16-01-2012, 04:25 PM
SATELLITE COMMUNICATIONS - AN OVERVIEW
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INTRODUCTION
The outer space has always fascinated people on the earth and communication through space evolved as an offshoot of ideas for space travel. The earliest idea of using artificial satellites for communications is found in a science fiction Brick Moon by Edward Evert Hale, published in 1869-70. While the early fictional accounts of satellite and space communications bear little resemblance to the technology as it exists to day, they are of significance since they represent the origins of the idea from which the technology eventually evolved. In the area of satellite communications, the technology has been responsive to the imaginative dreams. Hence it is also expected that technological innovations will lead the evolution of satellite communications towards the visions of today.
REALISATION OF CONCEPT TO REALITY
In October 1957, the first artificial satellite Sputnik -I was launched by former Soviet Russia in the earth’s orbit and in 1963 Clark’s idea became a reality when the first geosynchronous satellite SYNCOM was successfully launched by NASA.
The realization of the concept of satellite communications from an idea to reality has been possible due to a large number of technological breakthroughs and practical realization of devices and systems, which took place during and after the World War II. The pressures of international military rivalry during cold war period were also able to a great extent to push scientific and technological research and development far faster than it would have been possible if applied for peaceful purposes.
Passive Satellites
The principle of communication by passive satellite is based on the properties of scattering of electromagnetic waves from different surface areas. Thus an electromagnetic wave incident on a passive satellite is scattered back towards the earth and a receiving station can receive the scattered wave. The passive satellites used in the early years of satellite communications were both artificial as well as natural.
Experimental Satellite Communications Systems
For the purpose of test and evaluation of new technologies a number of satellites have been designed and operated for technical experiments. Various experiments have also been conducted using these satellites for demonstrating different applications of communications satellites. Prominent among these experimental satellites are:
