What are the essential condition the for geostationay satellite?

Answer 1

Because of the inverse square law of gravity, an object close to the Earth's surface feels a greater pull than an object further away. This would mean an artificial satelite in an orbit near Earth would have to travel faster to remain in orbit. One further away would travel slower. Close to the earth, a satelite might complete an orbit in, for example, 90 minutes; but the earth rotates once on its axis in 24 hours. This would mean the satelite would always have to travel faster than the Earth spins. Too far away, and the satelite would take longer than a day to orbit the Earth - so the planet would spin faster than the satelite's orbit. For a geostationary satelite, it would need to be at just the right distance, in an orbit that keeps it at the same place as seen from the rotating Earth - orbiting as fast as the Earth is spinning.

Geostationary satelites get parked a little over 22,200 miles above the Earth's surface and in orbits the same direction as the Earth spins - and are thus useful for communication and weather functions.