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An Overview of Air Traffic Control

(Courtesy of Fundamentals of Air Traffic Control by Michael Nolan)

Air traffic control is the organization and scheduling of all air traffic. Air traffic control technologies address issues such as collision avoidance, aircraft spacing, and traffic control.

Early aviation, pre-air traffic control

In the early days of aviation, demand for air travel was low and commercial aviation was slow to catch on. With the low volume of traffic, air traffic control in any form was not needed. World War I triggered the first significant demand for the mass production of aircraft, in a military context. Pilots began bringing radios, a relatively new invention, into planes with them to communicate with people on the ground. After the war ended, airlines began to use radios to transmit weather information from the ground to the pilots.

The first commercial use of aircraft began with the Post Office's creation of Air Mail to speed up the delivery of cross-continental mail over specific routes. Initially, these planes could not fly at night, and would hand off the mail to trains at the end of the day. In 1921, the Army deployed a line of rotating beacons from Columbus to Dayton OH that were visible to pilots at 10 second intervals, allowing for night time travel. The Post Office took over these beacons in 1923, eventually constructing a line that stretched from coast to coast. The next step in air traffic control came with the construction of 83 radio beacons by the Department of Commerce. These beacons automatically transmitted directional beams that pilots could follow to their destinations, allowing for navigation in poor visibility conditions. These were followed by the deployment of beacons to mark the location of airports. These initial technologies were all used for navigation purposes.

In 1935, the first air traffic control tower was established at the Newark International Airport. Early air traffic control units such as this one relied on radio communications and maps to keep track of planes in the air. Pilots would file an instrument flight plan with an airline dispatch office, including the type of aircraft, the departure and arrival airports, departure time, time en route, flight number, requested route of travel, airpseed, and requested altitude. The ACTU would be forwarded this information by the airline dispatcher. The controllers determined if there were any conflicts in route or altitude, and modified the flight plan to ensure safe separation of aircraft. When this was completed, the controllers would issue a clearance to the dispatcher and on to the pilot. The air traffic controllers would then write the flight plan information on a chalkboard and a note card, which was attached to a brass holder - the "shrimp boat". The shrimp boat was moved along an airway map to approximate the positions of aircraft in the sky, as the pilot called in his position. If the controller detected a potential conflict, it notified the aircraft of appropriate altitude and route changes through the dispatcher.

World War II brought significant technological developments to aviation, particularly with the invention of radar, which allowed surveillance, as well as navigation, to be done. Radar was first developed by the British before the war as a solution to the problem of early detection of enemy aircraft. By 1940, the British had deployed a line of radar transceivers along their east coast to detect German aircraft. The Americans adapted radar for use in distinguishing Allied aircraft from enemy aircraft by installing transponders in all Allied aircraft. These transponders would respond to the radar transmissions by emitting pulses that identified that plane as friendly. This system, IFF (Identification Friend or Foe) was used by the military and then adopted into a similar system by civil aircraft.

Early Air Traffic Control System

In 1956, Congress was woken up to the fact that aviation was growing at a tremendous rate and that the skies were getting crowded when two aircraft collided over the Grand Canyon, killing 128 people. The Federal Aviation Act was passed in 1958, which mandated the creation of a new safety regulatory agency, the Federal Aviation Agency. When the Department of Transportation was created in 1967, the FAA was placed under its control and renamed the Federal Aviation Administration. While previously air traffic control had been done on a smaller scale, the FAA was charged to develop and maintain a broad air traffic control system that would maintain safe separation of all commercial aircraft in all stages of flight.

At this time, the air traffic control system was primarily manual, comprised mostly of rules and procedures. The primary method of surveillance/control was radar, and allowed for separation based on altitude and distance. The airspace was divided into different levels of control - Positive Control, Controlled or Mixed, and Uncontrolled - each of which received different ATC services. Pilots in Positive Controlled Airspace were under IFR (Instrument Flight Rules) control, required to file an IFR flight plan with the air traffic control system, which then provided separation service between all aircraft. In Controlled/Mixed airspace, separation service was provided between all IFR-controlled aircraft. Additionally, controlled aircraft also received radar advisories regarding the bearing, distance, heading, and speed class of uncontrolled aircraft detected by the ATC radars. Uncontrolled aircraft in Mixed airspace could request radar advisory service, which would be provided contingent on controller workload. Equipment requirements followed from the type of airspace in which an aircraft primarily operated. Aircraft in uncontrolled or mixed airspace were not required to carry navigation, communications, or transponder equipment unless they were at a tower-equipped airfield or desired advisory service.

The 1958 formation of the FAA, followed by the midair collision of two commercial aircraft over NYC in 1960, led to the beginning of ATCRBS deployment as a secondary surveillance system. While primary radar shows where all the planes were relative to each other horizontally, it does not provide horizontal separation information. Prior to the use of secondary surveillance radar, air traffic controllers also had no reliable way of positively identifying a plane on the radar screen. Instead, using voice communications, they would essentially use the "shrimp boat" method, though on the radar screen instead of a map (see top of page). Read more about ATCRBS or the Project Beacon task force that led to its deployment.

By 1967, however, it was clear that the ATCRBS system was not adequate to future growth. There is more information on this on the Collisions, ATCAC and Other Factors page. These changes would lead eventually to the development of Mode S and its deployment.

Air Traffic Control Tomorrow

Automatic Dependent Surveillance-Broadcast (ADS-B) is a new air traffic technology. It represents a significant advancement over existing ATC system by providing increased accuracy and safety, all without the assistance of ground stations. It is a major step in the direction of the notion of "free-flight", the ability for planes to fly safely and without the manual control of ground stations. ADS-B will enable planes to send messages to each other to provide surveillence and collision avoidance. The FAA has identified ADS-B as the future model for ATC. The FAA is currently investigating the technology necessary to implement ADS-B. As part of this examination, the FAA is considering three alternatives for the choice of the data link technology for ADS-B. Those technologies are: Mode S extended squitter (also known as GPS Squitter), VDL and UAT. All three of these technologies leverage the global positioning system (GPS) to allow planes to determine their location with high accuracy. Aircraft then can broadcast this information to surrounding planes, enabling them to be aware of planes in the near vicinity and coordinate flight maneuvers.

The Story of Mode S: An Air Traffic Control Data Link Technology last modified: 12.06.2000