Stands for "radio detection and ranging." Radar was developed in World War II. Pulses of ultra-high frequency radio waves emitted by a transmitter aboard ship are reflected by solid objects and are detected upon their return to the sending location. The time it takes for the wave to return gives the objects range or distance, and the direction from which it comes gives the bearing. A visual picture of objects projecting above the surface of the sea is presented to the sending ship.