SATELLITE COMMUNICATION
P. BANERJEE Professor Amity School of Engineering and Technology Amity University, Noida and Former Scientist CSIR–National Physical Laboratory New Delhi
Delhi-110092 2017 Contents
Preface...... xi Acknowledgements...... xiii 1. Overview of Satellite Communication...... 1–7 1.1 Introduction 1 1.2 Evolution of Satellite Communication 1 1.3 Historical Development of Satellite Communication 2 1.4 Advantages of Satellite Communication 2 1.5 Growth of Satellite Communication 3 1.6 Frequency Allocation for Satellite Systems 4 1.7 Applications of Satellite Communication 5 Summary 6 Review Questions 7 2. Orbital Mechanics...... 8–34 2.1 Introduction 8 2.2 Kepler’s Laws of Planetary Motion 8 2.3 Description of the Orbit of a Satellite 11 2.4 Kepler’s Equation of Motion 14 2.5 Orbital Elements 16 2.5.1 Definitions of Six Orbital Elements 17 2.5.2 Comparison of Orbital Elements 18 2.6 Look Angles 20 2.6.1 Determination of Elevation Angle 20 2.6.2 Determination of Azimuth Angle 21 2.7 Solar Day and Sidereal Day 25 2.8 Time 26 2.8.1 Atomic Clock 26 2.8.2 Calendar 27 2.8.3 Local Time 27 Summary 27 Review Questions 28 Annexure 2A: Conic Section 29 v vi Contents
Annexure 2B: Periapsis and Apoapsis 31 Annexure 2C: General Expression of Velocity of a Satellite in an Orbit 32 Annexure 2D: Few Terminologies Related to an Oribit 33 Annexure 2E: Orbit for Interplanetary Flights 34 3. Orbits...... 35–49 3.1 Introduction 35 3.2 Earth’s Oblateness 35 3.3 Atmospheric Drag 37 3.4 Third Body Effects 38 3.5 Radiation Pressure 39 3.6 Earth Coverage 39 3.7 Different Satellite Orbits 41 3.7.1 LEO 41 3.7.2 MEO 41 3.7.3 Geosynchronous Orbits 42 3.8 Highly Eccentric Orbit (HEO): Molniya Orbit 43 3.9 Polar Orbit 45 3.10 Sun-synchronous Orbit 45 3.11 Hohmann Transfer Orbit 46 Summary 47 Review Questions 48 Annexure 3A: Van Allen Radiation Belt 49 4. Launchers...... 50–58 4.1 Introduction 50 4.2 Launch Vehicle 51 4.3 Efficiency of Rocket/Launch Vehicle 52 4.4 Fuel 53 4.5 Launch Sites 53 4.6 Satellite Placement in Geostationary Orbit 54 Summary 55 Review Questions 56 Annexure 4A: Derivation of Ideal Rocket Equation 57 5. Satellite Sub-systems...... 59–75 5.1 Introduction 59 5.2 Mechanical Structure 60 5.3 Propulsion Sub-system 60 5.4 Thermal Control 60 5.5 Tracking, Telemetry, Command, and Monitoring 62 5.6 Power Sub-system 63 5.6.1 Solar Cells 64 5.6.2 Total Solar Eclipses for Geostationary Satellites 64 5.6.3 Battery 65 Contents vii
5.7 Attitude and Orbit Control System 66 5.8 Antenna Sub-system 67 5.8.1 Types of Beam Pattern 68 5.9 Communication Sub-system 70 5.10 Space Specification, Reliability and Redundancy 71 5.11 Lifespan of a Satellite 72 5.12 System Configuration of INSAT 72 Summary 74 Review Questions 74 6. The Space Link...... 76–98 6.1 Introduction 76 6.2 Calculation of Received Power 77 6.3 System Noise Considerations 81 6.3.1 Noise Temperature 81 6.3.2 Amplifier Noise Temperature 82 6.4 Noise Figure 84 6.5 Carrier-to-Noise Ratio (C/N) 86 6.6 Overall Performance 87 6.7 Antenna Losses 88 6.7.1 Noise Due to Antenna 88 6.7.2 Antenna Pointing Loss 88 6.7.3 Loss Due to Surface Irregularities of Antenna 89 6.8 Propagation Effects 90 6.8.1 Atmospheric Absorption 91 6.8.2 Ionospheric Effects 91 6.8.3 Ionospheric Scintillation 92 6.9 Rain Fade (Rain Attenuation) 92 6.10 Sun Outage 94 6.11 Faraday Rotation 94 Summary 95 Review Questions 95 Annexure 6A: Low Noise Amplifier 96 Annexure 6B: Decibel Units 97 Annexure 6C: Concept of EIRP 98 7. Satellite Access...... 99–118 7.1 Introduction 99 7.2 Frequency Division Multiple Access (FDMA) 99 7.2.1 Characteristics of FDMA 102 7.2.2 Overall Carrier-to-Noise Ratio 102 7.3 Time Division Multiple Access (TDMA) 102 7.3.1 TDMA Frame Structure 105 7.3.2 TDMA Frame Efficiency 106 viii Contents
7.3.3 TDMA Superframe 107 7.3.4 TDMA Frame Synchronisation 108 7.4 Code Division Multiple Access (CDMA) 110 7.4.1 PRN Sequence 111 7.4.2 Maximum Number of Users in CDMA 113 7.4.3 Carrier-to-Noise Ratio in CDMA 114 7.5 Spatial Division Multiple Access (SDMA) 114 7.6 Assigning Slots in Multiple Access 114 7.6.1 Pre-assigned (Fixed Assigned) Multiple Access (PAMA or FAMA) 115 7.6.2 Demand Assigned Multiple Access (DAMA) 115 7.6.3 Random Multiple Access (RMA) 116 Summary 116 Review Questions 117 Annexure 7A: Processing Gain 118 8. Global Navigation Satellite System (GNSS)...... 119–142 8.1 Introduction 119 8.2 Global Positioning System (GPS) 120 8.3 GPS Constellation 121 8.4 Working Principle of GPS 122 8.5 Signal Structure 123 8.5.1 Generation of the Course Acquisition (C/A) Code 126 8.6 Navigation Data 127 8.7 Navigation Solution 128 8.8 Design of a GPS Receiver 129 8.9 Carrier-to-Noise Ratio of GPS Signal at the Receiving End 130 8.10 GPS Errors 132 8.11 Global Navigation Satellite System (GNSS) 135 8.11.1 GLONASS 135 8.11.2 Galileo 136 8.11.3 BeiDou 136 8.11.4 IRNSS (India) 136 8.11.5 GNSS Comparison 136 8.12 Some Issues Related to GNSS 137 8.12.1 Interoperability and Compatibility 137 8.12.2 Political Decisions 137 8.12.3 Financial Uncertainty 137 8.13 Applications 137 8.13.1 Navigation 137 8.13.2 Surveying, Mapping 138 8.13.3 Precise Time Reference 138 Summary 138 Review Questions 139 Annexure 8A: Least Square Method 140 Contents ix
