According to a spokesman for Tokyo’s foreign ministry, Japan discovered the radar in late June and issued a protest on Friday through its embassy in China, urging Beijing to explain the purpose.

Japan has been calling on China to halt construction of oil and gas exploration platforms in the East China Sea, accusing it of unilateral development despite a 2008 agreement to maintain cooperation on resources development in the area, where no official border between them has been drawn.

https://www.theguardian.com/world/2016/aug/07/japan-protests-over-chinese-radar-in-disputed-east-china-sea-drilling-rig