Dennis Roddy i
Satellite Communications Dennis Roddy
Fourth Edition
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Contents
Preface xi
Chapter 1. Overview of Satellite Systems 1 1.1 Introduction 1 1.2 Frequency Allocations for Satellite Services 2 1.3 INTELSAT 4 1.4 U.S. Domsats 9 1.5 Polar Orbiting Satellites 12 1.6 Argos System 18 1.7 Cospas-Sarsat 19 1.8 Problems 25 References 26 Chapter 2. Orbits and Launching Methods 29 2.1 Introduction 29 2.2 Kepler’s First Law 29 2.3 Kepler’s Second Law 30 2.4 Kepler’s Third Law 31 2.5 Definitions of Terms for Earth-Orbiting Satellites 32 2.6 Orbital Elements 35 2.7 Apogee and Perigee Heights 37 2.8 Orbit Perturbations 38 2.8.1 Effects of a nonspherical earth 38 2.8.2 Atmospheric drag 43 2.9 Inclined Orbits 44 2.9.1 Calendars 45 2.9.2 Universal time 46 2.9.3 Julian dates 47 2.9.4 Sidereal time 49 2.9.5 The orbital plane 50 2.9.6 The geocentric-equatorial coordinate system 54 2.9.7 Earth station referred to the IJK frame 56 2.9.8 The topocentric-horizon coordinate system 62 Dennis Roddy iv
2.9.9 The subsatellite point 64 2.9.10 Predicting satellite position 66 2.10 Local Mean Solar Time and Sun-Synchronous Orbits 66 2.11 Standard Time 70 2.12 Problems 71 References 75 Chapter 3. The Geostationary Orbit 77 3.1 Introduction 77 3.2 Antenna Look Angles 78 3.3 The Polar Mount Antenna 85 3.4 Limits of Visibility 87 3.5 Near Geostationary Orbits 89 3.6 Earth Eclipse of Satellite 92 3.7 Sun Transit Outage 94 3.8 Launching Orbits 94 3.9 Problems 99 References 101 Chapter 4. Radio Wave Propagation 103 4.1 Introduction 103 4.2 Atmospheric Losses 103 4.3 Ionospheric Effects 104 4.4 Rain Attenuation 106 4.5 Other Propagation Impairments 111 4.6 Problems and Exercises 111 References 112 Chapter 5. Polarization 115 5.1 Introduction 115 5.2 Antenna Polarization 120 5.3 Polarization of Satellite Signals 123 5.4 Cross-Polarization Discrimination 128 5.5 Ionospheric Depolarization 130 5.6 Rain Depolarization 131 5.7 Ice Depolarization 133 5.8 Problems and Exercises 133 References 136 Chapter 6. Antennas 137 6.1 Introduction 137 6.2 Reciprocity Theorem for Antennas 138 6.3 Coordinate System 139 6.4 The Radiated Fields 140 6.5 Power Flux Density 144 6.6 The Isotropic Radiator and Antenna Gain 144 Dennis Roddy v
6.7 Radiation Pattern 145 6.8 Beam Solid Angle and Directivity 146 6.9 Effective Aperture 148 6.10 The Half-Wave Dipole 149 6.11 Aperture Antennas 151 6.12 Horn Antennas 155 6.12.1 Conical horn antennas 155 6.12.2 Pyramidal horn antennas 158 6.13 The Parabolic Reflector 159 6.14 The Offset Feed 165 6.15 Double-Reflector Antennas 167 6.15.1 Cassegrain antenna 167 6.15.2 Gregorian antenna 169 6.16 Shaped Reflector Systems 169 6.17 Arrays 172 6.18 Planar Antennas 177 6.19 Planar Arrays 180 6.20 Reflectarrays 187 6.21 Array Switching 188 6.22 Problems and Exercises 193 References 196 Chapter 7. The Space Segment 199 7.1 Introduction 199 7.2 The Power Supply 199 7.3 Attitude Control 202 7.3.1 Spinning satellite stabilization 204 7.3.2 Momentum wheel stabilization 206 7.4 Station Keeping 209 7.5 Thermal Control 211 7.6 TT&C Subsystem 212 7.7 Transponders 213 7.7.1 The wideband receiver 215 7.7.2 The input demultiplexer 218 7.7.3 The power amplifier 218 7.8 The Antenna Subsystem 225 7.9 Morelos and Satmex 5 227 7.10 Anik-Satellites 231 7.11 Advanced Tiros-N Spacecraft 232 7.12 Problems and Exercises 235 References 236 Chapter 8. The Earth Segment 239 8.1 Introduction 239 8.2 Receive-Only Home TV Systems 239 8.2.1 The outdoor unit 241 8.2.2 The indoor unit for analog (FM) TV 242 8.3 Master Antenna TV System 243 Dennis Roddy vi
8.4 Community Antenna TV System 244 8.5 Transmit-Receive Earth Stations 246 8.6 Problems and Exercises 250 References 251 Chapter 9. Analog Signals 253 9.1 Introduction 253 9.2 The Telephone Channel 253 9.3 Single-Sideband Telephony 254 9.4 FDM Telephony 256 9.5 Color Television 258 9.6 Frequency Modulation 265 9.6.1 Limiters 266 9.6.2 Bandwidth 266 9.6.3 FM detector noise and processing gain 269 9.6.4 Signal-to-noise ratio 272 9.6.5 Preemphasis and deemphasis 273 9.6.6 Noise weighting 274 9.6.7 S/N and bandwidth for FDM/FM telephony 276 9.6.8 Signal-to-noise ratio for TV/FM 278 9.7 Problems and Exercises 279 References 281 Chapter 10. Digital Signals 283 10.1 Introduction 283 10.2 Digital Baseband Signals 283 10.3 Pulse Code Modulation 288 10.4 Time-Division Multiplexing 292 10.5 Bandwidth Requirements 293 10.6 Digital Carrier Systems 296 10.6.1 Binary phase-shift keying 298 10.6.2 Quadrature phase-shift keying 300 10.6.3 Transmission rate and bandwidth for PSK modulation 302 10.6.4 Bit error rate for PSK modulation 303 10.7 Carrier Recovery Circuits 309 10.8 Bit Timing Recovery 310 10.9 Problems and Exercises 311 References 313 Chapter 11. Error Control Coding 315 11.1 Introduction 315 11.2 Linear Block Codes 316 11.3 Cyclic Codes 321 11.3.1 Hamming codes 321 11.3.2 BCH codes 322 11.3.3 Reed-Solomon codes 322 11.4 Convolution Codes 324 11.5 Interleaving 328 11.6 Concatenated Codes 330 Dennis Roddy vii
