Bai Giang Thong Tin Ve Tinh

8/7/2019 Bai Giang Thong Tin Ve Tinh

1/68

Nguyn Trung Tn Trung tm K thut Vin thng

Khoa V tuyn in t Hc vin K thut Qun s

Bi ging thng tin v tinhdng cho h o to di hn

(45 tit- d tho)

Chng 1: Tng quan v thng tin v tinh (8 tit)1.1. Lch s pht trin thng tin v tinh. Vo cui th k 19, nh bc hc Nga Tsiolkovsky (1857-1035) a ra cc khi nimc bn v tn la y dng nhin liu lng. ng cng a ra tng v tn la y nhiutng, cc tu v tr c ngi iu khin dng thm d v tr.

Nm 1926 Robert Hutchinson Goddard th nghim thnh cng tn la y dng nhinliu lng.

Thng 5 nm 1945 Arthur Clarke nh vt l ni ting ngi Anh ng thi l tc gica m hnh vin tng thng tin ton cu, a ra tng s dng mt h thng gm 3v tinh a tnh dng pht thanh qung b trn ton th gii.

Thng 10 / 1957 ln u tin trn th gii, Lin X phng thnh cng v tinh nhn toSPUTNIK 1. nh du mt k nguyn v TTVT.

Nm 1958 bc in u tin c pht qua v tinh SCORE ca M, bay qu othp.

Nm 1964 thnh lp t chc TTVT quc t INTELSAT.

Nm 1965 ra i h thng TTVT thng mi u tin INTELSAT-1 vi tn gi EarlyBird.

Cui nm 1965 Lin X phng TTVT MOLNYA ln qu o elp.

Nm 1971 thnh lp t chc TTVT quc t INTERSPUTNIK gm Lin X v 9 ncXHCN.

Nm 1972-1976 Canada, M, Lin X v Indonesia s dng v tinh cho thng tin nia.

Nm 1979 thnh lp t chc thng tin hng hi quc t qua v tinh INMARSAT. Nm 1984 Nht bn a vo s dng h thng truyn hnh trc tip qua v tinh.

Nm 1987 Th nghim thnh cng v tinh phc v cho thng tin di ng qua v tinh.

Thi k nhng nm 1999 n nay, ra i tng v hnh thnh nhng h thng thngtin di ng v thng tin bng rng ton cu s dng v tinh. Cc h thng in hnh nhGLOBAL STAR, IRIDIUM, ICO, SKYBRIGDE, TELEDESIC.

Mt s mc nh du s pht trin ca h thng TTVT Vit Nam :

Nm 1980 khnh thnh trm TTVT mt t Hoasen-1, nm trong h thng TTVTINTERPUTNIK, c t to lng Do L Kim Bng H Nam. i Hoasen-1 l sn


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http://www.ebook.edu.vn Trang: 1phm ca nh nc Lin X tng nhn dn Vit Nam v kp thi truyn i trc tipnhng hnh nh v Olimpic 1980 t chc ti Lin X.

Nm 1984 khnh thnh trm mt t Hoasen-2 t ti Tp. HCM.

Ngy 24/9/1998 Th tng chnh ph ra quyt nh 868/Q-TTG v vic thng quabo co tin kh thi d n phng v tinh vin thng ViNaSAT ln qu o a tnh dotng cng ty Bu chnh vin thng Vit Nam lm ch u t.

Hin nay mng li thng tin v tinh ca vit nam ang ch s dng cc trm mt tnh: i truyn hnh Vit Nam thu v tinh MEASAT v THAICOM-3, i tig ni VitNam thu v tinh PALAPA pht thanh chng trnh quc t...

1.2. Qu o ca v tinh.

cc v tinh trn qu o c phn bit bi cc tham s sau y:

- Dng ca qu o (trn hay ellip)- cao ca qu o so vi mt t.

- nghing ca mt phng qu o so vi mt phng xch o.

S chuyn ng ca v tinh vng quanh tri t c tun theo nh lut Kepler, y lnh lut xc nh quy lut chuyn ng ca hnh tinh xung quanh mt tri. Nh vy, vtinh qu o tri t buc phi chuyn ng theo mt qu o m mt phng qu o can i qua tm tri t.

nh lut kepler th nht:V tinh chuyn ng vng quanh tri t theo mt qu o Ellip vi tm tri t

nm mt trong hai tiu im ca Ellip. im xa nht ca qu o so vi tm tri t

nm pha ca tiu im th hai, c gi l vin im cn im gn nht ca qu o

c gi l cn im

a: Bn trc di

b: Bn trc ngn

ha: cao vin im

hb: cao cn im

e: lch tm xc nh hnh dng ellip .

a

bae

22 =

ngha:

- V tinh chuyn ng theo qu o trn hoc Ellip.

- Tm tri t nm 1 trong 2 tiu im ca qu o Ellip.

- Nu l qu o trn th tm ca qu o trng vi tm ca tri t.

- Khi e = 0, th qu o v tinh l qu o trn.b. nh lut kepler th hai:

2a2b

ha hb


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http://www.ebook.edu.vn Trang: 2V tinh chuyn ng theo mt qu o vi vn tc thay i sao cho ng ni gia tm

tri t v v tinh s qut cc din tch bng nhau khi v tinh chuyn ng trong cng mt

thi gian nhnhau.

Vi: T1 = T2 th S1 = S2 ngha:

- V tinh chuyn ng vi vn tc nhanh hnkhi gn tri t v chm hn khi xa tri t.

- Vn tc chuyn ng ca v tinh trn quo trn l khng i v c xc nh:

V tinh bay qu o trn c bn knh R sl mt i lng khng i, c xc nh khi thc hin php ly cn bng lc ht v lc

ly tm RmV

R

mMG2

2 = , c vn tc l ( )sKmRRGM

V /630

== v chu k l

(s)102 32

RV

RT

=

.

Trong :

+ Gl hng s hp dn (6,674.10-8 cm3/gs2)

+ Ml khi lng ca tri t (5,974.1027g)

+ m l khi lng ca qu v tinh (g)

+ R l khong cch t tm tri t n v tinh (km)c. nh lut kepler th ba:

Bnh phng ca chu k quay t l thun vi lu tha bc ba ca bn trc ln ca qu

o Ellip.

32kaT = ,

kl h s t l, c gi tr khng i i vi mt vt th xc nh trn qu o.

1.2.2. Cc qu o

a. Qu o ng b - a tnh (GEO - Geostationalry Earth Orbit):

L qu o tho mn cc iu kin sau:

S1S2Vmax Vmin

T2

T1

Qu t

Qu o a tnh

ng xch o

62.000Km

36.000Km

rE

mMG/R2

-mV2/R

V


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http://www.ebook.edu.vn Trang: 3

- L qu o ng b vi tri t: c ngha l chu k quay bng chu k quay ca trit xung quanh trc Bc Nam.

- Mt phng qu o nm trong mt phng xch o ca tri t: c ngha l gcnghing bng 0.

- C cng chiu quay vi chiu quay ca tri t: t Ty sang ng.p dng cc cng thc trn ta c Km164.42

4 2

2

==

MGTR , cao ca qu v tinh so

vi mt t l 42.164 6378 = 35.786 Km 36.000Km, vn tc V = 3,074662 Km/s v

chu k T = 23h 56 4,096 24 gi .

u im:

- Hiu ng Dopler rt nh do vic iu chnh an ten trm mt t l khng cnthit.- V tinh c coi l ng yn so vi trm mt t. Do vy, y l qu o l tngcho cc v tinh thng tin, n m bo thng tin n nh v lin tc sut 24 gi trong ngy.

- Vng ph sng ca v tinh ln, bng 42,2% b mt tri t.- Cc trm mt t xa c th lin lc trc tip v h thng 3 qu v tinh c th phsng ton cu.

Nhc im:

+ Qu o a tnh l qu o duy nht tn ti trong v tr v c c l mt ti nguynthin nhin c hn. Ti nguyn ny ang cn kit do s lng v tinh ca cc nc phngln ngy cng nhiu.

+ Khng ph sng c nhng vng c v ln hn 81,30.

+ Cht lng ng truyn ph thuc vo thi tit.

+ Thi gian tr truyn lan ln, theo ng ngn nht c:

T: trm - v tinh - trm (72.000Km) 240ms.

T: trm - v tinh - trm Hub - v tinh - trm (154.000Km) 513ms.

T: trm - v tinh - v tinh - trm (134.000Km) 447ms.+ Tnh bo mt khng cao.

+ Suy hao cng sut trong truyn sng ln (gn 200dB).

ng dng: c s dng lm qu o cho v tinh thng tin bo m thng tin cho cc

vng c v nh hn 81,30.

b. Qu o Ellip:

L qu o tho mn cc iu kin sau:

- Mt phng qu o nghing so vi mt phng xch o 630 26- C vin im = 40.000Km v cn im 500Km.

Qu o Ellip

40.000


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- V tinh quay t Ty sang ng.u im:

- Ph sng c cc vng c v cao > 81,30.-

Gc ngng ln nn gim

c tp m do mt t gy ra.Nhc im:

- Mi trm phi c t nht 2 an ten v an ten phi c c cu iu chnh chm tia.- m bo lin lc lin tc 24h th phi cn nhiu v tinh.ng dng: c s dng lm qu o cho v tinh thng

tin bo m thng tin cho cc vng c v ln hn 81,30.

c.Qu o LEO v MEO:

L qu o trong tho mn cc iu kin sau:- L qu o c cao 500Km < h < 20.000Km.( 500Km < h < 10.000Km l LEO v 10.000Km < h < 20.000Km l MEO)

- C vn tc gc nh ln hn vn tc gc ca tri t.- C chiu quay t Ty sang ng.u im:

- Tn hao ng truyn nh do v tinh bay cao thp, nn ph hp vi thng tin ding.

- Tr truyn lan nh.Nhc im:

- m bo thng tin lin tc trong 24h v ph sng ton cu th cn rt nhiu v tinh( ph sng ton cu h thng Globalstar cn n 48 v tinh (v 8 v tinh d phng) ccv tinh thng tin bay qu o trn cch mt t 1410km, nghing 520, cc v tinh nybay trn 8 mt phng qu o mi mt phng c 6 v tinh, chu k v tinh 114 pht. Tpon Irdium (ca Motorola) cn 66 + 6 v tinh bay qu o trn nghing 84,6 0 cch mt

t 780km, cc v tinh bay 11 mt phng qu o, chu k v tinh 106 pht).- Mi trm phi c t nht 2 an ten v an ten phi c c cu iu chnh chm tia.- iu khin h thng TTVT rt phc tp- Tui th ca v tinh khng cao khi bay qu o LEO do thuc vnh ai Ion ho.ng dng: c s dng lm qu o cho v tinh thng tin bo m thng tin cho cc

trm mt t di ng.

1.3. H thng thng tin v tinh.

Mt h thng thng tin v tinh bao gm phn khng gian v phn mt t.


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http://www.ebook.edu.vn Trang: 5a. Phn khng gian: bao gm v tinh thng tin v cc trm iu khin TT&C(Telemetry, Tracking& Command: o lng t xa, bm v lnh) mt t. i vi vtinh bao gm phn h thng tin (payload) v cc phn h ph tr cho phn h thng tin:

- Phn h thng tin bao gm h thng anten thu pht v tt c cc thit b in t htr truyn dn cc sng mang.- Cc phn h ph tr gm:+ Khung v tinh

+ Phn h cung cp nng lng

+ Phn h iu khin nhit

+ Phn h iu khin qu o v t th ca v tinh

+ Phn h y

+ Thit b TT&C

- Nhim v phn h thng tin:+ Khuych i sng mang thu c phc v cho vic pht li trn ng xung. Cng

sut sng mng ti u vo ca my thu v tinh nm trong khong 100 pW n 1 PW.

Cng sut sng mang ti u ra b khuch i cng sut cao nm trong khong 10 W n100 W. Do vy, b khuch i cng sut ca b pht p v tinh khong 100dB n130dB.

+ Thay i tn s sng mang trnh mt phn cng sut pht i vo my thu v tinh.

thc hin cc chc nng trn, v tinh hot ng nh mt trm chuyn tip ngii. Thay i tn s trn v tinh c thc hin bng cc b i tn. v tinh loi ny cgi l Transparent satellite. Nu cc sng mang c gii iu ch trn v tinh, thay itn s s t c bng cch iu ch cc sng mang mi cho ng xung. Cc v tinh

loi ny

c trang b cc b x l bng gc v

c gi l Regenerative satellite.b. Phn mt t: bao gm tt c cc trm mt t, nhng trm ny thng c ni trctip hoc thng qua cc mng mt t n cc thit b u cui ca ngi s dng.

Trmmt t1

Qu oV tinh

Trmmt t2

ng ln(Uplink) ng xung

(Douwlink)

Phn mt t

Phn khng gian

Trm iu khin

TT&C


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Nhiu v trm mt t pht: Tip nhn cc tn hiu t mng mt t hoc trc tip tcc thit b u cui ca ngi s dng, x l cc tn hiu ny trong trm mt t sau pht tn hiu ny tn s v mc cng sut thch hp cho s hot ng ca v tinh.

Nhiu v trm mt t thu: Thu cc sng mang trn ng xung ca v tinh tn schn trc, x l tn hiu ny trong trm chuyn thnh cc tn hiu bng gc sau cung cp cho cc mng mt t hoc trc tip ti cc thit b u cui ca ngi s dng.

Mt trm mt t c th c kh nng thu v pht lu lng mt cch ng thi hoctrm ch pht hoc ch thu.

1.4. a truy nhp trong thng tin v tinh.

