A PRC Sukhoi Su-27 force is based at Hainan.[6] The island also houses a large signals intelligence facility that tracks civil and military activity in the area and monitors traffic from commercial communications satellites.[7] The United States has long kept the island under surveillance; on May 22, 1951, for example, RAF Spitfire PR Mk 19s out of Hong Kong's Kai Tak Airport flew photo-reconnaissance missions at the behest of U.S. naval intelligence.[8]