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Satellite Data System
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''.〔(Jonathan's Space Report: ''NRO Satellite Names'' )〕 The first generation were named simply 'SDS', the second generation were named 'Quasar' and the third generation each carried their own designation names.〔(Jonathan's Space Report: ''Satellite Catalogue'' )〕
==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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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