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CPSR Newsletter Vol 19, Number 2
Volume 19, Number 2 The CPSR Newsletter Spring 2001

National Missile Defense: The Trustworthy Software Argument
by William Yurcik

The National Missile Defense (NMD) system being developed by the U.S. revisits many of the technical software engineering arguments surrounding the Strategic Defense Initiative (SDI, or known as "Star Wars" to critics). Unfortunately in the public NMD debate thus far, questions of technical capability have been ignored focusing instead on non-technical factors. While acknowledging non-technical factors are relevant and perhaps predominant in the final decision-making, we feel a special obligation to specifically raise technical software engineering issues in this CPSR forum because the NMD policy-making process needs input from computing professionals who both understand and can communicate these issues. I have included a list of these non-technical factors for completeness.

The stated purpose of NMD is to protect the US from a limited attack by strategic (long-range) ballistic missiles armed with weapons of mass destruction (conventional, nuclear, chemical, biological). The scenarios for such a limited attack of a few to tens of missiles include: (1) an accidental or unauthorized launch from Russia; (2) an accidental, unauthorized, or deliberate attack from China; or (3) a deliberate attack from other countries that have or may acquire long-range missiles (e.g., North Korea, Iraq, Iran). Ballistic missiles are rocket-driven missiles that have three distinct stages: (1) the boost stage from launch within earth's atmosphere to space; (2) the midcourse stage in the vacuum of space; and (3) the terminal stage reentering the earth's atmosphere to the target. Ballistic missiles propulsion occurs only in the boost stage then the missile follows a path according to the laws of physics in the Earth's gravitational field to its target. Thus NMD has only limited capability and is vulnerable to both being "underflown" by short-range missiles and cruise missiles as well as being "overflown" by intelligent ballistic missiles that do not follow the laws of gravity but rather can intelligently maneuver.

The NMD system being developed would use a combination of ground-based radars and satellite-based sensors to detect a missile launch and track the missile and its warhead(s). The ground-based missile interceptors will be layered to engage incoming attacks at different stages in their flight paths to enable a shoot-look-shoot strategy [9]. If one system fails to stop a missile at one stage, other systems may have another chance to shoot it down. Small land-based, aircraft-based, and sea-based interceptors will engage attacks closer to the source at the boost stage. Theater defenses are focused on defense at the terminal stage. Although space-based lasers are still many years away, the intermittent research of the past two decades seems likely to continue.

The NMD software engineering issues revolve around the issue of trustworthy software. The term "trust" in software can have different contexts. The oldest use of the term refers to the DOD "Trusted Computer Systems Evaluation Criteria" (the Orange Book) which defines levels of trust protection (C to A). To address software integrity issues during SDI, the Trusted Software Development Methodology (TDSM) was developed to define trust levels (1-5) based upon 25 trust principles [2]. TSDM has much in common with the Software Engineering Institute (SEI) Capability Maturity Model (CMM) for software development in that rather than being based on product testing it is focused on the software development process. It is generally accepted that reliability cannot be "tested into" a software system but rather planned and developed in parallel with the software system itself. Unfortunately, risks in the software development process have gotten worse since SDI with many examples of complex system software failures [4].

Even critics now concede that an NMD system is a possible task but the over-riding question is whether the software can confidently be expected to work when it is needed. If the NMD system cannot be trusted then it may actually reduce strategic security since subsequent actions by other nations will be based on prudent assumption of reliability. Factors that inspire human trust in software include: (1) extensive past experience under actual working conditions; (2) ultimate safeguarding of critical operations by human operators; and (3) a fully specified, predictable, and stable environment for design, testing, and actual operation. None of these factors will be present in the final NMD system.

