Detection Probability Analysis of Space-Based AIS Signal by Using Geometrical Model

Transcription

1 Journal of Shipping and Ocan Enginring 4 (2014) D DAVID PUBLISHING Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl Yuli Chn Mrchant Marin Collg, Shanghai Maritim Univrsity, Shanghai , China Abstract: To analyz th dtction probability of shipborn AIS (automatic idntification systm) signal from spac, a mathmatical modl dpndnt upon thr factors of mssag collision avoidanc, powr of signal rcivd by satllit and intrfrnc ratio of signal rcivd is prsntd in th papr. Th altitud and footprint ara of th AIS satllit ar discussd to ovrcom th collision of mssags transmittd in th diffrnt tim slots from diffrnt SOTDMA (slf organizing tim division multipl accss) cll aras, but arriv at th sam tim slot du to th diffrnt signal path lngths. Th simulatd rsult shows that compard to th normal LEO (low arth orbit) satllit systm, on avrag th imum signal covrag ara and th imum FOV (fild of viw) of th AIS satllit systm ar rducd by 74% and 38%. Th majority of powr of signal transmittd from shipborn 12W-powr AIS transmittrs locatd within th imum signal covrag ara may b rcivd with th sufficint margin of powr of signal by th LEO satllit, but th spac-basd AIS systm gnrally suffrs from th insufficint CIR (carrir to co-channl intrfrnc ratio) of signal rcivd sinc around 95% pairs of mssag simultanously rcivd by satllits may not b corrctly dcodd. Th insufficint CIR of signal rcivd is th bottlnck for th high mssag dtction probability. Thrfor, th masur of sparating th collision mssags should b furthr takn by th spac-basd AIS systm to incras th dtction probability. Ky words: AIS, LEO satllit, dtction probability, mssag collision, distanc dlay, signal margin, CIR. 1. Introduction Th maritim AIS (automatic idntification systm) is initially dfind as a navigation broadcasting systm oprating on th VHF (vry high frquncy) radio wav for th information communication btwn ship to ship and ship to shor in ral tim. Th Intrnational Convntion on th SOLAS (safty of lif at sa) has mandatd that vssls ngagd on intrnational voyags should carry AIS mobil quipmnt (known as Class A shipborn quipmnt) onboard by th nd of Practic shows that th introduction of AIS into th maritim community has mad th navigational information xchang btwn ship to ship and ship to shor mor appropriat and mor rliabl and furthrd th nhancmnt of navigational safty and marin nvironmntal protction. Corrsponding author: Yuli Chn, associat profssor, rsarch filds: marin navigation, maritim ducation and training. ylchn@shmtu.du.cn. Th spac-basd AIS systm is dfind as th systm of spcializd micro-siz satllits carrying AIS transcivrs in th LEO (low arth orbit) to dirctly captur th AIS mssags and transfr such to th shor station so that th global survillanc of maritim traffic is rally achivd from th spac. This concpt has continuously attractd Norway, th Unitd Stats, Canada, tc. to mak vry ffort to carry out th rlativ rsarch with th grat invstmnt sinc its incption in 2003 [1]. Tchnical rsarchs rgarding rcption of AIS mssags from spac, including link budgt, mssag dtction probability [2, 3], and satllit antnna dsign, Dopplr shift and atmosphric intrfrnc [4, 5] hav bn widly invstigatd. Howvr, th abovmntiond rsarchs rgarding mssag dtction probability ar dirctly conductd basd on a constant altitud of th satllit orbit, such as 600 km or 800 km and th fixd ship numbr within th satllit footprint ara. Actually, th

