Hpi nghi Khoa hpc fc^nipn 40 ndm ngay thdnh l4pVipiHdn lim KH&CN Vift Nam n&t ban Cdng nghf thdng tin. Dipt tit, TV dpng hda vd Cdng nghf vS tnf
Ha N&i, 7/10/2015
Tur nghien cuu ca ban din umg dung - goc nhin thuc te trong truyen songHF
Hoang Th5i Lan'", Vlnh Hao^ T5ng VSn "niit*, Nguyin Xfit Thinh^, Ph^ni Vfin Haf, Nguyin l ^ a n Hikng^, V5 Quang Kbi'
' Vifn Vgt ly TP H6 Chi Minh, Kfn HLKH & CN Vtft Nam ' Vifn Nghien ciiu & Ohg dyng Cong ngh4 Nha Trang. Vifn HLKH & CN Vift Nam
^Ddi Thong tin duyen hm Vung Tdu Trung tdm qudn ly tan so khu vyc 7 Ddi Thdng tin duyen hdi Nha Trang
*Email lien Itfc: thailanl64@gmaiLcom
Tii^ difn ly cCia Tr&i dit \k m6i trubng t&i nguy&i qu;^, n6 cung d p cho chiing ta khi n&ng tniy^ thSng tin nhanh ch6ng vk ti ti^ v6i q ; ly xa. T h ^ tiln ^ chihig minh r ^ g , h$ th6ng thdng tin lien 1^ v6 tuyin tan s&
cao (HF) h09t d$ng khfi hi^u qua trong viing xlch d ^ t£r do nhSng d|c d i ^ (fi$n ly ci^ viing n&y. T4ng di^n ly 6 khu vi^c Nam bp da du^c quan trie li&i tyc t^i TP. nh Chi Minh trong th£ri ^an hon m$t diu k;y hQst d$ng cua Mjt trM vfia qua. Cic d$c tnmg dja phuong vk nhitng quy lu§t b i ^ d6i cOa d i ^ ly b Nam b$ cQng da d u ^ n^gn clhi qua nhihig s6 Ii$u quan trie dugc
Vift Nam I^ m^t qu6c gia biSn voi ba bien dii hon 3200 km vk kinh tk b i ^ dupe xdc djnh Ik m$t nglnh kinh tk mui nhpn. Do v^y, thdng tin WSa l^c vin c6c phirong t i ^ fao^ d^ng tr&i b i ^ ddng m$t vai tr6 vd cfing quan tryng. Vdi ckc kk qak dieu tia vk n^i£n cihi co b&n dfit du^ic, nbdm nghi&i ci>u dS tiln h ^ thCr nghi|m dua vk qu& nghiSn cihi vl d i ^ ly vCuig xich d ^ tir cCa Vi^ Nam vto ihig dyng trong truyln sdng HF. T^y nhien, t^ kk qua dilu tra vk n^ifin cihi co b ^ vl mdi trudng d i ^ ly d$c thO cila dja phuong din v i ^ tilp c$n
>hi;c tl dl dua kit qui nghi£n cihi vao ihig d|;ng vln odn Is ck m$t quing dir6ng v6 ciing khd khSn.
Ttr gdc nhin thyc tl, b ^ cdo nky ph&n tich m$t cdch xuySn su6t lodn bd qui trinh tijr quan trie din nghi£n eihi CO b&n, qud trinh ch^ I9C thdng tin vk dm hilu quy Iu|t cua tvr nhite din tilp c$n thuc l l vdi cdc dU thdng tin diiy£n hjj.,. Tr£n ca sd dd, bdo cdo trao d6i quan dilm ciia nhdm nghi€n ciiu gim cdc nhd khoa hpc, chuySn
^a k; thu$t vd nhd qudn 1;^ vl ^di phdp ling dyng cdc kit qud nghi€n cuu t^i Vien H ^ Idm sao cho phu h^p vdi thyc tr?ng dilu kif n k j thugt va cdng nghf hifn nay it ^^ft Nam.
Titkhda. Votuyindiin: Tin s6 cao (HF); TSng diinfy vimg xich tir. Dm thdng tin duyinhM.
H^ Uiong th6ng tin v6 tuyIn di§n (VTB) s i dyng bSng t ^ s6 tilr 3 - 3 0 MHz, c6 birdc song tir 80,
^0, 30, 20, 17, 15, 12 vk 10m gfli \k hf th6ng thong tin VTD s6ng ngSn hay h^ tfiong thong tin HF {High Frequency).
