>>> 2025-05-11 air traffic control (PDF)

Air traffic control has been in the news lately, on account of my country’s
declining ability to do it. Well, that’s a long-term trend, resulting from
decades of under-investment, severe capture by our increasingly incompetent
defense-industrial complex, no small degree of management incompetence in the
FAA, and long-lasting effects of Reagan crushing the PATCO strike. But that’s
just my opinion, you know, maybe airplanes got too woke. In any case, it’s an
interesting time to consider how weird parts of air traffic control are. The
technical, administrative, and social aspects of ATC all seem two notches more
complicated than you would expect. ATC is heavily influenced by its peculiar
and often accidental development, a product of necessity that perpetually
trails behind the need, and a beneficiary of hand-me-down military practices
and technology.

Aviation Radio

In the early days of aviation, there was little need for ATC—there just
weren’t many planes, and technology didn’t allow ground-based controllers to do
much of value. There was some use of flags and signal lights to clear aircraft
to land, but for the most part ATC had to wait for the development of aviation
radio. The impetus for that work came mostly from the First World War.

Here we have to note that the history of aviation is very closely intertwined
with the history of warfare. Aviation technology has always rapidly advanced
during major conflicts, and as we will see, ATC is no exception.

By 1913, the US Army Signal Corps was experimenting with the use of radio to
communicate with aircraft. This was pretty early in radio technology, and the
aircraft radios were huge and awkward to operate, but it was also early in
aviation and “huge and awkward to operate” could be similarly applied to the
aircraft of the day. Even so, radio had obvious potential in aviation. The
first military application for aircraft was reconnaissance. Pilots could fly
past the front to find artillery positions and otherwise provide useful
information, and then return with maps. Well, even better than returning with a
map was providing the information in real-time, and by the end of the war
medium-frequency AM radios were well developed for aircraft.

Radios in aircraft lead naturally to another wartime innovation: ground
control. Military personnel on the ground used radio to coordinate the
schedules and routes of reconnaissance planes, and later to inform on the
positions of fighters and other enemy assets. Without any real way to know
where the planes were, this was all pretty primitive, but it set the basic
pattern that people on the ground could keep track of aircraft and provide
useful information.

Post-war, civil aviation rapidly advanced. The early 1920s saw numerous
commercial airlines adopting radio, mostly for business purposes like schedule
coordination. Once you were in contact with someone on the ground, though, it
was only logical to ask about weather and conditions. Many of our modern
practices like weather briefings, flight plans, and route clearances originated
as more or less formal practices within individual airlines.

Air Mail

The government was not left out of the action. The Post Office operated what
may have been the largest commercial aviation operation in the world during the
early 1920s, in the form of Air Mail. The Post Office itself did not have any
aircraft; all of the flying was contracted out—initially to the Army Air
Service, and later to a long list of regional airlines. Air Mail was considered
a high priority by the Post Office and proved very popular with the public.
When the transcontinental route began proper operation in 1920, it became
possible to get a letter from New York City to San Francisco in just 33 hours
by transferring it between airplanes in a nearly non-stop relay race.

The Post Office’s largesse in contracting the service to private operators
provided not only the funding but the very motivation for much of our modern
aviation industry. Air travel was not very popular at the time, being loud and
uncomfortable, but the mail didn’t complain. The many contract mail carriers of
the 1920s grew and consolidated into what are now some of the United States’
largest companies. For around a decade, the Post Office almost singlehandedly
bankrolled civil aviation, and passengers were a side hustle [1].

Air mail ambition was not only of economic benefit. Air mail routes were often
longer and more challenging than commercial passenger routes. Transcontinental
service required regular flights through sparsely populated parts of the
interior, challenging the navigation technology of the time and making rescue
of downed pilots a major concern. Notably, air mail operators did far more
nighttime flying than any other commercial aviation in the 1920s. The post
office became the government’s de facto technical leader in civil aviation.
Besides the network of beacons and markers built to guide air mail between
cities, the post office built 17 Air Mail Radio Stations along the
transcontinental route.

The Air Mail Radio Stations were the company radio system for the entire air
mail enterprise, and the closest thing to a nationwide, public air traffic
control service to then exist. They did not, however, provide what we would now
call control. Their role was mainly to provide pilots with information
(including, critically, weather reports) and to keep loose tabs on air mail
flights so that a disappearance would be noticed in time to send search and
rescue.

In 1926, the Watres Act created the Aeronautic Branch of the Department of
Commerce. The Aeronautic Branch assumed a number of responsibilities, but one
of them was the maintenance of the Air Mail routes. Similarly, the Air Mail
Radio Stations became Aeronautics Branch facilities, and took on the new name
of Flight Service Stations. No longer just for the contract mail carriers, the
Flight Service Stations made up a nationwide network of government-provided
services to aviators. They were the first edifices in what we now call the
National Airspace System (NAS): a complex combination of physical facilities,
technologies, and operating practices that enable safe aviation.

In 1935, the first en-route air traffic control center opened, a facility in
Newark owned by a group of airlines. The Aeronautic Branch, since renamed the
Bureau of Air Commerce, supported the airlines in developing this new concept
of en-route control that used radio communications and paperwork to track which
aircraft were in which airways. The rising number of commercial aircraft made
in-air collisions a bigger problem, so the Newark control center was quickly
followed by more facilities built on the same pattern. In 1936, the Bureau of
Air Commerce took ownership of these centers, and ATC became a government
function alongside the advisory and safety services provided by the flight
service stations.

