SATELLITE COMMUNICATION, ADVANCEMENT, ISSUES, CHALLENGES AND APPLICATION

SATELLITE COMMUNICATION, ADVANCEMENT, ISSUES, CHALLENGES AND APPLICATION

CHAPTER 1 INTRODUCTION Satellite communication is one of the most impressive spinoffs from the space programs and has made a major contribution to the pattern of international communications. A communication satellite is basically an electronic communication package placed in orbit whose prime objective is to initiate or assist communication transmission of information or message from one point to another through space. The information transferred most often corresponds to voice (telephone), video (television), and digital data. Communication involves the transfer of information between a source and a user. An obvious example of information transfer is through terrestrial media, through the use of wire lines, coaxial cables, optical fibers, or a combination of these media. Communication satellites may involve other important communication subsystems as well. In this instance, the satellites need to be monitored for position location in order to instantaneously return an upwardly transmitting (uplink) ranging waveform for tracking from an earth terminal (or station). The term earth terminal refers collectively to the terrestrial equipment complex concerned with transmitting signals to and receiving signals from the satellite. The earth terminal configurations vary widely with various types of systems and terminal sizes. An earth terminal can be fixed and mobile land based, sea-based, or airborne. Fixed terminals, used in military and commercial systems, are large and may incorporate network control center functions. Transportable terminals are movable but are intended to operate from a fixed location, that is, a spot that does not move. Mobile terminals operate while in motion; examples are on commercial and navy ships as well as on aircraft.

1.1. THE ORIGIN OF SATELLITES The Space Age began in 1957 with the U.S.S.R.s launch of the first artificial satellite, called Sputnik, which transmitted telemetry information for 21 days. This achievement was followed in 1958 by the American artificial satellite Score, which was used to broadcast President Eisenhowers Christmas message. Two satellites were deployed in 1960: a reflector satellite, called Echo, and Courier. The Courier was particularly significant because it recorded a message that could be played back later. In 1962 active communication satellites (repeaters), called Telstar and Relay, were deployed, and the first geostationary satellite, called Syncom, was launched in 1963. The race for space exploitation for commercial and civil purposes thus truly started. A satellite is geostationary if it remains relatively fixed (stationary) in an apparent position relative to the earth. This position is typically about 35,784 km away from the earth. Its elevation angle is orthogonal (i.e., 90_) to the equator, and its period of revolution is synchronized with that of the earth in inertial space. A geostationary satellite has also been called a geosynchronous or synchronous orbit, or simply a geosatellite. The first series of commercial geostationary satellites (Intelsat and Molnya) was inaugurated in 1965. These satellites provided video (television) and voice (telephone) communications for their audiences. Intelsat was the first commercial global satellite system owned and operated by a consortium of more than 100 nations; hence its name, which stands for International Telecommunications Satellite Organization. The first organization to provide global satellite coverage and connectivity, it continues to be the major communications provider with the broadest reach and the most comprehensive range of services. Other providers for industrial and domestic markets include Westar in 1974, Satcom in 1975, Comstar in 1976, SBS in 1980, Galaxy and Telstar in 1983, Spacenet and Anik in 1984, Gstar in 1985, Aussat in 198586, Optus A2 in 1985, Hughes-Ku in 1987, NASA ACTS in 1993, Optus A3 in 1997, and Iridium and Intelsat VIIIA in 1998. Even more are planned. Some of these satellites host dedicated military communication channels. The need to have market domination and a competitive edge in military surveillance and tactical fields results in more sophisticated developments in the satellite field.

CHAPTER 2 ELEMENTS OF SATELLITE COMMUNICATION2.1. ELEMENTS Two major elements of Satellite Communications Systems are Space Segment Ground Segment

The Space Segment includes

Satellite Means for launching satellite Satellite control centre

The function of the ground segment is to transmit the signal to the satellite and receive the signal from the satellite. The ground segment consists of Earth Stations Rear Ward Communication links User terminals and interfaces Network control centre

