Air traffic control was implemented starting in the 1920s. Its foundation was procedural control built around radio communication. At that time, controllers could not see the actual operation of aircraft, until radar was introduced into air traffic control, which drastically changed the landscape of air traffic control.

　　Radar is a military technology developed during World War II. After the war, it was adapted for use in civil aviation. Physicists had long known that radio waves, like light, are electromagnetic waves with the same physical properties. They are reflected when they encounter obstacles. Using pulse waves generated by radio waves with wavelengths under 1 meter, high-energy directional emission is used. When encountering objects ahead, they will be reflected back. Although the energy of the reflected wave is much smaller than the emitted wave, the reflected signal can be received using a very sensitive receiver. By measuring the time it takes for the wave to travel from emission to reception, during this time, the wave travels the round trip between the transmitter and the object being measured. Multiplying this time by the speed of light and dividing by 2 gives the distance between the object in front and the transmitter. A radar mainly consists of a transmitting part, a receiving part, and an antenna. The antenna's function is to gather the emitted radio waves into parallel beams and then emit them directionally; it also feeds the received reflected waves to the receiver, which then displays them on the screen. The transceiver can only detect the direction the antenna is pointed at. If you want to detect the situation in all directions around 360 degrees, the antenna must continuously rotate. A rotating light is displayed on the radar screen, indicating the rotating antenna. The object being measured is a spot of light on the screen, and the center of the screen is the location of the radar. The azimuth and distance of the detected object can be obtained based on the position of the spot on the screen.

　　After radar was introduced into civil aviation, controllers could "see" the aircraft, knowing their position and distance. This type of radar can only monitor aircraft activity, and it is far from enough to use it to precisely guide aircraft flight. In the civil aviation industry, this type of radar is called primary radar or surveillance radar. About 20 years later, a radar signal system was developed that could distinguish aircraft identification, altitude, and speed, and is used in conjunction with primary radar. It is called the air traffic control radar beacon system, or secondary radar. Secondary radar is a system integrating ground and onboard equipment. During operation, a pulsed interrogation signal is emitted by the ground system. When the transponder installed on the aircraft receives this signal, it automatically sends a coded reply signal containing the aircraft's identification, altitude, and speed. After the ground equipment receives this signal, it is superimposed and displayed next to the spot of light representing the aircraft on the primary radar screen. Controllers can comprehensively grasp the flight dynamics of this aircraft from the radar screen. When using secondary radar, the aircraft must be equipped with a corresponding transponder, otherwise, the aircraft will not respond to the secondary radar signal. Secondary radar must be used with primary radar, otherwise, the information obtained has no practical value.