The outer space has always fascinated people on the earth and communication through space evolved as an offshoot of ideas for space travel. The earliest idea of using artificial satellites for communications is found in a science fiction Brick Moon by Edward Evert Hale, published in 1869-70. While the early fictional accounts of satellite and space communications bear little resemblance to the technology as it exists to day, they are of significance since they represent the origins of the idea from which the technology eventually evolved. In the area of satellite communications, the technology has been responsive to the imaginative dreams. Hence it is also expected that technological innovations will lead the evolution of satellite communications towards the visions of today.
CONCEPT OF SATELLITE COMMUNICATIONS
Scientists from different countries conceived various ideas for communications through space along with the technological breakthroughs in different fields of science. The Russian scientist Konstantin Tsiolkovsky (1857-1935) was the first person to study space travel as a science and in 1879 formulated his Rocket Equation, which is still used in the design of modern rockets. He also wrote the first theoretical description of a man- made satellite and noted the existence of a geosynchronous orbit. But he did not identify any practical applications of geosynchronous orbit. The noted German Scientist and rocket expert, Hermann Oberth, in 1923 proposed that the crews of orbiting rockets could communicate with remote regions on earth by signaling with mirrors. In 1928, Austrian Scientist Hermann Noordung suggested that the geostationary orbit might be a good location for manned space vehicle. Russian Scientists in 1937 suggested that television images could be relayed by bouncing them off the space vehicles.
During 1942-1943, a series of articles by George O Smith were published in Astounding Science Fictions concerning an artificial planet, Venus Equilateral, which functioned as relay station between Venus and Earth Station when direct communication was blocked by Sun. However, Arthur C. Clarke, an electronic engineer and the well-known science fiction writer is generally credited with originating the modern concept of Satellite Communications. In 1945, Clarke, in his article 'Extra Terrestrial Relays: Can Rocket Stations give Worldwide Radio Coverage?' published in Wireless World outlined the basic technical considerations involved in the concept of satellite communications. Clarke proposed orbiting space stations, which could be provided with receiving and transmitting equipment and could act as a repeater to relay transmission between any two points of the hemisphere beneath. He calculated that at an orbital radius of 42,000 km. the space station's orbit would coincide with the earth's rotation on its axis and the space station would remain fixed as seen from any point on the earth. He also pointed out that three such synchronous stations located 120 degrees apart above the equator could provide worldwide communications coverage. The concept was later considered to be generating a billion dollar business in the area of communications. However, Clarke did not patent the most commercially viable idea of twentieth century as he thought satellites would not be technically and economically viable until the next century.
REALISATION OF CONCEPT TO REALITY
In October 1957, the first artificial satellite Sputnik -I was launched by former Soviet Russia in the earth's orbit and in 1963 Clark's idea became a reality when the first geosynchronous satellite SYNCOM was successfully launched by NASA.
The realization of the concept of satellite communications from an idea to reality has been possible due to a large number of technological breakthroughs and practical realization of devices and systems, which took place during and after the World War II. The pressures of international military rivalry during cold war period were also able to a great extent to push scientific and technological research and development far faster than it would have been possible if applied for peaceful purposes.
The successful launching of communications satellite in earth's orbit was possible because of keen interests shown by specific groups of people along with the developments in diverse areas of science and technology. Some of these factors, which are considered important in the realization of satellite communications, are:
· Development of high power rocket technology and propulsion systems capable of delivering satellites in high altitude orbits
· Scientific and military interests in Space Research
· Development of Transistors and miniaturization of electronic circuitry.
· Development of Solar Cells for providing sustained energy source for the satellite.
· Development of high-speed computers for calculating and tracking orbits.
· Government support in large-scale financial commitment to Space Technology Development for Military, Scientific Experiments and Civilian Applications.
· International military rivalry among super powers.
· The psychological impact of Sputnik Challenge leading to long range program of scientific research and development undertaken by US.
Before the transformation of the concept of communications by satellite to blue print and subsequent development of the hardware took place it was necessary to make the scientific communities convinced about the technical feasibility of such a system. In US J.R. Pierce, of Bell Laboratories initiated this by promoting the idea of transoceanic satellite communications within the scientific and technical communities. In 1955 Pierce in a paper entitled Orbital Radio Relays proposed detailed technical plan for passive communications satellites, disregarding the feasibility of constructing and placing satellites in orbit. He proposed three types of repeaters.
· Spheres at low altitudes
· A plane reflector
· An active repeater in 24 Hr. orbit
Pierce concluded his paper with a request to the scientific community to develop rockets capable of launching communications satellite. Fortunately, scientific and military interest in rocketry after World War II contributed in the development of a number of rockets like Atlas, Jupiter and Thor rockets in US and different multistage rockets in former USSR that ultimately made the launching of satellites in orbit possible.
On Oct. 4, 1957, Sputnik-1 was launched as part of Russia's program for International Geophysical Year. The launching of Sputnik marks the dawn of the space age and the world's introduction to artificial satellite. Mass of Sputnik was only 184 lbs. in an orbit of 560 miles above the earth. It carried two radio transmitters at 20.005 MHz and 40.002 MHz. However this space craft was far more than a scientific and technical achievement as it had a tremendous psychological and political impact particularly on United States resulting in a technological competition between United States and Russia, long term planning in Space Research and establishment of NASA.
Four months after the launch of Sputnik, US Explorer-1 was launched in January 1958 by a Jupiter rocket and the space race between Russia and US began.
EVOLUTION OF COMMUNICATION SATELLITES
During early 1950s, both passive and active satellites were considered for the purpose of communications over a large distance. Passive satellites though successfully used in the early years of satellite communications, with the advancement in technology active satellites have completely replaced the passive satellites.
The principle of communication by passive satellite is based on the properties of scattering of electromagnetic waves from different surface areas. Thus an electromagnetic wave incident on a passive satellite is scattered back towards the earth and a receiving station can receive the scattered wave. The passive satellites used in the early years of satellite communications were both artificial as well as natural.
In 1954, the US Naval Research Laboratory successfully transmitted the first voice message through space by using the Moon to scatter radio signal. These experiments resulted in the development of Moon-Relay System, which became operational in 1959 for communications between Washington, DC and Hawaii and remained operational till 1963.
The first artificial passive satellite Echo-I of NASA was launched in August 1960. Echo-I was 100-ft. diameter inflatable plastic balloon with aluminum coating that reflected radio signals transmitted from huge earth station antennas. 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.
Echo-I was followed by Echo-II in 1964. With Echo-II, Scientists of US and Soviet Russia collaborated for the first time on international space experiments. Signals were transmitted between University of Manchester for NASA and Gorki State University in Russia. The orbit of Echo-II was 600 to 800 miles.
