Layered Defenses and Interceptor Efficiency

Continued from:
THAAD: What It Can and Can’t Do

While two THAAD batteries can be deployed in such a way to cover all of South Korea, an additional critical question is how effective will the system be in destroying incoming missiles. Because THAAD intercepts targets at altitudes above 50 km and is capable of protecting large areas, it ideally complements the lower-tier PAC-3, which protects point targets. In essence, intercepting targets at multiple levels, or tiers, offers more opportunities to succeed and improves intercept efficiency, which is the calculated number of interceptors needed to achieve a specified measure of protection. Interceptor efficiency is governed primarily by the probability an individual interceptor will collide with and destroy a missile or warhead. It is often referred to as the “single-shot probability of kill,” or SSPk. Historically, missile-defense designers at the US Missile Defense Agency have sought to achieve SSPk values of between 0.8 and 0.9, which means a single interceptor should succeed 80 to 90 percent of the time. Recent development and validation testing of THAAD indicate a kill probability of 0.8 is feasible, though design goals and test results may not be replicated under wartime conditions. Nonetheless, assuming an SSPk of 0.8 offers a measuring stick for evaluating the theoretical benefits of deploying THAAD in South Korea.

It is unclear what performance criteria South Korea or the US military have established for missile defenses on the peninsula. Two criteria are posited here for purely illustrative purposes. The first criterion would require the missile defense architecture to intercept all attacking threats with a probability of 0.75, and the other would dictate a probability of 0.9 that no attacking missiles leak through the defenses. The latter criterion might be required as an absolute minimum if North Korea is launching nuclear-armed missiles; the former, more relaxed criterion, might be acceptable for conventionally-armed attacks.

If one further assumes that two interceptors are launched at each layer of defense, the SSPk requirement to meet the overall defense criterion that all warheads in an attack are destroyed with a probability of 0.75, or a more stringent probability of 0.90, can be calculated. For illustrative purposes, assume the attacks consist of either 20 or 50 missiles at a time, which is a small fraction (less that 10 percent) of the overall stockpile held by North Korea, but is reasonably consistent with the estimated number of trained and equipped firing brigades capable of launching Hwasong and Nodong missiles under wartime conditions.[1] The benefits of layering the defenses are captured in Table 1, where the calculated results for one- and two-tiered defenses are presented.

Table 1. Benefits of layering defenses.

  Probability of No Leakage
P(0) = 0.75
Probability of No Leakage
P(0) = 0.90
Warheads in Attack 20 50 20 50
One Layer SSPk = 0.881 SSPk = 0.924 SSPk = 0.928 SSPk = 0.954
Two Layers SSPk = 0.654 SSPk = 0.725 SSPk = 0.731 SSPk = 0.786

The Single Shot Probability of Kill (SSPk) requirement for individual interceptors is calculated for each scenario. For example, if the attack contains 50 warheads, and the overall defense criterion is that no warheads leak through a two- layer defense with a probability of 0.75, the SSPk requirement is 0.725. Two interceptors are allocated to each warhead at each layer of defense.

The results captured in Table 2 illustrate the conclusion that a layered defense is likely to be more effective. In a single-layer defense where two interceptors are fired at each of the 20 or 50 attacking warheads, the requirement that all warheads are destroyed 75 or 90 percent of the time cannot be satisfied unless the SSPk of each interceptor is significantly greater than 0.80. If two layers are operational when an attack of 20 or 50 warheads is executed, the SSPk requirement is less than 0.8. This suggests that if THAAD and PAC-3 can achieve the same degree of success on the battlefield as in validation testing to date, a two-tiered defense in South Korea can meet the notional requirements assumed here.

In addition to reducing the SSPk value needed to defend against 20 or 50 missiles, a layered defense can also reduce the total number of interceptors that must be fired, assuming the first intercept attempt occurs early enough to facilitate a “shoot-assess-shoot” strategy. Shoot-assess-shoot is possible if the upper-tier (THAAD) intercept attempt occurs early enough in the threat missile’s trajectory to allow the lower-tier defense (PAC-3) to determine if the THAAD succeeded before launching the PAC-3 interceptors. For each success by THAAD, the PAC-3 defense would not have to fire its interceptors, thereby preserving them for use against future attacks. This becomes increasingly important as North Korea increases the number of missile firings above the 20 or 50 launches assumed.

Also, if each of the PAC-3 batteries has access to THAAD radar data, it would be possible for them to be launched before the target enters PAC-3 radar coverage. This scenario is referred to as a “launch on remote,” where one system launches its missiles on data generated by a remote sensor. PAC-3 batteries with a launch on remote capability would, in principle, have the capacity to protect a larger swath of territory, in some limited cases nearly doubling its defended footprint.

Return to last section: Two Illustrative Layered Defense Deployments
Next section: Some Significant Caveats


  1. [1]

    The total number mobile launchers (transporter-erector-launchers or TELs) is not the limiting factor here. Rather, road-mobile missile operations are supported by a large logistics trail, including trucks for carrying the oxidizer and fuel, pumping trucks to transfer the propellants to the missile, surveying units to establish an accurate determination of location and missile alignment prior to launch, weather units to measure wind speed at various altitudes, repair and maintenance teams, trailers to carry spare missiles, cranes to transfer missiles to the TEL, command and control trucks and teams, security and protection teams. The overall logistics requirement involves tens of vehicles and hundreds of trained personnel.


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