Trong thc t, mt b pht p c th phc v cng mt lc nhiu trm mt t khcnhau. K thut a truy nhp l k thut cc trm mt t truy nhp b pht p v tinh,vi yu cu sng VT t cc trm mt t ring l khng can nhiu ln nhau.

a.a truy nhp phn chia theo tn s (FDMA):

Cc trm mt t s dng cc tn s sng mang khc nhau v cng chung mt b phtp.

u im:

- Th tc truy nhp n gin.- Cu hnh trm mt t n gin. Nhc im:

- Khng linh hot thay i tuyn.- Hiu qu thp khi s dng nhiu knh, dung lng thp v cht lng thp.b. a truy nhp phn chia theo thi gian (TDMA):

Cc trm mt t s dng cng mt tn s sng mang v cng chung mt b pht p

nhng da trn phn chia theo thi gian.

u im:

f

t

fA fB fC fD fA

Di thng b pht p

Frequency DivisionMultiple Access

f

t

f0

AB

CD

A

Thi gian1 khung

TF

cng tn s F

TF

pha pht pha thuA B C D E F


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- Linh hot trong thay i tuyn.- Hiu qu s dng tuyn cao ngay c khi tng s lng trm truy nhp Nhc im: Yu cu cu ng b cm.

c. a truy nhp phn chia theo m

(CDMA): Cc trm mt t s dng cng mt tn s sng mang v cng chung mt b pht pnhng da trn phn chia theo m.

u im:

- Chu c tp nhiu v mo.- Chu c s thay i cc thng s khc nhau ca ng truyn.- Dung lng cao.- Bo mt cao. Nhc im:

- rng bng tn truyn dn yu cu cao- Hiu qu s dng bng tn km.CDMA l k thut a truy nhp mi v cch lng tt nht hin nay!

d. a truy nhp phn chia theo khng gian (SDMA)

Vic ph sng cc vng khc nhau trn mt t v phng php s dng cc phncc sng khc nhau th vi ph tn ging nhau c th s dng li vi ln m can nhiu bhn ch gia cc ngi s dng

- Phn cc: c cc loi phn cc thng ng (VP) v phn cc nm ngang (HP).Phn cc trn c phn cc trn bn tri (LHCP) v phn cc trn bn phi (RHCP), cth c pht i cng tn s t v tinh nhng vi hai phn cc khc nhau m cc trm mtt thu ng tn hiu ca trm mnh m khng b can nhiu do s dng cc anten thu cphn cc khc nhau.

- V tinh vi vic s dng cc loi anten khc nhau c kch thc khc nhau, c th phsng ln mt t vi cc vng ph sng c din tch v hnh dng khc nhau. C bn dngph sng c bn l: ph sng ton cu, l vng ph sng rng nht m v tinh c thph c; ph sng bn cu l vng ph sng mt na bn cu pha ng v pha ty caqu t; ph sng khu vc l vng ph sng mt khu vc kh ln nh bc M, chu uhoc ng Nam v vng ph sng m l vng ph sng vi din tch nh nht sovi ba vng trn.

Nu cc vng ph sng khng chng ln ln nhau v nng lng bc x ca cc bp phph sng ln cc vng khc thp di mc cho php th trong mi vng ph sng c

th s dng ph tn nhnhau.

1.5. Phn b tn s trong thng tin v tinh.

ff0

A

B

CCode DivisionMultiple Access


9/68

http://www.ebook.edu.vn Trang: 8Ph tn s v tuyn in l mt ngun ti nguyn thin nhin hu hn, v vy nht

thit phi s dng ngun ti nguyn ny mt cch hp l, kinh t v c hiu qu.

a. Ca s tn s v tuyn in

ng truyn ca thng tin v tinh b nh hng ch yu do tng in ly tn s thp v

do ma tn s cao. Di tn 1GHz10GHz t b nh hng bi tng in ly v ma nnc gi l ca s tn s VT v c ch ra trong th sau:

Tuy nhin, di tn 1GHz10GHz cng c s dng nhiu cho cc ng thng tinvi ba trn mt t, do s c s can nhiu ln nhau gia hai h thng. Ngoi ra m rng bng thng ngi ta phi chp nhn s dng c di tn ngoi ca s.

b. Bng phn chia bng tn

K hiu Di tn Phm vi s dng

L (1 - 2)GHz TTVT di ng, pht thanh qung b, vtuyn nh v.

S (2 - 4) GHz TTVT di ng, hng hi

C (4 -8)GHz TTVT c nh

X (8 -12)GHz TTVT Qun s v chnh ph

Ku (12 - 18)GHz TTVT c nh, truyn hnh qung b.

K (18 -27)GHz Trm c nhKa (27 -40)GHz TTVT c nh, truyn hnh qung b, lin lc

gia cc v tinh.

Sng mm > 40GHz Lin lc gia cc v tinh.

c. Phn b tn s trong thng tin v tinh

- Khu vc I (V1): Bao gm Chu u, Chu Phi, mt phn Chu v Lin bang Nga.- Khu vc II (V2): Cc nc nam v bc M.

- Khu vc III (V3): Gm Chu c, phn cn li ca Chu v Thi Bnh Dng. Trong c Vit Nam.

GHz

Ca s tn s vtuyn in

1 10 100

dB

0,115

10

50

100Suy hao dotn i n l

Suy hao do tngin ly

Suy hao doma


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http://www.ebook.edu.vn Trang: 9ITU phn chia di tn theo vng nhbng sau.

1.6. Cc loi hnh dch v trong trong thng tin v tinh.

D vo c im ca v tinh thng ti l c kh nng pht qung b trn mt vng a lrt rng, thng tin v tinh c s dng thnh lp cc tuyn thng tin im niim v im ni a tim. Trn c s cc tuyn thng tin trn, thng tin v tinh c sdng cung cp cc dch v c nh v di ng. Mt s loi dch v sau:

a. Dch v in thoi ng di: cung cp cc tuyn ng trc m mng mt t chatrin khai ti hoc khi cc mng mt t qu ti trong gi cao im v lm tuyn dphng cho cc tuyn ng trc mt t khi c s c.

b. Dch v vin thng nng thn: cung cp cc dch v vin thng nh thoi, fax cho ccvng xa xi ho lnh, cc hi o nhng ni m mng mt t cha ti hoc xy dngkhng kinh t.

c. Mng dng ring: cung cp cc dch v vin thng nh thoi, fax, truyn s liu chocc c quan nh nc, cc cng ty cn ng truyn c sn sng cao.

d. Dch v lu ng: cung cp cc dch v truyn s liu vi tc thp gia cc i ding nhxe ti, tu bin ... vi trung tm iu hnh cc i di ng.

e. Chuyn tip chng trnh truyn hnh v pht thanh: cung cp ng truyn gia cctrm HUB ca trung tm truyn hnh n cc trm pht chuyn tip t ti cc v tr cchxa trung tm.

f. Truyn hnh trc tip: cung cp cc knh truyn hnh m ngi xem c th thu trc tipchng trnh t v tinh bng mt anten thu c ng knh 60 cm. Dch v ny khch hngtr tin cc pho hng thng tu thuc vo s knh.

g. Dch v bng tn theo yu cu: cung cp cc dch v theo yu cu ca khch hng nhtruyn s liu tc cao c giao tip hoc khng c giao tip. Dch v ny khch hng trtin theo s ln truyn.

h. Dch v Internet qua v tinh: cung cp ng truyn d liu tc cao t nh cung cpdch v Internet (IPS) n cc thu bao dch v.

i. Dch v chun on bnh t xa: cung cp cc dch v tvn y t cho cc bnh vin xatrung tm y t v gia cc trung tm y t vi nhau.

j. Dch v o to t xa: cung cp dch v o to t xa cho cc trung tm o to.

1.7. Phng v tinh, nh v v duy tr v tinh trn qu o.

a. Phng v tinh ln qu o a tnh

Phng tin phng:- Dng tu con thoi: Loi ny c dng li nhiu ln, tin cy cao, gi thnh t. Nhtu con thoi M, Colombia...


11/68


- Dng tn la y nhiu tng: Loi ny khng s dng li c, gi thnh phng r,ngy ny dng tn la y nhiu tng ang c s dng rng ri. Nhtn la Proton (Nga),Delta (My), Long March (Trung Quc).

Qu trnh phng v tinh ln qu o ph thuc vo loi tn l y, v tr a l ca bi

phng v cc vn lin quan n phn h thng tin. Song phng php phng kinh t vquy chun nht l d trn qu o chuyn tip Hohmann.

Phng php phng da trn qu o Hohmann:

- Giai on 1: Dng tn la y nhiu tng a v tinh ln qu o LEO c cao200Km, V = 7.784m/s.

- Giai on 2: Ti im nng ca qu o LEO, dng tn la y nhiu tng thc hintng tc vi VPhng= 10.234m/s a v tinh sang qu o chuyn tip Ellip c vinim thuc qu o a tnh (h=35.786 Km) v cn im thuc qu o LEO(h=200Km), cn c gi l qu o Hohmann.

- Giai on 3: Khi v tinh chuyn ng qua vin im ca qu o Hohmann th sdng ng c y vin im t trong v tinh a v tinh v qu o a tnh v v vtr ca n.

b.a v tinh vo qu o a tnh: Qu trnh nh v v tinh bt u khi v tinh c

a vo qu o chuyn tip bao gm qu trnh a vn tinh vo qu o xch o v sau t t a v tinh vo v tr a tnh ca n. thc hin c qu trnh ny, cc ngc y vin im APM, cc ng c phn lc ca v tinh c iu khin bng cc trungtm v cc trm iu khin t ti cc v tr khc nhau ca tri t. Cc trung tm iukhin thc hin cc chc nng sau:

- Xc nh t th ca v tinh

- Tnh ton cc thng s ti u cho qu trnh iu khin v tinh t qu o Hohmannsang qu o trn

- Xc nh cc thng s ca ng c hiu chnh hng ca v tinh

Q

Qu o LEOh=200Km

V=7.784m/s

Qu o GEOh=35.786KmV=3.075m/s

QuoHohma

nn

hV=35.786Km

hC=200Km

VPhng=10.234m/s

VPhng=10.234m/s


12/68

http://www.ebook.edu.vn Trang: 11- Gim st v o cc thng s qu o ca v tinh so snh vi trng thi cui cng ca vtinh nhd kin.

c. Duy tr v tinh trn qu o: cc cng vic chnh c thc hin trong qu trnh duytr v tinh trn qu o l:

- Cc dao ng ca v tinh xung quanh v tr qu o theo hng ng Ty, Nam Bcphi c duy tr trong khong 0.10.

- T th v tinh phi c gim st v hiu chnh bo m anten v tinh lun lunhng v cc vng mong mun ca tri t.


13/68


(W/Hz)

N0

B Tn s (Hz)

N0(f)

Hnh 2.1: Mt ph tp m ca tp m trng

Chng 2:

Nhiu v tp m trong h thng tTVT. (6 tit)2.1. Cc ngun nhiu trong h thng thng tin.

Tp m do can nhiu t cc h thng khc gy ra. cc ngun can nhiu c th chialm hai loi:- Can nhiu do cc h thng v tuyn in chuyn tip mt t c cng bng tn:- Can nhiu t cc h thng v tinh khc:

2.2. Tp m trong h thng thng tin.b. Ngun tp m:

Tp m l tn hiu khng nm trong ni dung thng tin, tc ng ln tn hiu cch. N lm gim kh nng khi phc li ni dung thng tin ca my thu.

Cc ngun tp m bao gm:- Tp m pht ra t cc ngun t nhin trong khu vc t anten thu.- Tp m to ra bi cc linh kin in t trong thit bCc tn hiu t cc my pht khc tc ng ln my thu cng gy nn tp m. Tp

m ny c gi l can nhiu.b. c tnh v xc nh tp m:

Cng sut tp m nm trong rng bng ca tn hiu c ch. Thng l bngthng ca my thu. Mt kiu tp m dng rt nhiu l tp m trng l mt ph tpm No (W/Hz) bng hng s trong bng tn yu cu (hnh 2.10).

Cng sut tp m tng ng N (W) o c trong rng bng BN (Hz) c gi tr :

N = NoBN (W) (4.38)Nhit tp m ca mt ngun tp m c cng sut tp m N cho bi:

T = N/kB = No/k (K) (4.39)

T R

R TR T R T

4GHz

4GHz

4GHz

6GHz

6GHz

6GHz

Trm viba4GHz

Trm mt t4/6 GHz

Trm viba6GHz

V tinh

Hnh 2,1: Can nhiu t cc h thngthng tin viba mt t

Hnh 2,2: Can nhiu t cc h thngv tinh khc

T R T R

R T R T R T R T Trm mt

t A1Trm mt

t A2Trm mt

t B1Trm mt

t B2


14/68


G l h s khuych icng sut ca h thng

nhit vt lT = Te

khng ctp mu vo.T = 0

H thngthc

h thngtp m tdo

Cng suttp m cgi tr nhnhau

Hnh 4.14: Nhit tp m u vo tng ng ca h thng

nhit vt l c thkhng phi l T

ngun

tp m

nhit v t l T

cng sut c gitr (W):

N = kTB

Hnh 4.13: Xc nh nhit tp m ca mt ngun tp m

y k l hng s Boltzmann = 1,379. 10 -23 = - 228,6 dB (W/HzK), T l nhit ca mtin tr to ra gi tr cng sut tp m nh ngun xem xt (hnh 4.13). Cng sut tp mc gi tr l cng sut to ra bi ngun c tr khng ph hp vi ngun.

Nhit tp m Te ca mt phn t 4 cc l nhit ca mt in tr t ti u vo caphn t gi nh vi tp m t do, to ra cng sut tp m c gi tr nh nhau ti u raca phn t khi phn t thc khng c ngun tp m u vo (hnh 4.14). Te l tp mbn trong ca phn t bn cc.

H s tp m ca phn t bn cc l t s ca tng cng sut tp m u ra caphn t trn thanh phn ca cng sut ny sinh ra bi mt ngun u vo ca phn t vi nhit tp m bng nhit tp m chun To = 290 K.

Gi thit rng phn t c h s khuch i cng sut G, rng bng tn B v b

tc ng bi mt ngun nhit tp m To; cng sut tng ti u ra l Gk(Te+T0)B. Gi trca cng sut ny ban u t ngun l GkT0B. Vy h s tp m s l:F = {Gk(Te+T0)B} = (Te+T0)/T0 = 1+Te/T0 (4.40)

2.3. Tp m nhit v cc ngun tp m nhita. Nhit tp m ca antenMt anten thu tp m t cc vt th bc x nm trong th bc x ca n. Tp m u raca anten l mt hm ca tnh hng , th bc x v trng thi ca mi trng xungquanh. Anten c coi nh mt ngun tp m c trng bi nhit tp m v gi l nhittp m ca anten TA (K).

