Basic to air traffic control are special air routes called airways. Airways are defined on charts and are provided with radio ranges, devices that allow the pilot whose craft has a suitable receiver to determine the plane's bearing and distance from a fixed location. The most common beacon is a very high frequency omnidirectional radio beacon, which emits a signal that varies according to the direction in which it is transmitted. Using a special receiver, an air navigator can obtain an accurate bearing on the transmitter and, using distance-measuring equipment (DME), distance from it as well.
The system of radio ranges around the United States is often called the VORTAC system. For long distances other electronic navigation systems have been developed: Omega, accurate to about two miles (3 km); Loran-C, accurate to within .25 mi (.4 km) but available only in the United States; and the Global Positioning System (GPS), a network of 24 satellites that is accurate to within a few yards and is making radio ranging obsolete.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.