Air Traffic Control by Electronics, January 1960 Electronics World

Air Route Traffic Control Centers, now using the acronym ARTCC rather than ARTC as used in this 1960 article, were and still are the human and computer command and control facilities responsible for safe and orderly flow of air traffic in the U.S., and a worldwide network of Area Control Center (ACC) handles everything else in a massive coordinated effort. The advent of radar during World War II and the ensuing evolution of it and electronic computers in the following years struggled to keep pace with the equally rapidly evolving aircraft design and capability. A simple control tower with air traffic controllers using binoculars and a radio mike could not handle the volume of airplanes and helicopters traversing the skies and patronizing busy terminals. Many forms of electronic navigation aids were developed including very high frequency omnidirectional range (VOR), direction finders (DF) using antenna nulling for finding radials to/from FM radio transmitter, long range navigation (LORAN), tactical air navigation (TACAN), and others up through modern day satellite positioning systems (GPS, GLONASS, Galileo, BeiDou-2). With the assistance of computers, radar systems got smarter with signal processing able to differentiate between weather phenomena, moving aircraft, and stationary ground clutter. Onboard transponders in aircraft in conjunction with ground-based equipment provided a pseudo (secondary) form of radar that used coded radio signals to calculate distance to target rather than relying on RF signals reflected off the aircraft structure. The info here represented a new paradigm in air traffic control in when originally published, and is still useful today as a snapshot of the state of the art at the time.

As an example of how terminology changes over time, most people these days when seeing Figure 2 likely automatically think of the "cloud" as referring to information stored on the Internet ("in the cloud").

With more and faster planes. the control of air traffic must be a precise task with no margin for error. The job is being done by electronics.

Many of us have forgotten that less than 20 years ago, air travelers "zipped" through the sky at what was then the fantastic speed of approximately 150 miles-an-hour. Not only the speed, but the altitude of 18,000 to 20,000 feet was sufficient to identify one as a seasoned air traveler.

In the short span of the past two decades, aviation has literally leaped from the two-engine aircraft to the four-engine jets flying five to six miles above the earth's surface at better than 500 miles-an-hour.

View taken from inside an airport traffic control tower. The tower controls air traffic within about 20 miles of airport.

A New York area traffic controller keeping track of aircraft movement by noting flight location on flight progress strip.

Fig. 1. The FAA air route traffic control center areas are shown above.

And while the speed of aircraft has increased, so has the number of aircraft flying the airways crisscrossing the United States. In 1939 there were approximately 29,000 licensed aircraft; there were upwards of 110,000 licensed to fly in 1959. Approximate figures in the various categories are: military 42,000, air carrier 2000, and general aviation over 65,000. The latter category includes all aircraft either privately or corporately owned. In short, it accounts for all aircraft except military and airline aircraft.

More and faster airplanes have posed a gigantic problem in air traffic control. It's not as simple as red and green lights on railroads or highways. There are no "caution lights" on the airways. It's a precise task with no margin for error, and the job is being accomplished by electronics.

The essential tools of air traffic control are radio and radar. Radio was first, and then radar became the second tool of the air traffic controller after World War II.

Control of aircraft is under two basic sets of rules: IFR (instrument flight rules) and VFR (visual flight rules). Under IFR control, the aircraft is guided by an Air Route Traffic Control Center (ARTC). This guidance assures the pilot that his aircraft is being monitored along its entire course. An aircraft flying VFR is operating on a "see and be seen" basis.

Airways, like highways on the ground, are the core of the air traffic control system. Just as a motorist follows a numbered highway, the pilot follows a numbered airway between terminal points. The numbered airways are just as exact as to turns and direction as are the highways on a road map.

Unlike the driver who selects his numbered route from a gas station road map, the aircraft pilot files a flight plan with an ARTC maintained by the Federal Aviation Agency. There are 31 such control centers (see Fig. 1) in the U. S. linked together by leased telephone lines.

Air Traffic Control Centers differ from airport control towers. Centers direct traffic between terminal areas; control towers guide air traffic in a radius of approximately twenty miles of the airport.

