. Geodetic datum l b mt ton hc, l hnh ellipsoid tham kh o, vi gc ta v nh hng r rng.

47 dhspkt-hcm-2009

GPS the global positioning systemV d: xt geocentric geodetic datum. y l geodetic datum vi gc ta trng vi tm ca Tri t. R rng rng c v s nhng geocentric geodetic datum nh vy vi nhng nh hng khc nhau. Do , mt geodetic datum ch duy nht c xc nh thng qua tm thng s : hai tham s xc nh kch thc ca hnh ellipsoid tham kh o; ba tham s xc nh v tr ca gc; v ba tham s xc nh nh hng ca ba trc c xt n Tri t. Bng di y lit k ba h thng tham kho ph bin v nhng hnh ellipsoid i km tng ng.

Thm mt v d b sung vo h thng geodetic datum, mt h thng gi l vertical datum c s dng trong th c t nhm xc nh cao ca im. Bi v cao ca mt im nh v trc tip trn vertical datum c gi tr l zeros, do h thng vertical datum cn gi l b mt vi cao zeros. Mt vertical datum thng c la chn nh l mt th a cu; b mt ny xp x tt nht vi mc nc bin trung bnh trn ton c u. Trong qu kh, nhng v tr lin quan n nhng datum ngang v d c thng c m t tch ri ln nhau. V i s ra i ca h thng nh v trc a khng gian GPS, c th m t nhng v tr 3-D s dng h thng tham kho 3-D.

5.10.3 Php chiu bn :Php chiu bn l php chuyn i t nhng c im vt l trn b mt cong ca Tri t thnh m t b mt phng, b mt phng ny gi l bn .

48 dhspkt-hcm-2009


Hnh 21: Khi ni m v php chiu bn Tuy nhin, theo quan im ton hc, php chuyn i t nhng ta trc a t c t GPS chng hn, thnh nh ng ta li hnh ch nht gi l hng ng (easting) v hng bc (northing). Php bi n i ny gi l php chiu bn trc tip. Php chiu ngc li, t nhng ta li sang ta trc a. Nhng ta li hnh ch nht c s dng rng ri trong th c t, c bit trong nhng cng vic lin quan n Geomatics. Nguyn nhn ch yu bi v vic tnh ton th c hin trn mt phng bn thng d dng hn l thc hin trn b mt tham kho. Khng may, do s khc bit gia b mt ellipsoid ca Tri t so vi b mt phng ca php chiu, kt qu gy ra s mo dng. iu ny tng t vi vic lm phng v ca mt na qu cam; chng ta s c nhng phn cng ra v nh ng phn co rt li, dn n kt qu lm mo dng v ca qu cam. C nhiu php chiu bn c s dng ti thiu ha mo dng.Trong hu ht nhng ng dng GPS, conformal l php chiu hay c s dng. Vi php chiu conformal, nhng gc trn b mt hnh elip c duy tr sau khi c chiu trn b mt chiu phng. Tuy nhin, c din tch v t l u b mo dng; nh rng din tch hoc l b cng ra hoc l b nn li. Php chiu bn

49 dhspkt-hcm-2009

GPS the global positioning system conformal ph bin nht l Mercator ngang, UTM, php chi u hnh nn conformal Lambert. Nh rng nhng php chiu bn ny khng ch h tr nhng ta li ca mt im m cn l c h thng tham chi u. Bi v nhng ta trc a ca mt im s thay i t h thng tham kho ny sang h thng tham kho kia UTM (Universal Transverse Mercator): l php chiu bn dng h MET m chng ta thng thy trong cc bn gn y. Mi: rng 6 hay 666Km, theo kinh , c nh s t 1 n 60, bt u t kinh tuyn T v 80 Nam n 80 Bc, Tri t c chia thnh 60 mi (d c) v 20 di (ngang). H thng vung U.T.M (Hnh 22):

180 i v ng. Di rng 8 hay 888Km, theo v , c

t tn bng mt mu t theo th t t Nam n Bc, bt u t C n X (b qua cc mu t A,B,I,O,Y). Mi v di ct nhau thnh nh ng vng li vung mang tn bng s v mu t: s l tn ca mi, mu t l tn ca di.

V d: nc Vit Nam nm trong li vung mang ch sngang l 6 v chiu dc l 8 (Hnh 23) Hnh 22:h thng vung UTM 47Q-48Q-49Q-48P-49P . Mi vung ca hnh bn c chiu

50 dhspkt-hcm-2009


Hnh 23: Vit Nam xt trong h t a UTM

5.10.4 H thng ta dc: cao ca mt im c xc nh l khong cch dc t vertical datum n im . Geoid thng c la chn tr thnh vertical datum. Chi u cao ca mt im bn trn geoid gi l chiu cao orthometric. N c th mang gi tr dng hoc m tuy thuc vo v tr ca im c nh v bn trn hay bn di geoid. Nh c nhiu ngha vt l, chiu cao orthometric thng rt cn thit trong thc t v thng c s dng trong nhng bn a hnh (Hnh 24-25).

51 dhspkt-hcm-2009

GPS the global positioning systemHnh 24: chiu cao orthometric

Hnh 25: chiu cao orthometric xt trong mi tng quan vi 3 h thng mt phng Trong mt vi trng hp, v d nh l GPS, chiu cao ca i tng s c tham chiu n hnh ellipsoid tham kh o, ch khng phi l geoid. Do nhng cao ny gi l chiu cao ellipsoidal. Chiu cao ellipsoidal c th mang gi tr dng hay m ty thuc vo v tr ca im nm bn trn hay bn di b mt ca hnh ellipsoidal tham kho. Khng may l nhng chiu cao ellipsoidal ch n thun mang tnh ch t hnh h c ch khng c ngha vt l no. Do , nhng dng c Geomatic khc nhau s khng s dng thng s chiu cao ny trc tip. Khong tch ri gia geoid-ellipsoid gi l chiu cao geoid undulation. Khong cch ny c th t n 100m, gi tr ca n c th dng hoc m, ph thuc vo geoid nm bn trn hoc bn di hnh elip tham kh o ti im kho st. Thng tin chnh xc v chiu cao geoid undulation dng m t cao orthometric thng qua cao ellipsoidal v ngc li. Nhng m hnh Geoid s dng m t chiu cao geoid undulation c pht trin trn khp th gii. Khng may, nhng m hnh ny khng c chnh xc ph h p v khng ng u trn khp mi ni, ch yu bi v thiu d liu trng lc a phng (local gravity) v thng tin cao i km ti mt s ni trn th gii. Nhiu b thu GPS v gi phn mm c nhng m hnh geoid xy dng bn