S6ng HF (3 - 30 MHz) dugc sir dyng nhilu trong thong tin hang hSi, hibg khfing nh& la voi cdc lifin 1 ^ t6c do thap, khoang cSch xa (hang trim ho$c hang nghin km). T n ^ l n s6ng HF phy thupc vho
^ g th^ ting diln ly f^a phuong. E)e khai thdc cd h i ^ qua t ^ g di^n ly phyc vy cho thdng tin lign
l9C c ^ phai hilu r5 cac quy lu^t biin thiSn, cac qui tritUi v|t I^ diSn ra ttong mdi tnrdmg khi quyln niy. Cflng v<H cdc md hinh IJ thuylt, cdc ket qua quan trSc da tiing budc giai ttifch c^u tnic vi qui lu§t bien 6&i dia (Efn ly vi cho phfip ra dM m^t ITnh vvtc mdi 14 dy bio tigng thai ting iB|n ly phyc vy cho vipc trt^ln sdng vfi tuyen.
Ngiy nay, m$c dii thong tin vd tuyIn bing tSn HF di 6uiyc thay thi 6 m$t so djch vy bing cic hf thong thdng tin v$ tinh, c ^ biin nhung nd vln li mpt phuong ti§n khdng thi thi^ trong phit thanh cy ly xa vi mpt s6 d^ng thdng tin d$c bi|t sur dyng trong an ninh qu6c phdng. Thdng tin sdng ngin phin X9 tii tlbg d i ^ ly cd Ipi thi li don giin, dl di d$ng, ri tiln vi khdng phy thu^c vio ngtrdi cung c ^ djch vy nhu thdng tin v$ tinh. Vi ngudi ta di nlun thiy nhChig triln vpng md r$ng ihig dyng cua bing tin sdng ngin trong nhieu linh v\rc kinh tl - x3 h§i. H^ thdng thdng tin sdng n ^ rat ph6 biin tr£n thi gidi vl cic dtrdng truyen &yc hi^n dupe cho khoing cich xa vdi gii thinh ti. W v$y, thdng tin sdng ngin hi|n nay vln dir^ t^ ci cic nude tfch eye khai thac sit dyng [1 ].
0 \i^ Nam,vi|c sii dyng phin x^ vd tuy^ tCt tang di|n ly cho cong tic thdng tin li&i 1^ d nude
ta di dupe thyc hi|n tiJr nhOng nim I960. Tuy nhiSn, trong cdng tie tn^ln tin sdng ngin, cin bp k^
thu|t thudng g$p khd khin vl tfnh bit 6n ciia (fi^ ly. Do chua cd m$t co sd dd li|u quan trie lien tyc, lau dii d cic dii ti^m dilu tra co bin ve tang d i ^ ly vi nhiing quy ]u$t dja phuong cy thi ciia Yi^
Nam chira dupe nghien cuu siu ndn vi|c thilt i&p dy bio chinh xic dl phyc vy cho tniyin sdng HF d Vift Nam hipn chua dupe triln khai bii bin. Bk nude ta cd mpt bd biin dii ban 3200 km, kinh tl biin li i^inh kinh tl dupe uu tiSn. Hi$n nay, s6 lirpng cic phuong ti|n nghe ci cua ngu din Vi^ Nam li r^ Idn vi trSn da si tiu thuyen thi vi?c trang bj cic thilt bj iifin 1^ bi§n dgi dit tiln cdn At h^n chi.
BSn c^nh nhihig no lyc vuon tdi trang bj thilt bj thdng tin lien lac hi|n d^i tht vi|c tim giii phap ti^
ki|m thfch hpp trong dieu ki|n kinh tl h i ^ nay ciia ngu din li vipc rit cln thilt
0 khu vvc phia Nam, d^ quan trie khf quyen Hdc Mdn (TP. Ho Chf Minh) dupe xSy d\mg nim
1999 vi thu thfc s6 Ii|u d i ^ ly lien tyc hon m§t chu kj MJt triii tit nim 2000 din nay. Cic kit qui nghi&n ciru tii so lipu quan trie dupe t^ dii Hdc Mdn <S thi hi|n nhOng quy Iu|t d$c trung cua di$n 1)^
v4ng xich d^o tii Vi§t Nam [2]. Myc tieu cua chiing tdi la tim hilu th\rc te thdng tin liSn I ^ HF d m i ^ Nam, thii nghifm dua cic kit qua nghiSn cthi dilu tra co ban vl tang (B^n ly vio ting dung dif bios thdng tin lifin I^c sdng ngin giita dit liln vi tiu thuyen dinh bit hii sin trong vCmg d$c quyln kinh tij biin Vi?t Nam dl giim gii thinh dau tu thilt bj lien l^c, nhim dem l^i hi|u qui thilt tinrc cho ddngl dio nguiri din bim biin khai thic hii sin vi cic ilng dyng thyc tl khic ciia Vi§t Nam. i 2. Kit qui nghi£n cihi cu bin vl cic Idp di|n ly khu vve miln Nam Vi^t nam 1
•I
2.1. B^c tnmg ciia cic Idp dlfn ly f2J: ]
Vimg khdng gian tii dp cao khoing 50 km din hon 500 km cua Trai dat, cic phSn tii khdng khi dj
diy dupe biic x? Mit trdi ion hda vi t?o thinh m^t viing khi quyln chiia ion gpi li ling di|n ly. C ^
nhi chuy£n mdn chia tang di|n ly ra lim 4 Idp la Idp D, lop E, Idp Fl vi lop F2. ^ L*p D li lap t h ^ nhit, nim d d§ cao khoing 60 - 90 km. Diy li Idp khd quan trie vi cQng li l<ip]
phiic tgp nhIt tren phuong di|n hda hpc do ip suit kha cao vi do nhilu nguSn ion hda khic nhau inU hudng ding kl d&i sy t?io thinh ion. Ldp D h ^ thy ning lupng cila hlu h ^ cic sdng cd tin s6 cao.