9. Internet and Satellite Links...... 143–148 9.1 Introduction to Internet 143 9.2 Layered Structure for TCP/IP 143 9.3 TCP Link 145 9.3.1 Flow Control and Congestion Control 145 9.4 Satellite Links and TCP 146 9.4.1 Round Trip Time (RTT) for Satellite Link 146 9.4.2 Bandwidth-Delay Product (BDP) 146 9.5 Solutions to TCP Problems over Long Delays 147 9.5.1 Split TCP with PEP 147 Summary 148 Review Questions 148 10. Direct Broadcasting Satellite (DBS) Television...... 149–163 10.1 Introduction 149 10.2 Digital DBS TV 149 10.3 HDTV Transmission 151 10.4 Compression in Digital TV 151 10.4.1 Necessity of Compression 151 10.4.2 Compression Standards 152 10.5 Reduction in Resolution 153 10.5.1 Colour Sub-sampling 153 10.5.2 Motion Estimation 154 10.5.3 Exploiting Spatial Redundancies 154 10.5.4 Discrete Cosine Transform (DCT) 155 10.5.5 Quantisation 155 10.6 Exploiting Statistical Redundancies 155 10.7 Hybrid Video Encoder 155 10.8 DBS Receiver 156 10.9 Link Budget for FM-FDMA Television 157 10.10 Pre-emphasis and De-emphasis 158 10.11 FM-FDMA Television 159 Summary 160 Review Questions 160 Annexure 10A: Characteristics of Analog TV Signal 161 Annexure 10B: Power Roll-off in dB Scale (dB/Octave) 163 11. Very Small Aperture Terminal (VSAT)...... 164–173 11.1 Introduction 164 11.2 Frequency Allocation 165 11.3 Network Architecture 165 11.3.1 One-way System 165 11.3.2 Split Two-way System 165 x Contents
11.3.3 Broadcast Network 166 11.3.4 Point-to-Point Network 166 11.4 Two-way Network 167 11.5 TDM/TDMA 168 11.6 Trans-receiver in VSAT Network 169 11.7 VSAT or Wireless Local Loop (WLL) Network 170 11.8 Calculation of Link Margin 171 Summary 172 Review Questions 172 12. Special Purpose Satellites...... 174–178 12.1 Introduction 174 12.2 Globalstar 175 12.3 Iridium 175 12.4 Orbcomm 177 12.5 Landsat 177 12.6 RADARSAT 177 12.7 Indian National Satellite System 177 Summary 178 Review Questions 178 Bibliography...... 179–180 Index...... 181–189 Preface
Communication has pervaded into every sphere of life. With the advent of the satellite, communication has become more accessible in almost all parts of the globe. Hundreds of channels we see on our TV are linked through satellites. Mobile phone, e-mail, WhatsApp, net browsing, googling—all are facilitated by communication satellites. Communications via satellites are being so extensively used that the subject ‘satellite communication’ is being taught as one full semester course or at least one or two modules of a semester in electronics and communication engineering department/space department of almost all universities. During the course of my teaching the subject in Amity University, I could realise the difficulties of students in having an in-depth understanding of the subject. I tried to clarify the doubts in my own way. Further, I observed that students could not comprehensively assimilate the subject from any one single book out of many well written and widely referred books available in the market. These observations prompted me to write one compact book, which would adequately address these issues. My notes for taking classes generated over the years have actually been transformed into a presentable form in the shape of this textbook. The contents of the book have also been enriched by my prior research experience of more than three decades in the related field. Most of the chapters are supplemented by one or few annexures that cover presentations of some additional interesting relevant information. These have not been tacitly included in the main body to avoid digression from the main topic. The annexures may be useful for teachers as well as for more curious students. The book starts with an introductory chapter that introduces the evolution of satellite communication. It covers the orbital mechanics, the perturbation factors of the orbit and different orbit configuration. It deals with launching mechanism and satellite sub-systems that configure a complete satellite system. The book elaborates the link calculation to facilitate the design aspect. Satellite access mechanism is also discussed. Internet linking via satellite is also outlined for completeness. The book also includes detailed deliberation on navigation satellites, direct broadcasting satellites, VSAT and other low orbit satellites. INSAT system is also discussed. xi xii Preface
This book is targeted for the undergraduate and postgraduate students of electronics and communication engineering/electronics and telecommunication engineering in Indian universities in particular. The book may also serve as a reference book for the faculty teaching the subject. Comments and suggestions from readers and experts would be highly appreciated.
P. BANERJEE 2 Orbital Mechanics
2.1 INTRODUCTION 1TDKVCNOGEJCPKEUFGCNUYKVJVJGUVWF[QHOQVKQPUQHVJGCTVKſEKCNUCVGNNKVGUCPFURCEGXGJKENGU 6JKU YCU UVCTVGF D[ -GRNGT CPF 0GYVQP CPF YCU HWTVJGT GZRCPFGF CPF GNCDQTCVGF D[ OCP[ GZRGTVUQHVJGQTGVKECNRJ[UKEUKPVJGGKIJVGGPVJCPFPKPGVGGPVJEGPVWTKGU+PEQWTUGQHVKOGVJG VJGQT[JCUDGGPUQTGſPGFVJCVKVKUCDNGVQFGſPGVJGCUEGPVVTCLGEVQTKGUTGGPVT[CPFNCPFKPI CPF KPVGTRNCPGVCT[ VTCLGEVQTKGU #NN VJGUG OC[ DG FQPG YKVJ VJG CEEWTCE[ CPF RTGEKUKQP CU TGSWKTGF HQT VJG URCEG OKUUKQP 6JKU EJCRVGT RTGUGPVU C RTGNKOKPCT[ DCEMITQWPF QH VJG QTDKVCN OGEJCPKEU
2.2 KEPLER’S LAWS OF PLANETARY MOTION ,QJCPPGU -GRNGT Ō C )GTOCP OCVJGOCVKEKCP CUVTQPQOGT CPF CUVTQNQIGTſTUV RTQRQWPFGFVJTGGNCYUFGUETKDKPIRNCPGVCT[OQVKQP-GRNGTFKUEQXGTGFVJGUGNCYUGORKTKECNN[ DCUGF QP NCTIG CPF GZVGPUKXG QDUGTXCVKQPU -GRNGTŏU NCYU CRRN[ IGPGTCNN[ VQ CP[ VYQ DQFKGU VJCVKPVGTCEVKPURCEGWPFGTVJGHQTEGQHITCXKVCVKQP5QKepler’s laws are equally applicable to satellites orbiting around the Earth, irrespective of natural satellite like Moon and man- made satellites.
-GRNGTŏUſTUVNCY 2CVJUHQNNQYGFD[RNCPGVU QTUCVGNNKVGU CTQWPFVJG5WP QT'CTVJ CTGGNNKRUG TGHGTVQ(KIWTG CPF#PPGZWTG# YKVJVJG5WP QT'CTVJ CVQPGQHVJGVYQHQEK +P%CTVGUKCPEQQTFKPCVGU[UVGOGNNKRUGKUYTKVVGPCUHQNNQYU xy ab where a is UGOKOCLQTCZKUCPFb is UGOKOKPQTCZKU 6JGGNNKRUGKPRQNCTEQQTFKPCVGKUIKXGPD[ ae(1 ) r = 1cos ev where e is GEEGPVTKEKV[#PINGvKUUJQYPKP(KIWTG#CPF(KIWTG 8 Orbital Mechanics 9
#P GNNKRVKECN QTDKV JCU KVU apogee TGHGT VQ #PPGZWTG $ YJKEJ KU VJG HCTVJGUV RQKPV QH the QTDKVHTQOVJG'CTVJŏUEGPVTGCPFperigeeYJKEJKUVJGPGCTGUVRQKPVQHVJGQTDKVHTQOVJG 'CTVJŏUEGPVTG UGG(KIWTG # circular orbit KU C URGEKCN ECUG QH CP GNNKRVKECN QTDKV 5Q VJG GEEGPVTKEKV[ QDXKQWUN[ DGEQOGU\GTQ6JWUQPGOC[CUUWOGVJCVKPVJGECUGQHEKTEWNCTQTDKVVYQHQEKOGTIGVQIGVJGT VQIKXGCUKPINGEGPVTCNRQKPVIn this case, apogee and perigee do not exist.
