Radar (Radio Detection And Ranging) is a remote detection system that is used to locate and identify objects. It consists of an electronic system in which radio waves are bounced off an aircraft or other object in order to detect its presence and locate its position.

A radar system uses four basic components: a transmitter, an antenna, a receiver, and a display. The transmitter generates radio signals. The antenna transmits these signals as electromagnetic radiation into the airspace. When a target, such as an aircraft, enters the airspace, it scatters some of these radio waves, which reach the receiving antenna. An electronic amplifier amplifies these returned signals and displays them on a cathode ray tube (CRT) display where a radar operator can examine them. The location of the object being detected is determined by measuring the time it takes for the radio wave to travel from the transmitter to the object and back to the receiver.

There are many types of radar: The most common is pulse radar where the radio waves are emitted in discrete pulses. Moving-target indication radar is a form of pulse radar can detect the location of moving targets. Airborne moving-target indication radar makes it possible to detect moving targets when the radar unit itself is moving, such as when a moving aircraft detects another moving aircraft. Various pulse radars use pulses that operate on different frequencies for different purposes. There are also various imaging radars that are used to produce images. The most common of these is synthetic aperataure radar (SAR), which is primarily used to map the Earth's surface.

Tracking radar can continuously follow a single target to determine its path and predict its future position. Automatic detection and tracking radar consists of targets that show up on a radar screen as tracks rather than as discrete blips. 3-D radar locates it target in terms of a reference point and the horizon. Phased-array radar can track many targets at the same time. Continuous-wave radar transmits and receives signals at the same time. It can distinguish the weak returning signal from the strong transmitted signal. Aircraft use a particular type of continuous-wave radar, called frequency-modulated continuous-wave radar, to determine their height above ground.

Radar came into wide use during World War II. Although many individuals contributed to its development, Sir Robert Watson-Watt is usually credited with developing the first practical system.