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The Myth of Missile Accuracy

Under further interrogation by Fitzgerald and others it emerged that 40 percent of the missiles were down because of failures in their guidance system.

Missiles, as Fitzgerald’s account suggests, do not exist in the orderly universe of the strategic theologians but in the actual world of contract mismanagement, faulty parts, slipshod maintenance, bureaucratic cover-up, and the accidents that have afflicted military equipment since the world’s first bow string got wet in the rain. The nuclear scenarists are impatient at citation of such quotidian mishaps which, to be sure, do not refute but merely impair the claim that a missile will successfully strike its objective 6,000 miles away.

As prologue to consideration of just this claim, it is useful to bear in mind the accuracy being envisaged. By the standards demanded of an ICBM fired from the United States to the Soviet Union (or vice versa), a shell from an artillery piece—fired with no preliminary spotting rounds—would fall no more than one yard from a target thirty miles away.

An intercontinental ballistic missile is far more like an artillery shell than might be supposed. What happens when a missile is fired? It is boosted into space by three or four rocket stages which fall away as they burn out. After the last stage burns out, or is turned off, the warhead (or “re-entry vehicle”) continues in free flight through space until it re-enters the atmosphere and descends toward the target. With the moment of final burn-out, or termination of the rocket, vanishes the last chance for the warhead to change direction. It is on its own, as is a shell leaving the muzzle of an artillery piece, or a stone leaving a person’s hand.

From the moment the missile leaves its silo, no exterior system is guiding its course. No human hand, back on the ground, can interfere or correct its flight. Nor is any piece of equipment inside the missile taking bearings from some external point of reference. The missile depends for its guidance on inertial sensing. The simplest way of understanding this is to think of yourself sitting in an airplane looking at a glass of water. The movement of the water will reflect the angular movements of the plane. All that the inertial system in a missile essentially does is to compare the movements of a more sophisticated equivalent of the water in the glass with its programmed version of what those movements should be, if the rocket is going in the right direction. Undesirable variations are corrected accordingly, up until the moment the rocket motor burns out.

Once that happens, there is nothing that can be done. The warhead can receive no signal, and contains no targeting mechanism of its own. As can be understood, everything depends on the accuracy of the programmed data in the missile’s computer. If the missile’s flight were to take place in entirely predictable conditions, the inertial system would be perfectly satisfactory and no silo would be safe. But in reality the missile’s journey takes it through forces that either cannot be compensated for, or are entirely unpredictable, or are not understood.

During the flight of the missile, from launch to target, it is under the influence of two principal external factors: the pull of gravity and the drag of the atmosphere. Since the earth is not a perfect sphere and varies in density, its gravitational field is not constant. The inertial guidance system cannot tell the difference between the effect of its own motion and the effect of gravity.

If an unprogrammed variation in the earth’s gravitational force pulls the missile fractionally down, the guidance system has no way of distinguishing that movement from an equivalent force produced by an upward motion of the missile. Detecting what it records as an unprogrammed upward motion, the system adjusts the missile’s trajectory accordingly—off course.

Extensive satellite observations enable the missile’s programmers to supply its computer with reasonably precise information about the gravitational forces it will meet while traveling over the North Pole toward the Soviet Union (or vice versa). Even so, scientists operating in this field concede that enough anomalies in the earth’s gravitational field occur for precise prediction to be impossible, as demonstrated by uncertainties about the position of satellites now aloft.

A solution being considered is for the missile to carry a device known as a gravity gradiometer, which would measure such anomalies during the flight, and pass the information to the guidance system. Although such an instrument is theoretically possible, and indeed a large-scale working model has been demonstrated in the benign conditions of the Draper laboratory in MIT, even scientists intimately involved with the program doubt whether a practical model of the size and sturdiness necessary for operational use can ever be developed.

The atmospheric forces affecting the missile present even greater problems, which appear insuperable even on a theoretical level. Detached from its rocket, the warhead hurtles toward its target at an initial speed of some 12,000 miles per hour, descending into the atmosphere at a relatively shallow angle. The atmosphere extends upward in irregular contours to anywhere from fifty to one hundred miles above the earth’s surface. Given its re-entry angle of about 25° the warhead has to penetrate the atmosphere for a distance ranging between 120 and 240 miles. This atmosphere is far from placid, and is largely unpredictable.

To take one example, which produces some twenty possible variables: a solar flare will drastically affect the density of the atmosphere. But the effect will not be constant; it will be determined by the season of the year, whether it is day or night, whether the patch of atmosphere in question is over snow or ice, and by meteorological conditions generally. Since it is impossible to isolate the effect of one variable—the interaction, let us say, between the solar flare and a positively charged cloud—and all the other variables (day/night, etc.) which condition the effect of the flare on the atmosphere, it is impossible even in theory to construct an accurate profile of the atmosphere.

