Theater Missile Defense and the Anti-ballistic Missile (ABM) Treaty
Published as "Highly Capable Theater Missile Defenses and the ABM Treaty" in Arms Control Today, April 1994.
Lisbeth Gronlund, George Lewis, Theodore Postol and David Wright
Since 1972 the Anti-Ballistic Missile (ABM) Treaty has been one of the fundamental building blocks of U.S.-Soviet and U.S.-Russian arms control efforts. By severely restricting the deployment of defensive systems that could undermine deterrent capabilities, the ABM Treaty removed a potential incentive to increase strategic nuclear forces and allowed substantial force reductions to be negotiated in the START I and II agreements. Although the Cold War has ended, the treaty may still have important roles today and in the future. In particular, future deep reductions in U.S. and Russian strategic forces, as well as in the nuclear forces of other countries, may be impossible without the assurances granted by the treaty.
The Clinton administration recently proposed to Russia that the ABM Treaty be modified and that new agreed definitions be adopted to "clarify" how to interpret the treaty. The administration's stated objective is to allow the United States and Russia to develop and deploy anti-tactical ballistic missile (ATBM) defenses capable of engaging theater ballistic missiles (TBMs) with ranges up to 3,500 kilometers. Warheads delivered by missiles of this range would reenter the atmosphere at a speed of 5 kilometers per second, compared to roughly 7 kilometers per second for warheads delivered by a strategic ballistic missile with a range of 10,000 kilometers.
Our analysis indicates that a highly capable ATBM system-one capable of protecting a large ground area against theater warheads reentering the atmosphere at up to 5 kilometers per second-would almost certainly have a significant capability against strategic warheads as well. These proposed changes to the treaty could therefore undermine the core of what the ABM Treaty was designed to prohibit.
Impact of the Gulf War
The Iraqi ballistic missile attacks on Israel and Saudi Arabia during the 1991 Gulf War increased concerns about the potential threat posed to U.S. troops and allies by TBMs. Although the Iraqi attacks were militarily ineffective and caused relatively few casualties, their psychological impact was enormous. Moreover, there is concern that additional countries are acquiring TBMs or weapons of mass destruction or may do so in the not too distant future. Consequently, the United States has shifted the focus of its missile defense program from countering strategic ballistic missiles to developing systems to counter TBMs.
The more advanced theater missile defense systems now being developed by the United States, such as the Theater High Altitude Area Defense (THAAD) system, would have capabilities so great that the ABM Treaty-as currently interpreted-would not allow their testing or deployment. To permit development and testing of such defenses to proceed, the administration has proposed that the treaty be modified to allow a missile defense system to be developed and deployed, provided it is not tested against a target with a peak reentry speed of greater than 5 kilometers per second.
The Clinton administration argues that the proposed changes are simply clarifications, and would not undermine the treaty's ability to limit defenses against strategic ballistic missiles. However, while administration officials and some independent analysts have asserted that these changes will preserve the original intent of the ABM Treaty while allowing highly capable ATBMs, they have presented no analysis to support this position.1
Our analysis demonstrates that the administration's assertion is wrong. Deployment of highly capable ATBM systems is the objective of the THAAD program and of U.S. ATBM programs more generally. Thus, the administration's proposals could well have the effect of eliminating the ABM Treaty as a practical mechanism for preventing the deployment of significant defenses against strategic missiles.
To understand the problems and issues posed by the proposed treaty changes, it is worthwhile to review the restrictions on testing and deployment of missile defenses contained in the ABM Treaty as it is currently written and interpreted, consider the effect of the changes proposed by the Clinton administration, and examine the operational and technical characteristics of missile defense systems that determine their defensive capabilities. Our analysis and calculations illustrate the relative effectiveness expected of a THAAD-like missile defense system against strategic targets, assuming that this defense system can be made effective against theater targets. In sum, if it is possible to develop highly capable ATBM systems of the type that would be allowed by the proposed treaty modifications, such systems would almost certainly have significant capabilities against strategic missiles as well.
