Aircraft

Guiding the motion of aircraft through the skies is a serious problem for two reasons. First, commercial and military aircraft now fly in all kinds of weather, often under conditions that prevent them from seeing other aircraft, the ground, or the airports at which they are supposed to land. Second, there is so much traffic in the air at any one time in many parts of the world that precautions must be taken to prevent collisions.

The crucial invention that made the control of air flight possible was the development of radar in the 1930s by the Scottish physicist Sir Robert Watson-Watt. Radar is a system by which radio beams are sent out from some central location (such as an airport tower) in all directions. When those beams encounter an object in the sky—such as an aircraft—they bounce off that object and are reflected to the sending station. A controller at the sending station is able to "see" the location of all aircraft in the vicinity and, knowing this, can then direct their movements to make landings and takeoffs possible and to prevent in-air collisions of the aircraft.

Today, every movement of every aircraft is constantly monitored using radar and radio signals. The moment a commercial airliner is ready to leave its gate for a flight, for example, the pilot notifies the control tower. One controller in the tower directs the airplane's movement on the ground, telling it when to back away from the gate and what runway to use for departure. Once the airplane is airborne, the pilot switches to a new radio channel and receives instructions from departure control. Departure control guides the aircraft's movements through the busiest section of the air near the airport, a distance about 30 mi (50 km) in radius. Finally, departure control "hands over" control of the airplane to a third system, one that guides the aircraft through various sections of the sky between its takeoff and landing points. As the plane approaches its final destination, the series of "hand offs" described above is reversed.