So far, this list has focused on aircraft structures, but one of the most important aviation innovations -- actually a collection of innovations -- is air traffic control, the system that ensures aircraft can take off from one airport, travel hundreds or thousands of miles and land safely at a destination airport. In the United States, more than 20 air traffic control centers monitor the movement of airplanes across the country. Each center is responsible for a defined geographic area, so that as an airplane flies along its route, it gets handed off from one control center to the next. When the airplane arrives at its destination, control transfers to the airport's traffic tower, which provides all directions to get the plane on the ground.
Surveillance radar plays a key role in air traffic control. Fixed ground stations, located at airports and at control centers, emit short-wavelength radio waves, which travel to airplanes, strike them and bounce back. These signals allow air traffic controllers to monitor aircraft positions and courses within a given volume of airspace. At the same time, most commercial aircraft carry transponders, devices that transmit the aircraft's identity, altitude, course and speed when "interrogated" by radar.