The Coverage Analysis for Low Earth Orbiting Satellites at Low Elevation

Transcription

1 (IJACSA) Intrnational Journal of Advancd Computr Scinc and Applications, Vol. 5, No. 6, 14 Th Covrag Analysis for Low Earth Orbiting Satllits at Low Elvation Shklzn Cakaj 1, Bxht Kamo 1, Algnti Lala 1, Alban Rakipi 1 1 Faculty of Information Tchnology Polytchnic Univrsity of Tirana Tirana, Albania Abstract Low Earth Orbit (LEO) satllits ar usd for public ntworking and for scintific purposs. Communication via satllit bgins whn th satllit is positiond in its orbital position. Ground stations can communicat with LEO satllits only whn th satllit is in thir visibility rgion. Th duration of th visibility and th communication vary for ach LEO satllit pass ovr th station, sinc LEO satllits mov too fast ovr th Earth. Th satllit covrag ara is dfind as a rgion of th Earth whr th satllit is sn at a minimum prdfind lvation angl. Th satllit s covrag ara on th Earth dpnds on orbital paramtrs. Th communication undr low lvation angls can b hindrd by natural barrirs. For saf communication and for savings within a link budgt, th covrag undr too low lvation is not always providd. LEO satllits organizd in constllations act as a convnint ntwork solution for ral tim global covrag. Global covrag modl is in fact th complmntary ntworking procss of individual satllit s covrag. Satllit covrag strongly dpnds on lvation angl. To conclud about th covrag variation for low orbiting satllits at low lvation up to 1º, th simulation for attituds from 6km to 1km is prsntd through this papr. Kywords LEO; satllit; covrag I. INTRODUCTION Gnrally, satllit circular orbits ar catgorizd as Gosynchronous Earth Orbits (GEO), Mdium Earth Orbits (MEO) and Low Earth Orbits (LEO). Th main diffrnc among thm is in th attitud abov th Earth surfac [1]-[3]. Th satllits travrsing in orbits of attituds up to around 14 km (limitd by Van Alln blt [4]) ar considrd as LEO satllits. LEO satllits ar moving at around 7.5 km/s vlocity rlativ to a fixd point on th Earth (ground station) [5]. Th charactristics of LEOs ar: th shortst distanc from th Earth compard with othr orbits and consquntly lss tim dlay. Ths charactristics mak thm vry attractiv vn for scintific applications or communications ntworking [5], [6]. Th singl satllit covrag ara is dfind as a rgion of th Earth whr th satllit is sn at a minimum prdfind lvation angl. For multi satllit covrag or global covrag th managmnt policy for satllit coordination must b applid. For global covrag, handovr and managmnt policis bcom mor critical undr too low lvation bcaus of natural barrirs. andovr policis and managmnt ar wll analyzd undr [7] and [8]. Analysis of random covrag tim in mobil LEO satllit communications is also wll tratd by [9]. This papr discusss th singl LEO satllit covrag aspcts as an ovrtur to th global covrag. Som charactristics of LEO satllits ar givn followd by covrag gomtry. Finally th rsults of covrag simulation undr diffrnt attituds from 6km to 1km at low lvation ar prsntd. II. LEO SATELLITES AND COVERAGE Microsatllits in Low Earth Orbits (LEO) hav bn in us for th past two dcads, mainly ddicatd for scintific purposs. LEO satllits hav vry wid scintific applications, from rmot snsing of ocans, through analyss on Earth s climat changs, Earth s imagry with high rsolution or astronomical purposs. Ths satllits provid opportunity for invstigations for which altrnativ tchniqus ar ithr difficult or impossibl to b applid. Thus, it may b xpctd that such scintific missions will b furthr dvlopd in th nar futur spcially in filds whr similar xprimnts by purly Earth-basd mans ar impracticabl. Ground stations hav to b stablishd in ordr to communicat with such satllits. Ground stations can communicat with LEO (Low Earth Orbiting) satllits only whn th satllit is in thir visibility rgion. Satllits in ths orbits hav an orbital priod of around (9-11) minuts. For satllits this is a short flyovr priod, which mans that th antnna at th ground station must follow th satllit vry fast with high pointing accuracy. Th contact communication tim btwn th satllit and th ground station taks (5-15) minuts 6-8 tims during th day [6]. Th ubbl Spac Tlscop, for xampl, oprats at an altitud of about 61 km with an orbital priod of 97 minuts [6]. Evry satllit (spcially, microsatllit whn is ddicatd for scintific purposs) carris spcial instrumnts that nabl it to prform its mission [5] (for xampl, a satllit that studis th univrs has a tlscop, a satllit that hlps forcast th wathr carris camras to track th movmnt of clouds). On othr hand from th communication prspctiv th goal of th futur communication systms is to provid high quality broadband srvics with global covrag [1]. Th satllit constllation is a convnint ntwork solution for ral tim global covrag. Th constllation is a systm of low Earth orbit (LEO) idntical satllits, launchd in svral orbital plans with th orbits having th sam altitud. Th 6 P a g