Fig 2.1. Elements of satellite communication

Fig 2.2. Overview of satellite communication

2.2. SPACE SEGMENTCommunication Satellite Communication satellites are very complex and extremely expensive to procure & launch. The communication satellites are now designed for 12 to 15 years of life during which the communication capability of the satellite earns revenue, to recover the initial and operating costs. Since the satellite has to operate over a long period out in the space the subsystems of the satellite are required to be very reliable. Major subsystems of a satellite are: Satellite Bus Subsystems Satellite PayloadsSatellite Bus subsystems: Mechanical structure Attitude and orbit control system Propulsion System Electrical Power System Tracking Telemetry and Command System Thermal Control SystemSatellite Payloads Communication transponders Communication AntennasSince the communications capacity earns revenue, the satellite must carry as many communications channels as possible. However, the large communications channel capacity requires large electrical power from large solar arrays and battery, resulting in large mass and volume. Putting a heavy satellite in geosynchronous orbit being very expensive, it is logical to keep the size and mass of the satellite small. Lightweight material optimally designed to carry the load and withstand vibration & large temperature cycles are selected for the structure of the satellite. Attitude and orbit control system maintains the orbital location of the satellite and controls the attitude of the satellite by using different sensors and firing small thrusters located in different sides of the satellite.Liquid fuel and oxidizer are carried in the satellite as part of the propulsion system for firing the thrusters in order to maintain the satellite attitude and orbit. The amount of fuel and oxidizer carried by the satellite also determines the effective life of the satellite.The electrical power in the satellite is derived mainly from the solar cells. The power is used by the communications payloads and also by all other electrical subsystems in the satellite for house keeping. Rechargeable battery is used for supplying electrical power during ellipse of the satellite.Telemetry, Tracking and Command system of the satellite works along with its counterparts located in the satellite control earth station. The telemetry system collects data from sensors on board the satellite and sends these data via telemetry link to the satellite control centre which monitors the health of the satellite. Tracking and ranging system located in the earth station provides the information related to the range and location of the satellite in its orbit. The command system is used for switching on/off of different subsystems in the satellite based on the telemetry and tracking data.The thermal control system maintains the temperature of different parts of the satellite within the operating temperature limits and thus protects the satellite subsystems from the extreme temperature conditions of the outer space.The communications subsystems are the major elements of a communication satellite and the rest of the space craft is there solely to support it. Quite often it is only a small part of the mass and volume of the satellite. The communications subsystem consists of one or more antennas and communications receiver - transmitter units known as transponders. Transponders are of two types, Repeater or Bent pipe and processing or regenerative. In Repeater type, communications transponder receives the signals at microwave frequencies and amplifies the RF carrier after frequency conversion, whereas in processing type of transponder in addition to frequency translation and amplification, the RF carrier is demodulated to baseband and the signals are regenerated and modulated in the transponder. Analog communication systems are exclusively repeater type. Digital communication system may use either variety. Fig.2.2.1 and 2.2.2 show the schematic diagrams of repeater type and regenerative type transponders respectively.

Fig 2.2.1. Schematic diagram of repeater type transponderFig 2.2.2 Schematic type of regenerative type transponderThe actual reception and retransmission of the signals are however, accomplished by the antennas on board the satellite. The communications antennas on board the satellite maintain the link with the ground segment and the communications transponder. The size and shape of the communications antenna depend on the coverage requirements and the antenna system can be tailor made to meet the specific coverage requirements of the system

Launch VehicleThe function of the launch vehicle is to place the communication satellite in the desired orbit. The size and mass of the satellite to be launched is limited by the capability of the launch vehicle selected for launching the satellite. The satellite launch vehicle interface is also required to be provided as per the launch vehicle selected. Satellite launch vehicles are classified in two types i.e. Expendable ReusableIn expendable type the launch vehicle can be used only once and most of the launch vehicles are expendable type. Space Transportation System (STS) or Space Shuttle of NASA, USA is the only available operational reusable launch vehicle. Although most of the launches take place from ground, Sea Launch has embarked on the launching of satellites from off shore platforms and Peagasus launch vehicles can launch small satellites from aircrafts. Launching of a satellite in orbit being a costly affair a number of programs have been undertaken by NASA to make the future launching of satellites in orbit as cost effective and routine as commercial air travel. Satellite Control CentreSatellite Control Centre performs the following function. Tracking of the satellite Receiving Telemetry data Determining Orbital parameters from Tracking and Ranging data Commanding the Satellite for station keeping Switching ON/OFF of different subsystems as per the operational requirements Thermal management of satellite. Eclipse management of satellite Communications subsystems configuration management. Satellite Bus subsystems configuration management etc.

