Momentum of summit helps arms negotiators gear up to cut down. `START' TALKS

As they enjoy the first vacation they've been granted this year, the hard working arms-control negotiators in Geneva are gearing up for the final race toward 50-percent cuts in long-range nuclear weapons. The superpower summit three weeks ago advanced the Strategic Arms Reduction Talks (START) more than is generally realized, said senior American officials interviewed in Washington after the summit. It is conceivable that a START treaty could be signed in Moscow in spring. But much work remains.

When they reconvene in mid-January the negotiators will benefit from progress made at the recent summit in four areas:

Sublimits.

Counting rules.

Sea-launched cruise missiles (SLCMs).

Verification.

The toughest remaining disputes, according to United States officials, will involve:

Verification of mobile missiles.

SLCMs.

``Star Wars'' (the US Strategic Defense initiative or SDI).

On sublimits, Soviet acceptance in Washington of a maximum of 4,900 ballistic-missile warheads under the previously agreed overall ceiling of 6,000 strategic warheads broke the impasse of the past year. Ballistic missiles are the most threatening weapons, since they pack greater explosive power and are much faster than ``air breathing'' cruise missiles and aircraft, and thus more suited to surprise attack.

The US is still asking for an additional ``nested sublimit'' (under the 4,900 ballistic-missile warheads) of 3,300 warheads on land-based intercontinental ballistic missiles (ICBMs), the war horses of the Soviet arsenal. But US concern here focuses primarily on the Soviet Union's unique heavy missile, the SS-18. Since the Soviets have now promised to halve their total missile throw-weight and limit their potentially ``first strike'' SS-18 warheads to 1,540 in START and not increase them thereafter, US officials have signaled that this basically meets US requirements even without an additional 3,300 limit on ICBM warheads. In Washington the Soviets further agreed to write these promises down in binding form.

Before the Washington summit the Soviets came up with a counter demand for a subceiling of 1,800 to 2,000 warheads on submarine-launched ballistic missile (SLBM) warheads, the war horses of the US arsenal. Soviet negotiators indicated at the summit, however, that their main interest is not SLBM warheads as such, but equality of treatment. They bridle at limiting the Soviets' chief weapon without simultaneously limiting the Americans' chief weapons. The implication is that they will drop their demand for a special subceiling on SLBMs as soon as the US drops its demand for a special subceiling on ICBMs.

The Soviet proposal also calls for a limit of 800 to 900 warheads on air-launched cruise missiles (ALCMs), a category in which the US leads. This is not far below the number the US would want to deploy and might vanish altogether once the US drops its attempt to limit ICBM warheads. What will be harder in the case of ALCMs will be devising mutually acceptable counting rules and verification.

At the Washington summit the two sides did agree how to calculate all multiple warheads on ballistic missiles: on the US side, three warheads for Minuteman 3, 10 for the MX, eight for the Trident I and II submarines, and 10 for the Poseidon; and on the Soviet side, four for the SS-17, six for the SS-19, 10 each for the SS-18 and SS-24, seven for the SS-N-18, 10 for the SS-N-20, and four for the SS-N-23. (The N refers to naval missiles.)

At America's request, counting rules will thus not follow the precedent of the unratified second Strategic Arms Limitation Treaty (SALT II) in attributing to each missile the greatest number of warheads it has been tested with. Furthermore, the counting rules agreed on in Washington are tentative and may be modified later. This would allow the US Navy, for example, to respond to US critics' charges that more than half the US strategic arsenal would be vulnerable being housed in only a dozen and a half submarines by decreasing the number of warheads on each sub and spreading the total out over more vessels.

Attribution of fewer deployed warheads to launchers than full capacity or the number tested will require innovative verification to deter cheating. In the case of submarines the superpowers began exploring at the summit, according to US negotiators, such methods as requiring an American sub to surface on Soviet request at coordinates to be named by the US for inspection by Soviet plane or helicopter, perhaps with the additional request that one particular tube be opened.

The verification it will be most difficult to institute will be on ALCMs. The disparity between full and actual ALCM loading is wide (20 to 6 or 8 for a B-52), since much of the limited space on aircraft must be devoted to ancillary weapons to suppress anti-aircraft missiles. Moreover, aircraft do not normally carry nuclear weapons in peacetime, so spotchecks prove nothing. The solution the US seeks, so far without Soviet agreement, is to attribute six or possibly eight ALCMs to all aircraft carrying them, whether the US B-52 or B-1 or the Soviet Blackjack.

Mobile ICBMs - signs here are that the US will relax its current total rejection of any mobile land-based missiles once the price is right - also pose daunting verification problems. But some experience will be gained in observation of the Soviets' mobile long-range SS-25s by ``enhanced National Technical Means'' (NTM) pioneered under the Intermediate-range Nuclear Forces (INF) Treaty signed at the summit. ``NTM'' is a fancy name for satellite surveillance. ``Enhanced'' means the side being observed cooperates by such acts as opening garages over missiles.

In general, the joint Soviet-US statement concluding the Washington summit laid the groundwork for even more intrusive verification in START than in the already unprecedented INF monitoring. The statement's eight points on verification indicate that besides carrying over the INF Treaty's on-site inspection of missile facilities and destruction and permanent monitoring of exits from critical missile plants, ``enhanced NTM'' will be expanded in START to cover bomber bases and submarine ports.

``We got three things beyond INF:'' commented one US official, ``suspect site, a ban on countermeasures to NTM, and enhanced NTM.'' On-site inspection of ``suspect sites'' means that not only designated missile facilities, but any place in the Soviet Union or the US suspected of holding missiles covertly, could be subject to inspection on short-notice challenge.

The outlawing of countermeasures would include, according to the summit statement, ``a ban on telemetry encryption ... during missile flight.'' Eight years ago, Soviet encryption of flight test data led to one of the fiercest Soviet-US rows over the SALT II Treaty.

Moreover, the data exchange under START will be even more comprehensive than that under INF - and more sensitive for the secrecy-obsessed Soviets.

Besides verification, the issue that could prove the stickiest within the framework of the START treaty itself is SLCMs. The US made a major concession at the Washington summit in agreeing to limits on nuclear SLCMs (with warheads counted apart from START's 6,000 strategic warheads) if verification can distinguish between nuclear- and conventionally armed boats, thus allowing the US to build conventional SLCMs unrestrained while limiting nuclear SLCMs.

The US expectation is that the Soviets will try to push very close to agreement by the spring summit on all of these points - and then say once again that they will not sign on to START cuts in offensive warheads without collateral restraints on testing and deployment of SDI. Whether they would then insist on mutually agreed constraints or settle for unilateral linkage - and what some US analysts term the Soviets' de facto technological veto over US SDI deployment by the threat to proliferate offensive warheads to overwhelm it - is a matter of dispute. Some US negotiators think Moscow might again be willing, as in Washington, to agree to disagree. Others expect the Soviets to use the momentum in START talks that are on the verge of achieving a treaty to force the issue on SDI.