Brown awoke in an ambulance. This accident, along with the many previous flights at high altitude, aggravated an ear injury he had received at RAF Martlesham Heath in 1939, and during the spring he was hospitalized for a mastoidectomy operation in Brighton. The operation was successful, but a post-operation infection caused him to go deaf in both ears and several follow-up visits were required. By the time he returned to the AMES research center, now in Worth Matravers and renamed the Telecommunications Research Establishment (TRE), major research on the early AI sets had ended in favour of new systems working at microwave frequencies using the recently invented cavity magnetron. Brown had missed most of the development of this system, and he was no longer allowed to fly at high altitudes, so his work on AI ended. He was instead placed in a new group led by John Pringle, a zoologist from Cambridge University, and the two began to study new applications for radar technologies. Beacons for Army Co-operation In June 1941, Brown visited the Army headquarters at Old Sarum Airfield to see if the RAF Army Co-operation Command's School of Army Co-operation might put radar to good use. The Army Co-operation squadrons carried out a variety of missions including artillery spotting, general reconnaissance, and ground attack. He found the group was only mildly interested in radar, thinking it might make a useful device for warning of the approach of enemy fighters, but were perfectly happy using flags and smoke signals for navigation and communications. Brown then visited a military exercise involving ground attacks in close coordination with the Army, and was convinced that radar systems could be used to improve these results. However, he also came to realize that almost all such missions would be carried out by aircraft of other forces, notably the RAF, so any system they proposed would have to be mounted in those aircraft. Pringle then arranged for Brown and himself to meet with the Commander in Chief (C-in-C) for Army Co-operation, Sir Arthur Barratt. In a long conversation, the two outlined the possibilities of radar for bombing, navigation and return-to-base roles, all of which proved to be interesting to Barratt. Barratt then stated that any system they did adopt would have to fit in single seat aircraft like the Tomahawk, which eliminated most of these possibilities. Both Pringle and Brown then focused on the use