There must be three satellites around the earth at equal distance from each other.
⇒ A geostationary satellite has to be orbiting Earth in the equatorial plane, but travelling at the same speed as the Earth's rotation, so that it is constantly over the same spot on Earth. They have to stay at a fixed height, about 36,00 kilometres or 22,000 miles above the earth. They can directly communicate with a station on Earth, which can aim it's communications, through antennae, directly at the satellite, without having to adjust the antennae.
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Geostationary satellites are those satellites that revolve around the geostationary orbit. In India, ISRO has GSLV rocket which helps geostationary satellite to place in their orbit.
Two essential requisites of geostationary satellites are:It is used to detect any behaviour in the outer space.It function is to keep an eye on black holes, meteors, asteroids, planets and stars etc.