The United States Missile Defense Agency (MDA) is developing a Space Tracking and Surveillance System (STSS) which it will use to research the space-based detection and tracking of ballistic missiles. Data from STSS satellites could allow interceptors to engage incoming missiles earlier in flight than would be possible with other missile detection systems. The STSS program began in 2001, when the "SBIRS Low" program was transferred to MDA from the United States Air Force.