A MEO Tracking and Data Relay Satellite System Constellation Scheme for China *

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1 Dec. 25 Journal of Electronic Science and Technology of China Vol.3 No.4 A MEO Tracking and Data Relay Satellite System Constellation Scheme for China * WU Ting-yong, WU Shi-qi, LING Xiang (School of Communication and Information Engineering, University of Electronic Science and Technology of China Chengdu 6154 China) Abstract A medium earth orbit (MEO) tracking and data relay system (TDRSS) constellation scheme for China is proposed. This system consists of MEO constellation, inter- links (ISLs) and terrestrial gateway station, which can provide continuous bidirectional data transmission links between low altitude spacecrafts and the terrestrial gateway station in China. Theoretical analysis and simulation results indicate that the proposed constellation can cover the global low altitude space sphere and earth surface of China continuously, and has a preferable practical perspective. Key words medium earth orbit; data relay system; inter- links; low altitude spacecraft Now communications and tracking to support China LEO spacecrafts are primarily accomplished with a network of terrestrial gateway station (TGS). Because the TGS network provides only limited coverage, the large volumes of valuable data generated by a LEO spacecraft have to be stored in the on-board memory, and dumped at high speed when the spacecraft is in view of one of TGSs [1]. In recent years, with the rapid development of space science and technology of China, the increasing need for continuous connectivity with LEO spacecrafts during the flying mission has resulted in greater interest in the deploying of data relay system. The data relay system provides communications and tracking services to LEO spacecrafts, and is aimed at providing advantages in the following areas [2] : 1) a significant increase in coverage area; 2) a reduction or virtual elimination of the requirement for on-board data storage; 3) a reduction of the time delay between transmission of the data from the user spacecraft and its reception by end user on the ground; 4) a potential minimization of secondary data distribution problems on the ground; 5) an increased flexibility in scheduling operations for users; 6) Lower costs. At present, geostationary orbit (GEO) is the primary orbit type used by existing data relay system such as NASA s tracking and data relay system (TDRSS), ESA s data relay (DRS), and Japanese data relay and tracking system (DRTSS) [1,3-4]. These relay systems have the common characteristic of using only one TGS. With the constraint that only the GEO relay in view of TGS can provide communications and tracking services, none of these systems has the capability to cover a global space sphere. A well-designed medium earth orbit (MEO) constellation with ISLs, forming an independent dynamic trunk network that can provide global coverage to space spherical surface, is a preferable alternative to GEO relay system. The use of ISLs given the capability of routing in the sky, and therefore increases the flexibility of the system and allows to minimize the number of TGSs required to support the constellation [5-6]. Theoretically, one regional TGS could be sufficient for global connectivity. In this paper a MEO-TDRSS constellation scheme that provides global coverage to space spherical surface is proposed. Furthermore, the basic system architecture and traffic distribution characteristic of MEO-TDRSS are discussed. 1 A Review of GEO Relay Satellite Systems 1.1 Geometry of LEO Spacecraft and Relay Satellite Communications between LEO user spacecrafts and relay are connected by inter-orbit link (IOL). The earth shadowing may prevent the implementation of the IOL according to simple geometric considerations as illustrated in Fig.1. Received * Supported by National Natural Science Foundation of China (No )

2 294 Journal of Electronic Science and Technology of China Vol.3 Considering an acceptable margin for extra attenuation through the atmosphere, a minimum protective height h p of 5 km between IOL and the earth s surface is reasonable [7]. S r h 1 In Fig.1, S r is the relay with altitude h 1 km and S L is a user spacecraft with altitude h 2 km. Re is the average earth radius. By applying trigonometry, the maximum earth central angle, α max, between S r and S L is: Re+ h p Re+ h p α max = arccos ( ) + arccos( ) (1) Re+ h Re+ h2 1 Given altitude of the relay and the user spacecraft, the equation can be solved for α max. In Fig.2, α max as function of h 2 with constant h 1 and h p is shown. From Fig.2, it can be seen that the coverage range of a relay to space spherical surface increases with increasing altitude of relay and user spacecraft. Based on Eq.(1), if h 1 is km (i.e. GEO altitude), h 2 is 3 km and h p is 5 km, α max is equal to It means that a single GEO relay can S L h p h 2 α man Fig.1 Geometry of LEO spacecraft and relay αmax and βmin/ ( ) Re h 1 =1 km h 1 =15 km h 1 =2 km h 1 =35786 km h 1 /km Fig.2 Maximum earth central angle α max and minimum slots separation β min Vs. LEO spacecraft altitude h 2 provide a max coverage earth central angle up to for 3 km spherical surface. Therefore, two GEO relay s with separation in longitude can cover the global 3 km spherical surface continuously. 1.2 Coverage Capability of GEO Relay Satellite System As mentioned before, a GEO relay system consisting of only two GEO s with proper slots separation can provide global coverage to space spherical surface. Base on the geometry illustrated in Fig.1, the minimum allowed slots separation, β min, of two GEO relay s guaranteeing global coverage to space spherical surface with altitude h 2 can be derived: β = 2(π α ) (2) min The relationship of minimum slots separation β min and the spherical altitude h 2 is also given in Fig.2. Fig.2 shows that β min decreases with increase of spherical altitude h 2. This results in a reduction of the implementation complexity of system. With the maximum slots separation 18, a relay system consisting of two GEO s can covers global space spherical surface down to 126 km. 1.3 Limitation of GEO Relay System Although GEO relay systems have been used to provide tracking and communications services to LEO spacecrafts for decades, none of these existing GEO relay systems provides global coverage capability to the LEO spherical surface due to the restrictions of GEO slots and location of TGS. For a relay system with two GEO s, the required orbit slots separation increases with decrease of low space altitude. For most countries in the world, the proper GEO slots with requisite separation are very difficult to obtain. On the other hand, only the relay in view of a TGS can provides data relay service to LEO spacecraft. Hence, even if these required orbit slots are permitted, the requirement for these GEO relay s to see TGSs within specified region with limited geographical span will constrain the deployment of the system. For a relay system with three or more s, the separation demand of the orbit slots can be relaxed, but at least one TGS located on the other hemisphere of the earth is required. In conclusion, it is very difficult to deploy a GEO relay system that can provide global coverage to LEO spacecrafts. max

