Chng I

t vn Phng php nh v dng xc nh v tr, o c, dn ng l vn rt cn thit i vi hnh hi, qun s cng nh cc hot ng kinh t vn ho x hi. i i vi s pht trin v khoa hc k thut, th cc phng php nh v cng pht trin t th s n hin i. Bt u t knh lc phn, la bn, cc bng v sao. Sau l p dng sng v tuyn in nh v nh h thng dn ng radio, Transit v cho n khi k thut phng v tinh nhn to c pht trin ( u nhng nm 1960 ) th phng php nh v ton cu c thc hin. H thng nh v ton cu GPS ( Global Positioning System ) ra i, n khc phc c nhng nhc im ca cc phng php nh v trc nh: chnh xc cao (n tng cm), t ng ho cao, hot ng theo chng trnh phn mm c lp trnh sn, s dng v bo dng thit b. c bit cc ch lm vic lun m bo, cc thit b lm vic n nh 24/24gi do my c cu to c bit. Mc d h thng nh v GPS c nhng u im trn nhng n vn c nhng nhn t nh hng n vic nh v v dn ng th d:nh cc sai s do s tn sc v khc x trong tng in ly, trong qu trnh truyn sng, sai s ng h v tinh v ng h my thu GPS, sai s do nh hng ca cc tia phn x t mt t ti my thu GPS ( Multi parth signal) m v tr nh v bng GPS c th b sai lch. khc phc v p ng yu cu i hi ngy cng cao v chnh xc v phm vi ng dng ca phng php nh v th phi ng dng cc cng ngh tin tin c k thut cao. Ngi ta a ra k thut DGPS (Differential GPS )l mt k thut hin i c s dng rng ri. Vic ng dng cng ngh ng hng c hiu qu cao trc tin phi nghin cu hiu r bn cht ca cng ngh trn c s hiu bit v lm ch n. Xut pht t ngng l do trn ti nghin cu ti h thng nh v v tinh c sa phn sai DGPS (Differential Global Positioning System ) nhm mc ch nm vng l thuyt thc hnh ng dng cng ngh DGPS v ang c trin khai rng ri trn th gii v khu vc ng Nam v ang s dng rng ri nc ta. Trong khun kh ca ti ny ti trnh by cc phn sau: 1. S lc v cc h thng nh v 2. Cng ngh DGPS (Differential Global Positioning System) 3. Mt s kin kin ngh thit lp mng DGPS Vit Nam 3

4. Kt lun K thut DGPS l mt ni dung cn mi m i vi ngi dng Vit Nam, v thi gian c hn, vic tm hiu ti liu cn kh khn nn khng trnh khi nhng thiu st. Ti rt mong c s ch bo, ng gp kin ca cc thy gio v cc bn ng nghip. hon thnh c kho hc m c th l bo v thnh cng lun vn tt nghip ngy hm nay. l s c gng hc tp ca bn thn cng vi cng lao to ln ging dy truyn t kin thc ca cc thy c gio trng i Hc Hng Hi ni chung, ca cc thy gio trong khoa in T Vin Thng ni ring v c bit l s ch bo hng dn tn tnh ca thy gio hng dn: i t PGS - TS - Nh gio u t o c Knh. Qua y ti xin by t lng bit n su sc ti thy o c Knh cng cc thy gio trong trng. Ti xin knh chc thy cng ton th gia nh an khang thnh vng.

Xin chn thnh cm n! Tc gi: Nguyn Vn Duy

4

Chng II Tng quan v cc h thng nh vnh v dn ng c t lu trong cuc sng. Ngi ta a ra rt nhiu phng php nh v. y ti gii thiu mt s h thng nh v dn ng chnh.2.1. Cc h thng nh v dn ng trc khi c h thng GPS

2.1.1 H thng nh v Radio (Radio navigation system) Nguyn l c bn la giao hi cnh t 3 im bit to xc nh to mt phng ca im t my (im cha bit to ) Ti t nht 3 im bit to d,thit lp 3 trm pht tn hiu :Radio,im cn nh v t my thu tn hiu t cc trm pht tn hiu radio trn.Khong cch t trm t my thu dn trn pht c xc nh bng tr thi gian ca tn hiu pht t trm pht ti trm thu.Da vo tr thi gian t nht 3 trm pht v to ca cc trm ny,s tnh c to mt pht phng ca trm t my thu.K thut o tr thi gian c th s dng k thut o xung hoc o lch pha ca tn hiu sng iu ho. Cc h thng radio navigation trn th gii thng s dng di tn s sng ngn hoc sng trung. Nhc im ca h thng ny l tm hot ng c hn, chnh xc nh v ph thuc vo thi im do v v tr cn xc nh tng i vi cc trm pht radio c nh. tng tm hot ng ca h thng,ngi ta lin kt tng cm h thng gm t nht 3 trm Radio Beacon thnh tng chui h thng cc chui ny c ph sng ln,thng vo khong 1.000 dm. Nh vy chui ny to thnh mt h thng nh v quc gia hay khu vc.Cc h thng Radio navigation System ln dc bit trn th gii l:H LORAN_C, H CHAYKA; H LORAN-CHAYKA; H DECCA 2.1.1.1 H LORAN_C : Mc ch s dng trong qun s v sau ny s dng trong nh v dn ng dn s trn ton b bin nc M.H LORAN_C do lc lng bin phng M thit lp.N l h thng Radio Hybecbol s dng tn s xung 100 khz. ph sng ca h thng ph thuc vo chnh xc ca my thu. H ny phc v 24/24gi 5

vi chnh xc khong 100 m v tng i khong 20m n 100m.Do c tnh k tht ca h thng LORAN_C ph hp vi yu cu hng khng nn c quan qun l hng khng chp nhn h thng LORAN_C c ph sng ton nc M.H thng ny c hon thnh vo thng 6 nm 1991. 2.1.1.2 H thng CHAYKA v LORAN/CHAYKA : CHAYKA l mt h thng radio navigation tng t nh h thng LORAN_C do Nga xy dng dc theo b bin Nga. Gia h thng LORAN_C Bc Thi Bnh Duong v h thng CHAYKA c mt vng trng khong 500 hi l ti vng bin Bering khng dc c hai h thng ny ph sng.S hp tc gia M v Nga xy dng h thng LORAN-CHAYKA ph sng ton b vng trng ny.H thng ny lin kt cc trm ATTA v ALASKA thuc h thng LORAN_C ca M v cc trm Ptopavlovsk v Aleksandorcska thuc h thng CHAYKA ca Nga. 2.1.1.3 H thng DECCA Tng t cc h thng trc h thng DECCA c xy dng vi mc ch nh v dn ng trn khng v trn bin.H thng c thit lp trn vng pha ty Chu u,vng Ban Tch,Nam Phi,Vnh Rp,Nht Bn,n D, Australia. H thng hot ng trn hai di bng tn s :Bng 1 t 70-90 khz v t Bng hai t 110-130 khz. chnh xc khong 20-50cm trong khong cch di 220km.Trn khong cch nay gim rt nhanh. Trc khi co h thng dn ng bng GPS thi cc h thng dn ng trc y c s dng rng ri trn th gii.H thng GPS ra i vi tnh nng k thut vt tri va cc dch v khc km theo, dn y li cc h thng trn vo qu kh. 2.1.2 H thng TRANSIT: H thng ny l h dnh v radio ton cu s dng k thut DOPPLE nh v trong to khng gian.H thng ny gm 4 v tinh hoc nhiu hn c b tr trn cc qu o vi cao khong 1080km, my thu tn hiu t trn tri t c th thu cc fix cch nhau t 1n 3 gi khi v tinh t trong tm nhn ca ni t my.Bnh thng th mi v tinh xut hin trong tm nhin khong 20 pht.Khi ch cn xc nh mt fix cho mi v tinh. H thng TRANSIT nh v trn h quy chiu WGS-72. 6

H thng hot ng trn 2 tn s 150 Mhz v 400Mhz.V l thuyt ch cn hot ng trn mt tn s l c th nh v dc,nhng lm gim nh hng ca tng ion ti chnh xc nh v,ngi s dng phi s dng c hai tn s trn. chnh xc ca h thng ph thuc vo s ln bt c v tinh a vo tnh ton nh v(s fix thu c) v vic s dng mt tn s hay hai tn s. chnh xc tiu chun ca mt ln bt v tinh a vo tnh ton nh v vi mt tn s l 500 m,nu s dng hai tn s chnh xc tng ln 25m.Vi vic o ng thi trn im cn xc nh to v im bit to , chnh xc s tng ln 10m do s dng k thut phn sai. chnh xc c th tng n 5m khi s dng cc phng php o dc bit.H thng TRANSIT c hi qun M xy dng vi mc ch s dng trong quc phng.Tuy nhin h thng ny cng thng c s dng trong dn s trc khi h thng GPS ra i.2.2. H thng nh v ton cu GPS (Global Positioning System )

i i vi s pht trin v khoa hc k thut l s pht trin v nh v dn ng. Yu cu v vic nh v dn ng thng nht trn phm vi ton cu c t ra trong cc ngnh kinh t quc dn, quc phng an ninh v ch c bt u khi k thut phng v tinh nhn to c pht trin. V n ch c th nh v c da vo cc chun ngoi tri t. Chnh v th m vic nghin cu xy dng h thng nh v ton cu ch mi bt u t nhng nm 1960. Cc h thng nh v ang c thc hin trn th gii l h thng NAVSTA, h thng GLONASS. 2.2.1. H thng NAVSTA H thng nh v ton cu GPS hin nay c s dng ph bin trn th gii l h thng NAVSTA. H thng ny do khng qun v hi qun M thit lp t nm 1978 vi mc ch ch yu dng trong qun s. n cui nhng nm 1980, mc ch s dng ca h thng NAVSTA GPS c m rng sang cc lnh vc dn s. Bn cht k thut ca vic nh v trong h thng nh v ton cu l vic giao hi cnh khng gian t v tinh nhn to ( c thit lp phc v nh v ton cu) ti im cn nh v trn tri t. Mun xc nh to khng gian ca mt im trn tri t phi giao hi t nht t 3 v tinh. Nh vy yu cu k thut ca h thng nh v ton cu l: - C mt h thng cc v tinh nhn to bay quanh tri t trn cc qu o chnh xc. Cc v tinh ny pht tn hiu 7

-

-

nh v (hay cn gi l s liu ng dn NAVSTA ) cho cc my thu t trn tri t. S lng v tinh v cc qu o v tinh cn tnh ton sao cho bt k thi im no trn tri t u c th nhn thy t nht 3 v tinh. My thu tn hiu pht t v tinh GPS, vi cc gii php k thut phn cng v phn mm thu tn hiu v tnh c khong cch t my thu ti tng v tinh GPS. H thng kim tra theo di, iu hnh, kim sot, v hiu chnh qu o ca cc v tinh GPS. H thng NAVSTA GPS c thit lp theo nguyn l trn gm 3 phn h thng (Segment): Phn khng gian, phn iu khin v phn s dng.

