* Before World War II, experiments were performed with electronic systems to find the range of aircraft or other targets by transmitting radio pulses and measuring the time delay in receiving an echo back from the target. During the war, "radio detection and ranging (radar)" went through a rapid evolution and became an important weapon and tool in the conflict, with the opponents developing radars of increasing sophistication -- as well as "electronic intelligence (ELINT)" systems to spot enemy radars and "electronic countermeasures (ECM)" systems to jam them.
By the end of the conflict, radar systems had advanced to a level of sophistication inconceivable before the war, and have continued to be refined in the postwar era. Radar remains an important military and civil technology. This document provides an introduction to radar concepts and terminology.