original question: Which satellite is primary used for [Communications satellite]
answer: tdrs-k, palapa-d, indostar-1...
answer type: Satellite.
auxiliary information: A communications satellite is an artificial satellite that relays and amplifies certain telecommunications signals, acting as a middleman between a source and a receiver. These satellites are used for television, telephone, radio, internet, and military applications. As of May 11, 2015, there are over 2,000 communications satellites in Earth‚Äôs 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. In order to communicate over great distances, satellites are used to redirect the signal. There are two major classes of communications satellites. The first class is called the ‚Äúpassive satellite‚Äù. This type of satellite only redirects the signal coming from the source, and points in the direction of the receiver. With passive satellites, the transmitting signal has the tendency to be weak. This is because, as the electromagnetic wave moves through the atmosphere, particles will interrupt the wave and cause it to be muffled. Active satellites, on the other hand, allow for a signal that is much more clear.
Satellites that are primarily used for [Communications satellite] are?