 Among the hundreds of satellites circling Earth are those that relay phone conversations, radio and television broadcasts around the globe. Most of these communication satellites are in what is called the geostationary orbit, 35,786 km directly above the equator. This means that they complete an orbit in exactly one day and so always stay in the same position relative to the planet as it turns. The advantage is that antennas on the ground do not have to move to track a satellite, but it also means that this part of space becomes crowded and because satellites must be kept a certain distance apart to avoid radio interference only a limited number of orbital slots is available. To solve this challenge, ITU is where positions for satellites are allocated and the use of particular radio frequencies is determined. Satellite communication systems consist of a ground station that sends signals by high-powered high-frequency radio. The satellite receiving the signals transmits them back to other ground stations that are within its footprint. The geostationary orbit is sometimes known as the Clark Belt after the British writer Arthur C. Clark. In 1945 he proposed using the orbit for global communications building on the ideas of earlier scientists. Orbits that are not geostationary are also used for communications. They involve constellations of satellites circling closer to Earth. More satellites are needed, but each is less expensive and relays signals more quickly.