Radar was first conceived in the 1920s and 30s, following the invention of radio. Early radio operators noticed how their signal was sometimes interrupted by a passing ship or aircraft. Soon, numerous countries began investigating whether radio waves could be used for detecting military targets. A key development was the invention of the magnetron tube, which was able to transmit more sensitive, higher frequency radio waves.
Radar was first put to practical use during World War II and by the end of the war, the new invention was being used reliably to detect aircraft and ships.