Don't short-sell SALT's value as window on USSR, some warn. Bucking the pact could cost US key data on Soviet arsenal

By walking away from SALT II, President Reagan will make it far more difficult for the United States to gather intelligence about Soviet nuclear arms, say critics of the move. As well as weapon limits, the second strategic-arms limitation pact contains provisions intended to make it easier for both sides to keep track of the other's strategic arsenal. Without this prod the Soviets will revert to their natural habits of secrecy, and do such things as cover missile silos to hide them from prying Western eyes, according to these critics.

``We won't be confronted with a blank wall. But it will be much harder to find out what they are doing,'' says Walter Slocombe, a top Defense Department official during the Carter administration.

One verification window Mr. Slocombe and other analysts fear will be closed is SALT's nonconcealment, noninterference rule. Basically, this provision states that neither side can hide from or spoof the other's spy satellites. Missile silos must not be roofed over; mobile missiles cannot be disguised as fuel trucks; lasers can't be used to blind satellite sensors.

Another window is the category of so-called ``cooperative measures.'' These are steps that both sides are required to take so that the other knows what its satellite cameras are looking at. B-52 bombers capable of carrying nuclear-tipped cruise missiles, for instance, must look different than B-52s that can't carry such weapons. Each side must give prior notice of weapon tests involving more than one missile launch, and confine them to certain areas. Retired silos and missile subs must sit for a while with their lids up, so destruction can be verified by overhead photography.

A third provision bearing on intelligence gathering is the list of nuclear forces that each side must provide to the other. This has provided the US with a ``data base'' on USSR weapons, Slocombe says.

Because of the SALT agreement, therefore, the US knows far more about Soviet strategic weapons than it does about Soviet conventional forces. The US estimate of USSR tank production, for instance, is at best general and is the result of ``enormous effort,'' says former CIA chief William Colby.

And without the help of SALT-provided verification aids, US estimates of Soviet strength may well be inflated, Mr. Colby says, because intelligence officers will be forced to make worst-case estimates with fuzzier data.

Administration critics admit that there is one SALT verification provision that the Soviets have flagrantly violated even while the treaty was tacitly being observed by both sides. Under SALT, much of the data beamed from missiles during tests is not supposed to be in code. But according to Reagan administration officials, the USSR encrypts up to 95 percent of such `` telemetry'' anyway.

But if the Reagan administration explicitly abandons SALT, then the US has no grounds for complaining about encryption.

For its part, the Soviet Union has asked the US for a list of information it feels it's missed because of encoding. But the US has refused on the grounds that by doing so, it would tip the Soviets off to US surveillance techninques.

Even with the encryption violation taken into account, the verification provisions of SALT work in the favor of the US, Colby and Slocombe say. The Soviets, they point out, would have no trouble keeping track of US forces even without the provisions, because of the relative openness of Western society. There is no Moscow edition of Aviation Week & Space Technology, however. The national technical means of spy satellites are the US's main source of information about Soviet forces.

Thus without SALT ``we will lose an important advantage we have in terms of intelligence,'' Solcomb says.