The American Space Surveillance and Tracking System was a mid 1990's programme to provide a space based acquisition, identification and tracking of incoming ballistic missiles and their re-entry vehicles. The satellites, to have been designated as LS-9A , would have carried long-wave infrared sensors. The system proved too expensive to deploy in numbers sufficient to provide adequate coverage and the program was cancelled.
AKA: Space Surveillance and Tracking System, LS-9.