In 1886, Heinrich Hertz demonstrated that radio waves that hit objects are reflected, and in 1897, the first accidental radar detection of a ship was performed by Alexander Popov. In 1904, Christian Hulsmeyer made a practical demonstration of intentional detection of metallic objects using a radar.
In 1936, a prototype radar was demonstrated in New York City and detected aircraft from up to 7 miles away. After witnessing the demonstration, the Secretary of War ordered the prototype to be fully developed, and Congress offered $250,000 as funds for its development. The U.S. Army deployed the first fully developed model in 1941 in Hawaii. On Dec. 7, 1941, at 7:02 a.m., the recently installed radar detected aircraft north of its position, and the information was sent to the aircraft warning center. However, the information was misinterpreted as friendly bombers, and 46 minutes later, Pearl Harbor was attacked by Japanese aircraft.