Satellite communication system
Abstract:
This paper mainly focuses on satellite through each & every Human. Satellite is the wireless medium. Satellite that oribit the earth do so as a result of the balance between centrifugal and gravitational forces.
A communication satellite is a spacecrafy that carries aboard communication equipment ,enabling a communication page link to be established between distant points .The need to carry fuel imposes one of the major must limitation on the useful life of a satellite.
In addition the satellite receives energy to power the electronic equipment on board. This is invariable supplied by solar cells. By attitude is meant the satellite orientation in space .Attitude control is necessary to keep the directional antennas aboard the satellite pointing to desired region of the earth . A satellite attitude can be altered along one or more of three axes, termel the roll , pitch & yaw axes.
Introduction
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. In this document, we will examine human-made satellites that orbit Earth. 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. Similar and maybe more familiar examples of wireless systems are radio and television broadcasting and mobile and cordless telephones. Systems of this type rely on a network of ground-based transmitters and receivers. They are commonly referred to as 'terrestrial' systems as opposed to satellite systems.
Satellite communication systems differ from terrestrial systems in that the transmitter is not based on the ground but in the sky: the transmitter here consists of a ground-based part called the uplink, and the satellite-based part that 'reflects' the signals towards the receivers. This part is called the transponder.
Purpose
Satellites come in many shapes and sizes and have many uses. The first artificial satellite, called Sputnik, was launched by the Soviet Union in 1957 and was the size of a basketball. Its purpose was simply to transmit a Morse code signal repeatedly. In contrast, modern satellites can receive and transmit hundreds of signals at the same time, from simple digital data to complex television programmes. They are used for many purposes such as television broadcasting, amateur radio communications, Internet communications, weather forecasting and Global Positioning Systems (GPS).
Basics of satellite communications
This chapter introduces satellite communication technology from a non-specialist point of view. The basic principles will be introduced along with a list of generic functions that satellite technology can perform (broadcast, unicast, bi-directional, multicasting). The chapter traces a broad history of satellite technology and discusses the developments currently taking place, covering broadcast type functions and additional applications and services.
It will further introduce general trends within the overall ICT sector that have an influence on the evolution of satellite technology. Although some issues and topics may not seem directly related to the use of satellites in an educational context, it is important to understand the fundamentals of the technology. This chapter is not the easiest part of the report and the reader may consider skipping to the following chapter which discusses the practical applications and return to this chapter at a later stage. Both chapters are self contained and can be read independently of the rest of the report.
A Mobile Services
In February of 1976 COMSAT launched a new kind of satellite, MARISAT, to provide mobile services to the United States Navy and other maritime customers. In the early 1980s the Europeans launched the MARECS series to provide the same services. In 1979 the UN International Maritime Organization sponsored the establishment of the International Maritime Satellite Organization (INMARSAT) in a manner similar to INTELSAT. INMARSAT initially leased the MARISAT and MARECS satellite transponders, but in October of 1990 it launched the first of its own satellites, INMARSAT II F-1. The third generation, INMARSAT III, has already been launched.
An aeronautical satellite was proposed in the mid-1970s. A contract was awarded to General Electric to build the satellite, but it was canceled--INMARSAT now provides this service. Although INMARSAT was initially conceived as a method of providing telephone service and traffic-monitoring services on ships at sea, it has provided much more. The journalist with a briefcase phone has been ubiquitous for some time, but the Gulf War brought this technology to the public eye.The United States and Canada discussed a North American Mobile Satellite for some time. In the next year the first MSAT satellite, in which AMSC (U.S.) and TMI (Canada) cooperate, will be launched providing mobile telephone service via satellite to all of North America.
Passive satellites:
It is just a plastic balloon having a metal coated over it. This sphere reflects the coming microwave signals coming from one part of the earth to other part. This is also known as passive sphere. Our earth also has a passive satellite i.e. moon.
Active satellites:
It basically does the work of amplifying the microwave signals coming. In active satellites an antenna system, transmitter, power supply and a receiver is used. These satellites are also called as transponders. The transmitters fitted on the earth generate the microwaves. These rays are received by the transponders attached to the satellite. Then after amplifying, these signals are transmitted back to earth. This sending can be done at the same time or after some delay. These amplified signals are stored in the memory of the satellites, when earth properly faces the satellite. Then the satellite starts sending the signals to earth. Some active satellites also have programming and recording features. Then these recording can be easily played and watched. The first active satellite was launched by Russia in 1957. The signals coming from the satellite when reach the earth, are of very low intensity. Their amplification is done by the receivers themselves. After amplification these become available for further use.
Microwave communication is possible only if the position of satellite becomes stationary with respect to the position of earth. So, these types of satellites are known as geostationary satellites.
What are the requirements for a satellite to be geostationary?
1. Its revolutionary direction must be same as that of the earth, i.e. from west to east.
2. The time period of satellite’s revolution must be same to the time period of the rotation of earth along its polar axis, which is equal to 24 hours.
3. The equatorial plane of earth must be coplanar with the orbital plane of the satellites revolution.
The name given to the orbit of the geo-stationary satellites is synchronous orbit. Due to this geo-stationary satellites are also called as geo-synchronous satellites. Geo-synchronous orbit is at a height of nearly 36000km from the surface of earth.
These orbits are capable of giving a successful communication page link between two stations present on the earth. These satellites can handle communication up to large distances. But it is impossible for a single geo-stationary satellite to cover the whole earth and provide a communication link. Due to curvature of earth the stations will be out of sight after covering some distance. If we want to cover the whole earth then we have to put three satellites onto the geosynchronous orbit. These satellites can cover the earth if all are inclined at an angle of 120o to each other.