52 dhspkt-hcm-2009

GPS the global positioning systemtrong nhm chuyn i mt cch t ng gia cao orthometric v cao ellipsoidal. Tuy nhin cn phi ht sc cn thn khi m y ch l nhng m hnh c chnh xc thp. Bng d liu trng lc a phng mt s khu vc trn th gii:

53 dhspkt-hcm-2009


5.10.5 H thng ta trc a:H thng ta l tp hp nhng quy tc ch nh nhng v tr (hay gi l ta ) ca im. H thng thng bao gm mt gc ta , tp hp cc trc theo mt hng xc nh. Hnh bn d i ch ra h trc ta 3-D, s dng 3 trc(x,y,z), im giao nhau ti 3 trc l gc ta C (Hnh 26).

Hnh 26: H thng ta 3-D H thng ta c th c phn loi theo 1-D, 2-D v 3-D, lin quan n s lng ta cn thit xc nh v tr ca mt im. V d: H thng ta 1-D c dng xc nh cao ca im so vi mt nc bin. C th phn loi h thng ta da vo dng ca b mt tham kho, hng ca trc v gc ta . Trong trng hp i vi h ta trc a 3-D, b mt tham kho l hnh ellipsoid. Hng ca trc v gc c xc nh bi hai mt phng: mt phng kinh tuyn i qua im cc hoc l trc z ( kinh tuyn l mt phng ko di t cc bc xung cc nam ca Tri t ) v mt phng xch o ca hnh ellipsoid.Formatted: Indent: First line: 0.5"

54 dhspkt-hcm-2009

GPS the global positioning systemi vi nhng ngi s dng GPS, h ta trc a 3-D l quan tr ng nht. Trong h thng ny, ta ca mt im c xc inh bi v trc a ( ), kinh chuyn i sang h ta Cartesian (x,y,z). thc hin c php chuyn i, phi trc a () v cao so vi b mt tham kho(h). Nhng ta trc a ( , ,h) c th

xem xt n tham s (a,f) ca hnh ellipsoid. Ngoi ra hon ton c th chuyn i ta trc a sang ta li hnh ch nht dng trn bn .

5.10.6 H thng ta CTRS (Conventional Terrestrial Reference System):CTRS l h thng ta 3-D dng geocentric, c gc ta trng vi tm Tri t. CTRS c nh vi Tri t, xoay cng vi Tri t.Ngoi ra CTRS cn gi l h thng ta ECEF (Earth-centered,Earth-fixed). Phng v chiu ca nhng trc trong CTRS c xc nh nh sau:trc z tr v pha CTP (Conventional Terrestrial Point), l v tr trung bnh c a cc. Trc x c xc nh bng giao nhau gia mt phng xch o mt t v mt phng kinh tuyn, trn l v tr trung bnh c a i quan st Greenwich (London-England). R rng r ng, t nh ngha trc x v trc z, mt phng xz s i qua kinh tuy n Greenwich. Trc y c la chn hnh thnh h thng ta cnh tay phi. Ba trc giao nhau ti trung tm c a Tri t.

55 dhspkt-hcm-2009

GPS the global positioning systemHnh 27: (a) khi ni m v h thng ta trc a (b)h thng ta trc a v Cartesian Khi nh v CTRS phi xt n Tri t khi ng dng kt qu nh v trong thc t.S dng phng php gn gi tr ta v mt con s c la chn trong nh ng trm tham kho phn b ri rc khp ni. Mt trong nh ng h thng CTRS quan trng nht l ITRS (International Terrestrial Reference System), cn gi l ITRF(International Terrestrial Reference Frame). ITRF da trn nhng s liu o c t nhng trm tham kho phn phi trn khp ton cu s dng GPS v nhng h thng trc a khng gian khc. Do , y c tha nhn l h thng ta chnh xc nh t. ITRF c cp nht trong mi khong t 1 n 3 nm t c chnh xc c th cao nht.

5.10.7 H thng WGS 84 v NAD 83:WGS 84 (World Geodetic System of 1984 ) l mt h thng tham kho 3-D,ly Tri t lm trung tm, c pht trin bi U.S Defense Mapping Agency trc y, by gi l NIMA (National Imagery and Mapping Agency). y l h thng tham kho GPS chnh thc. Ni cch khc, mt ngi s dng GPS s dng nhng ephemeris qung b trong tin trnh s dng, kt qu t c l ta trong h thng WGS 84. WGS 84 s dng CTRS kt hp vi mt hnh ellipsoid ng nht, cng vi ellipsoid ca GRS 80 (Geodetic Reference System of 1980). Ellipsoid ca GRS80 c ngh bi International Association of Geodesy s dng trong nh ng ng dng trc a. Ban u, WGS84 c thit lp, s dng rt nhiu trm Doppler. Sau , h thng c cp nht vi ln c th gn ging nht vi h thng ITRF. Vi bn cp nht gn y, WGS 84 gn nh tng ng vi ITRF vi chnh xc di decimet. Ngoi ra, Bc M cn hin din mt geocentric datum khc,NAD 83 (North American Datum of 1983), c s dng nh mt datum chnh th c nh v khng

56 dhspkt-hcm-2009

GPS the global positioning systemgian. NAD 83 s dng ellipsoid ca GRS 80, do c NAD 83 v WGS 84 gn nh l tng ng nhau c v hnh dng v kch thc.