Phn Khng Gian (Space Segment).

L h thng v tinh gm 24 v tinh c thit lp trn 6 qu o quanh tri t. Mi qu o v tinh gm 4 v tinh phn b u. Trn qu o mt phng xch o tri t gc nh din 550. cao ca v tinh khong 20190 Km, chu k ca mt v tinh bay quanh tri t l 12 gi. H thng v tinh ny c hon thnh vo thng 2 nm 1989. Vi cch b tr v tinh nh trn, v mt l thuyt mi im trn tri t, trong mi thi im trn tri t, trong mi thi gian u nhn thy t nht 4 v tinh. Trn c s kim sot c cc qu o trn v tinh c th bit c to tc thi ca tng v tinh trong mt h quy chiu nht nh (hin nay s v tinh GPS ln ti 32 v tinh). Mi v tinh pht hin lin tc cc tn hiu dn ng ( gi l NADATA) xung cc my thu tn hiu v tinh s dng. Cc thng tin NADATA bao gm : H thng thi gian; thng s ci chnh ng b v tinh; thng s v qu o v tinh; lch v tinh v cc thng s nh hng ca tng in ly (Inospheric Delay Model Parameters). Phng php truyn thng tin t v tinh l truyn tin theo m: Code C/A v Code P. Code C/A khng bo mt c dng trong dn s, Code P l code v bo mt c dng ch yu trong qun s ( hin nay cng c s dng vi mc ch dn s). Cc m code P c truyn i bng hai sng mang L1= 1575.42 MHz v L2= 1227.60 MHz tng ng vi bc sng 1=19cm v 2=24 cm.

8

Tn s L1c iu ch bi c hai code C/A v code P. Tn s c iu ch bi code P.Phn Kim Tra (Control Segment)

H thng kim tra c thit lp nhm theo di,kim sot v hiu chnh v tinh,nhm m bo h thng v tinh hot ng bnh thng,chnh xc.H thng ny bao gm 4 trm quan st v tinh (Monitor Station),3 trm ci chnh (Upload Station) v mt trm iu khin trung tm (Master Control Station). Nguyn l hot ng ca h thng nh sau: Bn trm quan st v tinh lin tc thu tn hiu t tt c cc v tinh v truyn s liu v trm iu khin trung tm.Trm trung tm,tnh ton ra cc h s ci chnh qu o,ng h v tinh v truyn cc h s ny t cc trm pht s ci chnh.Ti y s ci chnh c pht li cho v tinh t nht mt ngy mt ln.Vi phng thc qu o ny s m bo thng tin v qu o v tinh pht xung cho cc my thu GPS c chnh xc v sai s ng h v tinh s gim bt rt nhiu,trn c s chnh nh v s c nng cao.Phn ngi s dng (User Segment)

on ngi s dng v cc my thu GPS,t c nh hoc di ng trn b mt tri t hoc trong khng gian(Trng hp gn myGPS ) rn my bay,tu bin). Cc my thu GPS ny thu tn hiu t trn cc v tinh GPS vi mc ch nh v v dn ng.Vi mc ch ny cc my thu GPS phi x l cc tn hiu va thu c t cc v tinh GPS tnh ton ra to khng gian ca tm antena my thu GPS nh phn trn nu,nguyn l nh v ca GPS l :Nguyn l giao hi cnh t im cn nh v (tm antena ca my thu)n cc v tinh GPS.Nh vy my thu GPS phi tnh dc khong cch tc thi t tm antena n cc v tinh GPS : tnh c my thu GPS n cc v tinh, cng ging nh nguyn l o khong cch bng sng in t,c th c 2 phng php : 1.Tnh c tr thi gian ca tn hiu pht i t cc v tinh GPS n khi thu c tn hiu ti my thu GPS. 2.Tnh c lch pha (di pha ) t tn hiu pht i t cc v tinh GPS n khi thu ti my thu GPS. Gii php k thut o c tr thi gian trong h thng GPS l p dng k thut tng quan ngang (Cross 9

Correclation).Phng php ny gi l phng php Code Pseudornge nh sau : V tinh GPS pht i cc m code gi ngu nhin (PRN code). My thu GPS to ra cc bn sao m code gi ngu nhin tng ng i vi tng v tinh.So snh tong quan gia hai im m code trn s lm ra tr thi gian.Hnh v sau minh ho cc phng php Code Pseudorange:

Satellite (V tinh) Receiver Code (M thu) t Time Delay (Thi gian tr)

PSEUDORANGE=c.t

Nguyn l ca phng php o lch pha l :Khong cch t v tinh GPS n my thu c tnh bng lch pha ca sng mang(Khi truyn t v tinh GPS ti my thu GPS ) nhn vi chiu di bc sng mang.Cc my o pha tng ng vi phn l ca mi chu k giao ng sng.V vy lch pha s tinh bng s nguyn ln bc sng cng vi lch pha o c.Phng php xc nh khong cch t v tinh my thu n GPS bng cch xc nh lch pha gi l phng php tr o khong cch pha sng mang(Carrier Phase Pseudorange). Hnh v m t nguyn l ca phng php ny:

10

Ambiguit

Carrer (Sng mang)

i vi phng php ny cn to my thu GPS phi m bo gii quyt 2 vn : 1.To sng mang ng thi vi thi im cc sng mang pht i t v tinh GPS (gi l tn hiu i chiu ). So snh tn hiu i chiu v tn hiu thu c t v tinh o c phn l lch pha tng ng vi phn l ca mt chu k giao ng sng. 2. m c s nguyn ln bc sng t v tinh GPS ti my thu GPS. H trc a th gii WGS -84 V tr ca mt im no c xc nh (nh v ) phi hiu theo ngha: c nh v theo h quy chiu nht nh no . Vi mc ch l mt h nh v ton cu,h thng GPS phi xc nh s dng mt h quy chiu thng nht trn ton cu. H quy chiu c s dng trong h thng nh v GPS l Elip Soid GPS-80 (Geodetic Reference System 1980 H quy chiu trc a nm 1980) c nh v trn ton cu theo a tm. Elip Soid GPS80 c bn trc ln bng 6378.137 (m) v nghch o rt 1/f = 298257223563. H trc a s dng mt quy chiu GPS 80 c gi l h trc a th gii WGS -84 (World Greodtic System). To xc nh GPS trong h WGS l to khng gian 3 chiu. H to khng gian 3 chiu l h to c xc nh bng 3 trc to x,y, z. Tm ca h to 3 chiu l a tm. Trc x nm trn mt phng xch o v i qua kinh tuyn gc v a tm. Trc y nm trn mt phng xch o, vung gc vi trc y i qua a tm. Trc z i qua a tm v vung gc vi mt phng xch o. Z N z X y 11 x Y

H to thng thng s dng trong trc a bn khng phi l h to khng gian 3 chiu m l h to a l B,L,H v h to vung gc vi mt phng x,y,h. T to khng gian 3 chiu c th chuyn i v h to a l B L H v h to vung gc (mt phng ) x y h (theo mt phng php no ) y lu rng H l cao tnh trn ElipSoid (High Above ElipSoid) cao ny khc vi cao chun (Meal Sea Level).Quan h gia H v h c biu th bng biu thc H = H + N. y N l cao gia Geoid v Elipsoid. Thng th mi quc gia u c mt h quy chiu trc a quc gia ring bit.Khi GPS ,To GPS c tnh trn h WGS 84.Mun chuyn v h to tham chiu quc gia cn c tham s tnh chuyn. 2.2.2 H thng nh v ton cu GLONASS. H thng nh v ton cu GLONASS do Nga thit lp nm 1993.y l h nh v ton cu s dng h to khng gian,v c bn c cu trc v nguyn l hot ng ging nh h thng GPS.Nhng khc c bn v mt chi tit k thut ca hai h thng nh sau : 1.Trong khi h thng NAVSTAR GPS c 24 v tinh c b tr trn 6 qu o,mt phng qu o v tinh nghing so vi mt phng qu o tri t 55 , cao v tinh khong 20.190 km v chu k v tinh l 12 gi,thi h thng GLONASS GPS c 24 v tinh c b tr trn 3 qu o,mt phng qu o nghing so vi mt phng xch o tri t 64.8 , cao v tinh l 19.100 km v chu k v tinh l 11 gi 15 pht. 2. Cng ging nh cc v tinh ca h thng NAVSTAR GPS ,cc v tinh ca h thng GLONASS GPS cng s dng hai tn s sng mang (carrier frequenciens) S dng hai loi m code C/A v P code.Song hai tn s sng mang L1 v L2 trong h thng NAVSTAR GPS c s dng chung cho tt c cc v tinh. Trong khi i vi h thng GLONASS GPS mi v tinh s dng mt tn s sng mang khc nhau. Gii tn bng sng L 1 ca h thng GLONASS GPS t 1602.5625 MHz n 161505 MHz. Mi v tinh c 12