11.7 Link Parameters Affected by Coding 331 11.8 Coding Gain 333 11.9 Hard Decision and Soft Decision Decoding 334 11.10 Shannon Capacity 336 11.11 Turbo Codes and LDPC Codes 338 11.11.1 Low density parity check (LDPC) codes 341 11.12 Automatic Repeat Request (ARQ) 344 11.13 Problems and Exercises 346 References 348 Chapter The Space Link 351 12. 12.1 Introduction 351 12.2 Equivalent Isotropic Radiated Power 351 12.3 Transmission Losses 352 12.3.1 Free-space transmission 353 12.3.2 Feeder losses 354 12.3.3 Antenna misalignment losses 355 12.3.4 Fixed atmospheric and ionospheric losses 356 12.4 The Link-Power Budget Equation 356 12.5 System Noise 357 12.5.1 Antenna noise 358 12.5.2 Amplifier noise temperature 360 12.5.3 Amplifiers in cascade 361 12.5.4 Noise factor 362 12.5.5 Noise temperature of absorptive networks 363 12.5.6 Overall system noise temperature 365 12.6 Carrier-to-Noise Ratio 366 12.7 The Uplink 367 12.7.1 Saturation flux density 368 12.7.2 Input backoff 370 12.7.3 The earth station HPA 371 12.8 Downlink 371 12.8.1 Output back-off 373 12.8.2 Satellite TWTA output 374 12.9 Effects of Rain 375 12.9.1 Uplink rain-fade margin 377 12.9.2 Downlink rain-fade margin 377 12.10 Combined Uplink and Downlink C/N Ratio 380 12.11 Intermodulation Noise 383 12.12 Inter-Satellite Links 384 12.13 Problems and Exercises 393 References 397 Chapter Interference 399 13. 13.1 Introduction 399
13.2 Interference between Satellite Circuits (B1 and B2 Modes) 401 13.2.1 Downlink 403 13.2.2 Uplink 404 13.2.3 Combined [C/I] due to interference on both uplink and 405 downlink 13.2.4 Antenna gain function 405 Dennis Roddy viii
13.2.5 Passband interference 407 13.2.6 Receiver transfer characteristic 408 13.2.7 Specified interference objectives 409 13.2.8 Protection ratio 410 13.3 Energy Dispersal 411 13.4 Coordination 413 13.4.1 Interference levels 413 13.4.2 Transmission gain 415 13.4.3 Resulting noise-temperature rise 416 13.4.4 Coordination criterion 417 13.4.5 Noise power spectral density 418 13.5 Problems and Exercises 419 References 421 Chapter Satellite Access 423 14. 14.1 Introduction 423 14.2 Single Access 424 14.3 Preassigned FDMA 425 14.4 Demand-Assigned FDMA 430 14.5 Spade System 430 14.6 Bandwidth-Limited and Power-Limited TWT Amplifier 432 Operation 14.6.1 FDMA downlink analysis 433 14.7 TDMA 436 14.7.1 Reference burst 440 14.7.2 Preamble and postamble 442 14.7.3 Carrier recovery 443 14.7.4 Network synchronization 444 14.7.5 Unique word detection 448 14.7.6 Traffic data 451 14.7.7 Frame efficiency and channel capacity 451 14.7.8 Preassigned TDMA 452 14.7.9 Demand-assigned TDMA 455 14.7.10 Speech interpolation and prediction 455 14.7.11 Downlink analysis for digital transmission 459 14.7.12 Comparison of uplink power requirements for FDMA and 461 TDMA 14.8 On-Board Signal Processing for FDMA/TDM Operation 463 14.9 Satellite-Switched TDMA 467 14.10 Code-Division Multiple Access 472 14.10.1 Direct-sequence spread spectrum 473 14.10.2 The code signal c(t) 473 14.10.3 Acquisition and tracking 477 14.10.4 Spectrum spreading and despreading 478 14.10.5 CDMA throughput 481 14.11 Problems and Exercises 483 References 488 Chapter Satellites in Networks 491 15. 15.1 Introduction 491 15.2 Bandwidth 492 15.3 Network Basics 492 Dennis Roddy ix
15.4 Asynchronous Transfer Mode (ATM) 494 15.4.1 ATM layers 495 15.4.2 ATM networks and interfaces 497 15.4.3 The ATM cell and header 497 15.4.4 ATM switching 499 15.4.5 Permanent and switched virtual circuits 501 15.4.6 ATM bandwidth 501 15.4.7 Quality of service 504 15.5 ATM over Satellite 504 15.6 The Internet 511 15.7 Internet Layers 513 15.8 The TCP Link 516 15.9 Satellite Links and TCP 517 15.10 Enhancing TCP Over Satellite Channels Using Standard 519 Mechanisms (RFC-2488) 15.11 Requests for Comments 521 15.12 Split TCP Connections 522 15.13 Asymmetric Channels 525 15.14 Proposed Systems 527 15.15 Problems and Exercises 527 References 530 Chapter Direct Broadcast Satellite (DBS) Television 531 16. 16.1 Introduction 531 16.2 Orbital Spacing 531 16.3 Power Rating and Number of Transponders 533 16.4 Frequencies and Polarization 533 16.5 Transponder Capacity 533 16.6 Bit Rates for Digital Television 534 16.7 MPEG Compression Standards 536 16.8 Forward Error Correction (FEC) 541 16.9 The Home Receiver Outdoor Unit (ODU) 542 16.10 The Home Receiver Indoor Unit (IDU) 544 16.11 Downlink Analysis 546 16.12 Uplink 553 16.13 High Definition Television (HDTV) 554 16.13.1 HDTV displays 554 16.14 Video Frequency Bandwidth 555 16.15 Problems and Exercises 557 References 560 Chapter Satellite Mobile and Specialized Services 561 17. 17.1 Introduction 561 17.2 Satellite Mobile Services 562 17.3 VSATs 564 17.4 Radarsat 566 17.5 Global Positioning Satellite System (GPS) 569 17.6 Orbcomm 572 Dennis Roddy x
17.7 Iridium 576 17.8 Problems and Exercises 582 References 583
Appendix A. Answers to Selected Problems 585 Appendix B. Conic Sections 591 Appendix C. NASA Two-Line Orbital Elements 609 Appendix D. Listings of Artificial Satellites 613 Appendix E. Illustrating Third-Order Intermodulation Products 615 Appendix F. Acronyms 617 Appendix G. Logarithmic Units 625 Index 631 Dennis Roddy xi
Preface
As with previous editions, the fourth edition provides broad coverage of satellite communications, while maintaining sufficient depth to lay the foundations for more advanced studies. Mathematics is used as a descriptive tool and to obtain numerical results, but lengthy mathematical derivations are avoided. In setting up numerical problems and examples the author has made use of Mathcad™, a computer program, but the worked examples in the text are presented in normal algebraic notation, so that other programs, including programmable calculators can be used. The main changes compared to the previous edition are as follows. In Chap. 1 the sections on INTELSAT and polar orbiting satellites, including environmental and search and rescue, have been updated. Sun-synchronous orbits have been treated in more detail in Chap. 2. A new section on planar antennas and arrays, including reflectarrays, and array switching has