Mobile and Wireless Networks Course Instructor: Dr. Safdar Ali

Transcription

1 Mobile and Wireless Networks Course Instructor: Dr. Safdar Ali

2 Satellite Communication

3 INTRODUCTION

4 INTRODUCTION Communication satellite are off-course only one means of telecommunication transmission. The traditional means include copper wire and microwave point- to-point links. Newer techniques involves use of optics either point-to-point infrared or fiber optics. Point-to-point radio system such as short wave radio may also be used.

5 Why Satellite??

6 INTRODUCTION Inter- continent communication is difficult by using the guided mediums. The same feature enables satellites to provide communications links to remote communities in sparsely populated areas that are difficult to access by other means.

7 INTRODUCTION Very large areas of the earth are visible from a satellite, the satellite can form the star point of a communications net, simultaneously linking many users who may be widely separated geographically.

8 INTRODUCTION The information transferred most often correspondence to voice (telephone), video (Television) and digital data.

9 Satellite Disadvantages??

10 DISADVANTAGES OF SATELLITES COMMUNICATION Launching satellites into orbit is costly. There is a larger propagation delay in satellite communication than in terrestrial communication.

11 What is Satellite??

12 INTRODUCTION A communication satellite is basically an electronic communication package placed in orbit whose prime objective is to initiate or assist through space.

13 INTRODUCTION Satellite contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.

14 INTRODUCTION

15 ELEMENTS OF SATELLITE COMMUNICATIONS The basic elements of a communication satellite service are divided between; Space Segment Ground Segment

16 ELEMENTS OF SATELLITE COMMUNICATIONS The space segment consist of the spacecraft & launch mechanism. Ground segment comprises the earth station and network control center of entire satellite system.

17 INTRODUCTION ECHO 1 TELSTAR

18 INTRODUCTION SYNCOM 2 Asiasat 2

19 EXAMPLES OF SATELLITE RADIO SERVICES Fixed Satellite Service Mobile Satellite Service Broadcast Satellite Service Radio Navigation Sat. Serv. Radio location Sat. Service Space Operation Service Earth observation Sat. Serv.... FSS MSS BSS RNSS RSS SOS ESS In total more than 18 radio services

20 SATELLITE OPERATING FREQUENCY BANDS

21 APPLICATIONS Communication (truncking call) Teleconference TV Broadcasting Data communication Weather telecast Navigation GPS Security/Calamity monitoring

22 Orbital Mechanics

23 THE ORIGIN OF SATELLITE The concept of using object in space to reflect signals for communication was proved by Naval Research Lab in Washington D.C. when it use the Moon to establish a very low data rate link between Washington and Hawaii in late 1940 s. Russian started the Space age by successfully launching SPUTNIK the first artificial spacecraft to orbit the earth, which transmitted telemetry information for 21 days in Oct

24 THE ORIGIN OF SATELLITE The American followed by launching an experimental satellite EXPLORER In In 1960 two satellite were deployed Echo & Courier In 1963 first GEO Syncom. The first commercial GEO (Intelsat & Molnya) in 1965 these provides video (Television) and voice (Telephone) for their audience

25 ORBITAL MECHANICS Satellite technology has progressed tremendously,since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World. When Clarke wrote, there were no satellite in orbit nor rockets were powerful enough to launch them. After 20 years, his idea was proven, when early bird was launched. It cost roughly $ 25,000 per kg to get a geostationary satellite in orbit.