tn s khc nhau 0.5625 MHz. Gii tn bng sng mang L2 ca h thng GLONASS GPS t1246.4375 MHz ti 1256.5 MHz. Mi v tinh c tn s khc nhau 0.4375 MHz. S khc bit gia hai h thng l: i vi NAVSTAR GPS th mi v tinh s dng mt code khc nhau cn vi h thng GLONASS th code c s dng chung cho tt c cc v tinh trong h thng. 3. H thng NAVSTAR GPS s dng h quy chiu WGS -84 cn h thng GLONASS GPS s dng h quy chiu SGS 85. V chnh xc ca h thng GLONASS GPS tuy cha c c nhng cng b y , xong c th coi hao h thng ny tng ng vi nhau v chnh xc. H thng GLONASS GPS c thit k t thng 5 nm 1993 khi c 12 v tinh. Cui nm 1993 (theo k hoch) s c 20 v tinh v n nm 1995 th hon thnh h thng. Thc t th h thng NAVSTAR GPS c s dng rng ri trn ton th gii (k c Vit Nam) hn h thng GLONASS GPS vi cc l do sau: 1. H quy chiu WGS 84 quen thuc hn v c s dng rng ri hn h quy chiu SGS -85. Hn na vic chuyn i gia hai h thng ny vn cn l mt vn nghin cu, thng nht. Vi nhng thc nghim ban u th hai h thng chnh lch nhau t 510 m. 2. Khi hai h thng coi l c chnh xc ging nhau, th ngi s dng ch quan tm n cc thit b thu tn hiu GPS. Cc my thu GPS tng ng vi h thng NAVSTAR GPS ph bin trn th trng rt nhiu so vi cc my thu tn hiu GPS tng ng vi h thng GLONASS GPS. 3. Cc thng tin v h thng NAVSTAR GPS v cc dch v km theo rng ri hn nhiu so vi cc thng tin v dch v ca h thng GLONASS GPS (bao gm c v tip th qung co). 2.2.3. H thng nh v ton cu ci chnh phn sai (Differential GPS). Do nh hng ca nhiu thnh phn sai s th d nh sai s do nh hng ca s tn sc v khc x trong bu kh quyn, sai s ca ng h v tinh v ng h my thu GPS. Sai s do nh hng ca tia phn x t mt t bi my thu GPS (multipart signal), sai s qu o v tinh, v tr nh v bng v tinh GPS c th b sai lch. 13

V vy trong ng dng ngi ta phi tm ra cc gii php gim thiu cc phn sai trn. l k thut o GPS phn sai. Nguyn l ca phng php ny l ngi ta s dng my thu GPS chun bit chnh xc to , thu tn hiu t v tinh GPS tnh ra s ci chnh phn sai. Sau gi s phn sai ti cc my thu ng dng GPS s tnh ra to chnh xc ca n. Chng ta s nghin cu c th k thut DGPS chng 3.

14

Chng III Cng ngh DGPS3.1. C s l thuyt:

Mun xc nh to khng gian ca mt im trn tri t phi giao t nht t 3 v tinh. Nu t to khng gian l XYZ. Khi to ca v tinh nh v v im thu GPS k hiu tng ng XS, YS, ZS v x, y, z th phng trnh khong cch t my thu GPS n v tinh l: P = ((XS-x)2+(YS-y)2+(ZS-z)2)1/2 (1-1) Cn vn tc hng tm nhn c bng cch ly vi phn phng trnh trn (1-1).* * P* = r 1 ( x S x)( x s* x * ) + ( y S y )( y S y * ) + ( z S z )( z S z * ) (1-2)

R rng cc i lng (1-1), (2-2) cn tng thch vi mt thi im o. V vy nn thang o ca h s dng v du thi gian ca v tinh phi c lin h trc tip. tnh to h s dng ti thi im Ti th to ca v tinh phi chn thi im (ti- r/c). Trong li h thng nh v v tinh p dng phng php o c ly vi vic gi im tnh u. C ly o gn ng s lch vi gi tr thc mt gi tr m gi tr ph thuc vo dch chuyn tng i ca h thng thi gian. Vi v tinh nh v c lch thang thi gian T Khi truyn sng in t trong khng gian kh quyn c gi chm tm so vi trong khng gian t do. Cc sai s . Do php s l ta c ly (c ly gn ng) ch khng phi l c ly P. Khi khong cch (psuedorange) l: P = ((XS-x)2+(YS-y)2+(ZS-z)2)1/2 + C( t- T) + atm+ Trong XS, YS, ZS l to khng gian ca v tinh. X, Y, Z l to khng gian ca my thu GPS. C tc nh sng trong chn khng P : tr o psuedorange t, T: l sai s ng h my thu GPS v sai s ng h v tinh GPS. atm : l sai s do nh hng ca kh quyn. : Sai s tng hp khc C( t- T); atm; c gi chung l cc phn sai. Cc phn sai ny dao ng t mt vi mt n mt vi trc mt. 15

T cng thc ta thy nu t my thu GPS trn mt im bit to l X,Y,Z c th tnh ra phn sai bng cch ly khong cch psuedorange tr i khong cch c tnh bng to v tinh v to antena my thu GPS. L thuyt v thc nghim chng minh rng nh hng phn sai ca cc im cch nhau trong khong 300 dm (500 km) l gn bng nhau.3.2. Nguyn l o DGPS

1. t my thu trn mt im bit to chnh xc, thu tn hiu t cc v tinh GPS v tnh ra cc s ci chnh phn sai. My thu ny c gi l my Base 2. Cc my thu ng dng gi l my Rover, thu tn hiu GPS ng thi vi my Base, ci chnh vo cc tr o c tnh t my Rover tnh ra ta chnh xc ca my Rover. Tnh ci chnh phn sai c hai phng php:* Phng php POSITION:

S ci chnh phn sai c tnh di dng thc o kinh , v , v cao (B, L, H), bng cch ly hiu s to bit ti trm BASE v to c tnh ra theo tr o GPS ti trm BASE.*Phng php MEASURMENT:

S ci chnh phn sai c tnh ra di dng ci chnh chiu di vo khong cch psuedorange (P), bng cch ly hiu khong cch tht t v tinh ti tm im antena ca my thu GPS ti trm BASE v khong cch Psuedorange ti cc v tinh, tnh t trm BASE. S ci chnh phn sai tnh theo phng php POSITION ti trm BASE ch ng i vi trm ROVER khi ta c ti trm BASE v ta tnh c ti trm ROVER trn cng mt chm v tinh GPS (GPS constellation), chnh v vy thng thng ngi ta s dng phng php MEASURMENT. Tu theo cch x l s ci chnh phn sai, c th chia phng php DGPS thnh hai loi* Phng php POST PROSESSING DGPS

Phng php ny s dng phn mm x l hai files s liu thu c t my thu BASE v ROVER ( ng thi thu tn hiu trong

16

cng mt khong thi gian), tnh ton ra s ci chnh phn sai v ta c ci chnh phn sai ca trm my ROVER. Nh vy to tc thi ( c ci chnh phn sai ) khng c tnh ton ti hin trng khi o, m phi tnh ton trong phng sau khi o. Phng php ny hin nay t c s dng.* Phng php REAL TIME DGPS

Phng php ny s dng phn mm lin tc tnh ra cc s ci chnh phn sai. Cc s ci chnh phn sai ny c pht ti cc my ROVER thng qua thit b truyn thng. Cc my thu GPS ROVER ng thi thu tn hiu t v tinh GPS v tn hiu ci chnh phn sai pht t trm BASE tnh ra ta chnh xc ( c ci chnh phn sai ). Nh vy khc vi phng php POST PROSESSING, ta tc thi ( c ci chnh phn sai ) c lin tc tnh ton my ROVER ti thi im nh v S hot ng ca phng php REAL TIME DGPS SATELLITE SATELLITE GPS Signal GPS Signal GPS Signal SATELLITE

SATELLITE

GPS Signal

GPS Signal

GPS Signal

Data Link Reference Station USERS

DIFFERENTIAL GPS Differential GPS GEOMETRY GEOMETRY

17

Cu hnh thit b ca phng php REAL TIME DGPS gm 3 phn chnh: + My thu GPS BASE + Thit b truyn tin thu v pht (Datalink) + My thu GPS ROVER

18

S khi ca thit b trm BASE (cn gi l trm Reference)GPS SATELLTE SCNALS

GPS RECEIVERS

DATA LINK

GPS SENSOR

V MEASUREMENT DPROCESSOR Differentia l ATA MEASUREMENT Correction Message PROCESSOR

DATA FORMATTER

MODULATOR

DATA LINK TRANSMITTER

REFERENCE STATION COORDINATES

Theo s khi trn, chu trnh vn hnh ti trm tnh nh sau : - My thu GPS thu cc tn hiu t ccv tinh v tnh ra cc Pseudorange tng ng n cc v. - Thit b phn mm (Correction Generator) s dng to bit ti trm tnh (to tm anten ca my thu GPS ) c cc Pseudorange tnh ra s ci chnh phn sai. - S ci chnh phn sai c tnh dng chun bng thit b Data Formater - S ci chnh phn sai c sng ti (carrier) truyn vo khng gian ti trm ROVER qua anten pht - sng ti truyn c s ci chnh phn sai th sng ti c iu ch bi dng tn hiu c nh dng chun ca s ci chnh phn sai Cng vic ny c thc hin thit b iu ch (Modulation ). - Yu cu k thut ca trm tnh l : 19

+ To chun c xc nh chnh xc lo b sai s to chun. + Thit b GPS v Correction Generator phi chnh xc, tnh chnh xc s ci chnh phn sai v kim sot c s ci chnh ny. + Format tn hiu ci chnh phn sai phi c nh dng chun c tnh ph thng. + Phng php iu ch phi m bo an ton,chnh xc truyn dc trn gi thng tin ca s ci chnh phn sai.GPS SATELLTE SCNALS

