Satellite Projects

7/31/2019 Satellite Projects

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ABSTRACT:

Satellites have evolutionized communication. Satellite communication has served mankind

in many ways for instance its is used to predict weather and broadcast storm warnings andalso provides a wide range of communication services in the fields of relaying televisionprograms, digital data for a multitude of business services. It might not surprise us if, innear future satellite links are used for voice and fax transmission to aircraft on internationalroutes. Communications satellite systems have entered a period of transition from point-to-point high-capacity trunk communications between large, costly ground terminals tomultipoint-to-multipoint communications between small, low-cost stations. As anyinvention develops with the passage of time, satellite communication has also moved a stepahead from what it was in the past with the use of several techniques such as frequencyreuse, interconnecting many ground stations spread over the world, concept of multiplespot beam communications, these days lasers are effectively used for transmission through

satellites. The latest development in satellites is the use of networks of small satellites inlow earth orbits.

In this present era, communication plays a vital role. We use a wide range of devices tocommunicate between two persons placed at different places (irrespective of the distancebetween them). Any earth-orbiting spacecraft that provides communication over longdistances by reflecting or relaying radio-frequency signals. Satellites have evolutionizedcommunication by making worldwide telephone links and live broadcasts commonoccurrences. A satellite receives a microwave signal from a ground station on the earth (theuplink), then amplifies and retransmits the signal back to a receiving station or stations onearth at a different frequency (the downlink). A communication satellite is in

geosynchronous orbit, which means that it is orbiting at the same speed as the earth isrevolving. The satellite stays in the same position relative to the surface of the earth, so thatthe broadcasting station will never lose contact with the receiver.

DEVELOPMENT IN SATELLITE COMMUNICATION:

Some of the first communications satellites were designed to operate in a passive mode.Instead of actively transmitting radio signals, they served merely to reflect signals that werebeamed up to them by transmitting stations on the ground. Signals were reflected in alldirections, so receiving stations around the world could pick them up. Echo 1, launched bythe United States in 1960, consisted of an aluminized plastic balloon 30 m (100 ft) in

diameter. Launched in 1964, Echo 2 was 41 m (135 ft) in diameter. The capacity of suchsystems was severely limited by the need for powerful transmitters and large groundantennas.Score, launched by the United States in 1958, was the first active communications satellite.It was equipped with a tape recorder that stored messages received while passing over atransmitting ground station. These messages were retransmitted when the satellite passedover a receiving station. Telstar 1, launched by American Telephone and TelegraphCompany in 1962, provided direct television transmission between the United States,


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Europe, and Japan and could also relay several hundred-voice channels. Launched into anelliptical orbit inclined 45 to the equatorial plane, Telstar could only relay signalsbetween two ground stations for a short period during each revolution, when both stationswere in its line of sight.Hundreds of active communications satellites are now in orbit. They receive signals from

one ground station, amplify them, and then retransmit them at a different frequency toanother station. Satellites use ranges of different frequencies, measured in hertz (Hz) orcycles per second, for receiving and transmitting signals. Many satellites use a band offrequencies of about 6 billion hertz, or 6 gigahertz (GHz) for upward, or uplink,transmission and 4 GHZ for downward, or downlink, transmission. Another band at 14GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed(nonmobile) ground stations. A band at about 1.5 GHZ (for both uplink and downlink) isused with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energycells mounted on large panels attached to the satellite provide power for reception andtransmission.

GEOSYNCHRONOUS ORBIT:

A satellite in a geosynchronous orbit follows a circular orbit over the equator at an altitudeof 35,800 km (22,300 mi), completing one orbit every 24 hours, in the time that it takes theearth to rotate once. Moving in the same direction as the earth's rotation, the satelliteremains in a fixed position over a point on the equator, thereby providing uninterruptedcontact between ground stations in its line of sight. The first communications satellite to beplaced in this type of orbit was Syncom 2, launched by the National Aeronautics and SpaceAdministration (NASA) in 1963. Most communications satellites that followed were alsoplaced in geosynchronous orbit.

COMMERCIAL COMMUNICATIONS SATELLITES:

Deployment and operation of communications satellites on a commercial basis began withthe founding of the Communications Satellite Corporation (COMSAT) in 1963. When theInternational Telecommunications Satellite Organization(INTELSAT) was formed in 1964, COMSAT became the U.S. member. Based inWashington, D.C., INTELSAT is owned by more than 120 nations. Intelsat 1, known asEarly Bird, launched in 1965, provided either 240 voice circuits or one two-way televisionchannel between the United States and Europe. During the 1960s and 1970s, messagecapacity and transmission power of the Intelsat 2, 3, and 4 generations were progressivelyincreased.

The first of the Intelsat 4s, launched in 1971, provided 4,000 voice circuits. With theIntelsat 5 series (1980), innovations in signal focusing resulted in additional increases incapacity. A satellite's power could now be concentrated on small regions of the earth,making possible smaller-aperture, lower-cost ground stations. An Intelsat 5 satellite cantypically carry 12,000 voice circits. The Intelsat 6 satellites, which entered service in 1989,can carry 24,000 circuits and feature dynamic on-board switching of telephone capacityamong six beams, using a technique called SS-TDMA (satellite-switched time divisionmultiple access). In the early 2000s, INTELSAT had 21 satellites in orbit, providing the


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world's most extensive telecommunications system. Other systems also provideinternational service in competition with INTELSAT. The growth of international systemshas been paralleled by domestic and regional systems, such as the U.S. Telstar, Galaxy, andSpacenet programs and Europe's Eutelsat and Telecom.

SERVICES:

Broadcasters use data from meteorological satellites to predict weather and to broadcaststorm warnings when necessary. Satellites such as the Geostationary OperationalEnvironmental Satellite (GOES) collect meteorological and infrared information about theatmosphere and the ocean. A camera on the GOES is continuously pointed at Earth,broadcasting satellite images of cloud patterns both day and night. Here, the GOES-Csatellite is being encapsulated inside its payload fairing aboard a Delta rocket.

Commercial satellites provide a wide range of communications services. Televisionprograms are relayed internationally, giving rise to the phenomenon known as the "global

village." Satellites also relay programs to cable television systems as well as to homesequipped with dish antennas. In addition, very small aperture terminals (VSATs) relaydigital data for a multitude of business services. Intelsat satellites now carry over 100,000telephone circuits, with growing use of digital transmission. Digital source coding methodshave resulted in a ten-fold reduction in the transmission rate needed to carry a voicechannel, thus enhancing the capacity of existing facilities and reducing the size of groundstations that provide telephone service.

Weather satellites carry cameras and other instruments pointed toward Earth's atmosphere.They can provide advance warning of severe weather and are a great aid to weatherforecasting. NASA launched the first weather satellite, Television Infrared Observation

Satellite (TIROS) 1, in 1960. TIROS 1 transmitted almost 23,000 photographs of Earth andits atmosphere. NASA operates the Geostationary Operational Environmental Satellite(GOES) series, which are in geostationary orbit. GOES provides information for weatherforecasting, including the tracking of storms. Meteosat 3, a European weather satellite alsoin geostationary orbit, augments GOES. The National Oceanic and AtmosphericAdministration (NOAA) operate three satellites that collect data for long-term weatherforecasting.

The International Mobile Satellite Organization (INMARSAT), founded in 1979 as theInternational Maritime Satellite Organization, is a mobile telecommunications network,providing digital data links, telephone, and facsimile transmission, or fax, service between

ships, offshore facilities, and shore-based stations throughout the world. It is also nowextending satellite links for voice and fax transmission to aircraft on international routes.

RECENT TECHNICAL ADVANCES:

Launched on October 4, 1957, the Sputnik 1 was the first craft in Earth's orbit. Named afterthe Russian word for "traveling companion of the world" (Sputnik Zemli), it was a smallsatellite measuring only 58 cm (23 in) across. It circled the earth once every 96.2 minutes


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and transmitted atmospheric information by radio. After three months aloft, it wasdestroyed while reentering the atmosphere.Communications satellite systems have entered a period of transition from point-to-pointhigh-capacity trunk communications between large, costly ground terminals to multipoint-to-multipoint communications between small, low-cost stations. The development of

multiple access methods has both hastened and facilitated this transition. With TDMA,each ground station is assigned a time slot on the same channel for use in transmitting itscommunications; all other stations monitor these slots and select the communicationsdirected to them. By amplifying a single carrier frequency in each satellite repeater, TDMAensures the most efficient use of the satellite's onboard power supply.

