Most of the attention given the US Navy’s Aegis Ballistic Missile Defense (BMD) program focuses on its various versions of the SM-3 anti-missile interceptor. These missiles, the SM-3 Block IA, the SM-3 Block IB and the forthcoming SM-3 Block IIA, intercept their targets above the atmosphere and are intended to provide coverage over large areas (particularly the Block IIA). However, this post discusses the Missile Defense Agency’s and the Navy’s Sea-Based Terminal (SBT) program, which is developing and deploying lower-altitude, within-the atmosphere ballistic missile defense interceptors.
An SBT capability offers several possible benefits. First, it provides a second layer of ballistic missile defense for Navy ships and nearby areas, thereby potentially increasing the overall effectiveness of the Aegis BMD system. SBT interceptors operate in a completely different way than the SM-3 interceptors. The SBT interceptors home in on their target using radar, maneuver using atmospheric forces, and kill with a high-explosive fragmentation warheads, while SM-3 interceptors use infrared homing, maneuver using rocket thruster (divert thrusters), and kill using direct high-speed collisions. These differences in operating principles may make it less likely that a countermeasure (or other circumstance) that defeats an SM-3 interceptor can also defeat an SBT interceptor. Second, SBT interceptors can potentially intercept shorter-range missiles, such as Scuds (or longer range missiles on depressed trajectories) that do not leave the atmosphere (that is, rise above about 100 km altitude) and thus cannot be intercepted by SM-3 interceptors. Third, SBT interceptors are likely to be much less expensive, by a factor of three or more, than SM-3 interceptors. Finally, SBT interceptors can also be used to intercept aircraft (and soon ships), allowing more efficient use of the limited number of vertical launch tubes on Navy ships.
SM-2 Block IV, Block IVA, and Block IV (modified)
In the 1990s, the Ballistic Missile Defense Organization and the U.S. Navy planned to develop a terminal phase ballistic missile defense system known as Navy Lower Tier. (In parallel, BMDO was developing the Navy Upper Tier interceptor, which was renamed Navy Theater Wide, and eventually evolved into the current SM-3 Aegis BMD interceptors.) This program, which was renamed Navy Area Defense in 1996, was based on a new version of the Navy’s existing extended-range air defense missile the SM-2 Block IV. The SM-2 Block IV operates within the atmosphere, uses semi-active radar homing and has a high-explosive fragmentation warhead.
The new missile, to be known as the SM-2 Block IVA, would have had a new side-mounted infrared terminal seeker as well as modifications to the Block IV’s guidance system, fuzing system and warhead (see figure 1 below). However, in December 2001, the Department of Defense cancelled the Navy Area Defense system, citing “poor performance and projected future cost and schedules” following a Nunn-McCurdy cost breach.
Figure 1. Modifications to the SM-2 Block IV to produce the SM-2 Block IVA. Missile Defense Agency image (http://www.mda.mil/global/images/system/navy_tbmd/sm2-block-IVA.jpg)
Despite the cancellation of the Navy Area Defense Program, the Navy retained interest in developing a Sea-Based Terminal capability and considered, for example, the possibility of developing a sea-based version of the Patriot PAC-3 system. Ultimately, following a successful intercept test of a modified SM-2 Block IV interceptor against a short-range ballistic missile in 2006 (see list of SBT tests below), the MDA and the Navy decided to develop an interim sea-based terminal missile defense system by modifying existing SM-2 Block IV interceptors. The modifications to the Block IV interceptors involved changes to the missile’s fuze and autopilot and were less extensive than the changes that would have been made to produce the cancelled SM-2 Block IVA missile. At the time, the Navy had about 100 Block IV interceptors in inventory, and ultimately 75 were modified for ballistic missile defense (5 of which have subsequently been expended in tests). The modified SM-2 Block IV was first intercept tested (successfully) against a short-range ballistic missile target in May 2006 (Pacific Phoenix) and has had three other successful ballistic missile intercept tests since (one test involved two interceptors).
Deployment of the modified Block IV missiles began in 2008 on ships equipped with the basic Aegis BMD 3.6.1 system. At the end of 2008, there were about 18 ships with this version of the Aegis system. This number that would grow to about 24 in 2012 and then decline as 3.6 ships were upgraded to higher version of the Aegis BMD system.
The Block IV interceptor was only intended to provide an interim “gap-filler capability” until a new (then undefined) SBT interceptor could be deployed. Accordingly, and as a cost-saving move, ships that were upgraded to Aegis BMD 4.x and early versions of 5.x (Phase II of the European Phased Adaptive Approach (EPAA)) would not be equipped for the Modified Block IV interceptors and thus would not have any SBT ballistic missile defense capability. A sea-based terminal capability would then be restored with Aegis 5.2 as part of phase III of the EPAA using the new SBT interceptor. However, a recent MDA slide now indicates that ships with either Aegis BMD 4.1 or 5.0 CU will be able to operate SBT interceptors.