2 (IJACSA) Intrnational Journal of Advancd Computr Scinc and Applications, Vol. 5, No. 6, 14 satllits mov in a synchronizd mannr in trajctoris rlativ to Earth. Th application of low Earth orbit satllits organizd in a constllation is an altrnativ to wirlss tlphon ntworks. Satllits in low orbits arrangd in a constllation, work togthr by rlaying information to ach othr and to th usrs on th ground. If satllits within a constllation ar quippd with advancd on-board procssing, thy can communicat dirctly with ach othr by lin of sight using intr-satllit links (ISL) [5]. To provid a global covrag to a divrs usr population a numbr of LEO satllit ntworks hav bn proposd and implmntd. Th LEO satllit ntworks can support both th aras with trrstrial wirlin and wirlss ntworks that lack any ntwork infrastructur [7]. Nowadays Svral LEO constllations (Globalstar, Iridium, Ellipso) ar activ and oprational [7], [8]. For complt covrag of th Earth s surfac som ovrlapping btwn th adjacnt satllits is ncssary, to kp th continuity of ral tim srvics [9]. Th global covrag can b considrd as an introprabl complmntary ntworking procss of multipl satllits organizd in constllation, ach of thm contributing with its individual covrag. This is achivd bcaus LEO satllits mov with rspct to a fixd obsrvr on Earth surfac, and along with satllit movmnt also th covrag ara changs its position continually crating a covrag blt as in Fig.1. Satllits undr th sam attitud undr diffrnt inclination mak diffrnt blts, nabling global covrag. Individual satllit covrag for fw LEO satllits is prsntd in Fig.. Fig. 1. Fw LEO covrag ara (Sourc: NOAA, 9) Whn dsigning a satllit ntwork som dcisions such as th slction of th orbit paramtrs, covrag modl, th ntwork connctivity and routing modl must b mad. LEO satllit ntworks as Iridium, Tldsic, Globalstar, hav architctur diffrncs affcting th capabilitis and srvics. Th main problm for th global covrag is th handovr procss from on covrag ara to anothr on. This is calld satllit hand ovr procss [7]. Diffrnt dtrministic modls for covrag tim valuation of low Earth orbiting satllit ar dvlopd. Modls involv statistical covrag tim assssmnts [8]. Th analyss ar particularly usful for probabilistic invstigation of intrsatllit handovrs in LEO satllit ntworks [8]. Th probability of srvic intrruption and hand ovr mchanism bcoms important for th ovrall systm prformanc [9]. Achivmnts in antnna tchnology ladd to multibam LEO systms whr th footprint or covrag ara is dividd in many clls (multibam arrays) in ordr to nhanc frquncy rus policis. Applying spac divrsity policy is achivd frquncy rus insid a footprint. andovr from on cll to anothr is dfind as cll handovr. Particularly th intrfrnc problms hav carfully to b tratd [11] - [13]. III. LEO COVERAGE GEOMETRY Th position of th satllit within its orbit considrd from th ground station point of viw is dfind by Azimuth (Az) and Elvation ( ) angls. Th azimuth is th angl of th dirction of th satllit, masurd in th horizon plan from gographical north in clockwis dirction. Th rang of azimuth is º to 36º. Th lvation is th angl btwn a satllit and th obsrvr s (ground station s) horizon plan. Th rang of lvation is º to 9º. Th covrag ara of a singl satllit is a circular ara (Fig. ) on th Earth surfac in which th satllit can b sn undr an lvation angl qual or gratr than th minimum lvation angl dtrmind by th link budgt rquirmnts of th systm. Th largst covrag ara is achivd undr lvation of º, but in ordr to avoid obstacls causd by natural barrirs at too low lvation, usually for th link budgt calculations it is dtrmind th minimal lvation angl which rangs on (-1)º. For simulation purposs of covrag it is considrd th lvations up to 1 º. Th satllit s covrag ara on th Earth dpnds on orbital paramtrs. Ground stations (GS) can communicat with LEO (Low Earth Orbiting) satllits only whn th ground station is undr covrag ara (satllit footprint) as prsntd in Fig. 3. Th duration of th visibility and consquntly th communication duration vary for ach LEO satllit pass ovr th ground station, sinc LEO satllits mov too fast ovr th Earth. Along with satllit, th footprint movs also, laving th GS out of th footprint and consquntly loosing th communication with th GS, as prsntd in Fig. 4. Fig.. Fw LEOs covrag ara (Sourc: NOAA, 9) 7 P a g