In 1963, US Air Force under Project West Ford launched an orbital belt of small needles at 2000 miles height to act as a passive radio reflector. Speech in digitized form was transmitted intelligently via this belt of needles. However, further work in this area was discontinued due to strong protests from the astronomers.
Although passive satellites were simple, the communications between two distant places were successfully demonstrated only after overcoming many technical problems. The large attenuation of the signal while traveling the large distance between the transmitter and the receiver via the satellite was one of the most serious problems. The disadvantages of passive satellites for communications are:
· 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.
· Communications via the Moon is limited by simultaneous visibility of the Moon by both the transmit and the receive stations along with the larger distance required to be covered compared to that of closer to earth satellite.
· A global system would have required a large number of passive satellites accessed randomly by different users.
· Control of satellites not possible from ground.
In active satellites, which amplify and retransmit the signal from the earth have several advantages over the passive satellites. The advantages of active satellites are:
· Require lower power earth station
· Less costly
· Not open to random use
· 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
World's first active satellite SCORE (Satellite Communication by Orbiting Relay Equipment) was launched by US Airforce in 1958 at orbital height of 110 to 900 miles. It transmitted a pre-recorded message of Christmas Greetings from US President Eisenhower. However, the satellite did not function as a true repeater.
The first fully active satellite was Courier launched into an orbit of 600 - 700 mile, by Department of Defense in 1960. It accepted and stored upto 360,000 Teletype words as it passed overhead and rebroadcast them to ground station farther along its orbit. It operated with 3 watts of on-board output power and it was also the first satellite to use solar cells for generating electrical power.
In July 1962 AT&T's active satellite Telstar was developed and launched. Telstar was placed in an elliptical orbit with orbital height of 682 to 4030 miles circling the earth in 2 hours and 40 min. Through Telstar, the first live transatlantic television was transmitted. Voice, television, fax and data were transmitted between various sites in UK, France, Brazil Italy and US at 6/4 GHz frequency range.
Relay-I satellite of RCA & NASA, was the first satellite to carry redundant system for increasing the reliability. Telephone & Television signals were transmitted to Europe, South America and Japan. Frequency bands of 4.2/1.7 GHz and orbit heights of 942 to 5303 miles were used.
Syncom, the first geosynchronous satellite of NASA was built by Hughes Aircraft Co. and was launched in July 1963 and was used for conducting many experiments. Most famous of the series Syncom-III was launched in 1963 and was used to transmit Tokyo Olympic games to United States, demonstrating the commercial market for space technology. Syncom-I and-II were used by Department of Defense for military purpose. The Syncom Satellites marked a turning point in the development of Satellite Communications as most of the commercial satellites that followed were designed to operate from geosynchronous orbit.
SATELLITE COMMUNICATIONS SYSTEMS
Historically, commercial operational satellite communications systems were developed after having working experience with a large number of experimental satellite systems launched to demonstrate the various aspects of satellite communications. Initially the commercial satellite communications systems were meant for meeting the needs of international transoceanic communications. The trend was for establishing only a few large earth stations in any country for overseas communications. In the early years of satellite communications, the earth stations were large due to low transmit power available from the satellites. Over the years the trend has been reversed as with the advancement of technology, higher transmitted power is available from the satellites. This has reduced the size and the cost of the earth stations. Thus the trend is now on the use of thousands of small earth stations and portable hand held terminals, for meeting the various specialized communications needs. Moreover, apart from international system a number of Regional, Domestic, and military systems are now in operations worldwide. From the traffic point of view, emphasis was initially more on point-to-point telephone, telex etc, and to some extent on Television broadcasting. The present trends however are on Direct To Home television broadcasting and VSAT based data communications using small antenna systems deployed on rooftops or on one's backyards. Finally, it is expected that the satellite communications will meet the ultimate goal of hand held personal communications of voice and data for anyone from anywhere and anytime.
Different types of Satellite Communications Systems are:
· Navigational and Radio Determination
· Personal Communications System
· Broadband Satellite System
Experimental Satellite Communications Systems
For the purpose of test and evaluation of new technologies a number of satellites have been designed and operated for technical experiments. Various experiments have also been conducted using these satellites for demonstrating different applications of communications satellites. Prominent among these experimental satellites are:
· Applications Technology Satellite Series (ATS-1, ATS-3, ATS-5 & ATS-6) of NASA.
· Joint Canadian - US Communications Technology Satellite (CTS or Hermes)
· Advanced Communications Technology Satellite (ACTS) of NASA.
· APPLE (Ariane Passenger Payload Experiment) Satellite of India.
· Symphonie Satellite (France & Germany).
· SIRIO (Italy)
· LES (US military)
· OTS (ESA)
· JBS, CS (Japan)
International Satellite Systems
Currently only a part of the world's long distance telecom traffic is handled by different international satellite communications systems. However, for international broadcasting of television there is no alternative to satellite communications. Examples of various international satellite systems are:
· New Skies Satellites
INTELSAT: Recognizing that Satellite Communications would be an important means for international cooperation, in July 1961, President Kennedy of US invited all nations to participate in a communication satellite system in the interest of world peace and brotherhood among peoples throughout the world.
In Dec. 1961, UN endorsed the US proposal regarding the desirability of a global system of communication satellites because it could
· Forge new bonds of mutual knowledge and understanding between people everywhere
· Offer a powerful tool to improve literacy and education in developing areas
· Support world weather services by speedy transmittal of data
· Enable leaders of nations to talk face to face on a convenient and reliable basis
The UN unanimously adopted General Assembly Resolution, which stated that:
'Communications by means of satellite should be available to the nations of the world as soon as practicable on a global and nondiscriminatory basis'.
In August 1962, US Government passed Communications Satellite Act. Its purpose was to establish a commercial communications system utilizing satellites, which would serve the needs of the US and other countries and contribute to world peace and understanding. The significance of the choice of a single system for international communications is economic, technical and political.
In August 1964, the final negotiations for the international satellite system were completed and nineteen nations became the founding members of International Telecommunications Satellite Organization (INTELSAT) with Headquarters in Washington D.C, USA. These nineteen nations are Australia, Austria, Belgium, Canada, Denmark, France, Germany, Ireland, Italy, Japan, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, the United Kingdom, United States and Vatican City. Over the years the number of member governments grew to 144.
