History of air traffic control: from beacons to radar

Today, at every big airport and in every control center, a controller sits in front of screens on which every aircraft appears with identification, altitude and speed. In the beginning, by contrast, there was one man with two flags. The history of air traffic control is a history of improvisation, military technology and lessons from serious accidents. This overview takes you from the first towers and beacons via radio ranges and radar to transponders, data link and remote towers – and shows what of it you can still hear and see as a spotter today.

The short answer

Air traffic control emerged in the early 1920s, when scheduled traffic between London and Paris was increasing. Croydon Airport put its own control tower into operation on February 25, 1920, and is therefore regarded as the birthplace of airport air traffic control. Several technical generations followed: beacon chains for the night, radio ranges for navigation without visibility, radar from World War II, secondary radar with transponders from the 1950s and today satellite-based positioning and data link. Many of these steps came only after accidents had exposed gaps in the system.

The beginnings: towers, flags and rules of thumb

After World War I, the first regular passenger and mail flights started in Europe, above all on the London–Paris axis. Croydon, south of London, became the most important British airport of this period. From February 1920, a plain tower a good four and a half meters high with windows on all sides stood there. From it, pilots received information on traffic, weather and their position. Jimmy Jeffs is named as the first air traffic control officer. Croydon also developed procedures for determining position by radio direction finding, and the airport is credited with the origin of the distress call “Mayday” – more on that in the article on Mayday and Pan-Pan. In 1928, Croydon received a new terminal with a much larger tower.

How urgently rules were needed was shown on April 7, 1922: over Picardy, in fog and drizzle, a de Havilland DH.18A of the Daimler company and a Farman Goliath of the Compagnie des Grands Express Aériens collided, flying toward each other in opposite directions on the same route. All seven occupants died. The consequences: airlines and the British Air Ministry agreed on a keep-to-the-right rule in the air, demanded better forward visibility from the cockpit and radio equipment for all airliners. In addition, fixed air routes were agreed over Britain, France, Belgium and the Netherlands.

In the US, airport air traffic control began more pragmatically. In 1929, St. Louis hired Archie League, who is now regarded as the first American air traffic controller. He stood on the airfield with a checkered flag for “go” and a red one for “wait,” and his first “tower” was a wheelbarrow with a beach umbrella. Only after a radio tower was built in the early 1930s did he work by radio. The first American airport control tower went into operation in Cleveland in 1930.

Light beacons: the highway of the night mail

The real problem of the early years was not the traffic at the airport but the route in between. Without sight of the ground, no pilot could orient himself. In the US, the Post Office and the Department of Commerce solved this between 1923 and 1933 with an enormous chain of light beacons: some 1,500 towers marked about 29,000 kilometers of airway. Each carried a rotating searchlight that flashed briefly every ten seconds and could be seen a good 60 kilometers away in clear air. Colored course lights flashed a Morse identifier, red between landing fields, green at a landing field. On the ground, a large concrete arrow showed the direction to the next beacon – many of these arrows still lie in the landscape today.

The system had an obvious weakness: in fog, low cloud or snow, it was useless. But that was exactly when pilots needed guidance most.

Radio ranges: navigation by ear

The answer was radio. From 1928, the first four-course radio ranges (Low Frequency Radio Range) went into service in the US. The principle was brilliantly simple: the station transmitted the Morse signal A (short-long) into two opposite sectors and the signal N (long-short) into the other two. Where the two overlapped with equal strength, the pilot heard a continuous tone – that was the course, only about three degrees wide. If he heard “A” or “N,” he knew which side he was on. At the peak, there were over 400 such stations in the US alone.

It was not comfortable. Pilots often had to listen for hours to Morse signals in crackling headphones, thunderstorms disrupted reception, and terrain or snow could bend the course lines. In addition, each station offered only four courses.

In Europe, a beam-guidance procedure for landing caught on in parallel: in 1932, an early Lorenz landing beacon went into operation at Berlin Tempelhof airport, regarded as a forerunner of the instrument landing system. In the US, on January 26, 1938, a scheduled Boeing 247 landed in a snowstorm in Pittsburgh solely with the help of an instrument landing system. How today’s ILS works is described in the guide ILS approach explained.