GPS RECEIVERSV DMEASUREMENTNAVIGIATION DATA POSITION CORDI NATES

GPS SENSOR

ATA

PROCESSOR

MEASUREMENT PROCESSOR

DISPLAY AND OUTPUT

DIFFENTIAL GPS DATA LINK

DIFFERENTIAL CORRECTION

DATA LINK

DATA LINK RECEVER

DEMODULATOR

DATA FORMATTER

20

Nguyn l hot ng ca trm ROVER : - Datalink Receiver thu c sng mang,ti s liu ci chnh phn sai t trm tnh thng qua anten DGPS. - B tch sng (Demodulation) tch s hiu ci chnh phn sai t sng ti thu nhn dc t Datalink Receiver. - S liu ci chnh phn sai c nhn dng chun bng Dataformater v truyn n thit b x l s liu ci chnh phn sai (Differential Dataprocessor). - Thng qua anten GPS ,GPS Sensor thu c tn hiu t cc v tinh GPS. - Thu tn hiu GPS c truyn ti b phn x l o (Measurement processor ) tnh cc Pseudorange. - Kt hp nhng s ci chnh phn sai v Pseudorange tnh c s tnh ra to trm ROVER c ci chnh phn sai.To ny c hin th trn mn hnh v c ly ra qua cng Output ca my thu GPS. V mt cu to thit b a s cc my ROVER thng dng hin nay u ghp Datalink v GPS Receiver thnh mt my (k c anten ).3.3 Phng php iu ch,nh dng Format s hiu v cc loi thng bo (Message type) c gi i trong tn hiu ci chnh phn sai.

3.3.1 Phng php iu ch. Trong k thut truyn thng,ni dung thng tin c truyn ti trn cc sng mang (sng ti ).Bn thn sng ti khng cha ng thng tin,cn truyn ti i. thc hin c nc ch ny,sng ti s b iu ch (Modolation) theo tn hiu cha thng tin. C nhiu phng php iu ch.Trong k thut DGPS ,thng tin cn truyn ti i l cc gi s liu.m bo tnh chnh xc 100%,ca gi s liu truyn i l yu cu khng th thiu,trong phng thc truyn thng i vi phng thc DGPS.iu ny ph thuc phng php iu ch c p dng.Qua thit k v thi nghim,t chc bin phng b bin M chng minh rng phng php iu ch Minimum Shift Kying (MSK), thch ng vi k thut truyn s ci chnh phn sai DGPS.Do cc trm BEACON DGPS u s dng k thut MSK v c gi l cc trm MSK BEACON DGPS. 3.3.2. nh dng Format s liu.

21

nh dng Format s liu l mt vn rt quan trng trong k thut truyn thng tin.Tt c thng tin truyn i trong k thut DGPS (ni dung thng tin tng s liu ci chnh phn sai ) c truyn i bng mt dy cc s t nhin.Nh vy,ni dung thng tin phi c truyn ti i theo mt quy lut xp xp ca dy s t nhin c truyn ti i.Ngi nhn c thng tin (cc my thu s liu ci chnh DGPS ) phi gii c m truyn tn hiu hiu c cc thng tin nhn c.Nh vy nh dng thng tin (nh dng Fomat s liu ) c hiu theo kho m ca tn hiu truyn tin. DGPS l mt dch v cng cng.V vy nh dng Format s liu cng phi mang tnh ph bin,cng cng c th ng dng rng ri trn ton cu. Chuyn ban k thut 104 thuc u ban k thut Radio v cc dch v hng hi (RTCM-104-Radio Technical Comnission for Maritime Services.Special 104) thit k mt chun Format d liu cho s ci chnh phn sai s dng trong k thut DGPS. Ta thng thy cc my GPS c ci t Option RTCM-104, ni dung l cc my ny c ci t phn mm nh dng s liu theo chun RTCM -104. Cu to c bn ca Format nh dng s liu chun ca s liu ci chnh phn sai theo RTCM-104 c gii thch ngn gn nh sau : 1. Ni dung ca s liu ci chnh phn sai bao gm cc thng tin bng s (V d : S ci chnh phn sai Pseudorange; to trm Referencev.v) v cc thng tin khng bng s (Vi d : Thng bo tnh trng v tinh; tnh trng ca trm Reference; sai s ca s ci chnh phn sai vv ). Ni dung thng tin bng s c gi nguyn theo thng tin gc; ni dung thng tin khng bng s s c quy nh tng ng vi cc code (c nh dng di dng s ). Th d : Sai s ci chnh cnh c code ho nh sau : Code Number One signal Difference error 00 (0) 1m 01 (1) >1m 4m 8m 2. Sau khi quy nh code ca tt c cc thng tin phi s th ton b thng tin c gi i l mt chui s t nhin. Cc chui 22

s t nhin ny c th hin trn cc frame s liu c di 30 bit.Mt ni dung,hoc mt vi ni dung trong s liu ci chnh phn sai c gp thnh mt dng thng bo (Message type).i vi mi Massage Type s c kho m nhn dng frame s liu. Th d : S ci chnh phn sai Pseudorange c nh dng nh sau :

1 2 3 4 5 6 7 8 9 1 0 1 1 . 23 24 25 .30 SCALE UDRE SATELITEID PSEUDORANGE FACTOR CORRECTION PARITY

Trn s trn thi Scale factor v UDRE c th hin bng code; cc ni dung thng tin khc th hin bng s. 3.3.3. Cc loi thng bo (Massage type ). Thng tin ci chnh phn sai gi i t trm Beacon khng n thun ch l s ci chnh phn sai m gm nhiu loi thng tin khc. c th nh dng s liu thng tin, ngi ta chia ra cc ni dung thng tin thnh cc nhm thng bo (Massage type ) v nh dng format cho tng nhm thng bo ny. RTCM-104 chia cc ni dung thng tin trong k thut DGPS thnh 26 massage v nh dng cho 26 massage ny. Di y em ch trnh by ni dung thng tin v nh dng format cho cc masage chnh,ng dng trong k thut DGPS 3.3.3.1 Loi thng bo 1 (Massage Type 1). Ci chnh phn sai DGPS (Differrential GPS Corrections) Thng bo 1 cha ni dung chnh l s ci chnh phn sai tc thi c tnh v pht t trm Beacon.S ci chnh ny gm hai thnh phn : 1. S ci chnh Pseudorange i vi tng v tinh tnh c ti trm Beacon v khong cch tht tnh c t to khng gian ca anten trm Beacon v to khng gian ca cc v tinh tng ng.S ci chnh ny c k hiu l PRC(t 0) (Pseudorange correction at t0 of time ). 2. V s ci chnh PRCt 0 truyn t trm Beacon ti trm Rover no (ngi s dng) mt mt khong thi gian nhi nh.V vy trm Rover nhn c s ci chnh PRC(t0) khng phi ti thi im t0 m ti thi im t.Ti thi im t, s ci chnh tnh c ti 23

trm Beacon s khc s ci chnh t l khong cch (Range Rate Correction). S ci chnh ny dc k hiu l RRC. Nh vy s ci chnh Pseudorange ti thi im t cho cc my GPS ng s l: PRC (t) = PRC (t 0 ) + RRC (t t 0) Ti thi im t, khong cch Pseudorange tnh c ti cc my thu GPS ng s l : PR (t ) = PRM (t ) +PRC (t ) = PRM (t ) + PRC (t 0 ) +RRC (t t 0) Vi : PR (t) l khong cch Pseudorange c ci chnh phn sai ti thi im t. PRM (t ) l khong cch Pseudorange cha c ci chnh phn sai ti thi im t (tnh t vic thu tn hiu t v tinh GPS ). S ci chnh phn sai PRC (t ) c tnh cho tng v tinh i vi tt c cc v tinh trong tm nhn ti trm Beacon tithi im t. V vy trong format s liu thng bo 1 bao gm c thng tin v s liu v tinh (Satellte ID ) format s liu ca thng bo 1 c cu to nh sau:

24

1 2 3 4 5 6 7 8 9 1 0 1 1 . 23 24 25 .30UDRE SATELITE ID PSEUDORANGE CORRECTION PARITY Words 3 8.13 or 18

SCALE FACTOR RANGERAE ISSUE OF CORRECTI0 DATA N (IOD) PSEUDORANGE CORRECTION SATELITE ID PARITY Words 4 9.14 or 19 Words 5 10.15 or 20

R E RANGE RATE CORRECTION

PARITY

SCALE FACTOR ISSUE OF DATA (IOD) PSEUDORANG E CORRECTION (LOWER BYTE) RANGE RATE CORRECTION UDRE SATELIT E ID PSEUDORANG PARITY E CORRECTION (UPPER BYTE) ISSUE OF PARITY DATA (IOD)

Words 6 11.16 or 20 Words 7 12.17 or 23 Words N+2 If Ni=1.4.7 or 10

RANGE RATE CORRECTION

ISSUE OF DATA (IOD)

FILL

PARITY

ISSUE OF DATA (IOD)

FILL

PARITY

Words N+2 If Ni=2.58 or 11

TYPE ! MASSAGE FORMAT Trong cu to Format c thng bo di dng code ca thng bo 1 nh trn ta cn thy tri s UDRE (USER DIFFERENTIAL RANGE ERRO). Tr s ny thng bo sai s ca ci 25

chnh phn sai pht t trm Beacon.UDRE l tng hp cc nh hng sai s ca hin tng a tia (multipath ); nhiu tn hiu GPS (Signal to noise radio) v cc nh hng sai s khc. Tr s IDO (Issue of data) trong cu to Format s liu ca thng bo s 1,thng bo cho my GPS ng phi dng mt Set s liu thu c t v tinh c s dng ti trm Beacon nh v v tnh ra s ci chnh phn sai.