Yet such a profile is essential if the guidance system is to release the warhead at the correct instant and angle to secure the desired results. What is true of solar flares is true of innumerable other, constantly changing meteorological events: the jet streams above 30,000 feet, barometric pressure, a thunderstorm anywhere along the re-entry trajectory, and so forth.

A layman might regard the variations in wind and atmospheric density as immaterial to the direction of a missile decelerating from 12,000 miles per hour toward its target. This view is not shared by guidance specialists,13 who have long acknowledged that one of the main problems in designing a re-entry vehicle is to strike a balance between streamlined high-speed shapes (which cause fatal overheating) and blunt, low-speed shapes, which can be blown disastrously off course by the wind.14

Such are the conditions and uncertainties which determine the flight and affect the accuracy of a missile. The testing of these missiles, a process which produces the apparently confident CEPs and kill probabilities, has not alleviated the problem.

US strategic land-based missiles are test-fired from Vandenberg Air Force Base in southern California to Kwajalein lagoon in the Marshall Islands. Soviet missiles are test-fired from nothern European Russia to the Kamchatka peninsula, at the eastern end of Siberia, or beyond this point into the northern Pacific.

Testing conditions are very different from the realities of a nuclear exchange, and the problems have been pithily summed up for us by Dr. Richard Garwin, of the IBM Research Center, former presidential scientific adviser, currently a member of the Defense Science Board, and professor of public policy at Harvard University:

In every ICBM you have an inertial package. Accelerometers and gyros and things like that are mounted in your missile. You’ve got to fire your missiles from operational silos to points in your enemy’s country. Now, obviously you’ve never done this before and so you have to base your calculation on test shots—in our case from Vandenberg to Kwajalein lagoon, that is, east to west; and in the Russians’ case from northern European Russia to Kamchatka in the northern Pacific, west to east. Judging from how far each test shot falls from the target, you adjust your accelerometer or your gyro, to compensate for the inaccuracy, until in the end your test shots are landing within the prescribed area. But every time you fire a new-model missile over the same range or the same missile over a slightly different range, the bias changes. Sometimes it is greater, sometimes it is smaller, but it never has been calculated beforehand.

So you have to go back to readjusting-the gyros and so on, to try and eliminate the novel bias. But if we were firing operationally, both we and the Russians would be firing over a new range in an untried direction—north. And a whole new set of random factors would come into play—anomalies in the earth’s gravitational field, varying densities of the upper atmosphere or unknown wind velocities. They may adjust and readjust in testing and eventually they might feel sure that they have eliminated the bias. But they can never be absolutely certain. We certainly cannot be: and although we are less well informed about the Russian ICBM test program than our own, there is no reason to suspect that they are any more successful than we are at dealing with the problem. If you cannot be sure that you would be able to hit the enemy’s silos, then there is no point in even trying—because the idea is that one side could wipe out the other’s missiles before they are launched in a first strike.15

It goes without saying that there is one further difficulty. The laborious adjustments necessary to reduce bias described by Dr. Garwin will be difficult to achieve over the novel northern trajectory, since no previous test data are available. The first shot is presumptively the only shot.

Such plain speech as that of Dr. Garwin is rare. Imputations of inaccuracy and discussion of the bias factor do not regularly take their place among the public pronouncements of the Defense Department. To a recent inquiry Colonel Alan MacLaren (USAF), in the Office of Dr. Seymour Zeiberg, deputy under-secretary for Strategic and Space Systems, replied, “I do not know what prompts Dr. Garwin to make these statements. You cannot say that these factors [gravity, atmosphere] can be neglected, but they are unimportant if we have done our homework right…. We understand these factors well enough so that they are not so serious than we do not have [silo-busting] accuracy.” But it appears unlikely that Dr. Garwin’s views will be lightly dismissed. Dr. Albert C. Vosburgh, former deputy for Strategic and Space Systems, Office of the Assistant Secretary of the Air Force for Research, Development, and Logistics, said recently, “Dr. Garwin is a brilliant scientist, whose arguments on any scientific or technical subject should be taken seriously.”

The problems have been forcefully acknowledged by one senior Defense Department official in a position to know. On March 4, 1974, in secret testimony before the Arms Control Subcommittee of the Senate Foreign Relations Committee, subsequently declassified, then Secretary of Defense James Schlesinger said:

I believe that there is some misunderstanding about the degree of reliability and accuracy of missiles…. It is impossible for either side to acquire the degree of accuracy that would give them a high confidence first strike, because we will not know what the actual accuracy would be like in a real world context. As you know, we have acquired from the western test range a fairly precise accuracy, but in the real world we would have to fly from operational bases to targets in the Soviet Union. The parameters of the flight from the western test range are not really very helpful in determining those accuracies to the Soviet Union. We can never know what degrees of accuracy would be achieved in the real world….