26 KU- BAND SATELLITE IN GEO

27 MAJOR PROBLEMS FOR SATELLITE Positioning in orbit in-term of Frequency & Orbit Selection Stability Power Harsh environment Interference Problem High initial investment New investment require in Ground Segment Short life time Spectrum crowding Regulatory aspects (landing rights etc.) Launch vehicle reliability

28 ELECTROMAGNETIC SPECTRUM

29 TYPES OF SATELLITE ORBITS Based on the inclination, i, over the equatorial plane: Equatorial Orbits above Earth s equator (i=0 ) Polar Orbits pass over both poles (i=90 ) Other orbits called inclined orbits (0 <i<90 ) Based on Eccentricity Circular with centre at the earth s centre Elliptical with one foci at earth s centre

30 TYPE OF ORBITS LEOs - Low Earth Orbit MEOs - Medium Earth Orbit GEO - Geostationary Earth Orbit

31 GEOSTATIONARY EARTH ORBIT (GEO) Originally proposed by Arthur C. Clarke Circular orbits above the equator

32 GEO Orbital height above the earth about miles/ km Round trip time to satellite about 0.24 seconds

33 GEO GEO satellites require more power for communications The signal to noise ratio for GEOs is worse because of the distances involved A few GEOs can cover most of the surface of the earth Note that polar regions cannot be seen by GEOs

34 GEO Since they appear stationary, GEOs do not require tracking GEOs are good for broadcasting to wide areas

35 GEO Angular separation about 2 degrees - allows 180 satellites

36 THE VIEW FROM 36,000KM

37 SOME GEO S ABOVE US Optus AsiaSat PAS Intelsat Inmarsat Palapa

38 LEO Low Earth Orbit 200-3,000 km High orbit speed Many satellites Predominately mobile Iridium, Globalstar (space shuttle orbit)

39 MEO Medium Earth Orbit 6,000 12,000km About 12 satellites Voice and mobile ICO (Odyssey), Orbcomm, Ellipso

40 WHY SATELLITE REMAINS IN THE ORBIT

41 SATELLITE ORBITS AND PERIODS

42 SATELLITE ORBITS

43 ASSIGNMENT # 2 Why Modulation?? Why do we use Decibel in Telecommunication?? Difference between the optical and radio horizon?? What is Multi-level signalling???

44 Kepler s Laws

45 KEPLER S LAWS LAW 1: The orbit of a planet about the Sun is an ellipse with the Sun's center of mass at one focus (1609) LAW 2: A line joining a planet and the sun sweeps out equal areas in equal intervals of time (1609) LAW 3: The squares of the periods of the planets are proportional to the cubes of their semi-major axes (1619)

46 MAJOR AND MINOR AXIS

47 KEPLER S FIRST LAW Kepler s first law states that the path followed by a satellite around the primary will be an ellipse. An ellipse has two focal points shown as F1 and F2.

48 KEPLER S FIRST LAW The center of mass of the two-body system, termed the barycenter, is always centered on one of the foci. Because of the enormous difference between the masses of the earth and the satellite, the center of mass coincides with the center of the earth, which is therefore always at one of the foci.

49 KEPLER S FIRST LAW The semi-major axis of the ellipse is denoted by a, and the semi-minor axis, by b. The eccentricity e is given by: For an elliptical orbit, 0 < e < 1. When e= 0, the orbit becomes circular. Earth s orbit has an eccentricity of (nearly circular)

50 KEPLER S SECOND LAW Kepler s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter.

51 KEPLER S SECOND LAW Assuming the satellite travels distances S1 and S2 meters in 1 s, then the areas A1 and A2 will be equal.

52 KEPLER S SECOND LAW The average velocity in each case is S1 and S2 m/s, and because of the equal area law, it follows that the velocity at S2 is less than that at S1.

53 KEPLER S SECOND LAW

54 APOGEE AND PERIGEE Satellites go faster at Perigee than at Apogee. Apogee: A point for a satellite farthest from the Earth. Perigee: A point for a satellite closest from the Earth.

55 KEPLER S THIRD LAW Kepler s third law states that the square of the periodic time of revolution of the smaller body about the larger body equals a constant multiplied by the third power of the semi major axis of the orbital ellipse. Kepler's third law of orbital motion gives us a precise relationship between the speed of the satellite and its distance from the earth.