FIGURE 2.1 5JCRGQHCV[RKECNGNNKRUGYKVJVJG'CTVJ YKVJQRGPEKTENG QPVJGHQEWUaKUUGOKOCLQT CZKUbKUUGOKOKPQTCZKUraKUCRQIGGTCFKWUCPFrpKURGTKIGGTCFKWU
-GRNGTŏUUGEQPFNCY 6JGTCFKWUXGEVQTHTQOVJG5WP QT'CTVJ VQVJGRNCPGV QTUCVGNNKVG UYGGRUQWVGSWCNCTGCKP GSWCNVKOG
FIGURE 2.2 'ZRNCPCVKQP QH -GRNGTŏU UGEQPF NCY HQT UCVGNNKVGŏU QTDKV CTQWPF VJG 'CTVJ (Area EAB equals area ECD and they are swept in the same amount of time. So, the satellite moves distance AB and distance CD in the same amount of time. The satellite has to move faster to cover larger distance AB than to cover shorter distance CD. Thus, the satellite moves fastest when it is nearest to the Earth.)
&KCITCOOCVKECNN[ -GRNGTŏU UGEQPF NCY JCU DGGP GZRNCKPGF KP (KIWTG +P VJKU ſIWTG VYQUJCFGFRQTVKQPU QHGSWCNCTGC QHVJGGNNKRVKECNQTDKVKPYJKEJCUCVGNNKVGOQXGUCTGUYGRV KPGSWCNVKOG 10 Satellite Communication
1PGOC[PQVG KP(KIWTG VJCVVJGFKUVCPEG#$KUNQPIGTVJCPVJGFKUVCPEG%&VJQWIJ VJG[CTGVTCXGNNGFD[VJGUCOGCOQWPVQHVKOG5QVJGUCVGNNKVGOQXGUYKVJVJGnon-uniform velocityThis very law further infers that when the satellite goes farther away from the Earth (i.e., around apogee), the satellite slows down (i.e., covers shorter distance CD in the same time). When the satellite comes closer to the Earth (i.e., around perigee), the satellite moves faster (i.e., covers longer distance AB in the same time). When the orbit is circle (as a special case of ellipse with zero eccentricity), the satellite moves in the orbit with a constant velocity.
-GRNGTŏUVJKTFNCY 6JGUSWCTGQHVJGQTDKVCNRGTKQFQHVJGQTDKVKURTQRQTVKQPCNVQVJGEWDGQHVJGUGOKOCLQTCZKU QHVJGQTDKV The ratio Ta3 KU EQPUVCPV HQT CNN RNCPGVU CPF UCVGNNKVGU YJGTG T KU QTDKVCN RGTKQF VKOG VCMGPVQEQORNGVGQPGTGXQNWVKQP CPFaKUUGOKOCLQTCZKUQHVJGQTDKV and T = (4p/m a3 13 3 5 3 where m = G Me = ª m /s ª MO /s = -GRNGTŏUEQPUVCPV –11 *GTG G is ITCXKVCVKQPCN EQPUVCPV ª 0O MI ) and Me ª MI KU VJGOCUUQHVJG'CTVJ Note: 6JG XCNWG QH m IKXGP CDQXG KU XCNKF HQT VJG UCVGNNKVG QTDKVKPI CTQWPF VJG 'CTVJ 6JG EQTTGURQPFKPIXCNWGUQHmHQTQVJGTRNCPGVUCTGIKXGPKP6CDNG
TABLE 2.1 )TCXKVCVKQP2CTCOGVGTUQH5GNGEVGF%GNGUVKCN$QFKGU
Body m MO3/s) 5WP ª11 'CTVJ ª5 Venus ª5 Mars ª4 ,WRKVGT ª6 5CVWTP ª7 Moon ª3
PROBLEM 2.1: %CNEWNCVG VJG RGTKQF QH IGQUVCVKQPCT[ QTDKV KH m ª 5MO3/s and a MO Solution: T = (4pa3)/m;
ËÛ T = 3 U ÌÜ5 ÍÝ T U JOKPU Orbital Mechanics 11
2.3 DESCRIPTION OF THE ORBIT OF A SATELLITE 5KT +UCCE 0GYVQP Ō 'PINKUJ RJ[UKEKUV OCVJGOCVKEKCP CUVTQPQOGT VJGQNQIKCP FGTKXGF -GRNGTŏU NCYU HTQO JKU QYP NCYU QH OGEJCPKEU Satellites orbiting the earth follow the same laws that govern the motion of planets around the Sun 0GYVQP FKUEQXGTGF VJG NCYU QH ITCXKVCVKQP. *G GZRNCKPGF RNCPGVCT[ CPF UCVGNNKVG QTDKVU KP VGTOU QH VJG DCNCPEG QH VJG VYQ HQTEGU DCUGF QP JKU NCYU QH OQVKQP CPF VJG EQPEGRV QH ITCXKVCVKQPCNCVVTCEVKQP
%GPVTKRGVCNCEEGNGTCVKQP
6JG NCY QH ITCXKVCVKQP UVCVGU VJCV VJG ITCXKVCVKQPCN HQTEG QH CVVTCEVKQP Fin) between two DQFKGUXCTKGU CU VJG RTQFWEV QH VJGKT OCUUGU CPF KPXGTUGN[ CU VJG USWCTG QH VJG FKUVCPEG r) DGVYGGPVJGO+HMeKUOCUUQHVJG'CTVJCPFmKUVJGOCUUQHVJGUCVGNNKVGVJGP
Mme Pm Fin = G rr where G KUITCXKVCVKQPCNEQPUVCPVCPFm = G Me Fin KU HQTEG QH CVVTCEVKQP D[ VJG 'CTVJ VQ VJG UCVGNNKVG UGG (KIWTG 6JKU KU CNUQ ECNNGF RCTVKEWNCTN[ HQT QTDKVKPI DQF[ centripetal force 6JG EGPVTKRGVCN HQTEG CVVGORVU VQ RWNN VJG UCVGNNKVGFQYPVQVJGGCTVJ
FIGURE 2.3 $CNCPEKPIQHEGPVTKRGVCNHQTEGCPFEGPVTKHWICNHQTEGHQTQTDKVKPIUCVGNNKVG
%GPVTKHWICNHQTEG 6JG UGEQPF NCY QH OQVKQP UVCVGU VJCV VJG CEEGNGTCVKQP QH C DQF[ KU RTQRQTVKQPCN VQ VJG HQTEG CEVKPIQPKVCPFKUKPXGTUGN[RTQRQTVKQPCNVQKVUOCUU6JGHQTEGECWUGFD[VJKUCEEGNGTCVKQPKU called centrifugal force (Fout HQTCQTDKVKPIDQF[CPFKUIKXGPD[ dv v F = mm ¹ out dt r +VJCUDGGPCUUWOGFVJCVVJGUCVGNNKVGKUOQXKPIYKVJCXGNQEKV[v in a circular orbit with radius rCTQWPFVJG'CTVJ6JGEGPVTKHWICNHQTEGFWGVQVJGMKPGVKEGPGTI[QHVJGUCVGNNKVGCVVGORVUVQ VJTQYVJGUCVGNNKVGQWVQHVJGQTDKV 0GYVQP GZRNCKPGF VJCV KH VJGUG VYQ HQTEGU CTG GSWCN VJG UCVGNNKVG YKNN TGOCKP KP C UVCDNG QTDKVCUUJQYPKP(KIWTG 3 Orbits
3.1 INTRODUCTION Kepler’s laws assume that the Earth is uniform and the only force acting is the centrifugal force resulting from the satellite motion balancing the gravitational pull of the Earth. But practically, when the satellite is placed in its orbit, it experiences various perturbing forces that perturb the orbit with time. Additional forces that act on the satellite are called perturbing forces, as they change the motion (orbit) of the satellite. These forces have a variety of causes/origins and effects. The major perturbations are as follows: Earth’s oblateness, i.e. effect of non-spherical Earth (or asymmetry of the Earth’s ITCXKVCVKQPCNſGNF Atmospheric drag Third body effects (i.e., gravitational forces from other bodies like solar and lunar attraction) Solar wind/radiation pressure This chapter elaborates on these factors that perturb the orbit. .CVGT RCTV QH VJKU EJCRVGT FGCNU YKVJ FKHHGTGPV QTDKV EQPſIWTCVKQPU VJCV CTG PQTOCNN[ WUGF This includes three types of orbits that are based on altitudes like LEO, MEO and GEO. Some special orbits like Molniya Orbit, Polar Orbit, Sun-synchronous orbit and Hohmann orbit are also discussed.
