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A communication satellite is an artificial satellite stationed in space for the purpose of telecommunications. Modern communication satellites use a variety of orbits including geostationary orbits, Molina orbits, other elliptical orbits and low earth orbits. For fixed services, communication satellites provide a microwave radio relay technology complimentary to that of submarine communication cables. They are also used for mobile applications such as communications to ships, vehicles, planes and hand held terminals and for TV and radio broadcasting, for which application of other technologies such as cable is impractical or impossible.
HISTORY OF COMMUNICATION SATELLTES
The first artificial satellite was the soviet sputnik 1, launched on October 4, 1957, and equipped with an onboard radio transmitter that worked on two frequencies, 20.005 and 40.002 MHz. The first American satellite to relay communication was project score in 1958, which used a tape recorder to store and forward voice messages. After that NASA launched an Echo satellite in 1960.
SATELLITE COMMUNICATION FOR COMPUTER NETWORKS.
A multi-node, satellite communication system employing a modified broadcast system is disclosed for used with distributed computer networks. The system involves a plurality of network nodes (computer systems) each capable of transmitting to any other node at a single unique frequency, but capable of receiving from all other nodes simultaneously. Each node in the n-node network comprises a single transmitter with up to n-1 receivers, with each node capable of arbitrating a plurality of requests for transmission access. The invention encompasses a method for communicating digital information in a network of geographical distributed computers.
In 1945, Arthur C. Clarke describes a complete communication system that used manned geostationary satellites, including the orbits, solar panels, radio frequencies and launch procedures. But it was failed, because these satellites were impractical due to the impossibility of putting power-hungry, fragile, vacuum tube amplifiers into orbit. The first communication satellite, Telstar was launched in July 1962. Then communication Satellites have become a multibillion dollar business and the only aspect of outer space that has become highly portable. These high-flying satellites are called GEO (Geostationary Earth Orbit) satellites.
Geostationary satellites space much closer than 2 degrees in the 360-degree equatorial plane, to avoid interference. With a space of 2 degrees, there can only be 360/2 =180 of these satellites in the sky at ones. However, each transponder can use multiple frequencies and polarization to increase the available bandwidth. Modern satellite was weighing up 4000kg and consuming several kilowatts of electric power produced by the solar panels. The effect of solar and planetary gravity tends to move them away from their assigned orbit slots and orientations, an effect countered by on-board rocket motors. This tuning activity is called Station Keeping. ITU has allocated certain frequency bands to satellite users. The main bands are listed as C band was the first to be designated for commercial satellite traffic and the upper case one for uplink traffic. To allow traffic to go both ways at the same time, two channels is required, one going each way. These bands are overcrowded because they are also used by the common carriers for terrestrial microwave links. The L and S bands were added by international agreement in 2000.
Low bandwidth; crowded
Low bandwidth; crowded
Rain, equipment cost
The next highest band available to commercial telecommunication carriers is the Ku band. Bandwidth allocated in the Ka band for commercial traffic, but the equipment needed to use it is still expensive. A modern satellite has around 40 transponders, each with an 80-MHZ band width. The first geostationary satellites had a single beam that illuminated about 1/3 of the earth's surface called footprint. A new development in the communication satellite world is the development of low-cost micro stations; it is called Very Small Aperture Terminals (VASATs). In VSATs systems, the micro stations don't have enough power to communicate directly with one another. VSATs have great potential in rural areas.
MEDIUM-EARTH ORBIT SATELLITES
MEO (Medium-Earth Orbit) satellites tracked as move through the sky. These are lower than GEOs; they have smaller footprint on the ground and require less powerful transmitters to reach them.
LEO (LOW EARTH ORBIT) satellite due to their rapid motion, large numbers of them is needed for a complete system. In this section we will examine, two aimed at voice communication and one aimed at internet service.
An iridium satellite was launched in 1997. Communication service began in November 1998. Iridium's business was providing worldwide telecommunication service using hand-held devices that communicate directly within the iridium satellites. It provides voice, data, paging, and fax and navigation service everywhere on land, sea, and air. These satellites are positioned at an altitude of 750km, in circular polar orbits. Each satellite has a maximum of 48 cells, with a total of 1628 cells over the surface of the earth. Each satellite has a capacity of 3840 channels, or 253440 in all.
An iridium alternative design is global star. It based on 48 LEO satellites. But it uses a different switching scheme than that of iridium. Global star uses a traditional bent-pipe design. The call originating at the North Pole is sent back to earth and picked up by the large ground station at Santa's Workshop. The call is routed via a terrestrial network to the ground station nearest the cal lee and delivered by a bent-pipe connection. The advantage of global star scheme is that it puts much of the complexity on the ground, where it is easier to manage.
Teledesic is targeted at bandwidth-hungry internet users all over the world. It was conceived in 1990 by mobile phone pioneer Craig McCaw and Bill Gates. The goal of this system is to provide millions of concurrent Internet users with an uplink of as much as 100Mbps and a downlink of up to 720 Mbps using a small, fixed, VSAT-type antenna, completely bypassing the telephone system. The original design was for a system consists of 288 small-footprint satellites arranged in 12 planes below the lower Van Allen belt at an altitude of 1350km. The design was later changed to 30 satellites with larger footprints. Transmission occurs in the uncrowned and high-bandwidth Ka band. When a user needs bandwidth to send packets is request and assigned dynamically in about 50 msec.
LEO EARTH ORBIT
HOW SATELLITE COMMUNICATION WORKS?
The concept of satellite based network is to transmit and receive signal from ground stations. The purpose of satellite communication is to use it for video transmission and sharing. In simple words a satellite is a device which revolves around the earth either for collecting useful information or for helping transfer of information.
HOW DO SATELLITES COMMUNICATE?
Satellites communicate through a complex system of telemetry that connects and transmits signals between satellites and earth-bound stations. This system begins with telemetry, tracking and command (TT&C) subsystem which tracks and directs signals. TT&C produces signal that are directly commensurable to the quantity that is being measured, encoding and transmitting these signal to distant stations on earth.
APPLICATIONS OF COMMUNICATION SATELLITES
The applications of communication satellites are,
Fixed Service Satellite
Direct Broadcast satellites
Mobile satellite technologies
The first application for communication satellites was in intercontinental long distance telephony. The public switched telephone network relays telephone calls from landline telephones to an earth station where they are transmitted to geostationary satellite.
The satellite television is relatively few signals of large band width to many receivers being a more precise match for the capabilities of geosynchronous comsats. Two satellite types are used for North American Television and radio that are Direct Broadcast Satellite (DBS) and Fixed Service Satellite (FSS).
FIXED SERVICE SATELLITE
It uses the c band and the lower portion of the Ku bands. It is normally used for broadcast feed to and from television networks and local affiliate stations as well as being used for distance learning by schools and universities, business television, video conferencing and general tele- communications. It is also used to distribute national cable channels to cable television head ends.
DIRECT BROADCAST SATELLITE
It is a communication satellite that transmits to small DBS satellite dishes. It is usually 18 to 24 inches or 45 to 60 cm in a diameter.
MOBILE SATELLITE TECHNOLOGIES
It was using antena's.