Limits on Missile Defenses
By signing the ABM Treaty, the United States and Soviet Union pledged "not to deploy ABM systems for a defense of the territory of its country and not to provide a base for such a defense." The treaty defines an ABM system as "a system to counter strategic ballistic missiles or their elements in flight trajectory," and commits each country "not to develop, test, or deploy ABM systems or components which are sea-based, air-based, space-based, or mobile land-based." However, the treaty does not ban all testing, development and deployment of ABM systems. No limits are placed on the development and testing of ABM systems and components, as long as these are fixed, ground-based and located at agreed test ranges. In addition, each country is allowed to deploy one limited ABM system, with up to 100 interceptors, at a single site on their territories.
The ABM Treaty was not intended to constrain the development and deployment of air defenses or defenses against shorter-range TBMs. Defensive systems that are not ABM systems may be freely developed and deployed as long as they are neither given "capabilities to counter strategic ballistic missiles or their elements in flight trajectory" nor are tested "in an ABM mode." Thus, in its present form, the ABM Treaty not only forbids the testing of ATBMs against strategic missiles, but also forbids the development, testing and deployment of ATBMs that are capable of intercepting strategic missiles or their elements even if they are never tested in such a mode. The treaty also places limits on the size and power of radars that could be used as part of an ATBM system.
However, the treaty leaves ambiguous the distinction between theater and strategic defenses. It neither specifies what a "capability to counter strategic missiles" consists of nor defines the difference between a strategic and theater ballistic missile. Moreover, there is no agreed, or even national, consensus on how to measure "capability." For example, the United States has in the past raised concerns that some Soviet ATBM systems-- which are far less capable than those the United States is now developing-- "may have some ABM capabilities."2
For some time the United States has used the so-called "Foster Box"--a criteria established in 1972 by then-Director of Defense Research and Engineering John Foster-- to distinguish between tests against strategic and theater missile targets. Under this approach, tests of ATBMs are permitted against targets below an altitude of 40 kilometers with reentry speeds less than 2 kilometers per second. This was the understanding presented to Congress by administration officials during treaty hearings in 1972, which has been widely accepted in the United States, but it has neither been formally adopted as U.S. policy nor proposed to Moscow.
The Clinton administration's motive in proposing some clarifications and modifications to the ABM Treaty is to allow the development of ATBMs-- such as THAAD or the Navy's Lightweight Exoatmospheric Projectile (LEAP) system-- that are more capable than would be permitted under the current interpretation of the treaty. Specifically, the administration has proposed to the Russians that any target with a reentry speed of less than 5 kilometers per second (corresponding to a missile range of 3,000 to 3,500 kilometers) be considered non-strategic. In addition, it has proposed that the "capability" prohibition on ATBMs be dropped in favor of a "demonstrated capability" prohibition. Thus, independent of its actual capabilities, a missile defense system would be regarded as a theater, and not a strategic, system as long as it is never tested against a target reentering the atmosphere at a speed greater than 5 kilometers per second.
The administration's justification for setting the threshold between strategic and theater targets at a 5 kilometer-per-second reentry speed is its desire to develop a system that could counter missiles such as the 3,000-kilometer range CSS-2 missile sold by China to Saudi Arabia. The Clinton administration has concluded that such an agreed interpretation, together with the change to a "demonstrated capability" prohibition, is necessary to allow the United States to conduct tests of the THAAD system-scheduled to begin later this year-- without violating the ABM Treaty.
Our analysis shows that these two proposals, taken alone, would effectively eliminate the ABM Treaty as a mechanism for restricting the development and deployment of strategic defenses. Reportedly, the United States and Russia are discussing additional limits on ATBMs, which, when taken together with the two proposals, might allow a meaningful distinction to be made between theater and strategic defenses. However, the two U.S. proposals are so permissive that unless additional limits are extremely restrictive they will not be effective in preserving the original intent of the ABM Treaty.
A Generic Ground-Based Defense
To understand what kind of additional restrictions one might try to place on an ATBM system to limit its capability against strategic missiles, it is useful to look first at some of the technical characteristics of a generic high-altitude, ground-based missile defense system operating without the benefit of a highly sophisticated space-based sensor system such as the proposed "Brilliant Eyes" system (which would raise other questions about treaty compliance). This discussion thus applies to an ATBM comparable in sophistication to THAAD. Such a system consists of several components, including interceptor missiles and launchers, a ground-based radar, and a command and control center. U.S. systems will almost certainly use additional information obtained from space-based sensors, such as the Defense Support Program (DSP) satellites that provided notice of Scud launches during the Gulf War.