3.2 EARTH’S OBLATENESS The basic assumption that the Earth is a perfect sphere is not true. Earth is neither homogeneous nor spherical +V KU UQOGYJCV ƀCVVGPGF CV VJG RQNGU CPF DWNIGU CV VJG equator. The shape is more close to oblate spheroid. The equatorial radius is approximately 6378 km, and the polar radius is 6356 km (about 22 km smaller). The average density of the Earth is not uniform. The potential generated by the non-spherical Earth causes periodic variations in all the orbital elements.
35 Orbits 41
Case I: .GVWUſPFQWVVJGmaximum coverage of geostationary satellite. For geostationary satellite, re = 6378 km and h = 35768 km. amax = 8.72, and bmax = 81.28. 2 amax = 17.44; this is the beam width of the antenna aboard the geostationary satellite required for the maximum coverage. 2 bmax = 162.56; this indicates the maximum area covered along the equator for geostationary satellite. Case II: .GVWUſPFQWVVJGOCZKOWOEQXGTCIGQHMEO in equatorial plane. For MEO satellite, re = 6378 km and h = 20183 km. a max = 13.89; so, in this case, b = 76.11. 2 amax = 27.78; this is the beam width required for the antenna aboard the MEO satellite for maximum coverage. bmax = 152.2; this indicates the maximum area covered along the equator. Case III: .GVWUſPFQWVVJGOCZKOWOEQXGTCIGQHLEO in equatorial plane. For LEO satellite, re = 6378 km and h = 1000 km amax = 59.82, and bmax = 30.18. 2 amax = 119.64; this is the beam width required for the antenna aboard the LEO satellite for maximum coverage. 2 bmax = 60.34; this indicates the maximum area covered along the equator.
3.7 DIFFERENT SATELLITE ORBITS Satellites may be categorised based on altitude, KPENKPCVKQP GVE 1PG RQRWNCT ENCUUKſECVKQP KU based on the altitude of the orbit. These categories are low Earth orbit (LEO), medium Earth orbit (MEO) and geostationary orbit (GEO).
3.7.1 LEO A low Earth orbit (LEO)KUIGPGTCNN[FGſPGFCUCPQTDKVDGNQYCPCNVKVWFGQHCRRTQZKOCVGN[ 2000 km. The altitude of LEO is usually not less than 500 km because of the orbit decay due to larger atmospheric drag. An LEO is an orbit around the Earth between the atmosphere and below the inner Van Allen radiation belt (see Annexure 3A). This region of the belt is not used for the placement of satellite, as it is full of high energy particles that may harm the satellite. Iridium, Orbocomm, Globalstar satellites (see Chapter 12) are LEO satellites.
3.7.2 MEO Medium Earth orbit (MEO) is the region of space around the Earth above the LEO (altitude of 2000 km) and below the geostationary orbit (altitude of 35786 km). The MEO lies within outer Van Allen radiation belt (see Annexure 3A). Global navigation satellites in GPS and GLONASS constellations (see Chapter 8) are MEO satellites. 42 Satellite Communication
3.7.3 Geosynchronous Orbits A geosynchronous satellite is a satellite in geosynchronous orbit whose orbital period is exactly one sidereal day (see Section 2.7). This makes the satellite to rotate around the Earth with the same speed as the rotation of the Earth around its own axis. The Kepler’s laws dictate that the satellite would necessarily be at approximately 35768 km above the Earth. This orbit is in safe zone as it is outside the van Allen radiation belt. Normally, geosynchronous satellite has non-zero inclination and is also a circular orbit. IRNSS constellation (see Section 8.11) has two geosynchronous satellites. A special case of geosynchronous satellite is the geostationary satellite, where the inclination is zero. INSAT, GSAT (see Section 12.3) satellites and satellites used for VSATs are in the category of geostationary satellite. The orbit of a geosynchronous satellite is not exactly aligned with the Earth’s equator. In this condition, the orbit is known as an inclined orbit. In this case, satellite would not be stationary with respect to the Earth. Instead, the latitude and longitude deviation of the sub-satellite point (for geostationary circular orbit) would be governed by the following relations:
FIGURE 3.6 Figure of eight of the sub-satellite point for inclined orbit of a geosynchronous satellite.