^ Ldp E nim tilp giip vdi ldp D, bit dIu tfr d§ cao 90 km vi k6o dii din dp cao khoing IIO km
T ^ s6 tdi h^n cd gii tri trong khoing 2,6 - 4,3 MHz. U>p niy xult hien sau khi M?t trW mpc, bie
mit sau khi Mjt trdi lin vi ddi khi xuit hi^n rat ylu dt vio ban d6m. Trong 1 ^ E, d dp cao khoing 9
- 120 km, thudng xult hi?n nhiing khu vyc cd sy ting ion hda bit thudng vdi nhOng vCing rdi r^c a
m$t d0 cao. Diy li ldp E dj thudng (Sporadic E - dupe kj^ hi|u la Es). Ldp Es thdng thudng li mp
viing r^ng khoing 0,6 - 2 km, tbn tai t ^ thdi vdi mat dp d i ^ tii tuong doi cao. Hinh d?ng ciia nd cd
liic li mOt ldp phing vi dong nhit tiieo phuong ngang, cd liic lai gjong nhu nhtJng dim may r^ng^
Tit nghien cuu ca ban din img dyng - goc nhin thuc te trong truyen sdng HF
(Icich c5 2 - 100 km) v i di chuyin theo phuong ngang vdi vfn tdc khoing 20 - 130 m/s, rit khd xac dinh nd xuat hi^n a dau, khi nio v i ton t ^ bao liu. Ldp Es co thi phan X9 tin hieu vd tuyen d khoing tin so phan xa ciia Idp F. Es ddi khi frong su6t v i cho p h ^ hiu hit cic tin hi?u xuyfin qua, v i ddi khi
^ cfai trong suot timg phan.
Ldp F l nim d dp cao khoing 190 - 250 km. Hinh thinh vio khoang 7 gid sing v i bien mat vio khoang 4 - 5 gid chieu. Tan s6 tdi ban d khoing gii tn 4 - 6 MHz, d^t eye dai vao gid tnra. Phin tich chi tilt so li§u quan trie, ta thay die trung m i Idp Fl thi hi?n d thinh pho Uh chi Minh la sy biin ddi mau Ip hong ngiy v i ludn thay ddi tit ngiy niy qua ngiy khic.
Ldp F2 nam d d§ cao khoang 250 - 380 km. Trong biin trinh ngay dem, eye tiiu ciia m|t d$ di^n ly ldp F2 quan trie dupe vao trudc binh minh (khoang 4 gid sang), eye dai thii nhIt vao 8 - 9 gid sing, tilp theo l i m^t sy giam nhp vao budi trua v i dat cue dai thur hai v i o khoang 4 gid chilu. Tin so tS l^in foF2 trung binh trong khoang 2,5 - 11,5 MHz. No ton t ^ suot ngiy dfim, do dd, F2 l i ldp quan trpng nhat doi vdi truyen sdng v6 tuyen tin s6 cao,
D$ cao cua F2 thich hpp cho cac dudng truyen dii nhIt v i nd khiic xa cic tin sd cao nhit trong vLing HF. V I vgy, ndi din truyln sdng HF chii yeu la ndi den phin x j tir ldp F2. Tuy nhifin, Idp F2 cd cic biin thien phirc t ^ v i sy phy thupc chat chS vao cic boat d$ng ciia M^it trdi (hinh la, lb).
ih hE. h f 1 Ji h F ; eoe rnin^ binh foE. l e f 1 * l o H a
Hinh I a: D$c trung dp cao ciia cic ldp di^n ly quan trie t ^ TRHCM
Hinh lb; E>Jc trung tin so ciia cic ldp di^n ly quan frlc t ^ TP.HCM 2.2. Tinh cong thuc Ihffc nghifm tir so lifu quan trie cho du boo truyen song:
Quan trie thuOc v l cdng t i c dilu tra co ban, li m^t cong vipc thirdng xuyen v i lau d^. Trong nghien cuu v|t 1^ dja cau thi chuoi s6 lifu phai dii dai mdi co thfi riit ra dupe cac quy lug.t (it nhat mpt chu ky ho^t dpng ciia M^t trdi). D I tai sii dyng so li^u tham do thang dimg ting dien ly bang cic may lonosonde t ^ TP. Ho Chi Minh v i Nha Trang. S6 lieu a TP. Ho Chi Minh dupe cic can bp nghien ciru cua Vi^n Vgt ly TP. H6 Chi Minh thu thap lien tyc tai Dai Hoc Mdn trong thdi gian 2002 - 2013 (m^t chuky MJtTrdi). So lieu d Nha Trang dupe cic nha khoa hpc Phi^) thu thip trong thdi gian 1951 - 1956 (nira chu ky- Mat Trdi).
