Nuclear arms: let's keep open the 'window of verifiability'

President Reagan has recently made public his plans for modernization of US strategic forces. One of the principal justifications given for the accompanying defense budget is an alleged ''window of vulnerability'' which the administration wants to ''slam shut.''

But while debate swirls around the issues of alleged Soviet ''superiority,'' ''spending gaps'' and the ''window of vulnerability,'' a far more significant and much less known ''window of verifiability'' may be quietly and forever eased shut.

When the United States first sought agreements with the USSR on limiting the arms race an obstacle presented itself which was then insurmountable: the problem of verification. The USSR especially would not accept the only available practicable measure - on-site verification.

Then the steady advance of technology brought us, fortunately, into the window of verifiability. Using advanced remote systems, especially satellite-borne, it became possible for both powers adequately to verify the other's compliance and the stage was set for mutually acceptable agreements. Noteworthy are the ABM and limited test ban treaties. To date there have been no significant compromises by either party of the terms of these treaties.

However, the inexorable process of technological advance which brought us past one edge of the verifiability window is now finally about to bring us past the edge out of the era of verifiability, probably forever.

The accuracy of strategic weapons has steadily increased. The size of a warhead for a given yield has steadily decreased. The cruise missile, for example, is a mere 21 feet long, is easily concealed, and can deliver a warhead of 16 times the yield of that placed on Hiroshima to within 100 feet of the intended target.

This, and devices of the same class, can be hidden in a wide variety of numerous and otherwise nonthreatening vehicles - 747 aircraft, cargo ships, tractor-trailers, etc. Once tested and deployed in this way the other party can no longer verify compliance. Thus the relentless application of new technology to weapons systems inevitably brings about a circumstance in which negotiations based on verification become impossible.

Essentially simultaneously, the same process of technological advance has conspired to undermine the survivability of land-based strategic systems, also probably forever.

In the early 1960s the state of US and Soviet technologies was such that a very good case could be made for deploying a system of land-based strategic missiles to deter attack. The required missiles were large. They each carried only a single warhead. They were not easily targetable. Guidance systems were crude by comparison with current technology. It wasn't even remotely possible for either the US or the USSR to launch a preemptive countersilo strike against the other. The kind of coordinated, precision targeting required to disable a field of hardened silos just did not exist. Thus it was possible for both sides to erect a stable and credible land-based deterrent force. A telling measure of this stability is that it was possible to maintain our Minuteman land-based force at a constant level of about 1,000 missiles for two decades.

The true significance of the long search for an MX basing mode is that technological advances have rendered all land-basing schemes ultimately vulnerable. Land-based strategic weapons have become as obsolete as the crossbow.

A telling measure of this fact is that, in spite of an extraordinary effort, no acceptable basing on land has yet been found for the MX. Moreover, those schemes which have been contemplated soon require ABM defenses and indefinite increases in the number of primary missiles, in contrast with the stable past history of US land-based forces. Once the obsolescence of land-basing is recognized it is easy to see how unenviable is the Soviet position with 75 percent of its deterrent on land and just now completing an expensive modernization of that component.

While the administration appropriates massive sums to the modernization of what has become a problem without a solution and forestalls serious arms negotiations, the window of verifiability is slowly closing. With it will close also the fundamental basis upon which nuclear arms negotiations rest: verification of compliance by nonintrusive measures not requiring cooperative acts by the examined party.

It is in the interest of the US, to say nothing of the Soviet Union or the rest of the world, to recognize the intrinsic vulnerability from now on of all land-based strategic forces and to get on with arms negotiations before the verifiability window has finally closed completely and forever.