Nu gi Tb(,)l nhit chiu sng ca mt vt th bc x nm trong hng(,) m hng h s tng ch ca anten c gi tr G(,). Nhit tp m ca anten


15/68


Hnh 4.15: Mt h thng thu

TR

TA

TFLRXphide

T2

T1

My thu

thu c bng vic kt hp tt c cc vt th bc x trong vng bc x ca anten. Nhittm m ca anten s l:

TA = (1/4)Tb(,)G(,)db. Nhit tp m ca b suy hao

Mt b suy hao l mt phn t bn cc gm nhng phn t th ng (c th l cc inkhng) tt c c nhit TF l nhit xung quanh. Nu LF l suy hao gy ra bi b suyhao, th nhit tp m ca b suy hao s l:

Te = (LF - 1)TF (K) (4.42)Nu TF = T0 bng cch so snh 4.40 v 4.42 th h s tp m ca b suy hao bng:

FF = LF

c. Nhit tp m ca mt thit b c nhiu phn t trong tng

Gi s mt thit b bao gm mt dy N phn t bn cc trong tng, mi phn t jc h s khuch i cng sut Gj (j = 1,2,3, ... N) v nhit tp m l Tj.

Nhit tp m ca thit b l:Te = Te1 + Te2/G1 + Te3/G1G2 +...+ TN/G1G2 ....GN-1 (K) (4.43)

H s tp m nhn c t (2.19)F = F1 + (F2 - 1)/G1 + (F3 - 1)/G1G2 + ....+ (FN - 1)/G1G2....GN-1 (4.44)

d. Nhit tp m ca my thu

Xt thit b thu ch ra trn hnh 4.15 gm mt anten ni n mt my thu qua phide. Vicu ni gy mt mt v nhit o c TF (gn vi T0 = 290 K). Gy ra suy hao LFRXtng ng vi h s tng ch GFRX = 1/LFRX v nh hn 1. Nhit tp m ca h thng sc quyt nh hai im sau:

- Ti u ra anten, trc mt mt u ni, nhit T1- Ti u vo my thu, sau mt mt, nhit T2.

Nhit tp m T1 ti u ra anten l tng ca nhit tp m anten TA v nhit tp m caphn h bao gm u ni v my thu trong cc tng. Nhit tp m u ni cho bi 4.42.T 4.43 nhit tp m ca phn h l (LFRX - 1)TF + TR/GFRX.. Thm vo anten, tng tnhmt ngun tp m, iu c c:

T1 = TA + (LFRX - 1)TF + TR/GFRX (K) (4.45)By gi xem xt u vo my thu, tp m ny b suy hao bi mt h s L FRX . Thayth GFRX bng 1/LFRX, nhit tp m u vo my thu s l:


16/68

http://www.ebook.edu.vn Trang: 15T2 = T1/LFRX = TA/LFRX + TF(1- 1/LFRX) + TR (K) (4.46)Nhit tp m ny a ra cho vic tnh tp m to ra bi anten v s u ni ng

thi vi tp m my thu c gi l nhit tp m h thng. Ch rng vic o lng tpm ti im ny ch phn nh tp m ca anten v u ni.

Tm li: tp m trong mt h thng thu

c quyt nh bi nhit tp m ti mt im cho trong h thng, hu ht thng u vo my thu; nhit tp m ny c gi l nhittp m h thng. N l tng ti im , tt c nhit tp m tng ng to ra tp m phatrn v tt c nhit tp m tng ng i vi tp m to ra pha di ca ca imxem xt.

2.4. Tp m anten thuC hai trng hp c xem xt:

- Anten v tinh (tuyn ln)- Anten trm mt t (tuyn xung)

a. Anten v tinh tuyn lnTp m nhn c bi anten l tp m t mt t v tp m t khng gian bn

ngoi. rng bp sng ca anten v tinh bng hoc nh hn gc quan st t v tinhxung qu t, vi v tinh a tnh gc quan st l 17,5o. Vi cc iu kin nh vy

nh hng ch yu l t mt t. Vi anten c rng bp sng 3dB bng 17,5o th

nhit tp m anten cho hnh 4.16. Ph thuc vo tn s v v tr qu o ca v tinh.Khi rng nh hn ( mt bp sng hp) nhit ph thuc vo tn s v vng phsng; t lin bc x tp m ln hn i dng. Thng ly gi tr tp m anten bng

290 K. Nhit tp m u vo b khuch i tp m thp ca v tinh bng:

TSL = T1/LFRX + TF(1 - 1/LFRX) + TR (4.50)

b. Anten trm mt t (tuyn xung)

Tp m gy ra cho anten bao gm tp m t bu tri v tp m do bc x t mtt.

Trng hp tri trong. nhng tn s ln hn 2 GHz nh hng khngphi vng ion ca kh quyn m l mi trng hp th, nh l mt ngun tp m. Khikhng xy ra cc hin tng kh tng (c gi l bu tri trong) nhit tp m antenbao gm nhit tp m ca bu tri v mt t xung quanh.

Tp m bu tri c xc nh t biu thc 4.41, trong Tb(,) l nhit

vng chiu sng ca bu tri hng (,). Trong thc t ch c mt phn ca butri nm trong hng anten c h s tng ch ln. Nh vy nh hng ca tp m butri trong Tsky ch c th c tc dng nhit vng ph sng i vi gc ngng caanten. Hnh 4.18 ch ra nhit tp m bu tri trong nh l hm ca tn s v gc ngng

[khuyn ngh 720, CCIR]


17/68


Bc x t mt t vng ln cn trm gy ra do cc bp ph ca anten v mtphn bi bp chnh khi gc t nh. Nhit tp m do mi bp ph c tnh bi Ti =

Gi(/4)TG, trong Gi l gi tr h s tng ch ca bp ph c gc c i v TG lnhit vng chiu sng ca mt t. Tng ca cc nh hng ny l gi tr Tm.t

bng:

=

=i

n

idatmTT

1

. (4.51)

Sau y mt s gi tr gn ng ca TG [khuyn ngh 390 CCIR]

- TG = 290 K i vi cc bp ph c gc t E nh hn - 10 o- TG = 150 Kivi - 10o < E < 0o- TG = 50 Kivi 0o < E < 10o- TG = 10 Kivi 10o < E < 90o

Nhit tp m anten s l:

TA = Tb.tri + Tm.t (K) (4.52)

Tp m ny c th tng thm bi cc ngun ring l nm trong khu vc ln cn

ca tnh hng anten. i vi mt ngun v tuyn ng knh gc biu kin vnhit tp m Tn ti tn s xem xt v o mc mt t sau suy hao bi kh quyn th

nhit tp m thm vo TA i vi mt anten c rng bp sng 3dB

c cho bi :TA = Tn(/3dB)

2 nu 3dB > (K)

TA = Tn nu 3dB < (K) (4.53)

Ch c mt tri v mt trng c k n i vi cc trm mt t hng v vtinh a tnh. Mt tri v mt trng c mt ng knh gc tng ng 0,5o. Nhit tpm s tng ln khi c cc vt th trn bu tri nm thng hng vi trm mt t v vtinh. iu kin c bit ny c th bit trc. r hn, ti tn s 12 GHz mt anten

13 m nhit tp m tng ln do mt tri ti thi gian c gi tr bng 12.000 K. Cciu kin xy ra v gi tr ca TA l hm ca ng knh anten v tn s. i vimt trng tng ln khong 250 K ti 4 GHz [theo khun ngh 390, CCIR].

Tb.tri

/

Tm.tTm = (1- 1/Am)

bu tri

Tmt

Hnh : nh hng ca nhit tp m ln anten trm mt t

Tbtribu tri

ma = b suyhao Tm Ama


18/68


Trng hp c ma. Nhit tp m anten tng do iu khin kh tng nh myv ma lm tng hp th. T biu thc 4.45, nhit tp m anten tr thnh:

TA = Tb.tri/Ama + Tm(1 - 1/Ama) + Tm.t (K) (4.54)

Trong Ama l suy hao trong ma v Tm l gi tr ca nhit trung bnh hiudng. Tm c gi tr bng 260 K n 280 K [khuyn ngh 564, CCIR].

Tm li, nhit tp m anten TA l mt hm ca :

- Tn s- Gc ngng- iu khin kh quyn (bu tri trong hay ma)

Do vy, h s phm cht ca mt trm mt t cn phi c xc nh r iu

kin thc t v tn s, gc ngng, v tnh trng ca kh quyn.

2.5. H s tp m:- H s tp m ca mng 4 cc.

Tp m sinh ra trong mt mng bn cc (c th l b khuch i hay b tiu hao) thngc biu th bi h s tp m F

H s tp m ca mt mng 4 cc bng t tn hiu trn tp m u vo trn t stn hiu trn tp m u ra ca mng

00 NS

NSF

ii=

Tuy nhin khi lm vic vi cc tn hiu yu nh trong thng tin v tinh th tp mN thng c biu th bi nhit tp m tng ng T (K: kelvin)

Tt c cc vt th c nhit vt l T (K) s bc x ra sng in t, mt phn cabc x ny nm trong bng tn viba v s gy ra tp m nhmt ngun tp m. Vy:

Nhit tp m tng ng l nhit ca mt in tr tng ng sinh ra tp mc cng sut nh ca ngun tp m to ra, nhminh ho trn hnh 3.37

Ngun tp m vi mt nhit vt l no c th khng phil T c tp m N (W)

nhit vt lT (K)

Gi tr cng sut tp mto ra:

N = kTB (W)

Hnh 3.37: Nhit tp m tng ng

So/NoSi/Ni mng 4 cc

Hnh 2.36: h s tap m


19/68


G1

T1

G2

T2

G3

T3

Hnh 3.39: Mng 4 cc c nhiu tng

Cng sut tp m trong mt rng bng tn B s l:N = kTB (W) (3.23)

Trong k l hng s Boltzmann:

k = 1,3806 10-23+ W s/K = - 228,60 dBW/Hz.K (3.24)

T l nhit tuyt i (K), B gii thng ca tp m (Hz)Vi nhit 290 K trong rng bng 50 MHz cng sut tp m s l:

0,2 10-12 = 0,2pW. Mt anten c nhit mt ngoi l 290 K s thu c cng sut tpm 0,2pW trong bng tn 50 MHz.

Quan h gia h s tp m v nhit t m ca mt mng 4 cc s l:Nu nhit tp m u vo l To , mng 4 cc c h s truyn t l G, nhit tp m tngng ca mng 4 cc l Te nhbiu th trn hnh 3.38 th:

Si /Ni = Si/kToB, So/No = GSi/Gk(To + Te)B thay vo cng thc 3.22 nhn c:

( ) 000

0

0 1T

T

T

TT

BTTGkGS

BkTSF

ee

ei

i +=+

=+

= (3.25)

T 3.25 c th rt ra:

Te = (F - 1)To (3.26)- H s tp m ca mng 4 cc th ng.- H s tp m ca mng 4 cc mc ni tip.

Nu mng 4 cc gm nhiu tng nh ch ra trn hnh 3.39 th tp m u ra s c tnh:Tp m do ngun ngoi tc ng vo u vo u ra ca mng 4 cc 3 tng:

N01 = G1G2G3kT0BTp m ca tng th nht u ra ca mng, c khuch i bi tng th hai vba:

u voNi =kToB

nguntp m

Nhit To

My thu

h s k.i Ggii thng Bnhit tp mtng ngTe

u ra

Hnh 3.38: h s tp m v nhit


20/68

http://www.ebook.edu.vn Trang: 19N02 = G2G3 (kTe1G1B)

Tp m ca tng th hai c c khuch i bi tng th ba:N03 = G3 (kTe2G2B)

Tp m bn thn tng th ba u ra ca mng:

N04 = kTe3G3B)Tp m tng u ra ca mng s l:

N0T= N01 + N02 + N03 + N04N0T= G1G2G3 kB (T0 + Te1 + Te2/ G1 + Te3/ G1 G2)

Nhit tp m tng ca 3 tng u ra ca mng:TeS = Te1 + Te2/ G1 + Te3/ G1 G2

Tng qut mt mng c n tng th nhit tp m ca h thng s l:TeS = Te1 + Te2/ G1 + Te3/ G1 G2 + ... + Ten/ G1G2...Gn-1 (3.27)

Trong Te1...n l nhit tp m tng ng ca cc tng 1, 2, 3, ...n - 1G1...n l h s khuch i ca mi tng

T cng thc 3.27 ta c nhn xt l: Tp m u ra ca mt mng 4 cc c nhiutng c quyt nh bi tp m ca tng u tin

Phn h thu ca trm mt t mun c tp m nh th tng u tin phi l tngkhuch i tp m thp (LNA). gim mc tp m u vo ca LNA, chng c tgn anten gim tp m ca on fide tip in t anten n LNA

H s phm cht ca trm mt t c nh gi bng t s h s tng ch caanten thu G trn nhit tp m ca h thng TeS (G/TeS) hu nh c quyt nh bi h stp m v h s khuch i ca LNA cng vi h s tng ch ca anten. Bi vy vic s

dng mt LNA c h s tp m cng nh cng tt. Sau y chng ta s nghin cu mt sb khuch i tp m thp s dng cc linh kin khuch i khc nhau c s dng phbin cc trm mt t trong h thng thng tin v tinh

2.6. nhy my thu.a. nh ngha

T s tn hiu trn tp m biu th mi quan gia ln ca tn hiu thu v tp m ti uvo my thu. Sau y ta xem xt mt vi t s sau:

- T s cng sut tn hiu trn cng sut tp m; iu ny c coi nh l bn cht vhai ln cng loi c so snh vi nhau. N thng c nh r l cng sut ca

sng mang iu ch k hiu C trn cng sut tp m N,

c vit C/N- Thc t t s cng sut tn hiu trn mt ph tp m, c vit C/N0 c n v l Hz.u im hn so vi vi t s C/N v n biu th cho bt k mt rng bng s dng.Thc t th hin rng bng tp m tng ng BN ca my thu c iu chnh n rng bng B chim bi sng mang iu ch- T s cng sut tn hiu trn nhit tp m; t s ny nhn c t C/N0 qua nhn vihng s Boltzman k, vit l C/T v c n v W/KT s C/N0 phhp nht vi hu ht cc trng hp thc t bi vy n c chn sau ny

b. Biu thc

Cng sut ca tn hiu thu ti u vo my thu cho bi 4.37. Trong :C = PRXMt ph tp m ti cng im l N0 = kT, cho bi 4.45. V vy:


21/68

http://www.ebook.edu.vn Trang: 20C/N0 = [(PTXGTmax/LTLFTX)(1/LFSLA)(GRmax/LRLFRXLPOL)

/[TA/LFRX + TF(1 - 1/LFRX) + TR](1/k) (Hz) (4.47)Biu thc ny c th c gii thch nh sau:C/N0 = (EIRP my pht)(1/mt mt ng truyn)(h s tng ch ca my thu/nhit tp

m)(1/k ) (Hz) (4.48)C/N0 cng c th biu th nh l hm ca mt thng lng nng lng ;

C/N0 = (2/4)(h s tng ch ca my thu/nhit tp m)(1/k (Hz) (4.49)

Trong :

= (EIRP my pht)/ (4R2) (W/m2)Cui cng, c th xc nh c C/N0 khng ph thuc vo im chn dy my

thu cng nh cng sut tn hiu v mt ph tp m c tnh ti cng mt im

c. H s phm cht ca thit b thu

Biu thc 4.48 ca C/N0 ara ba h s:- Th nht (EIRP) c trng cho thit b pht.- Th hai (1/L = 1/LFSLA) c trng cho mi trng truyn dn

- Th ba (h s tng ch my thu/nhit tp m) c trng cho thit b thu. N c gil h s phm cht, hoc G/T ca thit b thu.