An ARTC is a facet of air travel not known to most plane passengers. Unlike the airport control tower visible to all, the FAA ARTC is usually some distance from the ramps and runways. At New York's International (Idlewild) Airport, it is housed in Hangar 11 about two miles distant from the passenger terminal and control tower. Although designated as a "hangar," it is actually a three-story building with more than half its area given over to air traffic control and electronic equipment.

The New York ARTC guides IFR air traffic between Salisbury, Maryland and Montauk Point on the eastern tip of Long Island. Inland its area of control extends west to Phillipsburg, Pennsylvania and northwest to Elmira, New York. To the east the ocean control section guides traffic over the Atlantic Ocean in an area bounded by Nova Scotia, the Azores, and Bermuda.

All IFR traffic landing or departing from Idlewild, La Guardia, and Newark airports clears through the New York ARTC. To accomplish this seven-day-a-week, around-the-clock job, more than 500 people are employed, working in three eight-hour shifts.

Controllers check computer-prepared flight data and weather findings in the computer room of FAA's New York area Traffic Control Center at International Airport.

Air route surveillance radar (ARSR) antenna.

Radar signal reflects from clouds - RF Cafe

Fig. 2. Radar signal (left) reflects from clouds, buildings as well as from plane to produce scope clutter. Beacon (right) transmits interrogating signal which triggers plane's transponder into transmitting strong answering signal on different frequency. Beacon receiver does not pick up reflected signals, hence clouds, buildings will not produce any indication on the beacon scope.

Communication Communication is, of course, the heart of air traffic control. New York's air traffic controllers have at their disposal air-to-ground and ground-to-air radiotelephone circuits for communication with aircraft, leased telephone landlines for communication with other Air Traffic Control Centers, and remote communication stations along the airways. Landline teletypewriters provide flight-plan information and weather reports.

The direct radiotelephone communication between pilots and air route traffic controllers is accomplished by means of 45 separate channels of individual frequencies, both u.h.f. and v.h.f. For ocean control there are two high-frequency channels. The ocean control transmitter is located at Sayville, Long Island, the receiver at Barnegat, New Jersey, and both are connected to the New York Center by radio relay.

Within the Center there are 260 miles of wiring involved in the interconnection and interlocking of switching controls, microphone connections, earphones, and speakers. Added to this are 1700 miles of landline circuits linking remote transmitters and receivers. The Center has 115 leased telephone circuits.

In New York, as in all other Air Traffic Control Centers, the pilot files an instrument flight plan at least one hour before departure. The plan indicates his departure time, destination, requested altitude, and airway. This is almost like planning a cross-country trip by motor car. First you decide the time you start, and you have your numbered highway route marked in advance. The big addition, of course, is altitude.

From departure to destination, the aircraft flying an IFR flight plan is under positive control. An aircraft flying from New York to Cleveland is controlled by the New York Center until it passes Phillipsburg, Pennsylvania, where it is turned over to the FAA Air Traffic Control at Cleveland. Before the plane departed from New York, the Cleveland Center was advised of its flight plan and had reserved the requested altitude and airway for the aircraft when it was scheduled to enter the Cleveland control area. This fundamental principle is carried on in non-stop transcontinental flights, or anywhere within the U. S. and its territories.

There was a time many years ago when aircraft on cross-country flights were guided in the daylight by arrows painted on roofs of barns, and at night by rotating beacons on mountain tops. Although there are a few beacons still in operation and some air-minded citizens still paint arrows on their rooftops pointing to the nearest airport, navigation of aircraft from the ground both night and day is now being accomplished by electronics.

Spaced approximately every hundred miles along the airways are v.h.f. omni-range (VOR) stations. Wherever located, they are an object of curiosity. Their location, dependent on the straight-line airway, might place them in a remote mountain spot, the edge of a town, or near an airport. They are painted in a red and white checkerboard style and have an unusual appearance.

The system is an all-directional range station, giving bearings for the entire 360 degrees around the compass.

