As one who has tracked the progress of the Strategic Defense Initiative since its inception, I take strong exception to the negative portrayal of the promising new Brilliant Pebbles space-based missile defense system {"Pentagon Increases SDI Push," front page, Feb. 18}.

The article correctly described Brilliant Pebbles as small, space-based interceptors designed to destroy ballistic missiles by ramming into them -- that is, destruction by impact rather than by nuclear explosives. But in denigrating Brilliant Pebbles' capabilities, the article is completely misleading.

Moving our country toward increased reliance on defenses, rather than dependence on retaliatory offensive forces for deterrence, is an extremely significant and necessary policy shift begun by President Reagan and now being continued by President Bush.

The article claims there is a broad consensus that the "impregnable ballistic missile shield" envisioned by former President Reagan is an impossible dream because the U.S. concept for a first-phase Strategic Defense System would still allow a huge number of warheads to reach the United States. Denouncing the notion of an impregnable shield is a red herring. It ignores the the fact that the greatest contribution of effective defenses would be to strengthen deterrence. Preventing an attack in the first place is, and always has been, the goal of SDI.

Should deterrence fail, defenses could, depending on the size of an attack, provide important protection for our society. This contribution could grow as technology improves. One thing is certain, though: with no defenses, all of the strategic nuclear warheads launched in an attack would detonate in the United States.

The Post incorrectly reports that even if Brilliant Pebbles were fielded, the system could not react quickly enough to a missile attack. First, the claim that the interceptors would take too long to cover the distance to the target is simply not true. Research clearly shows that it would be feasible for the pebbles to reach attacking missiles in time to block release of multiple warheads.

Second, the command and control for sending Brilliant Pebbles into action would not be a time-consuming operation. Even with the positive control provided by a man-in-the-loop decision process -- which we would insist upon -- it would take just seconds to activate the Brilliant Pebbles system once a detection of launch occurs. The image of operators "caught by surprise" and not having enough time to respond is simply not credible to anyone familiar with the operation of strategic warning systems.

This responsiveness will become increasingly important. The threat of surprise missile attacks launched by Third World countries or terrorists is likely to grow even though the Soviet missile threat may decrease in the 1990s. An additional benefit afforded by a quick-reaction space-based system such as Brilliant Pebbles is that its global coverage can protect our allies and our forces overseas as efficiently as it provides coverage for the U.S. mainland.

Last year, the distinguished JASON panel of scientists was asked to conduct an independent study to determine if there were any fundamental violations of the laws of physics required for a Brilliant Pebbles system to work. They found none and reported that a program such as Brilliant Pebbles "deserves continuing support." Despite the panel's upbeat findings, The Post implies that the report was, overall, negative.

Further, other studies, including an in-depth review by the Defense Science Board, have found no fundamental flaws with the engineering required to build Brilliant Pebbles.

Finally, The Post's reporting on the cost of SDI is completely off the mark. The acquisition cost of Brilliant Pebbles, as estimated by two independent cost estimates this past summer, is $12 billion, not $55 billion. This latter number would buy a system that includes the entire first phase SDI deployment -- much more than just the Brilliant Pebbles hardware. Given the minimal amount of ground support that Brilliant Pebbles requires, the lifetime operations and maintenance cost is estimated to be only an additional $3 billion -- far short of the dollar amount cited.

-- Edward L. Rowny The writer is special adviser to the president and secretary of state for arms control matters.