The Patriot anti-missile defense system has been successful in the gulf war. The "Star Wars" weapons proposed by the Strategic Defense Initiative are anti-missile weapons. Therefore, SDI will be successful. Right?

Whoa! It's not so simple. We can all be proud of the Patriot system. It is the kind of practical, workable anti-missile system we should be producing. But those who are trying to piggy-back their own pet programs on the back of this success story are sadly misinformed.Here are the facts about Patriot and SDI.

- The Patriot program has nothing to do with SDI. It is funded and managed by the Army and has not received a penny of the $23 billion Congress gave to SDI since 1984. Congress had authorized about $1 billion a year on strategic defense research for decades prior to the start of the SDI program.

- The Patriot counters tactical ballistic missiles. These are the immediate threat our troops face. Despite congressional efforts over the years, SDI has refused to seriously address this problem. Rather, it has spent almost all its time and money on trying to counter Soviet intercontinental ballistic missiles.

- Shooting down intercontinental ballistic missiles is an extremely difficult, and as yet unsolved, problem. ICBM warheads span the oceans. They are about the size of a man, arrive in a barrage of up to 10 at a time and may be hidden among dozens of decoys. These missiles we do not know how to shoot down with confidence and more research is needed.

Over the years SDI has promoted one system after the other as an answer to this problem. All have been funded by Congress. None has worked.

To give some idea of the scope of the problem SDI faces compared to Patriot, consider that a battery of 32 Patriot missiles can defend an area of roughly 40 square miles. If this system were capable of shooting down long-range nuclear missiles, we would need more than 90,000 batteries with almost 3 million Patriot missiles to defend the territory of the United States. At present, there are about 53 Patriot batteries in existence.

Defenses against the short-range tactical missiles are legal. A space-based defense against ICBMs would violate the 1972 Anti-Ballistic Missile Treaty signed by the United States and the Soviet Union. This treaty recognizes the still valid premise that trying to erect a shield against nuclear attack would only provoke the other side to increase the number of attacking missiles and to devise ways to counter any defense.

Finally, there is the issue of cost. The Patriot program is not cheap. Congress has authorized $12 billion for this system to date. But that is peanuts compare to the $120 billion SDI estimated it would need to build, deploy and operate its so-called Phase I System of thousands of "Brilliant Pebbles" satellite weapons.

Now, for the fourth time in six years, SDI has overhauled its proposed system. Having virtually admitted that it is impossible to really protect the country against a Soviet nuclear attack, it now proposes instead to try to shoot down accidentally launched Soviet ICBMs or ICBMs from a Third World nation.

That is a more realistic, but still a very demanding mission. We are no longer talking about trying to shoot down thousands of Soviet warheads, but about one, two or, at most, a few dozen warheads. To do this, SDI officials say they would need to put about 1,000 Brilliant Pebbles weapons in orbit and deploy about 1,000 interceptor rockets around $6 billion.

Can we do this new mission? What will it cost? We just don't know yet. But the SDI program is like an 800-pound gorilla; when it plops down into the middle of the budget, everyone else get squashed.

The Patriot is a real success story, and its lessons should be used by SDI. But SDI, like all federal programs, must be kept to a reasonable dollar figure, lest more urgent competing defense needs be slighted. In addition, the SDI program should not proceed to undermine the ABM Treaty, at least until Congress directly addresses that question.