Radar
In the mid-1930's, in response to developing situations in Europe, the British government commenced research into methods of detecting aircraft using radio waves. On February 26th, 1935, the British successful detected an airplane using a primitive radar (RAdio Detection And Ranging) system.
Although various governments and their scientists had investigated numerous methods of using radio waves, the British were the first to build a large-scale system, and by 1939 had established the Chain Home radar system along the southern and eastern coasts of Britain. These early radar systems were continually upgraded and enhanced, and played an important role in Britain's ability to defend her skies from German attack during the Battle of Britain in 1940.
Notes
While searching for Private Ryan, Captain Miller and his squad encounter a heavily damaged radar site defended by a small number of German soldiers.