The primary purpose of SBIRS Low was the tracking of ballistic missiles .
The primary purpose of SBIRS Low discriminating between the warheads and other objects , such as decoys , that separate from the missile bodies throughout the middle portion of their flights .
The primary purpose of SBIRS Low was the tracking of ballistic missiles and discriminating between the warheads and other objects , such as decoys , that separate from the missile bodies throughout the middle portion of their flights .