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An airport surveillance radar (ASR) is a radar system used at airports to detect and display the presence and position of aircraft in the terminal area, the airspace around airports. It is the main air traffic control system for the airspace around airports.

At large airports it typically controls traffic within a radius of 60 miles (96 km) of the airport below an elevation of 25,000 feet. The sophisticated systems at large airports consist of two different radar systems, the primary and secondary surveillance radar.

The primary radar typically consists of a large rotating parabolic antenna dish that sweeps a vertical fan-shaped beam of microwaves around the airspace surrounding the airport. It detects the position and range of aircraft by microwaves reflected back to the antenna from the aircraft's surface.

The secondary surveillance radar consists of a second rotating antenna, often mounted on the primary antenna, which interrogates the transponders of aircraft, which transmits a radio signal back containing the aircraft's identification, barometric altitude, and an emergency status code, which is displayed on the radar screen next to the return from the primary radar.

It operates at a frequency of 1.03 - 1.09 GHz in the L band with peak power of 160 - 1500 W.

The positions of the aircraft are displayed on a screen; at large airports on multiple screens in an operations room at the airport monitored by air traffic controllers who direct the traffic by communicating with the aircraft pilots by radio. They are responsible for maintaining a safe and orderly flow of traffic and adequate aircraft separation to prevent midair collisions.