2 162 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl prformanc of spac-basd AIS systm is highly dpndnt upon th dtrmination of th satllit orbit altitud. Th footprint ara, powr margin of signal rcivd, mssag dtction probability, tc., vary with th satllit orbit altitud and bcom unusual if AIS tchnical charactristics is also considrd. Th papr basd on th AIS tchnical charactristics prsnts th mathmatical modl that AIS signal can b corrctly rcivd and dcodd from spac rlativ to th factors of mssag collision avoidanc, powr of signal rcivd and intrfrnc ratio of signal rcivd. Comparison of covrag ara btwn spac-basd AIS satllit and normal LEO satllit as wll as th quantity of powr of signal rcivd and CIR of signal rcivd that ar both rquird within th availabl AIS satllit footprint ara ar analyzd as wll at th various altituds of satllit orbits so as to dmonstrat th appropriat orbit altitud for spac-basd AIS systm or th systm prformanc proprtis which would b achivd as th satllit altitud rmains constant. 2. AIS Tchnical Charactristics Totally 27 diffrnt typs of mssags can b transmittd by using th various AIS application platforms, such as Class A AIS quipmnt, Class B AIS quipmnt, AIS bas station, SAR (sarch and rscu) aircraft AIS quipmnt, AtoN (AIS Aids to Navigation), tc., btwn ship to ship, ship to shor and ship to spac [6]. Each AIS application platform is abl to transmit and rciv svral typs of mssags. For th Class A shipborn quipmnt which is mandatorily rquird to b fittd on th SOLAS Convntion vssls, th mssag s information xchangd is classifid into four catgoris, i.., static, dynamic, voyag rlatd and short safty rlatd. Th dtails including th componnt and th rfrshing transmission rat for ths four catgoris of mssags ar illustratd in Rf. [7]. To nabl th automatic and autonomous opration for AIS without any organization of th bas station and simultanously avoid th collisions btwn mssags transmittd by diffrnt AIS application platforms, th communication protocol so-calld SOTDMA (slf organizing tim division multipl accss) is applid. Fig. 1 shows th principl of th SOTDMA which spcifis that on-minut, also rfrrd to as on fram, is dividd into 2,250 slots for ach of two dsignatd VHF channls, AIS1-87B ( MHz) and AIS2-88B ( MHz), to altrnativly transmit but simultanously rciv AIS mssags. Each tim slot is thn qual to ms long for 256-bit information communication. AIS automatically and autonomously dtrmins th slots unusd to transmit its mssags at th rat of 9.6 Kb/s using a binary GMSK (gaussian minimum shift kying) modulation and manwhil rsrvs othr slots for nxt mssag transmission. Multipl slots for th transmission of th longr AIS mssag ar also allowd. Th SOTDMA schm that rquirs AIS to rsrv thir transmission slots ahad of tim is abl to nsur that AIS mssags ar not transmittd at th sam tim slots within a SOTDMA cll. Morovr, as a channl approachs an ovrloadd stat, th SOTDMA algorithm is capabl of producing a progrssiv rduction of th SOTDMA cll siz in ordr to drop AIS rports from vssls farthst from th cntr of oprations, whil maintaining th intgrity of th (mor important) closr rang rports. Fig. 1 Principl of SOTDMA protocol.