i2 sinOQ sin 2 4 and sin q i sin n where l is longitude deviation of sub-satellite point, q is latitude deviation of sub-satellite point, n is true anomaly and i is inclination. It will appear (when viewed by someone on the ground) that the sub-satellite point traces a ſIWTG QH GKIJV (Figure 3.6) CTQWPF C ſZGF RQKPV QP VJG GSWCVQT 6JG sub-satellite point reaches a maximum latitude of ±i. As inclination of the orbit decreases, the magnitude of this oscillation becomes smaller. But, when the orbit lies entirely over the equator in a circular orbit as the case of a geostationary satellite, the satellite remains stationary relative to the Earth’s surface. Hence, there is permanent visibility of a geostationary satellite within its coverage area. Geostationary satellites, thus, are widely used for communication. Limit of visibility, i.e., the maximum value of b for geostationary satellite is 81.3° as shown in Case I of Section 3.6. This implies that the satellites cover up to 81.3° (practically 70° excluding elevation lower than 10°) of latitude. In other words, geostationary satellite does not have coverage in higher latitude near poles. This very fact prompted Russia to design Orbits 43 highly eccentric orbit (HEO) like Molniya orbit (see Section 3.8) for better coverage of higher latitude region of northern hemisphere. Along the longitude, the satellite covers the span of around 160° degree. Thus, minimum three satellites are required to cover the globe (i.e., 360°). Most commercial communications satellites, broadcast satellites and satellite based augmentation system (SBAS) satellites operate in geostationary orbits. But, for communication VJTQWIJCIGQUVCVKQPCT[UCVGNNKVGUKIPCNUVTCXGNCNQPIFKUVCPEGECWUKPICUKIPKſECPVFGNC[(QT geostationary satellite, the one hop (i.e., transmitted to satellite and satellite to receiver) delay lies between 238 ms to 275 ms. For example, one may calculate the delay by using Figure 3.5 [also refer to Eq. (2.27)]. The delay d (delay from transmitter to receiver via geostationary satellite) may be calculated as 2 d (OP22 OG 2OP * OG cosE ) c where, c is the velocity of light. Let us assume b = 45°. For geostationary satellite, let us assume OP = 42164.17 km, OG = 6378.137 km. d comes out to be around 269 ms. This large value of delay may pose a problem for latency-sensitive applications or interactive communication like for voice communication. This issue does not make any difference in case of non-interactive systems such as television broadcasts. Table 3.2 describes comparison of characteristics of LEO, MEO and GEO.
TABLE 3.2 Major Differences between LEO, MEO and GEO Satellite Systems
Parameter LEO MEO GEO Satellite Height (h) 200–2000 5000–25000 35768 km (SP in Figure 3.5) (km) Orbital period 95–130 min 3–12 h 23 h 56 min 4.09 s Surface coverage (ȕ) 59.82° 76.11° 81.28° (see Figure 3.5) (for h = 1000 km) (for h = 2083 km) Orbital speed (km/s) 7.6–6.9 6–3.8 3.07 Visibility time <15 min 2–4 h Always One hop delay 3–6 30–135 235–275 (approximately) (ms)
3.8 HIGHLY ECCENTRIC ORBIT (HEO): MOLNIYA ORBIT Highly eccentric, inclined and elliptical orbits are used to cover higher latitudes, which are otherwise not covered by geostationary orbits. A practical example of this type of orbit is the Molniya orbit. It is widely used by Russia and other countries of the former Soviet Union to provide communication services. Molniya orbits are designed so that the perturbations in argument of perigee are zero. So, typical eccentricity and inclination of the Molniya orbits are 0.75 and 65° (or 116.6°), respectively (see Section 3.2). The apogee and perigee points are about 40000 km and 400 km, respectively, from the surface of the Earth. 5 Satellite Sub-systems
5.1 INTRODUCTION Passive satellites being obsolete, the active version has grown with advancement of technology and with its wide applicability. The cost of an active satellite is very high. Further, the launching of a satellite is a quite expensive project. The technology is so advanced that the life of a communications satellite may range from 10 to 15 years. A typical large geostationary satellite is estimated to cost around $125 million. In order to support the successful operation of a satellite and to serve the purpose of the respective objective during the intended lifespan, the total system has to be carefully designed and engineered. The entire satellite may be divided into different sub- systems for convenience of design. The particular application of the satellite system, for example, ſZGFUCVGNNKVGUGTXKEGOQDKNG service, or broadcast service, or a weather forecasting satellite or even a remote sensing satellite, FGVGTOKPGUVJGURGEKſEGNGOGPVUQHVJGU[UVGO$WVCIGPGTKEUCVGNNKVGU[UVGOKTTGURGEVKXGQH the intended application, may broadly be divided into following functional sub-systems. This is shown in functional block diagram in Figure 5.1.
FIGURE 5.1 Schematic block diagram of satellite system consisting of few sub-systems (excluding mechanical structure and thermal control sub-systems that cannot be shown as separate block). 59 Satellite Sub-systems 63
The current orbit status of the spacecraft needs to be determined regularly. This is actually the tracking function. The beacon signal is very widely used for tracking a satellite. The Doppler shift of the beacon (or the telemetry carrier) is monitored to determine the location of the satellite. Acceleration and velocity sensors on the satellite can be used to monitor orbital location and changes in orbital location. The telemetry and monitoring part of the sub-system monitors health of all sub-systems. Its function involves the collection of data from sensors onboard the spacecraft. It relays VJKUKPHQTOCVKQPVQVJGITQWPFUVCVKQPQPVJG'CTVJ6JGTGCTGCHGYUGPUQTUKPVJGUCVGNNKVGVQ assess the respective health status. The telemetry data most commonly include data from few sensors like Pressure sensor in fuel tank to determine the amount of fuel available Voltage sensors to know the voltage of different electronic unit and battery %WTTGPVUGPUQTUVQCUUGUUVJGEWTTGPVƀQYKPIKPFKHHGTGPVUGEVKQPQHVJGRC[NQCF Temperature sensors to know the temperature of critical sub-systems Status of switches and relays in the communications and antenna sub-systems Attitude control sensor status, and Velocity and acceleration sensors data The telemetry carrier modulation is typically frequency shift keying (FSK) or phase shift keying (PSK), with the telemetry channels transmitted in a time division multiplex (TDM) format. Telemetry channel data rates are low (usually 1500–100 bps). Command is the complementary function to telemetry. The command system transmits URGEKſE EQPVTQN CPF QRGTCVKQPU KPHQTOCVKQP HTQO VJG ITQWPF VQ VJG URCEGETCHV RTKOCTKN[ DCUGF on telemetry information received from the spacecraft. Following parameters are mostly studied: Changes and corrections in attitude control and orbital control Antenna pointing and control Transponder mode of operation $CVVGT[XQNVCIGEQPVTQN Normally, the command signals are encrypted with a secure code format to maintain the health and safety of the satellite so that it may protect from intentional unauthorised commands or unintentional signals that may malfunction the satellite operation. Command data are at low TCVGHTQO'CTVJUVCVKQPTGSWKTKPIPCTTQYDCPFYKFVJ Tracking of satellite is necessary to monitor the orbit status to ensure that orbit parameters TGOCKP YKVJKP URGEKſGF NKOKVU; otherwise, necessary action may be taken to correct the orbit. Doppler shift measurement determines the range rate. Multi-tone method of ranging is used VQ ſPF TCPIG 6JG C\KOWVJ CPF GNGXCVKQP QH 'CTVJ UVCVKQP CPVGPPC FGVGTOKPGU VJG CPIWNCT QTKGPVCVKQPQHVJGUCVGNNKVG$[VJGEQODKPCVKQPQHVJGUGOGCUWTGOGPVUQPGOC[VTCEMCUCVGNNKVG quite precisely.
5.6 POWER SUB-SYSTEM Power sub-system supplies power to all sub-systems required for their operation. Mainly, solar cells are used to convert sunlight into electric power that is used for the operation of sub-systems. 64 Satellite Communication
There are also batteries essentially needed to serve as electric power when the energy from the Sun is absent. The solar panels also charge the batteries whenever they need it. The power sub-system (PS) mainly consists of solar cells, batteries, battery charger and power conditioning unit, as shown in Figure 5.3.
FIGURE 5.3 Arrangement of power supply in a satellite.