been added in Chap. 6. Chapter 8 includes additional details on C-band reception of television signals. In Chap. 11, a more detailed description is given of linear block codes, and new sections on the Shannon capacity and turbo and low density parity check (LDPC) codes have been included. Chapter 12 has a new section on intersatellite links (ISLs), including optical links. Chapter 15 has been extensively rewritten to include more basic details on networks and asynchronous transfer mode (ATM) operation. Chapter 16 covers high definition television (HDTV) in more detail, and the Iridium mobile satellite system, which is now under new ownership, is described in Chap. 17. In this age of heightened security concerns, it has proved difficult to get detailed technical information on satellite systems and equipment. Special thanks are, therefore, due to the following people and organizations that provided copies of technical papers, diagrams, and figures for the topics listed:
Planar antennas and arrays, reflectarrays, and array switching: Jacquelyn Adams, Battelle/GLITeC; Dr. Luigi Boccia, Universita della Dennis Roddy xii
Calabria; Thomas J. Braviak, Director, Marketing Administration, Aeroflex/KDI-Integrated Products; Dr. Michael Parnes, Ascor, Saint-Petersburg, Russia; Dharmesh Patel, Radar Division Naval Research Laboratory; Professor David Pozar, Electrical and Computer Engineering, University of Massachusetts at Amherst; Dr. Bob Romanofsky, Antenna, Microwave, and Optical Systems Branch, NASA Glenn Research Center; Dr. Peter Schrock, Webmaster/Publications Representative, JPL. ATM: William D. Ivancic, Glenn Research Center, and Lewis Research Center, NASA; Dr.-Ing. Petia Todorova, Fraunhofer Institut FOKUS, Berlin. Turbo and LDPC codes: Dr. Alister G. Burr, Professor of Communications, Dept. of Electronics, University of York; Tony Summers, Senior Applications Engineer, Comtech AHA. HDTV: Cathy Firebrace, Information Officer, the IEE, London U.K. INTELSAT: Travis S. Taylor, Corporate Communications, Intelsat, Washington, DC. Iridium system: Liz DeCastro, Corporate Communications Director, Iridium Satellite. Cospas-Sarsat: Cheryl Bertoia, Principal Operations Officer, Deputy Head, Cospas-Sarsat Secretariat, London, U.K., and Hannah Bermudez also of the Cospas-Sarsat Secretariat.
And from the third edition, thanks to Dr. Henry Driver of Computer Sciences Corporation for details relating to the calculation of geodetic position in Chap. 2. Thanks also to the users and reviewers who made suggestions for corrections, additions, and improvements. Errors should not occur, but they do, and the author would be grateful if these are drawn to his attention. He can be reached at [email protected]. The editorial team at McGraw-Hill has contributed a great deal in getting the fourth edition into print: thanks are due to Steve Chapman, the sponsoring editor; Diana Mattingly, editorial assistant; and Gita Raman, project manager, for their help, and their gentle but persistent reminders to keep the book on schedule. Dennis Roddy Thunder Bay, Ontario Chapter 1 Overview of Satellite Systems
1.1 Introduction The use of satellites in communications systems is very much a fact of everyday life, as is evidenced by the many homes equipped with anten- nas, or “dishes,” used for reception of satellite television. What may not be so well known is that satellites form an essential part of telecom- munications systems worldwide, carrying large amounts of data and telephone traffic in addition to television signals. Satellites offer a number of features not readily available with other means of communications. Because very large areas of the earth are vis- ible from a satellite, the satellite can form the star point of a commu- nications net, simultaneously linking many users who may be widely separated geographically. The same feature enables satellites to provide communications links to remote communities in sparsely populated areas that are difficult to access by other means. Of course, satellite sig- nals ignore political boundaries as well as geographic ones, which may or may not be a desirable feature. To give some idea of cost, the construction and launch cost of the Canadian Anik-E1 satellite (in 1994 Canadian dollars) was $281.2 million, and that of the Anik-E2, $290.5 million. The combined launch insurance for both satellites was $95.5 million. A feature of any satel- lite system is that the cost is distance insensitive, meaning that it costs about the same to provide a satellite communications link over a short distance as it does over a large distance. Thus a satellite com- munications system is economical only where the system is in contin- uous use and the costs can be reasonably spread over a large number of users. Satellites are also used for remote sensing, examples being the detection of water pollution and the monitoring and reporting of
1 2 Chapter One weather conditions. Some of these remote sensing satellites also form a vital link in search and rescue operations for downed aircraft and the like. A good overview of the role of satellites is given by Pritchard (1984) and Brown (1981). To provide a general overview of satellite systems here, three different types of applications are briefly described in this chapter: (1) the largest international system, Intelsat, (2) the domestic satellite system in the United States, Domsat, and (3) U.S. National Oceanographic and Atmospheric Administration (NOAA) series of polar orbiting satellites used for environmental monitoring and search and rescue.