3 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl 163 Th dtaild tchnical charactristics for th Class A AIS quipmnt ar summarizd in Tabl 1 [8]. 3. Two Scnarios for Spac-Basd AIS Mssag Collision As prviously statd, AIS mssag tim slots conflict is abl to ffctivly b avoidd during th communication btwn ships as th SOTDMA schm which is dsignd for th coordination of th tim slot usag in a givn ara (a circular ara with th radius of approximat 40 nm) is applid. Howvr, this is not th cas for th spac-basd AIS systm. As th footprint ara from th AIS snsor installd on a LEO satllit covrs a numbr of SOTDMA cll aras, thr is no coordination btwn ths cll aras in gnral. Thrfor, tim slots conflicts may occur for many AIS mssags rcivd by a LEO satllit at th sam tim. Th first issu to b considrd for dsigning th spac-basd AIS systm is thn that AIS mssags from diffrnt SOTDMA cll aras may simultanously arriv at th satllit so that AIS mssags collision ariss, which will los th contnt of AIS mssag. In thory, two scnarios of spac-basd AIS mssag collision xist. Scnario on is that AIS mssags ar transmittd at th sam tim slots from diffrnt SOTDMA cll aras with th sam signal path lngths. Rgarding this cas, many AIS mssags transmittd from diffrnt AIS-quippd ships locatd in th diffrnt SOTDMA cll aras ar rcivd by a LEO satllit at th sam tim slot as both thir transmission tim slots and distanc from ach ship to satllit ar xactly sam. Fig. 2 shows two mssags collid in this cas. Ship A and ship B ar locatd in diffrnt SOTDMA cll aras du to th sufficint spac btwn thm but both cll aras ar within th covrag of AIS satllit footprint. As th corrsponding distanc btwn satllit to ship A and ship B is xactly sam. If ach transmits a mssag at th sam tim slot and ths two mssags will undoubtdly also arriv at th satllit at th sam tim slot, which will caus th mssag collid. Scnario two is that AIS mssags ar transmittd in th diffrnt tim slots from diffrnt SOTDMA cll aras, but arriv at th sam tim slot du to th diffrnt signal path lngths. Th only diffrnc of this cas with th prvious cas is that vn th massag transmittd by ship A and ship B, rspctivly, at diffrnt tim slots, but will also suffr from th collision whn rcivd by satllit bcaus th diffrnc of transmission tim slot is compnsatd by th diffrnc of distanc of ach ship to satllit, s Fig. 3, which only shows two mssags collid. Tabl 1 Paramtr Class A AIS quipmnt tchnical charactristics paramtr. Charactristics standard Basband MHz and MHz (87 B and 88 B) Wavlngth 1.85 m (of 162 MHz) Transmittr powr 2 W or 12.5 W (by dfault) Bandwidth 12.5 khz or 25.0 khz (on high sas) Modulation GMSK Modulation indx 0.25 for 12.5 khz or 0.5 for 25 khz Bit rat 9,600 bits/s ± 50 ppm Fram lngth 1 min Slot lngth 26.7 ms or 256 bits Capacity 4,500 slots pr min for two channls RS (rcivr snsitivity) -107 dbm for 25 khz bandwidth undr normal tst conditions -104 dbm for 12.5 khz bandwidth undr normal tst conditions CIRT (CIR thrshold) 10 db

4 164 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl Fig. 2 Scnario on for AIS mssag collision. Th powr of signal rcivd by satllits is of sufficint margin; Th co-channl intrfrnc ratio of signal rcivd by satllits is satisfid. Th mathmatical modl that AIS signal can b corrctly rcivd and dcodd from spac hnc can b xprssd by F MPT (1) whr, F rprsnts th st that AIS mssag can b rcivd and proprly dcodd by satllits, M mans th st that mssags collision dos not occur, P is th st that powr of AIS signal rcivd by satllits is of sufficint margin, and T is th st that th AIS signal rcivd by satllits mts th rquirmnts of th co-channl intrfrnc ratio. Th probability and th rliability of AIS signal rcption from spac is apparntly th function of th mssag collision avoidanc, powr of signal rcivd and intrfrnc ratio of signal rcivd. As far as th avoidanc of mssag collision is concrnd, th rsarch associatd with scnario on is focusing on th statistical analysis with rspct to th ship s numbr, swath width and satllit obsrvation tim [2, 4]. Th papr will considr th massag dtction probability of scnario on basd on th mthod of th avoidanc of scnario two by using th AIS mssag buffr structur. 