Beginner's Guide to Air Traffic Control Systems

When a technology is fully assimilated, it is relatively invisible. Such is the way with air traffic control. It seems to go on all around us without our caring much about it, even if we fly. Air traffic control began via the mail service. The airmail had to be delivered, and many new routes in strange places were pioneered by the need to fly mail. Mail pilots used writing on building roofs below to guide them during the day and various lights at night. However, most of the congestion was at the airports themselves. Controllers would be stationed in a tower or at the end of a runway and would use light signals (still the same ones used in a communications failure today) to guide landings or to hold aircraft for a break in the traffic.

When airplanes were rare, the threat of collision, even in cloudy weather, was low. As the number of commercial passenger airlines grew in the 1930s, a method of separating them from one another and a way of proceeding as safely in the clouds was necessary. Radio was not in widespread aerial use, and radar had not yet been invented. In this depression era, the U.S. government had little money, so it required the airlines to establish Air Traffic Control Units (ATCUs) to separate and space the traffic. Markers, called ''shrimp boats,'' which could carry information about an airplane's intended altitude and air speed, represented an aircraft on a flight plan. The controllers moved the shrimp boats on maps following the approximate motion of the planes. In 1937, the federal government had enough money to incorporate the ATCUs. Units were renamed Air Traffic Control Stations (ATCS) and most controllers became civil service employees.