5.10.8 Php chuyn i datum:nh v vi nhng datum ngang v datum d c c m t mt cch c lp vi nhau.Ngoi ra, nh ng datum ngang khng ph i l dng geocentric v c s dng thit lp ti mt s khu vc trn th gii. Do , nhng datum ny gi l datum cc b (local datum). Hn 150 datum cc b c s dng trn khp th gii (Hnh 28). Mt v d ca datum cc b l NAD 27 (North American Datum of 1927 ). Vi s ra i ca h thng nh v geodetic nh l GPS, by gi ta c th m t nhng datum geocentric 3-D ton cc.

Hnh 28: nhng datum geocentric v datum cc b

57 dhspkt-hcm-2009

GPS the global positioning systemNhng bn c c sn xut vi datum cc b, trong khi ,hu ht nhng bn mi c sn xut vi datum geocentric. Do , m bo tnh chc chn,cn thit phi xy dng mi lin h gia datum cc b v datum geocentric,nh l WGS 84. Mi lin h nh vy gi l php chuyn i datum (Hnh 29). NIMA (National Imagery and Mapping Agency) xut bn nhng tham s chuyn i gia WGS 84 v nhiu datum cc b c s dng trn nhi u nc. Hin ti, nhiu nh sn xut GPS ang s dng nhng tham s ny bn trong nhng gi phn mm x l ca h.Tuy nhin, r rng r ng, nhng tham s chuyn i ny ch gn chnh xc v khng nn c s dng trong nhng ng dng GPS chnh xc. V d nh Toronto, s dng nhng tham s NIMA (WGS 84 n NAD 27) to ra s sai bit n vi met i vi nhng ta ngang (kinh v v ) khi so snh vi phn mm NTv2 (National Transformation) c sn xut bi Geomatics Canada. Thm ch, s sai bit c th ln hn trong nhng vng khc. Phng php tt nht c c nhng tham s chuyn i l so snh nh ng ta hay dng c nh sn trong c hai h thng ta .

Hnh 29: Php chuyn i datum

58 dhspkt-hcm-2009

GPS the global positioning system6. Nhng ch nh v GPS:nh v GPS c tin hnh theo m t trong hai cch: nh v im hoc l nh v tng i. nh v im GPS s dng mt b thu GPS -b thu ny c s dng o lng tm gi s dng m (code pseudorange)-nhm m t v tr ca ngi s dng ngay tc thi, min l b thu c th nhn th y t bn n nm v tinh. chnh xc nh v theo tiu chun ngang t b thu m dn d ng C/A gim xung khong 100 m khi SA cn tn ti,v gim i khong 22 m khi SA b loi b. nh v im GPS c s dng khi khng i hi chnh xc cao. Bao gm nhng ng dng gii tr v iu hng chnh xc thp. Tuy nhin,nh v tng i GPS s dng hai b thu quan st ng thi cng mt v tinh.Nu c hai b thu theo du vt t nht bn v tinh, ta c th t c mc nh v chnh xc cao t di centimet n vi mt. Nhng s liu o c pha-sng mang hoc/v tm gi c th c s dng trong ch nh v tng i GPS, ph thuc vo yu cu v chnh xc. i vi hai phng php o c ny, s dng o c pha-sng mang cho chnh xc cao hn. nh v tng i GPS c th c thc hin trong ch thi gian thc hoc ch post-mission. nh v tng i GPS c s dng trong nhng ng dng i hi chnh xc cao nh l trc a hoc v bn , GIS, v iu hng chnh xc.

6.1 nh v im GPS:nh v im GPS cn gi l nh v t tr bao gm ch duy nht mt b thu GPS. Trong b thu GPS theo du vt ng thi 4 v tinh GPS hoc hn m t ta ca n so vi gc ta l trung tm c a Tri t (Hnh 30). Hu nh tt c nhng b thu GPS c mt trn th trng u c kh nng hin th ta nh v im.

59 dhspkt-hcm-2009


Hnh 30: nguyn tc nh v im GPS Bt c lc no, m t v tr im ca b thu, i hi phi c ta ca nhng v tinh cng nh l khong cch t b thu n bn v tinh . B thu c c ta ca v tinh thng qua thng ip iu hng, cn khong cch th c c t m C/A hoc l m P , ph thuc vo dng ca b thu (dn dng hay qun s). Nh cp trc , o lng tm gi b lm sai lch i bi li ng b gia ng h b thu v ng h v tinh. iu chnh li ng h v tinh c th c thc hin trong thng ip iu hng; li ng h b thu c xem l mt tham s ph no trong tin trnh nh gi. Nh vy, c tng cng 4 tham s : ba tham s l ta ca b thu v tham s cn li l li ng b ng h b thu.y l l do ti sao ta cn ti thiu n bn v tinh. N u nh c nhiu hn bn v tinh th k thut lc Kalman c s dng. Khi m t a v tinh thu c v h thng WGS 84 th ta b thu cng tng t. Tuy nhin, hu ht nhng b thu GPS u cung cp tham s chuyn i gia WGS 84 v nhiu datum cc b trn khp th gii.

6.2 nh v tng i GPS:

60 dhspkt-hcm-2009

GPS the global positioning systemnh v tng i GPS cn c gi l nh v sai phn, s dng hai b thu GPS theo du vt cng mt v tinh nh m m t nhng ta tng i ca n. Mt trong hai b thu ny gi l b thu nn, n duy tr v tr c nh vi mt ta c xc nh trc. K thut nh v th rt a dng, chng c s dng nh v thi gian thc hoc l nh v postprocessing (postmission). nh v tng i GPS c chnh xc cao hn nh v t tr GPS. Ph thuc vo s o c no c s dng trong nh v, o lng pha-sng mang hoc o lng tm gi, ch nh v ny s c mt chnh xc t di 1 cm n vi met. Nguy n nhn ch yu l do nhng thng s o c c ca hai b thu c cng dng li, ch l nhiu hn hoc t hn thi.Khong cch gia hai b thu cng ngn th tnh t ng ng ca hai b thu cng cao.Do ,nu ta ly sai phn nhng thng s o c c t hai b thu th nh ng li tng t s b loi b hoc b suy gim.

