Early on the morning of June 6, 1944, 1st Lt. Stanley Fine climbed into the B-17 designated as the lead aircraft, sat down at his radar set, and led the 401st Bombardment Group from its base at Deenethorpe, England, over solid cloud cover to Normandy.
The weather over the English Channel was dreadful. Gen. Dwight Eisenhower, supreme allied commander in Europe, had agonized throughout the night about his decision to proceed with the invasion of Normandy.
Now, in murky skies above German fortifications and just ahead of Allied ground troops on the beaches, Lieutenant Fine studied the smudges on his screen and began his bombing run. From the copilot's seat, the air commander's voice crackled over the intercom, "Blind. Going in 'Mickey.' "
Sixty years later, as Americans commemorate D-Day, few know about Mickey - the nickname for the latest in a newly developed family of microwave radars that helped change the course of the war.
Small enough to be mounted in aircraft and accurate enough to "see" individual targets, microwave radar allowed the Allies to clear the sea lanes, get American troops and supplies to England, and then land those troops and weapons safely on a coastline occupied by the enemy. Without microwave radar, it's not clear that the Allies could have mounted the invasion of France in 1944.
"The thoughts of America were too much with the men going through the Normandy surf for a remarkable thing to get much notice," wrote Gen. Henry H. "Hap" Arnold, commander of the US Army Air Corps, afterward. "The final bombs that paved the way for them, dropping only a few yards ahead of the first men to hit the beaches, went down through a solid overcast of clouds, without ... so much as scratching the paint on a single rowboat in that packed armada below.... [O]ur scientists had taken from Hitler even the comfort of bad weather."
The technology - still used in some radars today - had its own heroes, including a handpicked group of British and American civilian scientists who crisscrossed the Atlantic past marauding U-boats; flew in and out of London during the Blitz to confer with fellow scientists; and flew test, training, and combat missions in military aircraft with military personnel.
As with many key inventions, the radar's beginnings were accidental. In the 1920s, scientists working on radio transmissions and communications noticed changes in their signal reception when planes flew by. Most of them recorded these observations as incidental nuisances in their notebooks, but some speculated about the cause. They soon realized that radio energy, like light, could be reflected by the surface of large objects. The audible echo - sound reflected back to its source from a large surface - works the same way.
In the decade leading up to the war, most of the advanced nations were developing radar. However, the equipment was too large and cumbersome to be mobile, and it was unable to show detail.
Scientists knew they would have to operate at higher microwave frequencies to improve the performance of their radar, but no device existed capable of transmitting sufficient power at those frequencies.
Then one afternoon in November 1939 two physicists at the University of Birmingham in England, John Randall and Henry Boot, sketched out the resonant cavity magnetron.
It took three months of shop work to build a prototype. No one knew what to expect, and at first it proved impossible to test. A blue-violet electric arc sizzled from the output lead. Then the lead melted.
Lab assistants were sent on a succession of errands to the local garage for higher and higher wattage bulbs to connect to the output, but each was burned to a crisp. The resonant cavity magnetron was generating so much power it was burning up everything they connected to it. It was working beyond their wildest dreams. Randall and Boot had created the single component destined to unlock the enormous potential of radar.
Resembling a tin of tuna fish with protruding spines, the resonant cavity magnetron was small enough to fit in the palm of a man's hand. In the hands of American and British scientists it would equip the two nations with a secret and exclusive capability - microwave radar.
The British brought the magnetron to the United States in great secrecy. And the newly organized National Defense Research Council convened a group of civilian scientists who, early in 1941, converged on Cambridge, Mass., from all over the US to begin work on a crash program at the Massachusetts Institute of Technology.
Working in tandem with the British, MIT's new Radiation Laboratory aimed to refine microwave radar so that it could be mounted in aircraft and accurately distinguish individual targets. Under intense pressure, the scientists spent day after grinding day in cold makeshift laboratories, designing, building, testing, and redesigning equipment that had never before existed.
By 1942, they had furnished the Air Force with hand-built microwave radars based on British designs to search for U-boats off the Atlantic coast. In 1943, awaiting orders to join the Eighth Air Force in Europe, Fine suddenly received secret orders to be trained, along with nine other navigators, on the Radiation Lab's new blind-bombing radar, Mickey.