Bng vic kho st 4.47 c th thy rng h s phm cht G/T ca thit b thu lhm ca nhit tp m anten TA v nhit tp m tng ng TR ca may thu. Gi tr can s c xc nh di y

d. Nhit tp m anten

C hai trng hp c xem xt:- Anten v tinh (tuyn ln)- Anten trm mt t (tuyn xung)e. Nhit tp m ca my thu

Hnh 4.20 ch ra cu trc ca my thu. p dng cng thc 4.42 nhit tp m TR ca mythu c th biu th:

TR = TLNA + TMX/GLNA + TIF/GLNAGMX (K) (4.55)V d: B khuch i tp m thp (LNA) : TLNA = 150 K, GLNA = 50 dBB trn: TMX = 850 K, GMX = - 10 dB (LMX = 10 dB)

B khuch i trung tn IF: TIF = 400 K, GIF = 30 dBV vy

TR = 150 + 850/105 + 400/105.10-1 = 150 K

Ch : nu b khuch tp mthp c h s khuch i cao s hnch nhit tp m TR ca my thuxung bng TLNA ca b khuch itp m thp.

TIF , G IFGM XTM

TL NA,GL N ATR

B trnIFLN A

LO

B h tn

Hnh 4.20: Cu trc mt my thu


22/68


Chng 3:(6 tit)K thut truyn dn v a truy nhp

trong thng tin v tinh.

3.1. Cc c tnh tn hiu:- Tn hiu thoiPhm vi tn s m con ngi c th nghe ln ti trn 20KHz, tuy nhin ph

ca tn hiu thoi tp trung ch yu trong di 0,33,4 KHz . Do vy trong hu htcc thit b vin thng ngi ta ct b tn s ngoi di ny m khng gy moln.

Cht lng ca tn hiu thoi tng t v l thuyt c quy nh bi CCITT(U ban t vn quc t v in thoi in bo), l t s tn hiu trn tp m khibng gc b gy nhiu trng hp ti nht, nh khi truyn tin trn c ly di. Tuynhin khi xem xt tn hiu chun test tone th tp m cc i cho php trong bng

gc l 10.000 picowatts.c tnh ca ting ni l c phm vi bin ng ln ti 50dB gia ni thm v

la ht. Ngi ni thng hay dng trong qu trnh m thoi, trung bnh ch chim40% thi gian m thoi.

Vic truyn tn hiu s c nh gi qua tye s bt li trn tng s bt thuc (BER). Thng thng vi mt h thng c nh gi l tt khi tho mBER10-4, tuy nhin cht lng ca nhiu h thng c yu cu cao hn trn 10 -5.

- S liuTn hiu d liu c chia lm 3 loi:

+ D liu bng hp (narrow): 300bit/s+ D liu bng thoi (voice): 300bit/s 16.000bit/s+ D liu bng rng (wide): > 16 k bit/s

Vic phn loi theo tc bt ny ph hp vi cc yu cu cung cp cacc phng tin truyn thng. Cht lng c nh gi qua BER.

- Tn hiu hnh truyn i hnh nh ngi ta thc hin bin i hnh nh thnh cc tn hiu

in bng vic qut hnh theo mt t l ph hp, tu thuc vo cc chun khcnhau nhM hay Chu u... bao gm c qut dng v qut mnh.

Tn hiu video c pht i bao gm 4 thnh phn tch bit:+ m thanh+ Tn hiu chi (brightness) cha ng thng tin+ Tn hiu ng b cho qut dng v qut mnh+ Tn hiu ng b mu

Vi cc h thng 625 dng Chu u (PAM v SECAM) v h thng 525dng M (NTSC) th tn hiu video tng hp cha cc thng tin v mu c iuti mt sng mang con ti tn s 3,5MHz hoc 4,5MHz tu thuc vo tng hthng3.2. M hnh knh v truyn dn:

- M hnh knh vcc ch tiu cht lng


23/68


- B bin i thc hin bin i thng tin truyn i (ting ni, hnh nh, vv...)thnh mt tn hiu thch hp vi ng truyn, cn c gi l qu trnh iuch.

- My pht s gi tn hiu ny qua ng truyn dn VT hay HT.- My thu bin i tn hiu thu c thnh tin tc ban u ca n, sau phn

pht ti ni nhn tin- Thng thng tp m gy ra do ng truyn dn, my pht, my thu v b

bin i nn lm cho cht lng tn hiu thu thp hn tn hiu truyn i.

Trong qu trnh iu ch ngi ta ti thng tin trn mt tn hiu tn s cao c mt dng ph hp vi ng truyn dn. Tn hiu cao tn ny c gi lsng mang. Ni chung sng mang c biu th theo cng thc

a(t) = A.sin(2fct + )

Trong TTVT th thng s dng h thng iu ch FM theo kiu iu chtng t v s dng h thng iu ch PSK theo kiu iu ch s.

H thng FM c u im l s dng hiu qu ph tn. cn h thng PSK c

t s li nh hn so vi cc h thng iu ch khc khi cng t s C/N.- Truyn dn tng t

Ngunthng tin

B bini

Mypht

ng truyn(VT HT)

Mythu

B bini

Ni nhntin

Ngun tp m

Hnh : Cu trc m hnh ca h thng thng tin

iu ch

Hnh : Cc h thng iu ch khc nhau

iu chtng t

iu chs

AM

FM

PM

ASK

FSK

PSK

Thay i bin (A)

Thay i pha ()

Thay i tn s (fc)

vm=m.VeBin

Ph sng AM

Sng mang

Bng tn thp

Bng tn cao

mVe/2mVe/2

(fe-fm) (fe+fm)fe

Hnh : Dng sng iu bin


24/68


+ H thng iu ch bin : lm thay i bin sng mang t l vi inp (dng in) ca tn hiu truyn i.

+ H thng iu tn lm thay i tn s sng mang t l vi in p (dngin) ca tn hiu truyn i. Theo l thuyt, ph ca n bao gm v s cc sng bin tn cao v bin tn thp vi trung tm l sng mang (f

c). v vy chim mt

bng tn rng.

+ iu ch pha: rt ging vi iu tn, iu pha thc hin vi mt tn hiuiu ch a qua mt mch vi phn cho ra cng kt qu nh iu tn thc hin bitn hiu iu ch ban u. iu tn v iu pha i khi c gi l iu gc.

- Truyn dn s.

+ Kho dch bin ASK: Trong truyn dn tn hiu s, ASK lamd thay ibin ca sng mang theo tn hiu cn truyn i. Khi tn hiu c truyn i lnh phn th bin c thay i thnh A0 v A1 tng ng vi gi tr 0 v 1.

Khi A0 bng 0, h thng c gi l h thng OOK (on off keying).

Bin

Ph sng FM

fe

Hnh : Dng sng iu tn

Dng sng iu ch 1 0 0 1 0 1 1

Sng mang

Kho dch bin

Bin On On On OnOff Off Off

Kho dch tn

Tn s f1 f1 f1 f1f0 f0 f0

Kho dch pha

Pha 0 0 0 0


25/68


+ Kho dch tn FSK: Lm thay i tn s ca sng mang theo tn hiu scn truyn i, khi truyn dn mt tn hiu nh phn, tn s sng mang thay ithnh f0 (tn s thp) v f1 (tn s cao) tong ng vi bit u vo 0 hay 1.

+ Kho dch pha nh phn BPSK: Lm thay i gc pha trong phng trnh

biu din sng mang, gc 0= v =0 hoc ng

c li t

ng ng vi bit u vo1 v 0.+ Kho dch pha cu phng QPSK: S dng 4 trng thi pha c dng

t hiu qu s dng tn s cao hn so vi h thng BPSK. Tn hiu truyn i cbin i t ni tip sang song song, hai song smang khc pha nhau /2 c iuch hai tn hiu nh phn trn v chng c kt hp li vi nhau thnh tn hiu ra.Tc tn hiu trong ng truyn dn bng tc chui u vo. Qu trnh giiiu ch QPSK c coi l hai qu trnh gii iu ch BPSK c lp. Hai tn hiubng gc I(t) v Q(t) l kt qu so pha tn hiu thu c vi hai sng mang chunlch pha vi nhau mt gc /2.

+ Cc c tnh li m ho: Kt qu sau tch sng ng b c nh gi quat s li bit Pe=0,5 * erfc(sqrt(Eb/N0)), trong Eb l cng sut trn bit ti u vomy thu (W), N0 l mt cng sut tp m ti u vo my thu (W/Hz) v erfc lhm li.

+ K thut ng b: bao gm ng b mng, ng b bit v ng b khung. ng b bit: Ti to v gi tn hiu thu c bng cch s dng mt tn hiu

ng b ti sinh t cc tn hiu thu c pha thu ti to cc v tr bit gingnhau c pha pht v thu.

ng b khung: c thc hin h thng thng tin ghp knh theo thi gian,cc xung ng b khung phi ch ra chnh xc im bt u ca mt khung cchn vo chui xung pht ra v pha thu xc nh thi im ng m cc cng phnknh theo cc xung ng b knh ny; th t u ni v nh thi cc thng tinc thit lp ging nhau c hng thu v pht.

ng b mng: to c tn s ng b nh nhau trn ton b mngtruyn dn.

+ Sa li m: Khng th trnh c tp m gy ra cc li m trong TTVT v cctrm mt t thu c cc tn hiu cc k yu. C hai cch sa li c bn

/2

Cu hnh b iu ch QPSK

BPF

LPF

Khi phcng h

Khi phcsng mang

/2

LPF

TS

TS

I(t)

Q(t)

Cu hnh b gii iu ch QPSKI(t)

Q(t)01

00 10

11

Biu sao tn hiu


26/68


Sa li hng i FEC, ch c pha thu xc nh cc v tr li s liu thu cv thc hin sa n. Ph hp vi ng truyn tn hiu thoi v truyn hnh vchng i truyn dn thi gian thc.

M khi dng sa li hng i: v d xt m khi Hamming k=3, N=7, m

ny c chiu di 7 bit trong c 4 bit tin v 3 bit m, kh nng s li l mt lin. My thu nhn chui m thc hin m li v kim tra xem ba bit V5, V6 v V7c ging vi ba bit V5, V6 v V7 ca khi tin thu c khng, nu thy sai th trabn li xc c chnh xc bit sai v sa, do l m khi nh phn nn vic sac thc hin bng cch o ngc bit b sai (0->1 hoc 1->0)

M xon: M xon (n, k) c to ra bng cch cho chui thng tin truyn qua hthng cc thanh ghi dch tuyn tnh c s trng thi hu hn. Cho s lng thanh ghi dchl L, mi thanh ghi dch c k nh v u ra b m xon c n hm i s tuyn tnh. Tc m l R = k/n, s nh ca b ghi dch l Lkv tham sL cn gi l chiu di rngbuc ca m xon.

7-bit khiCc bit tn hiu Cc bit d

0 0 0 0 0 0 00 0 0 1 0 1 10 0 1 0 1 1 0

0 0 1 1 1 0 10 1 0 0 1 1 00 1 0 1 1 0 10 1 1 0 0 0 00 1 1 1 0 1 11 0 0 0 1 0 01 0 0 1 1 1 11 0 1 0 0 1 01 0 1 1 0 0 11 1 0 0 0 1 01 1 0 1 0 0 1

1 1 1 0 1 0 01 1 1 1 1 1 1V1 V2 V3 V4 V5 V6 V7

a thc to mV5 = V1 + V2 + V3V6 = V2 + V3 + V4V7 = V1 + V2 + V4

Quyt nh

Bit li V5 V6 V7V1 sai ung saiV2 sai sai saiV3 sai sai ungV4 ung sai saiV5 sai ung ung

V6 ung sai ungV7 ung ung sai

u vo1

2

3

u rab: 01

c: 10

a: 00

c: 10

000

000

000111

111

111

001

110

011

100

001

010

101

(L-

0

1

a: 00

b: 01

d: 11

a: 00

b: 01

c: 10

d: 11

d: 11

Hnh : a) S b m xon vi L = 3, k= 2,n = 3 v a thc sinh G(D) = [D2 1+D2

1+D+D2

]. b) S hnh cy.


27/68


Yu cu pht li t ng ARQ, pha thu ch pht hin ra cc li v yu cubn pht pht li on s liu. Ph hp vi ng truyn s liu v khng cn thitphi truyn dn trong thi gian thc.3.3. a truy nhp trong thng tin v tinh:

L k thut cc trm mt t truy nhp b pht p v tinh, vi yu cu sngVT t cc trm mt t ring l khng can nhiu ln nhau. C cc k thut truynhp thng c s dng trong TTVT nh FDMA, TDMA, CDMA, SDMA, truynhp gi (Packet) v truy nhp ngu nhin.1.3.1.a truy nhp phn chia theo tn s (FDMA):

Cc trm mt t s dng cc tn s sng mang khc nhau v cng chung mtb pht p.

u im:- Th tc truy nhp n gin.- Cu hnh trm mt t n gin.

Nhc im:- Khng linh hot thay i tuyn.- Hiu qu thp khi s dng nhiu knh, dung lng thp v cht lng thp.

