An unmanned artificial satellite sent by rocket into a geostationary orbit (see synchronous orbit) around the earth to enable television broadcasts and telephone communications to be made between points on the earth’s surface that could not otherwise communicate by radio owing to the earth’s curvature. Modulated microwaves are transmitted to the satellite, which amplifies them and retransmits them at a different frequency to the receiving station. The satellites are powered by solar cells. Three or more satellites in equatorial orbits can provide a world-wide communications linkage. The satellites are placed well above the ionosphere (see earth’s atmosphere) and therefore the carrier waves used have to be in the microwave region of the spectrum in order to pass through the ionosphere.