Satellites are bodies that revolve around planets. All the planets in Solar System, except Mercury, have natural satellites. Moon is a natural satellite of the Earth. But we have artificial satellites launched by several countries.
The first artificial satellite was by name Sputinik-1 and launched by USSR on October 4, 1957. It carried a radio transmitter. The first American satellite to relay communications was Project Score in 1958. India launched its first artificial satellite Aryabhata from a USSR launching facility on 19th April, 1975.
This was followed by Bhaskara-I (7th June, 1979). After developing indigenous launch vehicle SLV-3, India launched 35 kg Rohini-I satellite (18th July, 1980) using a 4-stage SLV-3 vehicle followed by 2 more in the Rohini series. The next was Apple (Arianne Passenger Pay Load Experiment). These were followed by many satellites like Bhaskara-II and INSAT (Indian National Satellite) series which have been used for communication, TV and radio broadcasts.
In 1988, the first satellite of IRS series was launched aimed at serious remote sensing work and applications. Since then, India has successfully launched many satellites for remote sensing and communication.
Having satellites in space places us in a privileged position. If we are on ground there is a limit up to which Earth's features can be seen by us. But viewing Earth from a distance has an advantage. It allows us to look at up to half of the planet if the distance is sufficiently large. We can send electromagnetic signals to the other side of the globe through the satellites in space. Therefore, the artificial satellites have come to play a very important role in any country’s infrastructure. They serve very important role in communication, space research, survey of natural resources like minerals on Earth, weather prediction including movement of clouds, also change in course of rivers, and disaster monitoring (floods, cyclones, tsunami).
Communication becomes important for imparting education. The idea that satellites could be used for communication came from Arthur C Clarke in mid forties. That is the reason the geostationary (or geosynchronous) orbit is also called Clarke orbit. Clarke rose to become one of the greatest science fiction writers.
Electromagnetic waves sent from any part of Earth can't reach just any part of Earth. If sent downwards, they will be limited to a small distance due to curvature of Earth. If transmitted up, they will keep going straight and hit ionosphere, a layer of charges in the space, at 50 km and more above the ground. Then they will be reflected to the Earth and reach some part which is far away from the source. Thus, a huge area in-between will be a dark zone where the signal wouldn't reach.
Instead of the ionosphere, one may use satellites to retransmit the signals. But we need more than one satellite which can receive the signal sent from ground and re-transmit it in different directions. Therefore, it was thought that several satellites in space could together cover whole of Earth and facilitate communication.
A satellite's position and orbit are critical. The satellite has to be launched using a rocket, lifted into the correct orbit and given suitable energy and momentum in the right direction so that it keeps moving. A satellite could be geostationary which remains stationary with respect to Earth. A satellite in a geostationary orbit keeps moving at the same angular speed as Earth and in the same direction as rotation of Earth. So a geostationary satellite has a revolution time which equals the rotation time of Earth - 24 hours. It appears to be in a fixed position to an observer on Earth.
It can keep looking at the same spot on earth for a very long time, monitor the changes and transmit the data to ground station. Thus, to direct antennas towards the satellite to receive the signal, one doesn’t have to keep tracking a moving satellite. That would have demanded expensive instruments on ground for direct TV transmission. This means huge savings because it does away with the need for too many ground antennas.
Placing a satellite at 36,000 km has added advantage that it just falls under the gravitational pull of Earth and is energy - economical though it is more expensive to launch compared to low orbit satellites. Low orbit satellites are placed about 400 km above ground. But being low, they can see only a small portion of the ground below.
There are Polar Satellites that move over the poles. The remote sensing satellites have been placed in comparatively low (less than 1000 km high) orbits in contrast with communication satellites in geostationary satellites which move at 36,000 km above Earth. The remote sensing satellites should be launched such that they make observation at any place between 10 AM and 2 PM so that the ground is illuminated from the top and images come out clearer.
A geostationary satellite is useful for countries at low latitude such as India. The satellite is placed at an altitude of 36,000 km, going around in the plane of equator and making one revolution around Earth in 24 hours. However, as Earth also makes one rotation in 24 hours, the satellite looks at the same place on ground all the time. From this altitude, it can view about one-third of Earth. The signal is sent from ground to the satellite as microwave at certain frequency and the satellite re-transmits it to the other parts of Earth at a different (but still as microwave) frequency.
Microwave is at wavelengths of the order of a millionth of a meter. The highly directional antennas on Earth receive these microwave signals. Thus, satellites make it possible to send TV, radio signals to far away places on Earth, even on the other side of globe.