S6 Ii|u d i dupe xu ly theo nhung thong sd co ban ciia cac ldp di?n ly v i phin lo^i theo dieu ki?n i3i?n ly yen tinh v i diSn ty nhilu logn. Dl phyc vy cho npi dung tiiir nghiem truyen sdng ciia de thi, a
^ y chi sii dyng s6 li?u ciia Idp F2 trong dilu ki^n dien ly yen tmh dd tim cdng thuc dy bio truyln Tinh toan cong thiic thyc nghi?m ciia tan sd tdi han foF2 thyc chat l i giai bii toil hoi quy dya ti'en cic s6 li§u quan trie cho dang phirong trinh sau:
foF2 = a ( ! + b R ) ' M H z Ci)
a, b, X l i h? so thyc nghiem; R l i so vet den M?it Trdi i Ddi vdi CO sd diJ li|u khdng l6 cua ting di|n ly thi vi§c sii dyng cic phin min cho p h ^ phin tiq
hoi quy nhu phin mim Excel, Medcalc hay Rigmath ddi hdi q u i nhilu cdng sure v i thdi ^ a n no chiing tdi d i vilt phin mem lEE_CaI (lonspheric Empirical Equation Calculator) dl tinh toan cod thiic thyc nghipm cho cic tin s6 tdi ban ciia ting di$n ly tryc tilp tir cic file so Ii?u luu trii [3]- 2.3, Kit qud tinh cdng th&c thtfc nghifm:
Bing 1. Cdng thiic thyc nghi§m tinh cho 24 gid ngiy dfim ciia Idp F 2 1 ^ TP. H& Chi Mmh Gid dja
phucmg (LT) tiF2=a(l+bR)°^
Oglir 1 gi& -
zgei
3ff*
4 g i 9 ,
sm
g^ier Vgicr Sgid 9 giii • ipgift
11 m lagift:
\l9it
M p 15^8
»«»*-
I7gii»
ISgio . i9giei aogiii
%\0t'
22814 23gie>
4.228 3.872 3.194 2.254 1.267 . 0.557 3.393 5.755
• > 6.746 ,• 7.139
; 6.746 , 6.573 6.747 7.071 7.449 7.967 8.401 8.591 -v 8.562
• 8 094 7.330 6.617 5.719 4.877
0.035 0.035 0.037 0.053 0.142 0.541 0.016 0.013 0.014 0.014 0.015 0.014 0.011 0.010 0.010 0.009 0.008 0.007 0.006 0.006 0.008 0.011 0.018
foF2 - 4.228 (1 + 0 . 0 3 5 R ) "
foF2 = 3.872 (! + 0.035R)'°- foF2 = 3.194 ( 1 + 0 . 0 3 7 R ) "
foF2 = 2.254 (1 + 0 . 0 5 3 R ) "
f o F 2 - 1 . 2 6 7 ( 1 + 0 . 1 4 2 R ) "
foF2 = 0.557(1 + 0 . 5 4 1 R ) "
foF2 - 3.393 ( 1 + 0 . 0 1 6 R ) "
foF2 - 5.755 ( 1 + 0 . 0 1 3 R ) "
foF2 - 6.746 (1 + 0 . 0 1 4 R ) "
foF2 - 7.139 ( l + O . O H R ) "
foF2 = 6 . 7 4 6 ( l + 0 . 0 1 5 R ) "
foF2 = 6.573 ( 1 + 0 . 0 1 4 R ) "
foF2 = 6.747(1 + 0 . 0 1 1 R ) "
foF2 = 7 . 0 7 1 ( 1 + 0 . 0 1 0 R ) "
f o F 2 - 7 . 4 4 9 ( 1 +0.01 O R ) "
foF2 = 7.967(1 + 0 . 0 0 9 R ) "
foF2-8.401(1 + 0 . 0 0 8 R ) "
foF2 - 8.591(1 + O.OOTR)"^
foF2 - 8.562 (1 + 0 . 0 0 6 R ) "
foF2 = 8.094 (1 + 0 . 0 0 6 R ) "
foF2 = 7.330 (1 + 0.008R)"' foF2 = 6.617(l +0.011R'°-' foF2 = 5 . 7 1 9 ( 1 + 0 . 0 1 8 R ) "
foF2 = 4.877 (1 + 0 . 0 2 7 R ) "
0.54529 0.47983 0.58384 0.67111 0.62901 0.62550 0.31772 0.61243 0.70405 0.62653 0.66916 0.71276 0.66522 0.59753 0.58107 0.47076 0.40984 0.41837 0.39820 0.38103 0.41769 0.41014 0.45419 0.50124
a ban den img dung - goc nhin thuc te trong truyin song HF
Bing 2, Cdng thiic thyc n g h i ^ tinh cho 24 gid ngiy dem cua ldp F2 tai Nha Trang t •' Gift dja •
rpliucmg(LT) . Ogicr
flgii' • -.i
2gi<'-, .