In April 1965, Early Bird (INTELSAT-I) was launched starting the commercial international satellite services. Within four years the INTELSAT system grew from the single transatlantic link to the global network with high capacity INTELSAT satellites positioned over the Atlantic, Pacific and Indian Ocean Regions. The 240 voice circuit capacity of the Early Bird is miniscule in comparison to the channel capacity of the latest INTELSAT satellites which caters to tens of thousands of telephone channels in addition to providing TV, data, fax, telex and Internet services to more than 200 countries and territories.
With the improvement in life of the satellites, introduction of latest communication techniques, and the availability of more channel capacity, the tariff of Intelsat has also been reduced considerably over the years.
In the 1960's at the time of establishment of Intelsat, the satellite Communication Industry was not well developed. The international telecommunications was also not considered suitable for handling by private companies. However, the skepticism changed after successful privatization of telecommunications departments in many countries during the last few decades of the twentieth century. Since in highly competitive telecommunications market, private enterprises are in a position to provide better and cheaper services compared to the international organizational set up of Intelsat, ideas for privatization of Intelsat were mooted. Privatization places Intelsat on a level playing field to better address opportunities of the telecommunications marketplace. Streamlined decision-making is expected to make it easier to expand the business and introduce new services. Considering these, in November 2000, the representatives of all member governments of Intelsat unanimously approved a plan to privatize Intelsat.
The approved plan endorses the transfer of all assets, liabilities and operations to a private Bermuda based company known as Intelsat Ltd., and its 100 % subsidiaries. In accordance with its heritage as a global satellite communications services provider to all countries, Intelsat Ltd. will continue to honour a clear set of public service commitments on a commercial basis. These include
· Global coverage and global connectivity
· Service to "lifeline" customers around the world according to specific Lifeline Connectivity Obligation Contracts
· Non-discriminatory access to the Intelsat Ltd. Satellite fleet
A small separate and independent inter governmental office will monitor the private company's implementation of these public service commitments. Privatization of INTELSAT is expected to be completed in 2001.
New Skies Satellites N.V (New Skies) is formed through the partial privatization of Intelsat. It is a wholly independent satellite service provider starting its services through five in orbit satellites transferred from Intelsat fleet. New Skies, with headquarters at The Hague, Netherlands, began operations as a commercial spin off from Intelsat in November 1998, with three satellites in Atlantic Ocean Region, one each in the Indian & Pacific Ocean Region and appropriate ground facilities around the world. These satellite and the ground facilities provide complete global coverage at C band and high powered Ku band spot beams over most of the world's principal population centers. It offers video, voice, data and Internet communications links for broadcast networks, telephone carriers, enterprise customers and ISPs.
PanAmSat Corporation, based in Greenwich, Conn. / USA, is a leading provider of global video and data broadcasting services via satellites. The company builds, owns and operated networks that deliver entertainment and information to cable television systems, TV broadcast affiliates, telecommunications companies and corporations with a large fleet of 21 satellites in orbit as on 2001.
INTERSPUTNIK: The Intersputnik International Organization of Space Communications with headquarters at Moscow was established in 1971, according to an intergovernmental agreement submitted to UN, for operating a global satellite communication system. However, Intersputnik became operational only in 1974 with Molniya and Statsionar Satellites for providing telephony, telegraph, radio, data, telex and direct broadcasting of television. Initially Intersputnik had nine member countries, which has grown to twenty-three over the years. These are Afghanistan, Belarus, Bulgaria, Cuba, Czech Republic, Georgia, Germany, Hungary, Kazakhstan, Kyrghizstan, DPR Korea, Laos, Mongolia, Nicaragua, Poland, Tajikistan, Turkmenistan, Romania, Russia, Syria, Ukraine, Yemen, Vietnam. Intersputnik's user base exceeds 100 state run and private companies.
Due to the changes in the geopolitical conditions in the 1990's and rapid development of telecommunications market with the introduction of new services created severe competitions among the telecom service providers. Under the changed circumstances, Intersputnik reviewed its strategy to get adapted to the dynamically developing environment. In order to keep the competitive edge, Intersputnik established strategic alliances with different satellite communications operators, manufactures, launch vehicle service providers, ground equipment manufacturers and international entities. One of these alliances is the joint venture Lockheed Martin Intersputnik (LMI) established in 1997. First LMI satellite with 44 high power C and Ku band transponders and lifetime of 15 years was launched at 75 deg E. in September 1999. With strategic alliance with Lockheed Martin corporation, Intersputnik is able to provide high quality services, which include digital video, high bit rate access to Internet, use of VSATs, Telemedicine, Tele-education, banking on a global scale etc.
INMARSAT: INMARSAT was established as an international cooperative organization similar to INTELSAT, for providing satellite communications for ships and offshore industries. INMARSAT, a specialized agency of UN, was established in 1979 and became operational in 1982 as a maritime focused intergovernmental organization with headquarters located at London. INMARSAT has forty-four members and also provides services to nonmember countries.
INMARSAT has become a limited company since 1999. INMARSAT Ltd is a subsidiary of the INMARSAT Ventures plc holding company, which operates a constellation of geosynchronous satellites for worldwide mobile communications. The satellites are controlled from INMARSAT headquarters at London, which is also home to INMARSAT ventures and a small Intergovernmental office created to supervise the company's public service duties for the maritime community i.e. Global Maritime Distress and Safety System and Air traffic Control communications for the aviation industry.
Starting with a user base of about 900 ships in early 1980's, the user base of INMARSAT grew to 210,000 ships, vehicles, aircrafts and portable terminals in 2001. INMARSAT Type A mobile terminals meant for installation in large ships are quite expensive, whereas, portable INMARSAT mini-M terminals are small, cost effective and easy to operate. The services provided by INMARSAT include telephone, fax and data communications up to 64 kbps. Other services include videotext, navigation, weather information and Search & Rescue. INMARSAT Satellites can also be used for emergency Land Mobile Communications for relief work and to re-establish communications or to provide basic service where there is no alternative. INMARSAT can also be used to alert people on shore for coordination of rescue activities. Apart from maritime and Land Mobile Satellite Service, INMARSAT also provide aeronautical satellite service for passenger communications.
INMARSAT system operates at C-band and L-band frequencies. The INMARSAT system uses allocations in the 6 GHz band for the ground station to satellite link, 1.5 GHz for satellite terminal downlink, 1.6 GHz for terminal to satellite uplink and 4 GHz for the satellite to ground station down link.
COSPAS-SARSAT: COSPAS-SARSAT is a joint venture of Canada, France, Russia and US. It is a satellite based international Search and Rescue, alert and location system using different low earth orbit satellites operating in the frequency of 120 MHz and 406 MHz. Local Users Terminals operating in different parts of the world pick-up the alert signals from Emergency Location Beacons carried by ships and airplanes and pass on the information regarding the location of accident to the nearest rescue centres for carrying out the rescue operations. Since the operationalisation of the satellite based Search and Rescue System, hundreds of lives have been saved due to timely deployment of prompt rescue operations.
