STSS is designed to be the low earth orbiter within the layered Ballistic Missile Defense System. It complements the geosynchronous Defense Support Program, the Space-Based Infrared System, and other Overhead Non-Imaging Infrared systems[11] (ONIR)[12] and provides tracking cues to systems on the surface. The STSS program is developed in phases, the first of which is the launch of two demonstrator satellites. The demonstrators will perform experiments and prove out systems and processes to establish a knowledge base for future operational designs.[13] The demonstration satellites, built by Northrop Grumman and Raytheon detected and tracked a two-stage Ground-Based Interceptor (GBI) during a U.S. Missile Defense Agency flight test on June 6, 2010.