satellite communication

The first satellite that was used for communication purpose in INDIA was ARYABHATTA and it was launched in 19th April.1975. It was made and assembled by an organization called Indian Space Research Organization (ISRO). In the year 1981, a satellite named APPLE was launched in space which was the first Indian Experimental communication satellite. The unique feature of it was that it was a three axis stabilization geosynchronous satellite and weighed around 645 kg. The term APPLE is an abbreviation for Ariane Passenger Payload Experiment. It consisted of a (6/4 Ghz) processing equipment called Transponder. Various experiments were carried out with APPLE, [SITE, STEP (Other satellite telecommunication experiment projects)] and the results obtained from these experiments provided an impetus for Govt. of India to have its own multipurpose Geosynchronous Earth Orbit satellite under INSAT (Indian National Satellite) program.

The first satellite INSAT-1A was launched in the year 1982 which was under this INSAT program, but this effort went in vain as the power house of this satellite consisting of solar cells did not operate properly( failed to open) and this satellite was unused latter on. The average electrical power required by INSAT-1 was approximately 1000W and was provided by the power house subsystem of the satellite. The payload was one C-band transponder and two S-band transponders. Later succession of INSAT-1 series was launched like INSAT-1B, INSAT-C and INSAT-D. After this due to the success of the first generation satellites, INSAT-2 series was launched viz. INSAT-2A, INSAT-2B, INSAT-2C, INSAT-3D and INSAT-2E which provided variety of services.

A Satellite is a solid object which revolves around some heavenly body due to the effect of gravitational forceswhich are mutual in nature. We can categorize satellites in two types, namely Passive Satellites and Active satellites. Passive satellites are not like active satellites. Even a moon can be a passive satellite. Thus passive satellites are relay stations in space. A passive satellite can be further subdivided into two types, namely Natural satellites and artificial satellites. A moon is a natural satellite of earth. But spherical balloon with metal coated plastic serve as artificial satellites.Active satellites are complicated structures having a processing equipment called Transponder which is very vital for functioning of the satellite. The term Satellite communication is very frequently used, but what is satellite communication? It is simply the communication of the satellite in space with large number of earth stations on the ground.

The earth station then receives the signal from the satellite through parabolic dish antenna and processes it to get back the baseband signal. This baseband signal is then transmitted to the respective user via dedicated link or other terrestrial system. Previously satellite communication system used large sized parabolic antennas with diameters around 30 meters because of the very faint and weak signals received. But nowadays satellites have become much stronger, bigger and powerful due to which antennas used have become automatically smaller in size. Thus the earth station antennas are now not large in size as the antennas used in olden days. A satellite communication system operates and works in the millimeter and microwave wave frequency bands from 1 Ghz to 50 Ghz. There are various frequency bands utilized by satellites but the most recognized of them is the uplink frequency of 6 Ghz and the downlink frequency of 4 Ghz. Actually the uplink frequency band is 5.725 to 7.075 Ghz and the actual downlink frequency band is from 3.4 to 4.8 Ghz. The major components of a Satellite Communication system is spacecraft and one or more earth earths.