The Early Days of the ATC System
The origins of today's air traffic control system date back to the 1920s, when the first air mail routes were being developed between cities in the U.S. At that time, pilots flew close to the ground so they could navigate visually, zig-zagging from one familiar landmark to the next. At night, this method was particularly challenging. In 1926, the Air Commerce Act charged the federal government with the operation and maintenance of the airway system and of all aids to air navigation, through a system of regulation to be carried out by the Department of Commerce.
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| A 1940s advertisement for an airway beacon |
Borrowing a page from maritime navigation, the Lighthouse Division of the Department of Commerce set a series of bonfires along the most heavily traveled routes, allowing pilots to navigate from one fire to the next. This rudimentary system was replaced by the 1930s with a series of light towers which formed the Transcontinental Lighted Airway. These beacons (such as the one pictured at the right) dotted the nation, and created railway-like lines in the nation's skies. With over 1,500 beacons in service, 18,000 miles of airways were available to the burgeoning aviation industry. Some of these light beacons are still maintained today – but mostly for their historic value.
From Lights to Radios
By the 1930s, radio beacons began to replace the light beacons. While the technology was better, the fundamental concept remained the same: navigate from one ground-based waypoint to the next. These radio beacons became very numerous,and the successors to these original radio beacons -- "non-directional beacons" (NDB) and "VHF Omnidirectional Ranges" (VOR) -- are still quite common in the United States.
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Ground-based Navigational Aids (image courtesy Flight Explorer) |
In fact, we have over 11,000 ground-based navigation facilities still active. The map to the left shows this staggering array of navigation aids still active. These beacons and radio towers form thousands of high- and low- altitude airways. The technology has evolved but the operating principle - navigating from one fixed-waypoint on the ground to another fixed-waypoint on the ground - remains the same as that used in the original system.
Today's Airways
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| Arrival traffic to Chicago, May 2006 (credit: Flight Explorer) |
Much like the paths that connected the 1920s lighted beacons, today's airways are essentially fixed routes in the skies. As a result, traffic forms lines as it approaches airports. In the picture at right, airplanes queue-up for arrival at Chicago's O'Hare airport. The blue dotted lines show the airways. At busy times, air traffic controllers will instruct pilots to slow down in order to create space between the flights. This has the effect of metering the traffic flow into the airport's congested airspace. The impact of such measures can include delays to customers and shippers and missed connections.
GPS and Navigation Today
Ground-based beacons are not needed for most commercial aircraft, today, but they continue to be used as waypoints. With modern satellite based navigation, airlines have the ability to fly the optimal route from one city to another. However, this is not always possible because the ATC system still is based on the old waypoints.