"At Langley, where I was sent for training, a civilian from MIT showed me what knobs to twist and how to interpret the scope images," he recalls. "Then they took me up, covered my windows, and told me to direct the pilot to the Chesapeake and drop a dummy bomb into the bay. All I could think was, 'Please, God, don't let me drop it on the White House.' "
The technology helped set the stage for D-Day. Before mounting the invasion, the Allies had to accomplish two things: First, transport American personnel, supplies, and weapons to England to fight the war. Second, land American and British personnel, supplies, and weapons on mainland Europe.
Obstructing the first goal - transport - were the German U-boats. In just six months, from January to June 1942, U-boats sank a total of 585 Allied ships. But with the fragile, handcrafted radar mounted in slow-flying aircraft, the Allies discovered they could locate German U-boats at night when they surfaced to recharge batteries.
The vulnerable U-boat would suddenly find itself bathed in powerful searchlights from directly above and attacked before it could submerge. The German captains wondered: How were they being located? The first U-boat located by a plane equipped with microwave radar built by MIT's Radiation Lab was sunk on April 1, 1942.
Slowly, steadily, the mayhem inflicted by US and British planes equipped with microwave radar increased. In May 1943, the mystified German High Command, still assured by their scientists that radar was incapable of detecting an object as small as a U-boat, was forced to withdraw all its submarines from the North Atlantic.
"Radar location by aircraft had ... robbed the U-boats of their power to fight on the surface," Admiral Karl Doenitz, commander in chief of the German Navy wrote later. "Wolf-pack operations against convoys in the North Atlantic were no longer possible.... We had lost the Battle of the Atlantic."
With the opening of the shipping lanes, troop transports carried hundreds of thousands of military personnel, and freighters transported millions of tons of equipment to the British Isles in preparation for the D-Day landings. The German Air Force, however, remained a formidable obstacle. Before attempting to land troops, the Allies had to control the skies.
In England and western Europe, dirty weather is the rule throughout the year. During the winter of 1942-43, the Eighth Air Force discovered that visual bombing missions were possible only 20 percent of the days. By the time weather conditions allowed the bombers back into the air, they discovered that most of the damage inflicted by previous missions had been repaired. Their concept of strategic bombing was not succeeding.
Not only were the Germans saving their aircraft to oppose the anticipated Allied landings, their factories were building even more planes. Any possible chance for an Allied invasion in 1943 had evaporated.
However, with the new radar-equipped Pathfinder bombers, operations during the winter of 1943-44 were vastly more successful. Able to navigate in bad weather and "see" strategic targets through cloud cover, American and British Pathfinders led wave upon wave of heavy bombers over enemy territory around the clock, regardless of the weather.
If the formation couldn't bomb visually using bombsights, the Pathfinder lead ship with its blind-bombing radar dropped its load through the clouds. The rest dropped their bombs on the Pathfinder's markers. Although the radar-guided results weren't nearly as accurate as visual bombing, the relentless pounding still forced the German Air Force to send its planes into the skies to challenge the bombers and their fighter escorts.
"We were the bait to get the German planes into the air so we could shoot them down," recalls Fine. By June 6, 1944, there was hardly a German plane left in the sky to oppose the landings.
Even so, the D-Day landings were brutal. In one day, the Allies lost some 2,500 troops of the 156,000 who stormed the beaches. But without the heroism of a few civilians with slide rules, their brave campaign might have proved far more costly.
• The author is the nephew of Captain Fine.
• Atomic bomb - The nuclear weapon was dropped by the US on the Japanese cities of Hiroshima and Nagasaki in August 1945. Japan surrendered shortly thereafter. [Editor's note: The original version misstated the year the bombs were dropped on Japan.]
• Resonant cavity magnetron - Used in the first portable microwave radar on planes, it enabled around-the-clock bombing missions and could locate enemy U-boats. It was invented in November 1939 by scientists at the University of Birmingham, England.
• Water-launched air attacks - In 1941, Japanese airplanes are launched from aircraft carriers in a surprise attack against Pearl Harbor.
• V-2 Missile - This longer-range rocket was first developed in 1942 for the Germans. Hundreds fell on England.
Sources: encyclopedia.com, The Oxford Dictionary of World War II