Radio signals do not all travel the same way, the path a signal follows depends heavily on its frequency. Very low and medium frequency signals hug the ground and are called ground waves, while medium and high frequency signals can bounce off a charged layer high in the atmosphere called the ionosphere, and this bouncing path is called a sky wave. Satellite communication, however, uses very high frequency, ultra high frequency, or even higher microwave bands, and waves at these frequencies simply pass through the ionosphere instead of reflecting off it. Because of this, a satellite signal cannot rely on ground hugging or ionospheric bouncing, it needs a clear, unobstructed, essentially straight path between the ground antenna and the satellite in space, or between two satellites. This direct, line of sight style of travel is what is called a space wave. So the propagation mode responsible for carrying satellite communication signals is the space wave.