Basics of Satellite communication

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A brief history of space communication !
The idea of radio transmission through space was first conceived in 1911.In 1945, Arthur C. Clarke proposed the idea of using earth-orbiting satellite as a relay point for communication between two earth stations.This idea later led to the great success of satellite communications.In 1957, the Soviet Union launched Sputnik I , which transmitted telemetry signals for 21 days.In 1958, United States launched Explorer I, which transmitted telemetry signals for 5 months.

In July 1964, INTELSAT, a multinational organization, was formed with the purpose to design, develop, construct, establish, and maintain the operation of the space segment of a global commercial communications satellite system.
In April 1965, Early Bird (INTELSAT I), a geostationary communications satellite was launched.
In a span of seven years, four generations of satellites – INTELSAT I through IV were launched and placed in commercial operation.

Satellite communication utilizes electromagnetic waves to carry information from the ground to space and back. An electromagnetic wave consists of an electric field and a magnetic field that are perpendicular to each other and to the direction of propagation

The frequency of an electromagnetic wave is defined as the number of times it cycles in one second and is measured in Hertz (Hz).
1 Hertz = 1Hz (one hertz)
1,000 Hertz = 1kHz (one kilohertz)
1,000,000 Hertz = 1MHz (one megahertz)
1,000,000,000 Hertz = 1GHz (one gigahertz)

What is a Satellite?
A satellite is any 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 SATCOM parlance it’s a man- made machine orbiting Earth / Sun. Acting as a repeater in the sky, permitting communication over long distances at high bandwidths and relatively low cost.

Types of Satellites
Communications Satellites
Remote Sensing Satellites
Weather Satellites
GPS Satellites
Science Research Satellites – SOHO

Communications Satellites
Communications satellites act as relay stations in space.
Used for bouncing messages from one part of the world to another.
The messages can be telephone calls, TV pictures, or even Internet connections.

Remote Sensing Satellites
Remote-sensing satellites study Earth’s surface.
From 300 miles (480 km) up, these satellites use powerful cameras to scan the planet.
The instruments on remote-sensing satellites study Earth’s plant cover, chemical composition, and surface water, among many other features.
The satellite then sends back valuable data about global environments.
Farming, fishing, mining, and many other industries find this information very useful.
Remote sensing satellites can also be used to study changes in the Earth’s surface that are caused by people. Examples of this include the parts of West Africa that are turning into desert (desertification), and the destruction of the rainforest in South America (deforestation).

Weather Satellites
Also called TIROS (Television Infrared Observational Satellite).
It records weather patterns around the world. Many countries use TIROS data to forecast weather, track storms, and do scientific research.

GPS Satellites
GPS satellites are a group of satellites that can tell you your exact latitude, longitude, and altitude.
People everywhere are using these satellites to determine where in the world they are.
GPS satellites are used for navigation almost everywhere on Earth — in an airplane, boat, or car, on foot, in a remote wilderness, or in a big city.

Science Research Satellites
Many satellites in orbit conduct scientific experiments and observations.
For example SOHO, which studies the Sun.
Hubble Space Telescope is a satellite in orbit around Earth, and its mission is to study distant planets, stars and galaxies.

Satellite Orbits
Definition :
Orbit – path of a satellite around a body (e.g. the earth) which is stable under the influence of gravity.
Types :
Equatorial / Geosynchronous Orbit :
A satellite in geosynchronous equatorial orbit (GEO) is located directly above the equator, exactly 22,300 miles out in space. At that distance, it takes the satellite a full 24 hours to circle the planet. Since it takes Earth 24 hours to spin on in its axis, the satellite and Earth move together. So, a satellite in GEO always stays directly over the same spot on Earth.It is also called a Clarke Orbit or Geostationary Orbit.
Elliptical Orbit :
A satellite in elliptical orbit follows an oval-shaped path.
One part of the orbit is closest to the center of Earth (perigee) and the other part is farthest away (apogee).
A satellite in this orbit takes about 12 hours to circle the planet. Elliptical orbits move in a north-south direction. Also known as Super-synchronous Orbits or Molniya Orbits.
Polar Orbit :
A Polar orbit is a particular type of Low Earth Orbit. The only difference is that a satellite in polar orbit travels a north-south direction, rather than the more common east-west direction.
These orbits are at a typical height of 850 Km and a period of 100 minutes.
Polar Coverage
While most communications satellites are in Clarke Orbit, the footprints of GEO satellites do not cover the polar regions of Earth. So communications satellites in elliptical orbits cover the areas in the high northern and southern hemispheres that are not covered by GEO satellites.

Low Earth Orbit (LEO)
When a satellite circles close to Earth it is said to be in Low Earth Orbit (LEO).
Satellites in LEO are just 200 – 500 miles (320 – 800 kilometers) high.
Satellites in LEO speed along at 17,000 miles per hour (27,359 kilometers per hour)!
They circle Earth in about 90 minutes.
A Low Earth Orbit is useful because its nearness to Earth gives it spectacular views.
Remote Sensing and Weather satellites often travel in LEOs because from this height they can capture very detailed images of Earth’s surface.

For more articles on Satellite communication, look for other articles in satellite communication section!

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