3 No.4 WU Ting-yong et al: A MEO Tracking and Data Relay Satellite System Constellation Scheme for China Generic Architecture and Traffic Distribution Characteristic of MEO-TDRSS 2.1 The Limitation of GEO Relay System There are three s: space, user and ground. The space is composed of MEO relay s that are connected via relay ISLs. The user includes all kinds of spacecrafts whose altitude range from 3 km to 1 km. The ground consists of TGSs, a network control center (NCC) and operation control centers (OCCs). The NCC and OCCs handle overall network control functions, including resource management, operation, and orbiting control. The TGSs act as network interface between various external networks and the network. They also perform protocol, address and format conversions. There are three types of communication links used in MEO-TDRSS-inter-orbit links (IOLs) between LEO spacecrafts and relay s, inter- links (ISLs) between MEO relay s, and up-down links (UDLs) between relay s and GTS. 2.2 Traffic Distribution Characteristic According to the architecture of MEO-TDRSS, we can see that: Relay Manned spacecraft ISLs Space IOLs IOLs UDL Meteorological PSTN / ISDN Reconnaissance TGS Fig.3 MEO-TDRSS architecture User Ground 1) An IOL bears the transmission of data from a spacecraft to relay, and the tracking and telemetric signals from relay to a spacecraft. The traffic load of IOLs is relatively low. 2) A single relay will serve more than one spacecrafts. All received data must route to the proper landing relay that transmits data to the GTS. Hence, the traffic load of an ISL, as shown in Fig.3, is relative heavy than IOLs. 3) The UDLs have the heaviest traffic load because all relayed data will land to GTS from 1 or 2 relay s, depending on system design. It is obvious that the traffic load of the MEO-TDRSS has a convergent characteristic, which must be taken into account during the system designing process. 3 A MEO Constellation Scheme for TDRSS of China 3.1 Constellation Parameters The parameters of a MEO-TDRSS scheme for China based on common-track constellation are presented in Tab.1. Tab.1 Satellite index Satellite parameters of MEO constellation Altitude / km Satellite Parameters Inclination / ( ) RAAN / ( ) Initial phase angle / ( ) Sat Sat Sat Sat Sat Sat The MEO constellation utilizes 6 s configured in six planes of one each. The s will be at an altitude of km in circular orbits inclined at 55. Due to the careful selection of the separation of right ascension of ascending node (RAAN) and phase offset between s in adjacent planes, all s follow the same track on the earth s surface hence the name common-track [8-9]. 3.2 Coverage Performance In particular, according to the special user characteristic and functionality, the coverage performance of a MEO-TDRSS includes two aspects - the coverage performance to space spherical surface with chosen altitude and to earth s surface of China region Coverage Performance to Space Spherical Surface Because the LEO spacecrafts can be at any position on a space spherical surface, MEO-TDRSS should provide global coverage to that surface. Furthermore, a well-designed MEO-TDRSS should be

