SATELLITE COMMUNICATION

ByRavikiran S. AnandeSATELLITE COMMUNICATIONINTRODUCTIONCommunication satellites bring the world to you anywhere and any time..

BACKGROUND:Telecommunication system made it possible to communicate with anyone at any time.Before the telegraph & telephone, all communication was face-to-face, or in writing. If you wanted to talk someone, you had to travel to meet with that person.If you wanted to send information, it had to written down on papers & carried to destination.Telegraph cables laid across the oceans as early as the mid-1800s.Telegraph system used copper wire to carry signals over earths surface & across oceans , made possible intercontinental links.But cables capable of carrying voice signals did not begin service until 1953.In 1945, HF radio was only available method for transcontinental distances.HF is commonly called as Short Wave (SW).Operates in 3 to 30 MHzNot reliable.Because Sunspots & ionospheric disturbances disrupt HF radio links.

ORIGIN OF SATELLIE COMMUNICATION:In 1945, Arthur C. Clarke presented an article named Wireless world in the British Radio magazine.At that time Arthur C. Clarke was serving in British Royal Air Force.

Clarke suggested that a radio relay satellite in an equatorial orbit with a period of 24 h would remain stationary with respect to earths surface and make possible long distance radio links.Clarke wrote, there were no rockets powerful enough to launch them.

HISTORY OF SATELLITE COMMUNICATION:Satellite communication began in October 1957 with the launch by Russia of a small satellite called Sputnik I.Its a first artificial earth satellite and it sparked the space race between United States and Russia.Sputnik I carried only a beacon transmitter, did not have communication capability.The first satellite successfully launched by USA was Explorer I on January 1958.The first voice heard from space was that of US president Eisenhower, who recorded Christmas message that was transmitted back to earth from SCORE satellite in December 1958.The world's first commercial communications satellite was Early Bird (INTELSAT I).Launched on April 16, 1965 and placed in commercial service after moving into geosynchronous orbit 22,300 miles above the equator. The satellite weighed a mere 36 kg & incorporated two 6/4 GHz transponders, each with 25 MHz bandwidth.Early Bird didn't have a battery - and worked only when its solar panels were exposed to the sun. High quality satellite communication within borders of large countries.1960s First satellite communications:

1960 First passive communication satellite (Large balloons, Echo I and II).1962: First active communication satellite (Telstar I , MEO).1963: First satellite into geostationary (GEO) orbit (Syncom1, communication failed).1964: International Telecomm. Satellite Organization (INTELSAT) created.1965 First successful communications GEO (Early Bird / INTELSAT 1).

1970s GEO Applications Development, DBS:

1972 First domestic satellite system operational (Canada).1975 First successful direct broadcast experiment (USA-India).1975 First successful direct broadcast experiment (USA-India).1979 International Mobile Satellite Organization (Inmarsat) established.

1980s GEO Applications Expanded, Mobile:

1981 First reusable launch vehicle flight.1982 International maritime communications made operational.1984 First direct-to-home broadcast system operational (Japan).1987 Successful trials of land-mobile communications (Inmarsat).1989-90 Global mobile communication service extended to land mobile and aeronautical use (Inmarsat)

1990+s NGSO applications development and GEO expansion

Proposals of non-geostationary (NGSO) systems for mobile communications.Continuing growth of VSATs around the world.Spectrum allocation for non-GEO systems.Continuing growth of DBS. DirectTV created. Launch of first batch of LEO for hand-held terminals (Iridium).Voice-service portables and paging-service pocket size mobile terminals launched (Inmarsat).And continuesWhat is satellite ?In astronomical terms, a satellite is a celestial body that orbits around a planet.Example: The moon is a satellite of Earth.In Aerospace terms, a satellite is a space vehicle launched by humans and orbits around Earth or another celestial body. Communications Satellite: It is a microwave repeater in the sky that consists of a diverse combination of one or more components including transmitter, receiver, amplifier, regenerator, filter onboard computer, multiplexer, demultiplexer, antenna, waveguide etc.A satellite radio repeater is also called transponder. This is usually a combination of transmitter and receiver.

HOW A SATELLITE WORKS ?

Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.The two stations can use a satellite as a relay station for their communication.One Earth station transmits the signals to the satellite at Up link frequency.Up link frequency is the frequency at which Earth station is communicating with a satellite.The satellite transponder process the signal and sends it to the second Earth station at another frequency called downlink frequency.

Advantages of satellite communication:Global Availability- Communications satellites cover large geographical area. Customers in rural and remote regions around the world who cannot obtain high speed Internet access from a terrestrial provider are increasingly relying on satellite communications.Cost Effectiveness-Cost of satellite capacity does not increase with the number of users/receive sites, or with the distance between communication points.

17iSuperior Reliability-Satellite communications can operate independently from terrestrial infrastructure. When terrestrial outages occur from man-made and natural events, satellite connections remain operational.Immediacy and Scalability-Additional receive sites, or nodes on a network, can readily be added, sometimes within hours. All it takes is ground-based equipment.Superior Performance- Satellite is unmatched for broadcast applications like television.18Disadvantages of satellite communication-Huge initial cost-Satellites are large and expensive, thus there is a large capital cost in building and launching a satellite.Propagation delay-Impossibility to repair and maintain-Noise and interference-

APPLICATIONS:

SATELLITE ORBITS:Orbit is path traversed by a satellite.

Satellite

Low earth orbit (leo):Altitude -> 600 1000km.Revolution time: 3hours.

Advantages-Reduces transmission delay Low transmission power required. Disadvantages-Smaller coverage area.A network of at least 6 LEO satellites is required to cover a region continuously.Shorter life span(5-8yrs) than GEOsSubdivisions- Little, Big and Mega LEOs.

Medium earth orbit:Altitude -> 8000-20000 km.Revolution time- approx. 6hrs

These orbits are primarily reserved for communications satellites that cover the North and South Pole. Unlike the circular orbit of the geostationary satellites, MEOs are placed in an elliptical (oval-shaped) orbit.Approximately a dozen medium Earth orbiting satellites are necessary to provide continuous global coverage 24 hours a day.Advantages-1. Slightly longer propagation delays (~40 msec).2. Slightly higher transmission power required.Disadvantages-1. Coverage spot greater than a LEO, but still less than a GEO.2. Multiple MEO satellites are still needed to cover a region continuously.3. Handovers and satellite tracking are still needed, hence, high complexity.

GEOSTATIONARY EARTH ORBIT(GEO):Altitude-> 35786 km above earth surface.Revolution period- Same as period of earth.Satellite in GEO appears to be stationary over a fixed point on the ground.

One GEO satellite covers 1/3 rd of the earth surface using fixed antennas at the earth stations.So three satellite requires to cover the entire earth. Geostationary satellites are commonly used for communications and weather-observation.The typical service life expectancy of a geostationary satellite is 10-15 years.Advantages-Large coverage area.GEO satellites have a 24 hour view of a particular area.These factors make it ideal for satellite broadcast and other multipoint applicationsDisadvantages-Propagation delay is more.As the distance is more, signal at receiver is very weakGEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions.

FREQUENCY BANDS FOR SATELLITE SERVICES:

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C band:Features->Large Dish antenna required (3m diameter)Low rain attenuationUses->TV signal reception

KU-BAND:Features->Small antenna size, high antenna gainRain, snow, ice (on dish) susceptibilityUses->Remote TV broadcasting