The Satellite Data System (SDS) is a system of

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communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. C ...

s. At least three generations have been used: SDS-1 from 1976 to 1987; SDS-2 from 1989 to 1996; SDS-3 from 1998 to the present. It is believed that these satellites were known by the code name ''Quasar''. The first generation was named simply 'SDS', the second generation was named 'Quasar' and the third generation each had their own designations.

Orbital characteristics

SDS satellites have a highly elliptical

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

, going from about 300 kilometers at

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

to roughly 39,000 km at

apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

in order to allow communications with

polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates * Polar climate, the c ...

stations that cannot contact

geosynchronous A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbital ...

satellites. The high apogee meant that the polar regions were visible for long amounts of time, and only two satellites were required in order to achieve constant communications ability. In addition, two geostationary satellites appear to be part of the system. The SDS satellites were constructed by

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Mission

The primary purpose of the SDS satellites is to relay imagery from low-flying

reconnaissance satellite A reconnaissance satellite or intelligence satellite (commonly, although unofficially, referred to as a spy satellite) is an Earth observation satellite or communications satellite deployed for military or intelligence applications. The ...

s, notably the

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optical reconnaissance and Lacrosse/Onyx radar reconnaissance satellites to ground stations in the

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SDS-1

Each SDS-1 satellite had 12 channels available for

Ultra high frequency Ultra high frequency (UHF) is the ITU designation for radio frequency, radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one ten ...

(UHF) communication. They were

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in shape, roughly long. 980

watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...

s of electrical power were available from

solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...

s and

batteries Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...

. The SDS-1 had a mass of and was launched on Titan-3B rockets. The SDS-1 satellites had similar orbits to the Air Force's

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ELINT Signals intelligence (SIGINT) is intelligence-gathering by interception of ''signals'', whether communications between people (communications intelligence—abbreviated to COMINT) or from electronic signals not directly used in communication ( ...

satellites.National Reconnaissance Office: "NRO Brochure circa 1997"
/ref> It has been speculated that the early satellites served as data relays for the first

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reconnaissance satellites.

SDS-2

The SDS-2 is significantly more massive at , with three separate communication dishes, including one for a K-band downlink. Two dishes are in diameter, while the third is in diameter. The solar arrays generate 1238 watts of power. It is believed that the

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has been used to launch several satellites, possibly on missions

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STS-38 STS-38 was a Space Shuttle mission by NASA using the Space Shuttle ''Atlantis''. It was the 37th shuttle mission, and carried a classified payload for the U.S. Department of Defense (DoD). It was the seventh flight for ''Atlantis'' and the se ...

, and

STS-53 STS-53 was a NASA Space Shuttle ''Discovery'' mission in support of the United States Department of Defense (DoD). The mission was launched on December 2, 1992, from Kennedy Space Center, Florida. Crew Mission highlights ''Discovery'' c ...

. Other launches have used the

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launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pad, launch pads, supported by a missile launch contro ...

SDS-3

Satellites

References

* Vick, Berman, Lindborg, Fellow (March 19, 1997
SDS-1 Military Communications Satellite
''Federation of American Scientists'' Accessed April 24, 2004 * Vick, Berman, Lindborg, Fellow, Pike, Aftergood (March 19, 1997

''Federation of American Scientists'' Accessed April 24, 2004 {{NRO satellites Communications satellites Hughes Aircraft Company Military satellites