The United States of America maintains the world’s most complex and effective intercontinental ballistic missile (ICBM) defence system, the Ground-based Midcourse Defense System (GMD). The US government has spent $53 billion USD on the system [source]. American decision-makers have justified these expenditures as a result of its criticality to the US’ response to “nuclear blackmail”. Nuclear blackmail is the use of nuclear threats by minor rival states like in order to control US policy. Aiming to defeat limited nuclear ICBM attack, the GMD system exists to accentuate US defences against nuclear attack.
1.0. What is the Ground-based Midcourse Defense System?
The GMD system consists of a complex package of multiple interlocking systems. Core to the system is the Ground Based Interceptor (GBI) booster rocket. Within this rocket is the Exoatmospheric Kill Vehicle (EKV) which is taken to an ICBM in space. Supporting these are the GMD Fire Control (GFC) systems that synthesise data from hundreds of satellite and ground radars [source]. These radar assets include strategic installations using phased array radars like the AN/FPS-132 Upgraded Early Warning Radar (UEWR) [source].
The GMD’s core system remains the GBI, with its EKV payload, but their use would be impossible without the sensor and radar arrays, as well as the command and control systems that back it up. The Missile Defense Agency has deployed the GMD to two sites on the Western Seaboard of North America. 40 GBI emplacements are located at Fort Greely in the state of Alaska with the 49th Missile Defense Battalion. An additional 4 are located with the 100th Missile Defense Brigade at Vandenberg Air Force Base in California [source].
2.0. How Does the Ground-based Midcourse Defense System Work?
GMD systems very specifically target ICBMs when they’re most vulnerable and least likely to damage their targets. This is during an ICBM’s “midcourse” phase, leading to the name of the system as the Ground-based “Midcourse” Defense system. All known ICBM designs operate in three separate stages, known as the boost phase, the midcourse phase, and the terminal phase [source].
The boost phase, occurring directly during and after the launch of the missile, involves the generation of a staggering amount of thrust in order to escape Earth’s gravity and reach space, after which point the missile’s nose cone is released. This nose cone possesses the missile’s warhead, or in the case of Multiple Independent Reentry Vehicle (MIRV) ICBMs, warheads. Boost-phase missile defences exist in prototype forms, but these have not been successful due to the technical limitations of placing missile defences close to launch sites and the short period of time available to intercept the missile, as boost-phases can last less than 5 minutes [source].
2.1. So Why Not Terminal Phase Defence?
On the other hand, the terminal phase, which generally lasts a minute long, is less of a challenge to intercept a ballistic missile in comparison to interception in the boost phase. The US possesses a number of systems that target ballistic missiles in this phase. These include units like the Terminal High Altitude Air Defense (THAAD) system or the AEGIS Sea-based Terminal Defense system. However, these target short, medium and intermediate range missiles such as Scud missile family [source]. For ICBMs, targeting the terminal phase seriously risks massive damage to targets on the American homeland.
As a result, the GMD targets ICBMs while they are midcourse, in space. This allows safer targeting, with a longer period to intercept. It does so by launching the GBI system towards a predicted position for a detected ICBM. These positions are predicted at the GFC sites. These GFC prediction engines use data collected from the radar and sensor arrays present across the world. The GBI is a three stage rocket booster, and will discard most of its body after launch. By the point of deploying its EKV load, it will have reached the exoatmosphere. The EKV then targets the ICBM using data being delivered to it from the GMD’s associated sensor arrays. Locating the missile with this associated data, it intercepts its target at 24,000 kilometres per hour [source]. If it makes contact, it is highly likely to tear apart the approaching missile and neutralize it.
3.0. Does it Succeed?
The GMD’s effectiveness remains controversial, as many have claimed that its kill-testing has shown how the system cannot reliably defend against even a limited attack. The GMD’s 57% kill rate, even if this value includes tests conducted, ensures that in case of a limited attack, it is highly likely that several ICBMs would be able to strike the homeland [source]. An alternative is that the GMD remains only part and parcel of the full package of missile defence systems used by the United States, and would be able to rely on terminal-phase and boost-phase defences in the future as well. However, the overall test record of the hit-to-kill intercept programs that tests the effectiveness of the entire integrated missile defence system, including the THAAD and AEGIS systems, is only 78.8% [source]. Even through the integrated systems, it’s likely that several ICBMs in a limited wave conducted by an actor like North Korea, against which many of the improvements to the GMD system were directed, would bypass these defences [source].
Worse yet, the GMD system suffers from the possibility of being overwhelmed by missiles. Even the Iron Dome system in Israel, which targets non-precision guided short and medium range missiles with an effectiveness of 97.6%, can be overwhelmed [source]. North Korea, which drove increases in the number of GMD emplacements to 64 after its nuclear test in 2017 [source], now possesses enough mobile launchers for solid fuel nuclear capable ICBMs that it could overwhelm the current number of GBIs even with 100% hit-to-kill interception rate [source].
4.0. Deployment Consequences
The US’ decision to expand the GMD program resulted in heightened tensions with Russia and the People’s Republic of China. American decision-makers during the Cold War had feared these and saw their possibility as a reason to not pursue missile defence capabilities and resulted in the signing of the Anti Ballistic Missile Treaty in 1972 [source]. Russian and Chinese officials have previously stated that they view the development of additional GMD capabilities to be destroying deterrence-balance, threatening the credibility of their second strike capability. By doing so, the US has greater leeway in aggression towards these states in their eyes [source].
As a result, Russian and Chinese missile expansion programs began in the aftermath of the 2017 expansion. These programs attempted to produce greater numbers of missiles and improvements to attempt to bypass missile defence platforms [source] [source]. As a result, the US’ deployment of the GMD comes with the threat of greater instability in great power relations.
