In a comment to my post of July 16 about the THAAD deployment in Guam being made permanent, a question was raised about why THAAD was proposed for South Korea and Aegis Ashore for Romania and Poland (and why not vice versa).
There are two main technical issues that almost certainly drove the decision of which system went where:
(1) Europe can be almost completely covered by two Aegis Ashore sites but achieving similar coverage with THAAD would require a prohibitive number of THAAD batteries. On the other hand, S. Korea is small enough to be covered by one or two THAAD batteries.
A single Aegis Ashore site (with the Block IIA interceptor) can cover a much larger geographical area than a single THAAD deployment. The Block IIA interceptor is scheduled to begin deployment in 2018. This larger coverage area occurs because the Aegis Block II interceptor has a much higher burnout speed (likely about 4.5 km/s) than a THAAD interceptor (likely about 2.6-2.8 km/s) and thus can reach out to make intercepts at much greater ranges.
This is illustrated in two 2007 Missile Defense Agency Briefing slides. The yellow “footprints” in Figure 1 below shows the area that could be covered by three THAAD batteries in eastern Turkey against Iranian ballistic missiles. For THAAD, this situation — in which the attacking missiles are launched from a country bordering the country targeted – is closely analogous to the North Korea-South Korea situation. However, the three THAAD batteries together cover only a small fraction of Turkey.
Figure 1. Coverage of Europe against Iranian ballistic missile by THAAD, Aegis (Block IB), and two-stage GBI interceptors. Slides from MDA Executive Director Patricia Sanders, “Missile Defense Program Overview For The 4th International Conference On Missile Defense,” June 26, 2007. Available at: https://mostlymissiledefense.files.wordpress.com/2013/06/bmd-overview-sanders-june2007.pdf
What Figure 1 makes clear is that attempting to cover all of Europe using THAAD would require a prohibitive number of THAAD batteries, far more than the U.S. plans to buy. Current U.S. plans are to buy seven batteries, although there is a stated requirement for nine batteries.
On the other hand, the green shaded area in Figure 1 shows the footprints for Aegis Block IB interceptors deployed on four ships in the Mediterranean Sea (including the Adriatic Sea) and Black Sea. This slide, which assumes the launch of the interceptors is supported by external radars such TPY-2 X-band radars, shows that four ships can cover a significant fraction of Europe.
The much faster Aegis Block IIA interceptors scheduled to begin deployment in 2018, would allow all of Europe (except eastern Turkey, for reasons discussed below) to be defended from only two interceptor launch sites as shown in Figure 2 below. Note that both Figures 1 and 2 predate the decision to deploy the European Phased Adaptive Approach (EPAA) system with its land-based Aegis Ashore sites. Figure 2 also assumes that the Aegis interceptors are supported by external radars.
Figure 2. Slide from MDA Deputy Director Major General Patrick O’Reilly, “Missile Defense Program Update For The National Defense University “Road To Bucharest” Conference, February 20, 2008. (link coming soon.)
Thus is it possible to attempt to defend almost all of Europe (the NATO portion of it, at least) from Iranian missiles using a few Aegis sites, but similar coverage cannot be achieved using any plausible number of THAAD batteries. (The development of a faster, extended-range version of the THAAD interceptor could change this situation. Such an extended-range version of the THAAD interceptor is under consideration as part of a THAAD follow-on concept development and risk reduction program that MDA initiated in FY 2016.)
On the other hand, South Korea is small enough that covering it with THAAD is feasible.
Each of the three THAAD batteries’ footprints in Figure 1 are roughly 430 km long (east-west in this geometry) by 340 km wide. This is larger than the dimensions of mainland (leaving out the island of Cheju) South Korea – which is about 370 km north-south and 270 km east-west. Thus it should be possible to cover South Korea with no more than one or two THAAD batteries. This is consistent with defended footprints, reportedly provided by THAAD manufacturer Lockheed-Martin, published in a South Korean newspaper last month. These footprints are shown in Figure 3 below.
Figure 3. Coverage of South Korea by one (left figure) or two THAAD batteries. The coverage is said to be for missiles with ranges between 300 and 1,000 km. Image Source: “Simulation Shows How THAAD Would Defend S. Korea, The Chosunilbo (English Edition), June 15, 2015. Available at: http://english.chosun.com/site/data/html_dir/2015/03/25/2015032500896.html.
(The distinction between the green and blue coverage areas was not specified. One possibility is that green area is against missiles with ranges close to the lower limit of the 300-1,000 km attacking missile range band, while the larger blue area is for coverage against longer-range missiles for which a longer interceptor flyout time is available.)
(2) Due to the short distances involved, Aegis BMD cannot defend South Korea against short range ballistic missile attack. This problem arises because the Aegis SM-3 interceptor (all versions) can only intercept above the atmosphere (exo-atmospheric). While the precise lower altitude limit for the SM-3 interceptor is not publicly available, it is generally taken to be at about 100 km or higher.
Since no part of mainland South Korea is more than about 380 km from North Korea, most of South Korea can be targeted by 300 km Scud-class missiles and all of it can be targeted by 600 km extended-range Scuds. The Scud missiles never rise as high as the SM-3 interceptor’s lower altitude limit and thus cannot be engaged by Aegis BMD. An extended-range Scud might be high enough to be engagable over part of its trajectory, but the possibility of engaging such a missile can be reduced or eliminated by flying it on a lower-altitude depressed trajectory.
On the other hand, THAAD could attempt to intercept missiles with ranges as short as a Scud, since it is able to conduct intercepts in the upper layers of the atmospheres, as well as above it. The lower altitude limit of THAAD is also not publicly available, but is generally taken to be about 40 km, well below much of the trajectory of a Scud.
This capability to intercept at lower altitudes is the main technical reason that THAAD is more appropriate than Aegis BMD for defending South Korea against North Korean ballistic missiles. This is the same reason why Figure 1 includes THAAD batteries covering eastern Turkey, since eastern Turkey could be targeted by shorter-range Iranian missiles that Aegis SM-3 interceptors cannot operate low enough to engage.
The decision to base THAAD on Guam, however, was not determined by these two technical issues. From a technical perspective, Guam could have been covered by either a THAAD battery or an Aegis Ashore facility. Thus other issues such as cost and availability likely determined this decision. In particular, the original deployment to Guam in 2013 was made in response to North Korean threats and subsequent questions about whether or not the GMD national missile defense system could cover Guam (It can’t). It was possible to deploy a THAAD battery to Guam within days, since a THAAD battery’s equipment is air-transportable and can be operational with four hours of arrival. Building an Aegis Ashore facility there would likely have taken a year or more, particularly if this was to be done without disrupting the schedule of the European Phased Adaptive Approach. An Aegis BMD-equipped ship could have been deployed there much more quickly than an Aegis Ashore, but the cost (both financial and in terms of the limited numbers of Aegis BMD ships) of maintaining an Aegis ship permanently off Guam would have been much greater than that of maintaining a THAAD battery there.
 THAAD batteries deployed further from Iran could attempt to defend somewhat larger areas, particularly if supported by external radars, but such larger areas would not change the conclusion that the required number of batteries would much greater than the total the U.S plans to procure.
 The blue-shaded area shows the coverage of the now-cancelled plan to deploy two-stage versions of the U.S. national missile defense Ground-Based Interceptors in Poland.
 Although Figure 2 shows northern Scandinavia uncovered, this area would be covered by the planned Aegis Ashore site in Poland.