To intercept a ballistic missile target, an ATBM system must detect the incoming target and then track it to estimate its future trajectory and determine when and in which direction to launch interceptor missiles. The ATBM system must then launch one or more interceptor missiles and guide them toward the intercept point until they are close enough to be able to acquire the target with their on-board sensors. The interceptors then maneuver themselves close enough to the target to destroy it.
A key measure of the capability of a defense system is its defended "footprint"-- the ground area it can protect against an attacking missile. A defense system does not have a unique footprint; for a given system the footprint will depend on characteristics of the incoming target. Footprint size is determined by a number of parameters, only some of which are under the control of the defense. The footprint depends primarily on the range at which an incoming target can be detected, its speed and radar cross section, and the characteristics of the interceptor. Understanding how these parameters influence the footprint size is important because it indicates those parameters that would need to be controlled in establishing any effective agreement that limits the strategic capability of ATBMs.
Detection Range
The range at which the approaching warhead can be detected and tracked has a powerful influence on the overall performance of a defense system because interceptors cannot be launched until this occurs. The greater the detection range, the more time available for the interceptor to fly to the intercept point, and thus the larger the footprint will be. Alternatively, if detection ranges are too short for an intercept to occur, the footprint will shrink to zero.
Since the trajectory of the attacking missile is not known exactly, the radar must search the area of sky from which the warhead might be arriving. The detection range thus depends on the capability of the radar, the nature of the attacking missile, and the availability and quality of cueing information provided to the radar by other sensors about the missile's trajectory, which helps reduce the sky area the radar must search.
A radar's capability depends strongly on its power and the size of its antenna-- usually expressed in terms of its "power-aperture product," the product of its average power output (in watts) and the area of its antenna (in square meters). The detection range increases as the power-aperture product is increased either by deploying a different radar with greater size or power (or both) or by linking multiple radars electronically (assuming they have been designed to allow this), which will increase the search capability as the square of the number of linked radars.
Two characteristics of the incoming target that affect the detection range are its radar cross section and velocity. The smaller the radar cross section of the target, the shorter the detection range and the smaller the footprint. Faster targets can travel farther during the time it takes for the radar to scan the required search area, thereby decreasing the detection range and the footprint. The detection range can be increased if the radar is given additional information by other sensors. During the Gulf War the United States demonstrated that its early warning satellites could detect Iraqi Scud missiles during their boost phase. By providing information about the time and location of a missile launch, these satellites can limit the area of sky the radar must scan and thus increase the detection range. If additional information about the incoming target's trajectory were available from other sensors, such as an improved DSP satellite or the Distant Early Warning radars used by the United States and Russia to provide warning of missile attacks, the required search area would be further decreased and the footprint increased.
Characteristics of the Interceptor
The most important characteristics of the interceptor for determining the footprint size are its acceleration and maximum speed. In addition, many types of interceptors, such as those using infrared homing, will have a minimum altitude at which they can intercept a target, which will also affect the footprint. Below this altitude, the high speed of the interceptor through the dense atmosphere causes heating that can blind its sensor so it is unable to see and home in on its target. The size of the defended footprint depends on the distance the interceptor can fly from the time it is launched until the target reaches the minimum intercept altitude. The greater the acceleration and speed of the interceptor, the farther it can fly in a given time and the larger the footprint will be.
Separate from the problem of whether there would be adequate time for target acquisition and intercept is the important issue of whether the interceptor will be able to destroy the target once it reaches it. Properties such as the response time and maximum lateral acceleration of the interceptor and the resolution of its sensors will determine the probability of a successful intercept (the "kill probability").
