SSATELLITEATELLITE CCOMMUNICATIONOMMUNICATION

Introduction:- The use of satellite in communication systems has

become very common now - a- days. This is because the satellite can “see” a very large area of the earth.

Hence satellites can form a star point of a communication net, to link many users together, simultaneously. This will include users widely separated geographically.

A communication satellite is a station in space that is used for telecommunication, radio and television signals.

The construction and launch cost of a satellite are extremely high.

These costs are “distance insensitive”, that means the cost of a short distance satellite link is approximately same as that of a long distance link.

Therefore a satellite communication system is economical only where the system is used continuously and a large number of users use it.

HISTORY• 4th October 1957 first Satellite SPUTNIK-1 (Russia)

was launched.

• 3rd November 1957 launched Second satellite SPUTNIK-2 (Russia) carried with female DOG (Laika).

• 1958 first active communication satellite TELSTAR-1.

• 1960 ECHO 1(U.S).

• 1963 first geostationary satellite SYNCOM 2(NASA).

• 1965 The world's first commercial communications satellite was “Early Bird”, built for the Communications Satellite Corporation (COMSAT) 240 duplex telephone channels or 1 TV channel by huges.

• 19th April 1975 India’s first satellite launched by Aryabhatta.

• 1979 first mobile satellite telephone system INMARSAT.

• 1987 TVSAT first DBS- satellite (Direct Broadcast satellite, Television broadcasts directly to home).

• 1993 ACTS (NASA).

How does a satellite communication work?

1. Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.

2. An Earth Station sends message in GHz range. (Uplink)

3. Satellite Receive and retransmit signals back (Downlink)

4. Other Earth Stations receive message in useful strength area

. 5. Main Earth Station is in Delhi, Kolkata, Chennai, Mumbai and Shilling

TTYPESYPES OFOF S SATELLITEATELLITESatellites are divided in four major categories as follows:

Communication Satellite. Weather satellite. Remote-sensing satellite. Scientific satellite.

Communication satellite:- Geostationary satellites when used for the

communication application are called communication satellite.

They are used in applications such as point to point communication radio broadcasting, TV transmission, military application, Commercial application. Communications satellites are classified based on the coverage are as global, regional or domestic satellites.

A term “INTELSAT” is often used in satellite communication. INTELSAT means International Telecommunication Satellite.

These are communication satellites which are used for communication of telephony, computer data, TV

signals etc… More than 140 nations are interconnected via the INTELSAT system.

Weather satellite:-

These satellites are used for weather prediction, for taking the photographs of clouds. TIROS-1 was the first weather satellite. Other weather satellites are Nimbus and Meteosat.

India’s “INSAT” satellite is being used for three application i.e. communications. TV transmission and meteorology or weather prediction. Thus it is s multipurpose satellite.

Remote-sensing satellites:- This type of satellites can be used to find out the

condition of crops , forests or minerals underground , condition of soil etc. Indian satellite IRS is a remote sensing satellite.

From the information obtained from the remote sensing satellite detailed maps can be prepared.

Scientific Satellite:- These satellites are relatively simpler and of short life

span. They are used to carry out various scientific studies.

India’s “Aryabhatta” was a scientific satellite.

PPRINCIPLERINCIPLE OFOF SATELLITESATELLITE COMMUNICATIONCOMMUNICATION:-:-

A geostationary communication satellite is basically a relay station in space.

It receives signal from one earth station, amplifies it, improves the signal quality and radiate the signal back to other earth stations.

Such a relay system allows us to communicate with any corner of the world.

AAPPLICATIONPPLICATION OFOF SATELLITESATELLITE COMMUNICATIONCOMMUNICATION:-:-

The main application of satellite communication is in the field of communication. The communication of video signals (TV), audio signals (telephones, satellites phones) and computer data (internet).

To gain meteorological or weather information. The photographs taken by the satellites are analyzed for predicting weather.

To monitor the status of earth’s resources such as land, forests and oceans. We can get very important information about crops, lakes, rivers, forests, fire etc…

To spot our mineral resources, polluted areas, sources of pollution etc….

Satellite Orbits:- In space, satellites move in certain specific

paths. These paths are called as orbits. A result stays in an orbit because the two

forces acting on it namely the centripetal force and the gravitational force are equal.

The selection of a particular orbit depends on the following factor:

Transmission path loss. Earth coverage area. Delay time. Time period for which the satellite

should be visible.

Types of Satellite Orbits:-

1. Low Earth Orbit (LEO) satellites.2. Medium Earth Orbit (MEO) satellites.3. Geosynchronous Earth Orbit (GEO)

satellites.

Low Earth Orbit (LEO) satellites:

LEO is the short form of low earth orbit satellites. They revolve around the earth in the orbits

which are 500 to2000km above the earth. LEO satellites don’t stay in fixed position

relative to the surface, and are only visible for 15 to 20 minutes each pass.

The time taken for one revolution around the earth is about one to one and half hours.

Medium Earth Orbit (MEO) satellites:

The medium earth orbit satellites (MEO) are at much lower altitudes as compared to GEO satellite.

MEO satellites are similar to LEO satellites in functionality

Typically the MEO satellites operate in the range of 5000km to 15000km above the earth.

As observed from the earth, they drift slowly in longitude, taking about 6hours to circle the earth.

MEO satellites have a larger coverage area than LEO satellites.

Geosynchronous Earth Orbit (GEO) satellites:

The satellites orbiting in the geostationary orbit are called geostationary satellites. They travel at the velocity of revolution of earth, hence complete one revolution around the earth in one day i.e. 24hours.

This is the reason why geostationary satellites appear to be stationary.

These satellites are at about 36000km above the earth’s surface.

Satellite Applications Overview:

1. Systems using geostationary satellites:

Inmarsat (International Maritime Satellite Organization) used for voice, data, especially for ships worldwide except Polar Regions.

MSAT (Mobile Satellite) used for voice data, mainly for land mobile western hemisphere.

2. Systems using low-earth-orbit (LEO) satellite:

Iridium: major uses are voice, paging, low speed data.

Globalstar: major uses are voice paging, low speed date.

Teledesic: major uses high speed data, voice.

3. Systems using little LEO satellite: ORBCOMM: major uses are paging, short

messaging, e-mail, vehicle location. LEO: one major uses are paging, short

messaging, e-mail vehicle location. E-sat: major uses are remote meter

reading.

4. Systems using Medium earth orbit (MEO):

Ellipso: major uses are voice communication using portable and mobile terminals. ICO (Intermediate Circular Orbit): major uses are satellite to mobile links.

Advantages of Satellites:-

• The advantages of satellite communication over terrestrial communication are: The coverage area of a satellite greatly exceeds

that of a terrestrial system. Transmission cost of a satellite is independent of

the distance from the center of the coverage area.

Satellite to Satellite communication is very precise.

Higher Bandwidths are available for use.

Disadvantages of Satellites

The disadvantages of satellite communication:

Launching satellites into orbit is costly. Satellite bandwidth is gradually becoming

used up. There is a larger propagation delay in satellite

communication than in terrestrial communication.

How Satellites are used?

• Service Types:-

Fixed Service Satellites (FSS)

• Example: Point to Point Communication

Broadcast Service Satellites (BSS)

• Example: Satellite Television/Radio

• Also called Direct Broadcast Service (DBS).

Mobile Service Satellites (MSS)

• Example: Satellite Phones