Domestic Satellite Communications Systems
In the initial years of implementation of commercial Satellite Communications the emphasis was mainly on the transoceanic and international communications. However use of satellite communications for improvement of domestic communications also emerged as a distinct possibility. Countries like, USSR, Canada, Indonesia took initiatives in implementing domestic satellite communications systems for the respective countries. Satellite Communications System for domestic communications is cost effective compared to the conventional terrestrial systems under the following conditions.
· A large country without basic terrestrial communications
· Population is spread over mountains, deserts and a large number of islands
· Thinly populated remote areas
Former USSR was the first country to adopt satellite communications for its domestic use. However, because of its geographic location, where large landmass was in the high latitude region, the geosynchronous satellite systems were not found to be suitable. Thus a system with a series of Molniya Satellites operating in highly elliptical non-geosynchronous inclined orbits was introduced in 1965 to meet the country's domestic requirement of telecommunications and television transmission.
Canada became the first country to use a geosynchronous satellite for domestic communications with the launching of Anik-1 in 1972. With the advent of Anik Satellite it was possible to cover for the first time the whole of Canada particularly thinly populated northern region under the live TV coverage. Apart from TV, Anik Satellites are also used for radio broadcasting to remote locations and interactive distance education.
Indonesia is the first developing country to have its own domestic satellite system. Because of its limited infrastructure and widely scattered population dispersed over more than 13,600 islands, a satellite communications system is an ideal technology to deliver telecommunications and broadcasting throughout the country. Telecommunications services using PALAPA-A, the first Indonesian domestic satellite started in 1976.
Some of the other countries with their own domestic satellite communications systems are:
· United States of America (Wester, SBS, Etc.)
· India (INSAT)
· Brazil (Brazilsat)
· Mexico (Morelos)
· China (Chinasat)
· Japan (CS, BS)
Many countries where operating an exclusive domestic satellite communications system is not economical, the domestic requirements of communications can be met by leasing capacity from Intelsat or other satellites.
Regional Satellite Communications Systems
Regional Satellite Communications Systems have been an ideal means to deliver telecommunications and broadcasting services to a number of countries in a region for meeting their domestic and regional telecommunications and broadcasting requirements rather than having separate domestic system for each of these countries. A number of regional satellite communications systems are presently in operations and quite a few of them are in the planning stages.
Some of the regional Satellite Communications Systems are:
Eutelsat is a consortium of twenty-six European nations, established for operations and maintenance of space segment of the Eutelsat Satellite System, and providing its members with the space segment capacity necessary for meeting their telecommunications services requirements. Eutelsat provides services that are not available via Intelsat in Europe. These include:
· Transmission of television networks to eighteen countries for cable distribution and transmission of Eurovision programs
· Intra-European telephony and telegraphy
· Multi-service data communications for computer networking, facsimile, remote printing of newspapers, teleconferencing etc.
Eutelsats' space segment is coordinated by the European Space Agency (ESA) which procures spacecraft from European manufactures.
Arabsat evolved from 1953 Arab league agreement to develop regional telephone, telex and telegraph communications. Arab Space Communication Organization was established in 1976 and had twenty-two members. However, by the time two Arabsat Satellites were launched in 1985, Egypt, a leader in the System's planning had been expelled from Arab league. Each Arabsat Satellite has twenty-five C-band transponders and one C/S band transponder for community television reception. However, the Arabsat Satellites are extremely under utilized as only six countries have earth stations. Several countries are still working with Intelsat lease rather than switch to Arabsat.
AUSSAT systems of Australia designed for meeting domestic communications needs of Australia for Radio, TV broadcasting and long distance Telephony is also used by Papua New Guinea for telecommunications and broadcasting services.
Since 1979, PALAPA system of Indonesia became a regional satellite system, after Philippines, Thailand and Malaysia signed agreement to use PALAPA.
Military Satellite System
For military communications Army, Air force and Navy use both fixed and mobile satellite systems. In addition to the normal communications, military communications are also required for tactical communications from remote and inhospitable locations.
The special requirements of military communication terminals are high reliability, ruggedness, compact, operations under hostile environment, immunity to jamming, ease of portability and transportation, etc. Examples of military satellite communications systems are:
· DSCS (US AF)
· SKYNET (UK)
· NATO (NATO)
· FLTSATCOM (US NAVY)
Because of the special frequency band used in Military satellite system and other special requirements, Military satellite Systems are always much costlier and it takes longer time to design and develop compared to commercial satellite communications systems. Realizing that not all communications are strategic in nature, there is a trend now to use commercial communications system as far as possible. US Department of Defense is one of the major users of commercial Iridium satellite system with their own gateway.
Satellites have now replaced the stars and terrestrial systems for the purpose of navigation and radiolocation. The Transit Satellite system of US Navy was the first satellite navigational system with satellites orbiting in low polar orbits. By means of triangulation, the crews could establish the location of the ship and submarine by picking up the signals transmitted by different Transit satellites. US agreed to allow civilian use of Transit Navigational System for use by merchant marine shipping industry throughout the world.
Transit system is now replaced by the Global Positioning System (GPS) of US Navstar Satellite System consisting of eighteen low earth orbiting satellites operating at L-band. GPS receiver calculates the position (latitude, longitude, height) with extremely high accuracy by receiving signals from at least three-satellite passes. Apart from its use in ships, the miniaturized GPS receiver has also found many applications related to land based fleet monitoring.
The Russian Glonass system is the other navigational satellite system. However, the system is not being maintained properly by timely replacement of the satellites.
Personal Communications System
Introduction of terrestrial personal communications in many countries since 1980's saw a very rapid growth of wireless telecommunications. Considering that the terrestrial cellular telephone systems are limited only to urban and sub-urban areas, the business potential in providing global satellite based personal communications system was realized by many. It was thought that a satellite based personal communication system would provide not only communications to remote locations from anywhere, it would also provide a seamless roaming system integrating the scattered pockets of terrestrial system. A large number of global and regional personal satellite communications systems using both geosynchronous and non-geosynchronous satellites were planned during the 1990's. Most of these proposed systems were for voice communications with non-geosynchronous satellites in order to avoid the long delay associated with geosynchronous satellites. However, quite a few of these proposed systems, never took off and a few ran into financial problems at the implementation stage casting serious doubts about the commercial viability of satellite based personal communications system using non geosynchronous satellites.