The kill probability against a target will depend in part on the closing speed between the target and interceptor. A 10,000-kilometer range strategic target has a reentry speed roughly 40 percent greater than that of a 3,000- to 3,500-kilometer range theater target, which would correspond to a roughly 30 percent greater closing speed during an intercept attempt. If uncompensated for, this higher closing speed would be expected to result in a somewhat lower kill probability. However, a highly effective ATBM interceptor could easily be overdesigned-- and, in fact, is likely to be overdesigned-- to give it a high kill probability against a variety of targets and countermeasures. Without the ABM Treaty's prohibition against giving a system ABM capability, there would be nothing to restrict such overdesign, which would be difficult to verify in any event. Thus, if an interceptor has a high kill capability against a 5 kilometer-per-second theater target it can also be expected to be effective against a strategic target.
Target Characteristics
The characteristics of the target will also affect the size of the defended footprint. Target reentry speed, which increases with the missile's range, will directly affect the footprint. The faster the target, the shorter the time between its detection and the time it reaches the minimum intercept altitude. As discussed earlier, a faster reentry speed or a smaller radar cross section will also reduce the detection range, thus shrinking the size of the footprint.
Substantial trade-offs can be made between various parameters to maintain a defense system's performance under quite varied conditions. A longer detection range can compensate for a less capable interceptor or a faster target, and a more capable interceptor can compensate for a shorter detection range or faster target. Increasing the power-aperture product of the radar can compensate for a slower interceptor or for a target with a smaller radar cross section or greater reentry speed. This ability to vary a number of parameters of the system is one of the core problems in trying to establish a set of limits that are sufficiently restrictive to substantially limit the strategic capability of a system like THAAD.
Highly Capable ATBMs
A key question raised by the administration's proposals is how effective a highly capable ATBM system would be against a strategic target. That is, what is the inherent ABM capability of a system that can achieve a high success rate against ballistic missiles with ranges of 3,000 to 3,500 kilometers? Our analysis assumes an ATBM system roughly similar to THAAD, and further assumes it can be tested against targets with reentry speeds of 5 kilometers per second, as would be allowed by the U.S. proposal. It does not attempt to assess the effectiveness of such an ATBM system, particularly in the face of countermeasures, but simply considers how effective such a system would be against strategic targets if it is effective against targets reentering the atmosphere at 5 kilometers per second.
The key parameter in this analysis is the size of the ATBM's defended footprint against a strategic target compared to its footprint against a theater target. If the ATBM could defend a significant area against a strategic missile, it would clearly undermine or violate the intent of the ABM Treaty.
To assess the relative performance of a highly capable ATBM system against theater and strategic warheads, and how this performance depends on various parameters, we have constructed a simple -computer model of a THAAD-like ATBM system and calculated the defended footprint against different attacking missiles.3 Although few detailed characteristics of THAAD are available, public descriptions of the system indicate that its interceptors will reach speeds of 2.5 to 2.7 kilometers per second and will be able to make intercepts at ranges of up to 200 kilometers. The system's radar reportedly has a nominal detection range of about 500 kilometers. However, both the intercept range and detection range depend on the target that is assumed, and the nature of the assumed target has not been publicly discussed.
The model's ground-based radar is assumed to have a power-aperture product of 500,000 watts-meter squared. The radar, which looks forward in a 120 degree sector, is assumed to be located 20 kilometers behind the interceptor launcher. The approximate launch point of an attacking missile is assumed to be provided by DSP satellites, which reduces the angular search required. The search area in each case depends not only on the anticipated detection range, but also on the size of the footprint to be defended, and is determined iteratively. A five-second delay after detection is assumed for tracking and system computations before an interceptor can be launched. The interceptors are assumed to boost for 17 seconds and achieve a maximum velocity of 2.6 kilometers per second. The system is also assumed to have a minimum intercept altitude of 40 kilometers-the assumption of a lower minimum intercept altitude would somewhat increase the size of the defended footprints.
The results of the model should be regarded only as a qualitative picture of the potential performance of a defense system, since a number of simplifying assumptions have been made. However, the focus here is on the relative performance of the system against strategic versus theater missiles, so the conclusions should be relatively insensitive to the details of the model.