The SM-6 Interceptor
Ultimately, the Navy decided that the longer-term SBT interceptor would be based on its new SM-6 new extended-range air defense interceptor. The SM-6 combines the airframe and propulsion system from the SM-2 Block IV interceptor with the active radar seeker of the advanced Medium-Range Air-to-Air Missile (AMRAAM) missile. See figure 1 below. Like the SM-2 Block IV, it operates within the atmosphere and uses a high explosive warhead. As of 2013, the U.S. Navy planned to procure total of 1,800 SM-6 missiles by FY 2024 The SM-6 was first deployed in December 2013 (on DDG-100) and the Navy has so far accepted deliveries of over 250 missiles.
Figure 2. SM-6 compared to SM-2 Block IV. Image source: Laura DeSimone, “Aegis BMD; The Way Ahead,” MDA Briefing Slides, December 6, 2011 (available at: http://www.dtic.mil/ndia/2011PEO/DeSimone.pdf )
The SM-6 has an over-the-horizon capability against aircraft using targeting data provided by other ships or by aircraft, such as the Navy’s E2-D Hawkeye airborne warning and control system (AWACS) airplane. An upgraded version, the SM-6 Block IA, adds “guidance section hardware and software modifications,” including the incorporation of GPS guidance, to enable attacks on sea or land targets. In a January 2016 test, an SM-6 struck and sank a decommissioned U.S. Navy frigate. Current plans call for SM-6 missiles with an anti-ship capability to begin deployment in fall 2016.
The SM-6 Dual I and Dual II Interceptors
The initial version of the SM-6 did not have a capability against ballistic missiles. The SM-6 is to be given a terminal-phase anti-ballistic missile capability in two phases. The first phase, the SBT Increment 1, includes an upgraded missile known as the SM-6 Dual I. A Dual I interceptor successfully intercepted a short-range (less than 1,000 km) ballistic missile target in a July 2015 test and its deployment is expected to begin in 2016. This is to be followed in 2018-2019 by SBT Increment 2 which will include the Dual II version of the SM-6. The SM-6 Dual interceptors will eventually supplant the modified Block IV interceptors as the Navy’s SBT interceptors.
There appears to be little publicly available information about how the Dual I interceptor differs from the baseline SM-6 or SM-6 Block I, or how the Dual II missile differs from the Dual I. Press reports indicate that the Dual I has a more powerful processor and additional software. A Raytheon spokeswoman described the Dual I as containing both ballistic missile defense and anti-air warfare software in the same missile.
The modified SM-2 Block IV has had four successful intercept tests against ballistic missile targets and the SM-6 Dual I had its first successful test in July 2015. All of these tests were against short-range ballistic missile targets. The next SM-6 Dual I test will be against a medium-range (1,000-2,000 km) ballistic missile. The tests are summarized below:
Pacific Phoenix (May 24, 2006): A modified SM-2 Block IV launched from the cruiser Lake Erie successfully intercepted short-range ballistic missile in the terminal phase of its flight in a joint U.S. Navy and MDA test. This was the first sea-based intercept of a ballistic missile. The cruiser was equipped with an interim Aegis missile defense capability known as “Linebacker.”
Flight Test Standard Missile-14 (FTM-14) (June 5, 2008): A short-range ballistic missile target was successfully intercepted in its terminal phase by two modified SM-2 Block IVs salvo fired by the cruiser Lake Erie which was equipped with Aegis BMD 3.6.1. The intercept occurred at an altitude of about 12 miles.
Stellar Daggers (March 26, 2009): A short-range ballistic missile target was successfully intercepted by a modified SM-2 Block IV interceptor launched from the destroyer Benfold. The destroyer simultaneously intercepted a cruise missile target with an SM-2 Block IIIA interceptor. The test was conducted by the U.S. Navy rather than by MDA.
Multi-Mission Warfare Event 1 (MMW E1) (July 29, 2015): A SM-6 Dual I missile, launched from the destroyer John Paul Jones (Aegis BMD 5.0 CU, Aegis Baseline 9.C1), successfully intercepted short-range ballistic missile (SRBM). This was the first flight and intercept test for the SM-6 Dual I.
In two separate tests on July 31 and August 1, SM-6 Dual I interceptors successfully intercepted cruise missile targets (although in the second test, the interceptor warhead was intentionally not detonated in order to be able to reuse the target).
Multi-Mission Warfare Event 2 (MMW E2) (July 30, 2015): A SM-2 Block IV missile, launched from the destroyer John Paul Jones (Aegis BMD 5.0 CU, Aegis Baseline 9.C1), successfully intercepted a SRBM.
