DESPITE the important progress made at the summit, deep divisions over testing of President Reagan's Strategic Defense Initiative (SDI) still stand in the way of an agreement to reduce long-range arms. But the American and Soviet positions are more compatible than either side seems to realize. General Secretary Mikhail Gorbachev now admits that the Soviet Union is pursuing its own research on SDI technologies, confirming what we have always known. More important, the Soviets no longer demand elimination of SDI as a condition for deep cuts in offensive arms. They now ask only that both sides adhere to the traditional interpretation of the 1972 Antiballistic Missile (ABM) Treaty. This position is consistent with what the Congress has recently voted and the President signed into law, and with the ``common sense'' approach recommended by Margaret Thatcher and other NATO allies.
But the President and his aides have argued that abiding by its ban on testing of space-based ABM systems and components would kill SDI. We believe they are profoundly mistaken. Abiding by the traditional view of the ABM Treaty for another decade would do little if anything to slow the most interesting SDI developments.
Less than two years ago, the SDI office itself was telling Congress that the traditional interpretation would pose no problems for SDI testing for years to come. Unfortunately, that scientifically and economically sensible position has been lost in the rush for ``early deployment'' of a first-phase SDI system.
Why doesn't acceptance of the ban on space testing of antimissile systems pose an immediate threat to SDI?
Much of any future strategic defense would be anchored to the ground, and the ABM Treaty permits testing of ground-based defenses. Weapons designed to intercept Soviet missiles just after they are launched - in the ``boost phase'' - would, however, have to have key components based in space.
But those laser beam weapons that show any promise at all for boost-phase intercept are only distant possibilities, as pointed out in a recent authoritative report of the American Physical Society. It will be at least a decade before these potential weapons could be ready for the sort of testing limited by the ABM Treaty.
The space-based rocket interceptors SDI is proposing for ``early deployment'' use more mature technologies, which could be tested sooner. But testing of these rockets would be banned even under the administration's ``broad'' interpretation, since the ABM Treaty explicitly lists such ``interceptor missiles'' among the items that cannot be tested in space. Moreover, even if these rockets could be effectively drawn together into a total system - unlikely anytime soon - they could quickly be rendered obsolete by the next generation of faster-burning Soviet missiles. Spending billions on such space-based rockets would only divert funding and expertise from more important long-range technologies.
The sensor satellites being developed under SDI need not be hampered by the traditional interpretation, either. These were planned long before the SDI program began, for permitted tasks such as surveillance and early warning. Even the space-based particle-beam experiment intended to explore ways to discriminate between warheads and decoys could be done within the traditional interpretation, as long as the experimental device could not serve as a complete ABM ``component.''
Indeed, much of the most critical SDI work - devising the fantastically complex computer software for ``battle management,'' developing new techniques to ensure the survivability of defense satellites, and drastically reducing the costs of putting large payloads in orbit - falls into areas entirely unlimited by the ABM Treaty.
Admittedly, the precise limitations the treaty imposes on such new technologies as lasers and particle beams could stand clarification. The Soviets propose negotiating specific limits on new technologies, and this is surely worth pursuing. Limits could be devised that would allow enough testing to answer fundamental scientific questions, while serving the ABM Treaty's purpose of preventing either side from threatening the other with rapid deployment of a nationwide defense.
We often lose sight of the reality that the ABM Treaty serves US security by limiting Soviet defenses as well as our own. By agreeing to abide by the traditional view of the accord, President Reagan would be following the advice of six former secretaries of defense, Democrat and Republican alike, and of his own Scowcroft Commission, which warned against proceeding beyond the treaty's limits. And accepting the traditional interpretation would pave the way for an agreement on deep reductions in strategic nuclear forces, without in any way crippling what is worthwhile in the SDI program. President Reagan can have his SDI cake and eat it too.
Jack Ruina, professor of electrical engineering at the Massachusetts Institute of Technology, was director of the Advanced Research Projects Agency, the federal agency that managed the last major US effort to develop a space-based missile defense. Matthew Bunn is a senior research analyst at the Arms Control Association.