The first control centers

With more aircraft on the same airways, control at the airport was no longer enough. Who took care of separation en route, especially in bad weather? The first American en route control center opened in Newark in 1935, followed by Chicago and Cleveland in 1936. The controllers worked without radar: they merged pilots’ position reports on boards and strips and calculated when which aircraft had to be where. This principle of procedural control with flight strips lasted in many regions for decades. In the Soviet Union, a first air traffic service on the Moscow–Irkutsk route had already emerged in 1929.

Radar: seeing through the clouds

World War II brought the great leap. Radar had originally been developed for air defense, but physicists in the US recognized early that it could also make landings safer. Luis Alvarez and Lawrence Johnston developed radar-guided approach control, GCA (Ground-Controlled Approach): a controller watches the course and altitude of the approaching aircraft on a precision radar and talks the pilot down by radio to just short of the runway. On January 1, 1943, GCA brought a flying boat low on fuel safely down in bad weather. By the end of the war, such systems stood at most airfields in Europe and the Pacific.

The technology faced its great civil test in the Berlin Airlift of 1948/49. On the narrow corridors to West Berlin, transport aircraft landed in dense sequence and in all weather – without radar guidance, that would hardly have been possible. You can read more about the setting in the history of Tempelhof Airport.

In civil air traffic control, radar systems caught on in the 1950s. With them came dedicated approach and departure control between tower and en route control – the basic structure you still hear on the frequencies today and that the article Delivery, Ground, Tower, Approach, Center explains.

Worldwide rules: Chicago and the ICAO

Air traffic control does not end at borders. On December 7, 1944, 52 states signed the Convention on International Civil Aviation in Chicago. It entered into force on April 4, 1947, and on the same day the International Civil Aviation Organization, ICAO, began its work. Its annexes still govern the basics: Annex 11 the air traffic services, Annex 10 the radio and navigation technology.

The ICAO standardized the most important systems of the postwar period. The ILS became the first international standard for precision approaches and was published in Annex 10 in 1950. In the same year the ICAO standardized the VHF omnidirectional range VOR together with distance measuring equipment, DME. VOR works in the range of 108 to 117.95 MHz and shows the pilot not just four but any number of courses to the station. In the US it went into service from 1949 and displaced the old Morse radio ranges within a few decades.

The turning point: Grand Canyon 1956

Despite radar and control centers, there were big gaps: away from the airways, airliners often flew in uncontrolled airspace on the principle of “see and be seen.” On June 30, 1956, a TWA Lockheed Super Constellation and a United Air Lines Douglas DC-7 collided over the Grand Canyon. All 128 people on board died. The investigating authority, the CAB, concluded that the pilots had not seen each other in time and named, among other possible contributing causes, inadequate traffic information from the overloaded en route control.

The consequences reached far: in 1957 Congress approved money for more controllers and radar systems, and the Federal Aviation Act of 1958 created a federal agency with full responsibility for US airspace, including military flights: the Federal Aviation Agency (FAA), which was renamed the Federal Aviation Administration in 1966. Research on onboard collision warning systems also got a boost from the accident.

Secondary radar and transponders

Primary radar sees an aircraft as an echo – but not which aircraft it is or at what altitude it is flying. The solution again came from the military: the wartime identification friend or foe (IFF) was developed further into secondary radar. A ground station interrogates on 1030 MHz, and the transponder in the aircraft replies on 1090 MHz. From 1954, the ICAO began standardization. Mode A transmits the four-digit code you know today as a squawk, and Mode C adds the altitude.

As traffic kept growing, that was no longer enough. The US and Britain jointly developed Mode S with a fixed 24-bit address for every aircraft and selective individual interrogation, standardized by the ICAO in 1987. ADS-B, which you use with every flight tracking service, also comes from this technology. You will find details in Transponder Modes A, C and S explained and in Squawk codes 7500, 7600, 7700.