3 (IJACSA) Intrnational Journal of Advancd Computr Scinc and Applications, Vol. 5, No. 6, 14 Fig. 5. Ground station gomtry Fig. 3. Th ground station (GS) undr th LEO covrag ara. Th most ndd paramtr is th slant rang d (distanc from th ground station to th satllit). This paramtr will b usd during th link budgt calculation, and it is xprssd through lvation angl. Applying cosins law for triangl at Fig. 5 yilds out: r R d Rd cos(9 ) (4) Solving (4) by d, substituting, r R at (5) and applying (1), (), (3) finally w will gt th slant rang as function of lvation angl [14]. R d ( ) R cos sin (5) R is th satllit attitud abov th Earth s surfac. Transforming Fig. 5 from th covrag point of viw it looks lik in Fig. 6. Fig. 4. Th ground station (GS) out off th LEO covrag ara. Th basic gomtry btwn a satllit and ground station is dpictd in Fig. 5. Th points indicat th satllit (SAT), ground station (P), and thn th third is th Earth s cntr. Th lin passing at point P rprsnts horizon plan. Th subsatllit point is indicatd by T. Two sids of this triangl ar usually known (th distanc from th ground station to th 3 Earth s cntr, R E 6378x1 m and th distanc from th satllit to Earth s cntr-orbital radius). Thr ar four variabls in this triangl: - is lvation angl, - is nadir angl, - is cntral angl and d is slant rang. As soon as two quantitis ar known, th othrs can b found with th following quations [14]: 9 (1) d cos r sin () d sin R sin (3) Fig. 6. Covrag gomtry 8 P a g

4 Covrag (IJACSA) Intrnational Journal of Advancd Computr Scinc and Applications, Vol. 5, No. 6, 14 In Fig. 6, thr ar two triangls. Th largr on rprsnts th cas of full covrag undr lvation of º. Gnrally, from th smallr triangl applying sinus thorm yilds out: sin sin 9 R R R sin cos R Th maximal covrag is achivd for, thus for known attitud, asy is calculatd th covrag angl for maximal covrag, as: (6) (7) (sin ) R MAX, R (8) Similarly, for diffrnt lvations ( ) can b calculatd and thn basd on (1) also. Th surfac of th covrag ara dpnds on angl and it is [15]: S Covrag R (1 cos ) Usually, th satllit covrag ara or th satllit s footprint is xprssd (in prcntag) as a fraction of th Earth s ara. (1) S Covrag (%) S Covrag Earth R 1 Covrag(%) 1 cos IV. LEO COVERAGE SIMULATION 4R (9) (1 cos ) (11 Basd on (7), (8) and (11) it is obvious that th satllit covrag strongly dpnds on lvation angl. To conclud about th covrag variation for low orbiting satllits at low lvation, th simulation for attituds from 6km up to 1km is furthr discussd. For a givn satllit attitud and a givn lvation angl firstly should b calculatd, and finally th covrag basd on (1). For attituds of =6km, 8km, 1km and 1km which ar typical low orbit attituds it is simulatd and calculatd th covrag ara for lvation of (- 1) by stps of, and rsults ar prsntd in Tabl I and Fig. 7. Tabl I and Fig. 7 confirm th dcras of covrag ara as lvation angl incrass for th alrady dfind attitud, and th incras of th covrag ara as attitud incrass kping th fixd lvation. TABLE I. Orbital Attitud Elvation ( ) 8.% 6.% 4.%.%.% COVERAGE ARE AS A FRACTION OF EART AREA. 6 Covrag [%] 8 Covrag [ % ] Th LEO covrag as a fraction of th Earth's surfac Elvation 1 Covrag [ %] 1 6Km 8Km 1Km 1Km Fig. 7. Covrag ara variation for diffrnt attituds at low lvation 1 Covrag [ % ] Fig. 8. LEO covrag ara 9 P a g

