Editing Spacecraft (section)

=== Communications satellite ===
{{Main|Communications satellite}}

A communications satellite is an [[artificial satellite]] that relays and amplifies [[radio]] telecommunication signals via a [[Transponder (satellite communications)|transponder]]; it creates a [[communication channel]] between a source [[transmitter]] and a [[Radio receiver|receiver]] at different locations on [[Earth]]. Communications satellites are used for [[television]], [[telephone]], [[radio]], [[internet]], and [[military]] applications.<ref>{{cite encyclopedia |title=satellite communication |encyclopedia=Britannica.com |url=https://www.britannica.com/EBchecked/topic/524891/satellite-communication |access-date=2016-02-10 |last=Labrador |first=Virgil |date=2015-02-19}}</ref>  Many communications satellites are in [[geostationary orbit]] {{convert|22,300|mi|km}} above the [[equator]], so that the satellite appears stationary at the same point in the sky; therefore the [[satellite dish]] antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite. Others form [[satellite constellation]]s in [[low Earth orbit]], where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.

The high frequency [[radio wave]]s used for telecommunications links travel by [[Line-of-sight propagation|line of sight]] and so are obstructed by the curve of the Earth.  The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points.<ref>{{cite web |title=Satellites - Communication Satellites |url=http://satellites.spacesim.org/english/function/communic/index.html |access-date=2016-02-10 |publisher=Satellites.spacesim.org}}</ref>  Communications satellites use a wide range of radio and [[microwave]] [[frequencies]]. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.<ref name="aerospace.org">{{cite web |date=2010-04-01 |title=Military Satellite Communications Fundamentals &#124; The Aerospace Corporation |url=http://www.aerospace.org/2013/12/12/military-satellite-communications-fundamentals/ |url-status=dead |archive-url=https://web.archive.org/web/20150905170449/http://www.aerospace.org/2013/12/12/military-satellite-communications-fundamentals/ |archive-date=2015-09-05 |access-date=2016-02-10 |website=Aerospace}}</ref>