11-09-2013, 04:28 PM
Satellite Communication
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Introduction to Satellite Communication
Satellite is powerful long distance and point-to multipoint communication system. A communication satellite is an R.F (Radio Frequency) repeater. To overcome disadvantage of Line of sight communication which is only 45-55km, the transmitting antenna is placed on the satellite and the satellite is placed in the orbit high above the earth. The function of satellite is to communicate between different earth stations around the earth, thus with the help of satellite, it is easy to communicate over thousands of km, a com-satellite is a combination of ROCKET to put the satellite in the orbit, micro wave electronic devices for the communication, solar cells are used to convert the solar energy into a power supply (ELECTRICAL ENERGY) for the electronic equipment.
The satellite placed in GEO- STATIONARY and placed at an altitude of 22300 miles or 35900km above the ground level.
The satellite travels at the same speed at which the earth rotates around the sun. The rotation of satellite is synchronized with earth rotation as a result satellite appears to be stationary in the sky w.r.t the earth station is constant.
Geostationary Satellite
The satellites were placed in low earth orbit. as a result the satellite at a such high speed that it visible to the ground only for a short time at each day , the satellite appeared below the horizon and dies appear below the opposite horizon , the ground station was cut-off for long time in day , to maintain the communication link another station had to be activated , this problem was solved by placing the satellite in circular orbit of approximately 22300 miles or 35900 km radius, as the satellite height increases from the earth surface , the speed of satellite decreases by the same manner , at that height the angular velocity of satellite will be proportional to the angular velocity of earth , the satellite rotates with the same speed as that of the earth due to which the satellite will always be at the same place where it has been fixed , this type of satellite is called geo stationary satellite.
Telephone Link via Satellite
The satellite communication can be used for Telephone telecom. Around the world, the block diagram of such a system is shown in fig. The block diagram of earth station working with three satellites here, the national long distance Telephone network of a 4 countries (A,B,C,D,) through international switching center are connected, consider country “A” the 0/p of the Telephone exchange is applied to the MUX, the multiplexed signal is send to the micro-wave station and from there to the satellite earth station , at the earth station the signal is multiplexed and directly applied to the modulator stage of earth station where it demodulated with a high frequency signal and transmitted towards the satellite as uplink , in other case the earth station “A” receive three down link signal , the 3 carriers are demodulated and then transmitted toward the micro-wave station and from there international switching center.
Merits
1. Geostationary satellites require no tracking.
2. Satellite communication has the ability of multiple access points.
3. With the help of satellite the 24hour communication can be achieved.
4. Satellite communication has a higher signal quality.
5. BROAD—BAND can be obtained to put more information on the carrier.
6. For long distance communication or across oceans a Satellite Communication is used.
7. Satellite in close elliptical orbits required low transmitting Power and low receiver sensitivity.
Demerits
1. Satellite communication need a large diameter antenna high power and most sensitive transmitter and receiver are required.
2. Satellite communication is disturbed by solar activities and cyclones in the space.
3. Satellite efficiency drops due to the ageing effect of the components.
4. The longer propagation times (APPOX,300ms)are a disadvantage of satellite communication.