3gici "4gj6 5 gid 6gic<
7gi6 . -' -' 8 gia ] '9 pi!
lOgiif i l l gid. . i, '. 12 gic> ' • ' 13 gio -;
14gicr.^
15 giii 16 gid 17gi!l 1 8 | a 19 giii .20gi6 21 gid 22 gii, 23 giii
2.4. Tinh tan so Tir cac c6n
a 4.228 3.872 3.194 2.254 1.267 0.557 3.393 5.755 6.746 7.139 6.746 6.573 6.747 7.071 7.449 7.967 8.401 8.591 8.562 8.094 7.330 6.617 5.719 4.877
difbao/BFlchc g thiic thirc nghi^
b 0.035 0.035 0.037 0.053 0.142 0.541 0.016 0.013 0.014 0.014 0.015 0.014 0.011 0.010 0.010 0.009 0.008 0.007 0.006 0.006 0.008 0.011 0.018 0.027
(rmsela ' cdc nam 2015 m nay, dtra so 1
f J ^ - a ( l + B R ) " - foF2 - 4.228 (1 + 0.035Rf' foF2-3.872 (l+O.OSSR)"
foF2-3.194 ( 1 + 0 . 0 3 7 R ) "
foF2 - 2.254 ( 1 + 0 . 0 5 3 R ) "
f o F 2 - 1 . 2 6 7 ( l + 0 . 1 4 2 R ) "
foF2 - 0.557 ( 1 + 0 . 5 4 1 R ) "
f o F 2 . 3.393 ( 1 + 0 . 0 1 6 R ) "
foF2-5.755 ( 1 + 0 . 0 1 3 R ) "
foF2 - 6.746 (1 + 0.014R)"
foF2 - 7.139 ( 1 + 0 . 0 1 4 R ) "
foF2 - 6.746 ( 1 + 0 . 0 1 5 R ) "
foF2-6.573 ( 1 + 0 . 0 1 4 R ) "
f o F 2 - 6 . 7 4 7 ( 1 + 0 . 0 1 I R ) "
foF2-7.071(1+0.010R)°' foF2 - 7.449 (1 + 0.010R)"
foF2 - 7.967(1 + 0.009R)"
f o F 2 - 8 . 4 0 1 ( l + 0 . 0 0 8 R ) "
foF2 - 8.591(1+0.007R)"
foF2 - 8.562 (1 + 0.006R)"
f o F 2 - 8 . 0 9 4 ( l + 0 . 0 0 6 R ) "
foF2 - 7.330 (1 + 0.008R)"
f o F 2 - 6 . 6 1 7 ( l + 0 . 0 1 1 R ) 0 . 5 f o F 2 - 5 . 7 1 9 ( l + 0 . 0 1 8 R ) "
f o F 2 - 4 . 8 7 7 ( l +0.027R)°-' sai s6 chuan)
nnse
0.54529 0.47983 0.58384 0.67111 0.62901 0.62550 0.31772 0.61243 0.70405 0.62653 0.66916 0.71276 0.66522 0.59753 0.58107 0.47076 0.40984 0.41837 0.39820 0.38103 0.41769 0.41014 0.45419 0.50124
- 2019 Ifi Nha Trang vi TP. Hi Chi Minh [4J.
i?u dir bao do ho^t dgng Mat trdi vao (R) ta tinh diroc s6 li?u dy bao foF2 cho cac thdi gian tuong ling. •
138 Hodng Thdi Lan
2.5. Tinh tan sS truyin xien:
Cic kit qua tin so foF2 thu nhan bdi miy tham dd ting di§n ly tir dii quan trie l i tin so tdi^B thing dihig fv (tiic la phit v i thu t ^ I dilm). E)l truyen sdng di tdi m<it khoing cich D cln p h a ^ dyng m^t tin so khic - tan so tdi h^n truyen xign fob.
Cic djnh ly cua Breit v i Tuve, Martyn v i djnh li4t Secant v l ra6i quan h# tuong duong trc ting di?n ly [5] cho thiy ring, doi vdi m§t ting di^n ly phlng, q u i trinh phin x? tuong duong' phin xa qua guong tgi dp cao i o h trong phin xa ciia tan so thing dimg tuong duong. Sy biin tli cua h theo tin so cd thi dupe xic djnh bing thyc nghipm thdng qua mpt m i y thim dd.
