 In 1955 "Cook" began to carry out trials of the Decca Two Range Survey System, which fixed the ship's position by radio signals from two shore stations.She recommenced survey operations in 1956, with further radio trials and survey work in the Hebrides.In March 1957 she sailed to the Pacific, via the Azores, the West Indies, and Panama Canal, to observe the first British thermonuclear bomb test, code-named "Operation Grapple", on 15 May.