Radar Illuminates the Night

Key to opening the black sanctuary that shielded night bombers was the development of radar, both ground-based and units carried aloft in aircraft. The sixteenth-century English dramatist John Lyly could write that the “night hath a thousand eyes,” but until radar, aircrews knew that night interception was the product of luck rather than any number of eyes. Though the properties of radio waves had been known and understood since the late nineteenth century, not until 1922 did the United States Navy begin active research into radio detecting and ranging. (Radar-a Navy term-was adopted officially in 1940.) Successful experiments with aircraft soon attracted Army interest. The key development for World War II night fighters was the accidental discovery by Lawrance A. Hyland in 1930 that radio waves reflecting off an aircraft in flight, previously believed too small to measure, actually could be collected and gauged. By 1936 Americans were testing pulse radars, allowing a transmitter to receive its own signals bounced off an airplane in flight, for measuring distances. The U.S. Army began deploying ground radars in 1940, but the large amounts of electrical power needed to generate the radio waves (in wavelengths of one to three meters) and the size of the antennas precluded their use in aircraft. The technology also suffered from antennas being fixed in a stationary position. Aircraft flying through the resulting directional radio beam created a temporary blip on a cathode ray scope that disappeared once the aircraft had overflown the beam. The sweeping hand of a rotating radar beam was unknown at the time.

Under the threat of aerial attack from the continent, Britain made a considerable investment in the new technology. Robert Watson-Watt of the National Physical Laboratory and Hugh Dowding of the RAF constructed a belt of fixed warning radars able to detect incoming aircraft at over one hundred miles. Like U.S. radars, they required enormous towers (up to three hundred feet) and power consumption equal to that of a small town. These early warning radars provided range, altitude, and bearing data, allowing the Ground Control of Interception (GCI) radar controller to vector a night fighter by radio to within several miles of a target. At that point another means of detection had to be used. Meanwhile, under the codename MAGIC MIRRORS, British researchers strove to develop a radar set small enough to fit into an aircraft but with minimal power demands.

By August 1937 a handmade, experimental model was ready. The Mark I Airborne Interception Radar entered combat in September 1939, searching the North Sea for minelaying seaplanes at night. It had restricted range and suffered from excessive interference on the radar scope from ground returns. The Mark II and III versions showed little improvement. Then in November 1940, after three years of development, the new Mark IV airborne radar, mounted in twin-engine Beaufighters and Douglas Bostons, was ready to operate in Britain’s night skies. Unfortunately, ground returns on the Mark IV, which used 1.5 meter wavelengths, created target-obscuring clutter on the radar scope to the distance the aircraft was above the ground. Also, returns were too vague to make accurate determinations. At this point, radar was still more art than science. Nevertheless, Mark IV-directed night fighters achieved their first victory in November 1940 and went on to claim 102 victories out of 200 airborne radar contacts during the Night Blitz over England from March to June 1941. Despite this success, the Mark IV’s limitations underscored the importance that luck still claimed in night fighting.

The technological solution to these problems involved centimetric or microwave radar (wavelengths below 10 centimeters). These narrow beams were inherently more accurate and also minimized ground interference without requiring huge antennas. The answer to the problem of the large electrical demands of microwave radar came from the British team of John T. Randall and Henry A. H. Boot, who developed a resonance cavity magnetron to produce the necessary power. In September 1940, more than a year before the United States entered the war, the British Tizard Mission shared its radar achievements with the U.S. National Defense Research Committee (NDRC)-an unselfish display of good faith. Though Americans had made great progress in many areas of radar, they lacked the magnetron breakthrough necessary to power microwave airborne radar. The NDRC established Division 14 in October 1940 to produce a U.S. 10-centimeter radar, under the direction of the newly established Radiation Laboratory at the Massachusetts Institute of Technology (MIT) in Boston.

The U.S. commitment of resources to this project soon surpassed the small British development program. By March 1941 an MIT microwave airborne radar was flying in an Air Corps bomber and detecting aircraft at slant ranges up to eight miles. At first, wartime demands forced the Western Electric Company to produce the SCR-540 as a 1.5-meter radar set (equivalent to the British Mark IV), but soon the contractor converted to the 10-centimeter SCR-520 (British Mark VII), powered by one hundred kilowatts from Randall and Boot’s magnetron. Though heavier than the 540 by six hundred pounds, the SCR-520 provided a more refined target and suffered less from ground reflections. Meanwhile, the serious U-boat threat in the North Atlantic diverted most initial production of the airborne radar from aerial night fighting to antisubmarine operations. By late 1942, technology advanced even further, as MIT, Western Electric, and Bell Telephone Laboratories introduced the 10- centimeter SCR-720 (British designation Mark VIII and X), a system with a 6.5 mile range and generally invulnerable to enemy jamming.

Armed with airborne radar and assisted by ground-based systems, U.S. night fighters could now penetrate the darkness that offered sanctuary to enemy night bombers. The next requirement was an aircraft with sufficient speed and firepower to catch Axis enemy planes and knock them from the sky.