Stacks are "built" from the bottom up by the Air Route Traffic Control Center. The New York Air Route Traffic Center, in building its various stacks (some twenty-five in all), coordinates its work with the control towers at Idlewild, LaGuardia and Newark airports. In essence, the center builds the stack and the tower unloads it.

Stacks are built at thousand-foot levels starting from 3000 feet and ranging up to 10,000 feet altitude. Jets with their high rate of fuel consumption prefer to be held at higher altitudes, which are most economical for fuel consumption. Jets are usually given expeditious handling when placed in a holding pattern.

The VOR sends out static-free radio signals for the guidance of pilots, transmitted on frequencies between 112 and 118 mc.

Each VOR transmits repeated identification signals either in code or recorded voice. These signals enable the pilot to establish his course along an airway. From time to time the identification signals go off the air in order to broadcast weather reports. The weather reports are broadcast 15 and 45 minutes after the hour. Within the last few years this weather data was made public. As a result, farmers, operators of small marine craft, and trucking concerns have been tuning to their nearby VOR for weather reports.

This is a stack. Depending on the amount of traffic, an airport may have one or many stacks. Stacks are used in bad weather when planes cannot land as frequently as in good weather. They are also used in good weather when heavy traffic prevents a plane from landing immediately. Stacks, resembling race track patterns. are permanently established near an electronic "fix," which may be a VOR, a low-frequency range station, or a homing beacon. Each leg is designated as either one or two minutes before turn at the standard rate of five degrees per second. In the New York area most holding patterns are one minute per leg because of the heavy traffic.

The Instrument Landing System (ILS) now in use at most airports has been an electronic boon to air travelers and airlines. Although not specifically identified as a safety measure, the ILS System has enabled airlines to make scheduled landings under adverse weather conditions.

The system consists of two radio transmitters at an airport sending out radio beams to guide the aircraft on its instrument approach and landing. One beam, called the localizer, tells the pilot whether he is to the right or left of the centerline of the runway. The other beam, the glide slope, shows him the correct angle of descent. This information is displayed in the cockpit of the aircraft by two needles on a single meter face, one vertical, the other horizontal. The vertical needle moving right to left indicates the plane's position in relation to the centerline of the runway. The horizontal needle tilting up or down indicates the position of the aircraft on the glide slope, either low or high. With both needles crossed at right angles, the aircraft is on course and making an accurate landing, even though the pilot may not be able to see the tips of his own plane's wings.

The final decisions in air traffic control are made by the air traffic controllers in the control centers and airport towers. Their responsibility is heavy. The present and future tools of the air traffic controller are electronic instruments. The Federal Aviation Agency, which is responsible for the nation's air traffic control system, has pointed out that modernization of the airways involves "increased installations of radar and other electronic equipment."

The FAA has installed in the past six months "Univac" File-Computers in its air traffic control centers in New York, Washington, Boston, Pittsburgh, and Cleveland. The computers will not replace the air traffic controllers but will relieve them of their "bookkeeping chores," and lessen fatigue.

The computer performs the necessary calculations of an aircraft's passage along the airways. Instead of the air traffic controller computing speed of the aircraft and estimating time of arrival over various check points, the electronic computer does it for him. All the information is punched out on a rectangular strip containing flight-plan information for each airplane. Under the old system the data was calculated by the controllers and recorded by hand.

While the computer is doing the bookkeeping, a radar beacon system recently installed at Newark, La Guardia,

and Idlewild airports as well as in the New York Traffic Control Center, is aiding the traffic controller to identify aircraft on radar screens.

Traffic controllers at display scopes of airport surveillance radar (ASR) equipment.

VOR transmitter building located at New York's International Airport (Idlewild).

Fig. 3. Functional block diagram of the scan conversion system. Heart of unit is special cathode-ray tube (TMA 403X) that writes radar information on internal target grid with one electron beam and reads TV information with another electron beam.

