compiled June 2005

1.    The Early Days

It all started with the first commercial Air Mail services : both civil aviation and air traffic were born together as the result of the use of the plane for commercial reasons. Although the first powered flight took place in USA , it was WW1 who forced the European combatants to search deeper into the airplane evolution and design. By the end of WW1 a number of light but rather reliable European bombers have been converted to light cargo carriers ; mail was the first and most prominent cargo which ensured some regular income to the few of the many unemployed ex-‘eagles of the skies’ . The idea to carry passengers too , is definitely European : the Germans made the start with their Zeppelins but even as early as 1920 both in France and UK there were airmail and passenger flights with some special ‘huge’ biplane multi-engine converted bombers of that time. This created some primitive traffic density that resulted soon on a mid-air collision in 1922 due to poor visibility. 60 nm north of Paris a Farman Goliath and a De Havilland DH-18 were lost with 7 people on board.  The first rules were established then considering a mutual exchange of weather information between aerodromes and using separated inbound and outbound tracks over the Channel – there are plenty of them nowadays over there. 

The De Havilland DH-4 used extensively by the US Air Mail services in the 20s. Most probably the first plane ever that received some ATC-type service

Just few years from then , in UK , local rules for light signals guiding planes near the aerodrome were established. By 1923 the radio message ‘Mayday’ was adopted to signal an emergency and in 1926 the body of ‘radio operators’ has been created responsible mainly for the radio transmissions. These rules , however , it did not have yet a global acceptance and did not go deeply enough to predict the future evolution. It was in USA where airmail thrived due to the longer distances and a numerous population and there this business went booming. Around 1920 , few of the pilots whose names were to become historic later – Lindberg was one of them -  were working with the US Airmail Service. They were flying mostly the DeHavilland DH-4 two-sitter biplane having converted one sit to a cargo compartment. 

People that serviced and maintained the light beacon system of the US Airways Division

No doubt they were flying under visual conditions but mail delivery demanded an almost all-weather duty. To ensure some kind of safety and better service quality, radio contact became necessary and then just few years later, in 1927, a system of light beacons was installed , working more or less as with the navy shipping Around the 30s a modification of the light beacons created a directional device . The beacon was rotating at a steady specified speed while a second ,steady non rotating light beacon, would light only when the rotating beacon passed through the north. By measuring with a normal watch the time difference between the two flashes, the first of the non-rotating beacon and the second when the rotating one faced the pilot, one could tell with some fair accuracy on what direction from the beacon the plane was positioned at that time : the first tracks so created were definitely ‘visible’ and helped getting the bearing to the airfield. This allowed for some steady directions to be established as permanent tracks for navigation and the ‘Airways’ were born and continued to be used until today in the same context , although with different beacons. Thus the ‘US Airways Division’ was created ,a kind of an early form of an ATS Department. At this stage US was more advanced than Europe in the civil aviation domain. 


2.   The first Controller : Archie League  


No doubt flights have increased to a significant point and more than often help for runway conditions and guidance were necessary. Despite the virtual lack of runways the way we know them today , flights needed to know the surface wind as well as information for any other close traffic or what to do in emergency conditions. The name that remained in history was the one of Archie League , who may be considered as the father of aerodrome control and/or the pioneer of Air Traffic Control. Archie started his career in St Louis around 1920. 

Archie League in the 1920s

Archie League's 'Tower' environment ; come rain , come shine


He would take a chair and sit close to the runway threshold together with his gear playing the role of the wind observer and runway conditions reporter. When aircraft would arrive without radio , Archie would use a set of flag signals to warn the pilots. His gear and position would change according to the weather and the wind direction and this went on until around 1930 proper wooden structures housing  all the necessary devices of the future Archie’s colleagues were organized : the first aerodrome towers. 

The Cleveland Ohio tower was known to be the most modern aerodrome equipment around 1930. It was surrounded by the ‘french style windows’ , was fully equipped with radio devices and used a huge ventilation duct on top. 

There was another name that stayed in history : Larry Jewell , who was photographed in 1933 while perfectioning the use of the light signals with an ALDIS lamp. Radios were known but not all planes using an airfield were so equipped while radio devices were far from reliable : the ALDIS lamp , or ‘light gun’ , was an essential tool of those days and provided better protection than some unreadable distorted radio signals. . 

