10-11-2012, 04:13 PM
Satellite Communication
62749392-Satellite-Communication-Revised-3-1-5-09.ppt (Size: 747.5 KB / Downloads: 57)
Introduction
Today the whole world has become a global village.This has been possible due to the tremendous advancement of electronic communication systems, particularly the satellite communication systems. Today, satellites, like clocks, telephones, and computers, are commonplace tools of technology. They help us navigate, communicate, monitor the environment, and forecast weather. Appropriately, the word satellite means an "attendant."
Within only 50 years after the launch of the first satellite , the Sputnik-1, in 1957,equipped with on-board radio-transmitters by the Soviet Union there has been a revolution in communication.
Space communication showed phenomenal growth in the 1970s, and will continue to grow for some years to come. The growth has been so rapid that there is now danger of overcrowding the geo-stationary orbit.
communication capabilities with Earth
a power source
a control system to accomplish its mission
Communications antennae, radio receivers and transmitters enable the satellite to communicate with one or more ground stations, called command centers. Messages sent to the satellite from a ground station are "uplinked"; messages transmitted from the satellite to Earth are "downlinked."
Why Satellites for Communications
The greater amount of information required to transmit television pictures required that they operate at much higher frequencies than radio stations. A typical television station would operate at a frequency of 175 MHz. As a result, television signals would not propagate the way radio signals did.
Both radio and television frequency signals can propagate directly from transmitter to receiver. But it is more or less limited to line of sight communication. For long distance radio communication a signal is reflected back by the ionosphere. The higher frequency television signals penetrates the ionosphere and is not reflected. This is shown in the next slide.
Advantages of satellite communication
The laying and maintenance of intercontinental cable is difficult and expensive.
The heavy usage of intercontinental traffic makes the satellite commercially attractive.
Satellites can cover large areas of the Earth. This is particularly useful for sparsely populated areas.
For fixed services, communications satellites provide a technology complementary to that of fiber optic submarine communication cables. They are also used for mobile applications such as communications to ships and planes, for which application of other technologies, such as cable, are impractical or impossible.
Brief History
The first satellite equipped with on-board radio-transmitters was the Soviet Sputnik 1, launched in 1957.
The first American satellite to relay communications was Project SCORE in 1958. It carried a tape recorder which would record messages as it passed over an originating station and then rebroadcast them as it passed over the destination. However, it appeared only briefly every 90 minutes - a serious impediment to real communications.
In 1960, the simplest communications satellite ever conceived was launched. It was called ECHO, because it consisted only of a large (100 feet in diameter) aluminized plastic balloon. Radio and TV signals transmitted to the satellite would be reflected back to earth and could be received by any station within view of the satellite.