The advanced surface-to-air missile system China has deployed onto a disputed island in the South China Sea is a variant on the United States Patriot missile battery and the Russians' S-300.

The HQ-9 missiles were placed on the eastern part of Woody Island in the Paracel Island chain sometime between February 3 and February 14. Both Taiwan and Vietnam have claimed Woody Island as their own.

In late January, a US guided missile destroyer passed within 20km of Triton Island in the same chain, in what is known as a freedom of navigation patrol. The exercise drew condemnation from China at the time.

Both the US and Russian systems can engage targets at what is known as a "beyond the horizon" range using a set of sensors and radars that allow missiles to track and hit targets more than 160km away from the launch site. The HQ-9 is mounted to a truck chassis, which allows it be highly mobile when not in use.

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"From their perspective this is a defence system designed to protect their sovereign territory," said Neil Ashdown, a deputy editor at IHS Janes, a military research group.
Not only can the HQ-9 hit aircraft, but like the S-300 and Patriot, it can also hit ballistic missiles. The system can also engage targets at more than 24,000m high.

The deployment of advanced surface-to-air systems has been the go-to move for countries to project their military might in recent months, and it seems China has ripped a page out of Russia's playbook. In December, following the shoot-down of a Russian strike fighter by Turkish F-16s, Russia deployed S-400 missile systems onto their airbase in northern Syria.

According to Ashdown, the decision to deploy the HQ-9 is more a symbolic move than a military escalation. The Chinese could have deployed other systems that were less high-profile than the HQ-9, which is just short of the deployment of land-based anti-ship cruise missiles, something Ashdown believes would be an extremely aggressive move on China's behalf.

Regardless of the Chinese Government's intent, the HQ-9 could be the first part of a deployment of systems that could create a sophisticated anti-access/anti-denial (A2/AD) network in the region. In short, the HQ-9 could interface with other systems to create a sort of defensive bubble over the disputed island chains, making any type overflight for US aircraft extremely dangerous and problematic in the event of an actual conflict.

"[The deployment of the HQ-9] reflects a harder step by Beijing towards these Maritime claims," said Ashdown. "This is a harder step than what we've seen in 2015."