4 296 Journal of Electronic Science and Technology of China Vol.3 a fairness system that can provide identical quality of access service to any spacecraft anywhere on the space spherical surface. Coverage ratio merit is used to evaluate the overall coverage capability of a constellation to space sphere. Fig.4 shows the coverage ratio of the proposed MEO constellation to space sphere as function of spherical altitude. It can be seen that the proposed constellation provides global coverage to space sphere higher than 5 km, which ensures the communication service availability to most LEO spacecrafts. Probability / (%) %) ( y bili ba o P Mean optimal timal access access distance distance / km (km) Fig.5 The PDF of the MOAD for 3 km space sphere 6 Coverage ratio / (%) Sphere altitude / km Fig.4 Space sphere coverage ratio vs. spherical altitude We define the optimal access distance (OAD) of a spacecraft as the length of the shortest IOL between spacecraft and relay s at a given time instant. The time average of OAD is called mean optimal access distance (MOAD). For the proposed constellation, the probability density function (PDF) of MOAD for 3 km space sphere is illustrated in Fig.5. The global sphere mean and standard deviation of MOAD is listed in Tab.2. Tab. 2 Statistic characteristics of MOAD Statistic Characteristics Value / km Global Sphere Mean of MOAD Standard Deviation of MOAD 53 The 3 km sphere altitude is chosen because it is the lower limit of altitude for most existing LEO spacecrafts. Fig.5 and Tab.2 indicate that MOAD is with the range around km. The standard deviation is really small comparing with the global sphere mean value of MOAD. It means that the proposed constellation provides satisfactory fair coverage performance to space sphere with little fluctuation in MOAD. Latitude / ( ) ) ã u d e( Latit ) ã u d e( Latit Latitude / ( ) ? Longitude /( ) ( ) (a) Mean OE for earth surface of China Longitude /(( ) ) (b) Minimum OE for earth surface of China Fig.6 The contour plots for earth s surface of China Coverage Performance to Earth s Surface of China Due to the heavy traffic burdened by UDLs, the locations selection of TGSs becomes a crucial problem. It means that the MEO-TDRSS should provide the best possible coverage performance to TGSs to minimize the length of the UDLs, thus avoiding bottleneck link. Since the TGSs should be located in China, then coverage performance of constellation to TGSs is equivalent to coverage performance to earth surface of

5 No.4 WU Ting-yong et al: A MEO Tracking and Data Relay Satellite System Constellation Scheme for China 297 China. At a given time instant, a terrestrial point may see more than one relay s. The elevation to the nearest is called optimal elevation (OE) for a terrestrial point. The time average and minimum value of OE are called mean OE and minimum OE, which describe the overall and the worst case performance, respectively. In this paper, China region ranges from north latitude to 6 and east longitude 7 to 14. Fig.6 shows the contour plot of the mean OE and minimum OE for earth surface of China. It can be seen that the proposed constellation provides excellent coverage performance to China region. Most part of China region has a mean OE more than 5. Even in the worst case, the minimum OE for most part of China region is greater than 15, and more than half of China has a minimum OE greater than 2. So the TGSs can be located in a wide area flexibly. 4 Conclusions In this paper, a MEO-TDRSS constellation scheme for China is proposed. The main advantage of a MEO-TDRSS is to provide global coverage to space sphere. The generic architecture of this kind of system is also given, and the convergent characteristic of traffic load is discussed. The coverage performance of the proposed constellation has been evaluated. The simulation results show that the proposed constellation provides satisfactory overall coverage performance and fair access service to LEO user spacecrafts. It is expected that MEO-TDRSS will play an important role for space-based data relay services in future. References [1] Holmes W. NASA s tracking and data relay system[j]. IEEE Communications Magzine, 1978, 16(5), 13-2 [2] Berretta G, Agostini A D, Dickison A. The European data relay system: present cpncept and future evolution[j]. Proceedings of the IEEE, 199, 78(7): [3] Brandel D L, Watson W A, Weinberg A. NASA's advanced tracking and data relay system for the years 2 and beyond[j]. Proceedings of the IEEE, 199, 78(7): [4] Jackson A H, Guion W S, Chang R W. ATDRSS Program Overview[Z]. IEEE GLOBECOM 91, Phoneix, Arizona, [5] Zaim A H, Perros H G, Rouskas G N. Computing call-blocking probabilities in LEO constellation[j]. IEEE Transactions on Vehicular Technology, 22, 52(3): [6] Ibnkahla M, Rahman Q M, Sulyman A I, et al. Hogh-speed mobile communications: technologies and challenges[j]. Proceedings of the IEEE, Feb. 24, 92(2): [7] Werner M, Frings J, Wauquiez F, et al. Topological design, routing and capacity dimensioning for ISL networks in broadband LEO systems[j]. International Journal of Satellite Communications, 21, 19(6): [8] He J, Wu J, Ma Y. A New Mobile Communication Constellation for Chinese Area[Z]. WCC-ICCT 2, Beijing China, [9] He Jiafu, Wu Jiuyin Ma Yifei, et al. Design of the Mobile Satellite Communications Constellations with Common Ground Track[Z]. APCC/OECC 99, Beijing China, Brief Introduction to Author(s) WU Ting-yong ( 吴廷勇 ) was born in Sichuan Province, China, in He received his B.S.E. and M.S. degrees in communications and information system from University of Electronic Science and Technology of China (UESTC) in 1998 and 21, respectively. Now, he is a Lecturer with UESTC. His research interests lie in mobile communication, including NGEO constellation design and resource management in system. WU Shi-qi ( 吴诗其 ) was born in Sichuan province, China, in Now, he is a professor, Ph.D. supervisor with UESTC and a member of China Institute of Communications. His research interests include information and communication theory, mobile communication and personal communication. LING Xiang ( 凌翔 ) was born in Sichuan province, China, in He received his B.S.E., M.S. and Ph.D. degrees in communications and information system from UESTC in 1997, 2 and 24, respectively. In 23, he was a research assistant with Hong Kong University working in the area of wireless networks. Now, he is a lecturer with UESTC. His research interests lie in wireless networks, broadband access networks and SOC.