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Interceptors, kill vehicles, boosters, warheads, decoys. These are the principal topics in the political debate over what kind of national missile defense system the United States should build, if any. Yet before any interceptor leaves its silo or any kill vehicle homes in on a missile, an extraordinarily complex system of sensors, battle management computers, and software will have to find, track, and predict the trajectories of incoming warheads. Those systems get little public attention, but without them, even the best interceptors or kill vehicles would be virtually useless.

The most ambitious sensor package in the Pentagon's future plans is the Air Force's Space Based Infrared System, known as SBIRS (pronounced "sibbers"). If fielded as planned, SBIRS would consist of two sets of satellites. The so-called SBIRS High constellation, consisting of four satellites in geosynchronous Earth orbits and two sensors in elliptical high Earth orbits, would primarily provide early warning of missile launches and track rockets until their heat-generating boosters burn out. SBIRS Low, consisting of about two dozen satellites in low Earth orbit, would then track the warheads from their point of separation from a booster until they neared re-entry. Combined with powerful radars, SBIRS will provide "birth-to-death tracking" of ballistic missiles, says Col. Michael W. Booen, the Air Force's SBIRS program manager.

The $8 billion program could experience some difficulties. The Air Force already has delayed the deployment of both segments by two years. Plans now call for launching the first SBIRS High satellite in 2004 and the first SBIRS Low satellite in 2006. That delay sparked concerns in Congress that the Air Force might be neglecting the system. As a result, lawmakers last year ordered that by this October the Air Force hand over program management responsibility for SBIRS Low to the Pentagon's Ballistic Missile Defense Organization. Moreover, there are major questions about whether the technology for SBIRS Low is mature enough to be fielded in just five more years. In a February report, the General Accounting Office found that SBIRS Low "is at high risk of not delivering the system on time or at cost or with expected performance."

Different Lineages

SBIRS High and SBIRS Low, though they have been grouped together in the same program, have distinctly different lineages and missions.

The SBIRS High system will be the next-generation replacement for the Air Force's venerable Defense Support Program sensors. The first DSP satellite was placed in orbit in 1970, and at any given time, the on-orbit constellation comprises about five spacecraft. The DSP originally was intended to provide early warning of a Soviet ICBM launch. Over the decades, however, the satellite has been upgraded to provide several types of technical intelligence that would be hard to get any other way. Military experts regard an enhanced space early warning system as a high priority. "If I could only build one space system for the next 20 years, it's SBIRS High," says retired Gen. Thomas S. Moorman Jr., former vice chief of staff of the Air Force and former commander of Air Force Space Command. "It's a matter of survival."

During the 1991 Persian Gulf War, DSP satellites provided the primary means for detecting launches of Iraqi Scud missiles. The Gulf War also highlighted the limitations of the aging DSP system. It took a full 10 minutes to transmit missile-launch data from DSP through the ground stations to troops operating Patriot anti-missile batteries in the theater. That severely limited the time available to launch the Patriot weapons. The DSP satellites were unable to pinpoint launch areas, a fact making it difficult for coalition troops to find and destroy mobile Scuds-nor could they accurately predict a Scud's impact point.

SBIRS High is expected to eliminate or at least greatly diminish these problems. Booen says SBIRS High probably will need less than a minute to transmit missile-launch data from space to troops in the theater. The new system will do better pinpointing a launch location. David R. Tanks, a space analyst with the Institute for Foreign Policy Analysis, estimates that the new infrared sensors should be able to get within a kilometer, whereas that figure today is five kilometers. With the proliferation of mobile ICBM systems, shrinking the size of the possible launch area could be crucial to targeting a "shoot-and-scoot" launcher before it can scoot. Experts believe SBIRS High should throw out fewer false alarms than DSP, which sometimes "detects" a missile launch in the flare of an oil derrick or the lighting of a fighter's afterburner. SBIRS High ought to be sophisticated enough to identify what type of missile has been launched.

Moreover, SBIRS High will also be able to track missiles much more accurately in the early part of flight. Unlike the DSP satellites, whose sensors conduct a sweep at 10-second intervals, SBIRS High has both a "scanning" sensor and a "staring" sensor. That means it can simultaneously sweep a broad area and focus on a small area. When a missile is launched, the SBIRS High scanning sensor will quickly detect the sudden hot plume of exhaust as the staring sensor follows the plume continuously. That technique should allow SBIRS High to keep an accurate record of the missile trajectory until the booster burns out at an altitude of 100 miles or so above Earth.

It is at that point that the second constellation, SBIRS Low, is supposed to take over. This network of 24 satellites in low Earth orbit-the progeny of the Brilliant Eyes component of Ronald Reagan's Strategic Defense Initiative-is meant to track missiles during the midcourse portion of their flight, after the booster has burned out and its heat plume has disappeared. The innovation of SBIRS Low is that in addition to being able to detect hot objects, like the plume of fire belching from a missile, it will also be able to track very cold ones--like warheads flying through the vacuum of space.