The point I would like to make is that if you have any degradation in operational accuracy, American counter-force capability goes to the dogs very quickly. We know that, and the Soviets should know it, and that is one of the reasons that I can publicly state that neither side can acquire a high confidence first strike capability. I want the President of the United States to know that for all the future years, and I want the Soviet leadership to know that for all the future years.

The situation, as Dr. Garwin acknowledges, has not changed significantly since Schlesinger made these amazingly forthright observations.

The only way out of the impasse currently being considered is to contrive a method of guiding the warhead through the terminal stages of its descent. One method is to employ a navigational system (known as NAVSTAR, or Global Positioning System) now under development for nonstrategic military and civilian purposes. The system is to employ a series of satellites, transmitting radio signals which will allow the receiver to determine its exact position. A maneuverable warhead, equipped with such a receiver, could correct its course as it descended. In fact two missile-borne receiver sets have reportedly flown on Minuteman test missiles.16

Alluring as such a solution may seem, it runs into the objections which forced the reliance on an inertial guidance system for ICBMs, rather than on exterior radio communication of the sort used to control the initial ascent of the German V2 during the Second World War. Such exterior communications are inevitably vulnerable to interference by the enemy, in the form of jamming. Since it is unlikely that the satellite could generate power greater than 200 watts for transmission of the signals, a series of ground-based jammers, of one kilowatt each and a hundred miles apart, could effectively neutralize the system for guiding re-entry vehicles right to their targets. Dr. Garwin believes that before the re-entry vehicles are released from the missile the NAVSTAR could be used to give submarine and other mobile missiles accuracies at least as great as present land-based missiles. However, this could make the missile crucially dependent on a satellite system itself highly vulnerable to enemy action.

Other schemes have been proposed to overcome accuracy deficiencies of the self-contained inertial navigation approach. Most of these rely on having the warhead of the missile home in on some “signature” of the target—for example, radar, infrared, or optical images of the area to be hit. None of these schemes has demonstrated that it could cope with simple enemy “spoofing” or jamming of the target signature—countermeasures that are vastly less expensive than the guidance system being proposed. Nor is the solution to be found in a bigger warhead to compensate for inaccuracy. An unverifiable amount of bias uncertainty remains, with the enormous added expense of building one launcher for each warhead—in the case of the MX, two thousand instead of two hundred.

We have concentrated on accuracy in guidance, for this is essential to all conceptions of nuclear war which go beyond the simple objective of leveling a city. Such a focus should not allow many other “data free” premises of the strategic thinkers to go unchallenged.

For example, the nuclear explosive deemed necessary to destroy a silo is a purely theoretical calculation. No practical tests have ever been carried out. The precise power of Soviet warheads, which inspire estimates of US silo vulnerability, is unknown. It is hard to find a convincing refutation of “fratricide”—the effect on an incoming warhead of the thermonuclear explosion caused by one that has already landed.17

So long as US and Soviet leaders pay heed to Schlesinger’s warning of 1974 there remains some hope that they will appreciate that there are no certainties in strategic nuclear warfare. Yet amid the mounting alarmism which has characterized the defense debate in the years since Schlesinger testified, and which has reached a peak of frenzy in this election year, it is hard to retain even that minimal confidence.

Letters

On Target March 5, 1981

  1. 13

    See D.G. Hoag, “Ballistic-Missile Guidance,” essay in Impact of New Technologies on the Arms Race (MIT Press, 1971).

  2. 14

    To obtain at least a rough estimate of how drastic these weather effects might be, some experts have calculated the effect on a re-entering warhead of a band of jet stream between 30,000 and 40,000 feet blowing at a speed of 180 knots. Assuming a drag coefficient of .4, this wind alone would cause a drift off target of 1,065 feet. This is without considering the effects of varying and unpredictable atmospheric densities and other winds.

    Another way of obtaining a rough estimate is to apply an old marksman’s rule for calculating the drift of a bullet in a cross-wind: multiply the speed of the wind by the difference between the real time of flight of the bullet to the target and the time of flight it would have taken if it had been traveling in a vacuum. Dr. Richard Garwin has stated that the re-entry of a warhead takes about 60 seconds. Without an atmosphere the warhead would maintain its re-entry speed of about 20,000 feet per second all the way to impact. Traveling at an angle of about 25° this means it would land in 30 seconds. Thus the difference, given atmosphere, is 30 seconds. Assume a wind of 30 miles per hour, or 44 feet per second, averaged throughout the height of the atmosphere—a relatively low estimate considering that winds aloft can reach three or four times the speed of surface winds. By multiplying this wind speed by the “slow down” time we get 30 × 44 = 1,320 feet. This error alone, which still ignores other important weather effects, is more than enough to cause the MX warhead to fall well outside its lethal radius from the target.

  3. 15

    Statement to the authors, August 5, 1980.

  4. 16

    Military-Electronics/Countermeasures, June 1980, pp. 63-70.

  5. 17

    See George Kistiakowsky, New York Review, May 3, 1979.

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