A satellite is an object that moves in a curved path around a planet. There are natural satellites like moon that orbits the earth and many other satellites that revolve around the other known planets. Apart from natural satellites, there are also man-made satellites which are launched in a desired orbit with the help of launch vehicle called a rocket.
Satellites come in different sizes and are used for different applications. There are used for weather prediction, communication, television broadcast, military services, navigation for planes and ships, rescue in case of distress and observation of changes in planet earth.
A satellite is a communication system that interprets signals from earth and responds back. It is subjected to radiations and extreme temperature in the orbit where it gets launched. There are three most important systems in a satellite – communication system, propulsion system and power system. The power system consists of solar panels, batteries etc which power the satellite. The communication system consists of antennas and transponders that help in reception and transmission of radio signals in the outer space. Remember that the radio signals travel in vacuum. The propulsion system which helps in placing the satellite in the correct orbital position includes the rocket and thrusters. The thrusters help in making adjustments in its position which may be caused due to the effect of gravitational pull of the sun and the moon. The life of a satellite depends on the amount of fuel it has got to power its thrusters.
The antennas that we see on the ground are the equipments which communicate with the satellite from earth. They transmit or send radio signals which are received by the communication equipments in the satellite. On receiving these signals, the satellite makes them stronger, and then re-transmits them back down to the Earth.