The SCR-584 (short for Set, Complete, Radio #584) was an automatic-tracking microwave radar developed by the MIT Radiation Laboratory during World War II. It was one of the most advanced ground-based radars of its era and became one of the primary gun-laying radars used worldwide well into the 1950s. A trailer-mounted mobile version was the SCR-784. In 1937, America's first fire-control radar, the SCR-268 radar, had proven to be insufficiently accurate due in part to its long wavelength. In 1940, Vannevar Bush, heading the National Defense Research Committee, established the "Microwave Committee" (section D-1) and the "Fire Control" division (D-2) to develop a more advanced radar anti-aircraft system in time to assist the British air-defense effort. In September of that year, a British delegation, the Tizard Mission, revealed to US and Canadian researchers that they had developed a magnetron oscillator operating at the top end of the UHF band (10 cm wavelength, 3 GHz frequency), allowing greatly increased accuracy. Bush organized the Radiation Laboratory (Rad Lab) at the MIT to develop applications using it. This included a new short-range air-defense radar. Alfred Lee Loomis, running the Rad Lab, advocated the development of an entirely automatic tracking system controlled by servomechanisms. This greatly eased the task of tracking targets and reduced the manpower needed to do it. They were also able to take advantage of a newly developed microwave switch that allowed them to use a single antenna for broadcast and reception, greatly simplifying the mechanical layout. The resulting design fit into a single trailer, could provide all-sky search and single-target tracking, and followed the targets automatically. In close contact with the Rad Lab, Bell Telephone Laboratories was developing an electronic analog gun director that would be used in conjunction with the radar and servo-actuated 90 mm anti-aircraft guns. The SCR-584 was intended to be introduced in late 1943, but due to delays did not reach field units until early 1944. They began replacing the earlier and more complex SCR-268 as the US Army's primary anti-aircraft gun-laying system as quickly as they could be produced. They proved easier to use in the field than the less advanced Canadian/British GL Mk. III radar, and many SCR-584s were rushed to England, where they were an important part of the defences developed to counter the V1 flying bomb. By the end of the war they had been used to track artillery shells in flight, detect vehicles, and reduce the manpower needed to guide anti-aircraft guns.
Background In September 1940, a group of British physicists and engineers visited their counterparts in the US in what became known as the Tizard Mission. The goal of the meetings was to exchange technical information that might be of use to the war effort. The British were hesitant to give away too much information without getting anything in return, and initial progress was slow. When they moved onto the topic of radar, the British team was surprised to learn that the US was in the process of developing two systems similar to their own existing Chain Home, the Navy's CXAM and the Army's SCR-270. This began to break the ice between the two groups. Two previous attempts at radar-controlled gun laying were notable. In Britain, the 75 MHz GL Mk. I radar was used in connection with a Vickers predictor; and in the U.S., the 200 MHz SCR-268 was combined with the Sperry M-4 predictor. Neither the US or UK systems had the accuracy needed to directly lay their associated guns, due to their long wavelengths. The US delegates then mentioned the Navy's work on a 10 cm wavelength radar, which could provide the required resolution with relatively small antennas, but their klystron tube had low power and was not practical. This was the moment the British team had been waiting for. Edward George Bowen produced one of the earliest cavity magnetrons from a box and showed it to the other researchers. He explained that it also worked at 10 cm wavelength, but offered higher power – not just than the Navy klystrons, but even the US's existing long-wave radars. One US historian later described it as the "most valuable cargo ever brought to our shores". The potential of the device was obvious, and the US group, informally known as the Microwave Committee, immediately switched their efforts to the magnetron. They had their own examples built in US labs within weeks. They also began developing the other technologies presented at that meeting, including an aircraft interception radar and a radio navigation system that became LORAN. The expansion of the Committee led to it being renamed the Radiation Laboratory (RadLab) in 1940.
Development A formal proposal for a SCR-268 replacement was made by the Signal Corps in January 1941, by which point the RadLab had already formed what they knew as Project 2 to develop this advanced gun-laying radar. MIT proposed an advanced system with automatic search, tracking and the ability to directly aim the guns. This was a field MIT was particularly knowledgeable in due to work in their Servomechanisms Lab. At the same time, British and Canadian teams began work on versions of a simpler system that they hoped to deploy by 1942 – the GL Mk. III, which was a microwave version of the earlier lobe-switching VHF radar sets. The Radiation Lab kept in close contact with the Canadian team during these developments. The RadLab team, overseen by Lee Davenport, had a prototype radar system running in April 1941. To test the automatic aiming system, they attached the outputs from the radar to a gun turret taken from a Boeing B-29 bomber, removing the guns and replacing them with a camera. A friend then flew his light plane around the area, while the camera periodically took photographs, and on 31 May the system was able to accurately track the aircraft. Work then started on making the system suitable for field use, mounting the entire system in a single trailer with the 6 ft (1.8 m) antenna on top. Known as XT-1, for experimental truck-1, the system was first tested at Fort Monroe in February 1942.
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