Radar is an acronym for RAdio Detection And Ranging. Its operating principle is analogous to the broader pulse-echo (or pitch-catch) technique, except that pulsed electromagnetic (EM) waves (short radio waves or microwaves) are used instead of stress waves. Radar was developed during World War II to provide early warning of approaching enemy aircraft. From there, it evolved into indispensable tools in air-traffic control, shipping, weather prediction, and a multitude of other applications that range from probing distant planets to law enforcement. The application of radar is based on the use of sensors (antennas) that transmit and receive EM waves. First, an EM wave is generated in the instrument's circuitry and propagated to the antenna through a transmission line. This wave is then launched by an antenna in some medium (in general free space but in some cases may be coupled directly into a medium such as soil). The wave propagates within the medium (with attenuation) until it reaches a target with different electrical characteristics than the surrounding medium.