Radar (radio detection and ranging), system that detects long-range objects and determines their positions by measuring the time taken for radio waves to travel to the objects, be reflected, and return. Radar is used for navigation, air control, fire control, storm detection, in radar astronomy, and for catching speeding drivers. It developed out of experiments in the 1920s that were measuring the distance to the ionosphere by radio pulses. R.A. Watson-Watt showed that the technique could be applied to detecting aircraft, and from 1935 Britain installed a series of radar stations that were a major factor in winning the Battle of Britain in World War II. From 1940 the United Kingdom and the United States collaborated to develop radar. Continuous-wave radar transmits continuously and detects the signals received by their instantaneously different frequency. Pulsed radar has a highly directional antenna, alternately a transmitter or a receiver. As a transmitter, it scans the area systematically or tracks an object, emitting pulses, typically 400 per second. As a receiver, the antenna amplifies and converts echo pulses to a video signal that is displayed on a cathode-ray tube. The time-lag between transmission and reception is represented by the position of the pulse on the screen. Various display modes are used: commonest is the plan-position indicator (PPI), showing horizontal position in polar coordinates.