SDS 1, 2, 3, 4, 5, 6, 7

Satellite Data System (SDS) spacecraft are communication relay satellites for transmiting real-time data from US reconnaissance satellites (KH-11) in polar area. They are also used for communications to USAF air craft on polar routes and connect the various ground stations of the Air Force Satellite Control Network (AFSCN). The spacecraft relay the downlinked KH-11 data to a ground station at Fort Belvoir, Virginia.

The first generation SDS satellites were launched from 1976 to 1987. Technical details have not officially published, so the following information might be based on speculation. The SDS-1 has a mass of about 628 kg, and possibly were based on a version of the Hughes spin stabilized Intelsat IV bus called HS-350. The main transmitting antenna of the SDS-1 was over 10 feet in diameter. The primary electrical power for the SDS-1 spacecraft was provided by solar charged batteries with a total of 980 watts of power. SDS-1 uses an elliptical 500 × 39200 km orbit at an inclination of 57° (Molniya-type) which have their apogee in the northern hemisphere for long relay durations in northern areas, which are not visible to geostationary satellites. A constellation of three SDS-1 satellites is needed for constant polar communications coverage. Each SDS-1 spacecraft had reportedly 12 communications channels in the ultra-high frequency bands.