Satellite Data System

[2] 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.

[3] Each SDS-1 satellite had 12 channels available for Ultra high frequency (UHF) communication.

[5] The SDS-2 is significantly more massive at 2,335 kg (5,148 lb), with three separate communication dishes, including one for a K-band downlink.

It is believed that the Space Shuttle has been used to launch several satellites, possibly on missions STS-28, STS-38, and STS-53.

The current SDS-3 constellation, consisting of three Molniya orbit type and two geostationary satellites
Releasable Picture of NRO satellite, possible SDS-1
The SDS 3-4 satellite (USA 179, 2004-034A) created a tiny trail perpendicular to the star trails in this 10 second exposure with a Zeiss Sonnar MC 2.8/180mm lens made by amateur satellite observer Marco Langbroek.