1.2 Frequency Allocations for Satellite Services Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. This is car- ried out under the auspices of the International Telecommunication Union (ITU). To facilitate frequency planning, the world is divided into three regions:
Region 1: Europe, Africa, what was formerly the Soviet Union, and Mongolia Region 2: North and South America and Greenland Region 3: Asia (excluding region 1 areas), Australia, and the south- west Pacific
Within these regions, frequency bands are allocated to various satel- lite services, although a given service may be allocated different fre- quency bands in different regions. Some of the services provided by satellites are:
Fixed satellite service (FSS) Broadcasting satellite service (BSS) Mobile satellite services Navigational satellite services Meteorological satellite services
There are many subdivisions within these broad classifications; for example, the FSS provides links for existing telephone networks as well as for transmitting television signals to cable companies for distribution over cable systems. Broadcasting satellite services are intended mainly for direct broadcast to the home, sometimes referred Overview of Satellite Systems 3 to as direct broadcast satellite (DBS) service [in Europe it may be known as direct-to-home (DTH) service]. Mobile satellite services would include land mobile, maritime mobile, and aeronautical mobile. Navigational satellite services include global positioning systems (GPS), and satel- lites intended for the meteorological services often provide a search and rescue service. Table 1.1 lists the frequency band designations in common use for satellite services. The Ku band signifies the band under the K band, and the Ka band is the band above the K band. The Ku band is the one used at present for DBS, and it is also used for certain FSS. The C band is used for FSS, and no DBS is allowed in this band. The very high fre- quency (VHF) band is used for certain mobile and navigational services and for data transfer from weather satellites. The L band is used for mobile satellite services and navigation systems. For the FSS in the C band, the most widely used subrange is approximately 4 to 6 GHz. The higher frequency is nearly always used for the uplink to the satellite, for reasons that will be explained later, and common practice is to denote the C band by 6/4 GHz, giving the uplink frequency first. For the direct broadcast service in the Ku band, the most widely used range is approxi- mately 12 to 14 GHz, which is denoted by 14/12 GHz. Although frequency assignments are made much more precisely, and they may lie somewhat outside the values quoted here (an example of assigned frequencies in the Ku band is 14,030 and 11,730 MHz), the approximate values stated are quite satisfactory for use in calculations involving frequency, as will be shown later in the text. Care must be exercised when using published references to frequency bands, because the designations have been developed somewhat differ- ently for radar and communications applications; in addition, not all countries use the same designations.
TABLE 1.1 Frequency Band Designations
Frequency range, (GHz) Band designation
0.1–0.3 VHF 0.3–1.0 UHF 1.0–2.0 L 2.0–4.0 S 4.0–8.0 C 8.0–12.0 X 12.0–18.0 Ku 18.0–27.0 K 27.0–40.0 Ka 40.0–75 V 75–110 W 110–300 mm 300–3000 μm 4 Chapter One
TABLE 1.2 ITU Frequency Band Designations
Frequency range Metric (lower limit Corresponding abbreviations Band exclusive, upper metric for the number Symbols limit inclusive) subdivision bands
4 VLF 3–30 kHz Myriametric waves B.Mam 5 LF 30–300 kHz Kilometric waves B.km 6 MF 300–3000 kHz Hectometric waves B.hm 7 HF 3–30 MHz Decametric waves B.dam 8 VHF 30–300 MHz Metric waves B.m 9 UHF 300–3000 MHz Decimetric waves B.dm 10 SHF 3–30 GHz Centimetric waves B.cm 11 EHF 30–300 GHz Millimetric waves B.mm 12 300–3000 GHz Decimillimetric waves
SOURCE: ITU Geneva.
The official ITU frequency band designations are shown in Table 1.2 for completeness. However, in this text the designations given in Table 1.1 will be used, along with 6/4 GHz for the C band and 14/12 GHz for the Ku band.
1.3 INTELSAT INTELSAT stands for International Telecommunications Satellite. The organization was created in 1964 and currently has over 140 member countries and more than 40 investing entities (see http://www.intelsat.com/ for more details). In July 2001 INTELSAT became a private company and in May 2002 the company began providing end-to-end solutions through a network of teleports, leased fiber, and points of presence (PoPs) around the globe. Starting with the Early Bird satellite in 1965, a succes- sion of satellites has been launched at intervals of a few years. Figure 1.1 illustrates the evolution of some of the INTELSAT satellites. As the figure shows, the capacity, in terms of number of voice channels, increased dramatically with each succeeding launch, as well as the design lifetime. These satellites are in geostationary orbit, meaning that they appear to be stationary in relation to the earth. The geostationary orbit is the topic of Chap. 3. At this point it may be noted that geosta- tionary satellites orbit in the earth’s equatorial plane and their position is specified by their longitude. For international traffic, INTELSAT covers three main regions—the Atlantic Ocean Region (AOR), the Indian Ocean Region (IOR), and the Pacific Ocean Region (POR) and what is termed Intelsat America’s Region. For the ocean regions the satellites are positioned in geostationary orbit above the particular ocean, where they provide a transoceanic telecommunications route. For example, INTELSAT satellite 905 is positioned at 335.5° east longitude. The foot- prints for the C-band antennas are shown in Fig. 1.2a, and for the Ku- band spot beam antennas in Figs. 1.2b and c. Figure 1.1 Evolution of INTELSAT satellites. (From Colino 1985; courtesy of ITU Telecommunications Journal.) 5 6 Chapter One
10° 5° 0°
Figure 1.2 INTELSAT satellite 905 is positioned at 335.5° E longitude. (a) The footprints for the C-band antennas; (b) the Ku-band spot 1 beam antennas; and (c) the Ku-band spot 2 beam antennas. Overview of Satellite Systems 7
Figure 1.2 (Continued).
