Sizing up the SS-20s

World superpowers are not going to negotiate about nuclear weapons in public. So when Leonid Brezhnev and Ronald Reagan square off with rhetorical statements about reductions, freezes, and the like, the public is right to maintain a certain skepticism. The urgency is for the two sides to keep plugging away at the problem behind closed doors - and not emerge until they have wrestled the issues to the ground and come to an agreement on nuclear arms which will enable humanity to breathe easier.

The difficulty with public posturing is that it hides legitimate concerns on both sides and the immense complexity of the issues under discussion. These cannot be dealt with casually or flippantly. In the heat of public debate, therefore, the ordinary citizen is wise to remember that even experts are baffled by the intricacies involved and not let himself be swept up by easy generalizations, by either the Soviet or the American side.

Take those notorious Soviet SS-20s, for instance. The NATO countries are understandably concerned about the massive deployment of these mobile missiles targeted on Western Europe. To keep things in perspective, however, it bears reminding that these have not so much enlarged Russia's basic arsenal as substantially modernized it. The Soviet Union, always concerned that Europe might become a land base for an invasion of Soviet territory, already had the capability of destroying Europe several times over with its existing missiles. The SS-20, however, makes the Russians more secure because it is mobile and therefore less vulnerable to a NATO strike. It can be argued that this is not necessarily a bad thing, for if the Russians feel safer they are less likely to launch a preemptive strike out of fear of attack. The situation is more stable, in other words.

Then there is the tricky issue of numbers. The 300 Soviet SS-20s are difficult to bargain about because one third are in European Russia, one third on the Sino-Soviet frontier, and one third can go either way. President Reagan is cofrect in calling attention to the fact that mobile missiles can be turned around and aimed at Western Europe and that sMNezhnev's moratorium on deployment of more SS-20s west of the Urals therefore rings hollow. But he is not precise when he says ''even if east of the Urals, they could still target most of Western Europe.'' Not all of them. The SS-20s targeted on China cannot reach Western Europe. And anyone familiar with Sino-Soviet history knows how worried Moscow is about a threat from its former Chinese ally and how important, therefore, it regards the SS-20s pointed at China.This is a factor Mr. Reagan did not mention.

It is also necessary to consider the SS-20 in the larger context of the strategic (not just theater) arms spiral. Recall what happened at Vladivostok in 1974 when President Ford and Mr. Brezhnev met and agreed on a framework for a SALT II treaty. One of the Soviet concessions at that time was agreement not to count the so-called Forward Based Systems (FBS) in Europe - the US land- and carrier-based bombers. Knowledgeable specialists believe the Russians conceded for two reasons: one, because the US gave up demanding equality in throwweight (in which the Russians have the advantage) and, second, because Moscow had the SS-20 as a counter to FBS. At the time the US apparently did not see the significance of the SS-20.

Now, of course, it does. But, wPile the SS-20 has become the subject of negotiations on theater (medium-range) weapons, it is clear that the underlyi g issues of the so-called ''Euro-balance'' cannot be resolved until the talks are expanded to include weapons in the intercontinental or strategic balance as well. A major difficulty at Geneva in fact is deciding which weapons to include as medium-range. Embracing the whole gamut of nuclear arms could help break this impasse.

Mr. Brezhnev's unilateral moratorium may be unacceptable and Mr. Reagan's response too facile. But we trust that behind the thrusting and parrying there lies a genuine desire to reduce the nuclear risks which will translate into hard and honest bargaining. The public, meantime, should be alert to exaggerations and oversimplications.

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