One of World War II's most important battlefields was in the air, and fought with invisible radio signals. The Battle of the Beams during the air war over Britain saw a succession of radio navigational aids, with names like Knickebein, X-Gerat, and Y-Gerat, and sophisticated means of disrupting them. These innovations led to the modern navigation system used in aircraft for the remainder of the 20th Century and beyond!
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