Examples of regional satellite based personal communications systems providing voice and data services through geosynchronous satellites are
· Asia Cellular Satellite (ACeS)
Both these systems use satellites with large antenna systems and cover a large area of Asia and Europe.
Thuraya: Thuraya Satellite Company is a regional satellite system that provides satellite telephone services to a region covering 99 countries through a dynamic mobile phone that combines satellite, GSM cellular system and GPS. Thuraya was established in 1997 in UAE as a private joint venture with shareholders from 18 national telecommunications operators and investment houses. Thuraya meets the demand for seamless coverage of mobile communications to 2.3 billion people residing in India, Middle East,Central Asia, North & central Africa and Europe. Thuraya handsets offer voice, data, fax messaging and position location. It enables the user to use GSM service in local networks and automatically switch on to satellite mode whenever out of local terrestrial reach. The first Thuraya satellite operating in L band has been launched in Oct 2000 and commercial service started from 2001.
ACeS is another regional geo-mobile personal satellite communications system providing digital voice, fax and data communications using small dual mode (satellite and GSM) handsets. Users of ACeS are able to roam between terrestrial GSM cellular and satellite networks and can interface with public switched telephone networks. ACeS is jointly owned by PT Pacifik Satelit Nusantara of Indonesia, Lockheed Martin Global Telecommunications of USA, Philippines Long Distance Telephone Company and Jasmine International of Thailand. ACeS coverage area extends from Pakistan & India in the west to Philippines & Papua New Guinea in the east and from China & Japan in the north to Indonesia in the south. ACeS started its operations from November 2000.
Examples of a few Personal Satellite communications Systems providing services or planning to provide services using non-geosynchronous satellite constellations are:
· Iridium (66 Satellites)
· Globalstar (48 satellites)
· Orbcomm (35 satellites)
· New ICO (10 satellites)
Non-geosynchronous satellites in low and medium earth orbits need a large number of satellites in a constellation to provide global coverage and the number of satellites in the constellation increases with decreasing orbital height. The non-geosynchronous low earth orbit satellites appear to be attractive for providing two-way voice and data communications and location positioning to small handheld terminals from the points of view of higher available power from the satellite, low time delay etc. However, launching and maintaining a large number of satellites on orbit and operations of corresponding ground system pose technical as well as operational problems. Financial problems faced by a few of these systems in providing services at the early stages of operations, have made people rethinking on the commercial viability of such systems. The frequency allocations for personal communications systems are in the VHF, L band and S band.
Iridium: The Iridium system was the first satellite based Personal communications system to start commercial global wireless digital voice communications operations in November 1998 with its 66 Low Earth Orbit satellite constellation. But the infamous original Iridium service did not pick up and it failed in its attempt to attract the target subscriber base. This caused financial problems and bankruptcy of the company within a few months of starting the operational service. Iridium's failure despite a sophisticated on board technology and compatibility of hand sets with different terrestrial mobile telephone standards, is considered largely due to poor marketing and a service that was too costly. Moreover, by the time Iridium system was launched the cellular phone coverage also improved worldwide, thus reducing the target service area of Iridium that was uncovered by terrestrial cellular service.
After acquiring the assets of the bankrupt Iridium LLC, a privately held corporation Iridium Satellite LLC, launched its commercial global satellite communications service in March 2001. Iridium Satellite provides voice, paging, and messaging services to mobile subscribers using handheld user terminals. Frequency of operations of Iridium system is L band.
Globalstar: Globalstar is a consortium of leading international telecommunications companies originally established in 1991 to deliver satellite telephony services through a network of exclusive service providers. Globalstar system designed with a constellation of 48 low-earth orbiting satellites, started its commercial phone service using multimode handsets from October 1999. Other Globalstar services include voice mail, short messaging service, fax and supporting terrestrial IS 41 and GSM systems.
Calls from a Globalstar wireless handsets are transmitted in L band to the satellite and the receive frequency is in S band. Calls via satellites are routed through the appropriate gateway, from where they are passed on to existing fixed and cellular telephone network. The service is available in more than 100 countries in 6 continents.
Orbcomm: Orbcomm Global LP is the first commercial provider of global low earth orbit satellite data and messaging communications system. Globalstar started its commercial service in November 1998 with 28 out of a constellation of 35 low earth orbit satellites and 14 gateway Earth stations in 5 countries. Orbcomm provides two-way monitoring, tracking and messaging services to both fixed and mobile terminals. The system is capable of sending and receiving two-way alphanumeric packets, similar to two-way paging and e-mail.
VHF frequency bands are used for providing two-way messaging services at low data rates. The orbiting satellites pick up small data packets from sensors in vehicles, containers, vessels or remote fixed sites and relay these to the destination through a tracking Earth Station and Gateway Control Centre.
Orbcomm was originally formed as a partnership company owned by Orbital Sciences Corp (USA), Teleglobe Inc (Canada) and Technology Resources Industries Bhd (Malaysia). In April 2001, International Licensees, a consortium of Orbcomm licensees and other investors purchased all the assets of Orbcomm Global LP and its other entities that were under protection of bankruptcy since September 2000.
New ICO: New ICO, formerly of ICO global communications is working on a Medium Earth Orbit(MEO)/ Intermediate Circular Orbit (ICO) satellite system designed for both fixed and mobile operations around the world. ICO Global Communications was founded in 1995 and contracts for satellites launch services and ICO network were awarded. In August 1999, due to financial problems, the company was declared bankrupt. However, with fresh investment from a group of international investors New ICO emerged from the bankruptcy. New ICO system consists of a constellation of 10 on-orbit ICO satellites, 2 on orbit spares at an orbit of 10,390 km. Target launch of service of New ICO system is 2003.
New ICO is based in London with offices in different countries. The goal of New ICO is to provide global Internet protocol services, including Internet connectivity, data, voice and fax services. The system operates in both circuit switched mode based on GSM standard and packet switched Internet protocol mode. New ICO plans to target markets like maritime, transportation Government, oil, gas, construction & other industries, individuals and small & medium size businesses.
Broadband Satellite System
Broadband satellite service is an emerging service which has caught the fancy of many for meeting the demand of worldwide fiber like access to telecommunications services such as computer networking, broadband Internet access, interactive multimedia and high quality voice. These systems use advanced satellite technology at Ka band or Ku band frequencies to achieve the high bandwidth requirements.
Examples of proposed Broadband Satellite systems are:
Teledesic satellite network is designed with 288 plus spare satellites at low earth orbits. The operating frequency is in the Ka band of the frequency spectrum with 30 GHz uplink and 20 GHz downlink. The network will enable millions of simultaneous users to access the two-way network using standard user equipment providing up to 64 Mbps on the down link and up to 2 Mbps on the up link. The fixed user equipment will be mounted out door and connect inside to a computer network or PC.
