Controlling Traffic by Numbers

At busy, big city airports where planes arrive and depart in droves, traffic controllers have powerful radars to help them keep track of the high-speed activity. But planes show up on the crowded radarscopes as small luminous blips that are sometimes difficult to identify properly, and a mix-up of blips may lead to disaster.

At the Atlanta airport a new system designed to dispel such confusion began advanced tests last week. Arriving airplanes make their appearances on the Atlanta scope as the usual blips, which look very much alike, but planes participating in the test carry electronic transponders that send back a coded signal along with their radar echoes. A computer built into the intricate electronic system provides information for a luminous square of letters and numerals that appears on the scope beside the blip. Called an "alphanumeric data block," it identifies the airplane and gives its altitude, which the transponder gets automatically from the plane's altimeter and sends along to a receiver on the ground.

Guided by the computer, which can track simultaneously any reasonable number of airplanes, the alphanumeric block follows the blip through all its turns and circlings. As the plane slants down for a landing, the blocks report its changes of altitude. Even when the scope is swarming with blips there is practically no danger that they will get mixed up, and the controller can always tell from the altitude figures whether planes that are approaching each other are actually in danger of colliding.

The new system is not officially in use yet at any airport, but on the basis of the Atlanta tests, the Federal Aviation Agency already considers it successful and hopes to install it soon at major U.S. fields.

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