Figure 1 shows the defended footprints for the hypothetical ATBM system against a theater warhead reentering the atmosphere at 5 kilometers per second and against a strategic warhead reentering at 7 kilometers per second. Both targets are assumed to have a radar cross section of 0.05 square meters, leading to a detection range of 300 kilometers against the theater target and a somewhat greater detection range of 360 kilometers against the strategic target (since the required search area is smaller in this case). The footprint against the theater target is 150 kilometers across, well beyond what is required to protect most metropolitan areas. This footprint is somewhat smaller than that usually attributed to THAAD, which may be because such footprints assume a target that is slower or has a larger radar cross section. For the same ATBM system, the defended area shrinks only slightly against a strategic target: it is 70 percent as large as that against the -theater target-a footprint that is still sufficiently large to protect a major metropolitan area.
The above figure shows the defended footprints calculated for a THAAD-like anti-tactical ballistic missile (ATBM) against a 3,000-kilometer range theater missile (solid line) and a 10,000-kilometer range strategic missile (dashed line). The footprints in this model assume the ATBM radar has a power-aperture product of 500,000 watts-meter squared and that the attacking reentry vehicles (coming from the top of the figure) have a radar cross section of 0.05 square meters.
The above figure shows the defended footprints calculated for a THAAD-like anti-tactical ballistic missile (ATBM) against a 3,000-kilometer range theater missile (solid line) and a 10,000-kilometer range strategic missile (dashed line). The footprints in this model assume the ATBM radar has a power-aperture product of 500,000 watts-meter squared and that the attacking reentry vehicles (coming from the top of the figure) have a radar cross section of 0.05 square meters.
The footprints shown in Figure 1 assume the system faces only one attacking warhead at a time. If more than one target is approaching a given radar roughly simultaneously, the defended footprints will generally shrink relative to those shown in Figure 1 because the radar must divide its search capability between the incoming targets, thereby decreasing its detection range. On the other hand, if more advanced early warning satellites are deployed that are able to provide more accurate missile trajectory data, the defended footprints will be larger than those shown in Figure 1. These factors, however, would reduce or enlarge the footprint against both theater and strategic missiles, and the relative size of the resulting theater and strategic footprints should be roughly comparable to those shown in Figure 1.
In the case of the strategic target footprint shown in Figure 1, the ATBM radar and launcher are not within the defended footprint. This is also true for the footprints against low-radar cross section theater warheads (shown in Figures 2 and 3). The lack of a self-defense capability is not necessarily as serious a problem as it might seem, since the high mobility of planned U.S. ATBM systems would make them difficult to attack. The footprint of the ATBM system could be extended forward by increasing the power-aperture product of its radar, increasing the speed of its interceptors, or lowering their minimum intercept altitude. The launcher and radar might even be protected by a shorter-range ATBM system. The most important point, however, is that even with this potential vulnerability, deployment of systems that could defend very large areas against strategic missiles runs counter to the basic purpose of the ABM Treaty.
Countermeasures
While there is little doubt at least moderately effective ballistic missile defenses could be built against threats that do not employ countermeasures to defeat the defense, an actual defense system must be designed with the assumption that an adversary will be uncooperative and will present the defense system with a variety of inherently unpredictable countermeasures. As amply demonstrated by Patriot's failure to destroy Scud missiles during the Gulf War, effective countermeasures do not require high technology. The inadvertent breakup of Scud missiles during reentry proved to be an effective countermeasure. According to one post-conflict assessment, "In effect, what Iraqi engineers had created, purely unintentionally and by poor workmanship and design, was a high-speed, low radar-cross-section maneuvering reentry vehicle (RV), accompanied by decoys."4 A country capable of building TBMs with ranges of up to 3,500 kilometers should have little difficulty implementing -relatively unsophisticated but potentially very effective countermeasures. The main uncertainty about the potential performance of a ballistic missile defense system will be its effectiveness against countermeasures-- intentional or otherwise.
One of the most effective and readily implemented countermeasures would be for an attacker to reduce the radar cross section of its reentry vehicle. This could significantly reduce the defended footprint, and possibly even prevent a successful intercept. One cannot assume that the radar cross section of a strategic target would be significantly lower than that of a theater target the ATBM might face. While current Russian strategic reentry vehicles may have, or could be given, significantly lower radar cross section values than those of existing theater RVs, the basic principles involved in shaping a reentry vehicle to reduce its radar cross section (bringing its nose to a sharp point and rounding its back edges) are well-known and relatively easily implemented. Moreover, when going against a high-altitude interceptor, it will be easier to use radar-absorbing materials since they would only need to function in the near-vacuum of space. By the time a reentry vehicle begins to reenter the atmosphere-- burning off its sharp nosetip and radar-absorbing coating-- these radar cross section reduction techniques will already have accomplished their objective of reducing the detection range. Certainly, these techniques would also be within the capability of a country that can build a 3,000-kilometer range missile.