Teledesic is a private company based in Bellevue, Washington (USA) attracting investment from many reputed companies and individuals. The ambitious Teledesic service targeted to begin in 2005 will enable broadband connectivity for businesses, schools and individuals everywhere on the planet and expected to facilitate improvements in education, healthcare and other crucial global issues.
SkyBridge is a satellite-based broadband global telecommunications system designed to provide business and residential users with interactive multimedia applications as well as LAN interconnection or ISDN applications, thus allowing services such as high speed Internet access and video conferencing to take place anywhere in the world. The system is based on a constellation of 80 low earth orbiting satellites, which link professional and residential users equipped with low cost terminals and terrestrial gateways. The satellite network operates at Ku band and will deliver asymmetric broadband connection to fixed network at up to 60 Mbps (in steps of 16 kbps) to the user and up to 2 Mbps (in increments of 16 kbps) on the return link via a gateway.
SkyBridge LP was formed in Delaware, USA in 1997. The partners of SkyBridge LP are Alcatel and leading industries from North America, Europe and Asia.
Spaceway is another advanced broadband satellite system offered by Hughes Network system of USA that will make high-speed broadband applications available on demand to the businesses and to consumers around the world. Operating in the Ka band spectrum, SPACEWAY will consist of interconnected regional satellite systems providing service to nearly all of the world's population. The first North American regional service will start in 2002 with two geosynchronous satellites plus an on orbit spare. Using a globally deployed system of satellites in conjunction with a ground-based infrastructure, users will transmit and receive video, audio, multimedia and other digital data at uplink rates between 16 kbps to 16 Mbps. The access to the system will be provided through a family of low cost easily installed 66 cm terminals.
ORBITS FOR COMMUNICATION SATELLITE
The 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 (<10,000 km) leading to lower on board power requirement
· Less delay in transmission path - reduced problem of echo in voice communications
· Suitability for providing service at higher latitude
· Lower cost to build and launch satellites at NGSO
· Use of VHF and UHF frequency bands at NGSO permits low cost antennas for hand-held terminals
Disadvantages of NGSO are:
· Requirement of a large number of orbiting satellites for global coverage as each low earth orbit satellite covers a small portion of the earth's surface for a short time.
· Complex hand over problem of transferring signal from one satellite to another
· Less expected life of satellites at NGSO requires more frequent replacement of satellites compared to satellite in GSO
· Compensation of Doppler shift is necessary
· Satellites at NGSO undergoes eclipse several times a day necessitates the requirement of robust on board battery system for the satellite for operations without solar power during eclipse
· Complex network management for a constellation of satellites and corresponding ground system
· Problem of increasing space debris in the outer space
There are different types of Non Geostationary Orbits (NGSO), depending on the orbital height and the inclination of the orbital plane. Inclination is the angle that the orbital plane makes with the equatorial plane at the time of crossing the equator moving from south to north of the earth and is measured from 0 to 180 degrees. NGSOs are classified in the following three types as per the inclinations of the orbital plane
· Polar Orbit
· Equatorial Orbit
· Inclined Orbit
In polar orbit the satellite moves from pole to pole and the inclination is equal to 90 degrees. In equatorial orbit the orbital plane lies in the equatorial plane of the earth and the inclination is zero or very small. All orbits other than polar orbit and equatorial orbit are called inclined orbit.
A satellite orbit with inclination of less than 90 degrees is called a prograde orbit. The satellite in prograde orbit moves in the same direction as the rotation of the earth on its axis. Satellite orbit with inclination of more than 90 degrees is called retrograde orbit when the satellite moves in a direction opposite to the rotational motion of the earth. Orbits of almost all communication satellites are prograde orbits, as it takes less propellant to achieve the final velocity of the satellite in prograde orbit by taking advantage of the earth's rotational speed. Example of retrograde orbit is the sun synchronous orbit where the orbital parameters are such that that the satellite crosses the same latitude at the same local time. This type of orbit is used for earth observation satellites where repeated observations are required to be made under the same sun angle. It needs more propellant to launch a satellite in retrograde orbit as it is launched in a direction opposite to the direction of the earth's rotation.
Satellite orbits are also classified in terms of the orbital height. These are:
· Low Earth Orbit(LEO)
· Medium Earth Orbit (MEO) / Intermediate Circular Orbit (ICO)
· Highly Elliptical Orbit (HEO)
· Geosynchronous Earth Orbit (GEO)
Satellite orbits with orbital height of approximately 1000 km or less are known as Low Earth Orbit (LEO). LEOs tend to be in general circular in shape. Satellite orbits with orbital heights of typically in the range of 5000 km to about 25,000 km are known as Medium Earth Orbit (MEO) / Intermediate Circular orbit (ICO). MEO and ICO are often used synonymously, but MEO classification is not restricted to circular orbits. Satellites in Highly Elliptical Orbit (HEO) are suitable for communications in the higher latitudes. Russian Molnya satellites have highly inclined elliptical orbits with a perigee of about 1000 km, apogee of 40,000 km, inclination of 63.435 deg and orbital period of 12 hours. In Geosynchronous Earth Orbit (GEO) the satellite is in equatorial circular orbit with an altitude of 35,786 km and orbital period of 24 hours. Three satellites in GEO placed 1200 apart over equator cover most of the world for communications purposes.
Geostationary Orbit (GSO)
There is only one geostationary orbit possible around the earth, lying on the earth's equatorial plane and the satellite orbiting at the same speed as the rotational speed of the earth on its axis. For a Satellite to have an orbital period equal to that of earth's rotation i.e. a sidereal day (23 Hrs 56 min. 4.09 sec.) an altitude of 35,786 km is required. Such a satellite orbiting at a velocity of 3.075 km/sec remains fixed relative to any point on earth or geostationary. With the idealized assumptions that the geostationary satellite is at rest relative to the earth the conditions required to be satisfied for geostationary orbit are:
· The orbit shall be circular
· The period of the orbit shall be equal to the period of rotation of the earth about itself
· The plane of the orbit shall be the same as the equatorial plane but the sub-satellite longitude, i.e. the longitude of the projection of the satellite on the Earth's surface can be selected arbitrarily.
The principle of satellite communications based on this concept of geostationary orbit was originated by Arthur C Clarke. Main advantage of geostationary satellite being the permanent contact between the ground segment and the satellite with fixed directional antennas at both the earth station and the satellite.