5. AIS Mssag Buffr Structur Fig. 3 Scnario two for AIS mssag collision. 4. Mathmatical Modl for Corrct Rcption of AIS Signal from Spac Considring th AIS SOTDMA schm and th radio signal knowldg, th following conditions should b simultanously mt providd that shipborn AIS mssag can b rcivd and proprly dcodd by th LEO satllit: Th mssags collision is avoidd; AIS mssag data transfr uss a bit-orintd protocol basd on th HDLC (high-lvl data link control). Th transmission packt with th total lngth of dfault packt is 256 bits quivalnt to on slot including svn parts, i.., ramp up, training squnc, start flag, data, fram chck squnc, nd flag and buffr, shown in Fig. 4. Among thm th buffr part is 24 bits long consisting of bit stuffing, distanc dlay, rpatr dlay and synchronization jittr. And th distanc dlay with 12 bits long quivalnt to nautical mils (nm) is rsrvd for avoiding AIS mssags collisions within on SOTDMA cll ara du

5 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl 165 AIS transmittrs within th satllit footprint rang is idntifid by: h SA SG (3) whr, SG is th satllit altitud abov th ground surfac. If point B is th intrsction point btwn SA and th arc of BG rlativ to point S as th cntr, i.., SB is qual to SG, hnc h SA SB nm (4) Fig. 4 AIS mssag buffr structur. to th diffrnt signal path lngths. That is to say in thory th SOTDMA protocol provids th Class A AIS quipmnt with th protction for th diffrnc of a propagation rang up to nm. 6. Rlation btwn Altitud and Footprint Ara of AIS Satllit Basd on AIS mssag structur, it is concludd that du to th 12-bit part of distanc dlay rsrvd in AIS mssag structur, AIS mssags collision is still abl to b avoidd as long as ths signal transmission path diffrncs ar not mor than nm vn though ths mssags ar simultanously rcivd from diffrnt SOTDMA clls. Fig. 5 shows th gomtrical rlations among th satllit altitud, footprint ara and signal path diffrnc, whr point S, G, O rprsnt LEO satllit, satllit s projction on ground surfac (known as th subsatllit point), arth s cntr, rspctivly. Supposing point A and S 1 ar two AIS transmittrs, thn thir signal path diffrnc ( h ) can b dfind by: Thn according to th spac gomtry, th mathmatical rlation btwn satllit altitud ( h ) and footprint ara radius ( x ) can b rfrrd to as h(2h h) x 0.54r arccos[ 1 ] (5) 2r ( h r ) whr, h and r ar th mathmatical symbol of SG and OG (th radius of th arth) in km, rspctivly, and x is th mathmatical symbol of th arc of AG rprsnting th radius of satllit footprint ara on th ground surfac in nm. As a rsult, in ordr to avoid th collision of mssag transmittd in th diffrnt tim slots from th diffrnt SOTDMA cll aras for all AIS transmittrs, as th AIS satllit altitud rmains constant, th satllit footprint ara should b dsignd to mt th following quation h(2 h h ) x x r arccos[1 ] 2 r ( h r ) (6) h SA SS (2) 1 And if point A and B ar just locatd on th dg of th satllit footprint ara, in ordr to avoid th collision of mssag transmittd in th diffrnt tim slots from th diffrnt SOTDMA cll aras, thn th imum signal path diffrnc ( h ) btwn all Fig. 5 Gomtry of satllit altitud, footprint ara and signal path diffrnc.