5.6.1 Solar Cells Solar cells convert incident sunlight into electrical energy. Gallium arsenide-based multi- junction photovoltaic solar cells have higher GHſEKGPE[CPFUQCTGV[RKECNN[WUGFKPUCVGNNKVG systems. 'HſEKGPE[ QH VJG CXCKNCDNG UQNCT EGNNU KU CDQWV Ō At the beginning of life, power up to 220 W/m2 OC[ DG CXCKNCDNG HTQO UWEJ UQNCT EGNNU #XGTCIG RQYGT ƀWZ QH VJG incident sunlight (about 1.4 kW/m2 OC[ RTQFWEG CDQWV 9 DC RQYGT $WV VJKU RQYGT OC[PQVTGOCKPEQPUVCPVDWVOC[TGFWEGFWGVQVJGFTQRKPGHſEKGPE[QHUQNCTEGNNUYKVJVKOG because of aging and etching of the surface. Solar panels need to have a large surface area to collect the maximum possible amount of sunlight. Large numbers of cells (close-packed solar cell rectangles) are connected in serial- RCTCNNGNCTTC[UVQFGNKXGTUWHſEKGPVRQYGT6JGURKPUVCDKNKUGFUCVGNNKVGWUWCNN[JCURCPGNUCTQWPF KVUE[NKPFTKECNDQF[$WVKVDGKPIE[NKPFTKECNKPUJCRGKVUGPVKTGDQF[UWTHCEGFQGUPQVCNYC[U receive the sunlight. It depends on the portion of the surface facing the Sun. So, this type of system requires more number of solar cells than the VJTGGCZKU UVCDKNKUGF UCVGNNKVG YJGTG ƀCV panels may receive solar energy round the clock. In many satellites, a pair of long wings of solar panels extending from the sides of the main body has been in use. The solar panels may extend to few tens of feet. In that case, solar RCPGNUCTGHQNFGFFWTKPINCWPEJVQſVKPUKFGVJGNCWPEJKPIXGJKENGVJQWIJFGRNQ[OGPVQHUQNCT panels later in orbit is critical.
5.6.2 Total Solar Eclipses for Geostationary Satellites All satellites in space have to undergo a period during which its solar power is blocked/eclipsed D[VJGUJCFQYQHVJG'CTVJQTVJG/QQP6JKUKUECNNGFeclipse QHVJGUCVGNNKVG'ENKRUGQEEWTU YJGPVJG'CTVJQTVJG/QQPCTGDGVYGGPVJG5WPCPFVJGUCVGNNKVG 6 The Space Link
6.1 INTRODUCTION The design of a satellite link is very important, as it gives the estimate of the power that would be received by the Earth station when the power is transmitted by the satellite transponder. Also, it gives the estimate of the power that would be received by the transponder when the power is transmitted by the Earth station. For the case of satellite link, it is quite reasonably assumed that a signal propagates in line-of-sight (LoS) mode. It also takes into account the gains and losses that are encountered due to various factors like propagation, antenna gains, feed line and miscellaneous losses. Some of their contributions may be assessed through standard formula and some may be evaluated through experiments. Link analysis is basically the estimation of power that is to be transmitted from an Earth station to the satellite (uplink) and from the satellite to the receiver (downlink), as seen in Figure 6.1, so that optimum performance is achieved.
FIGURE 6.1 Uplink and downlink system between the Earth station and the satellite.
The characteristics of the radio terminals (like transmitter, receiver and antenna) and the propagation medium should be known well for proper evaluation of link parameters. 76 Global Navigation 8 Satellite System (GNSS)
8.1 INTRODUCTION Humans have always been curious for exploration of land. In past years, some sailing was done only for the sake of exploration of land. Missionaries were interested for religious reasons. Some were interested for gold or other valuable goods. Trades were another good reason for exploration. Traders wanted to make money, and so, wanted to know faster and newer routes VQ EQWPVTKGU VQ VTCFG YKVJ +P VJG RTKOKVKXG FC[U ſPFKPI TQWVGU YCU FQPG VJTQWIJ NCPFOCTMU like mountains, trees, stones structure, ponds, rivers, etc. But, this only worked within a local area. These landmarks get changed due to environmental factors such as natural disasters. In course of time, attention was drawn to celestial bodies. Sailors could determine the latitude by measuring the angle of Pole star or Sun against the horizon. The Sun was most commonly used, but navigators could also use the Moon, a planet or one of 57 navigational stars whose coordinates are tabulated in the Nautical Almanac and Air Almanacs. 9KVJVJGCFXCPEGOGPVQHUEKGPEGCPFVGEJPQNQI[UEKGPVKſEVGEJPKSWGUYGTGGXQNXGF6JWU the ‘navigation’—a science of determining the position of a vehicle relative to the position of its destination—emerged. Navigation is the science of maneuvering from one point to other. (In Latin, ‘navis’ means boat and ‘agire’ implies guide). Navigation was originally coined for ship or any watercraft. But now, navigation is used for the determination of a body’s position, velocity and direction of the course of motion relative to some reference coordinate system so that destination is reached. Breakthrough in navigation was the radio navigation using the transmission of time signals around the period of 1920. Radio navigation is based on the measurement of arrival time of VKOGRWNUGUYKVJTGURGEVVQCKTVTCPUOKVVGT&GRGPFKPIQPVJGV[RKECNVGEJPKSWGVJGVTCPUOKUUKQP OGVJQFQNQI[FKHHGTURCTVKEWNCTN[KPUGNGEVKQPQHHTGSWGPEKGUV[RGQHOQFWNCVKQPCPFTGRGVKVKQP HTGSWGPE[QHRWNUGU &KUVCPEG OGCUWTKPI GSWKROGPV &/' YCU DCUKECNN[ WUGF VQ OGCUWTG VJG VKOKPI QH VJG propagation delay of radio signals in a slant range direction. It was operated in VHF or UHF TCPIG6JGCEEWTCE[QH&/'ITQWPFUVCVKQPUYCUO During World War II, .14#0 PCXKICVKQP VGEJPKSWG YCU FKUEQXGTGF YJKEJ YQTMGF based on the hyperbolic navigation method. Hyperbolic navigation refers to a class of navigation systems based on the difference in timing between the receptions of signals from two
119 Global Navigation Satellite System (GNSS) 135
Total standard error 'CEJ EQORQPGPV QH VJG UCVGNNKVG GTTQTU FKUEWUUGF KP VJG RTGXKQWU UGEVKQPU KU CUUWOGF VQ DG WPEQTTGNCVGFCPFQH\GTQOGCP'TTQTUEQPVTKDWVGFD[KPFKXKFWCNHCEVQTUCTGPQTOCNN[RTGUGPVGF in terms of standard deviation (1s 5QVQVCNGTTQTOC[DGHQWPFCUVJGTQQVUWOUSWCTGQHCNN these components. The estimates corresponding to the errors in the ranging values are tabulated KP6CDNGPQVKPIVJCVGCEJTGEGKXGTJCUKPJGTGPVFGNC[
TABLE 8.3 5VCPFCTF'TTQT/QFGN.%#
Source Errors ()mV Satellite clock 2.1 'RJGOGTKU 2.1 Ionosphere 10 Troposphere 2 Multi-path 1.2 Receiver noise 1 Total 5.1
Let us take into account the effect of satellite geometry. (QT8&12 VQVCNGTTQTKUO For HDOP = 2.0, total error is 10.2 m.
