Satellite Communication

SATELLITE COMMUNICATION

Communication is the exchange of information from one body to another, from one person to another person, from one place to another. It requires transmitter, receiver as well as the channel for communication. It varies accordingly with the mode of communication. This Mode of communication is too much evolved in recent centuries. Satellite communication plays an important role in the development of humanity. This technology has contributed a lot more in the field of communication. This technology has a lot more applications like television broadcasting, navigation, military applications, mobile communication, etc. In this article we will discuss about satellite communication, it's types and applications.

Satellite:

Satellite is an object which revolves around the planet in a certain orbit. It can be natural or artificial. But in this blog from the point of view of satellite communication, we are talking about artificial satellites. An artificial satellite is a human-made satellite that revolves around the earth, other planets or natural satellites. These satellites are mainly used for communication and research. Communication satellites are the artificial satellites which acts as a communication channel between source and receiver and which is mainly designed for data transfer purposes. It is used for mobile networking, the internet, television broadcasting, etc. Today about 2000 communication satellites are revolving in the earth's orbit.

Working:

The satellite communicates with the help of radio waves to transfer the data or signal to the receiver on the ground. The antennas or receivers receive these transmitted waves or signals and retrieve the information from these signals. In this mode of communication, there is a transmitter, satellite as a medium of communication and receiver or ground station. Transmitter situated on the ground sends the signal using radio waves or microwaves of wider range frequencies to the satellite. 

Communication sub-system of the satellite has different parts like transmit or receive antennas to transmit and receive a wide range of frequencies, transponders which is a group of transmitters and receivers. These transponders are used to amplify and retransmit the signal received from the ground station. It has a bandpass filter, a down converter, and an output amplifier. Satellites which operates in c - band has 12 transponders. 36MHz is mostly used bandwidth for transponders. Some satellites use 50 and 72 MHz bandwidth. The support sub-system of the satellite consists of structure, attitude and orbit control system, propulsion system, electric power system, thermal control, and telecommand system. Each one has its own specific function. A retransmitted signal is retrieved by receivers on the ground or earth stations. The main purpose of the Earth Station is to communicate with the satellite. It retrieves the signal when the satellite is within a line of sight of the earth station. One satellite can communicate with one or more than one earth station simultaneously.

Uplink and Downlink:

Satellite acts as a medium of communication between two ground stations. The link for the communication from the transmitter to the satellite is called uplink. The link for the communication form satellite to the ground station is called the downlink. When the transmitted signal is received by the satellite, simultaneously earth station receives a retransmitted signal. So we can call the satellite communication as a two-way communication.

Uses of Communication Satellite:

The Satellite communication is widely used for:

Ø  Television broadcasting

Ø  Telecommunication

Ø  Mobile networking

Ø  Internet Communication.

Ø  Satellite tracking and security

Ø  Military operations

Ø  Satellite Navigation

Ø  Climate monitoring

Ø  Global information system.

Ø  Direct broadcast satellite TV

Satellite Communication VS Terrestrial Communication:

The coverage area of satellite communication is very larger as compared with that of terrestrial communication. The cost for the data transmission doesn't depend on distance from the centre of coverage area. The accuracy of this mode of communication is much larger than terrestrial communication. The loss of data is very less in this case. Also in satellite communication, higher bandwidths are available for data transmission.

Types of Communication Satellites:

Depending upon services provided by the satellite there are three types of satellite:

A.     Fixed Service Satellite (FSS): It is the type of satellite which is mostly used for telephone calling and transmitting television signal. This type of satellite generally has a low power output and larger dish antennas. e.g. point to point Communication.

B.     Broadcast service satellites (BSS): This type of satellite is designed for radio communication service in which retransmitted signals by satellite are directly received by the general public. DTH or television broadcasting is the best example of broadcast satellite service.

C.     Mobile service Satellite (MSS): This type of satellite is used for communication between mobile earth station and space stations or the communication between more than one mobile earth stations or space stations.

Frequency bands used in satellite communication:

The main resource in satellite communication is spectrum. In this mode of communication higher range of frequencies are available. The spectrum available for communication increases with the frequency. The following table describes the frequency bands used for satellite communication.

Name of Band	Frequency	Application
L – Band	1.0 – 2.0 GHz	Mobile Satellite Services
S – Band	1.55 – 3.9 GHz	MSS, DARS – XM
C – Band	3.7 – 6.2 GHz	FSS, VSAT
X – Band	8.0 – 12.0 GHz	Military / Satellite Imagery
Ku – Band	11.7 – 14.5 GHz	FSS, DBS, VSAT
Ka – Band	17.7 – 21.2 GHz and 27.5 – 31 GHz	FSS broadband and Inter - satellite

Among these frequency bands, C - band, Ka - band and Ku - band are widely used in satellite communication. C - band is mainly used in FSS satellites. In this case, transmission depends upon atmospheric conditions. Ku - band generally used in VSAT. It is used in internet trunking and video transmission. In this case size of the antenna at the ground station is comparatively smaller than that of c - band. Ka - band has high-frequency bandwidth of 18 - 30 GHz. It is widely used for broadband and multimedia application. Due to higher bandwidth, providing services like high-speed internet, multimedia transmission is possible. Transmission of Ka - band is not too much affected by weather conditions when compared with Ku and C band.

Orbits of the satellite:

The specific path followed by satellite while revolving around the planet is called the orbit. The communication satellite revolves around the earth in a specific orbit. Satellites are classified into three types depending on the orbit in which they are revolving. The following table describes these satellite orbits.

Orbit	Altitude Range (km)	Period
Lower Earth Orbit (LEO)	150 – 1000	1.5 – 1.8
Medium Earth Orbit (MEO)	5000 – 10000	3.5 – 6
Geostationary Earth Orbit (GEO)	36000	24

1. Geostationary Earth Obit (GEO):

Satellite in this orbit revolves around the earth with the same speed of earth' rotation speed so it appears stationary from the earth’s surface that's why the orbit called geostationary orbit. The coverage area of the GEO satellite is very large due to its distance from the earth's surface. This orbit is a circular orbit in the equatorial plane above the equator. The angular velocity of the GEO satellite is the same as that of earth.

2. Lower Earth Orbit (LEO):

Distance of the Satellites in this orbit is very less than that of the GEO satellite. Satellite doesn't remain in a fixed position relative to the earth like GEO satellites. Two satellite networks of IRIDIUM consisting of 66 satellites and GLOBALSTAR consisting of 48 satellites lie in this orbit. These satellites have a smaller area of coverage with radius is about 8000 km and smaller bandwidth. This orbit is slightly elliptical in shape with the period is equal to 1.5 to 2 hours.

3.  Medium Earth Orbit (MEO):

This orbit is circular with a period of 6 hours. This obit contains satellite network ICO with 10 satellites. The visibility of the satellite in this orbit is longer than that of LEO satellites. The strength of the signal received in the case of MEO satellites is lower than that of the LEO satellite but better than the GEO satellite. The diameter of the coverage area of the MEO satellite is 10000 to 15000.

Disadvantages of the satellite communication:

This mode of communication is expensive to launch and maintain. Only the government and some larger private organizations can afford this. Satellite communication may affect due to poor weather and atmospheric conditions. This may lead to signal degradation and noise may enter in transmission due to the atmosphere. Meteors or rocks in space may harm these satellites.

So this satellite communication has a lot more advantages and applications over the disadvantages. This is an excellent mode of communication till nowadays due to its feasibility and compatibility. So satellite communication is the widely used and beneficiary mode of communication due to its speed and other advantages.