6 166 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl 7. Comparison of Covrag Ara btwn AIS Satllit and Normal LEO Satllit Th satllit FOV (fild of viw) is usd to xprss BSA th covrag ara on th ground surfac that th signal snsors installd on th satllits can provid. Fig. 6 illustrats th normal satllit spac gomtry, whr SP and SQ ar tangntial with th arth surfac, rspctivly. Thrfor, th imum satllit FOV ( ) can b dtrmind in radian by: r PSQ 2[arccos( )] h r PSQ (7) And th imum radius of satllit footprint ara on th ground surfac ( y ) can corrspondingly dtrmind in nm by: r y 0.54r arccos( ) h r (8) Howvr, for th AIS satllit systm, as th imum margin of th signal path diffrnc should b takn into account in ordr to avoid th collision of mssags transmittd in th diffrnt tim slots from th diffrnt SOTDMA cll aras, th imum radius of satllit footprint ara on th ground surfac x ) should b dtrmind by Eq. (6) whn th ( satllit altitud is fixd. As a rsult, th imum FOV ( BSA should b idntifid by: ) for AIS satllit systm in radian BSA r arcsin (2h h h ) r sinarccos[1 ] 2r ( h r ) arcsin (9) h h Consquntly, th comparison of covrag ara btwn spac-basd AIS satllit and normal LEO satllit by considring th satllit altitud (h) of intgr tims of on hundrd from 40 km to 1,500 km is summarizd in Tabl 2 basd on 6,371 km of th arth radius, whr r rprsnting th rlativ rror of th radius of th satllit footprint on th ground and d rprsnting th rlativ rror of th FOV ar dfind by Eqs. (1) and (2) rspctivly. y x r 100% (10) y Fig. 6 Maxi mum FOV for normal satllit systm. Tabl 2 Comparison of covrag ara btwn AIS satllit and normal LEO satllit. h (km) y (nm) x (nm) r (%) PSQ ( ) BSA ( ) d (%) 400 1, , , , , , ,000 1, ,100 1, ,200 1, ,300 2, ,400 2, ,500 2, Avrag Avrag hma sin arccos[ 1 2 h h m

7 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl 167 PSQ BSA d PSQ 100% (11) Thrfor, similar to th normal LEO satllit systm, th signal covrag ara of AIS satllit systm grows as th satllit altitud riss but th FOV rducs. Nvrthlss, as for th sam satllit altitud, th signal covrag ara and th FOV of th AIS satllit systm both ar much lss than thos of th normal LEO satllit systm. For xampl, at 700 km orbit altitud, th radius of th imum signal covrag ara and th imum FOV of th AIS satllit systm ar nm and 83.4 whras th normal LEO satllit systm rsrvs 1, nm and 128.6, rspctivly. That is to say whn dsigning AIS satllit systm at 700 km of th satllit altitud, it should b considrd that th satllit FOV may b fixd within 834 rathr than to avoid th collision of mssags transmittd in th diffrnt tim slots from th diffrnt SOTDMA cll aras. On avrag, compard to th normal LEO satllit systm, th imum signal covrag ara and th imum FOV of AIS satllit systm ar rducd by 74.4% and 38.2% at th satllit altitud from 400 km to 1,500 km. 8. Analysis on Powr of Signal Rcivd by Satllits According to th Friis transmission quation and considring th signal powr attnuation causd by th transmittr wavguid and th atmosphric influnc, th powr of signal ( P r ) transmittd from th shipborn AIS transmittrs on th arth ground and rcivd by th satllit AIS rcivr can b obtaind by: Pr Pt Gt Gr 20log 10( ) P (12) 4 d Whr, P is th powr of signal rcivd by th r satllit AIS rcivr in dbm, P t is th shipborn G r AIS transmission powr in dbm, G t and rprsnt th transmittr gain and th satllit rcivr gain in db, rspctivly, is th AIS radio wav lngth in km, d is th signal path distanc in km btwn th shipborn AIS transmittr and th satllit rcivr, P is th total signal powr loss in db causd by th AIS transmittr wavguid, th inonsphric polarization and th atmosphric absorption. As th signal path distanc ( d ) btwn th shiborn AIS transmittr and th satllit rcivr varis with th radius of satllit footprint ara on th ground surfac ( x ) at a satllit orbit, according to Fig. 7, whr point T is a shipborn AIS transmittr, mn is th horizontal lin tangntial with th arth surfac, OZ is th lin passing through T and vrtical with mn, thn th rlation btwn d and x can b dtrmind by x a (13) r d r ( h r ) 2r ( h r )cos (14) ( h r )sin a d (15) cos whr, is th cntral angl in radian corrsponding to th arc of x, is th signal radiation angl abov th horizontal plan in radian. As a rsult, suppos th satllit rcivr gain ( G ) bing 3 db, th total signal powr loss ( P ) is assumd by approximat 2.2 db [9], and th AIS transmittr gain ( G ) can b dtrmind by th t radiation pattrn quation of Eq. (16) basd on halfwav dipol antnna which is widly usd for th shipborn AIS transmittr, Fig. 7 Rlation btwn signal path distanc and footprint radius. r

8 168 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl cos[( )sin ] ( ) 2 cos f (16) Powr of signal rcivd by AIS satllit is obviously th function of th signal travling path and th transmittr gain at an orbit. Figs. 8 and 9 show th graphical rlation btwn th radius of th satllit covrag ara on th arth and th AIS mssag powr that could b rcivd by th satllit at various orbits from 400 km to 1,500 km with th dfault stting of 12 W and 2 W for shipborn AIS transmittr powr, rspctivly. Fig. 8 Covrag ara and mssag powr rcivd at orbits for 12 W powr. Fig. 9 Covrag ara and mssag powr rcivd at orbits for 2 W powr.

