19-02-2011, 10:01 AM
Satelite communication.doc (Size: 36.5 KB / Downloads: 131)
1. INTRODUCTION
In today’s communication plays very important role in all walks of life. The demands of communication have increase to such extent that a new word “Personal Communication has appeared on the scene”.When we talk to personal communication on a global basis we say that the communication satellite is a very simple concept which sometimes change the world. The credit of invention of modern satellite goes to the British scientist and fiction writer ARTHUR C. CLARK communication satellite is radio relay in the sky. Signals are send to its from antenna on the earth, it amplifies the signals and sends them back. Three satellite cover almost all the inhabited region of the earth. Satellite communication technology has proved to be the most advanced means of communication. Satellite can provide the capability of simultaneous transmission of many television voice and data channels. For economical effort the satellite expert arrived for conclusion that using non-Geo system economically viable.They considered three types of orbits, name low earth orbit ie LEO medium earth orbit or bit ie MEO and high elliptically earth orbit ie HEO.
2. SATELLITE ORBITS AND SYSTEM
Satellite are not stationary. They revolves around earth in certain orbit typical path of an orbiting satellite areEquatorialPolarInclinedThe orbit it self may be elliptical or circular. But satellite is mostly said to be two types according to its orbit and speed
.a. Geostationary Satellite
If the satellite orbits in an equatorial plane at exactly the same angular velocity as the earth rotates it will appear to be fixed at a specific point in the sky when viewed from the earth. Such a satellite is said to be Geo synchronous or geo-stationary satellite. For synchronous orbit to occur three times of orbiting satellite to complete one revolution must be equal to hours 56 minutes and 4 seconds. This will occur at an altitude of 35784 Km. For Geo-stationary satellites points on earth beyond about 80 altitude are not visible byt they require simple earth station, no tracking is required.
b. Inclined satellites
These satellite are other than Geo-stationary satellites but can provide visibility to the higher nor them as well as southern latitude.For inclined satellite earth stations have to continuously track satellite for the communication.SATELLITE SYSTEMSThere are three types of satellites systems
Ground-Ground
In this type or system an unlink ground based earth station to satellites and down link back to ground takes places that is communication between two remote earth station visible to the same satellites takes place.
Ground-Crosslink-Ground
This system consist of a satellite cross link between two satellites prior to down link transmission. Such system allow communication between earth station not visible to the same satellite. By spacing multiple satellite in proper orbit around the earth worldwide communication between remote earth station in different hemispheres can be performed via such cross link.
Ground-User relay
This systems is a satellite relay system involving earth station, near earth users like air craft, ship, cars etc. and satellites. An earth station communication to another earth station or to a near earth user by transmitting to relay satellite will have larger near earth visibility than the transmission earth station. A relay satellite allows communication to a wider range of users. The user responds by retranslating back through the satellite to the earth station.The link from earth station to relay to user is called forward link, while link from user to satellite back to the earth is called return link. 3. FUNDAMENTALS OF SATELLITE COMMUNICATIONA satellite communications systems basically consists of a satellite in space and many earth station on the ground which are linked with each other through the satellite. The components involved in satellite communication are ground segment equipment, free space and space segment.
Ground segment equipment
Ground segment equipment is basically a digital earth station whose functional block diagram is given in figure.The digital signal through the terrestrial network inters the base band equipment where it is processed like buffered, multiplexed, formatted etc. Error correction error rate to an acceptable level. There a buster it is modulated using intermediate frequency ie IF carrier to 70 mHz, a standard frequency in satellite communication for a communication channel using 36 MHz transpoder. The modulated a carrier is further modulated to a satellite uplink radio frequency ie RF carrier and is sent to HPA for transmission to satellite. Similarly, on the received by low noise amplifies which amplifiesit, keeping carrier-to-noise ration at an acceptable level to meet the error rate equipment. The down converter translates it to the IF level which is red to the demodulators where the digital steam at data is extracted. Personal earth station/micro earth station or central hub station are ground segment equipment working on the same basic principle.Free spaceFree space is the medium between the satellite and earth satellite station which others certain obstruction for RF in both the uplink and the down link paths. The transmitted received signals undergo energy loss as they pass though the free space, termed as free space less. This can be measured in terms of dB.
Free space loss dB = 32.4 + 20 log ‘d’ + 20 log f
Where,d – Maximum slant range in km f – up link of carrier frequency