What is Satellite Communication?


What is Satellite Communication?


For sky wave propagation, usually the frequency band extending from 3 to 30 MHz is employed. Radio links over long distance over the earth surface can be established by multi-hop transmission. But reliable communication by means of sky waves is hampered due to problems like fading, ionospheric disturbance and storms. Artificial satellite offer reliable communication links over long distance. An artificial satellite is hurled into space in a circular orbit in the equatorial plane at height of 36000 km above the surface of the earth. 
The period of revolution of the satellite round the earth is 24 hours, i.e., equal to the period of rotation of the earth about in own axis. So, to an observer on the earth’s surface, the satellite appears to be stationary. Such satellite are therefore referred to a geostationary satellite and the orbit as the synchronous orbit.

 The wave containing information is transmitted to the satellite from a transmitter located on the earth surface. The signal is processed by the equipment kept in the satellite, amplified and retransmitted towards the receiving point on the surface of the earth. The satellite being geostationary, the transmitting and the receiving antenna on the earth’s surface can be oriented in fixed directions.
The geostationary satellite has become the world standard for most communication satellites.
 The angle between the direction of propagation of the wave radiated from an earth station antenna and the horizontal is termed the angle of elevation. The horizontal pointing angle of the antenna as measured clockwise from true north, is called the azimuth. To orient an earth station antenna towards a satellite, the elevation angle and the azimuth must known. These angles are referred to as the look angles. For a smaller angle of elevation, the radiated wave travel a longer distance through the earth’s atmosphere, and thus suffers more absorption and greater noise contamination. To avoid an excessive determination of the wave, the angle of elevation is kept larger than 50   . The radiation pattern of a satellite antenna can  cover nearly one-third of the earth’[s surface. Three equally spaced geostationary satellites can hence cover the entire earth except for the unpopulated areas of the north and the south poles.

Satellites operating at or near the same frequency are separated in space by 30 to 60   to prevent mutual interface. Usually, the carrier frequency for the wave transmitted from the earth station to the satellite, termed the uplink frequency, is 6 GHz and the carrier frequency of the wave from the satellite to the earth station, called the downlink frequency, is 4 GHz. An uplink frequency of 14GHz and downlink frequency of 12 GHz are also employed in satellite communication. The available bandwidth is several hundred MHz . Different uplink and downlink frequencies are used to avoid an unwanted feedback or ring around from the downlink antenna into the receiver. Frequency-band separation also permits the same antenna to be employed for both reception and transmission, simplifying the satellite hardware. The downlink frequency is lower than the uplink frequency. As the absorption increase with increasing frequency, the signal received by the earth station suffers less attenuation.

For the carrier frequencies employed in satellite communication, the antenna size is not inconveniently large. FM or special modulation techniques for digital signals are employed in satellite communication. The use of the 6/4 GHz band offers the following advantages:               1. relatively  inexpensive  microwave equipment can be employed 2. The attenuation due to galactic, solar, and terrestrial noise sources is insignificant.
The 14/12 GHz band is used as the 6/4 GHz band coincides with that used for terrestrial microwave system. In the 14/12 GHz band, smaller and hence less expansive antenna can be employed.
A satellite system is schematically shown. Essentially the system consists of three primary sections: an uplink, a satellite transponder, and a downlink. The uplink section is the earth station transmitter radiating the wave to the satellite. The satellite transponder contains a frequency translator which converts the uplink frequency to the downlink frequency by feeding the incoming uplink frequency wave and the signal from a 2GHz shift oscillator in a mixer. The signal is amplified and transmitted to earth station receivers.
The circuits on board the satellite are powered by solar cells mounted on deployed panels in the satellite. The downlink section contains the earth station receiver which recovers the signal from the modulated carrier.
Communication between satellites is  possible via cross links or inter satellite links (ISIs). Conventional speech telephone signals, data (in analog or digital form), and television picture and sound are conveyed through the satellite system.

The advantages of satellite communication are as follows:

1.      The quality of reception is quit good, and the system is most reliable and flexible.
2.      The bandwidth is very large.
3.      A  geostationary  satellite can cover approximately one-third of the earth’s surface . The larger area coverage is a distinctive feature.
4.      As the satellite is stationary with respect to a given earth station, expansive tracking equipment is not needed at the earth station.
5.      The wideband go-anywhere service has opened up the possibilities for the satellite system to handle huge number of signal and data.

The system has the following disadvantages:

1.      The installation cost is high. Geostationary satellite need sophisticated and heavy propulsion devices on board to keep them in a fixed orbit.
2.      The high altitude of the  satellite introduce long propagation time. The round- trip propagation delay between two earth  station via a geostationary satellite is 500 to 600ms.
3.      The system requires high output power(several kW) of earth station transmitter and sensitive receivers.

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