9 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl 169 Thrfor, AIS mssag powr of signal rcivd by th satllit rducs as th orbit altitud riss. For a constant orbit, xcpt that it is null around th sub-satllit point du to th halfwav dipol proprtis, th graphical rlation btwn th signal powr rcivd by satllits and th radius of th satllit covrag ara on th arth is lik a lft-skwd curv, i.., th powr rcivd will quickly rach th pak bfor th gradual rduction as th radius of th satllit covrag ara on th arth incrass. For xampl, if th satllit is orbiting at th altitud of 700 km, th imum mssag powr to b rcivd for th AIS transmittr working on 12 W will b around 94 dbm transmitting from th position of about 280 nm from th sub-satllit point. As th radius of AIS satllit footprint ara should b limitd within x at various orbit altituds to avoid th collision of mssags transmittd in th diffrnt tim slots from th diffrnt SOTDMA cll aras according to Tabl 2, th prcntags that th powr of signal rcivd within x ara rcivd by satllits is mor than th AIS rcivr snsitivity rquird at various orbit altituds for th diffrnt shipborn AIS transmittr powr ar shown in Fig. 10. As a rsult, whn shipborn AIS transmittrs locatd within x ara ar working on th powr of 12 W, th majority of powr of signal rcivd by satllits orbiting from th altituds of 400 km to 1500 km is mor than AIS rcivr snsitivity. All mssags transmittd from th ground will b rcivd with th sufficint margin of signal powr by th satllits orbiting at km altituds. Evn for th 1,500 km altitud satllits thr is about 87% of mssags with th sufficint margin of signal powr can b rcivd. By comparison, most powr of signal rcivd by LEO satllits is rathr wakr than th AIS rcivr snsitivity whn shipborn AIS transmittrs locatd within x ara ar working on th powr of 2 W, spcially non of powr of signal rcivd by th satllits running at th altituds of 900 km or abov ar sufficint. 9. Analysis on CIR of Signal Rcivd by Satllits Evn though scnario two of mssag collision can b avoidd by using distanc dlay, scnario on may still occur. Th signal intrfrnc simultanously rcivd by satllits should b avoidd in ordr to corrctly dcod th mssags. Th minimum CIRT (carrir to co-channl intrfrnc ratio thrshold) for Fig. 10 Prcnt of powr of signal rcivd by satllits with sufficint margin varying from orbits.

