The Aegis Ballistic Missile Defense (BMD) System, a part of the Aegis Combat System, is the sea-based component of the Ballistic Missile Defense System (BMDS). It uses variants of the Standard Missile-3 (SM-3) to intercept short to intermediate-range ballistic missiles during midcourse. The system is integrated on certain U.S. Navy Ticonderoga-class Cruisers (CG) and Arleigh Burke-class Guided Missile Destroyers (DDG). The land-based variant, known as Aegis Ashore, is currently deployed in Deveselu, Romania as part of the NATO missile defense system.

More broadly, the Aegis Combat System also provides air and fleet defense against enemy aircraft and cruise missiles using variations of the Standard Missile-2 (SM-2), Standard Missile-6 (SM-6), and the Evolved SeaSparrow Missile (ESSM), and ship defense systems such as the Phalanx Close-In Weapon System (CIWS). It also controls the firing of Tomahawk land-attack cruise missiles.

Deployments The Aegis Combat System is deployed on 84 U.S. naval vessels: 62 Arleigh Burke-class Guided Missile Destroyers (DDG), and 22 Ticonderoga-class Cruisers (CG). Thirty-three of these ships have ballistic missile defense capability (28 DDG and 5 CG). This number is scheduled to increase by three by FY 2018 with the addition of the three new Flight IIA DDGs (DDG 113, 114 and 115). Aegis Ashore is currently deployed in Deveselu, Romania, with another site under construction in Redzikowa, Poland. The Romania site currently deploys SM-3 IB interceptors, and the Poland site is expected to employ the newer SM-3 IIA alongside . On April 26, 2017, U.S. PACOM commander ADM Harry Harris recommended to Congress that the U.S. operationalize the Aegis Ashore test facility in Hawaii to bolster the state’s defenses against a potential North Korean missile attack.

Aegis BMD Components

Aegis, like all missile defense systems, is made up of three basic components: sensors, interceptors, and command and control. The primary ballistic missile defense interceptor is the Standard Missile-3, of which there have been three block developments (SM-3 IA, IB, and IIA), with each block having increased range and overall capability from the previous. SM-3 uses hit to kill technology to destroy incoming missile warheads during midcourse in the exoatmosphere. The Aegis system also employs other atmospheric interceptors, including the SM-2, SM-6, and ESSM for air defense and terminal ballistic missile defense. These interceptors are fired from the Mark 41 Vertical Launching System (VLS), which is also used to fire Tomahawk cruise missiles, as well as antisubmarine weapons.

Currently, the primary Aegis sensor is the AN/SPY-1D, an S-band radar with near 360-degree coverage. Aegis can also launch using data from remote sensors, such as the TPY-2 X-band radar.

Onboard command and control is governed by the Aegis Combat System, which has been gradually improved through a series of hardware and software upgrades called “baselines.” The most recent version, Baseline 9, allows for a single ship to conduct both ballistic missile defense and air defense operations simultaneously. Prior baselines permitted only one of these missions at a time, usually requiring Aegis ships to operate in pairs.

On a strategic level, Aegis ships and Aegis Ashore sites are integrated into the broader BMDS through the Command and Control, Battle Management, and Communications (C2BMC) system. This linkage allows for Aegis ships and sites to be alerted to missile threats detected by other BMDS sensors, and to transmit its own sensor data to the BMDS, including providing sensor data to support the U.S. Ground-based Midcourse Defense (GMD) system.

Aegis BMD Development

The Aegis combat system was originally designed during the 1970s as means to to defend ships against aircraft and antiship cruise missiles. The system went into operation in 1983. Aegis has since undergone several upgrades, most notably the addition of ballistic missile defense capability. In 2004, the Aegis system began its support for ballistic missile defense missions by serving as a surveillance and tracking radar for intercontinental ballistic missiles (ICBM). The system then upgraded to a midcourse intercept capability in 2005 with the development of the SM-3, and gained terminal defense capability in 2006.

