TheSatellite Data System (SDS) is a system of United Statesmilitary 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 nameQuasar.^[1] The first generation was named simply 'SDS', the second generation was named 'Quasar' and the third generation each had their own designations.^[2]

SDS satellites have ahighly elliptical orbit, going from about 300 kilometers atperigee to roughly 39,000 km atapogee in order to allow communications withpolar stations that cannot contactgeosynchronous 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 byHughes Aircraft Company.

The primary purpose of the SDS satellites is to relay imagery from low-flyingreconnaissance satellites, notably theKeyhole optical reconnaissance andLacrosse/Onyx radar reconnaissance satellites to ground stations in theUnited States.^[3]

Each SDS-1 satellite had 12 channels available forUltra high frequency (UHF) communication. They werecylindrical in shape, roughly 25 ft (7.6 m) long. 980watts of electrical power were available fromsolar panels andbatteries. The SDS-1 had a mass of 630 kg (1,390 lb) and was launched onTitan-3B rockets. The SDS-1 satellites had similar orbits to the Air Force'sJumpseatELINT satellites.^[4]

It has been speculated that the early satellites served as data relays for the firstKH-11 Kennen reconnaissance satellites.^[5]

The SDS-2 is significantly more massive at 2,335 kg (5,148 lb), with three separate communication dishes, including one for aK-band downlink. Two dishes are 4.5 m (15 ft) in diameter, while the third is 2 m (6 ft 7 in) in diameter. The solar arrays generate 1238 watts of power. It is believed that theSpace Shuttle has been used to launch several satellites, possibly on missionsSTS-28,STS-38, andSTS-53. Other launches have used theTitan IVlaunch vehicle.

• Vick, Berman, Lindborg, Fellow (March 19, 1997)SDS-1 Military Communications SatelliteArchived 28 August 2016 at theWayback MachineFederation of American Scientists Accessed April 24, 2004
• Vick, Berman, Lindborg, Fellow, Pike, Aftergood (March 19, 1997)SDS-2 Military Communications SatelliteArchived 20 September 2016 at theWayback MachineFederation of American Scientists Accessed April 24, 2004

• Communications satellites
• Hughes Aircraft Company
• Military satellites

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