Hnh 31:nguyn tc nh v tng i GPS

6.3 Quan st GPS tnh (static GPS surveying):

61 dhspkt-hcm-2009

GPS the global positioning systemQuan st GPS tnh l mt k thut nh v tng i ,s dng phng php o c sng mang-pha.Phng php ny s dng hai b thu tnh (hoc nhiu hn) quan st cng mt v tinh (Hnh 32).

Hnh 32:quan st GPS t nh

B thu nn c thit lp ti mt im vi ta chnh xc xc nh.Cn b thu kia l b thu ng cng c t ti mt im nhng ta th khng bit trc v ang c xc nh.B thu tnh c th h tr bao nhiu b thu ng cng c min l c t nht 4 v tinh hi n hu i vi c hai loi b thu. V nguyn tc ,phng php ny da trn vic tp hp nhng kt qu o c tc thi t c hai b thu trong m t khong thi gian no ; sau khi x l s liu s c c ta ca im.Thi gian quan st thay i t khong 20 pht n vi gi ph thuc vo khong cch gia b thu nn v b thu ng (baseline length),s lng v tinh hi n hu,v khi hnh h c v tinh.o c c thc hin trong kho ng thi gian m i 15giy hoc 20 giy hoc l o c mu mi 15 giy hoc 20 giy.

62 dhspkt-hcm-2009

GPS the global positioning systemSau khi hon thnh o c,d liu s c download t b thu xung my tnh x l.Nhng ty chn x l sai phn c la chn ty thuc vo nhng yu cu ca ngi s dng ,chiu di baseline,v nhng h s khc.V d nh,trong trng hp chiu di baseline ngn,gii quyt nhng tham s ambiguity s l vn then cht m bo ch nh v chnh xc cao.i vi nhng baseline di, khong 1,000 km,ng i ta ngh rng ngi s dng x l d liu vi mt trong nhng gi phn mm khoa hc sn dng,nh l phn mm BERENSE,phn mm ny c pht trin bi i hc Bern,ng hn y l gi phn mm thng mi.Ngoi ra,nhng ephemeris chnh xc nn c s dng trong trng hp ny,khi m nh hng ca nhng li qu o thc s khc bit ti hai u cui ca baseline. Trc a GPS tnh kt hp vi phng php o c pha-sng mang l k thut nh v chnh xc nh t.K thut ny lin quan ch yu n s thay i ng k trong khi hnh hc v tinh qua m t khong thi gian quan st di.M c d c hai b thu n tn v song tn c th c s dng trong ch nh v tnh ,b thu song tn thng c s dng hn,c bit i vi nhng baseline c chiu di ln hn 20 km . chnh xc mong i t mt b thu GPS cht lng tt in hnh l 5 mm + 1ppm (rms);ppm (parts per million) v rms (root-mean-square).L y v d,i vi mt baseline di 10 km,tnh chnh xc mong i t h thng trc a GPS tnh l 1.5 cm(rms). chnh xc cao hn c th t c bng cch p dng,v d nh nhng ephemeris chnh xc .

6.4 GPS sai phn, thi gian thc (real-time DGPS):DGPS l mt k thut nh v tng i da trn m,s dng hai b thu hoc nhiu hn theo di ng thi cng mt v tinh (Hnh 33).DGPS c s dng khi ch cn chnh xc cp mt,thi gian th c.Phng php da trn mt thc t : nhng

63 dhspkt-hcm-2009

GPS the global positioning systemli GPS hin hu trong nhng tm gi ,v c bn l ging nhau trn c b thu nn v b thu ch ,min l chiu di baseline gii hn trong tm vi trm kilomet.

Hnh 33:hot ng GPS sai phn th i gian th c B thu nn duy tr v tr tnh ti mt im xc nh. Phn mm build-in ca b thu nn s dng nhng ta nn c xc nh mt cch chnh xc v t a v tinh, dn xut t thng ip iu hng, tnh ton kho ng cch n mi v tinh. Ngoi ra, phn mm cn c kh nng ly s khc bit gia khong cch c tnh ton v t m gi o c c t xc nh c li tm gi (hoc l thng tin iu chnh DGPS). Nhng thng tin ny c pht theo mt nh dng gi l RTCM (Technical Commission for Maritime Service) n ch thng qua mt ng lin kt thng tin. ch n s s dng thng tin iu chnh DGPS ny iu chnh li tm gi m b thu ch o c. Cui cng, nhng tm gi ny s c s dng tnh ton t a ca ch.

64 dhspkt-hcm-2009

GPS the global positioning system chnh xc t c vi phng php ny thay i trong khong t di mt mt n khong 5m, ph thuc vo khong cch gia nn - ch, tc truyn dn ca thng tin iu chnh RTCM DGPS,v kh nng thc thi ca nhng b thu m C/A. chnh xc cao hn t c vi khong cch nn-ch ngn, tc truyn dn cao. Khi m SA b loi b, tc d liu c th gim xung 10 giy hoc thp hn m khng lm suy gim chnh xc. Ngoi ra, chnh xc c th c ci thin nu b thu c kh nng lu tr nhng s liu tm gi th, nhng s liu ny sau c s dng trong ch postprocessing.

6.5 Tuyn lin kt thng tin (radio):Hot ng ca DGPS thi gian thc cn mt tuyn lin kt truyn thng tin t b thu nn n b thu ch. in hnh, thng tin iu chnh DGPS c pht ti 200 Kbps. C nhiu tuyn lin kt v tuyn khc nhau, s dng nhng thnh ph n khc nhau ca ph in t trng, lun sn dng h tr hot ng lin kt thng tin. Trong th c t, nhng di thng c s dng ph bin l di tn s trung bnh/th p (LF/MF) (30Khz n 3Mhz), di tn s rt cao v cc cao (VHF/UHF) (30Mhz n 3 Ghz).Thng th ng, ngi s dng GPS tn dng nhng tuyn lin kt thng tin v tuyn chuyn dng ca ring h truyn pht thng tin c a my thu nn. a s nhng tuyn lin kt v tuyn GPS chuyn dng, t ti mt t c thit lp s dng di VHF/UHF. Nhng tuyn lin kt v tuyn trong di ny cung cp mt bao ph thng theo tm nhn do n c kh nng xuyn qua ta nh v nhi u i tng cn tr khc. Mt v d v mt tuyn lin kt v tuyn nh vy l RFM96W, c s dng bi Pacific Crest Corporation m sn dng trong nh ng m hnh khc nhau d a trn tn s c h tr trong di VHF/UHF. Dng lin kt v tuyn ny i hi mt s cp php hot ng. Mt tuyn lin kt v tuyn khc cng c sn xut bi Pacific Crest Corporation gi l PDL (Position Data Link). PDL cho php tc baud l 19,200 v c trng ha bi kh nng tiu th in nng thp v tng cng giao din ngi s dng.