The ITU (International Telecommunications Union), recognizing the importance of the GSO along with the frequency spectrum as limited natural resources available on the earth, set out the procedures for all radio communications services, regarding the use of GSO/spectrum through ITU Radio Regulations, a binding international treaty. With respect to the use of the GSO and frequency spectrum, the ITU space regulations laid down in the ITU Constitution is as follows:
In using frequency bands for radio services, Member states shall bear in mind that radio frequencies and any associated orbits, including the geostationary-satellite orbit, are limited natural resources and they must be used rationally, efficiently and economically, in conformity with the provisions of Radio Regulations, so that countries or groups of countries may have equitable access to those orbits and frequencies, taking into account the special needs of developing countries and the geographical situation of particular countries.
GEOSTATIONARY SATELLITE ORBIT
Attitude: 35,786 km.
Period: 23 Hr. 56 min. 4.091 sec. (One sidereal day)
Orbit inclination: 00
Velocity: 3.075 km per sec.
Coverage: 42.5% of earth's surface.
Sub satellite point: On equator.
Area of no coverage: Beyond 810 North and South latitude.
(77º if angle of elevation below 5º are eliminated)
Advantages - Simple ground station tracking.
- No hand over problem
- Nearly constant range
- Very small doppler shift
Disadvantages - Transmission delay of the order of 250 msec.
- Large free space loss
- No polar coverage
A perfect geostationary orbit is a mathematical abstraction that could be achieved only by a spacecraft orbiting around a perfectly symmetric earth and no other forces are acting on the spacecraft other than the central gravitational attraction from the earth. The abstraction is however, useful as an approximate description of real case, since all other forces or perturbations due to attractive forces of the Moon and the Sun and the non-sphericity of the Earth's gravity are small.
In real life due to gravitational pull of the Moon & the Sun, the equatorial orbital plane of the satellite makes an angle of inclination with respect to the equatorial orbital plane. For a satellite with orbital period equal to a sidereal day and non-zero inclination, the footprint of the satellite will move in North-South direction over its sub satellite point instead of remaining stationary. The non-spherical shape of the earth also causes movement of the satellite in the east-west direction. Thus the trace of the satellite on earth appears to roam in both North-South and East-West direction around the sub-satellite point.
The inclination of the satellite can be corrected by firing appropriate thrusters on-board the satellite and is known as North-South station keeping. Similarly the correction of East-West drift of the satellite is called East-West Station keeping. Without any station keeping the inclination plane drifts to about 0.86 deg per year. Thus the satellite orbital position is required to be corrected periodically to keep the drift from the desired location within a certain limit. Considering the drift in the satellite position in North-South and East-West direction around the sub-satellite point, it is more appropriate to designate such an orbit as geosynchronous orbit.
GEOSYNCHRONOUS COMMUNICATION SATELLITE
Geosynchronous Satellites have now become almost synonymous for communications satellites, because of its wide use in telecommunications due to the advantages over non-geosynchronous satellites. Because of the availability of a number of communication satellites over the geosynchronous arc, the communications between different parts of the world have become possible and affordable. The communication satellites have played a significant role in converting the world into a global village.
Salient features of Geosynchronous Communications Satellite
Salient features of Geosynchronous Satellite are:
· Wide Coverage
· Stationary Position
· Multiple Access
· Suitability for transcontinental telecommunications, broadcasting, mobile and thin route communications.
· Frequency reuse capability
· Very low Doppler Shift
· Cost effectiveness.
Brief description of each of these features are given below:
Wide Coverage: From the geosynchronous orbit the satellite can cover an area equal to about 42% of the area of the earth (38% if angles of elevation below 5º are not used). Thus three satellites placed 120º apart can cover almost the whole world for the purpose of communications. INTELSAT Satellites strategically placed over Atlantic Ocean Region (AOR), Indian Ocean Region (IOR) and Pacific Ocean Region (POR) covers the whole world for International Telecommunications. With worldwide satellite TV coverage, any incidence happening in any part of the world can now be viewed live in the TV throughout the world.
Stationary Position: The orbital velocity of the geosynchronous satellite being equal to the rotational velocity of the earth on its own axis, the satellite in the geosynchronous orbit appears to be stationary with respect to any location from the earth. Thus the satellite is always visible from any earth station situated in its coverage region and the tracking of the satellite is simple and there is no hand over problem of transferring signal from one satellite to another as in the case of satellites in NGSO. The constant visibility of the satellite also enables both the satellite and the earth station to use highly directive antennas. High gain of the antennas on-board the satellite and the earth station, enhances the transmit and receive capabilities.
Multiple Access: Multiple Access is the ability of a large number of users to simultaneously interconnect their respective voice, data and television links through a satellite. The wide geographic coverage and broadcast nature of satellite channel are exploited by means of multiple access. Multiple access also helps in optimum use of satellite capacity, satellite power, spectrum utilization and interconnectivity among different users at reduced cost.
A satellite in geosynchronous orbit can link multiple earth stations within its coverage area and separated by great circle distances up to 17,000 Km. Multiple access is the unique feature of satellite communications not possible to get by any other means. For m earth stations visible from a Satellite, the number of potential available communication circuits is given by
n = m (m-1)/2
compared to non flexible 2-port network of conventional cable or land based networks.
Suitability for Transcontinental Telecommunications, Broadcasting, Mobile and Thin Route Communications: TV Broadcasting via Satellite is perhaps the most common use of geosynchronous satellite. In developing countries where the terrestrial TV distribution is very limited, the communications satellites can be very effectively utilized for TV distribution. Geosynchronous satellites handle a large portion of transcontinental telecommunications traffic.
Geosynchronous Satellites along with other NGSO satellites are found to be suitable for reliable mobile communications for ships and aircrafts, as the ship and the aircraft can continuously maintain the communication link with the satellites while moving. However, GEO based satellite systems are much simpler to operate and maintain compared to other system.
Geosynchronous Satellites are also the most suitable means of providing reliable and cost effective communications to thin route rural areas, interconnecting small islands, and providing communications to hilly and difficult terrain.
Frequency Reuse: The frequency bands of a geosynchronous satellite can be reused by different methods for increasing the channel capacities of the communications satellite. By using specially designed spatially separated shaped beams the same frequency and polarizations can be reused. By using orthogonal polarizations the same frequency bands can be reused for the same coverage area of the satellite. By using orthogonal linear and circular polarizations and shaped beams covering different regions, the same frequency band can be reused many folds thus increasing the communication capacity of geosynchronous satellite. Different techniques of frequency reuse of the same frequency band are found in INTELSAT series of satellite.
