How Do Missile Defence Systems Work?

You’re probably aware that developed nations such as Israel, Russia
and the United States have missile defence systems. But do you know how these systems work? Or how effective they are? They’re not the impenetrable shield you think they are.

via NATO

North Korea recently tested another missile, which has experts convinced the entire U.S. mainland is now range of their intercontinental ballistic missiles (ICBMs), assuming the missile carries a light payload. That’s drawing more spotlight to the U.S.’s missile defence systems and whether or not they actually work. After an alleged failed defence attempt in Saudi Arabia with a U.S. supplied Patriot missile system, many Americans are starting to wonder “Are we safe?” But before we get into how effective these things are, let’s talk about how they work.

How Missile Defence Systems Work

via NATO
The basic functions of a missile defence system are simple — you use ballistic missiles to shoot down other missiles — but there’s a lot going on during this process. Here’s a play-by-play of how midcourse missile defence systems — like the U.S. Ground-Based Midcourse Defence System (GMD) and the U.S. Navy’s SM-3 Aegis BMD system — are supposed to stop an incoming ICBM similar to what North Korea recently tested:

1. The threat missile is launched.
2. Satellites using infrared technology and radar detect the launch and track the missile’s trajectory.
3. Threat missile releases a warhead and decoys (known as the “threat cloud”).
4. Ground-based and sea-based radar continuously track the threat cloud, trying to identify the warhead (where the payload is).
5. The missile defence system launches an interceptor missile.
6. The interceptor’s payload, the “kill vehicle,” separates from the missile body.
7. The kill vehicle spots the threat cloud and attempts to intercept high above in the atmosphere.

If everything goes according to plan, the threat payload is destroyed in space before it can reach its target below.

via Fox News
These systems differ, however, from the Patriot, Arrow, and Iron Dome terminal-phase within-the-atmosphere missile defence systems (talk about a tongue twister). These systems work in a similar fashion, using radar and atmospheric guidance for tracking, but are only designed to intercept short and medium range missiles that move at much slower speeds and lower altitudes than ICBMs. They only cover areas tens of kilometers in size, so they’re nice to have on hand as a backup to midcourse systems, but they’re probably more useful in areas like South Korea and Japan than here in the states (assuming the launch comes from North Korea). The U.S. Army’s Terminal High Altitude Area Defence (THAAD) system is classified as terminal-phase system, but it has a few tricks up its sleeve. It works more like a midcourse system, and can destroy targets via direct collision in the upper atmosphere or above.

OK, So How Effective Are They?

Successful GMD test, May 2017, via AiirSourceMilitary
When it comes to midcourse ICBM defence, we have no idea. Maybe they will work, maybe they won’t. Why? As George N. Lewis, a physicist and senior research associate at the Judith Reppy Institute for Peace and Conflict Studies at Cornell University, explains in his paper “Ballistic Missile Defence Effectiveness,” there is no real-world experience with the use of midcourse ballistic missile defences. America’s GMD systems have been tested, of course, and been successful, but current midcourse defence intercept tests are few and far between, and tend to be a highly-scripted demonstration that reflect more on the reliability of these systems and not their actual effectiveness in a real-world scenario.

You see, attacking enemies are likely to employ “countermeasures” when firing a missile. These countermeasures include mechanisms, like decoys or a cooling shroud, designed to confused or disrupt the defence system so it fails. The U.S. doesn’t test its defence systems with these in mind, even though the technology to develop such countermeasures is widely known. Its GMD system is highly effective at detecting missile launches and tracking multiple targets at long ranges, but it is not great at knowing which objects are warheads and which are decoys. The system’s primary discrimination sensor in Honolulu, Sea-Based X-band (SBX) radar, has significant operational limitations. That said, the U.S. National Academy of Sciences book Making Sense of Ballistic Missile Defence suggests that countermeasures that are theoretically possible are not always easy to deploy, citing the U.S. and U.K.’s own experience attempting to develop such penetration aids during the Cold War. There’s also a good chance such countermeasures would reduce a missile’s payload, range, or reliability.

Nevertheless, while the USA’s GMD defence system is theoretically capable of protecting all fifty states from an ICBM missile attack, it is still largely untested against real threats, and its test stats are not exactly comforting:

As you can see, the GMD system succeeds only about half of the time, and it doesn’t seem to be improving. The Navy’s Aegis BMD systems fare better in their tests:

Not only do they have higher success rates, but they have been tested both during the day and night; tested against short, medium, and intermediate range targets; tested against both intact missiles and separated warheads; and been tested against two simultaneous targets. But unfortunately, Lewis says their lower speed prevents them being able to cover enough territory to be useful as a defence here in the homeland. Next year, the U.S. hopes to deploy new SM-3 Block IIA interceptor missiles that may be able to get the job done (though they also suffered a recent test failure in June).

So are citizens safe from a missile attack in the U.S.? It’s hard to say, but we’re probably not as safe as you think. The U.S. military has a history of claiming success rates much higher than reality. During the Gulf War, the U.S. Army claimed a 96% success rate against Iraqi modified Scud missiles, then later reduced that claim to 61%. Further analysis from experts suggested the success rate was very low and possibly 0%. After the recent missile attack in Saudi Arabia, President Trump was quoted as saying:

“Our system knocked the missile out of the air… That’s how good we are. Nobody makes what we make, and now we’re selling it all over the world.”

But further evidence analysis suggests the Patriot missile system failed, and that the warhead nearly hit the airport it was targeting where people heard and witnessed an explosion. In regards to home front GMD defence, Lewis points out that U.S. officials have overstated those success rates as well:

Statements from U.S. officials suggest, however, that a system intended to counter nuclear-armed missiles could be considered effective if its predicted effectiveness is greater than about 90%. On June 16, 2009, just a week after Defence Secretary Robert M. Gates told Congress that the current GMD NMD system was “fully adequate to protect us against a North Korean threat,” General James Cartwright, Vice Chairman of the Joint Chiefs of Staff, told a Senate committee that he assessed the effectiveness of the GMD system against a North Korean missile as “ninety per cent, plus.” A year later, amid continuing statements by U.S. officials about their confidence in the effectiveness of the GMD system, MDA Director Lt. Gen. Patrick O’Reilly told the House Armed Services Committee that the probability that the system could counter a single ICBM launched by Iran “would be well over into the high nineties.”

But here’s the rub: even if the GMD and Aegis BMD systems manage an 80% to 90% success rate down the road, that still might not be good enough. After all, a failure rate of 10% to 20% is not adequate against a small barrage of nuclear-armed missiles. As the U.S. has proven in the past, it just takes one to decimate an area. But to improve these defences and even get that far would require a lot more testing (ideally real-scenario testing), and that requires more focus and money. These tests aren’t cheap — the recent test in May cost $US244 ($322) million — but they’re necessary if we’re going to rely on these systems. Though, perhaps we’re better off trying to avoid any conflicts where we’d need such defences to begin with.