65 dhspkt-hcm-2009

GPS the global positioning systemMt v d khc l b thu v tuyn tri ph, khng cn cp php; hot ng trong khu vc 902-928Mhz trong di UHF. Tuyn lin kt ny c bao ph t 1-5 km v 3-15 km tng ng trong nh ng khu vc thnh th v nng thn. Gn y hn, cc nh sn xut GPS s dng cng ngh t bo, PCS s (Personal Communication Services) nh mt tuyn lin kt thng tin thay th . Trong tng lai gn, ngi ta hy vng rng nhng mng s bng thng rng 3-G (third generation) s c s dng rng ri nh l mt tuyn lin kt thng tin GPS. Cng ngh 3-G s dng nhng tiu chun ton cu ph bin gip gim chi ph dch v .Ngoi ra, cng ngh ny cn cho php cc thit b lun v tr ON sut thi gian truyn hoc thu d liu, trong khi chi ph ch tnh i vi nhng gi d liu c pht i hoc thu v. Trong thc t, nhng vt cn dc theo ng truyn tn hi u, nh l ta nh v a hnh a vt, s lm suy hao tn hi u c pht i, t dn n gii hn bao ph. Ngoi ra, tn hi u pht cn b suy hao bi phn x mt t (a ng), anten pht v m t vi thng s khc. tng bao ph ca mt tuyn lin kt v tuyn, ngi s dng c th s dng mt mch khuch i cng sut hoc cp ng trc cht lng cao, hoc c th tng chiu cao ca anten v tuyn pht/thu. N u s dng mch khuch i cng sut, ngi s dng nn cn thn vi qu ti tn hi u (against overload signal) thng xy ra khi sng v tuyn pht v thu th c s gn vi nhau. Ngi s dng c th tng bao ph tn hi u bng cch s dng mt trm repeater. Trong trng hp ny, tt hn ht l s dng mt anten n hng, nh l anten Yagi, ti trm nn v anten v hng ti trm repeater (Hnh 34).

66 dhspkt-hcm-2009


Hnh 34: s dng nhng b lp (repeater) tng bao ph tn hi u v tuyn

6.6 GPS RTK:Quan st RTK l mt k thut nh v tng i da trn pha-sng mang, gi ng nh nhng phng php trc , n s dng hai b thu (hoc nhiu hn ) quan st ng thi cng mt v tinh (Hnh 35). Ph ng php ny thch h p khi: 1) vic quan st c nhi u im cha xc nh nh ng c v tr gn mt im xc nh no (trong vng bn knh kho ng 10-15 km). 2) ta nhng im cha xc nh c yu cu trong thi gian thc.

67 dhspkt-hcm-2009

GPS the global positioning system3) tm nhn, ng truyn khng c chng ngi vt. Nh vo tnh d s dng cng nh kh nng m t ta trong thi gian th c,y l phng php hon h o i vi ngi s dng

Hnh 35:quan st GPS RTK Trong phng php ny, b thu base duy tr v tr tnh vi mt im xc nh v i km vi mt b pht sng v tuyn. B thu ch thng c mang vc trn vai v i km vi mt b thu sng v tuyn. Tc truyn d liu l 1Hz (1 mu/giy). S liu o c t b thu nn v ta c truyn n b thu ch thng qua tuyn lin kt thng tin v tuyn. Phn mm built-in trong b thu ch kt hp nhng s liu GPS o c c t b thu nn v b thu ch v x l, kt qu l ta ca b thu ch.

68 dhspkt-hcm-2009

GPS the global positioning systemS dng k thut gi l phn tch ambiguity OTF (on-the-fly) phn tch tham s ambiguity. Mt khi m nh ng tham s ambiguity xc nh vi gi tr nguyn, b thu (hoc thit b iu khin my tnh cm tay ca b thu) s hin th nhng ta ca my thu ch ngay ti v tr . V vy khng cn s dng ch nh v postprocessing. chnh xc nh v theo nh mong i trong khong t 2 n 5cm (rms). C th ci thin chnh xc nh v bng cch duy tr v tr ti mt im ch trong mt khong thi gian ngn, v d nh khong 30s, c th ly trung bnh v tr nh v. Nhng ta ch tnh ton c th c lu tr li v download vo phn mm CAD ti mt khong thi gian sau phn tch k cng hn. Phng php ny c s dng ch yu i vi b thu song tn,tuy nhin thi t k khng phi ch dnh ring cho n. Trong cng iu kin, chnh xc nh v ca phng php nh v RTK ch b suy gim khng ng k khi so snh vi phng php nh v GPS truyn thng. Nguyn nhn ch yu bi v thi gian trong ph ng php nh v GPS truyn thng t c b thu nn v b thu ch ph hp ln nhau m t cch hon ho trong tin trnh x l. Tuy nhin, vi RTK, d liu b thu nn sau mt khong thi gian tr hon m i tin n b thu ch. Tr hon d liu bao gm: nh dng, ng gi, pht, v m ha d liu nn. tha thi gian vi b thu ch, d liu nn phi c ngoi suy, do lm suy bin chnh xc nh v.

VI. Nhng nh dng tiu chun GPS:Khi m cc nh s n xut GPS c lp c nhng nh dng c quyn ca chnh h trong vic lu tr nhng s liu o c GPS, tht kh kt hp d liu t nhng b thu khc nhau. V n tng t khi s dng cc thit b khc nhau, bao g m h thng GPS. khc phc nhng gii hn ny, cc nhm nghin cu pht tri n nhng nh dng tiu chun i vi nhiu nhu cu ngi s dng khc nhau, bao g m: RINEX, NGS-SP3, RTCM SC-104, v NMEA 0183.