Very Low Doppler Shift: Compared to low earth orbit satellites, in geosynchronous satellite there is almost no Doppler Shift i.e. change in the apparent frequency of operations to and from Satellite, caused by the relative motion of the Satellite and the earth station. Satellites in elliptical orbits have different Doppler shifts for different earth stations and this increases the complexities of the receivers especially when a large number of earth stations intercommunicate.
Reliability: The reliability of long distance telecommunication links improves considerably when geosynchronous satellites are used. The path loss in the satellite links although very high; these remain almost constant, thus maintaining the performance quality of the link.
Cost effectiveness: The geosynchronous satellite because of its long life of twelve to fifteen years and wide-band operations shared by a large number of users, makes the point to point service very cost effective compared to the service provided by land based terrestrial system. No viable alternatives to geosynchronous satellites are presently available, so far as the broadcasting and mobile services are concerned.
Problems of Geosynchronous Satellite Communications Systems
The problems of geosynchronous satellite communications systems are:
· No coverage of polar region.
· Long time delay.
· Eclipse due to the earth and the sun.
· Sun Transit outage
No Coverage Region: The geosynchronous satellite from its location of 35,786-Km altitude above equator is not found suitable for communications beyond the latitude of 81 deg. Thus the polar region of the earth cannot be properly covered by geosynchronous satellite.
Time Delay: In Satellite Communications System using geosynchronous satellite, the signal has to travel a long distance while travelling from the transmit earth station to the receive earth station via satellite. From the geometry of the geosynchronous satellite orbit it is found that the single hop time required for the signal to travel from one point to another varies from 230 m sec. (90 deg. elevation) to 278 m sec. (0 deg. elevation). This time delay does not pose any problem in data and broadcasting services, but this delay is quite perceptible in two-way telephone conversations. ITU-T specifies a delay of less than 400 msec to prevent echo effects and delay variation of upto 3 msec. Although the propagation and intersatellite delays of LEOs are lower, LEO systems exhibit high delay variation due to connection handovers, satellite and orbital dynamics and adoptive routing.
Echo: Generally a long distance telephone circuit is accompanied by echo due to mismatch at the terminal point where circuits are converted from four wire to two wire system. As the delay of the echo is increased, the effect of the echo becomes increasingly disagreeable to the talker. The echo can be attenuated by using echo suppressor or echo canceller. By using echo suppressor of excellent quality, a two hop satellite link can be utilized for practical communications, provided the delay is acceptable.
Eclipse of Satellite: A Satellite is said to be in eclipse when the positions of the earth, the Sun and the Satellite are such that the earth prevents sun light from reaching the satellite i.e when the satellite is in the shadow of the earth. For geosynchronous satellites, eclipses occur for 46 days around equinox (March 21 and September 23). During full eclipse, a satellite does not receive any power from solar array and it must operate entirely from batteries. In case the power available from battery is not enough, some of the transponders may be required to be shut down during the eclipse period. The satellite passes through severe thermal stress during its passage into and out of the earth's shadow. The solar power also fluctuates sharply at the beginning and end of an eclipse. For these reasons the probability of failure of satellite is more during eclipse than at any other time.
Sun Transit Outage: Sun transit outage takes place when the sun passes through the beam of an earth station. During vernal and autumnal equinox, the sun approaches toward a geosynchronous satellite as seen from an earth station and this increases the receiver noise level of the earth station very significantly and prevent normal operations. This effect is predictable and can cause outage for as much as 10 min. a day for several days. The sun transit outage is about 0.02 percent in an average year. A receiving earth station cannot do anything about it except wait for the sun to move out of the main lobe.
ELEMENTS OF SATELLITE COMMUNICATIONS SYSTEM
Two major elements of Satellite Communications Systems are
· Space Segment
· Ground Segment
The Space Segment includes
· Means for launching satellite
· Satellite control centre for station keeping of the satellite
The functions of the ground segment are 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
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 Payloads
Satellite Bus subsystems:
· Mechanical structure
· Attitude and orbit control system
· Propulsion System
· Electrical Power System
· Tracking Telemetry and Command System
· Thermal Control System
· Communication transponders
· Communication Antennas
Since 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 te 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.
The 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.
The 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.
In 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 Centre
Satellite 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.
The 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.
In the earth station the base band signal received directly from user's premises or from terrestrial network are appropriately modulated and then transmitted at RF frequency to the satellite. The receiving earth station after demodulating the carrier transmits the base band signal to the user directly or through the terrestrial link.
The baseband signals received at the earth stations are mostly of the following types.
· Groups of voice band analog or digital signals
· Analog or digital video signals
· Single channel analog or digital signal
· Wide band digital signal.
In satellite communications, in early days FM modulation scheme was most frequently used for analog voice and video signal transmission. However, the trnd is now to use digital signals for both voice and video. Various digital modulation schemes like Phase Shift Keying (PSK) and Frequency Shift Keying (FSK) are adopted for transmission of digital signals.
The network operations and control centre for the communications network monitors the network operations by different users, distribution of different carriers within a transponder and allocation of bandwidth & EIRP of different carriers. Proper functioning of Network operations and control centre is essential where the number of users in the network is large. Network operations & control centre is also responsible for giving clearance to the ground system in respect of antenna radiation pattern, EIRP etc.
SATELLITE COMMUNICATIONS SERVICES
Different Satellite Communications services are classified as one way link and two way link.
Examples of satellite services where the transfer of information takes place through one way link are:
· Broadcast Satellite Service (Radio, TV, Data broadcasting)
· Data Collection Service (Hydro meteorological data collection)
· Space operations service, (Tracking, Telemetry, Command)
· Safety services (Search & Rescue, Disaster Warning)
· Earth Exploration Satellite Service (Remote Sensing)
· Meteorological Satellite Service (Meteorological data dissemination)
· Radio Determination Satellite Service (Position location)
· Reporting Service (fleet monitoring)
· Standard frequency and time signal satellite service
· Space Research Service.
In two-way Satellite Communications link the exchange of information between two distant users takes place through a pair of transmit and receive earth stations and a satellite.
Examples of two-way satellite services are
· Fixed Satellite Service (Telephone, telex, fax, high bit rate data etc.)
· Mobile Satellite Service (Land mobile, Maritime, Aero-mobile, personal communications)
· Inter Satellite Service.
· Satellite News Gathering (Transportable and Portable)
A new class of two-way fixed satellite network service known as Very Small Aperture Terminal (VSAT) service has became very popular among business and closed users group communities.
SAT networks are operated in two different configurations i.e. Mesh and Star. While in Mesh configuration a VSAT terminal can communicate with another VSAT terminal in a single hop connection, Star network involves two hops via satellite and the hub station.
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