26

3.3.3.2. Thng bo 2 (Massage type 2 ) Delta Differential GPS Correction. Trong cu to h thng GPS ,c 4 trm Monitor v 1 trm Master Control. Bn trm ny thu nhn s liu GPS v trruyn v trm s liu Master Control,tram Master Control x l s liu t 4 trm Monitors trnv tnh ton ra s liu ci chnh ng h v tinh v s ci chnh qu o v tinh.V s ci chnh ny c pht tr li iu chnh hot ng ca cc v tinh t nht 1 ln trong ngy. Nh vy sau khi trm Master Control Uploaded s ci chnh cho cc v tinh, th cc v tinh pht qu o mi (EPHEMERIDE) ti cc my thu.Cc my thu s Decode qu o v tinh mi v tnh Pseudorange theo qu o v tinh mi. Tuy vy i vi cc my thu GPS vic Decode qu o v tinh khng thc hin ngay tc thi m mt mtkhong thi gian nht nh. C ngha l mc d qu o v tinh mi c thng bo trong tn hiu GPS t cc v tinh,song my GPS vn tnh Pseudorange theo lch v tinh c (qu o v tinh c ). Cc trm Beacon u c trang b cc my GPS c th decode qu do v tinh mi ngay tc thi. Kt qu l trong mt s trng hp, thi im v tinh GPS pht i lch v tinh mi, thi c mt khong thi gian, trm Beacon nh v v tnh s ci chinh phn sai theo lch v tinh mi,con my thu GPS ng li nh v theo lch v tinh c. Nh vy s ci chnh phn sai c tnh ti trm Beacon theo lch v tinh mi s khng chnh xc i vi cc my thu GPS c nh v theo lch v tinh c.V vy cn phi c mt s ci chnh ph, b li s sai khc ni trn. S ci chnh ny gi l Delta Differential GPS Correction, v c k hiu l DELTA PRC. DELTA PRC c tnh theo cng thc : DELTA PRC = PRC (old IOD) RRC (new IOD ) Cng vi l do nh trn, khi tnh s ci chnh Range rate differential, trm Beacon cng s dng v tinh mi, do phi c thm s ci chnh DELTA RRC DELTA RRC c tnh theo cng thc : DELTA RRC = RRC (old IOD) - RRC (new IOD ) . Vi :PRC (OLD IOD ) l s ci chnh phn sai Psuedorange c tnh theo lch v tinh c. PRC (New IOD ) : L s ci chnh phm sai c tinh theo lch v tinh mi. 27

RRC (Old IOD ) :L s ci chnh phn sai Range-rate c theo lch v tinh c RRC (New IOD) : L s ci chnh phn sai Range rate c tnh theo lch v tinh mi. My thu GPS ng s c IO D (Issue of data ) trong thng bo s 1 la chn xem c s dng s ci chnh Delta Diffencetial GPS Correction hay khng. Khi s dng s ci chnh DELTA th s ci chnh phn sai ng dng ti trm my GPS ng s c tnh bng cng thc sau :PRC(t )

= PRC (NEW IOD) + DELTA (OLD IOD ) + RRC (NEW IOD ) +DELTA RRC( OLD IOD )

Cu to Format s liu Massagge Type 2 c trnh by trong hinh v di y : 1 2UDRE

3 4 5 6 7 8 9 1 0 16 17. 24 25 29 30SATELITE ID DELTA PSEUDORANGE CORRECTION SCALE FACTOR PARITY Words 3 8.13 or 18

DELTA RANGERAE CORRECTI0 N

ISSUE OF DATA (IOD)

UDRE

SATELITE ID

PARITY

Words 4 9.14 or 19

DELTA PSEUDORANGE CORRECTION

DELTA RANGE RATE CORRECTION

PARITY

Words 5 10.15 or 20

SCALE FACTOR ISSUE DATA (IOD) OF UDR E SATELITE ID DELTA PSEUDORANG E CORRECTION (UPPER BYTE) ISSUE OF

PARITY

Words 6 11.16 or 20

DELTA

DELTA

28

PSEUDORANGE CORRECTION (LOWER BYTE) DELTA RANGE RATE CORRECTION

RANGE RATE CORRECTION

DATA (IOD)

PARITY

Words 7 12.17 or 23 Words N+2 If Ni=1.4.7 or 10 Words N+2 If Ni=2.58 or 11

ISSUE OF DATA (IOD)

FILL

PARITY

ISSUE OF DATA (IOD)

FILL PARITY

3.3.3.3. Thng bo 3 (Massage type 3) Thng bo ca trm Refrence. Thng bo 3 gi i to khng gian 3 chiu ca trm Beacon To d c tnh trong h WGS 84. Thng bo v to (v tri ) ca trm Beacon gip cho my GPS ng khi thu c tn hiu ci chnh t mt trm Beacon no , bit c khong cch t trm Beacon n my thu GPS ng l bao nhiu. Trong trng hp thu c nhiu tn hiu ci chnh phn sai t nhiu trm Beacon, thi my thu GPS ng c th da vo khong cch ti cc trm Beacon quyt nh xem s dng s liu ci chnh phn sai pht i ti trm Beacon no thch hp nht. (Thng th s dng s ci chnh phn sai i t trm Beacon gn nht).. Tuy vy, i khi c th trn ch s dng s ci chnh phn sai t trm Beacon no m tn hiu ci chnh thu c ti my GPS ng ln nht. Format s liu ca Massege 3 c trnh by trong hnh v di y: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ECEFX-COORDINATE WORLD (UPPER 3 BYTE) PARITY 3 MSB ECEFXCOORDINATE (LOWER BYTE) ECEFY-COORDINATE (UPPER 2 BYTE) MSB 29 WORLD 4

PARITY

ECEFYCOORDINATE (LOWER 2 BYTE) LBS

ECEFZ-COORDINATE (UPPER BYTE) MSB

PARITY

WORLD 5

ECEFZ-COORDINATE MSB TYPE 3 MESSAGE PARAMETERS FORMAT

PARITY

WORLD 6

REFERENCE

STATION

3.3.3.4. Thng bo 5 (message Type 5) l chm v tinh kho (Constellation Heath) Thng bo ny, thng bo thng s ca tng chm v tinh ti trm Beacon, v tinh no trong tm nhn ti trm Beacon v c s dng nh v v tnh s ci chnh phn sai. Thng bo gip cho my GPS ng, t ng la chn c ng chm v tinh kho (constellation) c s dng nh v ti trm Beacon v trm GPS ng cng s dng mt GPS constellation nh v. Vic ny rt quan trng v nu trm GPS ng s dng mt GPS constellation khc vi trm Beacon, th s c cc Psuedorange tnh c t trm GPS ng c ci chnh phn sai. Gi s tt c cc Psuedorange tnh c t trm GPS ng c ci chnh phn sai m ch cn mt Psuedorange khng c ci chnh phn sai s gy nn sai s nh v rt ln. Thng bo s 5 ny khng c gi lin tc m c gi i theo chu k mi khi trm tnh thay i GPS Constellation. Format DATA ca Message Type 5 c trnh by trong hnh v sau: HEALTH ENABLE NEW NAVIGATION DATA LOSS OF SATELLITE WARING

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 30

SATELLIE

DATA HEALT ID H

c/N0

TIME TO UNHEALTHY

p PARITY

WORD 2 + Ni

Message Type 5 - - Constellation Health. 3.3.3.5. Thng bo 16 (Message Type 16) thng bo c bit. y l mt dng thng bo c bit bng m ASCII trm Beacon c th thng bo cho cc trm GPS ng cc thng tin c bit bng ch. Mi mt thng bo c th di 90 ch ci. thc hin c iu ny, ngi ta s dng code ASC II 7 bit trong FORMAT s liu Message Type 16. Th d hnh sau trnh by ch QUICK c cu to ra sao trong format s liu Message Type 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Q U I 26 27 28 29 30 PARITY WORLD 010100 010101 010010 3 01 01 01 MSB LSB 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 C K FILL PARITY WORLD 010000 010010 000000 3 Vi cc loi thng bo trnh by trn, vic ci chnh phn sai thi gian s c thc hin mt cch hon ho.

31

3.4. Cc phng php o DGPS.

3.4.1. K thut MSK DGPS vi cc trm Beacon. p dng k thut DGPS trn phm vi, mt vng mt quc gia hay mt khu vc, cc trm Reference DGPS c xy dng c nh trn cc v tr thit k nhm phc v tn hiu ci chnh phn sai cho bt k my ng no nm trong phm vi ph sng c thit k. Cc trm ny c gi l Beacon Control GPS Station. Nhiu trm Beacon Control GPS Station cng hot ng di mt s iu hnh chung c gi l Beacon Control GPS System. Cu hnh cc trm Beacon Control GPS rt linh ng. Cu hnh ny ph thuc vo quy m trm, quy m ca h thng cng nh yu cu ca i tng phc v. 3.4.1.1. Yu cu chung ca mt trm Beacon Control l: - S dng k thut iu ch MSK (Minimum Shift Kying Modulation). - S dng cc chun thng bo v cu hnh (Format) s liu chun quc t RTCM 104 - S dng h thng phn mm iu khin chun RSIM ca RTCM 104 - a s s dng sng mang trung tn, t 280 KHz n 325kHz. L do s dng phng php iu ch MSK v cc chun Format RTCM 104 c trnh by mc 2.1.5. Vic s dng sng mang trong di trung tn v cc l do sau: + Sng trung tn c bc sng di, v vy c th truyn sng vt cc chng ngi vt trn ng truyn sng. Do khng yu cu cc tm antena nhn thy nhau. Do v tr t anten trm Beacon khng cn phi cao, cng nh chiu cao anten my pht, tn s khng yu cu cao lm. + S suy hao trn ng truyn sng trung tn khng ln lm. iu ny m bo tm hot ng xa ca trm Beacon vi cng sut pht khng cao. + Trong k thut radio, do c tuyn tn s ca cc linh kin n nh vi tn s trung tn nn thit b radio d hot ng n nh vi gii sng trung. Yu cu dch v i vi trm Beacon control v ca h thng phi m bo tnh nng sau:

3 32

+ S ci chnh phn sai phi c pht i lin tc, khng gin on. S liu phi c lu tr trong DATABASE ca trm Beacon control nhm mc ch s l sau (Poss Processing) v kim tra. + m bo tm ph sng theo yu cu phc v. + Kim sot c chnh xc ca s ci chnh phn sai pht i nhm m bo chnh xc hot ng ca trm. + Kim sot t ng s vn hnh ca thit b ca trm. T ng pht tn hiu v thng bo cc s c trong qu trnh vn hnh c s chnh l khc phc tc thi. + Nu cc trm Beacon control hot ng trong mt h thng, th cc trm Beacon control ring l phi c kh nng iu khin t xa v c t di s kim tra iu hnh thng nht ti mt trm kim tra, iu khin trung tm. Cu hnh ca mt trm Beacon Control v hot ng ca trm Beacon Control. S ca mt trm Beacon Control c my thu GPS v my pht sng mang kp c trnh by hnh v di y: DUAL RS RECEIVERS AND TRANSMITTERS FOR RENDUNDANCY RS 1

A

B RSIM PC RS 2 Transmitt er

Ante n

Theo s cu to trn, trm Beacon Control c hai my thu GPS k hiu l RS 1 v RS 2 (Reaference Station ). Cc my thu GPS ny thng l cc my thu Trimble 4000 MSK c thit k b iu ch MSK trong my thu GPS. My thu RS1 v RS 2 thu tn hiu t v tinh GPS v tnh ra cc Pseudorange i vi tng v tinh GPS v tnh ra s ci chnh phn 3 33

sai Pseudorange cho tng v tinh. Option RTCS 104 c ci t trong my chun ho s ci chnh theo Format chun RTCM 104 dng lm tn hiu iu ch tn s sng mang pht ra t my pht tn s (Transmitter). Hai my thu GPS RS 1 v GPS RS2 hot ng c lp vi nhau v c ni vi my tnh PC iu khin c ci t phn mm chun RSIM. Cng nh vy my pht tn s cng gm hai my (hoc 2 block my pht trong mt my pht kp ). Vi cu to nh trn trm Beacon Control c tnh an ton cao trong vn hnh, do m bo c pht tn hiu ci chnh lin tc,khng b gin on do s c k thut. 3.4.1.2. H thng antena bao gm cc phn chnh sau: - Antena pht : C nhiu dng nh antena Diamon, antena hnh T, antena ct (S dng ngay ct lm yu t pht ). - Li i trng : L h thng dy ng trc c di chn chm di bi antena c tc dng lm tng h s in ly ca bi antena. - B phn iu chnh antena : C nhiu chc nng iu chnh cun cm (L) to ra tn s cng hng R - C ca my pht tn hiu. - Cc thit b an ton bao gm : H thng chng st, h thng n cao khng t ng vv. Hnh v di y m t mmt s loi antena v li i trng : Ct Dy pht S cch

Dy no ATU

A Antena hnh ch T

34

Antena hnh Thp S cch

Ct

Dy

S cch A ATU

Li i trng

Vi cu hnh nh trn trm Beacon Control mi ch m bo pht tn hiu an ton v lin tc ch khng kim sot c chnh xc ca s liu ci chnh pht i. V vy ti cc tm Beacon Control, thng thng c trang b thm cc my GPS Monitoring, lm nhim v theo di v kim sot chnh xc ca tn hiu ci chnh phn sai c pht i t trm Beacon Control.

35

S cu hnh ca mt trm Beacon Control c trang b my GPS Monitoring c trnh by nh hnh v di y :

THE INTRODUCTION OFAN INTERGRITY MONITOR RS 1 RS 2 A Ante n

RSIM PC

B Transmit ter

IM 1 Near field antena IM 2 Near field antena

3.4.1.3. Hot ng ca my GPS Monitor ti trm Beacon Control. My thu GPS IM (Integrity monitor GPS Receiver) l tch hp ca mt my thu GPS v my thu tn hiu ci chnh phn sai. Antena ca my thu GPS IM c t ti mt im bit trc to gn trm Beacon Control. My thu GPS IM khi hot ng nh mt my thu GPS ng. My thu GPS IM s tnh ra to c ci chnh phn sai. So snh to c ci chnh phn sai vi to bit ti ni t antena ca my thu GPS IM,s nh gi c sai s ca tn hiu ci chnh phn sai pht i t trm Beacon Control. Hot ng ca my GPS IM cng c kim sot qua my tnh iu khin PC c ci t phn mm RSIM. RSIM v nguyn tc hot ng ca RSIM : RSIM l mt phn mm chun ho do RTCM 104 xy dng nhm m bo thng tin trong h thng cc trm Beacon Control, bao gm cc trm Reference, trm Monitor v trm iu khin trung tm. 3 36

Rsim l mt cu thnh khng th thiu ca mt trm Beacon Control. N l phn mm chnh, ci t,qun l v vn hnh trm Beacon Control.Ngoi ra RSIM cn c mt s b tr khc cu thnh trong h thng phn mm cho trm Beacon Control. H thng phn mm cho trm Beacon Control c thit k chy trn cu hnh my tnh 32 bit trong mi trng Windows NT. Khi trm Beacon Control hot ng, phn mm RSIM qun l mi hot ng ca trm mt cch t ng v to ra cc thng tin v tnh trng hot ng ca trm bao gm : Ca s trng thi (Status Windows); Bo ng v cc thng bo (Alarms and massage). Tt c c 9 Status Windows,mi Status Windows cung cp mt thng tin qun l, theo di s hot ng ca trm v h thng. Cc thng tin bao gm : 1. IM Satellite Status : Thng bo thng tin v cc v tinh m my GPS Monitoring ang s dng. 2. PS Satellite Status : Thng bo thng tin v cc v tinh m my Reference GPS ang s sng. 3. IM DGPS IM Status : Thng bo thng tin lin quan n vic tnh ton inh v bao gm chnh xc, DOP, s v tinh c s dng nh v c xc nh bi my GPS Monitoring. 4. IM Correction Data : Thng bo v s ci chnh phn sai m my GPS Monitoring nhn c t trm Reference. 5. RS Correction Data : Thng bo v s ci chnh phn sai c to ra bi my Reference GPS. 6. IM Data Link Status : Thng bo v cht lng ca tn hiu v s liu RTCM v my GPS Monitoring nhn c. 7. Transmitter Status : Beacon thng bo tnh trng hot ng ca my pht tn s sng mang. 8. Bng nh v (Position Statistics Table) : Thng bo v tri c nh v ( c ci chnh phn sai ) bng my GPS Monitoring theo dng bng s. 9. Posotiong Statistics Scatter Plot : Thng tin v v tr c nh v ( c ci chnh phn sai ) bng my GPS Monitoring di dng th. 37

Cc thng tin c hin th trn mn hnh my tnh v c th c in ra qua Printer. Trong qu trnh vn hnh trm, bt k thi im no, ngi vn hnh cng c th la chn cc thng tin cn thit trong 9 Windows Status trn tham kho. 3.4.2. Chc nng bo ng v thng bo (Alams and message) ca RSIM. Vi mc ch bo m bo s an ton hot ng ca trm Beacon Control, RSIM t ng bo ng v gi cc thng bo c bit cn thit cho ngi vn hnh trm khi c s c k thut.Trong trng hp cc trm Beacon Control hot ng theo mng (Net working) th cc bo ng v thng bo gi v trm iu khin trung tm.Ti trm iu khin trung tm, ngi vn hnh s phn tch tnh trng bo ng v thng tin a ra cc gii php khc phc thch hp. Chc nng hu s liu ti trm Beacon control Ngoi chc nng chnh l pht s liu ci chnh phn sai, ti trm Beacon control cn c chc nng ghi li mt s loi s liu trong Data BASE Cc loi s liu bao gm : 1. S liu GPS th (Raw GPS Data), s liu ny l khong cch Pseudorange v s liu pha sng ti (Carrier phase) i vi mi v tinh trong tm nhn ca trm Beacon Control. S liu ny c ly t my Reference GPS v c gi trong Data BASE ca my tnh di dng file s liu (Data file). Cc Data file ny s c s dng Post Procesing i vi my GPS ng khng thu tn hiu ci chnh phn sai. 2. Cc thng tin thng k, thng tin ny thng k cc lch chun, cch trung bnh RMS tr s ti thiu, ti a v tng s cc quan st trong ng v. 3. Cc hng mc h thng s liu (System Data items) thng bo cc thng tin v v tinh, s liu ci chnh phn sai, thng tin v tnh trng ca h thng truyn tn hiu (Data link). 4. Bo ng v h thng bo ng (Alrams and system message) ghi li tt c cc bo ng v thng bo c bit ca RSIM trong qu trnh vn hnh trm. Tt c cc s liu c ghi li trn cho php ngi s dng bit c tnh trng hot ng ca trm trong qu kh, ng thi c th kim tra, x l s liu ng v DGPS khi c nghi ng gp s c k thut ti trm Beacon control. 38

i vi mi vng hoc mt quc gia th cc trm Beacon control c cu to thnh mng (Net working) v c iu khin thng nht ton mng bi trm iu khin trung tm. C ngha l tt c cc trm Beacon control ring l c iu khin t xa. iu khin t xa th phi thit lp h thng t trm iu khin trung tm n mi trm Beacon contrrol thnh vin. C hai phng thc truyn thng gia trm Control v cc trm Beacon. 1. Leased line (ng truyn thng ring). L mt knh truyn thng c thit lp ring gia trm iu khin v trm Beacon. Knh truyn thng ny c thit lp nh mt ng in thoi ring v thng c thc hin qua ng cp quang. 2. Quay s khi cn (Dial on Demad). iu khin cc trm Beacon control th thc t trm iu khin trung tm v cc trm Beacon khng cn thit phi ni thng tin vi nhau. Thng tin ch cn ni mi khi bo ng v cc thng bo c RSIM t ng gi v trm iu khin. V vy, phng thc tryn thng Dial on Demad c cu to nh sau: Khi cn truyn thng tin t trm Beacon Control ti trm u khin th knh truyn thng tin thit lp gia trm Beacon v trm iu khin c m ging nh ta gi in thoi. Ch khc l quay s in thoi c thc hin r ng qua thit b Routeer v Radio modem.