69 dhspkt-hcm-2009

GPS the global positioning system6.1 nh dng RINEX: tit kim khng gian l u tr, nhng nh dng c quyn pht tri n bi cc nh sn xut b thu GPS hu ht l nh phn, do khng th no c trc tip khi hin th. T to ra vn khi kt hp d liu t nhng b thu GPS khc nhau. khc phc vn ny, mt nhm cc nh nghin c u pht tri n mt nh dng trao i d liu c chp nhn trn ton th gii. nh dng ny gi l nh dng RINEX, ngun gc t nh dng ASCII tiu chun (vn bn c th c). Mc d mt file trong nh dng ASCII chim nhiu khng gian b nh hn trong nh dng nh phn, tuy nhin, file trong nh dng RINEX c kh nng phn ph i linh hot hn. Mt file RINEX l kt qu chuyn i file t nhng file nh phn nn c a chnh b thu . Phin bn phc tho ca nh dng RINEX c gii thiu vo nm 1989, sau phin bn c cp nht nhiu ln c th h tr nhiu dng d liu hn (vd:d liu GLONASS) v nhiu mc ch khc.Phin b n RINEX 2.10 hin hnh nh ngha su file RINEX khc nhau, mi file bao gm khu vc header v nhng khu vc d liu: 1) file d liu quan st, 2) file thng ip iu hng, 3) file kh tng , 4) file thng ip iu hng GLONASS, 5) file d liu v tinh a tnh 6) file d liu ng b gia ng h b thu v ng h v tinh. Phin bn 2.20 cung cp d liu t nhng v tinh LEO (Low Earth Orbit) trang b trong nhng b thu GPS hoc b thu GPS/ GLONASS. i vi ngi s dng, ba file

70 dhspkt-hcm-2009

GPS the global positioning systemRINEX c lit k u tin trn kia l quan tr ng nht. Chiu di mi dng ca tt c nhng dng file RINEX gii hn khong 80 k t. Quy tc t tn theo ngh i vi nhng file RINEX l ssssdddf.yyt. Trong : ssss : trnh by tn trm. ddd : trnh by s th t ngy trong nm ca bn ghi u tin. f : file sequence number(vd: f=0,file mang tt c d liu c trong ngy). yy v t: ln lt trnh by hai k s cui cng ca nm hin ti v dng file.Dng file bao gm nhng k t sau:O l file quan st;N l file iu hng;M l file d liu kh tng,Gl file iu hng GLONASS v H l file thng ip iu hng payload GPS tnh.(V d,vi file c tn abcd032.01o,file ny c quan st bi trm abcd, quan st vo ngy 1,thng hai,nm 2001). File quan st c cha thng tin tiu m t ni dung ca file nh l tn trm, thng tin anten, t a trm gn ng,s lng v dng quan st, kho ng thi gian quan st tnh theo giy, thi gian ca bn ghi quan st u tin, v nhi u thng tin khc. Nh ng dng quan st c xc nh bi L1 v L2, v s liu o c pha thc hin trn L1 v L2 (s chu k), C1 trnh by tm gi s dng m C/A trn L1 (mt); P1 v P2 trnh by t m gi s dng m P trn c L1 v L2 (mt); D1 v D2 trnh by t n s Doppler trn L1 v L2 (Hz). Khung thi gian GPS s dng i vi nhng file GPS trong khi khung th i gian UTC s dng i vi nhng file GLONASS.Khu vc header c th c mt vi bn ghi ty chn. Hai mi k t cui ca mi bn ghi (v d nh t ct 61 n ct 80) m t theo ng nguyn vn ca bn ghi . Bn ghi cui cng trong khu v c header phi l END OF HEADER. (Hnh 36)

71 dhspkt-hcm-2009


Hnh 36: v d v file quan st nh dng RINEX i vi d liu n tn Khu vc d liu chia thnh hai giai on; mi giai on u c gn th thi gian quan st (thi gian b thu thu tn hi u, theo khung th i gian GPS i vi nhng file GPS), s lng v danh sch cc v tinh, nh ng dng o c khc nhau m t trong cng mt chui hin din trong header, v chiu di tn hi u. Ngoi ra khu vc ny cn bao gm gi tr suy hao ca tn hi u ch th s tc nghn. sai lch ng h b thu tnh theo giy c th hin din hoc khng trong khu vc d liu. (Hnh 37 )

72 dhspkt-hcm-2009


Hnh 37: V d v file iu hng GPS Thng ip iu hng mang thng tin v v tinh. Trong phn tiu , mang nhng thng tin bao g m ngy to file, tn c quan qun l, v cc thng tin c lin quan khc. Ging nh file quan st, bn ghi cui cng trong khu vc header ca file iu hng phi l END OF HEADER. Ngoi ra, khu vc header c th mang nhng thng tin thm vo nh l nhng thng s ca m hnh t ng in ly i vi ngi s dng n tn. Tng t, nhng tham s almanac lin quan n thi gian GPS hoc thi gian UTM v leap second c th c ty chn bao hm lun trong khu v c header ca thng ip iu hng. Bn ghi u tin trong khu v c d liu mang thng tin v s PRN v tinh,th thi gian,v nhng tham s ng h v tinh (phn c c, tri git v tc tri git). Nhng bn ghi ph mang thng tin v qu o qung b ca v tinh, tnh tr ng v tinh,tun v tinh (GPS week), v cc thng tin c lin quan khc .

73 dhspkt-hcm-2009

GPS the global positioning systemBng thng tin GPS Week:

File kh tng mang thng tin c g n km thi gian nh l nhit (theo Celsius), p sut khng kh (theo milibar) v m (theo phn trm) ti cnh quan st. File kh tng bt u vi khu vc header mang nh ng dng thng tin quan st khc nhau (v d nh p sut), thng tin c lin quan n cm bin, v tr chnh xc c a cm bin kh tng, v mt vi thng tin c lin quan khc, b n ghi cui cng trong khu vc header phi l END OF HEADER. Khu vc d liu mang nhng th thi gian (theo thi gian GPS), theo sau l d liu kh tng c sp xp trong cng mt chui c trng nh trong khu vc header (Hnh 38).

