【教えて下さい】「it had tracked 」のitはなにを指しているのか

「Subi reef, located in the disputed Spratly Islands in the South China Sea, is shown in a satellite image taken August 8, 2012. A U. S. guided-missile destroyer sailed close to one of China's man-made islands in the South China Sea, drawing an angry rebuke from Beijing, which said it had tracked and warned the ship and called in the U. S. ambassador to protest.」
の翻訳結果について、
which said it had tracked and warned the ship and called in the U. S. ambassador to protest.のitはなにを指しているのか