The Satellite Data System (SDS) is a system of

United States military The United States Armed Forces are the military forces of the United States. The armed forces consists of six service branches: the Army, Marine Corps, Navy, Air Force, Space Force, and Coast Guard. The president of the United States is th ...

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 ...

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 ...

, 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 ell ...

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 ell ...

in order to allow communications with polar 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 Hughes Aircraft Company.

Mission

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

Keyhole A lock is a mechanical or electronic fastening device that is released by a physical object (such as a key, keycard, fingerprint, RFID card, security token or coin), by supplying secret information (such as a number or letter permutation or pas ...

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 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 tenth of a meter ( ...

(UHF) communication. They were cylindrical in shape, roughly long. 980

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s of electrical power were available from solar panels and batteries. 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

KH-11 Kennen The KH-11 KENNEN (later renamed CRYSTAL,p.199-200 then Evolved Enhanced CRYSTAL System, and codenamed 1010 and Key Hole) is a type of reconnaissance satellite first launched by the American National Reconnaissance Office (NRO) in December 1 ...

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 STS-28, STS-38, and STS-53. Other launches have used the

Titan IV Titan IV was a family of heavy-lift space launch vehicles developed by Martin Marietta and operated by the United States Air Force from 1989 to 2005. Launches were conducted from Cape Canaveral Air Force Station, Florida and Vandenberg Air Forc ...

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 pads, supported by a launch control center and ...

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