Satellite Communication

Posted: February 21, 2008 in info itch
Satellite and  Earth Station

Satellites are those equipment that revolve our earth ( like moon ) and act as a repeater section. The transponder section of it receives signal from the earth station, amplifies the signal and then transmit(broadcast) it in to required place. Satellite do exactly in the same way as hub do in the star topology of network. So, if transponder fail, then the all system will go down. The uplink and downlink frequencies of it are separated by 45 MHz to avoid interferance.

It is a microwave communication and hence requires Line Of Sight(LOS) to transmit and receive signal. LOS means the earth station in ground and satellite in sky could see each other. The frequencies that it utilizies falls on the category of GigaHertz(GHz). Among the several freq. band separation of microwave, C, Ku and Ka bands are allocated for commercial satellite communication.
To make satellite able to stay in space revolving round the earth, Rocket having the satellite equipment is required to launch to sky with velocity greater than escape velocity. Escape velocity means the velocity required against the earth’s gravitation.
The 1st satellite ,sputnik1, was launched by Soviet Union in october 4, 1957 which again lauched the first living creature(sputnik2), a dog named Laika in to space on November 3, 1957. Explorer 1 was the first satellite of United States, launched in January 31, 1958. The early communication satellites were in relatively low altitude orbits(below 36000 km) and consequently sped around the earth in a few hours. This required the ground antennas to move constantly in order to beam signals to them.
The orbit in range of 36000 km above the earth surface is called geostationary orbit as the satellite in that orbit do round the earth in the same time that the earth takes to round in it’s own axis, means 24 hours. So though both earth and satellite revolve, the satellite appears to be stationary from a particular earch surface. This provides easyness in work eliminating the need of ground antennas to move constantly regarding satellite movement.
The satellites in 36000 km are called geostationary or geosynchrounous satellite while satellites below 36000 km are called low earth satellite satellites. In theoritical concept, 3 satellites in geostationary orbit can cover the whole earth while 64 are required in low earth orbit.
In 1965, Early Bird satellite was launched in to a much higher equatorial orbit of 36000 km.
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