A technique called frequency reuse allows satellites to communicate with a number ofground stations using the same frequency by transmitting in narrow beams pointed towardeach of the stations. Beam widths can be adjusted to cover areas as large as the entireUnited States or as small as a state like Maryland. Two stations far enough apart canreceive different messages transmitted on the same frequency. Satellite antennas have been

designed to transmit several beams in different directions, using the same reflector.A method for interconnecting many ground stations spread over great distances wasdemonstrated in 1993 with the launch of NASA's ACTS (Advanced CommunicationsTechnology Satellite). The satellite uses what is known as the hopping spot beam techniqueto combine the advantages of frequency reuse, spot beams, and TDMA. By concentratingthe energy of the satellite's transmitted signal, ACTS can use ground stations that havesmaller antennas and reduced power requirements.The concept of multiple spot beam communications was successfully demonstrated in 1991with the launch of Italsat, developed by the Italian Research Council. With six spot beamsoperating at 30 GHZ (uplink) and 20 GHZ (downlink), the satellite interconnects TDMAtransmissions between ground stations in all the major economic centers of Italy. It doesthis by demodulating uplink signals, routing them between up- and downlink beams, andcombining and remodulating them for downlink transmission.Laser beams can also be used to transmit signals between a satellite and the earth, but therate of transmission is limited because of absorption and scattering by the atmosphere.Lasers operating in the blue-green wavelength, which penetrates water, have been used forcommunication between satellites and submarines.

The latest development in satellites is the use of networks of small satellites in low earthorbit (2,000 km (1,200 mi) or less) to provide global telephone communication. TheIridium system uses 66 satellites in low earth orbit, while other groups have or aredeveloping similar systems. Special telephones that communicate with these satellitesallow users to access the regular telephone network and place calls from anywhere on theglobe. Anticipated customers of these systems include international business travelers andpeople living or working in remote areas.CONCLUSION:

Looking at the rate of advancement in satellite communication one would foresee the useof satellites in every field where communication is required such as relaying television and


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radio signals. Special telephones that communicate with these satellites allow users toaccess the regular telephone network and place calls from anywhere on the globe.

Satellite TechnologyThe world changed on October 4, 1957, when the Soviet Union launched the Earth's firstartificial satellite (the Moon is a natural satellite). Sputnik, a Russian word meaning "fellow

traveler," was an 83 kilogram (183-pound) satellite the size and shape of a basketball. It didlittle except orbit the Earth every 98 minutes and emit a simple radio signal, a recording ofwhich can be downloaded from a National Aeronautics and Space Administration (NASA)web site (http://www.hq.nasa.gov/office/pao/History/sputnik/). Yet, this simple eventstarted what was to be known as "The Space Race" that eventually led to the lunar landingsas well as space shuttle missions, and weather and direct broadcast television satellites.Although the satellite concept is theoretically simplean object placed high enough aboveEarth's atmosphere to be moving at a speed of eight km/sec (17,280 miles/hour)thesuccessful launch, orbital insertion, and control of any satellite is extremely complex. Thisis evidenced by the many failures that occurred before Sputnik (and since). However, as of2002 there are more than 2,670 artificial satellites orbiting Earth. In addition to these, the

U.S. Space Command is also tracking 6,186 pieces of space debris. This demonstrates thathumans have not only mastered satellite technology but also succeeded in extending well-developed littering capabilities into space.A satellite's orbit is described in one or more of three dimensions: the perigee,its closestdistance from the Earth; the apogee, its furthest distance from the Earth; and, itsinclination, the angle the orbit makes with the equator. Satellites are put into particulartypes of orbit depending on their mission. In a geostationary orbit, the satellite speed issynchronized with the Earth's rotation so that the satellite stays in the same relativeposition. A polar orbit is characterized by its 90-degree inclination to Earth's equator.When a satellite is in low Earth orbit, the apogee and perigee are each only about 483kilometers or 300 miles.