74 dhspkt-hcm-2009


Hnh 38:v d v file kh tng RINEX Hu ht cc nh sn xut b thu GPS pht tri n nhng gi phn mm postprocessing c kh nng chp nhn d liu GPS theo nh dng RINEX. Ngoi ra, a s nhng gi ny u c kh nng chuyn d liu GPS theo nh dng c quyn ca nh sn xut sang nh dng RINEX.Tuy nhin, cc nh sn xut nn bit rng, mt vi gi phn mm thay i lun nhng quan st th ban u bn trong tin trnh chuy n i (v d: iu chnh li nhng gi tr o c tm gi th).

6.2 nh dng NGS-SP3:Vi t chc ang to ra d liu qu o chnh xc (ephemeris) h tr nhng ng dng i hi s nh v chnh xc cao. c th trao i mt cch thun tin nhng thng tin d liu qu o chnh xc nh vy, U.S.NGC pht tri n nh dng SP3, sau tr thnh tiu chu n quc t.SP3 l ch vit tt ca Standard Product #3, c gii thiu ln u tin vo 1985 nh l SP1. File SP3 l mt file ASCII, mang nhng thng tin d liu qu o chnh xc (trong khung tham chi u ITRF) v thng tin iu chnh ng

75 dhspkt-hcm-2009

GPS the global positioning systemh v tinh gn km. Chiu di mi dng ca nhng file SP3 gii hn trong 60 k t. Tiu chun d liu SP3 s dng h thng thi gian GPS. File ephemeris chnh xc trong nh dng SP3 bao gm hai khu vc: tiu v d liu.Khu vc tiu c 22 dng(Hnh 39). Hng u tin bt u vi biu tng c bit (#a) v mang thng tin nh l ngy Gregorian, th i gian xt trong ngy ca giai on qu o u tin v s lng cc giai on trong file ephemeris. Hng th hai bt u vi nhng biu tng (##) v ch ra tun GPS,ngy th hai ca tun, khong thi gian ca tng giai on v ngy Julian sa i. Hng 3-7 bt u vi biu tng (+), ch ra s lng v tinh t ng cng (trn hng th ba) cng nh l danh sch cc v tinh cng vi nhng nhn dng tng ng ca ring chng (s PRN). Dng 8-12 bt u vi biu tng (++) v ch ra nhng ae (accuracy exponent) chnh xc ca cc v tinh trn cc hng 3-7. ngha ca ae c gii thch nh sau: sai lch tiu chun ca mt li qu qu o c lch tiu chun l 2 = 64 mm hoc 6.4cm. Dng 13-19 trn tiu SP3 dnh cho nhng li bnh lun. o i vi mt v tinh =2 mm. V d: v tinh PRN1 c mt ae l 16, c ngha rng li c d tr dnh cho nh ng sa i tng lai, trong khi dng 19-22 c s dng t do

76 dhspkt-hcm-2009


Hnh 39: v d v khu vc tiu ca mt file SP3 Khu vc d liu ca nhng ephemeris chnh xc trong nh dng SP3 bt u ti dng 23, bao gm d liu v thi gian ca bn ghi u tin.Thc t,bn ghi ny cng th i im vi dng u tin ca khu vc header.Nhng dng sau mang thng tin t a v tinh v d liu ng h v tinh ca giai on hin hnh.M i dng gn vi mt v tinh no v bt u vi k t p,c ngha l hng v tr.Theo sau k t P l mt con s PRN v tinh ,t a x,y,z ca v tinh theo kilomet,v thng ip iu chnh ng h v tinh theo micro giy (Hnh 40).Trong vi trng hp, tc lc v tinh v t c ca thng ip iu chnh ng h c ghp hp li vi nhau cng vi thng tin ny.Bn ghi v tr v bn ghi iu chnh ng h s nm trn mt dng ,theo sau l mt dng khc mang gi tr tc lc v bn ghi iu chnh tc ca ng h ca cng mt v tinh. Dng mang b n ghi

77 dhspkt-hcm-2009

GPS the global positioning systemtc lc bt u vi ch ci V(V elocity).Nhng giai on theo sau s c cng cu trc ,v file kt thc vi biu tng EOF

Hnh 40: v d v khu vc tiu ca mt file SP3

6.3 Tiu chun RTCM SC-104 i vi dch v DGPS:Hot ng DGPS thi gian thc i hi mt s nh gi i vi thng tin iu chnh tm gi ti b thu tham kh o, thng tin sau c pht n b thu ch thng qua

78 dhspkt-hcm-2009

GPS the global positioning systemmt tuyn lin kt thng tin. m bo hot ng hiu qu,thng tin iu chnh tm gi c gi trong mt nh dng tiu chun cng nghip gi l RTCM SC-104. Dch v ny c xut bi RTCM (Radio Technical Commission for Maritime Services),y l mt t chc c vn c thnh lp vo nm 1947 khm ph nh ng vn lin quan n vin thng hng h i.y ban c bit No.104(SC-104) c thnh lp vo nm 1983 nhm pht tri n nhng khuyn co trong hot ng pht i nhng thng tin iu chnh sai phn i vi ngi s dng.Phin bn khuyn co phc tho c xut bn vo thng 10/1985,theo sau l nh ng phin bn cp nht khc.Ban u,nh dng RTCM SC-104 c thit k h tr vic qung b thng tin iu chnh DGPS trong ng dng dn ng hng h i cng cng. Tuy nhin, sau tr thnh m t nh dng tiu chun cng nghip pht i thng tin iu chnh DGPS thi gian th c. nh dng RTCM SC-104 bao gm 64 dng thng ip.Nhng thng ip ny bao gm cc thng tin nh l thng tin iu chnh tm gi PRC (Pseudorange correction) i vi mi v tinh trong t m nhn ca b thu tham kh o, tc thay i thng tin iu chnh RRC (Rate of change of the pseudorange correction), v ta ca trm thao kho. Thng ip m a s nhng ngi s dng DGPS thi gian thc a chung l thng ip dng 1 v thng ip dng 9. V c hai dng ny u mang thng tin iu chnh PRC, RRC. Tuy nhin, thng ip dng 1 mang thng tin iu chnh ca tt c v tinh trong tm nhn ca trm tham kho, trong khi , i vi thng ip dng 9, nhng thng tin iu chnh li c ng gi thnh nhm,m i nhm bao gm 3 thng ip. S dng thng ip dng 9 gy ra iu bt tin l: trm tham kho i hi mt ng h n nh hn. Mt vi thng ip thm d (tentative message) c a vo phin bn 2.2 h tr hot ng RTK v GLONASS sai phn.

