the BTL developed the FA, the first fire-control radar for the U.S. Navy, it improved this with the FC (for use against surface targets) and FD (for directing anti-aircraft weapons). A few of these 60 cm (750 MHz) sets began service in the fall of 1941. They were later designated Mark 3 and Mark 4, respectively. About 125 Mark 3 and 375 Mark 4 sets were produced. S-Band airborne For the Airborne Intercept radar, the Rad Lab 10 cm breadboard set was fitted with a parabolic antenna having azimuth and elevation scanning capabilities. Cathode-ray tube indicators and appropriate controls were also added. Edwin McMillan was primarily responsible for building and testing the engineering set. This was first flight-tested near the end of March 1941, giving target returns at up to five miles distance and without ground clutter, a primary advantage of microwave radar. Designated SCR-520, this was America's first microwave radar. It saw limited service on some larger patrol aircraft, but was too heavy for fighter aircraft. Improved as the much lighter SCR-720, thousands of these sets were manufactured and used extensively by both the U.S. and Great Britain (as the AI Mk X) throughout the war. S-Band Army Gun-Laying Microwave gun-laying system development had already started in Great Britain, and it was included with high priority at the Rad Lab due to its urgent need. The project, with Ivan Getting leading, started with the same 10-cm breadboard used in the AI project. Development of the GL system was challenging. A new, complex servomechanism was needed to direct a large parabolic reflector, and automatic tracking was required. On detection of a target, the receiver output would be used to put the servo control into a track-lock mode. The mount and reflector were developed with the Central Engineering Office of Chrysler. BTL developed the electronic analog computer, called the M-9 Predictor-Corrector, containing 160 vacuum tubes. The components were integrated and delivered in May 1942 to the Army Signals Corps for tests. Designated the SCR-584 Anti-Aircraft Gun-Laying System, about 1,500 of these were used in Europe and the Pacific starting in early 1944. S-Band Navy Search After the 10 cm experimental breadboard demonstration, the Navy requested an S-band search radar for shipboard and airborne applications. Under the leadership of Ernest Pollard, the 50 kW SG shipboard set was given sea trials in May 1941, followed by the ASG version for large patrol