39

S mt h thng trm Beacon Control Net Working.

Phng php MSK Beacon DGPS l phng php thng dng nht c s dng rng ri trn th gii. Cc trm MSK Beacon DGPS c ng k quc t v hot ng nh mt h thng dch v khu vc ton cu. Thng 5 1996 Trimble navigation thu thp c bng thng k cc trm MSK Beacon DGPS trn ton th gii ti 199 trm. Vi bng thng k trn, mi my GPS ng no c th bt tn hiu ci chnh phn sai DGPS t bt k trm Beacon no tng ng vi khu vc ph sng ca trm mt cch min ph. Di y trnh by mt phn bng thng k ca trm Beacon trong khu vc: Tram Beacon ca

Raffes 9 32 N

Trm Beacon Ca SINGAPOR Geo Position 40

10

Lighthose 103 44 26 E Reference Station ID TX Station ID Frequency(Khz) Nominal Range (Km) Message Type Bitrate (bit/s) Station in operation Integrity yes Comments :0

65 65 298 100 3, 5, 7, 9, 16 100 yes monitoring

Bintulu 11 9.510

Tram Beacon Ca MALAYSIA Geo Position N

30 11

3 2 3.32 E Reference Station ID TX Station ID Frequency(Khz) Nominal Range (Km) 450@40 /m Message Type 9-3, 16 Bitrate (bit/s) Station in operation yes Integrity monitoring yes Comments : Trm Beacon ca HONGKONG Kau Yi Chau 32 N 34.71 E Geo Position

821 290

1,3, 200

7,

1

0

9

114 0 4

Reference Station ID TX Station ID Frequency(Khz) 41

298

Nominal Range (Km) v/m Message Type 16

500@20 1, 2, 3, 5, 6,

Bitrate (bit/s) 200 Station in operation yes Integrity monitoring yes Comments : * Operated by Hydrographic office, Maritime * Department at Tel No.(852) 2504 0267 Trm Beacon ca TRUNG HOA DAJISHAN 48 36 N0

Geo Position

30 0 112

10 25 E Reference Station ID 624/625 TX Station ID 621 Frequency(Khz) 307 Nominal Range (Km) 300 Message Type 3, 5, 7, 9, 16 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring yes Comments : Norm al Operation with 24 hours integrity morning Assosi ated RDF Morse identifier / Frequency 42

e Stite RSIM Stite med Station and Frequency

Trimbl Full

Confir

BAOHUJIOA 20 N0

Geo Position

20 0 11

9 36 57 E Reference Station ID 652 / 653 TX Station ID 626 Frequency(Khz) 310.5 Nominal Range (Km) 300 Message Type ? Bitrate (bit/s) 0 Station in operation yes Integrity monitoring Comments : - Station exists in Admiralti List of Radio Signal, Vol 2, 1996 - Transmission format not confirmed - Lieca Stite

QAOGHUANDA 50 N

Geo Position

39 0 54 119 0

36 57 E Reference Station ID TX Station ID 43

606/ 607 603

Frequency(Khz) 287.5 Nominal Range (Km) 300 Message Type 3, 5, 7, 9, 16 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring Comments : - Normal operation with 24 hours integrity morning - Associated RDF Morse identifier / Frequence - Station exissts Admiralty List Radio Signal, Vol 2, 1996 - Frenquence not format not confirmed - Lieca Stite DASANSHAN 25 N 10

Geo Position

38 0 12

50 E Reference Station ID TX Station ID Frequency(Khz)

602 / 601

603

301.5 Nominal Range (Km) 300 Message Type 9, 16 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring Comments : Normal operation with 24 hours integrity morning Associated RDF Morse indentifier / Frequence Geo Position 39 0 11 7 430

3, 5, 7,

-

6

BEITANG N E

44

Reference Station ID 609

608 /

TX Station ID 604 Frequency(Khz) 310.5 Nominal Range (Km) 300 Message Type 3, 5, 7, 9, 16 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring Comments : - Normal operation with 24 hours integrity morning - Associated RDF Morse indentifier / Frequence

WANGIJAMAI N

Geo Position

36 0

1200 10 E Reference Station ID 614/ 615 TX Station 607 Frequency(Khz) 315.5 Nominal Range (Km) Message Type 5, 7, 9, 16 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring Comments : - Normal operation with 24 hours integrity morning - Associated RDF Morse indentifier / Frequence SHITSNG N 45 Geo Position

ID

300 3,

36 0 15 45

121 0 36 47 E Reference Station ID 628/629

TX Station ID 614 Frequency(Khz) 295 Nominal Range (Km) 300 Message Type 1 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring Comments : Frequencuency not confirmed - Transnission format not confirmed Message type Geo reference and bitrate not known at this time

YANWEIGANG 28 49 N0

Geo Position

340 11

9 46 50 E Reference Station ID 620/621 TX Station ID 610 Frequency(Khz) 291 Nominal Range (Km) 300 Message Type 1 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring yes Comments : Frequencuency not confirmed - Transnission format not confirmed Message type Geo reference and bitrate not known at this time

46

9

ZHENHAIIAO N

Geo Position

24 0 16 118 0

7 54 E Reference Station ID 632 / 633 TX Station ID 616 Frequency(Khz) 320 Nominal Range (Km) 300 Message Type 1 Bitrate (bit/s) 200 Station in operation yes Integrity monitoring yes Comments : Frequencuency not confirmed - Transnission format not confirmed Message type Geo reference and bitrate not known at this time

3.4.3. Phng php DGPS din rng (Wide area Differential GPS WADGPS) V nguyn l k thut th phng php WADGPS khng khc viphng php MSK Beacon DGPS. S khc nhau gia hai phng php ny la phng thc truyn tn hiu ci chnh phn sai. Cc trm Beacon control thng c tm ph sng khong 500 km. Cc trm ny phi xy dng trn t lin cho nn v nguyn l trn vng bin xa, nhiu vng khng c ph sng ca h thng Beacon control System. Hn na nu thit k ca trm Beacon control c tm hot ng xa hn th chnh xc ci chnh phn sai ngoi tm 500 km (cch trm Beacon ) cng b gim nhiu. tng khong cch dch v ci chnh phn sai ngi ta thay phng thc truyn tn hiu ci chnh phn sai trc tip t cc trm Beacon control ti my thu GPS bng phng thc truyn tin gin tip, thng qua v tinh truyn thng (Communication Satellite). V tinh ny c gi l v tinh inmarsat Satellite . Vi phng thc truyn tin qua v tinh truyn thng. Cc my GPS ng nm trong tm nhin ca v tinh truyn thng u nhn 47

c tn hiu ci chnh phn sai. Do vy khong cch dch v ci chnh phn sai tng ln ng k.Vi bin php s dng nhiu v tinh truyn thng trn cc qu o khc nhau th v mt truyn thng tin, v tinh truyn thng c th truyn thng trn ton cu. Tuy vy din tch dch v ca h thng WADGPS khng v th m c th m bo trn ton cu. Khong cch dch v khng ph thuc vo v tinh c th truyn thng ti u m ph thuc vo cc trm Reference trong h thng WADGPS. Khong cch hot ng ca h thng WADGPS bng kong cch t my thu DGPS n trm Reference gn nht trong h thng trm Beacon ca h thng WADGPS m t chnh xc ci chnh phn sai con chnh xc. S nguyn l hot ng ca phng php WADGPS c th hin trong hnh v di y :

Nguyn l hot ngca trm Reference : Trm Reference (cu to nh mt trm Beacon control) nhn tn hiu t v tinh GPS v tnh ra s ci chnh phn sai. S ci chnh phn sai tnh t trm Reference c truyn ti trung tm kim tra. Ti y s liu ci chnh phn sai c kim tra, x l v thng qua uplink centre ti cc my thu trn mt t.My thu trn mt t nhn c tn hiu ci chnh phn sai t v tinh 48

truyn thng Inmarsat v tn hiu t v tinh GPS tnh ra v tr chnh xc c ci chnh phn sai. My thu trn mt t l tch hp gia mt my thu GPS c kh nng c c Formats s liu RTCM v mt thit b thu v decode tn hiu pht i t v tinh thng tin Inmarsat. Mt h thng WADGPS ng dng gm nhiu trm Reference, mmt vi trung tm kim tra; uplink centrel, v khong 3-4 v tinh truyn thng phc v cho tng vng. Thc t th h thng WADGPS c mt vi cng ty o c ln, thit lp mc ch dch v. Vi phng thc dch v th ngi s dng phi mua my v thu tn hiu WADGPS ca hng dch v, cc hng dch v s m m kho, cho php my WADGPS no thu dch v ca h hot ng. Cc my khng c m m kho s khng thu c tn hiu ci chnh phn sai t v tinh truyn thng. Vic m m kho c thc hin trc tip qua tn hiu t v tinh truyn thng hoc phn mm ci t trong PC (Ch c nhn vin ca hng mi c phn ny). Cng bi mc tiu thng mi, th cc hng dch v WADGPS c kh nng kho vng i vi cc my thu WADGPS (Khng cho php my thu hot ng trong mt phm vi nht nh ). Cc hng dch v WADGPS cnh tranh nhau trn cc phwng din : Tm hot ng ca h thng; chnh xc ca h thng v cc dch v kem theo. Hin nay c ba h thng dch v WADGPS : H thng Omnistar, h thng Starfix ca hng Fugor v h thng Skyfix ca hng Racal. H thng Starfix gm 60 trm Reference c b tr thnh ba vng trn ton th gii, 3 trm kim tra v 4 v tinh truyn thng, 4 v tinh truyn thng m bo ph sng 700 v Nam ti 700 v bc trn ton cu, khong cch dch v l 2000 km. H thng Omnistar thc cht l h thng Starfix nhung ch dch v ven bin v trn t lin. H thng Skyfix c cu to, chnh xc v tm hot ng tng t nh h thng Starfix. 3.4.4. K thut DGPS truyn thng gii tn s cao UHF hoc VHF. Nh ta bit k tht DGPS trong ng v v tinh ton cu l s dng cc s hiu ci chnh phn sai tnh c t mt my thu nh v t c nh ti mt im bit trc to ci chnh cho mt my thu nh v khc (gi l my thu di dng) ti mt 49