The INTELSAT VII-VII/A series was launched over a period from October 1993 to June 1996. The construction is similar to that for the V and VA/VB series, shown in Fig. 1.1, in that the VII series has solar sails rather than a cylindrical body. This type of construction is described in more detail in Chap. 7. The VII series was planned for service in the POR and also for some of the less demanding services in the AOR. The antenna beam coverage is appropriate for that of the POR. Figure 1.3 shows the antenna beam footprints for the C-band hemispheric cover- age and zone coverage, as well as the spot beam coverage possible with the Ku-band antennas (Lilly, 1990; Sachdev et al., 1990). When used in the AOR, the VII series satellite is inverted north for south (Lilly, 1990), minor adjustments then being needed only to optimize the antenna pat- terns for this region. The lifetime of these satellites ranges from 10 to 15 years depending on the launch vehicle. Recent figures from the INTELSAT Web site give the capacity for the INTELSAT VII as 18,000 two-way telephone circuits and three TV channels; up to 90,000 two-way telephone circuits can be achieved with the use of “digital circuit mul- tiplication.” The INTELSAT VII/A has a capacity of 22,500 two-way telephone circuits and three TV channels; up to 112,500 two-way tele- phone circuits can be achieved with the use of digital circuit multipli- cation. As of May 1999, four satellites were in service over the AOR, one in the IOR, and two in the POR. 8 Chapter One
Figure 1.3 INTELSAT VII coverage (Pacific Ocean Region; global, hemispheric, and spot beams). (From Lilly, 1990, with permission.)
The INTELSAT VIII-VII/A series of satellites was launched over the period February 1997 to June 1998. Satellites in this series have similar capacity as the VII/A series, and the lifetime is 14 to 17 years. It is standard practice to have a spare satellite in orbit on high- reliability routes (which can carry preemptible traffic) and to have a ground spare in case of launch failure. Thus the cost for large international schemes can be high; for example, series IX, described later, represents a total investment of approximately $1 billion. Table 1.3 summarizes the details of some of the more recent of the INTELSAT satellites. These satellites provide a much wider range of services than those available previously, including such services as Internet, DTH TV, tele-medicine, tele-education, and interactive video and multimedia. Transponders and the types of signals they carry are Overview of Satellite Systems 9
TABLE 1.3 INTELSAT Geostationary Satellites
Satellite Location Number of transponders Launch date
901 342°E Up to 72 @ 36 MHz in C-Band June 2001 Up to 27 @ 36 MHz in Ku Band 902 62°E Up to 72 @ 36 MHz in C-Band August 2001 Up to 23 @ 36 MHz in Ku Band 903 325.5°E Up to 72 @ 36 MHz in C-Band March 2002 Up to 22 @ 36 MHz in Ku Band 904 60°E Up to 72 @ 36 MHz in C-Band February 2002 Up to 22 @ 36 MHz in Ku Band 905 335.5°E Up to 72 @ 36 MHz in C-Band June 2002 Up to 22 @ 36 MHz in Ku Band 906 64°E Up to 72 @ 36 MHz in C-Band September 2002 Up to 22 @ 36 MHz in Ku Band 907 332.5°E Up to 72 @ 36 MHz in C-Band February 2003 Up to 23 @ 36 MHz in Ku Band 10-02 359°E Up to 70 @ 36 MHz in C-Band June 2004 Up to 36 @ 36 MHz in Ku Band
described in detail in later chapters, but for comparison purposes it may be noted that one 36 MHz transponder is capable of carrying about 9000 voice channels, or two analog TV channels, or about eight digital TV channels. In addition to providing transoceanic routes, the INTELSAT satellites are also used for domestic services within any given country and regional services between countries. Two such services are Vista for telephone and Intelnet for data exchange. Figure 1.4 shows typical Vista applications.
1.4 U.S. Domsats Domsat is an abbreviation for domestic satellite. Domestic satellites are used to provide various telecommunications services, such as voice, data, and video transmissions, within a country. In the United States, all domsats are situated in geostationary orbit. As is well known, they make available a wide selection of TV channels for the home enter- tainment market, in addition to carrying a large amount of commercial telecommunications traffic. U.S. Domsats, which provide a DTH television service, can be classi- fied broadly as high power, medium power, and low power (Reinhart, 1990). The defining characteristics of these categories are shown in Table 1.4. The main distinguishing feature of these categories is the equivalent isotropic radiated power (EIRP). This is explained in more detail in Chap. 12, but for present purposes it should be noted that the upper limit 10
Figure 1.4 (a) Typical Vista application; (b) domestic/regional Vista network with standard A or B gateway. (From Colino, 1985; cour- tesy of ITU Telecommunication Journal.) Overview of Satellite Systems 11
TABLE 1.4 Defining Characteristics of Three Categories of United States DBS Systems
High power Medium power Low power
Band Ku Ku C Downlink frequency 12.2–12.7 11.7–12.2 3.7–4.2 allocation GHz Uplink frequency 17.3–17.8 14–14.5 5.925–6.425 allocation GHz Space service BSS FSS FSS Primary intended use DBS Point-to-point Point-to-point Allowed additional use Point-to-point DBS DBS Terrestrial interference possible No No Yes Satellite spacing degrees 9 2 2–3 Satellite spacing determined by ITU FCC FCC Adjacent satellite interference No Yes Yes possible? Satellite EIRP range (dBW) 51–60 40–48 33–37
NOTES: ITU—International Telecommunication Union; FCC—Federal Communications Commission. SOURCE: Reinhart, 1990. of EIRP is 60 dBW for the high-power category and 37 dBW for the low- power category, a difference of 23 dB. This represents an increase in received power of 102.3 or about 200:1 in the high-power category, which allows much smaller antennas to be used with the receiver. As noted in the table, the primary purpose of satellites in the high-power category is to provide a DBS service. In the medium-power category, the primary purpose is point-to-point services, but space may be leased on these satellites for the provision of DBS services. In the low-power category, no official DBS services are provided. However, it was quickly discovered by home experimenters that a wide range of radio and TV programming could be received on this band, and it is now considered to provide a de facto DBS service, witness to which is the large number of TV receive- only (TVRO) dishes that have appeared in the yards and on the rooftops of homes in North America. TVRO reception of C-band signals in the home is prohibited in many other parts of the world, partly for aesthetic reasons, because of the comparatively large dishes used, and partly for commercial reasons. Many North American C-band TV broadcasts are now encrypted, or scrambled, to prevent unauthorized access, although this also seems to be spawning a new underground industry in descram- blers. As shown in Table 1.4, true DBS service takes place in the Ku band. Figure 1.5 shows the components of a DBS system (Government of Canada, 1983). The television signal may be relayed over a terrestrial link to the uplink station. This transmits a very narrow beam signal to the satellite in the 14-GHz band. The satellite retransmits the television signal in a wide beam in the 12-GHz frequency band. Individual receivers within the beam coverage area will receive the satellite signal. 12 Chapter One
Figure 1.5 Components of a direct broadcasting satellite system. (From Government of Canada, 1983, with permission.)
