SDS (Satellite Data System)
SDS (Satellite Data System) are United States Air Force satellites, in Molniya-type orbits, used to relay images from optical and digital reconnaissance satellites. They also provide communications for Air Force units out of range of geostationary communications satellites, via an AFSATCOM (Air Force Satellite Communications System) transponder, and nuclear blast detection. Six first-generation SDS satellites were launched from 1976 to 1987. SDS-1 had a mass of about 630 kg, a cylindrical body about 4 m long and 3 m in diameter, and a main transmitting antenna over 3 m in diameter. SDS-2 satellites are much larger, with a mass of about 3,000 kg, and are equipped with several big antennas and the HERITAGE infrared missile launch detection sensors. Based on the TDRS-3 (Tracking and Data Relay Satellite 3) design, they are built to fit inside the Space Shuttle's payload bay. The first three were launched by the Shuttle in 1989–92, and a fourth by a Titan IV in 1996. See also communications satellites.