Mdi quan h? giOa tin s6 xien (vdi gdc tdi (|)D) v i tan so thing diing tuong duong t^i dp cao hdoi v dudng truyin khoang each D dupe cho bdi cdng thiic sau:
:^i
/„=/.sec(*.=/,,l+hri:; C2) Nhu viy, ttr tin so tdi hgn thing dung foF2 v i dp cao h' ciia ldp, ta co t h i tinh dupe tin s6 tdi hji
truyen xuyen cho khoang cich dudng truySn cy the. 1 2.6. Tinh MVF vd OWF cho nhitng khoang cdch truyen sdng cu thi, d(e bao cho cdc nSm 201 f^
20191^ Nha Trang va TP. Ho Chi Minh f6J.
3. Mpt so van d l cin luu y trong thyc t l truyin sdng HF
1 f i n s6 tdi h^n l i tin so cao nhit dupe khiic xa va phin x$ t^i nhiing ldp trong ting di^n l>
nhihig tin s^ Idn hon tin so tdi han s£ khong phin x^ d l trd ve m$t dat (bj truyen thing qua cac ldp).
Tan so sir d^ng toi da MUF (Maximum Usable Frequency) l i t i n 56 cao nhat m i mpt dii phi cd the truyen sdng giOa thilt bj dau v i thilt bj cuoi vdi khiic x? I lln bdi ting di^n ly, Vdi tin si thIp hon MUF, sdng dupe phin x? tir m^t dilm trong Idp di?n ly. Vdi tan so cao hon MUF, qui trini phan x? khdng dii khi ning d l dua tia sdng trd ve Trii dat v i nd thoit ra khdi ldp niy. MUF phy thu$i vio m^t dp ion t ^ cic ldp xiy ra khiic x^phin xa; thdi gian trong ngiy; thfn gian trong nim; d$ ho^
dpng ciia Mit trdi; vj tri ciia dii phit v i dii thu; gdc truyen din ciia anten. MUF khdng phy thu$c vd(
cdng suit cua dii phit v i m ^ d$ thu cfia miy thu.
Tan so sir dyng to! thilu LUF (Lowest Usable Frequency) l i tan so t h ^ nhit sau khi bj cic Id]
t h ^ (Idp D, E) hip thy nhung vln truyin din dupe miy thu v i b i o dim tl so S/N.
Diem phit Diem thu Hinh 2: Tin so lam vi§c tdi uu
Tu nghien cihi ca ban den ung dvng ~ goc nhin thuc ti trong truyin song HF 139
Tan so siJr d y n g toi uii OWF (Optimum Working Frequency; hoic FOW - Frequmcy of Optimum Working; hoac FOT - Frequency of Optimum Transmission) l i tin so lim w ^ h i ^ qui nhit dupe "dy bao d l sur dyng cho m$t dudng t n ^ l n v i thdi gian nio do d^t dupe hi?u qui 90% thdi gian trong thing. Do dO suy hao ciia sdng khi (fi qua cic ldp ciia ting difn ly t j I§ n^jch vdi binh phuong ciia tan sd, cho nfin sCt dyng tin s6 cing cao thi cang cd phin x? tot. Tuy nhi&i, cdc thay dii bit thudng trong tang di^n ly cd thi lim cho sdng MUF lech hudng v i di ra khdi Idp niy. Vi viy, tren tiiyc t l chi nSn dOng tan so t h ^ hon MUF 15% (hinh 2).
D$ dii^ciia hwitc n h i y (Hop lenghth) l i khoang cich trSn be mjt Trii dit dupe xac djnh bdi sdng v6 tuyen sau khi dupe phin x^ mpt lln tir tang dien ly v i quay trd l^i m§t dit.
Hinh 3: D$ dii budc nhay Hinh 4: Viing t^i
Budc nhiy Idn nhit dirpc xic djnh bdi d$ cao cua ting di^n ly v i dudng cong ciia bl m§t Trii dit, Doi vdi ldp E v i ldp F cd d$ cao tuong ling tir 100 km v i 300 km, chieu dii budc nhay Idn nh^
khoang 1800 km v i 3200 km tuong ling vdi gdc tin khoing 4° (hinh 3).
Khoing cich xa hon thi d n nhilu budc nhiy hon. VI dy d n tiaiyin thdng tin vdi khoang cich 6000 km cln 4 budc nhay d Idp E ho|c 2 budc nhiy d ldp F.
V i n g toi (Skip Zone) l i viing khdng thu dupe sdng. Kich cd c i a viing toi cung thay ddi trong ngay, theo mtia v i theo dO ho?t dpng ciia MJt tr£ri (hinh 4).
