During the last few years, China has apparently constructed a considerable number of radar installations on the South China Seas islands. These facilities, which could potentially be employed for a variety of purposes, drastically increase China’s real-time awareness and intelligence capabilities over a large portion of the region.
Furthermore, while radar installations may appear less threatening than, for example, surface-to-air missile systems or airbases, they in fact give China a significant edge over other powers that operate in the South China Sea, according to an article posted on the Australian Strategic Policy Institute’s blog website The Strategist.

This rapidly expanding radar network, further augmented by Beijing’s growing military and intelligence satellite network (as it appears that many of the aforementioned radar facilities are also being outfitted with satellite uplinks) will allow China to better track foreign military assets in the region.

http://sputniknews.com/asia/20160828/1044712959/china-radar-electronic-warfare.html