In 1935 , in Newark , the first Flight Monitoring Center was established and was housed just below the tower in the middle of the aerodrome terminal. There was a big clock on the wall , an area chart and a notepad to note all flights evolution and a radio transmitter / receiver . This primitive center was the forerunner of the later ATC Centers but on those days they called them , the ‘radio rooms’ 


3.   Earl Ward and Glen Gilbert, the fathers of ATC


Ear Ward was another airmail pilot who was working for the American Airlines at Chicago. He got worried for the constant increase of flights and he thought that sooner or later an air collision would happen unless rules were properly established . He then tried to figure out what these rules could be and how to implement them. He considered as vital to maintain radio contact between all flights and pass information on any one affected by the presence of the others. This was not however enough for the safety of all flights unless all other operating companies would also follow them. TWA , United and Eastern were convinced by Earl Ward to adopt his methods and in Chicago , 1935-36, these rules proved very successful in practice. 

Earl Ward while monitoring traffic from pilot reports. He was using small paper tags that were positioned on a map. He was also using pilot reports for visual bearings from various light beacons and he would estimate times-over-point using speed and distance. During the 70s this became the task of a computerised radar system


The solution in case of conflict was meant to be the result of pilot actions.  Ward’s assistant , however , Glen Gilbert, insisted that no solution can be really safe unless all pilots adhere to clearances provided by the ground personnel , such as altitude adherence and directions flown. Gilbert was assigned the task of publishing these rules and many of them still exist today on the basics learned by all student Traffic Controllers. This is why Earl Ward may be credited with the creation of the first Control Center and Glen Gilbert with the design of the Traffic Separation Rules. To us , these two , are considered as the fathers of Air Traffic Control mainly because of the essential difference from the service provided by the Airway Division until then. From now on an element of active control was introduced where the ground operator had to guide the flights to ensure separation and not only to inform the pilots for important weather and aerodrome conditions and/or other traffic in the area. This ground operator of the 20s has evolved to the Air Traffic Controller we know today.

 Some would put more credit to Gilbert than Ward because he detailed many of the rules that gave birth to Aerodrome , Approach and Area Control. He used the aerodrome circuit and its legs to sequence traffic for the aerodrome. The circuit was a natural track pilots would fly to first identify the airfield conditions and then use the landing direction. Gilbert established the downwind, the basic and the final legs , the extension of the downwind for sequencing and the ‘T’ landing indicator. He introduced the spacing of departing traffic due to time according to speed and departure tracks and the time separation along a track after reporting on significant visual waypoints. As for the Area he established the vertical separation based on 1000 feet -a round figure that later introduced the Flight Level system - and the 10 nm airway width . This is why Gilbert has left his 'prints' on what is now known as Air Traffic Rules


4.    The Air Traffic Control and its tools are born


In 1936 this ATC service was still run by the first involved air companies. To establish a global system however one needed the State to take over such a task for the entire country and all the flights. The Air Commerce Department , in 1936 , took over this job under the form of a public service that developed as the Civil Aviation Authority. In 1937-38 more and more technical applications were introduced as standard ATC equipment , like the communication boxes , the headsets, the teletype , the radio locators and the paper flight progress strip boards. It is worth noting that these tools have stayed in many countries and for many years as the standard ATC equipment ; for some areas of the world they are still used in exactly the same way ! As for the term Air Traffic Control it was officially adopted for the first time by the British Air Ministry in 1939. 

Communication terminals, radio frequencies, flight progress monitoring ( paper strips) and ...a clock. The eternal tools of the ATC 'trade' in their primitive form. On the left , a typical Approach unit in the 30s : there is a wind direction indicator in the upper-left corner. To the right an Area Control Center in the same period : many , many paper strips divided in different bays for different waypoints where the flight data and evolution was recorded. 

Despite the technology of the days the basics of Air Traffic Control were set then , just before 1940 ; one may argue that there have been no context changes on the job itself until the end of the century but merely an introduction of technological improvements to relieve the human factor but otherwise doing the same basic job.

It should also be mentioned that in 1939 , in UK at least , the first ATCO (Air Traffic Control Officer) school started its operations and the first terms of airspace organization appeared. The control has been divided initially in Aerodrome (or Tower) and Area Control. The Aerodrome was responsible for landing and taking-off traffic from ground and within an area of about 3 to 5 miles around the airfield. The rest was the care of the Area Control , which would not go that high as today : aircraft were flying visually and their performance was not enough for altitudes higher than about 10.000 feet.


