SCR-584 radar

The SCR-584 (short for Signal Corps Radio # 584) was a microwave radar developed by the MIT Radiation Laboratory during World War II. It replaced the earlier and much more complex SCR-268 as the US Army's primary anti-aircraft gun laying system as quickly as they could be produced. In service it proved to be an outstanding system, much more advanced than any other battlefield radar system deployed during the war.

Background
The genesis of the SCR-584 started with the Tizard Mission in September 1940, when a group of British scientists travelled to the US to present various advances useful to the war effort. The British were initially hesitant to give away too much information without getting anything in return, and initial progress was very 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 system, the Navy's CXAM and the Army's SCR-270. Neither of these systems had the accuracy needed to directly lay their associated guns, however. The US delegates then mentioned the Navy's work on a 10 cm wavelength radar, which would have the required resolution, but their klystron tube had very low power and was not practical.

Edward George Bowen had prepared for just this moment, and presented one of the earliest cavity magnetrons to the assembled researchers. It also worked at 10 cm, but offered dramatically higher power - not just than the 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 device was so much more advanced than anything in US use at the time that plans were immediately started to undertake a massive research and development program to quickly put it into production, leading to the creation of the Radiation Laboratory (RadLab) and its many developments. The US would take over leadership in the radar field by the end of the war.

Development
Along with its SCR-270 early warning radar, the Army was also in the process of building an associated gun laying system, the SCR-268. Based on similar longwave technology, the SCR-268 was only marginally useful in its current form, with accuracy too low to directly lay the guns. While the SCR-268 had its operational problems, the real complaint was its size.

It is a basic outcome of antenna design that the bearing resolution of a radar system is dependant on two primary factors, the size of the antenna and the wavelength of the radio being used. In the case of the SCR-268's ~1.5 metre wavelength (205 MHz) and ~0.1 degree resolution, the required antenna was several meters across and presented serious logistics problems. A microwave radar in the 10 cm range could be aimed much more accurately while using a parabolic dish of much smaller size, making it easily portable.

A formal proposal for a SCR-268 replacement was made by the Signal Corps in January 1941, by which point the RadLab was already in the process of developing useful radar systems based on the magnetron. But instead of a simple system similar to the SCR-268 in concept, MIT proposed a much more 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.

The RadLab team, overseen by Lee Davenport, had a prototype radar system running in April. 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, 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-foot antenna on top. Known as XT-1, for eXperimental Truck-1, the system was first tested at Fort Monroe in February 1942.

Work also started on a suitable gun-laying computer that could use electrical, as opposed to mechanical, inputs for pointing data. Bell Labs delivered an analog computer known as the M9 Director for this role, able to control four of the Army's standard 90 mm M3 guns. The entire system, including the M9, was demonstrated in complete form on 1 April 1942. A contract for over 1,200 systems arrived the next day. Bell also worked on their own microwave radar as a backup project.

The SCR-584 was extremely advanced for its era. To achieve high accuracy it used a conical scanning system, in which the beam is rotated around the antenna's axis to find the maximum signal point, thus indicating which direction the antenna should move in order to point directly at the target. This system was not new, having been introduced on the German Würzburg radar in 1941. However the SCR-584 developed the system much further, and added an automatic tracking mode. Once the target had been detected and was within range, the system would keep the radar pointed at the target automatically, driven by motors mounted in the antenna's base. For detection, as opposed to tracking, the system also included a helical scanning mode that allowed it to search for aircraft. This mode had its own dedicated PPI display for easy interpretation. When used in this mode the antenna was mechanically spun at 4 rpm while it was nudged up and down to scan vertically.

