(S (S (BASENP (DT The) (NNS bombers)) (VP (ADVP (RB eventually)) (VP (VP (VBD left) (BASENP (DT the) (NNP Nato) (NN zone))) (COMMA ,) (PP (IN under) (NP (NP (NP (NN tracking)) (PP (IN by) (NP (BASENP (NNP UK) (NN aircraft)) (CC and) (BASENP (NNP RAF) (NN radar))))) (PP (IN in) (BASENP (NNP Northumberland)))))))) (PERIOD .))