The Satellite Data System (SDS) is a system of United States military communications satellites. 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, going from about 300 kilometers at perigee to roughly 39,000 km at apogee in order to allow communications with polar stations that cannot contact geosynchronous 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 The Hughes Aircraft Company was a major American aerospace and defense contractor founded on February 14, 1934 by Howard Hughes in Glendale, California, as a division of Hughes Tool Company. The company was known for producing, among other produ ...

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 Keyhole optical reconnaissance and Lacrosse/Onyx radar reconnaissance satellites to ground stations in the United States.

SDS-1

Each SDS-1 satellite had 12 channels available for Ultra high frequency (UHF) communication. They were

cylindrical A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...

in shape, roughly long. 980 watts 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 Jumpseat ELINT 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 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 Space Shuttle has been used to launch several satellites, possibly on missions

STS-28 STS-28 was the 30th NASA Space Shuttle mission, the fourth shuttle mission dedicated to United States Department of Defense (DoD) purposes, and the eighth flight of Space Shuttle ''Columbia''. The mission launched on August 8, 1989, and travel ...

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 Titan IV launch vehicle.

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