im cn xc nh to . Nh vy v nguyn tc ci chnh c cho my thu nh v ti im cha xc nh c to (my di ng )cn cc yu t sau: - My thu nh v dng lm my ci chnh t chnh xc trn im c nh bit trc to . - My thu phi l loi my DGPS RS, to c cc gi dng chun RTCM SC 104ti cc cng giao tip SR 232 ca my thu. - C my pht sng Radio iu ch c dng tn hiu s, pht s liu ci chnh phn sai n my thu di ng. - My thu di ng phi l my thu DGPS, thu nhn cc gi thng tin dng chun RTCM SC 104 ti cc cng giao tip RS 232 ci chnh to m my tnh c, ti im cha bit to ch thi gian thc (Realtime). Nh vy k thut DGPS truyn thng gii tn s cao UHF hoc VHF c cu trc nhu sau :

50

A. Trm ci chnh RS c nh. Trm c cu trc nh sau :

GPS Antenna Radio Antenna

Trimble 4000/SSI_RS Radio Link

Mt my thu DGPS RS c chnh xc cao Antena ca my thu DGPS RS t chnh xc trn v tr bit trc to X, Y,Z. V tr ca antena phi trn ni cao khng b che khut bi cc to a vy xung quanh. - Mt my pht sng Radio link c b iu ch Modem k thut s trn di tn UHF hoc VHF. - Antena ca my pht Radio link cn t trn v tr cao khng b che khut bi cc a vt xung quanh. Vic xc nh to chnh xc ca v tr antena my thu DGPS tin hnh bng cc phng php o c nh v truyn thng. K thut DGPS truyn thng di tn cao UHF hoc VHF ch khc bit ci cc cng ngh DGPS khc l k thut thu pht sng. i vi di tm UHF cc gi tn hiu RTCM SC 104 t my thu DGPS RS c modem ca my pht Radio link iu ch trong di tn sng mang t 420 khz n 370 khz i vi di tn VHF cc gi tn hiu RTCM SC 104 t my thu DGPS RS c modem ca my pht Radio link iu ch trong di tn sng mang t 140 khz n 180 khz. Di thng ca cc my pht Radio link l 250 khz. Di thng ny tu thuc tng loi my pht c th la chn bng cc minicontac hoc bng phn mm. Di thng ca cc my pht Radio link cho bit s knh pht ca my trong di UHF hoc VHF. Cc trm ci chnh s dng k thut pht sng UHF hoc VHF c chia lm hai loi : 51

Loi c nh Loi xch tay di ng. Cc rm ci chnh c nh l cc trm c cng sut phat ln tm ph sng cho mt vng rng phc v vho nhiu mc ch v i tng s dng Cc trm ci chnh xch tay di ng l cc trm co cng sut pht nh tm ph sng hp s dng trong mt thi gian nht nh. B. Trm ci chnh di ng : Trm c cu trc nhu sau : Antenna P/N 23965 & 23513 (Re-order P/N 36654-00) Radio Modem TT 450S

Cable P/N 11820-11

NC to N Adapter P/N 32499

Cable P/N 30945 (Re-order P/N 36669) To a PC

Cc b phn chnh : * Mt my thu DGPS c chnh xc cao. - Antena ca my thu DGPS t trn phng tin o c. V tr ca antena phi t trn cao khng b che khut bi cc thit b khc trn phng tin o nh rada, antena v tuyn in vv. - Mt my thu sng radio link c b iu ch Modem k thut s trn di tn UHF hoc VHF. - Antena ca my pht radio link cn t trn v tr cao khng b che khut bi cc thit b gy nhiu xung quanh. i vi di tn UHF cc gi tin tn hiu RTCM SC 104 t my pht radio link c iu ch trong di tn sng mang t 420 khz 52

n 470 khz c my thu radio link c cng tn s cng tc UHF nh trn thu li a vo Modem gii iu ch hon li thnh tn hiu s. Sau a vo cng giao tip RS 232 ca my thu nh v DGPS. i vi di tn VHF cc gi tin tn hiu RTCM SC 104 t my pht radio link c iu ch trong di tn sng mang t 140 khz n 180 khz c my thu radio link c cng tn s cng tc VHF nh trn thu li a vo Modem gii iu ch hon li thnh tn hiu s. Sau a vo cng giao tip RS 232 ca my thu nh v DGPS. Di tn cng tc ca my thu radio link thng l 250 khz cng ging nhu gii thng ca my pht. Di thng ny tu thuc tng loi my thu m c th la chn bng cch ci t cu hnh bng minicintac hoc bng phn mm. Di thng ca my thu radio link cho bit s knh thu ca my trong di UHF hoc VHF. Cc knh ny c chn ng vi cc knh ca my pht radio link ca trm ci chnh m my di ng s dng. V nguyn tc my thu di ng phi c ci t knh lin lc radio trng vi knh ca my pht radio link ca trm ci chnh m n s dng. Nu chng khc nhau s dn n vic my thu radio trm di ng khng thu nhn c tn hiu ci chnh, nh vy my thu DGPS di ng khng c ci chnh to . nhn bit c mt my thu DGPS ng c c ca chnh to ch d thi gian thc hay khng ?(my thu Radio link c thu c tn hiu ci chnh ca trm c ffnh hay khng ?) thng trn mt s my tnh c hin th dng ch DGPS trn mn hinh tinh th lng hoc trn gi s liu $GPGGA ca chun NMEA 0183 xut hin ti cng giao tip RS 232 ca my ng ti trn th su nu nhn gi tr 1 l khng ci chnh. i vi cc my thu radio link di dng, nhy ca my thu l rt quan trng. N quyt nh n cht lng thu cc tn hiu ci chnh. Gi tr v nhy ca my thu Radio c tnh bng mV, i vi cc my thu c cht lng cao gi tr ny khong 0,20 mV n 0,30mV. Cc my thu c cht lng thp vo khong 0,5mV n 0,75mV. Mt s my thu cho php iu chnh nhy ny bng mt chit p iu chnh SEN.Trong trng hp tn hiu thu khng tt c th iu chnh nm SEN tng hoc gim nhy ca my thu nhm thu tn hiu tt nht. Vic iu chnh tn s knh thu ca my thu co hai cch : T ng xc nh tn s i pht. 53

-

Ci t tn s hoc d tn s sng mang ca i pht.

C. Cc tnh nng k thut DGPS s dng di tn s UHF v VHF. Nh trn trnh by, cng ngh DGPS s dng di tn s UHF v

VHF thc cht l s dng cc knh sng mang trong di tn s UHF v VHF truyn cc gi thng tin dng ci chnh dng RTCM SC 104 t trm ci chnh n cc my thu Radio ca trm di ng. Do vy cc tnh nng k thut ca cng ngh ny ph thuc ch yu vo tnh cht cc knh sng mang trong di tn UHF v VHF. V chnh xc ci chnh : Cng ngh DGPS s dng di tn s UHF v VHF hon ton ph thuc vo cht lng cc thit b nh v DGPS (DGPS mt tn hay hai tn, cng ngh Maxwell hay Super Trak v phng php ci chnh Realtime DGPS hoc RTK). chnh xc ci chnh dng phng php Realtime DGPS vo khong 1m n 3m, i vi phng php RTK khong 20 cm. * Maxwell Technology Th h my thu GPS mi nht va h Trimble 4000 c tha hng cng ngh Maxwell, cho php nng cao chnh xc nh v trn c s x l C/A code c mc nhiu thp. Trong lnh vc ci chnh phn sai nh v (DGPS ) cng ngh Maxwell cho php nng cao chnh xc t +-2- 5 m, nu l mt phng th cnh xc ln n +- 1 m vi cc my hai tn. Ngoi ra chnh xc c pha ca sng mang c nng ln gn vi l thuyt. * Super Trak TM Technology Cc my GPS loi 4000 SSi s dng cng ngh Super Trak. Cng ngh ny to kh nng o c vi tin hiu yu ca v tinh nh v, c kh nng thu c trong mi trng in trng RF cao. Cng ngh Super Trak dng kiu kt hp phn cng v phn mm iu khin (Hardware & Firmware), cng vi chc nng s l a tn hiu. Mt vi loi my s dng li th o trn L 2 vi m P code. Ngoi ra mt s my cn s dng hn hp c hai cng ngh Maxwell va cng ngh Super Trak tn dng th mnh ca hai cng ngh ny lm tng kh nng o c v nng cao chnh xc cho cc phng php o tnh v o ng.

54

* My thu pht Radio Link. V kh nng ph sng :Cng ngh DGPS s dng tn s UHF v VHF nm trong di sng cc ngn dm (tn s VHF : 140 Mhz n 180 Mhz v UHF 420 Mhz n 470 Mhz). Cc sng in t c bc sng dm c kh nang chng nhiu trn ng truyn cao c bit trn ng truyn khng ng nht. Do s dng tn s sng mang ln, nn cho php mang s lng ln thng tin v vy c th dng cc bin php bo mt thng tin khc nhau khi truyn. Kh nng nhiu do phn x sng in t khi gp vt cn ln. Do vy tm ph sng khng cao (thng