8.11 GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS) 6JG INQDCN PCXKICVKQP UCVGNNKVG U[UVGO )055 KU C IGPGTKE VGTO VJCV GPEQORCUUGU )NQDCN 2QUKVKQPKPI5[UVGO )25 QH75#).10#55QH4WUUKC)CNKNGQQH'WTQRGBeiDou (Compass) of China and also few regional satellite systems and satellite augmentation systems. All systems QH)055CTGKPFKHHGTGPVRJCUGUQHFGXGNQROGPV %WTTGPVN[)25CPF).10#55CTGHWNN[QRGTCVKQPCN%JKPCKURNCPPKPIVQJCXGVJGINQDCN PCXKICVKQP U[UVGO PCOGF CU %QORCUU $GK&QW QRGTCVKQPCN D[ 6JG 'WTQRGCP 7PKQPŏU )CNKNGQU[UVGOKUCNUQUEJGFWNGFVQDGHWNN[QRGTCVKQPCND[(TCPEG+PFKCCPF,CRCPCTG KPVJGRTQEGUUQHFGXGNQRKPITGIKQPCNPCXKICVKQPU[UVGOU)25JCUCNTGCF[DGGPGNCDQTCVGFKP previous sections.
8.11.1 GLONASS ).10#55 CNOQUV HQNNQY VJG VGEJPKSWG UKOKNCT VQ VJG )25 GZEGRV VJCV VJG ).10#55 WUGU VJG VGEJPKSWG QH FDMA, not CDMA. GLONASS has antipodal satellites, implying that two satellites in the same orbital plane, but are separated by 180o (i.e. in true anomaly). As the pair of satellites cannot be simultaneously viewed at the same place, they may use same frequency for transmission. 5Q KP URKVG QH WUKPI (&/# ).10#55 EQPUVGNNCVKQP FQGU PQV TGSWKTG HTGSWGPEKGUDWVTGSWKTGUQPN[HTGSWGPEKGUHQTUCVGNNKVGU 136 Satellite Communication
6JG).10#55EQPUVGNNCVKQPYCUEQORNGVGFKP*QYGXGTHQNNQYKPIEQORNGVKQPVJG U[UVGOFGITCFGFYKVJVJGEQNNCRUGQHVJG4WUUKCPGEQPQO[).10#55YCUTGXKXGFD[ and now operational with full constellation.
8.11.2 Galileo )CNKNGQKU'WTQRGŏUINQDCNPCXKICVKQPU[UVGO+VRNCPUVQJCXGUCVGNNKVGUKPQTDKVCNRNCPGU It is suitable for applications where safety is crucial such as air and ground transportation. )+18'# CPF )+18'$ VGUV UCVGNNKVGU CTG CNTGCF[ KP QTDKV %WTTGPVN[ UKZ UCVGNNKVGU CTG CXCKNCDNG)CNKNGQOC[DGHWNN[QRGTCVKQPCND[
8.11.3 BeiDou BeiDou is China’s global navigation system. BeiDou or compass will be fully operational by 2020 with the constellation of 35 satellites (mixture of 5 IGQUVCVKQPCT[ )'1 UCVGNNKVG inclined IGQU[PEJTQPQWU +)51 UCVGNNKVGU CPF /'1 UCVGNNKVGU A few of its satellites are planned for only regional service.
8.11.4 IRNSS (India) IRNSS is a regional service over Indian sub-continent. Its constellation has three geostationary UCVGNNKVGU CV NQPIKVWFGU QH u ' u ' CPF u ' +V JCU UCVGNNKVGU KP IGQU[PEJTQPQWU QTDKV CV KPENKPCVKQP QH u YKVJ VJG UCVGNNKVGU ETQUUKPI CV NQPIKVWFGU QH u ' CPF u ' The constellation is so designed that all seven satellites would always be visible with Indian control stations.
8.11.5 GNSS Comparison 6JG6CDNGUJQYUCEQORCTKUQPQHUQOGQHVJGMG[HGCVWTGUQHVJGUGFKHHGTGPVINQDCN)055 systems.
TABLE 8.4 %QORCTKUQPQH-G[(GCVWTGUQH5QOG)NQDCN)0555[UVGOU
Parameters GPS GLONASS GALILEO
Number of satellites 21 + 3 21 + 3 27 + 3
Number of orbital planes 6 3 3
5GOKOCLQTCZKU MO 26600 25440 29600 Orbital revolution period * * * Inclination (in degree) 55 64 56 Solar panel area (m2)142313 Direct Broadcasting Satellite (DBS) 10 Television
10.1 INTRODUCTION Around 1870s, television (TV) started in analog form. Though there were regular broadcasts, people at large did not adopt television until after World War II. Digital television (DTV), an advanced broadcasting technology, has revolutionised the television viewing experience. A digital signal can carry more information than the old analog network, implying better quality service. Service providers (i.e., broadcasters) may offer more channels and a range of new services. All-digital broadcasting opens up the valuable broadcast URGEVTWO HQT RQNKEG ſTG FGRCTVOGPVU CPF TGUEWG USWCFU #NUQ UQOG QH VJG URGEVTWO OC[ DG leased to companies to provide consumers with advanced wireless services. The old analog television (see Annexure 10A) network is being progressively switched off and replaced with a digital TV signal in course of time by most of the countries across the globe.
10.2 DIGITAL DBS TV The satellite broadcasting has the following advantages over the terrestrial broadcasting: Wide area coverage with a nationwide broadcasting using a single broadcasting frequency. Economical for not requiring terrestrial relay stations. A large capacity due to the availability of wide frequency band. Not affected by terrestrial disaster, etc. Thus, satellites are gradually being preferred for video and data delivery in addition to the telephone service due to their effectiveness and economical viability. Direct broadcast satellite (DBS) is also known as direct-to-home (DTH). In digital television, signals are broadcast in digital format at microwave frequencies. Geostationary satellites are used for majority of services. DBSTV is very popular, as it is directly accessible. This is also very attractive for the area which is not served by the cable TV. Pay-television services are quite common these days. So, some type of encryption is incorporated into the signal so that conditional access is achieved. &$5 UGTXKEGU CTG RTQXKFGF D[ UCVGNNKVG KP URGEKſE FQYPNKPM HTGSWGPE[ DCPFU VJCV CTG allocated for this purpose. The bands are as follows:
149 Direct Broadcasting Satellite (DBS) Television 151 antenna has normally a narrow beam pointing to a particular geostationary satellite without interfering with the adjacent satellites. The down link antenna should have wide beam to cover the assigned locations on the Earth. A number of companies provide DBS and DTH services throughout the world.
