If we throw a stone with some speed in a horizontal direction, it follows a curved path as it falls to the ground. If the stone is thrown with a higher speed it follows a path of bigger radius as it falls. We thus conclude that the higher the speed of the stone, the greater the radius of the curved path. If somehow we could throw the stone with such tremendous speed that the radius of its path became a little greater than the radius of the earth, the stone would fall around the earth, rather than on it. This is the principle of an artificial satellite.

In the case of a satellite, the centripetal force is provided by the gravitational pull of the earth. We can calculate the speed of a satellite at a distance r from the centre of the earth by equating the centripetal force with the gravitational force. Thus if m is the mass of the satellite and g be the acceleration due to gravity, we have

Satellites are the objects that orbit around the earth. Any satellite is kept in its orbit by the gravitational attraction of the body about which it is rotating. Many man made satellites are in circular orbits around the earth and once in orbit they do not need rocket motors to keep them in orbit. They are sufficiently far away from the earth's surface to have very little air resistance. They can remain rotating about the earth for many years. Satellites may have circular or elliptical orbits. Circular orbits are easier to handle mathematically and there many orbits, which are approximately circular.

A satellite orbiting near the earth would have a velocity of 7.9 kms-1 and would take 84 minutes to orbit the earth. It is impossible to have a satellite going around the earth faster than this speed unless it can keep its rocket motors going all the time to increase the force on it towards the earth. In practice it is impossible as there would be to great need of fuel. The higher the satellite, the slower will be required speed and longer it will take to complete one revolution around the earth.

Satellites are basically are objects that orbit around the earth. They are put into orbit by the help of rockets and are held there by the gravitational pull of the earth. The low flying earth satellites have an acceleration of 9.8 m/s2 towards the centre of the earth. If they do not, they would fly off in a straight line towards the earth. The minimum velocity required to a satellite into orbit is known as critical velocity. The higher the satellite, the slower will the required speed be and the longer it will take to complete one round around the earth. The close orbiting satellites orbit the earth at a height of about 400Km. twenty four such satellites form the famous known Global Positioning System (GPS).

Several geostationary satellites are set-up in orbit at various places. Each one covers 120 of longitude. The earth can be covered by three satellites place in correct positions. These satellites receive their energy for transmissions and receptions from solar panels fitted on them. The largest satellite system is managed by 126 countries and is known as International Telecommunication Satellite Organization (INTELSAT). It operates at microwave frequencies of 4,6,11 and 14 GHz and has a capacity of 30,000 two way telephone circuits and 3 TV channels