5.    And then there were radio beacons and radar


Although most of people believe that radar, radio navigation and ATC were born together , radar was introduced in ATC after WW2 and continued a long and fastidious way into getting integrated as its most prominent tool. Radio and Communications were and are still by far the major tools of the trade. The war , however , provided for some particular radio devices : other than the radio contact between pilots and controllers and pilot reports checking over significant visual points there was no other direct way of verifying aircraft positions. The war , therefore, brought along with it some benefits for the ATC that had to do with helping the pilot to navigate beyond visual conditions and controllers to detect planes positions on a screen. One might say that , although the ATC principles remained the same , these tools have changed drastically the character of the job : it was possible now to control the flights in a more direct way than ever before.  

The radio goniometers that initially helped E. Ward to plot aircraft positions on a map could help in almost the same way pilots to locate fixed radio beacons on the ground using radio signals only. It was possible now to navigate without having to check for the light beacons : this is the essence of Radio Navigation. Replacing the light beacons with radio beacons and positioning them along the standard routes gave the possibility of an extended airway network that could reach any possible direction in the air ,within the coverage of beacons.  This type of beacons , named Non Directional Beacons (NDB), were initially based on the Medium Frequency radio technology that was well known those days. They were very simple transmitters, easy to install and maintain with important coverage - but proved less reliable under bad weather and lots of interference. The start of such a structure that would allow flights to any point started in 1946 and it also gave the tools for a deeper airspace division : it was in UK that the first FIRs (Flight Information Regions) have been officially created.  

A small NDB used nowadays as a locator fix for approaching aircraft. In the 50s they were taller and much more powerful and enabled full Radio Navigation  along airways networks. They are less accurate and reliable than modern beacons , though , due to the fragility of Medium Waves propagation on long ranges. Yet , few are still in use in less critical distances

A GCA operator : the lines defining the vertical and horizontal limits of the final approach area are indicated with thin black lines on the screen. The task of the operator is to instruct pilots so as to maintain their track within that area while landing

The radar as was used during the Battle of Britain had a major disadvantage : it did not have a rotating antenna and the accuracy of the target direction was poor. In US , between 1943-46 , they decided to use it only for aircraft on their final landing track - some 10 to 20 miles before the runway - using antennae that would move on small angles covering horizontally and vertically the ‘cone’ within which the landing aircraft is found. This was the Ground Controlled Approach (GCA) and was the very first radar in ATC , used even to our days , although its technology has been abandoned. Using the GCA was very flexible : the plane had to be equipped only with radio and the controller was guiding the pilot to stay on its final three-dimensional path on course to the runway by advising for altitude and direction corrections. As for the GCA equipment could be carried on a track and be positioned near any runway in use : for years and years controllers would keep on loving this tool especially where the visibility conditions were poor : it was the unique ATC tool for landings. Heathrow was equipped with GCA in 1947. Its major disadvantage is that it can handle only one aircraft at a time and pilot has to use its own eyes to complete the last 2 miles before the runway

GCA was later named the Precision Approach Radar (PAR) ; this title was fully justified until about the late 50s when another type of a more precise system - nothing to do with radar technology -  was developed : the Instrument Landing System (ILS).  The ILS , more precise than PAR itself ,  still is the major radio landing tool for all important aerodromes until today. It provides automatically corrective signals for the accurate positioning of the aircraft relative to the horizontal and vertical path during the final landing and can be linked with the plane's autopilot without any other assistance from the controller. It  requires , however , more airborne and ground equipment and is more delicate during the installation because of  the surrounding obstacles , but ... It is usable for more than one landing aircraft at a time  and guides more close to the runway than a PAR. The ILS is practically used from about 20 up to 30 miles before the runway and guides the aircraft until few feet above the runway threshold. It is only due to extremely heavy fog that some ILS landings can not be completed . After the 80s some planes are equipped with Radio-Altimeters , a radar-like device , that provides information about the distance left below the plane and until the runway surface, which eventually limits , albeit not to zero , the lowest altitude the ILS can guide a plane safely to. However , many limitations during landings are nowadays due to obstacles close to the landing path , eg : populated areas , high buildings , the restrictions due to noise abatement and surrounding hills or mountains. It is not always the ILS to be blamed for all problems during landings !

The radar with a rotating antenna that could cover all directions around was not far away and near 1950 allowed the monitoring of flights approaching an aerodrome. The aircraft 'echoes' are displayed as small bright bars ; they are actually small arcs of a circle of some 2-3 degrees wide. The range marks , lines and other details of the background in the screen are a simple map transparency projected properly into the screen. At a time only one sector area of the screen is illuminated and some of the adjacent areas can still be seen due to the phosphorus effect . 

