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Aegis Ashore vs THAAD (July 27, 2015)
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.[1] 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.[2] 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.[3] 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.
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.)
[1] 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.
[2] 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.
[3] Although Figure 2 shows northern Scandinavia uncovered, this area would be covered by the planned Aegis Ashore site in Poland.
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