Primary radar

A primary radar or primary surveillance radar (PSR) is a conventional radar sensor that illuminates a large portion of space with an electromagnetic wave and receives back the reflected waves from targets within that space. The term thus refers to a radar system used to detect and localize potentially non-cooperative targets. It is specific to the field of air traffic control where it is opposed to the secondary radar which receives additional information from the target's transponder.

This type of radar uses low vertical resolution antenna but good horizontal resolution. It quickly scans 360 degrees around the site on a single elevation angle. It can thus give the distance and radial speed of the target with good precision but requires often one or more radars to obtain the vertical position and the actual speed.

The advantages of the primary radar are that no on-board equipment in the aircraft is necessary for detecting the target and that it can be used to monitor the movement of vehicles on the ground. The disadvantages are that the target and altitude can not be identified directly. In addition, it requires powerful emissions which limits its scope.