With such a radar only the position information is directly provided , the altitude has to be reported by the pilots and the identity to be kept thanks to the memory of the controller. yet it was a nice tool to have instead of no-radar at all. 

Such a device used for guiding traffic near aerodromes was later called the Terminal Radar. At that time a region of about 60 to 80 nm could be covered and it was at this year that a number of many important aerodromes , like Heathrow, was so equipped. Around 1951 a new unit appeared officially in ATC : the Approach . It was a division within the Area Control that took care of the arrivals and departures which , due to heavy traffic , needed a separate unit to handle them. 

The Area Controllers who were controlling far larger areas were not equipped with radar yet and had to wait for many more technical improvements to come, although their traffic was increasing as well. Around 1950 , in UK , the first names were officially adopted for some renown airways , the first one ever was the ‘Green One’ (G1) : it does exist until today bringing traffic from UK, Dover (DVR beacon), to Central Europe via Belgium , Kokseide (KOK beacon), although the official name now is GOLF ONE – the color names have been abandoned.     

The diagram above displays an airways system (UK). These airways are actually 'tunnels in the air' with specified dimensions and names. Standard commercial traffic is guided only within these tunnels-airways. Their corner points are defined by radio beacons or points defined with the use of radio beacons

The Atlantic crossings by air were not yet as frequent but were definitely expanding. In 1955  ICAO introduced the 1,000 feet vertical, 120 mile lateral and 30 minute longitudinal separation between trans-Atlantic aircraft. It is important that in those days , international rules have started appearing affecting all flights over very large regions and that was to continue ever since quite successfully : the Air Traffic Controller’s job became an international profession.


6.    A ‘leap’ forward in technology

 The standard NDB beacons working on Medium radio frequencies proved unreliable on long range flights and as higher frequencies were known to be more precise and less liable to errors due to weather phenomena and radio propagation , technology moved to the VHF radio band and developed another type of beacon , the VOR , to provide for directions coupled with another invention , the DME , to add the distance element to radio navigation. This couple of VOR+DME became the most important navigation element of the following years and helped design the airspace with more precision and therefore accommodate more traffic than before by limiting the safety distances kept for track separations due to the errors and tolerances of the NDB. 

The picture above displays the hut where a VOR is enclosed and the cylinder on top is the DME antenna


The early 60s saw another improvement in the radar technology , the Secondary Surveillance Radar (SSR). This was another component linked to the existing radars - renamed then as Primary Surveillance Radars (PSR) - which was triggering a specific airborne device to answer a signal call from the ground located SSR. The answer was a number that was properly displayed on the standard radar screen and was used as an identifier for a flight, followed by an automatic altitude report from the altimeter itself. 

A rotating aerial hosting a Primary (PSR) , below, and a SSR (above) , antennae


This made aircraft detection an easy thing and the altitude verification automatic , without having to ask the pilots often on the radio. Today the SSR is a necessity for any busy ATC environment while in some areas ,cheaper to buy and maintain, SSRs were enough to display aircraft positions and have even replaced totally the older PSRs in certain areas. Near the end of the 60s computers came into play trying to help the job of this new international profession. There were a number of tasks the human being had to execute daily that were not clearly and purely about controlling aircraft . 

The 4-digit numbers appearing next to the aircraft echoes of the PSR were the SSR codes used to identify flights , thus removing the difficulty of maintaining  identification by memory . The 3-digit number appearing below the 4-digit SSR code were the altitude in hundreds of feet ( the Flight Level actually). This removed the need for the pilots to report all level changes

Many secondary, yet absolutely necessary functions, were based on picking up a telephone line to pass information on outgoing and incoming traffic, reading teletype messages or writing paper strips, calculating speeds, distances and times; that was time wise a considerable part of the job, however, and was sacrificing a number of personnel to these duties away from the controlling positions. It was thus considered necessary to allocate most of these tasks to computers. It was around these days the first basic software appeared that allowed clear printing, calculation and distribution of paper strips to the appropriate sectors. Other software took care of linking the radio transmissions and ground messages between the various sectors of control. Soon the need to expand radar coverage to larger area control centers introduced the multi-radar coverage. Again, computers were necessary to combine the echoes received from many radar sites and optimize the various accuracies of the aircraft positions, eliminate intrinsic radar errors and display in a comfortable and clear way all related information on the controller screens. In some ATC centers the paper strips were totally forgotten and were replaced by electronic flight information displays. The ability to replace the sole SSR code number on the screens directly with the full flight's number was another achievement of the computers, known as the code to call sign correlation. In few other systems the Short Term Conflict Alert (STCA) was introduced: a computer warning of a potential loss of separation some 2 minutes in advance. 

