Communication Satellites And Computer Networks

A communication satellite which is sometimes abbreviated to COMSAT is an artificial satellite to engage in space for the purpose of telecommunications. Modern communication satellites use different orbits including geostationary orbits, Molina orbits, other elliptical orbits and low earth orbits. 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. It was used to send messages like greetings etc. After that NASA launched an Echo satellite in 1960. Telstar was the first active, direct relay communications satellite.

SATELLITE COMMUNICATION IN 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.

GEOSTATIONARY SATELLITES

A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer. 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. 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.

Band

Downlink

Uplink

Bandwidth

Problems

L

1.5GHZ

1.6GHZ

15MHZ

Low bandwidth; crowded

S

1.9GHZ

2.2GHZ

70MHZ

Low bandwidth; crowded

C

4.0GHZ

6.0GHZ

500MHZ

Terrestrial interference

Ku

11GHZ

14GHZ

500MHZ

Rain

Ka

20GHZ

30GHZ

3500MHZ

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.

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.

LOW-EARTH ORBIT

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. LEO earth satellites are less expensive to launch into orbit than geostationary satellites. A grou8p of satellites working in concert is a Satellite constellation.

GLOBAL STAR

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.

MOLNIYA ORBIT

The Molniya orbit is designed so that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprints moves only slightly. Its period is one half day, so that the satellite is available for operation for eight hours every second revolution. The Molniya satellites are typically used for telephony and TV services over Russia.

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,

Telephone

Satellite Television

Fixed Service Satellite

Direct Broadcast satellites

Satellite radio

Mobile satellite technologies

Amateur radio

Satellite internet

Military uses

Navigation

TELEPHONE

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.

SATELLITE TELEVISION

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).

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.

SATELLITE RADIO

A Satellite radio offers audio services in some countries, mostly in United States. Mobile services allow listeners to roam a continent, listening to the same audio programming anywhere. A Satellite radio or subscription radio(SR) is a digital radio signal that is broadcast by a communications satellite, which covers a much wider geographical range than terrestrial radio signals.

MOBILE SATELLITE TECHNOLOGIES

Initially available for broadcast to stationary TV receivers. Some manufactures have introduced special antennas for mobile reception of DBS television. Such mobile DBS antennas are used by jet-blue Airways for Direct TV, which passengers can view on board on LCD screens mounted in the seats.

CONCLUSION

Communication satellite has really paved way for many technologies to emerge and its advance in technology radically reduces the cost of deploying a satellite. Users of satellite systems will benefit from new service options and stability in terms current satellites and their replacement. It is very useful to economics of constructing and operating communication satellites. To conclude, I was really benefited by getting more knowledge by doing this assignment by researching on Communication satellite.