W. David Thompson took me to task in his June 6 commentary [“MDA Was Right To Kill STSS,” page 19] for writing a “badly misinformed” assessment of the Space Tracking and Surveillance System (STSS). I had argued that STSS technology is an essential component in any near-term missile defense architecture, and that flight tests of two demonstration satellites launched in 2009 prove the technology is viable.

Mr. Thompson begs to differ, asserting that, “the most underreported space story of the past two years was the miserable performance of STSS in its first year on orbit.” Among other things, he contends that STSS technology is antiquated and that various on-board subsystems are “dysfunctional.” He also alleges the satellites “would not work effectively in many scenarios.”

The reason this story has not been widely reported is that performance problems exhibited by STSS during its first year on orbit have been fixed. The fixes work so well that the director of the Missile Defense Agency told Congress on May 25, “Two recent flight tests demonstrated that STSS dramatically improved the precision of threat missile tracks and provided more accurate fire control quality data to the Aegis ships several minutes earlier than less accurate data provided by organic radars in the Aegis or THAAD (Terminal High Altitude Area Defense) systems.”

U.S. Army Lt. Gen. Patrick O’Reilly, the MDA director, went on to request $94 million for STSS in fiscal 2012, and to state that an Aegis intercept was planned for fiscal 2013 using STSS data. These plans reflect the fact that STSS has successfully demonstrated stereo tracking of ballistic warheads from birth to death, the main goal of the program.

If I had not been in the business for as long as Mr. Thompson, I might be troubled by his contention that the STSS architecture can’t cope with “many scenarios.” However, I have watched every missile defense concept proposed for the last three decades be assailed by critics because it couldn’t cope with stressing scenarios. What I have discovered is that it is always possible to dream up some set of circumstances that even the most capable system can’t address.

The problem with applying such worst-case reasoning to missile defense is that it results in nothing being deployed, so instead of having a system that can’t deal with some scenarios, we end up with no capacity to address any scenario — even the relatively easy ones. Is that really a better outcome than having a system deployed that isn’t perfect, but can gradually be expanded and refined?

Defense against nuclear attack is the toughest challenge that our military planners face. Having the capacity to track and discriminate hostile warheads from space is a crucial feature of any architecture designed to provide in-depth defense. Whatever the deficiencies of STSS may be, it is the only system that offers a partial solution to the space-based tracking requirement anytime soon, and we must get on with the business of protecting America.

Loren B. Thompson,

Arlington , Va ,