Some may argue that NMD testing is currently taking place. On closer inspection, the tests thus far have not been very meaningful. The tests are actually "hardware-in-the-loop" simulations with unrealistic conditions (reduced closing speeds, lack of credible countermeasures) and very few unknown variables (known launch times, target trajectories, target and decoy signatures) [8]. So far, 2 of 4 NMD tests have failed to intercept, 1 had a successful intercept despite a software failure, and 1 test was completely successful. For perspective, military systems that require destructive testing are rarely tested enough to provide a statistically significant measure of their reliability so this information must be gained from combat experience. Ironically, it is reasonable to assume that NMD will receive little if any combat experience but its development test schedule is being compressed. For perspective, the submarine-launched Polaris missile system (the backbone of US Cold War nuclear deterrence) failed its first 11 tests. On the other hand, the NMD requirement for an interceptor 85% kill probability can be contrasted with the well-maintained B-2 bomber which is capable of performing its mission only about 45% of the time. The Patriot anti-missile system actually had a perfect test record (17 for 17) but in the Gulf War the Iraqi Scud missiles did not fly predictable smooth test trajectories - they broke apart upon reentry and the Patriot success rate estimates vary widely from 70% to below 10% despite military and media hype [7].

The con-SDI trustworthy software argument first stated by David Parnas in 1985 [1,6] and later echoed by others (including CPSR [5]) are still valid for NMD [3]. Parnas stated in 1985 that it is not possible to construct SDI software that could confidently be expected to work when needed [1,6]. He makes this argument, independent of software size, based on (1) lack of specifications, (2) lack of realistic testing, and (3) lack of backup system capability given time constraints (no "real-time debugging" is possible). Since 1985, not much has changed to nullify this argument. Software engineering principles are still based on precise specifications for requirements. Although computing power has improved and formal methods have been introduced, testing multi-million line programs is still beyond our current capability. Errors in large software system is still the norm, Peter Neumann has a large archive of examples [4].

A new trustworthy software argument that was alluded to by Parnas in 1985 [1] has been introduced for NMD. This argument is based on the fallacy that since NMD is a subset of the SDI mission then it is possible to "overengineer" NMD into being trustworthy. "Overengineering" is based on the continuous system concept of tolerance in which small system changes do not result in large changes in output behavior due to overcapacity protection (e.g., extra safety margins for bridge construction). For discrete systems, the concept of tolerance does not hold -- an error of one bit may be as catastrophic as an error of 1000 bits. While new fault tolerant software techniques have been introduced to address software decay and software aging the dilemma is that adding functionality to solve these problems results in software that is even more complex and thus difficult to verify and validate [10].

Finally, NMD has the unique requirement that it must work perfectly on first use. This is an unprecedented requirement for any system approaching the size and complexity of NMD. Comparable systems include the US Navy's Aegis anti-missile system, Lucent's 5ESS telephone switch software, and the Microsoft Windows operating system. These systems have had decades to find and debug errors and yet all have had, and continue to have, significant failures. In fact, new software upgrades on these proven systems rarely perform reliably despite rigorous testing and simulation. Thus it is indeed likely that NMD will experience a software failure upon first use -- the question is will this software error prove catastrophic to overall intercept functionality.

In conclusion, an NMD system can be built but will its software ever be trusted? The idea of a national shield is a simplistic solution to a complex problem with many factors. Non-technical factors will dominate the final political decisions unless technical factors are introduced into the debate. I urge CPSR members to look closer at the technical software argument I have raised here and get involved in the policy process. There is the dual responsibility to protect against massive loss of human life, terrorist blackmail, and strategic military interests if it can be accomplished and yet the responsibility to prevent dependence upon untrustworthy software that actually decreases national security. The administration's current position is the rapid deployment of an expanded version of NMD regardless of test outcomes.

William Yurcik ( is an Assistant Professor within the Department of Applied Computer Science at Illinois State University in Normal Illinois USA. Prior to his academic career, he worked for organizations such as the Naval Research Laboratory, MITRE, NASA, Verizon, and MITRE.


[1] Dertouszos, M., D. Parnas, C. Seitz, J. Weizenbaum, and D. Cohen, Star Wars: Can the Computing Requirements Be Met?
Computer Professionals for Social Responsibility (CPSR) - MIT Debate, MIT, 1985.

[2] Leahy, P. J. The Need for Trusted Security Software,
National Security and Emergency Preparedness Telecom News, National Communications Systems, Issue 2, 1997.