56 KEPLER S THIRD LAW Satellites in circular orbits travel at a constant speed. Simple. We just specify that speed, and we're done. Satellites in non-circular (i.e., eccentricity > 0) orbits move faster when they are closer to the earth, and slower when they are farther away.

57 KEPLER S THIRD LAW The common practice is to average the speed. You could call this number "average speed", but astronomers call it the "Mean Motion. Mean Motion is usually given in units of revolutions per day.

58 NUMERICAL The earth rotates once per sidereal day of 23 h 56 min 4.09 s. Show that the radius of the GEO is 42, km.

59 NUMERICAL The Space Shuttle is an example of a low earth orbit satellite. Sometimes, it orbits at an altitude of 250 km above the earth s surface, where there is still a finite number of molecules from the atmosphere. The mean earth s radius is approximately km. Using these figures, calculate the period of the shuttle orbit when the altitude is 250km and the orbit is circular. Find also the linear velocity of the shuttle along its orbit.

60 WHAT IS A SATELLITE? A celestial body In astronomical term, e.g. Moon A space vehicle launched by humans and orbits the earth or another celestial body In aerospace terms Communication Satellite provides communication and other services to variety of consumers It is a microwave repeater in the sky A satellite radio repeater is called a transponder A satellite may have one to many transponders

61 WHAT IS A SATELLITE SYSTEMS? A Satellite System may consist of : one or more satellite space vehicles, a ground based control earth station, and a network of user earth stations that provides the interface facilities for the transmission and reception of terrestrial communications traffic through the satellite systems.

62 WHAT IS A SATELLITE SYSTEMS? Transmissions to and from the satellites are categorized as either bus or payload. The payload is the actual user information conveyed through the system. The bus includes control mechanisms that support the payload operation.

63 TYPES OF SATELLITES Passive Satellites Active Satellites

64 PASSIVE SATELLITES Simplest type of satellite is a passive reflector It simply bounces signals from one place to another. It reflects signals back to earth as there are no gain devices on board to amplify or modify the signals. The passive satellites used in the early years of satellite communications were both artificial as well as natural. Moon became the first passive satellite in 1954 when the U.S Navy successfully transmitted the first message over an Earth-to-moon-to-Earth communication system.

65 PASSIVE SATELLITES But moon proved to be unreliable communication satellite as it is above the horizon only half of the time and its position relative to earth is constantly changing. Radio beacon transmitters are required for tracking and ranging purposes. Beacon are used so that an earth station can lock on to and use to determine the exact location of a satellite so the earth station can align its antennas.

66 ARTIFICIAL PASSIVE SATELLITE The first artificial passive satellite Echo-I of NASA was launched in August Echo-I was 100-ft. diameter inflatable plastic balloon with aluminum coating that reflected radio signals transmitted from huge earth station antennas.

67 ARTIFICIAL PASSIVE SATELLITE Echo-I had an orbital height of 1000 miles. Earth Stations across US and Europe picked up the signal and contributed a lot in motivating research in communication satellite.

68 DISADVANTAGE OF PASSIVE SATELLITE Moon proved to be unreliable communication satellite as it is above the horizon only half of the time and its position relative to earth is constantly changing. Earth Stations required high power (10 kw) to transmit signals strong enough to produce an adequate return echo. Large Earth Stations with tracking facilities were expensive.

69 DISADVANTAGE OF PASSIVE SATELLITE Control of satellites not possible from ground. As little as 1 part in every 10 x 18 of an earth station s transmitted power is actually returned to earth station receiving antennas. A global system would have required a large number of passive satellites accessed randomly by different users.

70 ACTIVE SATELLITES It is capable of receiving, amplifying, reshaping, regenerating and retransmitting the information. Has sophisticated electronic equipment on-board.

71 ACTIVE SATELLITES Advantages of active satellites are: Require lower power earth station Directly controlled by operators from ground. Disadvantages of active satellites are: Disruption of service due to failure of electronics components on-board the satellites Requirement of on-board power supply Requirement of larger and powerful rockets to launch heavier satellites in orbit