A refinement of radar, the radar beacon system (see Fig. 2) permits the positive identification of aircraft. When a controller has two or more aircraft on his radar screen, identification of each aircraft is vital to their safe passage through the area. Frequently an aircraft which is not identified will appear on the screen. Prior to the installation of the radar beacon system, the controller would request the aircraft to execute a 180-degree turn. This took some time - only minutes, of course - but in handling fast-moving aircraft, minutes are precious.

With the radar beacon, no turns are needed. The radar blip of a plane can be identified electronically. A controller asks a pilot to identify his plane. The pilot then pushes a button and the blip on the controller's radar screen blooms and expands into an oval shape.

The radar beacon system is the outgrowth of the World War II military identification ("friend or foe") problems. Since 1953 Airborne Inetrwments

Laboratory has been a consultant to FAA and its predecessor, the CAA, in the over-all analysis of the use of the beacon system in high-density terminal areas.

The most recent electronic aid to air traffic control is the radar-to-television converter. It is best known as the scan converter (see Fig. 3) and was recently commissioned at the New York Air Traffic Control Center. Although on order from Intercontinental Electronics Corporation. since 1958, production and installation delayed the commissioning of the scan converters until a few months ago.

The basis of the scan conversion system is a cathode ray tube that converts radar information to television signals. These signals can be seen in full daylight on horizontal monitors upon which are placed video maps.

Since being placed in use, air traffic controllers have commented that the scan conversion system has been less fatiguing. They add that working under normal light conditions is easier on the eyes, and is therefore an aid in promoting safety.

This is but the start of many new uses of electronics in the field of air traffic control. The FAA Bureau of Air Traffic Management says that a computer is being developed that "will keep track of flights in progress and reject proposed paths that would conflict with those of aircraft already enroute."

Although this may appear to be automation in a sense, the final decision on the disposition of aircraft will always rest with the air traffic controller, aided by the accurate information provided him by the various electronic devices at his disposal.

Shown below is one of the horizontal consoles of the radar-to-television scan converters that have been installed in the enroute Air Route Traffic Control Center at New York's International (Idlewild) Airport. The system provides bright display of radar information that can be viewed in normal room light. Each plane has "tail" of light, allowing controller to determine speed, direction of turn.

Cover Story January 1960 Electronics-World - RF Cafe

Symbolic of the Jet Age, our cover shows a portion of one of American Airlines' new 707 Jet Flagships along with the new Newark (N.J.) Airport control tower. The $5 million jet plane is crammed full of electronic gear, not only for communications and radar, but also for air conditioning control, automatic pilots, fuel injection control, and many other special uses. It has been estimated that the total cost of all the electronic equipment in the 707 is $500,000.

The nose cone has been raised in order to show the antenna of the weather surveillance radar. This equipment has proved to be so valuable that the FAA has recently proposed a new safety regulation calling for the installation of this type of gear on all airliners. The particular radar installed in the plane on our cover is an RCA AVQ-10, operating in the 5 cm. band. The radar has a maximum range of 150 miles. The antenna rotates through 360 degrees, although the radar beam is cut off toward the rear by the plane's fuselage. The normal coverage angle is therefore from wingtip to wingtip, or about 210 degrees. The antenna can also be tilted downward or upward for better coverage. The nose cone is made of special material that is transparent to the radar beam.

The Newark control tower, recently built by the Port of New York Authority at a cost of $1-3/4 million, is one of the most modern and well-equipped structures of its type. Although Newark does not handle jet flights, except in an emergency, this tower is symbolic of the importance of electronics in air traffic control. About $1-million of electronic equipment is installed here, including 29 transmitters, 22 receivers, direction finding, and air surveillance radar gear. In addition, the tower controls two instrument-landing systems, one omni-radio range, distance measuring equipment, and a telecommunication network. To maintain and operate this complex network, the FAA has staffed the tower with 16 highly skilled electronic specialists and 58 air traffic controllers. (Tower photo by Bob Loeb; plane photo courtesy American Airlines.)

Air Traffic Control by ElectronicsJanuary 1960 Electronics World

Air Traffic Control by Electronics
January 1960 Electronics World