FILE - In this Tuesday, May 2, 2017 file photo, a U.S. missile defense system called Terminal High Altitude Area Defense, or THAAD, is installed at a golf course in Seongju, South Korea. Clashes between residents and police over the deployment of an advanced U.S. anti-missile system highlight a divisive issue ahead of South Korea’s presidential election on May 9. (Kim Jun-beom/Yonhap via AP, File)

Chinese officials have long protested the U.S. deployment of the Terminal High-Altitude Area Defense (THAAD) missile defense system to South Korea because they believe it can spy on its military activities deep inside its mainland. Well, on Tuesday, Beijing’s fears were pretty much confirmed when military officials in South Korea reported that they were in fact able to detect North Korea’s recent ballistic missile test Sunday with THAAD.

Reuters reports that South Korean officials were able to determine that the missile was an IRBM (intermediate range ballistic missile), which can travel between 1,860 to 2,485 miles. The country’s defense minister, Han Min-koo, added that the North’s missile program is developing faster than expected.

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While we are not sure how, exactly, the South used THAAD to track the north’s missile test, the accompanying X-band AN/TPY-2 radar may have played a role. To recap, THAAD uses powerful radar systems to track short, medium, and intermediate-range ballistic missiles similar to the ones North Korea could use in a hypothetical a nuclear shooting match. THAAD then uses non-warhead equipped missiles to destroy the enemy projectile.

To be sure, China is not worried about THAAD’s missiles; again, they are not armed with warheads, so they are not offensive weapons. What’s really at issue here is the radar.

At the same time, as The Diplomat explained in March, there are some technical issues countering the argument that the system is as powerful of the Chinese claim it is. For example, this isn’t the first time the U.S. has deployed AN/TPY-2 radar. There are already two in Japan, specifically the Shariki, Aomori prefecture. Also, the surveillance range of the AN/TPY-2 may not be able to monitor the locations where the Chinese do more of their missile testing, as The Diplomat explains:

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Second, while we have no watertight estimates on just how capable the AN/TPY-2 radar is and in what configurations, even the most generous estimates don’t leave the Gyeongsangbuk-do unit capable of any useful surveillance deep into the Gobi desert, where China has its most active and sensitive missile testing ranges. (AN/TPY-2 range estimates go from “several hundred miles” to 3,000 km.) I’ve mapped out the ranges below with the most generous range estimate of 3,000 km, using a Chinese ballistic missile impact range that Thomas Shugart at War on the Rocks recently revealed as a test-bed for potential People’s Liberation Army Rocket Force preempetive warfare tactics (i.e., a site of surveillance interest for the United States).

Adding the westernmost AN/TPY-2 in Japan — the Kyogamisaki Communications Site unit — the map doesn’t change drastically, either. (Incidentally, North Korea’s latest missile test resulted in three missiles splashing down in Japan’s exclusive economic zone, between the two AN/TPY-2s in the country — a less-than-subtle show of confidence.)

There is an argument that THAAD could threaten China’s second-strike capabilities—its ability to respond in kind to a nuclear attack, and minimize its chances of being obliterated or crippled by an enemy’s first strike.

Li Bin, a nuclear weapons expert at Tsinghua University in Beijing, wrote in March that THAAD’s radar would “would undermine China’s nuclear deterrence by collecting important data on Chinese nuclear warheads.”

More specifically, as the New York Times explains, Beijing fears Washington can use the radar to get a jump start on its nuclear weapons strike response (China as a no first use nuclear weapons policy), weakening its capabilities to the point of uselessness:

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He and other Chinese experts say the radar could identify which Chinese missiles are carrying decoy warheads intended to outfox foes. That would be like being able to see what cards China holds in a nuclear poker game, and that could weaken China’s deterrent, they say.

“For China this is a very important point, because its missiles are limited in number to begin with,” Wu Riqiang, a nuclear expert at Renmin University in Beijing. That meant, he said, “China could lose its nuclear retaliatory capacity.”

For China, it does not matter that the American and South Korean governments have said Thaad is meant only to foil North Korean missiles. Mr. Wu said.

“What we worry about is the ability. It doesn’t matter to us whether the United States says this is aimed at North Korea or China,” Mr. Wu said. “If there’s this ability, then China must worry.”

What this comes down to is trust. Beijing doesn’t believe that the U.S. will use THAAD solely as a defensive measure against a North Korean missile attack. If the Chinese truly believe THAAD can track which of its missiles is carrying a warhead, it is a moot conversation to argue that it will not be used for that.

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The fact that THAAD can determine the success of North Korea’s latest ballistic missile test will not make China feel any more secure about it being deployed in South Korea. If it can be used to track Pyongyang’s actions, to what extent can it be used to do the same against Beijing?

That is what has China up at night.