The first mission that Aegis BMD ships filled was as Long Range Tracking and Surveillance radars in support of the Ground-based Midcourse Defense system in 2004. This mission involved deploying Aegis ships at forward bases to pick up missile tracks for ICBMs directed at the United States. At present, each Aegis ship uses the AN/SPY-1 radar, which can track up to 100 targets simultaneously. Upgraded versions of the Aegis software like the Baseline 9.C1 software configuration allow the ships to simultaneously track ballistic missiles and air and cruise missile threats. The Flight III Arleigh Burke-class destroyers are also scheduled to feature the Air and Missile Defense Radar (AMDR/SPY-6).

Aegis systems have tested “launch on remote” capability where an interceptor can use off-board radars to pick up the initial track of a threatening missile and then on-board radars for final tracking and intercept. The first test of this concept coincided with the completion of Phase 1 of the EPAA in 2011, a test that included an SM-3 Block IA interceptor receiving information from am AN/TPY-2 radar. In 2013, another launch on remote test proved the capability to integrate Space Tracking and Surveillance System Demonstrators (STSS-D) data for an intercept with an SM-3 Block IA illustrating the expanded battlespace such a capability provides. Phase 3 of the EPAA is also scheduled to include an engage on remote capability for Aegis interceptors to conduct operations based entirely on off-board radar information, expanding the range of the Aegis systems even further.

Envisioned roles for the Aegis BMD ships have varied over the years. Some have proposed using the Aegis ships for homeland and coastal defense missions while Aegis ships are on mandatory rotations at home ports in the United States. There have also been proposals to station Aegis BMD ships close to adversary coasts and produce interceptor variants that would reach speeds sufficient to intercept ballistic missiles in the boost phase. The cancelled Phase 4 of the European Phased Adaptive Approach envisioned the SM-3 Block IIB interceptor, which would have had a limited capability to intercept an Iranian ICBM targeted at the United States mainland from forward deployments in Europe.

In 2008, the Aegis system was used to intercept a non-functioning and potentially toxic satellite as part of Operation Burnt Frost.

Testing

Although the Aegis Combat System has undergone development since the 1970’s, it did not begin testing against ballistic missile targets until the U.S. withdrawal of the Anti Ballistic Missile Treaty in 2002. Since then, Aegis has achieved 35 successful intercepts out of 42 attempts using a Standard Missile-3. The Missile Defense Agency and the Navy have also conducted terminal phase ballistic missile intercepts using a modified SM-2 Block IV on four occasions, and an SM-6 on two.

Aegis ships have also participated in at least ten tests of the GMD system, which supports U.S. homeland missile defense.

Modernization

The U.S. Navy is currently undertaking an effort to modernize ships with older 3.6 and 4.0 versions of the Aegis Combat System to the 5.0 Capability Upgrade (Baseline 9). Initial plans to upgrade all of the BMD ships to the version 5.0 floundered under the pressure of budget cuts imposed by budget caps, forcing the Navy to commit to only upgrading all current 3.6 version ships to the 4.0 version and then only upgrading seven ships to the 5.0 version configuration. These cuts affect the ability of the ships to process radar information from multiple sources and track aircraft, cruise missile, and ballistic missile threats simultaneously.

Budget issues have also affected the Navy’s projections of the number of Aegis BMD capable ships that it will be able to deploy in the future. The Navy has proposed to eliminate the BMD capability on five of its Aegis cruisers during scheduled modernization in favor of upgrading Aegis Destroyers with BMD capabilities. In addition, prior Missile Defense Agency estimates of the number of BMD capable ships it would procure did not account for the training time required for new hulls, meaning earlier estimates had to be revised downward from an earlier 48 to 39 in FY2020. These estimates were revised after the decision was also announced in the FY 2016 budget that the Navy would cut the BMD upgrades from the modernization plan of five of its ships.

The newest Aegis upgrade, Baseline 10, is also underway and will incorporate the newer SPY-6 (AMDR) radar. Certification is expected in 2023 timeframe.

Japan

In addition to the U.S. deployment of Aegis ships, Japan also currently deploys four Aegis-equipped KONGO-class destroyers with plans to upgrade its other two Aegis capable destroyers. Additionally, Japan plans to build two more BMD-capable Atago-class cruisers with BMD capability version 5.0 for a final total of eight. Japanese ships have conducted four flight tests of Aegis interceptors, including three successful intercept tests. Japan also cooperates with the United States on the development of the SM-3 Block IIA interceptor.