1.3.2. a truy nhp phn chia theo thi gian (TDMA):Trong TDMA mt s trm mt t dng chung mt b pht p v tinh bng cch

phn chia khong th gian. Mi trm mt t c n nh mt khe thi gian v chc pht tn hiu ca chng trong khe gi l cm. Tp hp tt c cc cm nnh cho mt s trm mt t to thnh mt khong thi gian gi l khung TDMA.

Mi trm mt t pht tn hiu c c cngtn s sng mang f0 v chim ton b bng tnca b pht p v tinh. V cc khe thi gian

khc nhau c n nh cho tt c cc trm mtt nn ch c tn hiu t mt trm mt tchim b pht p v tinh ti thi gian chophp v khng bao gi xy ra trng hp tnhiu t hai trm mt t chim b pht p vtinh trong cng thi gian.

lu ca mt khe thi gian n nh chomt trm mt t c xc nh trc tu theoyu cu lu lng ca trm . Mi trm mt t pht tn hiu ca n trong khe n nhcho n trong tt c cc khung TDMA . V vy, cc tn hiu pht i t mi trm mt t

nm trong mi cm c chu k ng bng chu k pht khung TDMA. Cc cm trong mtkhung TDMA c n nh sao cho khng chng ln ln nhau, nh ch ra trn hnh 5.3

f

t

fA fB fC fD fA

Di thng b pht p

Frequency DivisionMultiple Access

f

t

f0

A

BC

DA

Thi gian1 khung

Mt b pht p


28/68

http://www.ebook.edu.vn Trang: 27Mng TDMA bao gm cc trm lu lng v t nht l mt trm chun. Cc cm pht

i t trm lu lng gi l cm lu lng. S liu lu lng c pht bi cm lu lng.Trm chun pht mt cm c bit theo chu k gi l cm chun. Cm chun cung cpchun nh thi cho cc khung TDMA c chu k bng mt khung TDMA. Trm lu

lng pht cm lu lng v iu khin nh thi pht cm lun lun theo cm chun,nh ch ra trn hnh 5.4

Mi trm lu lng pht cc cm lu lng trong khe thi gian c n nh v tinh,nh thi pht cm c iu khin bi cm chun, cm chun c s dng nh lchun nh thi.

v tinh, cm chun v cc cm lu lng c sp t theo th t c nh khngchng ln nhau trong mi khung TDMA. Mi trm lu lng thu cc cm ny v ly racm lu lng cha cc tn hiu c a ch n trm lu lng

Trong TDMA s dng tn hiu s v cc tn hiu s d bin i thnh chui cccm v d dng iu khin nh thi pht v thu. V vy, cc tn hiu tng t nh tnhiu thoi trc ht c bin i thnh tn hiu s bng m ho PCM. Sau tn hiuthoi PCM v cc tn hiu s gc nh s liu c ghp knh phn chia theo thi gian(TDM). Tn hiu s ghp c bin i thnh dng cm. Sau a vo iu ch siucao tn vi k thut iu ch thng c s dng l bin i pha (kho dch pha: PSK).Tm li phng php truyn dn tn hiu trong TDMA l : iu xung m - ghp knhphn chia theo thi gian - kho dch pha - a truy nhp phn chia theo thi gian (PCM -TDM - PSK - TDMA).

V trong PCM tn s ly mu l 8 kHz nn di khung ca tn hiu PCM l

125s. Trong TDMA, di khung TDMA bng bi s ca di khung TDMA hay

125s, ph hp vi tn hiu PCM:

Khung TDMA = 125s m (m l s nguyn)

A B N... A

Cm lu lng

Khung TDMA

t

Mng TDMA

A B C D AD

Khung TDMA

t

cm

Khung TDMA v cm

cng tn sF

Cc trm lu lng pha thu

A B C D E FC

Trm chun

KhungTDMA

KhungTDMA


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5.1.2. u im ca TDMA

Thng tin v tinh vi k thut TDMA c cc u im so vi FDMA nh sau:

1/ S dng hiu qu cng sut v tinh

Trong FDMA khi nhiu sng mang ng thi c khuch i chung bng mtHPA th s gy ra nhiu iu ch. gim nh nht nhiu xuyn iu ch ta phi licng sut ra mt li kh ln t 6 dB n 10 dB so vi cng sut ra cc i. V vycng sut HPA khng c tn dng ht.

Cn trong TDMA s sng mang trong mt b pht p lun lun bng mt v vyc th cho HPA lm vic vi cng sut ra cc i m khng c nhiu xuyn iu ch.

2/ Tnh linh hot trong khai thc khi thay i nhu cu lu lng. Trong FDMA khithay i knh i hi phi thay i phn cng ca thit b. Trong TDMA, di v v tr

cc cm c xc nh theo quan h v yu cu lu lng ca mi trm mt t. Khi thayi n nh knh trong TDMA cn thay i div v tr cc cm. Trong trng hpny, khng cn phi thay th hoc sa i cc thit b thng tin ca trm mt t m chcn iu khin bng phn mm.

3/ Mt u im khc ca TDMA l c th tng dung lng truyn dn ln bngcch chn k thut ni suy ting ni s (DSI). DSI l k thut m cc knh v tinh cn nh cho ngi s dng ch khi c tn hiu thoi truyn dn, thay vo cc knh v tinhc nh n nh cho tng c nhn ring bit. DSI c th cung cp knh trn mt t gp

hai ln so vi s knh v tinh c. i vi cc tn hiu thoi c s ho, ta c th ddng thc hin ni suy ting ni. v th d dng p dng DSI trong TDMA. S kt hpTDMA v DSI cho php tng dung lng truyn dn ln ba hoc bn ln so vi dunglng truyn dn FDMA

4/ D dng thc hin u ni vi cc mng mt t, C kh nng tng thch tuytvi vi cc mng thng tin s v d dng u ni vi mng thng tin mt t m h thngs ang ngy cng pht trin

5.1.3: Nguyn l ng b cm:

Trong TDMA cc cm c pht i t cc trm lu lng phi c iu khinsao cho chng c sp xp ng vo khe thi gian c n nh v tinh. Vic iukhin nh thi ny c gi l ng b cm.

Nh ta bit khong cch gia v tinh v trm mt t lun lun thay i. vvy thi gian cn thit truyn mt cm lu lng t trm mt t n v tinh thay ilin tc. Ngoi ra trm chun v cc trm lu lng c cc ng h ring vi cc tns khc nhau. v vy nh thi pht cm gia cc trm l khc nhau, nu khng c

ng b cm th cc cm c pht i s lch khi thi gian c n nh v tinh,cc cm s chng ln ln nhau v tinh, thng tin s b mt.


30/68


cm lu lng cm chun

trm lu

lng

trm

chun

ng b vng kn trc tip

trm chuntrm chuntrm l

ulng

cm chuncm lu

ng b vng kn hi tip

V tr ca khe thi gian n nh cho mt cm pht i t mt trm lu lng cxc nh bng v tr tng i ca cm so vi cm chun. nh thi pht cm cn phic iu khin theo chu k ly cm chun lm chun nh thi.

C ba phng php ng b cm iu khin v tr tng i ca cc cm lu

lng so vi cm chun: ng b vng kn trc tip, ng b vng kn hi tip v ngb vng m

1/ ng b vng kn trc tip

ng b vng kn trc tip p dng cho hot ng vi vng ph sng ton cu mi trm lu lng c th lun lun thu c cc cm xut pht t trm qua v tinh.Trm lu lng thu c cm chun v cm lu lng ca chnh n. Khong thi gian giahai cm ny c o theo chu k ti trm lu lng. pht hin li trong khong thigian bng vic so snh chng vi gi tr danh nh cho. nh thi pht cm c a

ra hocb tr b cho li pht hin. Bng cch ny cm lu lng c th c gi vtr chnh xc trong cc khung TDMA.

2/ ng b vng kn hi tip c p dng cho hot ng ca bp sng du in, y mi trm lu lng khng th thu c cm khi u t trm pht i qua v tinh.Trm lu lng khng th o khong thi gian gia cm chun v cm pht i t trm v trm chun t trong vng i din, nm trong vng ph sng ca bp du in khc.Trm chun gi tr li cc kt qu o ti trm lu lng. Trm lu lng pht iu khinnh thi pht cm b li so vi v tr danh nh. Vic b tr da trn kt qu o c

trm chun gi tr li cho trm lu lng. Phng php ng b ny cng c th pdng cho hotng i vi bp sng ton cu.

3/ ng b vng m: nh thi pht cm c xc nh bng cch b sung mtlng tr ph hp vo nh thi thu cm chun. Lng tr ny nhn c nh vic tnhton da trn vic o khong cch gia mi trm mt t v v tinh. Mi trm lu lngthu cm chun v pht cm ca n sau mt tr c lng tnh t nh thi thu cmchun. ng b vng m c th p dng cho c bp du in v bp sng ton cu vn gin hn so vi cc phng php ng b cm khc. Tuy hin li v tr gia cc cmtrong ng b vng m c th ln hn.


31/68


trm lulng

trm chun

ng b vng m

3.3. Khung TDMA v cc khun dng cm

3.3.1: Khun dng khung v khung TDMA chung trong mt nhm

Cu trc ca khun dng ca khung TDMA s dng trong h thng Intelsat

di khung l 2 ms. Mt khung TDMA cha 2 cm chun (RB1 v RB 2) vmt s cm lu lng. RB1 hoc RB 2 hot ng nh cm chun s cp, cm chun cnli l cm chun th cp. S liu iu khin pht i bi cm chun ng b cm viu khin h thng c thc hin bi a khung.

Mt a khung gm 16 khung TDMA lin tip c di bng 32 ms. Cc t duynht ring bit gi l du a khung c s dng cho cc cm trong khung 0, l khungu tin ca a khung. Cc du a khung ny c s dng nhn bit khung 0 v chth khi u ca a khung.

Cmchun1: RB1

Cmchun2: RB2

Cmlu lng 1

TB1

Cmlu lng n

TBn

Cmchun1: RB1

Cmchun2: RB2

2 ms

Dng khung TDMA s dng trong h thng Intelsat

a khung (32

khung 15

cm lu lng

.........

khung 0 khung 1

Khun dng ca a khung (16 khung)

RB1

RB1

s dng cc t duynht ring bit gi l

du a khung


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http://www.ebook.edu.vn Trang: 31Mt nhm b pht p c ng b vi mt nhm khc nhy b pht p.

Nhm ny c gi l mt nhm cc b pht p ng b. Hnh 5.19 ch ra quan h nhthi gia mt khung TDMA chung trong nhm v cc cm chun ca mi b pht png b trong nhm .

Khung TDMA c xc nh duy nht cho cc b pht p trong nhm bngcch to ra khi u chnh xc tt c cc khung ging nh mt khung khc. Mt b phtp c chon lm b pht p chun nh thi. RB 1 ca b pht p ny xc nh khiu khung TDMA. TRT l b pht p chun nh thi v SOF l khi u ca khungTDMA cho tt c cc b pht p trong nhm. Vic xc nh v tr cm trong nhm davo SOF. Tt c cc b pht p trong nhm c th thit lp nh thi chung bng cch

xc nh khi u ca khung TDMA cho tt c cc b pht p.Biu thi gian cm quy nh khong thi gian gia RB1v SOF, v khong thi

gian gia RB2 v SOF cho mi b pht p. Mt trong hai trm chun pht RB1 v ttrmchun kia pht RB2 ti nh thi danh nh cho tt c cc b pht p trong nhm.

Cc cm chun ny bo m cc trm lu lng vi mt chun nh thi thchin ng b cm. RB1 phi ng trc RB2 trong mi khung nhng RB2 khng philun lun t ngay sau RB1. Mt hoc nhiu cm lu lng c th c xen vo giaRB1 v RB2. Cng c trng hp trong cc b pht p loi tr TRT, c mt hoc nhiu

cm lu lng c t gia SOF v RB1. Trong h thng TDMA Intelsat, ng bkhung c thit lp bng vic xc nh mt khung TDMA chung cho mt nhm cc bpht p ng b. Hn na, trong cng mt nhm, ng b a khung cng c thit lpsao cho cc du a khung phi xut hin trong cng mt khung TDMA tt c cc bpht p. Bng cch thit lp ng b khung v a khung theo phng php ny, nhybc pht p c th c thc hin. Ngha l cc trm lu lng v trm chun c thpht cc cm k tip nhau ti mt s b pht p s dng mt u cui TDMA v cngtheo cch tng t c th thu cc cm t mt s b pht p

nhm cc b phtp c ng b

WH EH

WH EH

WZ EZ

WZ EZ

u Nhy bc pht p, ch ra quan h nh thi gia mt khung TDMA chungtrong nhm v cc cm chun ca mi b pht p ng b trong nhm .


33/68


1.3.3 a truy nhp phn chia theo m (CDMA): Cc trm mt t s dng cng mt tn s sng mang v cng chung mt bpht p nhng da trn phn chia theo m.

u im:- Chu c tp nhiu v mo.- Chu c s thay i cc thng s khc nhau ca ng truyn.- Dung lng cao.- Bo mt cao.

Nhc im:- rng bng tn truyn dn yu cu cao- Hiu qu s dng bng tn km.CDMA l k thut a truy nhp mi v cht lng tt nht hin nay!

ff0

A

B

CCode DivisionMultiple Access

DNI: tn hiu s khng c ni suyDSI: tn hiu s c ni suy

M u S liu lu lng

S liulu lng DSI DSI DSI DSI

0 1 2 .... nCc knh v tinh

Cm con

1 2 3 .... 16Cc mu PCMMt knh

v tinh

1 2 3 4 5 6 7 8Mt muPCM 8 bit/mu

Bit c trng

s ln nht

Bit c trng

s nh nht

128 bit/knh (16 mu PCM)

S knh v tinh:Cc tiu: 1 knhCc i : 128 knh DNI

127 knh DSI

S cm con cc i : 8

Cu trc phn cp ca cm lu lng

iu ch

Bn tin

Sng mang

m1(t)

M a1(t)

Knhtruyn

a1(t).m1(t)

a2(t).m2(t) M a1(t)

ti giiiu ch

an(t).mn(t)

H thng DS/CDMA


34/68


Chng 4:V tinh thng tin (4 tit)

Trong h thng thng tin v tinh, v tinh ng vai tr l mt trm chuyn tip,lm chc nng ca mt trm lp (repeater). Thu tn hiu t cc trm mt t, khuch

i, bin i sang mt tn s khc v khuch i ln mt cng sut yu cu ri phttr li mt t. L mt trm rt xa qu t v bay xung quanh qu t, c iukhin t xa nn c cu to phc tp. Ngoi phn h thng tin (Payload) cn phi cnhng phn h ph tr khc o lng, iu khin, gim st v.v... t xa.4.1. Nhim v v cc c tnh ca phn h thng tin trn v tinh.a. Nhim v: Phn h thng tin ng vai tr nh mt trm chuyn tip gia cc

trm mt t vi nhau. Vi hu ht cc v tinh thng tin thng mi th phn hthng tin gm hai phn l l lp v phn h anten v tinh.

b. Chc nng:-

Thu cc sng mang

c pht t cc trm mt t trong mng vi bngtn v phn cc xc nh. Cc trm mt t ny c t trong vng phsng ca anten v tinh.