The system could be operated at four frequencies between 2,700 and 2,800 MHz (10–11 cm wavelength), sending out 300 kW pulses of 0.8 microseconds in duration with a pulse repetition frequency (PRF) of 1,707 pulses per second. It could detect bomber-sized targets at about 40 miles range, and was generally able to automatically track them at about 18 miles. Accuracy within this range was 25 yards in range, and 0.06 degrees (1 mil) in antenna bearing angle (See Table "SCR-584 Technical Characteristics"). Because the electrical beam width was 4 degrees (to the -3db or half-power points), the target would be smeared across a portion of a cylinder, so as to be wider in bearing than in range (i.e., on the order of 4 degrees, rather than 0.06 degrees implied by the mechanical pointing accuracy), for distant targets. Range information was displayed on two "J-scopes", similar to the more common A-line display, but arranged in a radial pattern timed to the return delay. One scope was used for coarse range, the other for fine.

Operational use
Although the first operational unit was delivered in May 1943, various bureaucratic problems led to it being delayed in being delivered to the front-line troops. The SCR-584 was first used in combat at Anzio in February 1944, where it played a key role in breaking up the Luftwaffe's concentrated air attacks on the confined beachhead. The SCR-584 was no stranger to the front, where it followed the troops, being used to direct aircraft, locate enemy vehicles (one radar is said to have picked up German vehicles at a distance of 26 kilometers), and track the trajectories of artillery shells, both to adjust the ballistic tables for the 90 millimeter guns, and to pinpoint the location of German batteries for counter-battery fire. The SCR-584 was not, however, used in the rapidly-shifting very front lines, where lighter, less accurate, radars such as the AN/TPS-1 were used.

The SCR-584 was so successful that it was adapted for use by the United States Navy. CXBL, a prototype of the navy version, was mounted on the carrier USS Lexington on March 1943, while the production version, the SM, built by General Electric, was operational on the carriers USS Bunker Hill and USS Enterprise by October 1943. A lighter version of the system was also developed, the SCR-784. The only real difference was that the new design weighed 12,000 lb, whereas the original was 20,000.

Davenport waterproofed a number of the radar sets so that they could be carried aboard the Allied armada launching the Normandy landings on D-Day.

Automatic gunlaying (using, among others, the SCR-584 radar) and the proximity fuze played an important part in Operation Diver, (the British operation to counter the V1 flying bombs). Both of these had been requested by AA Command and arrived in numbers, starting in June 1944, just as the guns reached their free-firing positions on the south eastern coast of England. Seventeen per cent of all flying bombs entering the coastal 'gun belt' were destroyed by guns in the first week on the coast. This rose to 60 per cent by 23 August and 74 per cent in the last week of the month, when on one extraordinary day 82 per cent were shot down. The rate increased from one V-1 for every 2,500 shells fired to one for every hundred.

After the war, the radar was adapted for use in the AN/MPQ-12, and AN/MPM-38 systems, a US Army field artillery missile system (MGM-5 Corporal). A modified version was also used to control and beacon-track (using an onboard transponder) the CORONA spy satellite.

In 1953, the SCR-584-Mod II was used for tracking the Redstone (rocket), its range extended to 740 km by the use of an onboard transceiver.

Despite using vacuum tubes and being powered by an analog computer, some specimens of the SCR-584 are still operational today. In 1995 the first Doppler On Wheels (DOW) radar adapted the MP—61 pedestal from an SCR-584 for use in a mobile weather radar. Using this pedestal, the DOWs created the first maps of tornado winds, discovered hurricane boundary layer rolls, and pioneered many other observational studies. The pedestal housed first a 6' then an 8' antenna. Later the original motors were replaced with more powerful brushless versions for faster scanning in high winds. Three DOWs are now operated as National Science Foundation facilities by the Center for Severe Weather Research. One is found at the National Severe Storms Laboratory in Norman, Oklahoma, where the 584 pedestal is the platform for the new Shared Mobile Atmospheric Research & Teaching Radar, or SMART-R.

K-83 dolly
General Electric constructed a dolly for the SCR-584, designated K-83. The K-83 was designed to attach to a semi-trailer hitch, allowing smaller vehicles to move the SCR-854.