The Critical Moment

When the missile is launched, a SBIRS Low satellite will either pick it up with its own sensors or get a location cue from SBIRS High. Then it will begin its own tracking. The critical moment comes when the missile's booster burns out and the missile ejects its warhead, decoys, and penetration aids onto a ballistic flight path through space. Pentagon officials hope that SBIRS Low at that point will be able to track all of the objects hurtling through space. With several different satellites looking on from different angles, defenders on Earth should be able to develop an accurate, three-dimensional view of where the warhead is, its location, and ultimate destination.

Those data will be crucial. In order to shoot down a high-performance warhead traveling at 15,000 miles per hour through space, interceptors will have to be launched shortly after launch of the missile itself. Unless interceptors are close enough to shoot down the ICBM in its very brief boost phase (DOD has not yet begun to develop that technology) it will be SBIRS Low data that guide the interceptor toward the incoming ICBM during its midcourse phase. Getting the interceptor close to the warhead is essential for guiding the kill vehicle--launched from the interceptor in the final moments of flight--into the missile to destroy it in space.

SBIRS Low could help solve another vexing problem-distinguishing the warhead from decoys and other countermeasures meant to confuse a missile shield and let the warheads leak through. Missile experts say the best way to do that is to begin tracking all of the objects the moment they have been ejected from the rocket. That lets different sensors gather data on the characteristics of each object and gives computers time to process it. While discrimination tactics are highly classified, missile experts say constant tracking is the key to determining what to worry about and what to ignore.

Critical Handoff

The final step in birth-to-death tracking of missiles will be the handoff of the tracking mission to powerful, surface-based X-band radars that follow incoming warheads once they come over the horizon. Current plans call for basing one such radar on Shemya Island, at the far western tip of Alaska's Aleutian Islands chain.

Missile defense experts say that, if a system can track a missile throughout its flight, there should be a high likelihood that it will be able to identify the warhead by the time it is picked up by the radars that will guide the kill vehicle to its target. "Once you fuse infrared [data] with radar," says Booen, the SBIRS program manager, "it makes it very hard to defeat [our system] with countermeasures."

Building a system to shoot down ballistic missiles, of course, ranks as one of the most ambitious projects the Pentagon has ever undertaken. Guiding a kill vehicle into an ICBM-at a combined speed of 17,000 miles per hour-is the easy part. (DOD still has to figure out how to do that with high confidence; in three tries to "hit a bullet with a bullet," one succeeded.) Harder still is developing the "system of systems" that, theoretically, would enable nearly 30 satellites, several ground-based radars, and numerous ground stations to shoot gigabytes of data back and forth in nanoseconds. During the 1999 NATO war against Serbia, it was common for hours to pass between detection and location of a critical target and a pilot's release of a bomb onto that target. That was simple in comparison to shooting down a ballistic missile with less than 20 minutes' notice.

Not surprisingly, SBIRS has encountered much of the same kind of skepticism that once attended Reagan's Strategic Defense Initiative. While SBIRS High is based on proven technology that has been fielded for 30 years, the two-year delay in the first launch reflected difficulties with software integration and other problems. "Continuing delays ... remain a concern," reported Philip E. Coyle, the Pentagon's top weapons evaluator during the Clinton Administration, in his annual report for 2000. Air Force officials are quite a bit more optimistic. Booen, for example, cited another factor that partly explains the delay in the SBIRS High schedule: The DSP satellites were lasting longer than expected, and thus there was no need to rush the program.

Fast Track?

Far more controversial is SBIRS Low, a technology the Pentagon has never before fielded. In its February report, the GAO cited many problems. Traditionally, the Pentagon requires satellite software to be finalized one year before the first satellite of a new system is to be launched. SBIRS Low is so complex that the software isn't set to be done until March 2010, more than three years after the first satellites are set to go into orbit. The current SBIRS Low schedule doesn't call for a series of key tests until more than five years after production of the satellites has begun. That means the Air Force would have to incorporate any needed design changes into satellites that already are in production, raising the danger of cost growth and delays. When queried by the GAO, the SBIRS Low program office explained that six critical technologies had to be in place for the system to work properly and that five were at maturity levels that constituted "high risk."

Senior Pentagon officials have instructed the Ballistic Missile Defense Office to review SBIRS Low to see whether there might be a less expensive or more effective way to conduct midcourse tracking of ICBMs. One alternative may be construction of a series of X-band radars that would be placed in strategic locations throughout the world. The obvious drawback of such a system is that, if based on land, it would require political cooperation from nations that may not even be US allies.

SBIRS Low has attracted the attention of members of Congress, especially those who are strong proponents of the Bush Administration's plans for missile defense. When the Air Force delayed from 2004 to 2006 the fielding of the first SBIRS Low satellite, it did so without consulting Congress. The result was a passage in the authorization bill for Fiscal 2001 that transferred management authority of SBIRS Low from the Air Force to BMDO. While the Air Force will still execute the program, it cannot make any further changes without consulting the director of BMDO.

Without doubt, much is riding on the program.

Rep. Jerry Lewis (R-Calif.), chairman of the House Appropriations Defense Subcommittee, in a March 2 statement, put the situation this way: "Our plans for a shield against attack by foreign nuclear missiles depend on a highly reliable and comprehensive detection. It is essential that we know that any such system will work when needed and provide the most accurate information possible. Without this detection system, we cannot be fully protected from foreign threats."