2.3 GROUND SEGMENTThe ground segment of satellite communications system establishes the communications links with the satellite and the user. In large and medium systems the terrestrial microwave link interfaces with the user and the earth station. However, in the case of small systems, this interface is eliminated and the user interface can be located at the earth station. The earth station consists of Transmit equipment. Receive equipment. Antenna system.

Fig. 2.3.1 shows the schematic block diagram of an earth station.

Fig.2.3.1 Schematic block diagram of Earth station

CHAPTER 3 ORBITS FOR COMMUNICATION SATELLITEThe path a Satellite or a planet follows around a planet or a star is defined as an orbit. In general the shape of an orbit of a satellite is an ellipse with the planet located at one of the two foci of the ellipse. The circular orbit may also be considered as an ellipse where the two foci of the ellipse coincide at the center of the circle. Satellite Orbits are classified in two broad categories i.e. Non-Geostationary Orbit (NGSO) Geo Stationary Orbit (GSO)

Non-Geostationary Orbit (NGSO)Early ventures with satellite communications used satellites in Non-geostationary low earth orbits due to the technical limitations of the launch vehicles in placing satellites in higher orbits. With the advancement of launch vehicles and satellite technologies, once the Geo Stationary Orbit (GSO) was achieved, majority of the satellites for telecommunications started using GSO due to its many advantages. During 1990s the interests in NGSOs were rekindled due to several advantages of NGSO in providing global personal communications in spite of its many disadvantages. Advantages of NGSO are: Less booster power required to put a satellite in lower orbit Less space loss for signal propagation at lower altitudes (20GHz) devices Materials for electronics devices Solar cell materials and structures Network technology for high data rate, integrated spac and terrestrial systems Optical components and sub systems Radiation resistant device structures and circuits Strong and light-weight material Thermal dissipation materials In addition experimental satellites are needed that can be used to test out new technology that cannot easily be tested on the ground. At the systems level, the future of satellites could also be impacted by high altitude, long endurance platforms which would operate from 65,000 to 1,00,000 feet such as airships and loitering aircraft. Such systems could be used to substitute for satellite communication in regional applications or could be used in conjunction with satellites as a system capacity multiplier over populated areas. 4) Policies and Regulatory issues In international satellite trade landing rights agreements, annual licensing fees for terminals, non-tariff barriers, allocation of frequencies and orbital slots, adequacy and effectiveness of intersystem coordination procedure, security and privacy of information being relayed on satellite system etc are some issues to be resolved. Most important of all is the need to develop protocols for seamless interconnection of satellite, wireless and terrestrial fiber networks. In the 21st century interconnection of satellite systems, particularly inter-satellite links will be a key challenge. Connecting them to low latency terrestrial network is truly a challenge.