Era	Technology	What the controller or pilot knew
1920s	visual signals, light beacons, first towers	position by sight, traffic at the field
1928 to 1950s	four-course radio ranges, beam guidance, position reports	course by ear, separation by calculation
from 1943 / 1950s	primary radar, GCA	echo with position, but without identification
from 1950s	VOR/DME, ILS as ICAO standard	any course, precision approach
from 1954	secondary radar Mode A/C	code and altitude for the echo
from 1987	Mode S, later ADS-B	unique address, GPS position, data exchange

Air traffic control in Germany

After 1945, the Allies controlled the airspace over Germany. The law of March 23, 1953, created the Bundesanstalt für Flugsicherung (BFS), to which the civil air traffic control services were transferred on July 7, 1953. Its seat was initially Frankfurt am Main. In 1990, the BFS also took over the airspace of the former East Germany. On January 1, 1993, responsibility passed to the newly founded DFS Deutsche Flugsicherung GmbH, which is wholly owned by the federal government and based in Langen. Today it works with control centers in Bremen, Langen, Karlsruhe and Munich; the former center in Berlin was dissolved from 2006. A detailed overview is offered by DFS German air traffic control explained, and the neighboring countries are covered in Austro Control and Skyguide.

At the European level, Germany, France, Britain and the Benelux states founded Eurocontrol in 1960. The organization operates the Maastricht control center for the upper airspace over the Benelux countries and northwestern Germany and, as network manager, coordinates the flow of traffic in Europe. More in the article Eurocontrol explained. Since the early 2000s, the EU has also pursued the goal of a single European airspace (“Single European Sky”).

Überlingen 2002 and collision warning on board

Onboard collision warning systems have complemented air traffic control since the late 1980s. TCAS II uses the transponder signals of other aircraft and gives avoidance advisories in case of danger. In the US it became mandatory for larger airliners in 1994, in Europe in 2000. That technology alone is not enough was shown on July 1, 2002: over Überlingen on Lake Constance, a Tupolev Tu-154 and a Boeing 757 collided. The Tupolev crew followed the controller’s instruction, the Boeing crew the TCAS advisory – both descended. Since then it has been clearly regulated that the system’s avoidance advisory takes precedence over a conflicting controller instruction, and the software was revised. The background is told in the article The Überlingen mid-air collision of 2002.

From radar to data: the present

The latest stage shifts air traffic control from echoes to data. Aircraft broadcast their satellite-determined position themselves via ADS-B, now also picked up by satellite receivers over oceans (space-based ADS-B). Routine radio is increasingly handled by text message (CPDLC). And radio beacon networks are shrinking: the US is reducing its VOR stations to a minimum network as a backup for GPS, and Britain cut the number of its stations from 44 to 19 by 2020.

Even the tower no longer has to stand at the airport. From April 2015, an airport was controlled remotely in regular operation for the first time in Örnsköldsvik, Sweden. In Germany, on December 4, 2018, an aircraft at Saarbrücken Airport received a landing clearance for the first time from the DFS remote tower center in Leipzig, about 450 kilometers away. A hundred years after Croydon, the tower has thus become a screen room.

What to watch for as a spotter

• Historic types in context: Super Constellation, DC-7 or Boeing 247 are not just beautiful museum pieces but witnesses of an era without nationwide radar.
• Putting flight tracking in perspective: Much of the data on flight tracking services comes from the same transponder technology that controllers use. How to use it at the spot is described in Flight tracking with ADS-B for spotters.
• Understanding airspace: The distinction between controlled and uncontrolled airspace is a direct lesson from accidents such as the Grand Canyon. Today’s classes are explained in Airspace classes A to G.
• The profession: If, after looking back, you are interested in the job itself, Becoming an air traffic controller shows the way into the profession.

Conclusion

In about a hundred years, air traffic control has grown from flags and beacons into a dense network of radar, transponders, satellite positioning and data link. The technology often came from the military, the drive for new rules often from accidents: Picardy 1922 brought the keep-to-the-right rule and radio on board, the Grand Canyon 1956 a federal agency with full airspace control, Überlingen 2002 clear precedence rules for collision warnings. Anyone who listens to air traffic radio at the fence today or reads squawks on flight tracking uses the results of this development every day.