Two types of radar are used by air traffic control to determine the position and altitude of air traffic, which are primary and secondary.

Primary radar involved the transmission of radar waves. An antenna is then used to look for any of that transmitted radio energy which has been reflected back to the transmitter site. A computer then interprets the data for display to the controller.

Because primary radar looks for reflected energy, the strength of a primary target varies with the size of the reflecting object and the materials from which it is made.

The azimuth of a target from the radar site is known based on the azimuth of the radar transmitter and antenna as they rotate together.

The distance of the target from the radar site is a function of the time it takes for the radio waves to make the round trip from the transmitter to the target and back to the antenna. Since radio waves travel at the speed of light, the target's distance can be known and displayed.

Secondary radar involves the use of a ground based interrogator and airborne transponder. The interrogator transmits radio signals which request replies from airborne transponders. The transponder receives these signals and replies with the assigned beacon code.

Secondary radar returns to not vary based on the size or composition of the aircraft, since they are transmitted by airborne equipment.

Since the radar energy is transmitted parallel to the surface and the earth is curved, the radar beam will climb away from the ground with increasing distance from the radar site. As a result, traffic near the radar site is visible all the way to the ground. However, as you move away from the radar site a larger and larger area becomes hidden beneath the radar beam. This is why you cannot be detected on radar until you climb high enough to be seen.

Radar is line of sight limited, and so is blocked by terrain. In mountainous areas, there may be areas where radar service is not available, as a result.

Radar beams can also be bent as they pass through varying atmospheric conditions. This might cause false returns to be displayed on the controller's display. These false returns are called anomalous propagation, or "AP".