- Hn ch ti a nhiu c hi t cc h thng v tuyn khc.- Khuch i cc sng mang thu c trong khi hn ch ti a tp m v mo.- Bin i tn s cc sng mang nhn c trn ng ln sang tn s sng

sng mang trn ng xung.- Cung cp cng sut ln trong bng tn xc nh ti u vo anten pht

v tinh.- Bc x sng mang trong bng tn v phn cc xc nh ti cc vng c xc

nh trn b mt tri t.Tuy nhin, vi v tinh ti to th b lp cn c chc nng gii iu ch, khi

phc xung v iu ch.Bng tn phn b cho trm lp ca v tinh c th t vi trm MHz ln n

vi GHz. thun li cho vic khuych i tn hiu, bng tn ny thng cchia thnh cc bng tn con vi cc dy khuych i ring bit cho tng bng con.c. Thng s k thut ca phn h thng tin :

Phn h thng tin c c trng bi cc thng s k thut trong tng bng tnsau:

-

Bng tn cng tc- S lng b pht p- rng bng thng ca mt b pht p.- Phn cc tn hiu ng ln, ng xung.- Cng sut pht x tng ng ng hng EIRP hoc l mt thng

lng cng sut to ra ti bin ca vng dch v.- Mt thng lng cng sut bo ho ti anten thu ca v tinh SFD.- H s phm cht G/T ca my thu v tinh ti bin ca vng dch v hoc

gi tr cc i.- Vng ph sng.- Cng sut u ra ca b khuch i cng sut.- Cu hnh d phng ca my thu v b khuch i cng sut cao


35/68


4.2. Cu trc ca tuyn thng tin trn v tinh thng tin.4.2.1. B pht p

n gin ta ch xt cu hnh ca tuyn thng tin (trm lp) ch c mt beam(chm, hng) pht v thu.

a. My thu:

Chc nng my thu: Khuch i tn hiu VT nhn c t cc trm mt tpht sau khi tn hiu ny b suy hao ln trong khng gian t do. B thu bao gmb khuych i tp m thp LNA ti tn s ln, b i tn xung nu nh tn sc i hai ln v b khuych i. B thu l my thu bng rng c rngbng thng 500 (bng C) n 1GHz (bng Ku) tu thuc vo bng tn s dngv loi LNA.

B LNA (Low Noise Amplituder):B ny quyt nh ch yu h s phm cht G/T ca b pht p. B khuyvhsi ny c h s tp m thp ng thi phi c h s K cao hn ch tp mphn b ca cc tng sau . Cc thng s c trng cho b LNA l:-Bng tn hot ng: C(6 GHz), X(8 GHz), Ku(14 GHz), Ka (30 GHz)-H s K: (50 60) 0.5 dB-H s tp m: 1,6 (6 GHz), 1,9 (14 GHz), 2,2 (30 GHz)

m bo h s tp m ca b khuch i thp th cn c nhit bn ngoi

b LNA thp, nn ngi ta t n trong Heli lng gn 0 tuyn i. B i tn xung: bao gm mt b trn (Mixer), mt b dao ng ni v cc b

lc thng di. Tn s ca b dao ng ni tu thuc vo bng tn cng tc cngnhb pht p thc hin i tn mt ln hay hai ln.

Khuch i trong my thu: B K tn hiu sau khi i tn nhm cung cpcng sut tn hiu ln cho cc tng sau. B K ny c thit k sao cho hs K c th iu khin c t xa bng lnh telecommand. Do hot ng trnbng tn rng v K mt cch ng thi nhiu sng mang nn c tnh truynt ca b K ny i hi rt cao. Hn na cng sut ca hi bc ba phi nh

hn cng sut ca sng mang t nht l 40 dB u ra b K.

Phnknhuvo

My thuAnten

Anten

Khuch i knhKhuch i knh

Khuch i knh

Ghpknhura

Bin i xung

K. iLNA

Khuchi

B lcgii

B lcgiiTrn


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Trong thc t, my thu c ch to theo modul phi bo m c h s tngch yu cu v khng i trn ton b bng thng hot ng (in hnh 500MHz). Gi tr in hnh v gn sng ca c tuyn truyn t nm trongphm vi 0,5 dB trn ton b bng thng cng tc. t c gn sng

nh, s dng cc b Isolator (phn ly) nhm suy hao sng phn x. tng tin cy ca h thng, my thu c th c du ni theo cu hnh d phng 1:1

a. B phn knh u vo.My thu l thit b bng rng, trong khi khuch i tn hiu ti cng sut

ln cho ng xung phi s dng cc b khuch i cng sut cao. Do vy, gim hi xuyn iu ch bng thng ca b pht p phi chia bng tn thnh ccbng con.

Nhim v: Phn bng tn ca my thu thnh cc bng con (36, 54 hoc 72MHz...). Yu cu: Ti tn s trung tm c tuyn cng hng ca b lc khng c thay

i trong sut qu trnh v tinh hot ng. Do vy, cc hc cng hng phi lmbng vt liu chu nhit, c h s dn n nh, dn tt.

Cu hnh: bao gm cc b Circulator (xoay vng) v cc b lc bng di, chnhcc b lc bng di ny quy nh bng thng ca b pht p. Trong hnh di ,cc knh c t chc thnh hai nhm chn v l. S dng mt b lai ghp(Hybrid) chia thnh hai phn dng chung cng sut u ra my thu. Suy haotn hiu trong b phn knh u ra ph thuc vo s ln tn hiu i qua b

B lc gii

B lc gii

B lc gii

B lc gii

B lc gii

B lc gii

Knh 1

Knh 3

Knh 5

Knh 2

Knh 4

Knh 6

K 1 K 2 K 3 K 4 K 5 K 6 f

RF vo

My thu(d phng)

My thu(d phng)

RF ra

Hnh : Cu hnh d phng 1:1


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Circulator v s ln phn c ti b lc thng di. Do vy, suy hao tn hiu ivi cc knh khc nhau l khc nhau v t gi tr cc i ti knh xa u voca b IMUX nht. Bng cch phn chia b IMUX thnh nhiu phn, trong mi phn h tr mt s knh c th lm gim chnh lch suy hao gia cc knh

b.

B khuch i knh.y l thut b quan trng nht ca h thng pht p, bao gm cc b khuchi cng sut cao (HPA).

Nhim v: Thc hin cung cp cng sut theo yu cu cho ng xung v do xc nh EIRP ca mi knh.

Yu cu: Khuch i phi bo m tnh cht tuyn tnh hn ch tp m xuyniu ch phn b ti u vo cc trm mt t thuNg ra cn c cu hnh dphng v n d b hng.

Gii php:- S dng cc b tim K trc b HPA, do tn hiu u vo my thu sau khi

K b suy hao trong b IMUX khng ln iu khin b HPA.- Mc cc b suy hoa ni tip vi b K vi h s suy hao c iu chnh t

0 n vi dB v bc nhy vi phn mi dB bng lnh t TT&C mt t- S dng cc b cn bng (hot ng ch bo ho) b c tuyn bin

tn s v pha tn s ca b HPA. (Nguyn l lm vic ca b cn bngthc cht l lm mo trc tn hiu sao cho c tuyn bin tn s, pha tns ngc vi c tuyn trn ca b HPA.

Thng s k thut: C hai loi HPA th

ng

c s dng trn v tinh l b Kn sng chy (TWTA) v b K bn dn (SSPA) c sau:Thng s k thut TWTA SSPA

Bng tn hot ng C, Ku, Ka C, KuCng sut bo ho ti u ra 20200W 2040WH s K im bo ho ~55dB 7090WT s cng sut trn tp m xuyn iu ch ti ch bo ho trong trng hp hai sng mang ccngbin (C/N)IM

1012 dB 1018 dB

H s chuyn i AM/PM 4,50/dB 20/dBHiu sut tng 5060% 3045%Khi lng bao gm c b ngun 1,52,2Kg 0,81,5KgXc sut hng trong 109 gi


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Gii php: Cc b lc u ra b khuch i cng sut s c ghp trc tip lnng dn sng chung trong o ng dn sng c ngn mch ti u cui vu kia c ni vi cng tip sng ca anten.

d. Phng php kt ni my thu/ pht vi h thng anten. Trong thc t trn mt v tinh c nhiu anten thc hin ph sng ti cc vng

min khc nhau trn tri t. Vic kt gia u ra b ghp knh vi cc antenny c thc hin theo hai k thut sau:

- Kt ni tnh: c kt ni c nh khng thay i c.- Kt ni ng: C th thay i bng cc chuyn mch v c iu khin bngcc lnh di trm iu khin TT&C di mt t.

Trn v tinh Intelsat VI c 50 b pht p lm vic bng C v bng Ku vi rng bng tng cng l 3,272 GHz. Bng C c hai b pht p ph sng bn cu, bnb pht p ph sng vng v hai b ph sng a phng (spot beam). Bng Ku gmhai b pht p ph sng a phng. Hnh 2.20 ch ra nhng vng ph sng in hnhca v tinh Intelsat VI

ng dn sng ngn mch cui

Ngn mch

Blcknh1

Blcknh3

Blcknh5

Blcknh7

Blcknh2

Blcknh4

Blcknh6

Blcknh8

iu hngthch ng vi 1b lc bng tn

iu hngthch ng vi 8b lc bng tn

Ti cngtip sngca anten

T cc HPA

T cc HPA

Hnh : B ghp knh u ra

S khi cc b phn chnh ca phn h thng tin trn v tinh Intelsat VI


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i vi v tinh c nhiu chm tn hiu (beam) th vic kt ni mt my thu ting thi mt vi my pht c thc hin bng cc b chuyn mch.+ Yu cu: Thc hin chuyn mch trong vi trm nano giy.+ Gii php: S dng cc b chuyn mch tch cc nh diode PIN hay cc

transistor hiu ng trng FET mt cng hoc hai cng, chng c mc theokiu ma trn gi l ma trn chuyn mch.

4.3. Anten thng tin ca v tinhPhn h anten trn v tinh ng mt vai tr quan trng trong chc nng

ca v tinh. Cc chc nng chnh ca anten trn v tinh

- La chn sng v tuyn c pht i trong bng tn cho vi phn cc cho t cc trm mt t nm trong vng ph sng ca v tinh.

- Pht sng v tuyn bng tn v phn cc cho ln khu vc quy nh

trn mt t. Yu cu:- Thu can nhiu cng nh cng tt.- Pht cng sut nh nht ra ngoi vng quy nh. Cc thng s c trng cho:Anten trn v tinh phi ph sng mt khu vc gi l vng phc v vi mc

cng sut yu cu n c c trng bi cc ng ng mc v tng ch caanten hoc l EIRPs (Equivalent Isotropic Radiated Power) ng mc v h sphm cht thu GR/TS ng mc (thng thng trong vng mt cng sutkhng gim qu 3 dB so vi hng cc i). Nh vy tt c cc trm mt t nmtrong vng ph sng u thu c mc cng sut yu cu khng nh hn mc cci 3 dB. Tu theo c im thng tin, loi dch v... m vng ph sng c hnhdng, khu vc ph sng v do anten s c cu to khc nhau. C bn loi vngph sng c bn ln mt t ca anten trn v tinh

- Ph sng ton cu: l vng ph sng rng ln nht ca mt v tinh ln mtt. Vi v tinh a tnh c cao bay 35.786 km, vng ph sng c c ln nhtbng 42% b mt qu t, vi gc nhn t v tinh l 17,4o.

Hnh 2.21: Ph sng ton cu i vi mt gc ngng cho v Ph sng bn cu


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-Ph sng bn cu: l vng ph sng cho mt na bn cu pha ng v mtna bn cu pha ty, nu quan st t v tinh, hai khu vc ph sng ny cch lynhau.

- Ph sng vng: khu vc ph sng c th nhiu khu vc khc nhau trn mtt nh vng ng-Bc, vng Ty-Bc... .

- Ph sng du (spot footprin): vng ph sng nh nh cc du in trn mtt, dng thng tin trong mt nc nh hay mt vng ca mt nc ln nh chra trn hnh 2.24. Trong h thng thng tin di ng qua v tinh vng ph sngdu phi k tip nhau v chng ln ln nhau c th thng tin mt cch lin tckhi u cui di ng trong vng phc v ca mng, nh ch ra trn hnh 2.25.Vngph sng c th l trn, elip hay dng tu

Hnh 2.23: Ph sng khu vc v ph sng du

Hnh 2.25: Vng ph sng li

- Gin bc x, mc bp sng ph v c tnh phn cc:+ Gin bc x c quy nh trong khuyt ngh ca ITU-R S.672-4


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+ Mc bp sng ph: khng lm nh hng ti cc vng khc.+ c nh phn cc: Phn cc ng, ngang, trn (quay tri hay quay phi), elp(quay tri hay quay phi).

