TABLE OF CONTENTS By the 1930s, the primitive biplanes of fabric and wood that had populated the skies above the battlefields of World War I had become all-metal monoplanes capable of carrying thousands of kilograms of explosives and traveling at hundreds of kilometers per hour: by the time observers could visually identify an incoming flight, it was too late to respond. Detection of airplanes beyond visual range was the task of microwave radar, also under rapid development in the 30s, but mere detection of the presence of aircraft begged the key question: whose side were they on? It was exactly this inability to identify aircraft that enabled the mistaken assignment of incoming Japanese aircraft to an unrelated United States bomber flight and so ensured surprise at Pearl Harbor in 1941. The problem of identifying as well as detecting potentially hostile aircraft challenged all combatants during World War II.
The Luftwaffe, the German air force, solved this problem initially using an ingeniously simple maneuver [1]. During engagements with German pilots at the beginning of the war, the British noted that squadrons of fighters would suddenly and simultaneously execute a roll for no apparent reason. This curious behavior was eventually correlated with the interception of radio signals from the ground. It became apparent that the Luftwaffe pilots, when they received indication that they were being illuminated by their radar, would roll in order to change the backscattered signal reflected from their airplanes (Figure 2.1). The consequent modulation of...