Table 1.5 shows the orbital assignments for domestic fixed satellites for the United States (FCC, 1996). These satellites are in geostationary orbit, which is discussed further in Chap. 3. Table 1.6 shows the U.S. Ka-band assignments. Broadband services, such as Internet (see Chap. 15), can operate at Ka-band frequencies. In 1983, the U.S. FCC adopted a policy objective, setting 2° as the minimum orbital spacing for satel- lites operating in the 6/4-GHz band and 1.5° for those operating in the 14/12-GHz band (FCC, 1983). It is clear that interference between satel- lite circuits is likely to increase as satellites are positioned closer together. These spacings represent the minimum presently achievable in each band at acceptable interference levels. In fact, it seems likely that in some cases home satellite receivers in the 6/4-GHz band may be sub- ject to excessive interference where 2° spacing is employed.
1.5 Polar Orbiting Satellites Polar orbiting satellites orbit the earth in such a way as to cover the north and south polar regions. (Note that the term polar orbiting does not mean that the satellite orbits around one or the other of the poles). Overview of Satellite Systems 13
TABLE 1.5 FCC Orbital Assignment Plan (May 7, 1996)
Location, degrees west longitude Satellite Band/polarization
139 Aurora II/Satcom C-5 4/6 GHz (vertical) 139 ACS-3K (AMSC) 12/14 GHz 137 Satcom C-1 4/6 GHz (horizontal) 137 Unassigned 12/14 GHz 135 Satcom C-4 4/6 GHz (vertical) 135 Orion O-F4 12/14 GHz 133 Galaxy 1-R(S) 4/6 GHz (horizontal) 133 Unassigned 12/14 GHz 131 Satcom C-3 4/6 GHz (vertical) 131 Unassigned 12/14 GHz 129 Loral 1 4/6 GHz (horizontal)/12/14 GHz 127 Galaxy IX 4/6 GHz (vertical) 127 Unassigned 12/14 GHz 125 Galaxy 5-W 4/6 GHz (horizontal) 125 GSTAR II/unassigned 12/14 GHz 123 Galaxy X 4/6 GHz (vertical)/12/14 GHz 121 EchoStar FSS-2 12/14 GHz 105 GSTAR IV 12/14 GHz 103 GE-1 4/6 GHz (horizontal) 103 GSTAR 1/GE-1 12/14 GHz 101 Satcom SN-4 (formerly 4/6 GHz (vertical)/ Spacenet IV-n) 12/14 GHz 99 Galaxy IV(H) 4/6 GHz (horizontal)/12/14 GHz 97 Telstar 401 4/6 GHz (vertical)/12/14 GHz 95 Galaxy III(H) 4/6 GHz (horizontal)/12/14 GHz 93 Telstar 5 4/6 GHz (vertical) 93 GSTAR III/Telstar 5 12/14 GHz 91 Galaxy VII(H) 4/6 GHz (horizontal)/12/14 GHz 89 Telestar 402R 4/6 GHz (vertical)/12/14 GHz 87 Satcom SN-3 (formerly 4/6 GHz (horizontal)/12/14 GHz Spacenet III-)/GE-4 85 Telstar 302/GE-2 4/6 GHz (vertical) 85 Satcom Ku-1/GE-2 12/14 GHz 83 Unassigned 4/6 GHz (horizontal) 83 EchoStar FSS-1 12/14 GHz 81 Unassigned 4/6 GHz (vertical) 81 Satcom Ku-2/unassigned 12/14 GHz 79 GE-5 4/6 GHz (horizontal)/12/14 GHz 77 Loral 2 4/6 GHz (vertical)/12/14 GHz 76 Comstar D-4 4/6 GHz (vertical) 74 Galaxy VI 4/6 GHz (horizontal) 74 SBS-6 12/14 GHz 72 Unassigned 4/6 GHz (vertical) 71 SBS-2 12/14 GHz 69 Satcom SN-2/Telstar 6 4/6 GHz (horizontal)/12/14 GHz 67 GE-3 4/6 GHz (vertical)/12/14 GHz 64 Unassigned 4/6 GHz (horizontal) 64 Unassigned 12/14 GHz 62 Unassigned 4/6 GHz (vertical) 62 ACS-2K (AMSC) 12/14 GHz 60 Unassigned 4/6 GHz 60 Unassigned 12/14 GHz 14 Chapter One
TABLE 1.6 Ka-Band Orbital Assignment Plan (FCC December 19, 1997)
Location Company
147°W.L. Morning Star Satellite Company, L.L.C. 125°W.L. PanAmSat Licensee Corporation 121°W.L. Echostar Satellite Corporation 115°W.L. Loral Space & Communications, Ltd. ∗ 113°W.L. VisionStar, Inc. 109.2°W.L. KaStar Satellite Communications Corp. 105°W.L. † GE American Communications, Inc. 103°W.L. PanAmSat Corporation 101°W.L. Hughes Communications Galaxy, Inc. 99°W.L. Hughes Communications Galaxy, Inc. 97°W.L. Lockheed Martin Corporation 95°W.L. NetSat 28 Company, L.L.C. 93°W.L. Loral Space & Communications, Ltd. 91°W.L. Comm, Inc. 89°W.L. Orion Network Systems 87°W.L. Comm, Inc. 85°W.L. GE American Communications, Inc. 83°W.L. Echostar Satellite Corporation 81°W.L. Orion Network Systems 77°W.L. Comm, Inc. 75°W.L. Comm, Inc. 73°W.L. KaStar Satellite Corporation 67°W.L. [under consideration] 62°W.L. Morning Star Satellite Company, L.L.C. 58°W.L. PanAmSat Corporation 49°W.L. Hughes Communications Galaxy, Inc. 47°W.L. Orion Atlantic, L.P. 21.5°W.L. Lockheed Martin Corporation 17°W.L. GE American Communications, Inc. 2°E.L. Lockheed Martin Corporation 25°E.L. Hughes Communication Galaxy, Inc. 30°E.L. Morning Star Satellite Company, L.L.C. 36°E.L. PanAmSat Corporation 40°E.L. PanAmSat Corporation 48°E.L. PanAmSat Corporation 54°E.L. Hughes Communications Galaxy, Inc. 56°E.L. GE American Communications, Inc. 78°E.L. Orion Network Systems, Inc. 101°E.L. Hughes Communications Galaxy, Inc. 105.5°E.L. Loral Space & Communications, Ltd. 107.5°E.L. Morning Star Satellite Company, L.L.C.
