US expert says THAAD can’t distinguish between real and decoy warheads

Posted on : 2016-10-04 16:24 KST Modified on : 2019-10-19 20:29 KST
South Korea’s Defense Ministry denies claim about THAAD’s detection range, saying system is limited to North Korean territory
Theodore Postol
Theodore Postol

Theodore Postol, an emeritus professor at MIT and an authority in the area of missile defense, said on Oct. 3 that the THAAD system, which is currently slated to be deployed in Seongju County, North Gyeongsang Province, “lacks the ability to distinguish between real warheads and fake warheads.” This suggests that North Korea could render THAAD helpless by launching decoys during a missile attack.

“The infrared seeker on THAAD interceptors is easily fooled by decoys,” said Postol, who was attending an academic debate on the 9th anniversary of the Oct. 4 Inter-Korean Summit Agreement that was held at the 63 Building in Seoul.

The THAAD interceptors are guided by the AN/TPY-2 radar, but in the end they home in on the target with their onboard infrared seeker. The reason that THAAD’s interception range is so high - between 40 and 150 km - is that the higher air density below 40 km causes friction between the cover of the seeker and the outside air. This friction acts as noise that prevents the seeker from functioning properly.

Postol also said that if North Korea fired a missile from a high angle, it could take advantage of the limitations of the infrared seeker on the THAAD interceptors. If a Scud-B were fired at 55 degrees (a high angle), for example, its range would decrease from 300 km to 280 km, but its maximum altitude would increase from 75-80km to 100km. Since the air density at 100k m is 1/2,000,000 to 1/3,000,000 of sea level, decoys that were deployed there could fall with the real warhead to 50-60km of altitude, Postol said.

“The infrared seeker on a THAAD interceptor cannot determine the distance from the target, and the THAAD radar cannot determine the precise azimuth of the target even if the decoys are only about 100 meters away from the real warhead,” Postol said. In other words, THAAD has no way to identify the real warhead. Postol explained that one way to achieve a decoy effect would be to cause the missile to fragment at the peak of its ascent.

“Scud and Rodong missiles do not carry decoys, and when the warhead separates from the booster after burnout occurs in the atmosphere, the warhead and booster do not fall close to each other,” South Korea’s Ministry of Defense said in response to Postol’s remarks.

Postol argued that the THAAD radar has the ability to detect Chinese ballistic missiles. This means that a THAAD system deployed in South Korea would function as part of an American missile defense system aimed at China. If the THAAD radar is deployed in “Terminal Mode,” which is used for fire control, it would create a fan-shaped searchable zone extending from Seongju County in North Gyeongsang Province through a wide swath of North Korea.

In this case, the detection range would be 500 km, but the detection range could be extended by narrowing the searchable area, Postol said. If the searchable area were narrowed by converting the THAAD radar to “Forward-Based Mode,” its detection range would expand to more than 2,000 km. Furthermore, China’s DF-5 ICBM has a large radar cross section, which would extend THAAD’s detection range to 3,000-4,000 km, Postol said. This means that THAAD could detect Chinese missiles flying over Manchuria and Siberia toward the US.

South Korea‘s Defense Ministry denied Postol’s claims. “The optimal detection range of the THAAD radar is limited to North Korean territory. 2,000-3,000 km is beyond the detection range,” the Ministry said. The US has avoided making comments more specific than its official position that the detection range of the THAAD radar is “more than 1,000 km.”

South Korean and American officials have stated that the THAAD radar that is to be deployed in Seongju County will be operated in Terminal Mode. But according to a report that the US Department of Defense submitted to US Congress in 2011, it is possible to switch THAAD between Terminal Mode and Forward-Based Mode in eight hours.

“The only difference between the two modes is the kind of commands that the software is giving to the radar,” Postol said.

By Park Byong-su, senior staff writer

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