GLOBAL SATELLITE COMMUNICATION SYSTEM

GLOBAL SATELLITE COMMUNICATION SYSTEM

A satellite is an object that orbits or revolves around another object. For example, the Moon is a satellite of Earth and Earth is a satellite of the Sun. The idea of satellite communication was concerned by Arthur C Clark in 1945. It was also stated by Clark that the electrical power for the satellite would be obtained by conversion of the sun's radiation by means of solar cells.

 In December, 1958 first man-made satellite called sputnik-I was launched. The communication satellite corporation (comsat) was established on 1993 in USA and then SYNCOM series of communication satellite were launched. They are highly specialized wireless receiver/transmitters that are launched by a rocket and placed in orbit around the Earth. There are hundreds of satellites currently in operation. Satellite communication is one particular example of wireless communication systems. They are used for many purposes such as mobile commmunication, television broadcasting, amateur radio communications, Internet communications, weather forecasting and Global Positioning Systems (GPS).

HISTORY OF COMMUNICATION

 In the early days, people used to send messages through trained pigeons. However, this mean of communication was not reliable. Therefore, this method also proved to be quite impracticable. As a remedy, the messenger was provided a horse so that he could move between two destinations quite swiftly. Thus, providing a horse to the messenger, the task of communication became much easier and quicker. As a matter of fact, this method overcame the problem of sending the messages from one place to other only to some extent. However, it laid the foundation of modern method of communication .With the advent of electronics, the messages in the form of electronic signals are made to sit on high frequency carrier waves (in place of putting a man on the back of the horse) and the person at the other end can pick the messages with the help of some electronic gadget. This mean of communication is known as radio communication. The tremendous advancement has been made in technology in the recent past and it has led to the development of a number of useful communication systems. Table gives a brief account of the advancement made with the passage of time in the history of communication. The above mentioned invention have led us to communication the messages instantly and efficiently. For example, telephone enables us to talk to any person anywhere in the world just by dialing a number. The television sets bring audio and visual information about the event happening anywhere in the world. Earlier, people located far away could communication with each other by gathering at the place of meeting. It meant long distance travel and thus wastage of a lot of time. However, video phone and satellite conferencing make such meetings between the concerned persons without having them to travel. Fax, e-mail, mobile etc. have brought a total revolution in the field of communication. 

A communication satellite is essentially a microwave link repeater. The communication satellite receives the energy beamed up at it by an earth station and amplifies it and returns to earth surface. The singal which is beamed up towards the satellite from earth station is called uplink and the signal which is beamed down towards earth's surface from satellite is called downlink. There is a frequency difference between the uplink and downlink of 2 MHz, so that there is no interference between the single of uplink and downlink. Applications of Satellite Communication are as given below:

1. Advances in satellite technology have given rise to a healthy satellite services sector that provides va rious services to broadcasters, Internet service providers (ISPs), governments, the military, and other sectors. There are three types of communication services that satellites provide: telecommunications, broadcasting, and data commu-nications. Telecommunication services include telephone calls and services provided to telephone companies, as well as wireless, mobile, and cellular network providers. Broadcasting services include radio and television delivered directly to the consumer and mobile broadcasting services. DTH, or satellite television, services (such as the DirecTV and DISH Network services in the United States) are received directly by households. 

2. Remote-sensing satellites study the surface of the Earth. From a relatively low height (480 km) up, these satellites use powerful cameras to scan the planet. The satellite then transmits valuable data on the global environment to researchers, governments, and businesses including those working in map making, farming, fishing, mining and many other industries.  

3. Remote-sensing satellites are also used to study changes in the Earth's surface caused by human activity. Examples of this kind of observation include investigations into presence of ozone and greenhouse gasses in the atmosphere, the desertification in West Africa and deforestation in South America .                                                . 

4. Weather satellites record weather patterns around the world. Almost all countries use the data coming from satellites like TIROS (Television Infrared Observational Satellite) ENVISAT to forecast weather, track storms and carry out scientific research.

5. Satellite communications technology is often used during natural disasters and emerg-encies when land-based communication services are down. Mobile satellite equipment can be deployed to disaster areas to provide emergency communication services.

6. The military have developed the Global Positioning System (GPS), but now people are using these satellite services to determine their exact latitude, longitude and altitude wherever they are in the world.                                   .

7. GPS satellites can be used for navigation almost everywhere on Earth: in an airplane, boat, or car, on foot, in a remote wilderness, or in a big city. GPS uses radio signals from at least three satellites in sight to calculate the position of the receiver.

8. Military government institutions make extensive use of satellites for a mixture of communication, remote sensing, imaging, positioning and other services, as well as for more secret applications such as spying or missile guidance.