NOTES: FCC—Federal Communications Commission; W.L.—West Longitude; E.L.—East Longitude. ∗ VisionStar will operate at a nominal orbit location of 113.05°W.L., and with a station- keeping box of ±0.05°, thereby increasing the separation from the Canadian filing at 111.1°W.L. by 0.1° relative to the worst case orbital spacing using the station keeping assumed in the ITU publications (±0.1°). †The applicants in the range 95° to 105°W.L. have agreed to operate their satellites with a nominal 0.05° offset to the west, and with a station-keeping box of ±0.05°, thereby increasing the separation from the Luxembourg satellite at 93.2°W.L. by 0.1° relative to the worst case orbital spacing using the station keeping assumed in the ITU publications (±0.1°). Overview of Satellite Systems 15
TABLE 1.6 Ka-Band Orbital Assignment Plan (FCC December 19, 1997) (Continued )
Location Company
111°E.L. Hughes Communications Galaxy, Inc. 114.5°E.L. GE American Communications, Inc. 124.5°E.L PanAmSat Corporation 126.5°E.L. Orion Network Systems, Inc. 130°E.L. Lockheed Martin Corporation 149°E.L. Hughes Communications Galaxy, Inc. 164°E.L. Hughes Communications Galaxy, Inc. 173°E.L. PanAmSat Corporation 175.25°E.L. Lockheed Martin Corporation
Figure 1.6 shows a polar orbit in relation to the geostationary orbit. Whereas there is only one geostationary orbit, there are, in theory, an infinite number of polar orbits. The U.S. experience with weather satel- lites has led to the use of relatively low orbits, ranging in altitude between 800 and 900 km, compared with 36,000 km for the geostation- ary orbit. Low earth orbiting (LEO) satellites are known generally by the acronym LEOSATS. In the United States, the National Polar-orbiting Operational Environmental Satellite System (NPOESS) was established in 1994 to consolidate the polar satellite operations of the Air Force, NASA (National Aeronautics and Space Administration) and NOAA (National Oceanic and Atmospheric Administration). NPOESS manages the
Figure 1.6 Geostationary orbit and one possible polar orbit. 16 Chapter One
Integrated Program Office (IPO) and the Web page can be found at http://www.ipo.noaa.gov/. As of 2005, a four-orbit system is in place, consisting of two U.S. Military orbits, one U.S. Civilian orbit and one EUMETSAT/METOP orbit. Here, METSAT stands for meteorological satellite and EUMETSAT stands for the European organization for the exploration of the METSAT program. METOP stands for meteorologi- cal operations. These orbits are sun synchronous, meaning that they cross the equator at the same local time each day. For example, the satellites in the NPOESS (civilian) orbit will cross the equator, going from south to north, at times 1:30 P.M., 5:30 P.M., and 9:30 P.M. Sun- synchronous orbits are described in more detail in Chap. 2, but briefly, the orbit is arranged to rotate eastward at a rate of 0.9856°/day, to make it sun synchronous. In a sun-synchronous orbit the satellite crosses the same spot on the earth at the same local time each day, so that the same area of the earth can be viewed under approximately the same lighting conditions each day. A sun-synchronous orbit is inclined slightly to the west of the north pole. By definition, an orbital pass from south to north is referred to as an ascending pass, and from north to south as a descending pass. The polar orbits are almost circular, and as previously mentioned they are at a height of between 800 and 900 km above earth. The polar orbiters are able to track weather conditions over the entire earth, and provide a wide range of data, including visible and infrared radiometer data for imaging purposes, radiation measurements, and temperature profiles. They carry ultraviolet sensors that measure ozone levels, and they can monitor the ozone hole over Antarctica. The polar orbiters carry a NOAA letter designation before launch, which is changed to a numeric designation once the satellite achieves orbit. For example, NOAA M, launched on June 24, 2002, became NOAA 17 when success- fully placed in orbit. The series referred to as the KLM satellites carry much improved instrumentation. Some details are shown in Table 1.7. Most of the polar orbiting satellites used in weather and environ- mental studies, and as used for monitoring and in search and rescue, have a “footprint” about 6000 km in diameter. This is the size of the antenna spot beam on the surface of the earth. As the satellite orbits the earth, the spot beam sweeps out a swath on the earth’s surface about 6000 km wide passing over north and south poles. The orbital period of these satellites is about 102 min. Since a day has 1440 min, the number of orbits per day is 1440/102 or approximately 14. In the 102 min the earth rotates eastward 360°×102/1440 or about 25°. Neglecting for the moment the small eastward rotation of the orbit required for sun syn- chronicity, the earth will rotate under the subsatellite path by this amount, as illustrated in Fig. 1.7. Overview of Satellite Systems 17
TABLE 1.7 NOAA KLM Satellites
Launch date NOAA-K (NOAA-15): May 13, 1998 NOAA-L: September 21, 2000 NOAA-M: June 24, 2000 NOAA-N: March 19, 2005 (tentative) NOAA-N’: July 2007 Mission life 2 years minimum Orbit Sun synchronous, 833 ± 19 km or 870 ± 19 km Mass 1478.9 kg on orbit; 2231.7 kg on launch Length/Diameter 4.18 m/1.88 m Sensors Advanced very high resolution radiometer (AVHRR/3) Advanced microwave sounding unit-A (AMSU-A) Advanced microwave sounding unit-B (AMSU-B) High resolution infrared radiation sounder (HIRS/3) Space environment monitor (SEM/2) Search and rescue (SAR) repeater and processor Data collection system (DCS/2)