Ang ten: Sdng HF phin x^ d ting di§n ly each m|t dit tii 200 - 300 km. Dl d^it dupe khoing cich li6n l?c theo </ mu6n, phai tinh gdc nging vi gdc phuong vi ciia tin hi|u phit; tinh toin gdc nging vi gdc phuong vj cua tin hi^u thu d l thu tin hi^u phin X9 tir ting di?n ly,
V{ dy, may phit each miy thu 350 km, tin hieu burc x? tii angten phit cd gdc nging tin hi^u phit la 60°, phin x^ d ting di?n ly 300 km, liic niy miy phit - diem phin x? d ting di§n ly - miy thu sS t?o thinh tam giic gin dIu, phuong thiic truyen sdng niy dupe gpi l i phucmg thto: m$t budc nhay. C ^ g ting khoang cich giihi may phat v i may thu, thi gdc nging tin hieu vi ^dc t i (li gdc t^o giua tin hifiu phan X91^ ting di|n ly vdi phuong nim ngang) cang giam, gdc ta cd thi gjam xuong 3° dl cd khoang cich truyen sdng Idn nhat ddi vdi phuong thirc truyin sdng mpt budc nhiy (3" l i gdc t i b6 nhit cd thi thyc hi^n truyen sdng, tuy nhiSn sS bj gidi h^n khi angten phit dat gin d6i nui, nhihig chudng ngai v ^ . . . ) . Thdng thudng, d l tang khoing each din miy thu ngudi ta thyc h i ^ phuong thirc truyin sdng nhieu budc nhay (phin x^ nhilu lln d ting difn ly), tuy nhien khi thyc h i ^ phuong thirc tiuyln sdng nhilu budc nhiy cOng cd the xay ra h i ^ tupng da dudng t^ti may thu (vl thu dupe tin hi$u tii mpt budc nhdy v i tin hi^u tir nhilu budc nhiy).
140 Hodng Thdi Lan
D I thu tit, dieu quan trpng li phii sii dyng Sngten thu cd hudng tinh dl ting ti sd S/N (Signal/Noise) vi m|t d$ t9p im giim khi chi thu theo m$t hudng nio dd. Mpt dai thu can phai trang bj mpt so ingten thu dl thu dupe cic ti'n hi^u HF. Cic ingten bao ghm ingten thu vfi hudng dl thu tin hi^u cd gdc ti Idn, tin sd thip vi nhiing angten thu cd hudng tinh dl thu cic tin hi^u cd tin so cao.
4. Sv khd kfaSn khi triln khai tbyr t l
Myc tieu cua nghiSn ciiu niy li hudng tdi phyc vy ngu din lim kinh tl Bioi, vi sd lupng cic phuong ti|n nghl ci ciia ngu din Vi^ Nam li rit Idn (khoing 130.000) vi vdi hp thi phuong thiic lien l^c re tiln li nhu du cin thilt Tuy nhien, khi tilp c ^ thyc te thi 1^ gkp phii nhieu khd khin, trong dd cd 3 vin dl mlu chit:
•nui nhat, vl kinh phi h^n chi ciia de tii nghifin oihi: ting kinh phi li 400 tri^u dong, I chuySn dl thii nghifm chi dupe toi da li 30 trifu, trong khi thuS 1 chile tiu toi thilu li 15 tri^u d^ng/ngiy, chua kl thug tiiilt bj thu phit.
Vdi khd khan niy, giii phip giim thilu chi phi li ph6i hpp vdi Dii thfing tin duydn hii (TTDH) Nha Trang vi Dii TTDH Vung Tiu, sii dyng miy phit vi ingten phit cua cic dii de phit sdng. Tuy nhien, hardng phit x? cua ingten tgi cic dii TTDH li vd hudng. Nhipm vy cua Dii TTDH la ph^t sdng theo Ijch cua h? tiidng dii TTDH, do v§.y dii vdi m^t dii bd trong h | thong, vipc tiiay ddi hudng vi gdc nging li khd cd the dip ting. E)l thyc hi|n thu phit sdng thu nghi^, de tii cln phii cd kinh phi cho vipc thilt kl angten chuygn dyng cd thi thay ddi dupe gdc nging cung nhu t|p trung vio hudng phit.
Thir hai, vl thdi gian thyc hi^n: vi I;? do an toin, vi§c khio sit, thur nghifm thu phit giOa dit liln vi trSn biin chi cd thi thyc hipn thugn tif n vio khoing thin gian biin khfing d^ng (thing 3 - 4 - 5 hing nim). Thdi gian biin d^ng khdng nSn thyc hi^n cic"cdng tic khio sit. Hai nSm li thdi gian khdng dii dl thyc hipn vi^ dua kit qui nghiSn cihi vio Cmg dyng.
