Arms control inches ahead on new fronts. Beyond Euromissile pact, superpowers grope toward `star wars' compromise to pave way for reducing long-range missiles by half

In addition to working toward a final pact eliminating intermediate-range nuclear missiles, the Soviets are inching toward a far more important arms control agreement that would halve intercontinental strategic offensive warheads. Moscow, however, is insisting on a price for a strategic deal: limitations on the American strategic defense initiative (SDI or``star wars'') and on comparable Soviet programs. And, so far, it is not clear if Moscow will end up offering a tradeoff that President Reagan could accept.

The talks last week in Washington, D.C., between Soviet Foreign Minister Eduard Shevardnadze and United States Secretary of State George Shultz were strikingly non-polemical. At these talks the Soviets showed some movement in both offensive and defensive strategic negotiations, according to the Sept. 18 press conference by Mr. Shultz and Western sources familiar with the negotiations.

Especially in the case of star wars this movement remains ambiguous, despite repeated American questioning of Sheverdnadze's team about the Soviet position.

In talks about a Strategic Arms Reductions Treaty (START), Shultz told reporters, the Soviets specified last week for the first time that if 50 percent cuts in total warheads are agreed on, Moscow will also halve its present heavy missiles down to 154, with 1,540 warheads. Making `narrow' interpretations

In space negotiations, Shultz said the Soviets made vague suggestions about abiding by the traditional or ``narrow'' interpretation of the 1972 Anti-Ballistic Missile (ABM) Treaty. Previously they have rejected this and insisted instead on writing additional restrictions to make the ``narrow'' interpretation still narrower.

The American response seems to have been as ambiguous as the Soviet offers. Shultz hinted publicly at some American flexibility, however, in addressing the Soviet concern about ``predictability''; and he lauded the ``tremendously productive'' summit at Reykjavik, Iceland, a year ago.

These comments suggested American consideration of a return to the 10 years tentatively set at Reykjavik as a period of initial agreed adherence to the ABM Treaty. Washington had reduced its offer to seven years after Reykjavik. Three key questions

At the heart of the sparring are three key questions:

What kind of SDI testing regime should prevail during any initial period of ``non-withdrawal'' from the 1972 ABM Treaty?

What unilateral freedom to deploy strategic defense should exist after that initial ``non-withdrawal'' period expires?

What linkage should exist between START and strategic defense?

On these points the Soviets may be signaling greater tolerance for SDI testing during a period of agreed ``non-withdrawal'' from the ABM Treaty - and possibly even greater tolerance for deployment of some kind of strategic defense thereafter.

Thus, in discussing the key list that they have proposed for magnitudes of technologies to be barred from space during the period of observance of the ABM Treaty, the Soviets clarified certain points in Washington, Western diplomats said. If specified parameters for such things as the power of a laser or the size of a mirror are not exceeded in space, they indicated, these would not be deemed violations of the treaty. In ``laboratories'' and test ranges on earth, however, these magnitudes could be exceeded without being deemed illegal. Linkages an issue

Still murky are the reasons for the differentiated Soviet formulas about linkage between strategic offensive and defensive arms control in the two draft treaties presented by the Soviets in Geneva last July. In their draft START treaty, the Soviets say that once the signatories have agreed to 50 percent cuts in offensive warheads, ``practical development and deployment'' of strategic defense by one side (i.e., Washington) would be grounds for releasing the other side (i.e., Moscow ) from its treaty obligations to reduce offensive weapons. In their draft space treaty, however, the Soviets do not use the phrase ``and deployment,'' leaving only ``practical development'' as grounds for resuming an all-out offensive arms race.

In this connection the Soviets seem to have talked somewhat more in Washington about ``gross violations'' of the ABM Treaty than about ``practical development'' of space defense. The implication was that the price for 50 percent offensive cuts would be no ``gross violation'' of the ABM Treaty - a considerably looser formulation. The Soviets declined to specify, however, what might constitute a ``gross violation.''

Nor have they ever explained what distinction they draw between ``practical development'' and ``practical development and deployment'' of SDI. An optimistic reading would be that they are insisting that no agreement on 50 percent cuts in offensive weapons is possible unless Washington accepts a 10-year restraint on SDI testing and a 10-year ban on SDI deployment - but that once the 10 years were over, some deployment might be allowed without triggering a race to redouble offensive warheads and overwhelm a deployed defense.

A cynical reading would be that the different semantics are nothing but sleight of hand. For without ``practical development'' (which the Soviets have never defined, but which in Western usage means testing of components and subcomponents), no SDI system ever could be brought to the stage of deployment.

The possible Soviet movement toward abiding by the traditional ``narrow'' interpretation of the ABM Treaty that Shultz referred to is also ambiguous.

In recent years, far from endorsing the ``narrow'' interpretation - which the US Senate insists on and the Reagan administration rejects - the Soviet Union has demanded that SDI tests be confined to an earthbound ``laboratory.'' What is at issue here is whether tests of ``exotic'' technologies (such as lasers) that are mentioned in a protocol attached to the main treaty - and not just traditional missile-and-radar combinations - are also subject to the basic ban on space-based missile defense stipulated in the main body of the ABM treaty. The Reagan administration maintains that its ``broad interpretation,'' exempting exotic defense from these restrictions, is the proper one. Negotiating leverage affected

In this context the administration argues that the Senate resolution of last week favoring the narrow interpretation ties its hands and takes away its leverage in negotiating with the Soviets. The Senate majority argues, on the contrary, that it is encouraging the Soviets to compromise by signaling that their offers will not be rejected out of hand but could lead to acceptance by the United States of constraints on SDI to create a predictable nuclear environment for the coming decade.

On START - where the outstanding issue is no longer broad concepts but sheer horsetrading with numbers - Western diplomats would not expect any major Soviet movement until the outlines of a possible offense-defense tradeoff have become clearer. They were therefore not surprised that the only Soviet offer in Washington in this area was to give a specific number on reductions in the heavy SS-18s, the missile that most worries the US because of its potential use in a surprise ``first strike.''

Even with this caveat, the specific Soviet offer of 1,540 warheads is interesting in actually starting the horsetrading. The comparable figure proposed by the US under the already-agreed overall ceilings of 6,000 strategic warheads is a ``subceiling''of 1,650 for heavy and multiple-warhead mobile missiles. These comprise the Soviets' SS-18s and new SS-24s, and the American MX. If the Soviets start with 1,540 SS-18 warheads, the American proposal would then permit them an additional 110 SS-24 warheads. This is fewer than the Soviets apparently now plan to deploy, but such a small gap is eminently negotiable.