A computer system like the one on the left is driving almost all the available ATC tools of today :

1. records the radar data , calculates the aircraft trajectories and generates alerts

2. stores , activates , calculates and updates all information related to flight planning

3. controls the display of all data on the screens

4. controls the communications

An Air Traffic Control Centers of today are similar to the picture shown to the right (London ACC). Many working positions subdivide the large area of a country or groups of countries into sectors all of them driven by a computer system. Each working position is manned by one controller and his assistant . The controller is mainly handling the air talk to aircraft and cares about separations guiding accordingly the traffic by assigning tracks , altitudes, speed limitations and headings. His assistant is responsible for the planning of the traffic prior to entering the sector and coordinates the situation with the other sectors where his traffic is then moving to. 

Modern Towers are equipped with similar tools to facilitate traffic monitoring in the aerodromes. Increase of traffic means all the more that runways and taxiways are heavily loaded. Controllers need faster monitoring and grouping of all data concentrated in front of them. On the left you may see a modern Aerodrome Control (Tower) working position


In the 90s these tools eased the job of the controller in a positive way but at the same time the traffic increase was more than ever expected: it was doubling at every 10 years! This factor brought to an impasse the capacity of the ATC systems and beginning of 1990 the first studies for a modern ‘intelligent’ ATC environment appeared to allow the same number of controllers to handle more traffic. Computers were of course everywhere the main tools but the designers were not sure on the proper direction to take. Yet, after some time they all started converging on certain issues: the computes systems were supposed to be able to issue warnings on the planning of traffic, analyze some traffic situations, offer solutions and minimize the delay of inputs and co-ordinations. Additionally satellite technology was called for help. Although satellites have no effect on traffic demand, they can help on channeling the pilot-controller communications and reduce radio-telephony occupancy, simplify the aircraft navigation and enhance the surveillance of flights in remote areas or over oceans were no radars can be installed.   The next 15 years saw an important evolution of tools under the collective term ‘ATC Modernization’. Additionally some other systems evolved , notably the one that centralised the flow management : it was possible to predict from flight plans traffic capacities and overflows along vast areas before departure and thus issue departing time periods to flights so as to avoid the danger of exceeding traffic capacities for the controllers. Another important characteristic was that the same system was redistributing the traffic flow so as to maximize the existing capacity and create traffic strategies for periods of the year over problematic regions. One such famous unit is run by Eurocontrol under the name Central Flow Management Unit (CFMU) in Brussels , responsible for the broader area of Europe. CFMU and some other areas of evolution created the term ATM ( Air Traffic Management) to include all related technical and organizational activities whose goal was to support the global ATC functionality and ultimately give an answer to the future capacity needs. Despite some occasional air company problems and despite some hasty pessimistic predictions the air traffic demand did not , but temporarily, leveled-off. At the moment - 2005 - the recorded increase is 4-5 % yearly and this means some  60% additional traffic around 2015 in the skies, for a system working already at maximum capacity !  

European Traffic Density

In the red areas the numbers of aircraft controlled

 ranges between 55-70 per hour in a single sector !

While some transport specialist saw the dead end on capacity in the air, the environmental and economic  restrictions did not allow for drastic measures in other spheres. Trains for example can no longer replace air transport although may serve better the shorter distances. And yet, trains are not cheaper while the new and very well and fast developed ‘low-fare’ air companies have shown that commercial airplanes may work like regular bus services with lesser cost both to them and to the passengers alike. In as much the same way as air mail in the 20s lead to the re-organization of ATC in the middle of the 30s , it might be that around 2030 , or a century later from its birthday, ATC will have to take another , more revolutionary form , if it is to tackle with the growth of commercial air transport.


The above article has used much of the information included on the following very interesting sites which are recommended for further reading on this topic :

 NATS, UK , History of ATC : http://www.nats.co.uk/library/history1.html

US Centenial of flight Commission : http://www.centennialofflight.gov/essay/Government_Role/Air_traffic_control/POL15.htm  

The 'ATC Café' (US) http://www.airtrafficcafe.com/atc_history.shtml


Other sites talking about the profession of ATC :



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