Quasar 1, 2, 3, 4, 5, 6, 7 (SDS 1, 2, 3, 4, 5, 6, 7)

SDS (Quasar) [NRO]

Satellite Data System (SDS) spacecraft are communication relay satellites for transmiting real-time data from US reconnaissance satellites (KH-11 Kennen / Crystal). 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 relayed the downlinked KH-11 data to a ground station at Fort Belvoir, Virginia.

The SDS system was developed by the US Air Forces SAMSO, not by the NRO, for both the Air Force had experience with communication satellites and to shroud the connection to the KH-11 program. 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.

For the relay role, SDS carried a payload communicating at a frequency of 60 GHz, which does not penetrate the Earth’s atmosphere, with the KH-11 satellites. The code-name QUASAR has been confirmed by leaked budgets in 2013 to be associated with these satellites.

Besides the KH-11 relay role, some secondary payloads were considered. Originally SDS was also considered to relay DSP early warning data, but this requirement was dropped early. A small S-band transponder was carried to relay communications between the headquarters of the Air Force Satellite Control and a remote tracking station at Thule, Greenland, ending reliance upon balky land lines. SIOP (Single Integrated Operational Plan) was a large payload for communications with Strategic Air Command B-52 Stratofortress bombers and KC-135 Stratotankers, but in 1971 it was considered to big to be integrated, but apparently was eventually included.

Each SDS-1 spacecraft had reportedly 12 communications channels in the ultra-high frequency bands.

The third and fourth satellite (and possibly later ones) were retrofitted with nuclear detection (NUDET) devices known as bhangmeters.

The first generation SDS satellites were launched from 1976 to 1987. The SDS-1 had a mass of about 628 kg, and 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 3 m 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.

Originally a structural test model designated X-1 was acquired, followed by a qualification model designated Y-1 equipped with most of the electronic systems to demonstrate that the satellite could perform the functions it was designed for. Then four flight spacecraft designated F-1 to F-4 were ordered and the Y-1 qualification model was refurbished to fligh standard.

Note: The identification of these satellites beyond the third flight is not definitely confirmed. Some may be in fact Jumpseat SIGINT satellites, which used the same Titan-3(34)B Agena-D launch vehicle and the same orbits.