Flight Test Other-21 (FTX-21) (May 17, 2016): An Aegis destroyer (John Paul Jones) successfully tracked a medium-range ballistic missile (MRBM), including its reentry and subsequent flight within the atmosphere. This is the first time Aegis had tracked an MRBM within the atmosphere during reentry. The plan for this test had originally included an intercept by an SM-6 Dual I, but for range safety reasons the intercept attempt was deferred until the FTM-27 test in late 2016. Department of Defense budget documents describe the test as “SM-6 Dual I missile simulated engagement of a Medium Range Ballistic Missile.”
FTM-27 (1Q, FY 2017): Two SM-6 Dual I interceptors will be salvo fired against a MRBM target.
FTX-24 (4Q FY 2017): Simulated Sea-Based Terminal (SM-6 Dual) intercept of separating short-range ballistic missile. Actual target, but no interceptor will be fired.
FTM-28 (3Q, 2018): SBT intercept test.
FTM-31 (1Q FY 2019): SBT intercept test
FTM-33 (1Q FY 2019): SBT intercept test
FTM-32 (3Q FY 2019): SBT intercept test
 Department of Defense, “Navy Area Missile Defense Program Cancelled,” News Release, December 14, 2001. Online at http://www.dau.mil/pubscats/PubsCats/PM/articles02/dodjf022.pdf.
 Malina Brown, “Navy Rebuilding Case for Terminal Missile Defense Capability,” Inside the Navy, April 19, 2004.
 John Liang, “Obering: ’Limited’ Sea-Based Aegis Terminal Competition RFP Out Soon,” Inside Missile Defense,
August 27, 2008.
 Figure 1 of Ronald O’Rourke, “Navy Aegis Ballistic Missile Defense (BMD) Program: Background and Issues for Congress,” Congressional Research Service, CRS Report RL33745, May 26, 2016. Figure shows a slide provided to the CRS by the Missile Defense Agency on March 25, 2016.
 Jason Sherman, “New Threat Analysis Prompts $3B increase in SM-6 Procurement Spending,” Inside the Navy, September 9, 2013.
 Lee Hudson, “Raytheon Exec: Standard Missile-6 Price Slashed by 32 Percent,” Inside the Navy, July 14, 2014; Raytheon Company, “SM-6 Shatters Engagement Distance Record,” News Release, March 7, 2016.
 Jason Sherman, “Navy, Raytheon Ready New Satellite-Guided, Standard Missile-6 Variant,” Inside the Pentagon, July 3, 2014.
 Sam LaGrone, “Navy Sinks Former Frigate USS Reuben James in Test of New Supersonic Anti-Surface Missile,” USNI News, March 7, 2016.
 Justin Doubleday, “Navy To Deploy Modified SM-6 with Anti-Ship Capability this Fall,” Inside Defense SITREP, March 10, 2016.
 According to MDA Director Vice Admiral James Syring’s 2016 Congressional Prepared Statement, the SM-6 Dual I “will enter the fleet inventory this spring.” Online at http://www.mda.mil/global/documents/pdf/FY17_Written_Statement_HASC_SF_Admiral_Syring_14042016.pdf.
 Sydney J. Freeberg Jr., “SM-6 Can Now Kill Both Cruise AND Ballistic Missiles,” breakingdefense.com, August 4, 2015; Kyong M. Song, “Naval Interceptor Takes on both Ballistic and Cruise Missiles, Aerospace America, September 2015, p. 6.
 Jason Sherman, “Aegis-Guided SM-6 Interceptor Shoots Down Ballistic Target,” Inside Missile Defense, August 4, 2015.
 Missile Defense Agency, “First at-Sea Demonstration of Sea-based Terminal Capability Successfully Completed,” News Release, May 24, 2006.
 Staff writers, “Aegis Destroys Ballistic Missile in Terminal Phase,” Space Daily, June 4, 2008.
 Missile Defense Agency, “Successful Sea-Based Missile Defense Intercept,” News Release, June 5, 2008; Staff writers, “Aegis Destroys.”
 U.S. Third Fleet. “Navy Completes Air and Ballistic Missile Exercise,” News Release. March 27, 2009; “Raytheon Standard Missile-2 Demonstrates Sea-Based Terminal, Fleet Protection,” PR Newswire, March 27, 2009.
 Missile Defense Agency, “Aegis Ballistic Missile Defense System Completes Successful Series of Intercept Flight Test Events,” News Release, August 3, 2015.
 MDA, “Aegis Ballistic Missile Defense System Completes.”
 Missile Defense Agency, “Aegis BMD System Completes Successful Tracking of Medium-Range Ballistic Missile Target,” News Release, May 17, 2016.
 Jason Sherman. “Aegis BMD Tracks Ballistic Missile Target Within Earth’s Atmosphere,” Inside the Pentagon, May 19, 2016.
 Department of Defense, President’s Budget (PB), FY 2017, MDA, RDT&E, p. 2a-656.
 PB FY 2017, pp. 2a-657, 2a-678, 2a-708.
 PB FY 2017, p. 2a-679.
 PB FY 2017, p. 2a-679.
 PB FY 2017, p. 2a-679.
 PB FY 2017, p. 2a-679.