 Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere. One as a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. The understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international short-wainth broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems. Several different types of propagation are used in practical radio transmission systems. Line of sight propagation means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used to medium-range radio transmissions such as cell phones, cordless phones, walkie-talkies, wireless networks, FM radio and televisions broadcasting and radar, and satellite communication, such as satellite television. Line of sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible at microwave frequencies and above. At microwave frequencies, moisture in the atmosphere rain fake and degrade transmission. At lower frequencies in the MF, LF, and VLF bands, due to diffraction radio waves can bend over obstacles like hills, and travel beyond the horizon and surface waves which follow the contour of the Earth. These are called ground waves. SAM broadcasting stations use ground waves to cover their listening areas. As the frequency gets lower, the attenuation with distance decreases, so very low frequency VLF and extremely low frequency DELF ground waves can be used to communicate worldwide. VLF and LF waves can penetrate significant distances through water and Earth, and these frequencies are used for mine communication and military communication with submerged submarines. At medium waves and short-way frequencies MF and HF bands radio waves can refract from a layer of charged particles ions high in the atmosphere, called the ionosphere. This means that radio waves transmitted at an angle into the sky can be reflected back to Earth beyond the horizon, at great distances, even transcontinental distances. This is called Skywave Propagation. It is used by amateur radio operators to talk to other countries, and short-way broadcasting stations that broadcast internationally. Skywave communication is variable, dependent on conditions in the upper atmosphere, it is most reliable at night and in the winter. Due to its unreliability, since the advent of communication satellites in the 1960s, many long-range communication needs that previously used Skywaves now use satellites. In addition, there are several less common radio propagation mechanisms, such as tropospheric scattering troposkatter and near-vertical incidence Skywave NVIs which are used in specialized communication systems.