January and a combined exercise with the Atlantic Fleet in March 1929. In 1927–1928 Frobisher was briefly fitted with a prototype F.I.H aircraft catapult and a crane on the quarterdeck, displacing the four-inch AA gun. Together with a fourth AA gun, the displaced gun was moved to a platform between the funnels. In 1929–1930 the ship served with the Atlantic Fleet. By June 1930 her aft superfiring gun had been removed to provide space for a floatplane and her superstructure was built up around the base of the mainmast. Later that year Frobisher was reduced to reserve and became the flagship of the Vice-Admiral Commanding the Reserve. The ship was converted to a cadet training ship in 1932 and the forward quarterdeck 7.5-inch gun and two 4-inch AA guns were removed. By July 1935 she had received her aircraft catapult where the aft superfiring 7.5-inch gun had been located. All of her guns were removed in 1936, as were the above-water torpedo tubes, and a single was added. Frobisher returned to reserve in 1937 and was stationed at Devonport. The ship was transferred to Portsmouth in early 1939 where she again served as a cadet training ship. Wartime service Shortly before the Second World War began in September, the Royal Navy decided to give Frobisher and her sister ship limited rebuilds along the lines of their sister , but with their original armament reinstalled. Work was planned to begin in September, but it did not start until 5 January 1940 at a very low priority. The original plan was to replace all seven 7.5-inch guns and the above-water torpedo tubes, increase the number of 4-inch AA guns to five, add two quadruple and two single two-pounder mounts, and three Oerlikon AA guns. To do this the catapult and crane were removed and the built up superstructure around the mainmast was removed. In 1941 the plan was revised in light of battle experience and another pair of quadruple two-pounder mounts replaced the wing 7.5-inch guns and the single two-pounder mounts were exchanged for four additional Oerlikons. Before the refit was completed in March 1942, a Type 281 early-warning radar, a Type 273 surface-search radar and a pair of Type 285 anti-aircraft gunnery radars were fitted on the roofs of the newly installed four-inch directors. A set of depth-charge rails was added on the stern and the ship was fitted with hydrophones