Answer:
Satellites are placed into orbit at least 150km above Earth's surface to ensure that they remain above the atmosphere.
The velocity required to keep a balance between the inertia of the motion of the satellite and the earth's gravitational pull, which is called the orbital velocity, essentially depends on its distance above the surface of the Earth. The higher this distance, the longer the satellite will stay in the orbit. Otherwise it might run into the atmosphere and could get burnt up there.