10 170 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl Fig. 11 Prcnt of CIR of signal rcivd by satllits mor than CIRT varying from orbits. a typical AIS rcivr is rquird as 10 db. To analyz th quantity that th diffrnc btwn any two signals transmittd within x ara and rcivd by AIS satllits is mor than th rquird CIRT as satllit altitud varis, th prcnt of CIR of signal rcivd by AIS satllit mor than CIRT rquird at an orbit altitud is dfind by x 0 x 0 g( x)dx CIR% % f ( x)dx (17) whr, g (x) is th function of th total amount that th diffrnc btwn any two powr of signals transmittd within x ara and rcivd by AIS satllits is mor than th rquird CIRT, and f (x) is th function of th total amount of th diffrnc btwn any two signals transmittd within x ara and rcivd by AIS satllits. Fig. 11 shows th prcntags that th diffrnc btwn any two signals transmittd within x ara and rcivd by AIS satllits is mor than th rquird CIRT at various satllit altituds whn shipborn AIS transmittrs ar working on th powr of 12 W and 2 W, rspctivly. Evn though th prcntags that th powr diffrnc btwn any two signals transmittd within x ara and rcivd by AIS satllits is mor than th rquird CIRT riss as th satllit orbits incrass, th spac-basd AIS systm gnrally suffrs from th insufficint CIR irrspctiv of satllit orbits and shipborn AIS transmittrs powrs. For instanc, if at th sam slot th AIS satllit orbiting at th altitud of 700 km simultanously rcivs on mssag transmittd from 12 W-powr shipborn AIS transmittrs locatd within x ara, about 95% of ths pairs of mssags cannot b corrctly dcodd du to th insufficint CIR. 10. Conclusions By using th distanc dlay in th AIS buffr structur to avoid th collision of mssags transmittd in th diffrnt tim slots from th diffrnt SOTDMA cll aras, compard to th normal LEO satllit systm, on avrag th imum signal covrag ara and th imum FOV of th AIS satllit systm ar rducd by 74% and 38%. Th appropriat orbit altitud of th AIS satllit systm

11 Dtction Probability Analysis of Spac-Basd AIS Signal by Using Gomtrical Modl 171 should b around km with th imum signal covrag of nm ara and th imum FOV of 80.0 as th majority of powr of signal transmittd from shipborn 12 W-powr AIS transmittrs locatd within th imum signal covrag ara can b rcivd with th sufficint margin of signal powr by th LEO satllit. But du to th insufficint CIR of signal rcivd, for th spac-basd AIS systm, th masur of sparating th collision mssags should b furthr takn to corrctly dtct and dcod th mssags and incras th probability and th rliability of AIS signal rcption from spac. Acknowldgmnt Th rsarch is sponsord by Scinc and Tchnology Commission of Shanghai Municipality (Projct No.: ). Rfrncs [1] T. Wahl, G.K. Høy, Nw possibl rols of small satllits in maritim survillanc, in: Procdings of th 4th IAA Symposium on Small Satllits for Earth Obsrvation, Brlin, Grmany, April 7-11, 2003, pp [2] T. Eriksn, G.K. Høy, G. Narhim, B.J. Mland, Maritim traffic monitoring using a spac-basd AIS rcivr, Acta Astronautica 58 (2006) [3] M.A. Crvra, A. Ginsi, On th prformanc analysis of a satllit-basd AIS systm, in: Procdings of 10th Intrnational Workshop on Signal Procssing for Spac Communications, Rhods Island, Grc, Octobr 6-8, 2008, pp [4] G.K.J. Høy, T. Eriksn, B.J. Mland, G. Narhim, Spac-basd AIS for global maritim traffic monitoring, Acta Astronautica 62 (2008) [5] K. Ritn, R. Schlanbusch, R. Kristiansn, F. Vdal, P.J. Nicklasson, P.C. Brntsn, Link and Dopplr analysis for th spac-basd AIS rcption, in: Procdings of 3rd Intrnational Confrnc on Rcnt Advancs in Spac Tchnologis, Istanbul, Turky, Jun 14-16, 2007, pp [6] Guidlins on th Univrsal Automatic Idntification Systm (AIS), IALA (Intrnational Association of Marin Aids to Navigation and Lighthous Authoritis),Volum 1-2, Paris, [7] Rcommndation on Prformanc Standards for an Univrsal Shipborn Automatic Idntification Systm (AIS) (Rsolution MSC.74 (69)), IMO, London, [8] Tchnical Charactristics for a Univrsal Shipborn Automatic Idntification Systm using Tim Division Multipl Accss in th VHF Maritim Mobil Band (ITU-R M ), ITU (Intrnational Tlcommunication Union), Gnva, [9] Y.L. Chn, H.J. Gng, Analyzing th possibility of signal rcption for spac-basd AIS systm, in: Procdings of th 10th Intrnational Confrnc of Chins Transportation Profssionals, Bijing, China, August 4-8, 2010, pp