A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver(s) at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are over 2,000 communications satellites in Earth’s orbit, used by both private and government organizations.
Wireless communication uses electromagnetic waves to carry signals. These waves require line-of-sight, and are thus obstructed by the curvature of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated points.
There are two major classes of communications satellites, passive and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, and only a very small amount of the transmitted energy reflects toward the receiver. Since the satellite is so far above Earth the radio signal is attenuated on its long journey due to free-space path loss, so the signal received on Earth is very weak. Active satellites, on the other hand, amplify the received signal before re-transmitting it to the receiver on the ground Passive satellites were the first communications satellites, but are little used now.
Communications satellites usually have one of three types of orbit (although several other types of orbits are sometimes used). Geostationary satellites have a geostationary orbit (GEO), which is 35,786 kilometres (22,236 mi) from Earth’s surface. This orbit has the special characteristic that the apparent position of the satellite in the sky when viewed by a ground observer does not change, the satellite appears to "stand still" in the sky. This is because the satellite's orbital period is the same as the rotation rate of the Earth. The advantage of this orbit is that ground antennas do not have to track the satellite across the sky, they can be fixed to point at the location in the sky the satellite appears.
Closer to the Earth is the medium Earth orbit (MEO). It ranges from 2,000–35,786 kilometres (1,243–22,236 mi) above Earth. Below MEO is low Earth orbit (LEO) and is about 160 to 2,000 kilometres (99 to 1,243 mi) above Earth. As satellites in MEO and LEO orbit the Earth faster, they cannot stay visible in the sky continually like a geostationary satellite, but appear to a ground observer to cross the sky and "set" while they go behind the Earth. Therefore, to provide continuous communications capability with these lower orbits requires more satellites, so one will always be in the sky when needed. However, due to their relatively small distance to the Earth their signals are stronger.
The electromagnetic signals that communication satellites work with, have a large spectrum of wavelengths and frequencies. To keep these waves from interfering with one another, international organizations have certain rules and regulations describing which wavelength a certain company or group can use. By separating out wavelengths, communication satellites will have minimal interference and be able to communicate effectively.