Line-of-sight (LOS) communication requires that the transmitter and receiver be in direct view of each other without any obstacles in between.
Space waves, which include microwave and UHF radio waves, travel in straight lines and are suitable for LOS communication such as TV transmission, satellite links, and mobile phones.
Sky waves reflect off the ionosphere, and ground waves follow the curvature of the Earth, both of which are not line-of-sight.