[3] Neumann, P. G. Inside Risks: Missile Defense,
Communications of the ACM, Vol. 43 No 9, Sept. 2000, p. 128.

[4] Neumann, P.G. moderator. Risks Digest: Forum on Risks to the Public in Computers and Related Systems,
ACM Committee on Computers and Public Policy. <>

[5] Parnas, D.L., D. Redell, Jim Horning, M. Rotenburg, G. Chapman, C. Johnson, and E. Roberts. CPSR Open Letter on the Strategic Defense Initiative,
CPSR News, Vol. 8, No. 1-2, Winter-Spring 1990.

[6] Parnas, D.L. Software Aspects of Strategic Defense Systems,
Communications of the ACM, Vol. 28, No. 12, Dec. 1985, pp. 1326- 1335.

[7] Performance of the Patriot Missile in the Gulf War, Activities of the House Committee on Governmental Operations, 102nd Congress,
Report 102-1086, April 7 1992, pp. 179-188.

[8] Sessler, A.M. et. al., Countermeasures: A Technical Evaluation of the Operational Effectiveness of the Planned US National Missile Defense System,
Union of Concerned Scientists/MIT Security Studies Program Report, April 2000.

[9] Special Issue on Ballistic Missile Defense: It's Back,
IEEE Spectrum, Vol. 34 No. 9, Sept. 1997.

[10] Yurcik, W. and D. Doss, Achieving Fault-Tolerant Software with Rejuvenation and Reconfiguration,
IEEE Software, July/August 2001, pp. 48-52.

Appendix: Non-Technical Factors in the NMD Debate

  • The planned NMD would violate the 1972 Anti-Ballistic Missile (ABM) Treaty which prohibits both the US and Russia from deploying nationwide defense against ballistic missiles. This may result in an unstable situation leading to an arms race but technically the USSR was the signatory to the ABM Treaty (not Russia) and much has changed since 1972 in terms of politics and technology. Simply, NMD is a clear step outside of mutually assured destruction (MAD) that has served as deterrent to nuclear war for 30 years.

  • Delivery systems for weapons of mass destruction (conventional, nuclear, biological, chemical) have become more flexible such that a ballistic missile attack may not be the preferred method used by terrorists or a rogue nation.

  • While numerous rogue nations hostile to the U.S. (North Korea, Iran, Iraq) are seeking to develop or acquire long-range ballistic missiles, ballistic missile attack is not a current threat and there are differences of opinion about when ballistic missile attack will become an actual threat.

  • Allies from Europe and the Middle East (especially Israel) are urging the U.S. to expand its national missile defense to an umbrella global missile defense since allied nations feel they may be targeted if left unprotected from missile attack. Prime targets include overseas U.S. military bases. The only known allied cooperative effort with the goal of an interoperable tactical missile defense was started in 1988, the joint U.S.-Israeli "Arrow" anti-ballistic missile program.

  • The U.S has already spent $5.5 billion on NMD research and development since 1991 and the final cost estimates for a deployed NMD system range from $20 billion to $200 billion. Critics say that this money would be better spent on conventional defense systems, and diplomatic efforts. To weigh the cost of NMD, the authors were unable to find any recent damage cost estimates for a hypothetically successful nuclear missile attack against a major U.S. city by terrorists or a rogue nation. One reason such studies may not exist is that such an attack would be so devastating in loss of human life that it should be avoided at all costs.

  • NMD does have some undeniable economic benefits. Some see NMD as an opportunity to develop expanded research funding for computer science and do not want to hinder this bonanza. Defense contractors have been among the most active supporters of NMD: in 1997 and 1998, Boeing, Lockheed Martin, Raytheon, and TRW spent $35 million on NMD lobbying. It is also clear that a continuous defense infrastructure is needed for the long-term development of systems rivaling the complexity of NMD and that benefits of such investment may not be immediately apparent.

  • While the U.S. has no missile defense system, Russia maintains a missile defense system of approximately 100 interceptors protecting hundreds of miles around Moscow, its national capital.

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