5 (IJACSA) Intrnational Journal of Advancd Computr Scinc and Applications, Vol. 5, No. 6, 14 Finally, in Fig. 8, applying satllit orbit analysis softwar is prsntd th cas of simulatd covrag ara for synchronizd orbits at an attitud of 6km for diffrnt inclination (fw orbits) at lvation of 1 º, as th smallst covrag ara stmmd from th simulation considrd in this papr. Small circls in Fig. 8 rprsnt LEO covrag ara on th Earth s surfac. For th ral tim srvics th main problm du to global covrag rmains th handovr procss from on covrag ara to anothr on. For ral tim srvics it is too important th rliabl communication what can b disturbd undr to low lvation bcaus of natural barrirs. Our futur work will b orintd on corrlation of singl covrag ara with GIS (Gographic Information Systm) in ordr to hav th xact information undr which lvation th saf communication could b providd. Th simulation tools will b ssntial part of forsn futur work. CONCLUSION Th satllit s covrag ara on th Earth dpnds on orbital paramtrs and usually is xprssd (in prcntag) as a fraction of th Earth s ara. LEO satllits organizd in constllations act as a convnint solution for ral tim global covrag. Satllit covrag strongly dpnds on lvation angl. Th largst covrag ara is achivd undr lvation of º, but in ordr to avoid obstacls causd by natural barrirs at too low lvation, usually for th link budgt calculations it is dtrmind th minimal lvation angl which rangs on (- 1)º. Through simulation for typical LEO attituds on rang of (6-1) km at low lvation of (-1) º, it is confirmd that th fraction of Earth covrd by satllits at appropriat attituds is from 1.69% to 7.95%. REFERENCES [1] M. Richharia, (1999), Satllit communication systms, McGraw ill, Nw York, [] D. Roddy, Satllit communications, McGraw ill, Nw York, 6. [3] G. Maral and M. Bousqut, Satllit communication systms, John Willy & Sons, Ltd, Chichstr, England,. [4] -blt [5] R. E. Z, t al, Th MOST Microsatllit: A low cost nabling tchnology for futur spac scinc and tchnology missions, Canadian Aronautics and Spac Journal, 48(1), Canada, pg. 1-11,. [6] J.E. Obrright, Satllit artificial, World Book Onlin Rfrnc Cntr, World Book, Inc, 4. [7] I. F. Alyildiz,. Uzunalioglu, M. D. Bndr,Turkiy, andovr managmnt in Low Earth Orbit (LEO) satllit ntworks Mobil Ntworks and Applications 4 (1999) [8] P. Papaptrou, S. Karapantazis, F.N. Pavlidou, andovr Policis in LEO Systms with Satllit Divrsity, Intrnational Confrnc on Advancd Satllit Mobil Systms (ASMS 3), 1-11 July, 3, Frascati, Italy. [9] Y. Sydi, S. M. Safavi, On th Analysis of Random Covrag Tim in Mobil LEO Satllit Communications, Communications Lttrs, IEEE, Volum 16, Issu 5, MAY 1. [1] A. Botta, A. Pscap, Nw gnration satllit broadband Intrnt srvic: should ADSL and 3G worry, TMA 13, co-lactd with IEEE INFOCOM 13, April 13, Turin, Italy. [11] S. Cakaj, Modulation Indx Application for Satllit Adjacnt Downlink Intrfrnc Idntification, Th 6th Europan Confrnc on Antnnas and Propagation EUCAP 1, IEEE, March 6-3, 1 Pragu, Czch Rpublic, pp. -4. [1] S. Cakaj, W. Kim, K. Malaric, Intrmodulation by Uplink Signal at Low Earth Orbiting Satllit Ground Station, 18 th Intrnational Confrnc on Applid Elctromagntics and Communications, ICECom, IEEE, 1-14 Octobr 5, Dubrovnik, Croatia, pp [13] S. Cakaj, K. Malaric, A. L. Schotlz, Modlling of Intrfrnc Causd by Uplink Signal for Low Earth Orbiting Satllit Ground Stations, 17 th IASTED Intrnational Confrnc on Applid Simulation and Modlling, ASM 8, Jun, 3 5, 8, Corfu, Grc, pp [14] G.D. Gordon, W.L. Morgan, Principls of communication satllits, John Wily & sons, Inc [15]. Curtis, Orbital Mchanics for Enginring Studnts, Elsvir arospac nginring sris, pg. 55, AUTOR PROFILE Shklzn Cakaj has rcivd his BSc and MSc dgrs from Prishtina Univrsity in Kosovo. Sinc 3 is cooprating with Institut for Communication and Radio Frquncy Enginring at th Tchnical Univrsity in Vinna, whr h has prpard his Mastr Thsis rlatd to th prformanc of th ground satllit station in July, 4. was awardd a PhD in ara of satllit communication from Zagrb Univrsity in January 8 with whom h has continud tchnical associations. has attndd courss on satllit communication and spctrum managmnt at USTTI. was awardd as Fulbright scholar rsarchr in 9 at NOAA (National Ocanic and Atmosphric Administration) at Maryland, USA. is th author of 45 paprs publishd in worldwid confrncs and journals; mostly IEEE. is ara of intrst is th prformanc of satllit ground stations for scintific satllits. is working at Post and Tlcommunication of Kosovo and lcturing satllit communications for mastr studnts at Prishtina Univrsity, Kosovo and Polytchnic Univrsity of Tirana, Albania. 1 P a g