Bng :nhng dng thng ip RTCM hin hnh

79 dhspkt-hcm-2009


Thng ip nh dng RTCM SC-104 khng th no c trc tip c; y l nhng chui s nh phn, 0 v 1. M i thng ip hoc khung RTCM SC-104 mang N +2 t, vi mi t di 30 bit; trong , N t trnh by s lng t mang d liu thc s bn trong thng ip v hai t cn li trnh by mt tiu hai t bt u ti mi thng ip. Kch thc ca N thay i, ph thuc vo dng thng ip v ni dung thng ip (v d khi m s lng v tinh thay i trong tm nhn ca trm tham kho). Kch thc t v thut ton kim tra chn tng t trong thng ip iu hng GPS.

80 dhspkt-hcm-2009

GPS the global positioning system6.4 nh dng NMEA 0183:NMEA l vit tt ca National Marine Electronics Association,c thnh l p vo 1957 bi mt nhm nhng ngi bun bn thit b in t nhm tng cng mi quan h ca h i vi cc nh sn xut thit b in t. Vo nm 1983, vi u vo t cc nh sn xut,t chc t nhn, chnh ph , hip hi tha nhn NMEA 0183 l nh dng giao tip vi nhng thit b in t hng hi.N c cp nht vi ln. Tiu chun NMEA 0183 l mt chui d liu trong nh dng ASCII, pht vi tc 4,800bps, t bn ni n bn nghe, trong , bn ni l thit b gi d liu n nhng thit b khc (v d nh: b thu GPS) v bn nghe l thi t b nhn d liu t thit b kia (v d nh laptop giao tip vi b thu GPS). Chui d liu NMEA 0183 bao gm thng tin v v tr, gc ta , su ca nc, v nhng thng s khc. D liu c gi di dng cu; mi cu bt u vi k hiu dollar $ v kt thc vi (carriage return line feed). Theo sau k hi u dollar $ l vng a ch nm k t, hai k t u tin nhn dng bn ni, ba k t cui nhn dng kiu d liu v nh dng chui ca nhng vng k tip. Vng cui cng trong bt c cu no u l vng checksum, vng ny theo sau mt k t phn cch checksum *. S lng k t ti a trong mt cu l 82; trong , ti a c 79 k t gia du phn cch bt u $ v du kt thc . Mt s cu c h thng GPS v GLONASS s dng, trong khi nh ng cu cn li c tc dng h tr nhng thit b khc nh l my pht m thanh echo v nhi u thit b khc. Chng ta s ch tho lun v mt cu duy nht, GGA: Global Positioning System fix data. Cu ny trnh by th i gian, v tr v thng tin c lin quan n gii php. Hnh 41 ch ra cu trc chung chung c a cu GGA v gii thch nh ng thut ng ca cu.

81 dhspkt-hcm-2009


Hnh 41:cu trc chung ca cu GGA Bng: Gii thch nh ng thut ng v k hiu trong cu GGA

82 dhspkt-hcm-2009


Hu ht cc b thu c mt trn th trng u h tr tiu chun NMEA 0183. Tuy nhin khng ph i tt c b thu vi port NMEA 0183 u xut ra mt thng ip GPS c th no. Ngoi ra, m t s nh sn xut b thu GPS c th thay i mt cht nh dng

83 dhspkt-hcm-2009

GPS the global positioning systemtiu chun ny. Tuy nhin, thng thng, h cung cp phn mm gii thch cu d liu.

7. K thut ambiguity- resolution: chnh xc nh v cp centimet c th t c vi nhng quan st phasng mang trong ch nh v tng i. Tuy nhin, vi iu kin tin quyt l phi xc nh thnh cng nh ng tham s ambiguity nguyn ban u. Tin trnh ny g i l ambiguity resolution. Phn tch nh ng tham s ambiguity mt cch chnh xc t ng ng vi hot ng xc nh tm thc s, chnh xc t b thu n v tinh, kt qu l nh v chnh xc. Ban u, nhng tham s ambiguity c xc nh bi phng php lc Kalman v phng php least - squared. Khng may, nh ng phng php ny khng th xc nh c nhng s nguyn ca tham s ambiguity. Chng ta t c g vi nhng con s thc i km ch duy nht vi nhng tham s khng xc nh. Trong thc t, nhng tham s thc ny kh m c phn tch dng ph ng php baseline. Nh vy, khi m chng ta bit c rng, tham s ambiguity l s nguyn, phng php phn tch c th c xc nh r rng. Thng thng, nh v tng i GPS chnh xc cao cng vi nhng quan st phasng mang c tin hnh trong m t khong thi gian quan st di (trong kho ng vi gi). iu ny khin cho khi hnh h c v tinh-b thu thay i mt cch ng k, do gip ch trong hot ng tch ri nhng tham s ambiguity t php phn tch baseline. Nh vy, mc d phng php least-square cn mang nh ng gi tr thc ca nhng tham s ambiguity, chng thc s gn vi gi tr integer. Do , nhng gi tr nguyn chnh xc c th t c mt cch n gin bng cch lm trn nh ng con s gi tr thc tr thnh gi tr nguyn gn nht. Sau tin hnh iu chnh least-square, th a nhn rng nhng tham s ambiguity gi tr nguyn l nhng gi tr c xc nh trong khi

84 dhspkt-hcm-2009

GPS the global positioning systemnhng thnh ph n baseline th cha. R rng rng, mc d phng php ny c kh nng xc nh