CHAPTER 8 APPLICATIONS Satellites are uniquely suited for certain application. These include broad casting, service to mobile users including ships, aircraft, land mobile and emergency services and providing nearly instant infrastructure in underserved areas. A significant factor in these plans has been the growth of the Internet which shows no sign of abating, despite the poor access that most users currently enjoy. Thus the fielding of some of these Ka-band systems could overcome, the last mile connection problem encountered in most developed countries. This would be major application previously not served by satellites system. Other applications are: 1) Traditional Telecommunication Telecommunication trend that are fuelling interest in satellite systems are direct-to-home television (DTH) or direct broadcast satellite (DBS), the enormous growth in wireless hand -held phone usage (cellular, personal communication services and paging) and the growth in the number of personal computer in the world, increasing numbers of which are multimedia ready and are being used to interconnect with Internet, maritime and aeronautical telephony, fleet broadcast communication etc. 2) Atmospheric, oceanic and terrestrial observation using Satellites i) Atmospheric observation Metrological satellites of major space bearing countries operating in the region are used to collect atmospheric data used in climate forecast. Indias INSAT-3 series, MATSAT, Chinas Feng Yung-1C, European metrological satellite NOAA series etc are operational for this kind of applications. ii) Oceanic observations Wind speed and direction near the oceanics surface are very important to forecast oceanic storm. Indias Oceansat-1 and Oceansat-2, QuikSat of China KOMPSAT for measuring ocean colour, Envisat to measure pigment concentrations, suspended sediments and dissolve organic matters are operational satellites for the oceanic observation purpose. iii) Terrestrial observation Terrestrial observation is carried through remote sensing satellite include crop management, fertility, pest and disease information to increase crop yields and profitability, flood forecast, forestry estimation, global change studies, land cover monitoring and assessment, large area mapping, cartography, search and rescue operation, emergency disaster communication and hazard mitigation, observation of environmental change occurring over land etc. India have Resource sat constellation of four satellite, Landsat-7 of NASA, ALOS,IKONOS-2,Quickbird1,Orbview-3&4 etc satellites are providing terrestrial observation need. 3) Satellite based navigation and positioning SPS satellites will spawn advanced applications that require very precise location and tracking such as precision mapping and surveying or tracking oil spills and hazardous icebergs. In addition the same satellites will provide advanced services for aircraft and vessel navigation and moving-map displays for motor vehicles. NAVSTAR and GLONASS constellations, GNSS-1 satellite are supporting this purpose. 4) Space Science and Solar terrestrial applications. i) Space science application With the construction of the International Space Station now under way, another door has opened not only to long duration examination of the space environment but also to research and manufacturing activities under microgravity conditions. Mir Space Station, the International Space Station will establish a more advanced platform to conduct space science and technology experiments. ii) Solar terrestrial applications Some countries are investigating the concept of solar power from space. The Solar Power system 2000 project would involve electrical power generation from solar cells on board satellites in low, equatorial orbits for transmission by microwave to specially designed power receiving antennas in countries lying the equatorial zone. 5) Satellite-based education and training i) Distance learning and teaching South-East Asian Ministries of Education organisation have implemented open learning and distance education programmes in their countries via satellite-based education and training in disciplinary open learning centres located in various countries in the region. PEACESAT and other similar satellites are used in Asia and Pacific region for education and training. ii) Engineering Research and Development At least nine countries are pursuing smallscale experimental missions that have the objectives of human resource and industry development. These include Badr-B, FedSat, TMSat and KITSAT series. 6) Military Application. Space plays an increasing role in military activities. They are widely used to provide support for military or security related activities such as verifying compliances with arms and control treaties. Military uses include imagery, navigation, signal intelligence, telecommunications, early warning and metrology. There are over 270 military satellites as well as ~600 civil, commercial and multipurpose satellites. These satellites serve military as well civil application 6.1. Future Application1) Frequency reuses application Since Satellite system serve large areas such as countries or continent, a large number of beams need to share the available beam width. To circumvent this frequency reuse scheme is often utilized. This is based on reusing the same frequencies in spatially isolated beams. Therefore available bandwidth is divided into a smaller number of beams in coverage area. The set of beams that share the total available bandwidth is known as cluster. The cluster is then repeated in the coverage area relaying on the fact that the beams operating at same bandwidth will be separated from each other sufficiently so that they do not interfere with each other. There are only a discrete set of possible cluster sizes, N, to accommodate a contiguous coverage of hexagonal geometry. The possible number of beams in cluster which would form tessellating shape is given by: N=i2 + j2 + i x j .. (1) Where N is the number of beams in cluster and i, j are non-negative integer numbers.2) Use of Spot Beam Concept and its applications. NASAs satellite ACTS uses hopping spot beam technique to combine the advantage of frequency reuse, spot beams and TDMA. Concept of multiple spot beams is also planned in future generation satellite of Italy, Italsat with six spot beam operating at 30GHz uplink and 20GHz downlink, the satellite interconnects TDMA transmissions between ground station in all the major economic centres of Italy.3) Other Applications. Laser beams based satellite communication. In finding Space debris solution Space Situational Awareness uses Use of constellation of spacecraft and inter satellite links for telecommunication purpose etc.

References[1] Deepak misra, Dinesh Kumar Misra, Dr. S. P. Tripathi, Satellite Communication Advancement,Issues,Challenges And Application, IJARCCE Vol. 2,Issue 4

[2] Clarke, `Extra Terrestrial Relays, Wireless World. Vol.51, pp 305-308, October 1945 [3] Morrow Jr. (Ed) Satellite Communications, Proc. IEEE Vol.59, No.2 Feb.1971. [4] Harry L. Van Trees (Ed), Satellite Communications, IEEE Press selected series [5] Dennis Roddys , Satellite Communication, 4th Edition

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