SBIRS (pronounced 'sibers') is an acronym for Space-Based Infrared Systems. This is a United States military program intended to replace the aging DSP satellite systems.

The first component, the Mission Control Station at Buckley AFB, reached IOC in 2001. This system gathers, correlates and processes sensor data from a variety of legacy systems, including DSP, and presents that information to military and NCA clients. The space segment of the system was originally divided into two parts, SBIRS-High and SBIRS-Low. SBIRS-Low, which was to consist of satellite sensors in Low Earth Orbits, has been renamed STSS (Space Tracking and Surveillance System) and transferred to Missile Defense Agency management from the Air Force.

The Mission

SBIRS-High (which is now just called SBIRS) is a set of satellites in both geostationary orbits (GEO) and highly elliptical orbits (HEO). These contain infrared sensors which watch the Earth for a variety of reasons. Here are four that the Air Force is willing to tell us about:

Missile Warning
Consisting of the same duties as the DSP satellites, this tasking uses IR sensors to watch the Earth for the particular infrared signature of a missile launch. Boosters large enough to throw even short-range ballistic missiles put out an enormous amount of IR energy, which can not only be detected but (most likely) profiled by SBIRS sensors. Use of the DSP satellites for this task provided Coalition forces during Operation Desert Shield and Desert Storm with warning of Iraqi SCUD missile launches, allowing forces in the calculated target areas to seek shelter and take precautions against the threat of chemical attack.
Missile Defense
SBIRS is fast enough and sharp-eyed enough to not only provide warning of missile launches, but to track those missiles through the whole of their flight path. This information allows missile defense systems to be targeted on them.
Technical Intelligence
In other words, straight up spying. The IR sensors on SBIRS are powerful enough to provide a wealth of technical information on their targets, whether those targets are missiles, or aircraft, or even ground targets - imagine having a very sharp-eyed infrared night vision camera view of an area on the ground. SBIRS is used to carefully observe missile tests by other nations, allowing its users to learn all manner of things about not only the missiles in question, but the rocket technology in use, the procedures, and probably the performance of the reentry vehicles.
Battle Space Awareness
A fancy term for reconnaissance. SBIRS can be used to provide information on the location of enemy troops (tanks, airplanes, trucks and even men give off IR energy). It can also be used to keep track of activity - in much the same manner as SLOW WALKER, SBIRS can no doubt track energetic jet exhausts and offer warnings when high-speed military aircraft are maneuvering in its area of vision. Perhaps, akin to the related FAST WALKER program, it can track spacecraft via their heat signatures as well.
Those aren't the only things SBIRS will do. For example, it is certain that SBIRS is also capable of performing the duties of the NUDETS systems aboard the DSP satellites - that is, acting as a bhangmeter for detecting and measuring atomic explosions on the Earth's surface.

The System

SBIRS-High (now SBIRS) consists of several satellites in GEO and several in HEO. The GEO satellites provide constant monitoring of the earth under their gaze - but ICBM launches generally take polar routes, and GEO satellites are least effective at seeing things at the poles. The HEO satellites are placed so that the 'long' portion of their orbit ellipses are over their designated target area, and they have an appreciable north/south component to their orbits. As a report from the Congressional Research Service notes:
HEO orbits can provide coverage of the polar regions. A classic HEO orbit (called a Molniya orbit after the Soviet communications satellite system that first utilized it), has an apogee (the highest point of the orbit) of approximately 40,000 kilometers, and a perigee (the lowest point) of about 500 kilometers, giving the orbit an elliptical shape. With an inclination of about 63 degrees (the angle at which it intersects the equator), such an orbit allows a satellite to linger or “dwell” over the northern hemisphere for several hours per orbit, viewing parts of the globe not observable from GEO. DOD reportedly uses this type of orbit for classified satellites.

The GEO satellites contain both 'scanning' and 'staring' sensors. The scanning sensor, which scans its field of view across the entirety of the visible earth using a movable mirror in front of the IR telescope, is constantly on the watch for missile launches or other events of interest. When it sees one - and the system is designed to completely scan its field of view every 20 seconds or so, meaning it should see an event within at most 20 seconds - the satellite can task the 'staring' sensor (which has greater imaging resolution but is slower) to watch the event. The staring sensor is also 'moveable' so that it can time-share and watch multiple points of interest.

HEO satellites are built slightly differently, since they need to realign their telescopes constantly in order to keep them pointed at the Earth as they move through their orbits. Their sensors are used as scanning sensors, but by moving the entire telescope using this slewing mechanism.

The Program

The SBIRS program was originally envisioned as the latest in a long line of attempts to replace the DSP satellite system. Many attempts were made; many projects initiated and then abandoned due to cost overruns and/or technology shortfalls over the years. As a result, essentially unchanged replacement DSP satellites continued to be launched as late as 2005.

SBIRS itself has a history of cost overruns. The program went into Nunn-McCurdy breach of cost overrun limits in 2001, forcing explanations and justifications to Congress for its continued existence. It has been restructured and redesigned since, and its timelines have slipped. But the mission for which it is intended is so critically important to U.S. national security that the various agencies have (somewhat grimly) gritted their figurative teeth and continued. As a result, the Air Force is hoping to launch its first dedicated SBIRS satellite in April of 2011, with the remaining satellites and ground spares funded if not complete.

Some Sources:

Iron Noder 2010

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