Figure 2 shows the footprint of the model ATBM system against theater and strategic targets with radar cross sections reduced from those used in Figure 1 by a factor of 10 (to 0.005 square meters). While both footprints shrink considerably, the footprint against the strategic warhead is still a substantial fraction of that against the theater warhead. If the radar cross section of the targets was further reduced, eventually it would reach a point where the system had no capability against the strategic target. At this point, it would retain some capability against the theater target, but the protected area would be much smaller than the footprint shown in Figure 2 (below) and those associated with THAAD and other advanced ATBM systems currently under development.
The reduction in footprint size against targets with low radar cross sections could be at least partly offset by better cueing or by increasing the capability of the radar so that the targets could be detected at greater distance.
The above figure shows the defended footprints calculated for a THAAD-like ATBM system against a 3,000-kilometer range theater missile (solid line) and a 10,000-kilometer range strategic missile (dashed line). The attacking reentry vehicles (coming from the top of the figure) have a radar cross section of 0.005 square meters, a reduction by a factor of 10 from those used in Figure 1.
The above figure shows the defended footprints calculated for a THAAD-like ATBM system against a 3,000-kilometer range theater missile (solid line) and a 10,000-kilometer range strategic missile (dashed line). The attacking reentry vehicles (coming from the top of the figure) have a radar cross section of 0.005 square meters, a reduction by a factor of 10 from those used in Figure 1.
Figure 3 shows the defended footprints against the same low-radar cross section target with the power-aperture product of the radar increased by a factor of four (to 2 million watts-meter squared). This increased power-aperture product would still fall well below the existing ABM Treaty limit of 3 million watts-meter squared. As Figure 3 shows, the defended footprints against the low-radar cross section targets are now substantially larger than those in Figure 2.
The above figure shows the defended footprints calculated for a THAAD-like ATBM system against a 3,000-kilometer range theater missile (solid line) and a 10,000-kilometer range strategic missile (dashed line) whose reentry vehicles (coming from the top of the figure) have a radar cross section (0.005 square meters). In this case the ATBM's power-aperture product has been increased by a factor of four.
The above figure shows the defended footprints calculated for a THAAD-like ATBM system against a 3,000-kilometer range theater missile (solid line) and a 10,000-kilometer range strategic missile (dashed line) whose reentry vehicles (coming from the top of the figure) have a radar cross section (0.005 square meters). In this case the ATBM's power-aperture product has been increased by a factor of four.
Under the modifications to the ABM Treaty the administration is proposing, systems more capable than THAAD could also be developed and deployed, as long as they were not tested against targets with reentry speeds of greater than 5 kilometers per second. Thus, the footprints could also be increased by means other than increasing the radar's power-aperture product. For example, faster interceptors could be deployed, better cueing information could be provided and interceptors could be deployed at additional sites away from the radar.
The key point is that if an ATBM system has a large footprint and is highly effective in dealing with plausible countermeasures that might be employed in designing a TBM, it is also very likely to retain its effectiveness against strategic missiles employing countermeasures. Conversely, if countermeasures exist that could be used by strategic missiles to defeat an ATBM system, similar countermeasures used by TBMs are also very likely to defeat the ATBM system or, at a minimum, greatly degrade its performance.
Consequences and Implications
The above analysis shows that highly capable ATBM systems that are limited only by a test ban against targets reentering the atmosphere at speeds greater than 5 kilometers per second would almost certainly have significant capabilities against strategic RVs. It thus clearly indicates that the Clinton administration's two proposals-- the 5 kilometer-per-second reentry speed threshold and the replacement of the "ABM capability" prohibition on ATBMs with a "demonstrated capability" prohibition-- would, if implemented in isolation, significantly erode the ability of the ABM Treaty to control strategic defenses by allowing systems that could defend areas of tens of thousands of square kilometers.
