The initial seven Tracking and Data Relay Satellite System (TDRSS) were built by TRW to improve communication coverage for the Space Shuttle, International Space Station (ISS), and U.S. military satellites. When first launched in 1983, the TDRS satellites were the largest, most sophisticated communications satellites built at the time. The seventh vehicle in the series was ordered as a replacement when TDRS-B was lost in the Challenger accident.