page_title: Radio
section_title: Space communication
hierarchical_section_title: Radio / Applications / Space communication
caption_attribution_description: Русский: ЦКС Дубна
context_page_description: Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz and 300 gigahertz. They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by a radio receiver connected to another antenna. Radio is very widely used in modern technology, in radio communication, radar, radio navigation, remote control, remote sensing and other applications.
  In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking and satellite communication among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal in the transmitter. In radar, used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location.
context_section_description: This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft.  
  Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft.  In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic "dish" antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwave band are used, since microwaves pass through the ionosphere without refraction, and at microwave frequencies the high gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF, L, C, S, kᵤ and kₐ band are allocated for space communication.   A radio link which transmits data from the Earth's surface to a spacecraft is called an uplink, while a link which transmits data from the spacecraft to the ground is called a downlink.
  Communication satellite – an artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth.  These are used because the microwaves used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth.  There are currently over 2000 communication satellites in orbit around the Earth.  Most are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it.  In a satellite ground station a microwave transmitter and large satellite dish antenna transmits a microwave uplink beam to the satellite.  The uplink signal carries many channels of telecommunications traffic, such as long-distance telephone calls, television programs, and internet signals, using a technique called frequency-division multiplexing (FDM).  On the satellite a transponder receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first.  There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines.  Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.
  Direct broadcast satellite – a geostationary communication satellite that transmits retail programming directly to receivers in subscriber's homes and vehicles on Earth, in satellite radio and TV systems.  It uses a higher transmitter power than other communication satellites, to allow the signal to be received by consumers with a small unobtrusive antenna.  For example, satellite television uses downlink frequencies from 12.2 to 12.7 GHz in the kᵤ band transmitted at 100 to 250 watts, which can be received by relatively small 43–80 cm (17–31 in) satellite dishes mounted on the outside of buildings.
caption: Russian satellite ground station
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