Arms control pacts can be verified - includes related article

The argument goes something like this: If we agreed with the Soviets to allow 10,000 nuclear bombs each, our security couldn't be affected very much if they secretly increased their inventory to 10,100. But if we agreed to reduce our total arsenals to only 50 bombs each, it would matter a great deal if they secretly tripled their number to 150, even though the actual increase -- 100 bombs -- is the same in both instances. (The opponents of arms control agreements say that what matters, at least for now, isn't whether those 100 secret bombs pose a military threat but that we be able to verify that there's no cheating at all.)

If we're to be assured that we can always monitor Soviet compliance with any arms control agreement, we should be able to determine whether an event has or hasn't happened, or is in the process of happening. There are two kinds of event: one, like the firing of a missile, changes the scene temporarily; the other, like the construction of a large radar or a missile silo in the middle of a forest, creates a permanent change.

Permanent changes, or even semi-permanent ones -- for example, the slow movement of a division of troops or a wing of mobile missiles from one part of the country to another -- are detected by before- and-after comparisons. Usually this is the task of photo reconnaissance satellites, which routinely photograph whatever comes into their field of view (it can also be done by high-flying planes like the U-2, whose ceiling is 90,000 feet, and the SR-71, or Blackbird, which can travel at Mach 4 at 125,000 feet, or by small remotely pi- loted vehicles). By comparing different images of the same scene, taken under similar lighting conditions over a period of time, we can detect changes -- the laying of a keel for a nuclear submarine, say.

But such transient events as the flight test of a new missile orthe underground detonation of a nuclear explosive must be observed while they're occurring. Therefore the detection systems (radars and telemetry receivers in the case of missile tests, seismographs for underground explosions) have to be on at all times. Such vigilance can be expensive. And so we depend on the synergy of a variety of intelligence-gathering tools, e.g., if satellite photos tell us the Soviets are preparing a missile for launch, we may be able to get the exact time of the test by listening in on their telephone calls. Or we can be on the lookout for the activation of Soviet radars whose beacons track their missiles in flight. Then we can turn on our own monitoring radars and telemetry receivers to ''see'' and ''hear'' the details of the test.

One such missile-watching installation is the Cobra Dane phased-array radar on Shemya Island in the western Aleutians, which tracks warheads as they head from their launch sites at Tyuratam or Plesetsk across eastern Siberia for impact areas on the Kamchatka Peninsula or in the Pacific. We may also send up reconnaissance planes, usually lumbering RC-135s called Cobra Ball aircraft, to track a warhead with a battery of cameras working in various wavelength ranges while the plane's ELINT equipment searches for the missile' telemetry. (Some U.S. officials think the Soviets may have shot down Korean Air Lines Flight 007 because they mistook the Boeing 747 for an American spy plane on just such a mission.)