Communication Satellites

COMMUNICATION SATELLITES

INTRODUCTION •

A satellite is an object that orbits or revolves around another object.

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They are highly specialized wireless receiver/ transmitters launched by a rocket & placed in orbit around the Earth.

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In 1950’s & 60’s, people tried to set up communication systems by bouncing signals off metallized weather balloons. Unfortunately received signals were too weak.

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The key difference between an artificial & a real satellite: the artificial one can amplify the signals before sending them back.

WHAT IS A SATELLITE ? •

A communication satellite can be thought of as a big microwave repeater in the sky.

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It contains several transponders, each of which listens, amplifies, and then rebroadcasts the signal at another frequency to avoid interference with the incoming signal.

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Telstar was the first artificial communication satellite to be launched in July 1962.

KEPLER’S LAW • According to Kepler's law, the orbital period of a satellite varies as the radius of the orbit to the 3/2 power. • Higher the satellite, longer the period. • Near the surface of the earth, the period is about 90 mins. • Consequently, low-orbit satellites pass out of view fairly quickly, so many of them are needed to provide continuous coverage. • At an altitude of about 35,800 km, the period is 24 hours. • At an altitude of 384,000 km, the period is about one month, as anyone who has observed the moon regularly can testify.

VAN ALLEN BELTS •

Layers of highly charged particles trapped by earth's magnetic field.

FREQUENCY BANDS •

C-band: The oldest! Freq. range: 6 GHz-uplink & between 3.7 & 4.2 GHz-downlink.

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Ku-band: Most common! Freq. range: 14 GHz-uplink & between 10.9 & 12.75 GHz-downlink.

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Ka-band: Frequency range: 30 GHz-uplink & between 18 & 20 GHz-downlink.

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C-band and Ku-band are becoming congested by an increasing amount of users, so Ka-band has become the preferred choice.

FREQUENCY BAND DIAGRAM

COMMUNICATION SATELLITE COMPONENTS

LAUNCHING OF A SATELLITE

GEO SATELLITES • A satellite at an altitude of 35,800 km in a circular equatorial orbit appears to be motionless in the sky & need not be tracked. • With a spacing of 2 degrees, there can only be 360/2 = 180 of these satel-lites in the sky at once. • Initially the footprints were small but due to multiple transponders a no. of beams called spot beams are possible.

GEO SATELLITES (Contd.) • A new development in the communication satellite world is the development of low-cost micro stations, sometimes called VSATs (Very Small Aperture Terminals). • VSATs communicate with each other through special ground stations: Hubs. • VSATs are a boon in rural areas where installing them is more feasible than telephone wires.

MEO SATELLITES • At much lower altitudes, between the two Van Allen belts, we find the MEO (Medium-Earth Orbit) satellites. • Viewed from the earth, these drift slowly in longitude, taking something like 6 hours to circle the earth. • They must be tracked as they move through the sky. • Since they are lower than the GEOs, they have smaller footprints on the ground & require less powerful transmitters to reach them. • The 24 GPS (Global Positioning System) satellites orbiting at about 18,000 km are examples of MEO satellites.

LEO SATELLITES • LEOs move in either elliptical or circular orbits at a height of less than 2,000 km above the surface of the earth. • The orbit period at these altitudes varies between ninety minutes and two hours. • The radius of the footprint of a communications LEO satellite varies from 3000 to 4000 km. • The maximum time during which a LEO satellite is above the local horizon for an observer on the earth is up to 20 minutes. • Iridium, Global star & Teledesic are a few important LEO satellites.

LEO SATELLITES (Contd.) IRIDIUM(1997): • In 1997, Motorola launched 77 LEO satellites for the Iridium project (element 77 is iridium). • Communication between distant customers takes place in space, with one satellite relaying data to the next one Iridium's business was providing worldwide telecommunication service using hand-held devices TELEDESIC(1990): • Teledesic uses 30 satellites & is targeted at bandwidthhungry Internet users Transmission occurs in the relatively uncrowded and high-bandwidth Ka band.

APPLICATIONS • • • • • • • • • • •

TV Broadcasting Radio Communications Internet Communications Telephony- Trunk, Mobile etc Weather Forecasting Global Positioning Military Applications Scientific Applications VSATs Remote Sensing Satellite News Gathering

CONCLUSION • The use of satellite technology, particularly in the use of communications satellites has grown rapidly in the past thirty years. • "What at the beginning of the decade, was no more than a concept in the minds of a few engineers had, by the end, become a fully commercial system providing global communication system". • This network will provide the framework and capability for anyone in the world to communicate with anyone else, regardless of location. • More and more satellites are being launched each year to support new and growing uses for business, military and communication needs. • Satellite communications will continue in the right direction, UP.

Thank You…