communication satellites

communication satellite is a spacecraft that orbits the earth.it has equipment called transponder to relay radio,telegraph,telephone and television signals transmitted by the earth station.many activities can by handled by communication satellites such as telephone conversations,telegraph messages,radio and tv programmers,and computers data.all the activities can be broadcasted by satellites from one country to other at the same time.
the development of communication satellis began in 1950's in response to the growing demand for better communications.now they become the basic part of worldwide communications.
communication satellites are launched by rockets.they orbit the earth at about 35.900 km height above the earth.during their orbits,they receive signals transmitted by the earth stations.then they relay the signals to the receiving earth stations.the satellites are powered mainly by means of solar panels and storage batteries.the solar panels collect energy from the sun which is converted into electric power while the batteries suply electricity when the satellites do not receive sunlight.each satellite's lifetime is about 7 to 10 years,so the expired satellite must be replaced by the new one.