Types of SatellitesMost satellites today are in place for communication, environmental monitoring, ornavigational purposes. There are both government and commercial satellites in space.Communication.Of all satellite technologies, communication technology has probably had the greatestimpact on our world. It has been called one of the greatest forces for the "super-tribalization" of the human species. Communication satellites have also proven to be one of


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the most successful commercial applications of space technology. In 2002 there are about200 communication satellites orbiting above Earth.The first telephone communication satellites, ECHO and Telstar, were launched in 1960and 1962, respectively. These and many subsequent satellites carried analog signals to allparts of the world. Because the computer resources in such satellites were minimal, in

computer terms they could be called "dumb," since they were, for the most part, simplepassive transceivers.However, the demands of the Internet and personal communication devices such as pagersand wireless phones have resulted in radically changed communication satellitetechnologies. Present satellites not only utilize digital signals and processing but also, insome systems, provide satellite-to-satellite communications. Perhaps this was nowheremore evident than in the Iridium system, a constellation of 66 Low Earth Orbit (LEO)communication satellites that featured sophisticated computer resources and protocols forinter-satellite communication.Television broadcast or, more precisely, relay, from satellite also began in 1962 with thelaunch of the Relay satellite. This technology has similarly improved, as is evidenced by

the shrinking sizes of the ubiquitous home satellite "dishes" that, owing to greater satellitetransmission power and other advances in technology, are now almost unnoticeable.Environmental Monitoring.The view from a geostationary environmental satellite is often shown during televisedweather broadcasts. The Geostationary Operational Environmental Satellites (GOES) orbitsimultaneously with the Earth's rotation at an altitude of just under 37,115 kilometers or23,000 miles. Typically, there are two such satellites in orbit stationed to offer views ofboth the eastern and western parts of the United States. The GOES are true environmentalsatellites; in addition to images of the clouds, they also provide information on water vapor,land and sea temperatures, winds, and estimates of precipitation.Landsat is also an environmental as well as a natural resource satellite. Over the last 30

years, seven Landsat satellites have been launched and two are currently in polar, sun-synchronous orbits. A sun-synchronous orbit is one in which the satellite passes over pointson the ground at the same local time. Landsat's multi-spectral scanner (MSS) and thematicmapper imaging systems provide digital imagery over discrete sections of the visible andinfrared portions of the electromagnetic spectrum. Such multi-spectral imagery and itsanalyses are routinely used in environmental studies such as deforestation, water pollution,tracking oil spills, and monitoring forest fires and droughts. It is also utilized in naturalresource studies such as land use classification, vegetation and soil mapping, andgeological, hydrological, and coastal resource studies.Navigation.Lines of latitude and longitude have been noted on maps since ancient times. Yet, the

accurate determination of one's exact position on the Earth has always been a vexingproblem. Perhaps at no time was this more evident than during the eighteenth century whenscholars and inventors vied to solve the problem of accurately determining longitude at sea.Such a determination required a highly accurate (and stable) clock, since it was necessaryto know simultaneously the time on the ship and at a land-based point of known longitude.It is interesting to note that the problem of determining accurate time was the basis ofaccurate position determination then, as it is also the basis of today's most accurateworldwide satellite navigation or position system. Indeed the navigation satellites of the


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twenty-first century have precise clocks that are accurate to within three nanoseconds, orthree billionths of a second.The navigational system known as the Global Positioning System (GPS) is a constellationof 24 NAVSTAR (NAVigation Satellite Timing And Ranging) satellites orbiting at 20,278kilometers or 12,600 miles altitude in six orbital planes. Each orbital plane is at an

inclination of 55 degrees with four satellites spaced so that a minimum of five satellites isvisible from any location on the planet. Each satellite broadcasts time and orbitinformation. Receivers on the ground also contain internal clocks. The difference in timebetween when a signal was sent and when it was received from each observable satellite isused to solve a spherical trigonometry problem to determine the exact location of theobserver. With certain receiver systems, this satellite technology means that someone'sexact position on the surface of the Earth can be determined to within one centimeter orfour-tenths of an inch!

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