10.3 HDTV TRANSMISSION With initiation of digital television, JKIJFGſPKVKQP 68 *&68 emerged. The objective of HDTV is to enhance particularly picture resolution and clarity. In HDTV, both the video and VJGCWFKQUKIPCNUOWUVDGFKIKVKUGFD[#&EQPXGTVGTUCPFVTCPUOKVVGFUGTKCNN[VQVJGTGEGKXGT Because of the very high frequency of video signals, special techniques must be used to transmit the video signal over a standard 6 MHz bandwidth TV channel. After much deliberation, the (GFGTCN%QOOWPKECVKQP%QOOKUUKQP (%% JCUſPCNKUGFVJGUVCPFCTFUQH*&686JG*&68 sets can now be purchased by the consumer. Initially, they were expensive, but with the increasing use of digital television, the cost of HDTV receiving sets has eventually dropped down to a reasonably affordable value. In digital technique unlike analog one (see Annexure 10A), TV picture is made up of thousands of tiny dots of light, called pixels. Each pixel can be of any colour (total 256 colours are there). These pixels can be used to create any image. The greater the number of pixels on VJGUETGGPVJGITGCVGTKUVJGTGUQNWVKQPCPFVJGſPGTKUVJGFGVCKNVJCVECPDGTGRTGUGPVGF6JG format of an HDTV screen is described in terms of the numbers of pixels per horizontal line and by the number of vertical pixels. In HDTV, progressive scanning is preferred to interlaced scanning. This makes the video signals compatible with computer video monitors so that it is possible to display HDTV on computer screens as well. Further, it may be noted that KPVGTNCEGFUECPPKPIOKPKOKUGUƀKEMGT but complicates the video compression process. Some of the most common HDTV formats are shown in Table 10.1.
TABLE 10.1 Most Common HDTV Formats
Standard Aspect ratio Horizontal line Vertical pixel 720 p 16 : 9 1280 720 1080 i & p 16 : 9 1920 1080 (Note: ‘p’ stands for progressive and ‘i’ for interleaving)
10.4 COMPRESSION IN DIGITAL TV
10.4.1 Necessity of Compression It is obvious from the following calculation that an uncompressed digital video requires an enormous amount of storage space: For *KIJFGſPKVKQPVGNGXKUKQP *&68 A resolution of 1920 * 1080 pixels (HDTV), with a refresh rate of 30 frames per second (FPS). and 8-bit colour depth, would require Very Small Aperture 11 Terminal (VSAT)
11.1 INTRODUCTION There is continuous development taking place in the telecommunications industry to cope with the changing needs of the users. Through new technologies, telecommunications service providers can Expand the network coverage to new areas Improve the quality of basic communications services Reduce the costs of services to allow more users Add new services to increase the value of telecommunications services to users To start with, satellite technology was meant for long distance communication with large and expensive Earth terminal. With the rapid development and advancement of satellite technology, a few remarkable developments have been seen. There has been increase in transponder output RQYGT#PVGPPCYKVJUGXGTCNURQVDGCOUJCUDGGPKPVTQFWEGF$GVVGTNQYPQKUGCORNKſGTUJCXG been developed. All these technological breakthroughs have gradually changed the satellite communication scenario. Thus, satellite communication can now be planned for regional systems requiring smaller coverage with affordable smaller even portable Earth station. Very small aperture terminal (VSAT) has, thus, been evolved as an important recent development KPVJGſGNFQHVGNGEQOOWPKECVKQPU The VSATs operate through a satellite for the distribution and/or exchange of voice, data and compressed video between the users. The VSAT systems have the capability of addressing the issue related to point-to-multi-point communication, which is the feature that cannot be handled easily in traditional terrestrial system. Because of these merits, VSAT systems have rapidly advanced technology, and thus, their applications have been expanding. So, this has ECWUGFCUKIPKſECPVITQYVJYKVJKPVJGUCVGNNKVGEQOOWPKECVKQPKPFWUVT[ VSAT network is an assemble of a large number of small and inexpensive Earth stations (VSATs). Each one of them is located at the respective location of customers affecting point-to- multi-point communication. Mostly, customers establish a link with a central large Earth station called the hub station, which also has the facility to communicate with the relevant terrestrial network. The entire network is organised by the hub station via the network management system (NMS). The network management system (NMS) is a software package, which is
164 170 Satellite Communication
FIGURE 11.6 )GPGTKEHWPEVKQPCNDNQEMUQHVTCPUTGEGKXGTKP85#6PGVYQTM85#6VGTOKPCNCPFJWD VGTOKPCN CTG CNOQUV UKOKNCT *WD KU OQTG GNCDQTCVG VQ GHſEKGPVN[ JCPFNG DQVJ KPDQWPF CPFQWVDQWPFVTCHſE
Geostationary satellite is used in VSAT network. So, the antenna dish of a VSAT Earth UVCVKQPPGGFUVQDGCNKIPGF D[CFLWUVKPIVJGelevation and the azimuth) only once to the desired UCVGNNKVGFWTKPIVJGRTQEGUUQHKPKVKCNKPUVCNNCVKQP The IDU is located inside a room or building. It consists of a demodulator (demod) in VJG TGEGKXG EJCKP CPF C OQFWNCVQT OQF KP VJG VTCPUOKV EJCKP6JG +&7 KPENWFGU C RQYGTHWN microprocessor, which helps in the processing of baseband sub-system. This also facilitates the GHſEKGPVUCVGNNKVGEJCPPGNCEEGUU The hub system has almost similar units, but has more elaborate arrangement to handle DQVJKPDQWPFCPFQWVDQWPFVTCHſECUUJQYPKP(KIWTG D (QTGZCORNGVJGJWDUVCVKQPKU usually a relatively large, high-performance Earth station with the CPVGPPCFKCOGVGTQHTCPIKPI between 6 m and 9 m. 6JGEWUVQOKUGFFCVCKPVGTHCEGKUFQPGVJTQWIJGHſEKGPVOKETQEQPVTQNNGT which establishes a link between a terrestrial protocol and the satellite link protocol.
11.7 VSAT OR WIRELESS LOCAL LOOP (WLL) NETWORK Public switched telephone network (PSTN) (or broadband) may also be accessed though Internet through a wireless technology without using the copper cable network. This scheme is called Special Purpose 12 Satellites
12.1 INTRODUCTION Satellites used for communication are mostly geostationary satellites. The International Telecommunication Satellite organisation (ITSO), previously known as INTELSAT, operates for YQTNFYKFGEQOOWPKECVKQPUGTXKEG+VKUWUGFHQTJWIGCOQWPVQHEQOOWPKECVKQPVTCHſE5KOKNCT satellite is INMARSAT, which is also used for modern communication services, including mobile communication. INMARSAT has also leased satellite service. The geostationary orbit around the equator is overcrowded because of its wide coverage and their nearly static position within the coverage area. Nevertheless, the low orbit (LEO and MEO) satellites have their own importance and application due to the following features:
Each low orbit satellite may have limited coverage, but a few such satellites taken together, i.e., in constellation, have global coverage in true sense, even covering higher latitudes beyond 80°, i.e., the area which is, otherwise, not covered by geostationary satellites. Path length between the satellite and the Earth is less. So, propagation delay and free space path loss are smaller. The antenna pattern need not be highly directive. So, antenna size is smaller.
Lower orbit satellites are specially used for navigation purposes, remote sensing applications, and mobile communication. Navigations satellites like GPS, GLONASS, Galileo and Beiduo have been amply discussed in Chapter 8. A few satellite constellations are already in operation for mobile communications and satphones. Some of them are Globalstar, Orbocomm, Iridium, etc. RADARSAT is a remote sensing satellite of Canadina Space Programme and Landsat series is an effort of National Aeronautics and Space Administration (NASA) for remote sensing. There are around 10 operational lower orbit Indian remote sensing satellites. This chapter covers a few lower orbit satellites in more detail. This chapter also includes Indian National Satellite (INSAT), which is in geostationary orbit, but has multi-purpose objectives other than just communication.
174