Satellite:

A satellite is an object which has been placed into the orbit by human endeavor. These objects are always called as artificial satellites. It is because to differentiate from natural satellites such as Moon. The world’s first artificial satellite, the sputnik 1, was launched by the Soviet Union in 1957. Since then, thousands of satellites have been launched into the orbit around the earth.

Satellite Communication:

Satellite communication is basically a microwave link repeater. A satellite receives energy from an earth station, amplifies it and returns it to each at a frequency about 2 GHz away from the upper frequency(earth to satellite). This prevents interference between the uplink and the downlink(satellite to earth). Satellite so used is a geostationary satellite which appears to be stationary at a given spot above the equator. Actually, it moves with the same angular velocity as the earth. i.e, it completes one revolution per 24 hours hence appears to be stationed over one spot on the globe. A satellite orbiting the earth will be geostationary when it is about 36,000 km away from the earth.

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configuration of satellite communication

The above flow chart gives the general structure of a satellite communications system.A Satellite in space links many earth stations.The user is connected to the earth station through the terrestrial network.This network may assume various configurations including a telephone switch or a dedicated link to the earth station.The signal generated by the user is processed and transmitted from the earth station to the satellite.The downlink frequencies are kept different from the uplink frequencies in order to avoid interference.

Advantages:

• Mobile communication can be easily established by satellite communication.
• Once the satellite is in position, Earth Station can be installed and communication may be established within some days or even hours.
• High bandwidth.
• Satellite communication is economical compared with terrestrial communication, particularly where long distances are involved.
• Communications satellites cover all land masses and there is growing capacity to serve maritime and even aeronautical markets. Customers in rural and remote regions around the world who cannot obtain high-speed Internet access from a terrestrial provider are increasingly relying on satellite communications.
• Coverage over a large geographical area.
• Satellite is unmatched for broadcast applications like television. For two-way IP networks, the speed, uniformity and end-to-end control of today’s advanced satellite solutions are resulting in greater use of satellite by corporations, governments, and consumers.
• Can be cheaper over long distances.
• Additional receive sites, or nodes on a network, can readily be added, sometimes within hours. All it takes is ground-based equipment. The satellite has proven its value as a provider of “instant infrastructure” for commercial, government and emergency relief communications.
• During critical condition, each Earth Station may be removed relatively quickly from a location and reinstalled somewhere else.
• Point to multipoint communication is possible whereas terrestrial relay is point to point, this is why satellite relay is wide-area broadcast.
• Compared to the optical fiber communication, satellite communication has the advantages that, quality of transmitted signal and location of sending and receiving stations are independent of distance.
• The user has control over their own network.
• For thin traffic remote areas, satellite communication is most economical.

For search, rescue, and navigation, satellite communication is far superior and economical compared to other systems.

Disadvantages: 

• Between talks, there is a time gap which becomes quite annoying. 
• Propagation delay
• This time delay also reduces the efficiency of communication satellite in data transmission.
• There is a delay of ¼ sec between the transmission and reception of a signal because the velocity of the electromagnetic wave is 3* 10^5 Km/second.
• Over-crowding of available bandwidth due to low antenna gains occurs.
• An imperfect impedance match may cause echo, received back after a delay.Echo suppressor has to be used.
• Huge initial cost.
• Noise and interference.

• Repair of the satellite is almost impossible, once it has been launched.

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