Figure 1.7 Polar orbiting satellite: (a) first pass; (b) second pass, earth having rotated 25°. Satellite period is 102 min. 18 Chapter One
1.6 Argos System The Argos data collection system (DCS) collects environmental data radioed up from platform transmitter terminals (PTT) (Argos, 2005). The characteristics of the PTT are shown in Table 1.8. The transmitters can be installed on many kinds of platforms, includ- ing fixed and drifting buoys, balloons, and animals. The physical size of the transmitters depends on the application. These can weigh as little as 17 g for transmitters fitted to birds, to track their migratory patterns. The PTTs transmit automatically at preset intervals, and those within the 6000 km swath are received by the satellite. As mentioned, the satel- lite completes about 14 orbits daily, and all orbits cross over the poles. A PTT located at the polar regions would therefore be able to deliver approximately 14 messages daily. At least two satellites are operational at any time, which doubles this number to 28. At the equator the situa- tion is different. The equatorial radius of the earth is approximately 6378 km, which gives a circumference of about 40,074 km. Relative to the orbital footprint, a given longitude at the equator will therefore rotate with the earth a distance of 40074 × 102/1440 or about 2839 km. This assumes a stationary orbital path, but as mentioned previously the orbit is sun synchronous, which means that it rotates eastward almost 1° per day (see Sec. 2.8.1), that is in the same direction as the earth’s rota- tion. The overall result is that an equatorial PTT starting at the west- ern edge of the footprint swath will “see” between three and four passes per day for one satellite. Hence the equatorial passes number between six and seven per day for two satellites. During any one pass the PTT is in contact with the satellite for 10 min on average. The messages received at the satellite are retransmitted in “real time” to one of a number of regional ground receiving stations whenever the satellite is within range. The messages are also stored aboard the satellites on tape recorders, and are “dumped” to one of three main ground receiving stations. These are located at Wallops Island, VA, USA, Fairbanks, Alaska, USA, and Lannion, France. The Doppler shift in the frequency received at the satellite is used to determine the location of the PTT. This is discussed further in connection with the Cospas-Sarsat search and rescue satellites.
TABLE 1.8 Platform Transmitter Terminals (PTT) Characteristics
Uplink frequency 401.75 MHz Message length Up to 32 bytes Repetition period 45–200 s Messages/pass Varies depending on latitude and type of service Transmission time 360–920 ms Duty cycle Varies Power Battery, solar, external
SOURCE: www.argosinc.com/documents/sysdesc.pdf Overview of Satellite Systems 19
1.7 Cospas-Sarsat* COSPAS is an acronym from the Russian Cosmicheskaya Sistyema Poiska Avariynich Sudov, meaning space system for the search of vessels in dis- tress and SARSAT stands for Search and Rescue Satellite-Aided Tracking (see http://www.equipped.com/cospas-sarsat_overview.htm). The initial Memorandum of Understanding that led to the development of the system was signed in 1979 by agencies from Canada, France, the USA, and the former USSR. There are (as of November 2004) 37 countries and organizations associated with the program. Canada, France, Russia and the USA provide and operate the satellites and ground-segment equipment, and other countries provide ground-segment support. A full list of participating countries will be found in Cospas-Sarsat (2004). The system has now been developed to the stage where both low earth orbiting (LEO) satellites and geostationary earth orbiting (GEO) satel- lites are used, as shown in Fig. 1.8. The basic system requires users to carry distress radio beacons, which transmit a carrier signal when activated. A number of different beacons are available: emergency locator transmitter (ELT) for aviation use; emer- gency position indicating radio beacon (EPIRB) for maritime use; and per- sonal locator beacon (PLB) for personal use. The beacons can be activated manually or automatically (e.g., by a crash sensor). The transmitted signal is picked up by a LEO satellite, and because this satellite is moving relative to the radio beacon, a Doppler shift in frequency is observed. In effect, if the line of sight distance between transmitter and satellite is shortened as a result of the relative motion, the wavelength of the emit- ted signal is also shortened. This in turn means the received frequency is increased. If the line of sight distance is lengthened as a result of the
∗ http://www.cospas-sarsat.org/
LEOSAR satellites
GEOSAR satellites Figure 1.8 Geostationary orbit search and rescue (GEOSAR) and low earth orbit search and rescue (LEOSAR) satellites. (Courtesy of Cospas-Sarsat Secretariat.) 20 Chapter One relative motion the wavelength is lengthened and therefore the received frequency decreased. It should be kept in mind that the radio-beacon emits a constant frequency, and the electromagnetic wave travels at con- stant velocity, that of light. Denoting the constant emitted frequency by f0, the relative velocity between satellite and beacon, measured along the line of sight as v, and the velocity of light as c, then to a close approxi- mation the received frequency is given by (assuming v c):