A crucial problem with the administration's proposal is that it establishes only a 40 percent gap between the reentry speed of strategic warheads and the maximum reentry speed that ATBM systems can be tested against. Without other limitations on the capability of the ATBM systems, this gap is not wide enough.
If an ATBM system is both robust against countermeasures and has a large footprint (with a radius of 100 kilometers or more) against 3,000- to 3,500-kilometer range TBMs, it will inevitably have a substantial capability against strategic targets. While its footprint against strategic missiles would be somewhat smaller, it would still be large enough to protect a city and its surrounding suburbs.
Discussions are apparently underway between Russia and the United States about limiting the maximum speed of the interceptor as a way of limiting the strategic capabilities of ATBMs, and there are some reports that Moscow may be willing to set a limit as high as 3 kilometers per second. Our analysis shows that an interceptor with a maximum speed of 2.6 kilometers per second, when used with radars of the size being discussed for THAAD, could lead to very large footprints against strategic missiles, indicating that the interceptor speed limit would have to be significantly less than 2.6 kilometers per second if the ABM Treaty is to remain meaningful. Further analysis must be done to establish how capable an ATBM system could be without undermining the treaty, but as is clear from this analysis, it must have much less capability than is proposed for THAAD.
By imposing limits on the number of ATBM radars or launchers, the effectiveness of an ATBM system against a large-scale strategic attack could be minimized. These systems (or any other missile defense system) will not be 100 percent "leakproof" in the face of enemy countermeasures, and if the systems were numerically limited they could be saturated by existing U.S. and Russian forces.
However, allowing the development of ATBM systems with the capability against strategic missiles described here would result in a situation far different from the original intent of the ABM Treaty, and would have important implications for nuclear targeting and strategy as well as for future efforts to reduce nuclear arms.
The importance of developing and deploying an ATBM system to counter long-range theater missiles should be fully debated, taking into account the consequences for the ABM Treaty and thus for efforts to reduce other threats facing the United States, such as the large nuclear arsenals remaining in Russia and other nuclear-weapon states. The current U.S. approach of developing and deploying ATBM systems designed to defend against long-range theater threats needs to be carefully assessed in the context of other means of addressing the problem of missile proliferation, including diplomacy, export controls and arms control.
The question of the compatibility of highly capable ATBMs and the ABM Treaty is a complex one and further work is needed in this area. In addition, until the results of the U.S. and Russian discussions are known publicly, it is premature to assess in detail their impact on the treaty. But as our analysis indicates, it is far from obvious that it will be possible to deploy highly capable ATBMs without seriously undermining the ABM Treaty. It also casts serious doubt on statements by administration officials that the proposed changes are only minor "clarifications" to the treaty.
The ABM Treaty has been central to U.S. security planning for more than 20 years, and changes to it should not be made without a full understanding of their implications and consequences. The Clinton administration needs to support its proposed changes to the ABM Treaty with a clear analysis documenting the impact of these changes on the treaty. In particular, if the administration believes that its proposed changes will allow the deployment of highly capable ATBMs without undermining the ABM Treaty, it should present a clear analysis to support this claim. Given the central role of the treaty in U.S. security planning, there should be a full and open discussion of the effects of the proposed treaty modifications before any changes are made.
Notes
1 For an argument that effective ATBMs and a strengthened ABM Treaty are compatible, see Sidney N. Graybeal and Michael Krepon, "It's not son of Star Wars," Bulletin of the Atomic Scientists, Vol. 50, No. 2, March/April 1994, p.16.
2 Matthew Bunn, Foundation for the Future: The ABM Treaty and National Security, Washington, D.C.: Arms Control Association, 1990, p. 82.
3 Details of the analysis will be published elsewhere. The computer model follows the general approach outlined in Jurgen Altmann, SDI for Europe? Technical Aspects of Anti-Tactical Ballistic Missile Defenses, Frankfurt Peace Research Institute, 1988.
4 Robert M. Stein, Manager of Advanced Air Defense Systems, Raytheon Company, "Correspondence: Patriot Experience in the Gulf War," International Security, Summer 1992, pp. 199-225 (p. 212).