2.4.3. Cc loi anten c c cc vng ph sng khc nhau anten trn v tinh thng s dng hai

loi chnh anten loa v anten mt phn x vi cc b chiu x khc nhau c tipsng theo cc phng php khc nhau

1)Anten loa: C u dim tin cy v n gin nhng tnh hng km nn cs dng ph sng vi bp sng ton cu

H s tnh hng c tnh theo cng thc:D = 4S/2 ( 2.4)

Trong : S l din tch ca ming loa l hiu sut s dng b mt bng 0,6 0,8

V d: Vi bng tn C, t c vng ph sng ton cu th cn anten loa c

ng knh 30m.2) Anten phn x (reflector): L loi anten thng c s dng nht to rabp sng dng vt v dng hnh th ring r (shaped). Anten ny bao gm mt mtphn x parabol v mt hoc nhiu ngun pht x t ti tiu im ca mt phn x. iu chnh c hng chm sng ca anten trn qu o bng cc lnh iukhin t xa. Vic thay i bp sng c thc hin bng cch thay i pha ca ccphn t bc x. Vic lp t b pht x c t theo kiu ng trc hay lch trc(offset). to ra bp sng dng trn hay elp c thc hin bng cch thay i hnhdng mt phn x cho ph hp vi vng ph sng. Cn to ra bp sng dng hnhth ring r hay phc tp th c th c thc hin bng cch t mt dy cc phn t

bc x ti tiu im ca mt phn x c tip in ca cng mt tn hiu nhng bin v pha lch nhau nh cc mch to bp sng.

3) Anten dy (array): s dng mt b rt nhiu cc phn t bc x to nnmt gc m bc x. Biu bc x ca anten ny c to ra bng cch kt hp bin v pha ca sng c bc x bi dy cc phn t bc x. Cc phn t bc x ct cch nhau 0,6, biu bc x c iu chnh bng cch thay i pha v bin ca tn hiu tip in bng mt dch pha, chia cng sut c th iu khin c.

4.4. Cc h thng b tr. v tinh hon thnh c chc nng quan trng nht ca n l m bo

thng tin lin lc lin tc 24/24 trong sut thi gian sng, ngoi phn h thng tincn c cc phn h ph tr cho hot ng ca phn h thng tin. Cc phn h cngc chc nng v tnh cht quan trng ca n v c lit k trong bng sau:

Hnh 2.27 : Anten phn x

Anten loa hnh ch nht


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Phn h Chc nng chnh c trng biPhn h iukhin qu o vt th v tinh

Duy tr t th c v tinh (t th an ten,cnh pin mt tri) v xc nh ccthng s v qu o

chnh xc v tr quo v vng ph sngca v tinh

Phn h ng c

y To ra gia tc cn thit

Cc loi ng c phn lc

v khi lng nhin liuPhn h o lngt xa, bm vlnh (TT&C)

Trao i cc thng tin lin quan nthng s qu o v tinh, thu nhn cclnh iu khin TT&C t mt t.

S lng knh TT&C,tnh bo mt thng tin

Phn h iukhin nhit

Duy tr nhit cc thit b trn vtinh trong gii hn hot ng cho php

Kh nng phn tn nhit

Khung v tinh H tr lp ghp cc thit b Cc tnh ca cht vt liuch toTrong qu trnh thit k v ch to v tinh, ba tnh cht sau lun phi c

cp n:

- Ti thiu ho trng lng ca mi phn h- Cng sut tiu th ti thiu- tin cy caoa. Phn h iu khin qu o v t th ca v tinh:Chuyn ng ca v tinh c th c chia lm hai thnh phn l chuyn

ng xung quanh tri t c tm t ti tm qu t (a tnh) v chuyn ng niti ca v tinh xung quanh n.

Nhim v: Duy tr cc bp sng ca anten v tinh lun hng ng vvng cn ph sng trong sut thi gian sng ca v tinh.

thc hin nhim v trn, phn h ny phi c kh nng b cc m menxon nhiu lon nh hng n trng thi ca v tinh nh lc hp dn, p sut bcx ca mt tri, cc ng c iu chnh trng thi trn v tinh khi ng khngng b hoc khng cn bng.

C th iu khin bng h thng th ng hoc tch cc. Vi h thng iukhin tch cuc c lin quan n qu trnh sau:

- o t th v tinh so vi cc im chun bn ngoi nh mt tri, tri t,cc ngi sao.

- Xc nh t th v tinh so vi gi tr c nh ngha- c tnh cc lnh iu khin ng c hiu chnh- Thc hin qu trnh hiu chnh bng cc ng c gn trn v tinh- Xc nh t th v tinh sau khi c tc ng ca ng c y.C hai k thut c s dng iu khin trng thi l:

- n nh theo kiu con quay: V tinh c ch to hnh tr sao cho thn v tinhcn bng xung quanh trc ng ca hnh tr. i vi v tinh a tnh trc c iuchnh song song vi trc Bc-Nam ca qu t, tc quay ca thn l 4060vng/ pht


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- n nh theo kiu ba trc: Thn v tinh c nh so vi tri t. T th v tinhc biu din theo cc tr l trc lch hng (yaw), trc quay (roll) v trc cao(pitch) ca h to c tm t ti trng tm ca qu v tinh.

b. Phn h ng c:Nhim v: to ra lc y, m men xon dng iu khin qu o cng

nh t th ca v tinh.- ng c y cng sut nh c vi millinewton n vi newton c dng

iu khin t th v qu o v tinh.- ng c y cng sut trung bnh c vi trm newton n vi ngn

newton c dng thay i qu o trong qu trnh phng v tinh

Hnh : Cc trc n nh ca v tinh

qu o bay v

trc quay trongmt phng qu o

qut

B

N

trc cao

n nh mthn u o

tr

nh


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c. Phn h cung cp nng lng in:Nhim v: Bo m cung cp ngun in cho v tinh hot ng n nh lintc 24/24 gi trong sut thi gian lm vic ca v tinh.- Ngun cung cp chnh l h thng pin mt tri- Ngun cung cp d phng l h thng cquy cung cp cho v tinh

trong thi gian v tinh b che khut.VD: V tinh Intelsat VI c cng sut h thng 2 kW sau 10 nm hot ng,din tch 59 m2 v trng lng 250 kgd. Phn h o lng t xa, bm v lnh (TT&C):Nhim v:- Thu cc tn hiu iu khin t di mt t thay i trng thi hoc

phng thc hot ng ca cavsd thit b trn v tinh thng qua tuyniu khin t xa (TC).

- Pht cc kt qu o, thng tin lin quan n hot ng ca v tinh, hotng ca cc thit b v cc kt qu thc hin cc lnh iu khin di

mt t thng qua tuyn o lng t xa (TM)- Cho php o khong cch gia mt t v v tinh, tc hng tm

xc nh thng s qu o.- Cung cp cc tn hiu chun cho cc trm mt t phc v cng vic bm.e. Phn h iu khin nhit:Nhim v: duy tr nhit ca v tinh trong mt gii hn cho php

cao ca qu o a tnh l chn khng. Nhit trung bnh ca v tinh do hpth trc tip nng lng mt tri, s tiu tn in nng ni ti Q, v s bc x nhitvo khng gian. Cc yu t ny ph thuc vo tnh cht b mt ca v tinh, v hnh

dng ca n. Cng sut bc x mt tri u vo bng tch ca hp th b mt vidin tch c chiu sng, v mc chiu x ca mt tri SC (gi tr trung bnh =


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1370W/m2). Cng sut bc x u ra ca v tinh bng tch h s pht x , tng din

tch b mt A, hng s Stefan-Boltzmann (=5,6703.10-8 W/m2.K4), v lu tha bc 4ca nhit tuyt i. S cn bng nhit gia cng sut hp th v cng sut bc xch ra hnh 2.11

Trong iu kin bnh thng, mt cng sut bc x ca mt tri u vo, stiu tn in Q phi bng cng sut bc x u ra ca v tinh Nhit trung bnh cav tinh T s l:

.T4 = (a/A).SC + Q/A (W/m2 ) (2.2)

Nhit trung bnh ca v tinh ph thuc vo t s h s hp th b mt trn

kh nng pht x , t s din tch chiu sng a trn tng din tch b mt A, v s tiutn bn trong v tinh. iu khin nhit trung bnh ca v tinh, cc c tnh nhit(pht x v hp th nhit i vi nh sng mt tri) ca cc b mt khc nhau phi cbit ch . C th thay i nhit ca cc b phn trong v tinh bng cch thay is kch thch nhit (thermal coupling) v s dng cc b nhit in. Mt vi nhim v

v nhit quan trng hn trong mt v tinh thng tin l bo qun ngun pin trong giihn nhit t thay i, gi nhin liu ca b phn y trong cc bung lnh, v tonhit hu ht nhit pht ra bi cc b khuch i cng sut.

f. Khung v tinh:Cu trc v tinh c thit k nhm bo m h tr v c kh cho tt c cc b phnca v tinh, bo m s ng chnh chnh xc ni cn thit v h tr cho iu khinnhit. Cu trc c quyt nh bi mt s iu kin bt buc. Cc gii hn phngtin phng, hnh dng, kch thc ca cu trc cng nh khi lng tng. H thng

iu khin t th thng xuyn gii hn s sp xp khi lng. Nhit b nh hngbi tnh cht b mt v dn nhit. S ng chnh anten, b cm bin, v b phn

T4 = /.(a/A).SC + Q/AHnh 2.11: S cn bng nhit ca v tinh

AT4

u rauvo

aSc

bc xnhitbc x

mt tri

Qtiu tnbn trong


46/68

http://www.ebook.edu.vn Trang: 45y yu cu cng trong cu trc hoc cc b phn ca n. Cui cng iu quantrng l d dng xm nhp trong khi tp hp, t hp, v kim tra.

Cu trc cn phi chu ng cc ti trong khi o th mi trng, iu khin mtt, phng ln qu o thp, s t chy vin im v cn im, v bt k s trinkhai anten hoc cc dy tm pin mt tri. Trong khi hot ng trn qu o cc ti lnh nht, nhng yu cu chnh xc phi cao hn. Bng 2.1 ch ra tm tt khilng ca mt v tinh thng tin.

Khi lng tng ca cc v tinh c th khc nhau, nhng t l ca cc phn hth tng t. Ngoi phn h thng tin, cc b phn quan trng khc trong khi lngtng bao gm nhin liu, cu trc, v ngun in. Nhiu v tinh thng tin c cc bphn y v nhin liu a v tinh ln qu o ti vin im (ng c kch hot).iu ny chim mt na khi lng tng ca v tinh trn qu o chuyn giao, trckhi s dng ng c kch hot vin im.

Bng 2.1: tm tt khi lng ca mt v tinh

Tn phn h Khi lng (kg)

Thng tin

iu khin v quyt nh trng thi

Ngun in

y qua li

o lng t xa, lnh, o c ly

Cu trc/nhit

Ton b in v c kh

Cc b phn khng chy ca ng c kch hotTng khi lng con tu khi s

dng ht (EOL)

Nhin liu/p sut

Tng phi thuyn khng gian (BOL)

B phi hp

Cc b phn ca ng c kch hot

Khi lng gi tr

Tng khi lng trn qu o chuyn giao

233

73

141

39

26

184

69

61

826

1871013

21

871

24

1929


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http://www.ebook.edu.vn Trang: 46Cc t vit tt: EOL (end of life): ht tui th; BOL (beginning of life): bt

u tnh tui th

Nhiu lc tc ng ln v tinh trong khi phng. Khi u tp m n rt ln.Khng kh do ng c tn la b t chy s pht ra tp m n rt ln. Khi lng tnla phng gim, gia tc tng ln vi ln so vi lc hp dn. Nu v tinh 1000 kg khi nng ln nhiu nghn kg. S rung ng ca nhiu tn s c pht ra thng qua gi phi thuyn khng gian t cc ng c tn la. Cc thit b pht ho dng tch cctng v bt u cc trin khai, gi cc va chm nh qua cu trc.

Hnh 2.10: Cu trc ca v tinh thng tin

Cu trc mt v tinh c ch ra hnh 2.10. V ngoi thit b thng tin, bnh chanhin liu, ngun pin d phng v.v...

Yu cu phi c cu trc chc chn cn thit gia cc ti v s tip xc tn laphng. Cu trc v c bn ch ra trn hnh l hnh vung vi cu trc anten tch ri gi cc b tip sng RF v mt phn x (gng). B tip sng c c nh ti v trca n, nhng cc mt phn x m ra sau khi phng. Cc dn pin mt tri gm cctm c gp li khi phng, v m ra sau khi phng


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Chng 5:Trm mt t v cu hnh mng VSAT. (8 tit)

5.1. Nhim v, chc nng trm mt t.Trm mt t (SES: Satellite Earth Station) l tip nhn cc lung tn hiu didng s hay tng t t mng mt t hoc trc tip t cc thit b u cui cangi s dng, x l n v pht ln v tinh tn s v mc cng sut thch hp chos hot ng ca v tinh. i vi trm mt t thu, thu cc sng mang trn ngxung ca v tinh nhng tn s chn trc, x l tn hiu ny trong trm chuyn thnh cc tn hiu bng gc sau cung cp cho mng mt t hoc trctip ti cc thit b u cui ca ngi s dng. Mt trm mt t c th c khnng thu v pht lu lng mt cch ng thi hoc trm mt t ch pht hay thu.5.2. Cu hnh trm mt t.Mt trm mt t c th truy nhp b pht p v tinh theo nhiu phng php khc

nhau (FDMA, TDMA, CDMA...) cho nn cu hnh cho tng loi a truy nhp cngkhc nhau.

Mt trm mt t ln bao gm bn khu vc chnh: phn h thng tin, phn h truyndn trn mt t, phn h ngun cung cp v phng iu khin,

Tn hiu t mng mt t a ti qua b u ni n b ghp knh s (TDM) hay ghpknh tng t (FDM), n b a truy nhp phn ra cc lung khc nhau a ncc b iu ch s (thng l PSK) hay l iu ch tng t (FM). Thng l iu ch

tn s trung gian gi l trung tn (70 18) MHz hay (140 36) MHz. Sau b iuch s l trung tn c iu ch bi bng tn c s, tn s trung tn c a n

b bin i tn s ng ln (U/C), nng tn ln tn s bc x (RF). in hnhbng C nng ln 6 GHz. Tn hiu cao tn iu ch sau cc b nng tn c an b kp hp thnh mt bng tn chung vi nhiu sng mang v rng bng ln

sau a n b khuch i cng sut c cng sut ra ln, gi l b khuch icng sut cao (HPA). Trng hp tn dng c h s khuch i ca b khuchi cng sut cao m khng gy nhiu do iu ch th mi b khuch i ch khuch

H thngphn phi

B

LNA

Bin i

xung

K.i

IF

iu

ch

Gii.Ch

Thit b atruy nhp

Ti bghpknh

bHPA

Bin iln

K.iIF

Thit b a truy nhp,iu ch v gii .ch

My thu tpm thp

My pht cng sut ln

Thit b

bm

Thit b an ten vbm


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http://www.ebook.edu.vn Trang: 48i mt sng mang, lc t mi b nng tn s a n b khuch i cng sut caotng ng, sau mi sng mang c khuch i ln cng sut theo yu cu sc a n b kt hp qua b lc phn hng a ra anten pht ln v tinh, nh

ch ra trn hnh 3.3

Khi thu c bng tn thu s c anten cm ng vi mc cng sut rt thp do c lythng tin ln, bi vy tn hiu thu c t anten sau khi qua on ng dn sng n blc phn hng (D) qua b khuch i tp m thp (LNA) nng mc tn hiu thu

c ln khong 40 dB 60 dB, v phi c mc tp m nh tp m tng tng lnkhng ng k. Bi vy LNA t cng gn anten cng tt gim nh nht tp m

a vo h thng. LNA s khuch i ton b bng tn thu, sau

a ti b chia phn ra cc bng tn con ri a ti b bin i h tn (D/C) xung trung tn (IF) 70MHz hoc 140 MHz, sau a ti cc b gii iu ch (DEMOD) ly ra tn hiubng tn c s, n thit b x l tn hiu, qua thit b phn knh n thit b u nira mng mt t i ti ngi s dng.

