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Original SBIRS Low[edit]

The SBIRS Low program was originally expected to consist of about 24 satellites in low earth orbit. 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. The system was to have two major sensors, coordinated by an on-board computer:

• a scanning infrared sensor, designed to acquire ballistic missiles in the early stages of flight.
• a tracking infrared sensor, designed to follow missiles, warheads, and other objects such as debris and decoys during the middle and later stages of flight. The tracking sensor would be cooled to very low temperatures.

SBIRS Low's original deployment schedule was 2010, the date when its capabilities were said to be needed by the National Missile Defense System.

Space Tracking and Surveillance System[edit]

In 2001, the Missile Defense Agency assessed the programs needed for a national ballistic missile defense system (BMDS) and found that they were lacking in the relatively new arena of space. The MDA decided to absorb the SBIRS Low constellation in its very early stages of development and renamed the program theSpace Tracking and Surveillance System (STSS). This transition changed the direction of the program somewhat, but the overall mission remained the same—detection and tracking of ballistic missiles through all of its phases of flight.