Thii ba, thyc tl ddi hdi nhihig giii phip ky tiiu§t gip nit cho nhu ciu cu^c sing. H| thing Bii Thdng tin duyen hii Vift Nam dupe Nhi nude dIu tu xiy dyng tii nim 1997. Diy li mpt h? thing gim 32 dii, dupe dIu tu kinh ph( rit Idn, bo til dpc tiieo bd biin tii Mdng Cii tdi Kien Giang, do Cdng ty TNHH MTV Thdng tin di?n tii Hing h^ Vift Nam (VISHPEL) tl chicc quin ly vi khai thic [7]. Tin so lifin t9c dupe phin b6 khic nhau dl t ^ tiiinh mgng \u6i bao phu viing biin Vi§t Nam. Tuy nhien, cic tin s6 lien l^c 6SL phin bo khdng trimg vdi cic tin s6 tr^ng thii thyc ciia dif n ly Vipt Nam da quan trie dupe vi phin Idn cin bp ky thu^t t^i cic d^ TTDH deu xa 1? vdi nhthig kiln thiic vl ting di^n ly.
5. IVao ddi tbio luSn
Tim ra dupe nhihig quy lu§t cua ty nhien khdng chi rat vat vi mi cdn cln din sy kien tri khdng m?t mdi cua nhihig ngudi lim khoa hpc nghian cihi co bin. Lim diu ky thu|t dl tim dupe nhihig gi^' phip hihi idi phyc vy cuOc sdng cung khdng hi don giin v^ nhOng ngudi lim cdng tic ky thu^. B&t c^nh do, cic quy djnh vi sii dyng pho tin sd vfi tuyIn cua Lira minh Vien Uidng qudc tl vi do Nhij nude quan ly, cung djtt ra nhOng tiiich thiic khfing nhd khi ting dyng cic kit qui nghifin cuu vio tiiyc tien.
Quylt djnh 125/2009/QD-TTg di ban hinh quy djnh phan chia dii tin HF tir 3 MHz - 30 MHz
thanh cic bSng tin dinh cho cic nghifp vy vd tuyIn difn vi quy djnh myc dich, dilu ki^n dl sit dyng
doi \6i timg bing tin. Hi§n nay, Cyc quin ly tin sd di c ^ hon 300 gily ph4p cho cic dii sir dyng
bing tan HF trong nghifp vy quing ba, cd djnh, luu dfing, luu dpng hing hii, luu dpng hing khong vi
hon 12.800 gily p h ^ cho cic phuong tifn nghe ci. Cic dii tan HF mi dl tai dy djnh phat tiiu tiiir
Tir i^hien ciiu ca ban din ung dyng-goc nhin thuc li trong truyen suns'"
nghi?m thyc ra deu nim trong phd tin sd vfi tuywi di§n qudc gia ^ ban hinh, nen c6 the xin cap p ep de thir nghipm v i sau dd cd t h i d l nghj cip phit cfi djnh.
D l tiiyc hi^n cd hifu q u i viec dua cic kit ciia nghien cihi co ban cia Vi?n HSn ' ^ J[^ * ? * ^ ^ d n thay ddi djnh miic kinh phi nghi«i cuu khi xet duyft dl tii, d n diy m^nh va khuy&i *f'J]'f^ , tii nghien ciiu ky thuit - cfing ngh§ sau nghiSn ciiu co bin va d n sy hd trp ve m& quin ly Nha nuoe ddi vdi v i ^ dua kit q u i nghiSn clhi vio img dung trong thyc tl.
T i i lieu tbam khao
[1] hUp:/Avww.itu.int ^, . . [2] Hoing Thii Lan (2013). Tdng di?n ly xich dgo tic a Vi^f Nam va du bdo Ihcri Hit va "T- NA« ^"°« "V*^ 'V
nhiSn vi Cdng n ^ ? . y, ri t'' id\
[3] VTnh Hio v i Hoing Thii Lan (2013). Phin mem lEE-CAL tinh toan cdng t h ^ thuc nghi^ ciio cac lan so tdi h^n ciia ting di^n ly. Tgp chi Khoa hpc vd Cong ngh^ Bien 13(3A), 34-44.
[4] Davies K.. (1965). Ionospheric Radio Propagation National Bureau of standards, U.S. Govemmen Printing OfTlCe. UC rhn khu virc [5] Hoing Thai Lan vi D$ng VSn Li§t (2013). Dy b&) foF2 ung dyng trong thdng tin hen l?c nr cno snu vy„
b\eRUiarT&n$,K.hii\i\Hba.T(ip chi Khoa hpc vd Cong nghf Biin 13(3A).86-P0. „„Ai.iv^nHF [6] Vi¥n V§t iy,,TP. HCM (2015). Bio c4o ting ket dl lai: "Nghi&i cim ung dgng si; phan x? song vo uiycu
tu ting di^n ly trong k j thu^t truyen thong tin l^i khu vyc Nam b?".
[7] http://vishipel.com.vn