Nguyn l hot ng ca cc trm mt t trong h thng thng tin v tinh lnh nhau, nhng cc ch tiu k thut, bng tn cng tc, h s phm cht G/T v ccng dng ch yu th c cc yu cu khc nhau. Da vo Intelsat phn loi cc

trm mt t t A n Z.5.3. Anten trm mt t.a. Nhim v anten trm mt t

Trong thng tin v tinh, anten trm mt t ng vai tr quan trng. Nhim v chungca anten l bin nng lng cao tn ca my pht thnh sng in t bc x v phaanten thu ca v tinh phn pht v thu sng in t trn ng xung a vo uvo my thu.

b. Cc yu cu i vi anten trm mt tCc yu cu i vi anten trm mt t theo khuyn ngh 390 ca CCIR (CCIR rep -390) l:

HP

bkthp

U/C

U/C

U/C

MOD

MOD

MOD

bkthp

U/C

U/C

U/C

MOD

MOD

MOD

HP

HP

HP

HPA khuych i nhiu

sng mang

HPA khuych i mt

sng mangHnh 3.3: HPA khuych i mt v nhiu sng mang


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- H s tng ch v hiu sut cao- Tnh hng cao, bp ph nh khng gy can nhiu ln cc h thng vi ba

khc- c tnh phn cc tt s dng cc dng phn cc khc nhau khi s dng

li tn s- Tp m thp, phi gim tp m xung mc thp nht c th c. Mc tp

m ca anten ph thuc vo nhiu yu t nh gc t, v tr t, hng v cuto anten

t c cc yu cu trn, trm mt dt thng s dng cc loi anten mtphn x cn gi l anten gng.

Anten gng parabol: nguyn l cu to gm mt mt phn xcong theo ng cong parabol, lm bng cc vt liu c h s phn x cao (Rpx1),thng bng nhm hay hp kim ca nhm, mt phn x phi nhn sng phn x

khng b tn x. Ti tiu im ca gng parabol t mt ngun bc x s cp(thng l mt anten loa: feed horn ) gi l b chiu x, sao cho tm pha ca b chiux trng vi tiu im ca gng, nh ch ra trn hnh 3.4

Khong cch t tiu im F n nh gng O gi l tiu c f, trc i qua nhgng v tiu im gi l trc quang (trc ox), nu gng parabol trn xoay th ngknh ming gng L c gi l khu (aperture).

Theo tnh cht ca gng parabol, cc tia sng xut pht t tiu im ca gngri phn x t mt gng s tr thnh cc tia sng song song nhau v c tng ngi t tiu im n mt phn x ti ming gng l bng nhau v bng mt hng s f+ h ( y h l su ca gng). Nh vy nu ngun s cp t ti tiu im gngbc x sng cu th sau khi phn x t mt gng s tr thnh sng phng. Bi vy

anten gng parabol c bc x n hng, vi tnh hng hp h s tng ch cao.

L F xO

r

f

Hnh 3.4: Anten gng parabol


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http://www.ebook.edu.vn Trang: 50Anten gng parabol c cu to n gin nht v gi thnh thp nht trong cc

anten dng cc trm mt t trong thng tin v tinh. N c nhc im b chu xt xa nh gng nn h thng b chiu x c kt cu phc tp, cng knh, cngvi b chiu x s chn i mt phn sng phn x t gng, gy ra hiu ng che ti

lm mo th tnh hng, tng bp ph v lm gim hiu sut ca anten. Fide tipsng cho b chiu x di gy nn tn hao v tp m ln. Do anten gng parabolkhng c s dng cc trm mt t thng thng, m ch c s dng ch yu cc trm thu v cc trm nh, dung lng thp.

Anten hai gng (anten Cassegrain)Nguyn l cu to gm hai gng, mt gng chnh vi ng knh ln l

gng parabol, mt gng ph nh l gng hypebol, c t sao cho tiu im cahai gng trng nhau ti F

1, nh ch ra trn hnh 3.5

B chiu x c t sao cho tm pha trng vi tiu im ca gng ph o

hypebol F2. Do tnh cht ca gng hypebol l hiu ng i ca tia sng t tiu imca gng o n mt phn x vi ng i t tiu im ca gng tht n mt phnx l mt hng s v tnh cht ca gng parabol nh ni trn, nn cc tia sngxut pht t b chiu x (c ngha l t tiu dim ca gng ph o hypebol, phn xln th nht ti gng ph Hypebol, ri phn x ln th hai ti gng chnh parabol str thnh cc tia sng song song vi nhau v c on ng i n mt phng songsong vi ming gng l hng s. Do ngun s cp bc x sng cu sau khi phnx t hai gng s tr thnh sng phng. Nh vy, anten Cassegrain cung c tc dng

nh

anten mt g

ng parabol, nh

ng n c

u im kch th

c theo h

ng trc quangngn hn so vi anten gng parabol (ngn hn mt on bng tiu c ca gnghypebol), b chiu x t gn nh gng parabol hn nn gi n n gin hn v

gngh H

gngchnh

FoF

z

Hnh 3.5: Anten hai gng Cassegrain


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http://www.ebook.edu.vn Trang: 51fide tip sng s ngn hn do tn hao v tp m s nh hn. Bi vy antenCassegrain c s dng ph bin cho cc trm mt t thng thng v vi cc antenc kch thc trung bnh v ln

Anten lch (offset antenna)Cc anten mt gng parabol v anten hai gng Cassegrain c mt nhc

im chung l b chiu x hay gng ph t thng hng vi nh gng lm chnmt b phn cc tia sng phn x t gng chnh parabol gy ra mt min ti phasau gng lm gim h s tng ch, hiu sut v tng bp ph. khc phc nhcim ny ngi ta s dng anten lch ngha l b chiu x c t lch ra ngoihng ca cc tia phn x t gng parabol, nh ch ra trn hnh 3.6

Cc anten lch c s dng trong trng hp yu cu cht lng cao nhkhicn phi gim can nhiu t cc mng vi ba trn mt t, hoc t cc v tinh khc nmgn nhau trn qu oc. c tnh phn cc

Nh nghin cu mn hc L thuyt trng in t, sng bc x ra t anten gmhai thnh phn in trng E h t trng H, vung gc nhau v vung gc viphng truyn lan. Theo quy c, khi xt phn cc ngi ta xt s bin thin ca vct cng in trng E. Phn cc c c trng bi cc thng s sau:- Phng quay ca vc t E: nu nhn t hng sng truyn lan i thng vo ngi

quan st th vc t E c th quay theo chiu kim ng h (t tri sang phi) gi lquay phi, hay quay ngc chiu kim ng h (t phi sang tri) gi l quay tri.

- T s trc (AR: Axial Ratio): l t s di gia trc chnh v trc ph ca hnhlp do u mi vc t E trong qu trnh bin i to nn:

AR = Emax/Emin.+ Nu Emax = Emin th AR=1 l sng phn cc trn+ Nu Emin = 0 th AR= l sng phn cc thng+ Nu Emax Emin th AR c gi tr bt k l sng phn cc lp

Hnh 3.6 : Cc anten lch (offset antenna)

anten parabol lch anten Cassegrain lch


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5.4. Bm v tinha. Ti sao phi c h thng iu khin bm

V tinh a tnh v nguyn l l ng yn khi quan st t mt im c nh trnmt t, nhng trong thc t n lun lun chuyn ng theo mi hng do tc ngca nhiu nguyn nhn khc nhau nh lc hp dn ca mt tri, mt trng v cc hnhtinh khc, tc ng ca lc bc x nh sng mt tri v trng hp dn ca qu t,lm cho v tinh trt theo cc hng Nam-Bc v ng-Ty so vi v tr nh trc,

gii hn cho php l 0,10. Hng nm mt phng qu o v tinh nghing i mt gckhong 0,9o theo hng Bc-Nam so vi mt phng xch o Vic duy tr v tinh vtr ban u, vi chu k hiu chnh c thc hin bi lnh pht i t trm o lng txa, bm, iu khin v gim st (TTC&M: telemetry, tracking, control andmonitoring) t trn mt t.

Vng cho php v tinh dao ng trong m khng cn iu chnh v tr v tinhgi l ca s gi trm (station keeping window)

Kch th

c ca s cho php v tinh chuyn ng t do trong c di cacung cc hng ng- Ty v Bc-Nam chn gc tm qu o l 0,1o, theo caothp l 0,001o, tng ng vi di cc cnh ca s l 75 km v 85 km theo cao.

Hnh 3.18: S nghing ca mt phng qu o a tnh

Qu o a tnh ng

Qu o b nghingBc

Na

x

E

E

anten

hngtruyn lan

Ex

P

T s trc (AR ) = Emax/Emin

Hnh 4.6: Tnh cht phn cc ca sng in t


54/68


Do s chuyn ng ca v tinh nn vi cc anten trm mt t c bp sng hpth khi v tinh chuyn ng lch khi hng thu cc i mc thu s b gim xungng k. bo m mc thu tt anten trm mt t phi c h thng iu khin bm

theo v tinh (h thng tracking)b. Cc h thng t ng iu khin anten bm v tinh:- Bm xung n (monopulse tracking)- Bm tng nc (steptrac tracking)- Bm theo chng trnh/b nh (program/memory tracking)- Bm bng tay (Manual Tracking)

Bm bng xung ny l thut ng thng dng trong k thut rada ch li gc do hng thu

lch s nhn c mt xung n. Khi hng thu tn hiu t v tinh n ng hngthu cc i ca anten (hng thng gc vi b chiu x), th trong ng dn sng ca

b tip sng s xut hin loi sng TE10, khi hng thu b lch th trong ng dn sngs xut hin loi sng bc cao hn l TE20 (nh ch ra trn hnh 3.20) . Pha ca loisng TE20 s thay i ph thuc vo hng ca gc lch t my thu iu khin bms ra lnh ph hp quay anten

Hnh 3.20: Cc loi sng thu c cc hng khc nhau

Cc loi sng bc cao m h thng bm thu c s to ra tn hiu li a ti

my thu ca h thng iu khin x l. Cui cng in p li s

c

a ti hthng tr ng (serve system) iu khin anten cng nh hu b in p li

0,1o0,1o

N

85 km

75 km

75 km

B

Hnh 3.19: Ca s cho php v tinh chuyn ng t do

mt phng

xch o


55/68


H thng iu khin bm theo chng trnhKhi cc v tinh qu o nghing xut hin th h thng iu khin bm theo

chng trnh c a ra

Anten c iu khin bi my tnh da trn s liu d on cc v tr v tinhdo Intelsat cung cp cho mi v tinh v cc trm mt t c lin quan, theo mt chu knht nh thng l hai tun mt ln. S liu d on v tr v tinh c ci t vophn mn ca my tnh iu khin, t bin i thnh cc lnh iu khin antenbm theo v tinh tng thi gian nht nh

H thng bm tng ncH thng t ng bm theo xung n c nhc im l m t iu khin anten

lm vic lin tc nn chng hng

Nguyn l chung ca h thng bm tng nc sau khi nhn c tn hiu iukhin bm v tinh (hay tn hiu hng dn) pht i t v tinh mc khng i trongbp sng ton cu. Mc tn hiu thu c so snh vi mc mu c quy nh, nunh hn mt gi tr quy nh no th anten s c lnh quay mt gc ban u.Bng cch so snh mc tn hiu thu trc v sau khi di chuyn, hng di chuyn tiptheo c quyt nh. C ngha l nu mc tn hiu c chiu hng tng ln th antentip tc di chuyn theo hng c, nu mc tn hiu c chiu hng gim th hng dichuyn ca anten s quay tr li. Cc x l ny lin tc v thay i lun phin gia haitrc vung gc vi anten.

u im ca anten bm tng nc l b phn m t quay anten khng lm viclin tc nn t b hng, nhc im l t khi anten hng ng cc i vo v tinh doh thng ch lm vic khi mc thu gim n mt gi tr quy nh

Bm bng tay (Manual Tracking)Ch nhng anten c ng knh ln l cn thit phi c h thng bm t ng.

Khi ng knh anten gim yu cu ny khng cn thit lm. Trong thc t i vi cc

anten c kch thc bng hoc nh hn 8m th bm t ng l khng cn thit m chcn iu chnh bng tay hng tun hoc hng thng bi v rng bp sng an tenln. oi chnh ny c th thc hin bng cch lm cho cc chuyn mch ph hp vim t gc phng v v gc ngng, hoc dng bin php c kh quay trc tipanten. iu khin bm bng tay cng thng c s dng nh l mt bin php dphng cho cc loi iu khin trn.

5.5. Cu hnh trm VSAT.a. S lc:

Nh s pht trin ca thnh tu khoa hc cng ngh, ch to ra

c ccb khuch i c cng sut ln, cc b khuch i c tp m nh, cng vi s rai ca cc trm v tinh c cng sut khuch i cao. Nn a ti s pht trin


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ca cc trm v tinh c khu an ten nh VSAT (VSAT: Very Small ApertureTerminal). H thng thng tin VSAT c s dng u tin (1980) ti M.

H thng VSAT cho php cc trm mt t c kch thc rt nh, gi thnhthp, ng knh anten c th nh t

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