Air Traffic Control

Overview | ATC Facilities | Radar

Radar

Air Traffic Control uses radar to track and monitor aircraft both in the air and on the ground. For air traffic control purposes, radar is considered a surveillance system because it watches aircraft move through the sky.

A radar system transmits ultra-high frequency (UHF) radio waves that bounce back to its antenna from objects they hit. These waves travel at precise speeds. By measuring the time it takes a radar �return� to arrive back at the antenna, computers calculate how far away the object is. (Radar returns are also called �echoes.�) Radar signals can bounce off other things besides aircraft, such as boats, mountains, buildings and even moisture-laden clouds. Weather radar systems make use of this fact.

Radar operates on a line-of-sight basis. A radar unit must have a straight line of sight to an aircraft to be able to track it. So a radar�s ability system to watch a flight depends, among other things, on the aircraft�s altitude, its distance from the antenna and the location of ground obstructions that create blind spots. Under the right atmospheric conditions, clouds or even temperature differences between layers of air can create blind spots by bending radar signals from their straight line of sight.

Air traffic controllers want to minimize the chance that altitude, distance or obstructions will interrupt radar coverage of a particular flight. For this reason, air traffic control radar systems often are designed to mesh the returns from several radar units at different sites around an airport or airway. By using several radars at once, the system makes it more likely that one antenna will always have a clear line of straight to aircraft in the coverage area.

There are two types of radar - primary and secondary .

Primary Radar

Primary radar gives an air traffic controller a �bird�s-eye� presentation of the echoes or returns from aircraft within the radar system�s detection range. The aircraft�s distance from the site is determined by measuring the difference between the time the radar signal is transmitted and the echo bouncing off the aircraft returns to the antenna. Primary radar does not enable controllers to identify specific aircraft. Primary radar will show a aircraft�s location but not its altitude.

A video mapping unit combined with the radar unit generates a map of the airways (or, in the case of an airport, the runways) that is displayed on an air traffic controller�s screen along with the radar returns. This helps the controller judge where the aircraft under his control are in relation to runways, navigation aids and hazardous ground obstructions.

Secondary Radar

Secondary radar operates separately from primary radar, but the two are typically used together. Secondary radar also is known as the Air Traffic Control Radar Beacon System.

This type of radar sends a coded signal to a receiver/transmitter, called a transponder, on each aircraft. (As a general rule, aircraft are not permitted to fly near the busiest airports in the U.S. unless they have a transponder installed and it is working.) The coded signal from the radar tells each aircraft to identify itself and provide information such as its altitude, heading and speed. Most aircraft have �Mode C� transponders with an altitude- reporting function.

Before takeoff, pilots of flights that will operate under a controller�s guidance are given a coded number to enter into the transponder. This creates a unique signature for that aircraft as far as the secondary radar is concerned. Transponders are required on aircraft flying above 10,000 feet MSL and in the airspace around airports with radar service. Information from the transponder helps generate the �data block� for each aircraft that controllers see on their screens.

Unlike primary radar, secondary radar can positively identify an aircraft for the controller using the system, and tell the controller the altitude, heading and speed of the aircraft.

As with the primary radar system, a video mapping unit combined with the secondary radar system generates a map of the airways (or, in the case of an airport, the runways). This map is displayed on an air traffic controller�s screen along with the radar returns. This helps the controller judge where the aircraft under his control are in relation to runways, navigation aids and hazardous ground obstructions.