En route center controllers worked off of position reports from pilots via
radio, but needed a way to visualize and track aircraft’s positions and their
intended flight paths. Several techniques helped: first, airlines shared their
flight planning paperwork with the control centers, establishing “flight plans”
that corresponded to each aircraft in the sky. Controllers adopted a work aid
called a “flight strip,” a small piece of paper with the key information about
an aircraft’s identity and flight plan that could easily be handed between
stations. By arranging the flight strips on display boards full of slots,
controllers could visualize the ordering of aircraft in terms of altitude and
airway.

Second, each center was equipped with a large plotting table map where
controllers pushed markers around to correspond to the position reports from
aircraft. A small flag on each marker gave the flight number, so it could
easily be correlated to a flight strip on one of the boards mounted around the
plotting table. This basic concept of air traffic control, of a flight strip
and a position marker, is still in use today.

Radar

The Second World War changed aviation more than any other event of history.
Among the many advancements were two British inventions of particular
significance: first, the jet engine, which would make modern passenger
airliners practical. Second, the radar, and more specifically the magnetron.
This was a development of such significance that the British government
treated it as a secret akin to nuclear weapons; indeed, the UK effectively
traded radar technology to the US in exchange for participation in US
nuclear weapons research.

Radar created radical new possibilities for air defense, and complimented
previous air defense development in Britain. During WWI, the organization
tasked with defending London from aerial attack had developed a method called
“ground-controlled interception” or GCI. Under GCI, ground-based observers
identify possible targets and then direct attack aircraft towards them via
radio. The advent of radar made GCI tremendously more powerful, allowing a
relatively small number of radar-assisted air defense centers to monitor for
inbound attack and then direct defenders with real-time vectors.

In the first implementation, radar stations reported contacts via telephone to
“filter centers” that correlated tracks from separate radars to create a
unified view of the airspace—drawn in grease pencil on a preprinted map.
Filter center staff took radar and visual reports and updated the map by moving
the marks. This consolidated information was then provided to air defense
bases, once again by telephone.

Later technical developments in the UK made the process more automated. The
invention of the “plan position indicator” or PPI, the type of radar scope we
are all familiar with today, made the radar far easier to operate and
interpret. Radar sets that automatically swept over 360 degrees allowed each
radar station to see all activity in its area, rather than just aircraft
passing through a defensive line. These new capabilities eliminated the need
for much of the manual work: radar stations could see attacking aircraft and
defending aircraft on one PPI, and communicated directly with defenders by
radio.

It became routine for a radar operator to give a pilot navigation vectors by
radio, based on real-time observation of the pilot’s position and heading. A
controller took strategic command of the airspace, effectively steering the
aircraft from a top-down view. The ease and efficiency of this workflow was a
significant factor in the end of the Battle of Britain, and its remarkable
efficacy was noticed in the US as well.

At the same time, changes were afoot in the US. WWII was tremendously
disruptive to civil aviation; while aviation technology rapidly advanced due to
wartime needs those same pressing demands lead to a slowdown in nonmilitary
activity. A heavy volume of military logistics flights and flight training, as
well as growing concerns about defending the US from an invasion, meant that
ATC was still a priority. A reorganization of the Bureau of Air Commerce
replaced it with the Civil Aeronautics Authority, or CAA. The CAA’s role
greatly expanded as it assumed responsibility for airport control towers and
commissioned new en route centers.

As WWII came to a close, CAA en route control centers began to adopt GCI
techniques. By 1955, the name Air Route Traffic Control Center (ARTCC) had been
adopted for en route centers and the first air surveillance radars were
installed. In a radar-equipped ARTCC, the map where controllers pushed markers
around was replaced with a large tabletop PPI built to a Navy design. The
controllers still pushed markers around to track the identities of aircraft,
but they moved them based on their corresponding radar “blips” instead of radio
position reports.

Air Defense

After WWII, post-war prosperity and wartime technology like the jet engine lead
to huge growth in commercial aviation. During the 1950s, radar was adopted by
more and more ATC facilities (both “terminal” at airports and “en route” at
ARTCCs), but there were few major changes in ATC procedure. With more and more
planes in the air, tracking flight plans and their corresponding positions
became labor intensive and error-prone. A particular problem was the increasing
range and speed of aircraft, and corresponding longer passenger flights, that
meant that many aircraft passed from the territory of one ARTCC into another.
This required that controllers “hand off” the aircraft, informing the “next”
ARTCC of the flight plan and position at which the aircraft would enter their
airspace.

In 1956, 128 people died in a mid-air collision of two commercial airliners
over the Grand Canyon. In 1958, 49 people died when a military fighter struck a
commercial airliner over Nevada. These were not the only such incidents in the
mid-1950s, and public trust in aviation started to decline. Something had to be
done. First, in 1958 the CAA gave way to the Federal Aviation Administration.
This was more than just a name change: the FAA’s authority was greatly
increased compared to the CAA, most notably by granting it authority over
military aviation.

This is a difficult topic to explain succinctly, so I will only give broad
strokes. Prior to 1958, military aviation was completely distinct from civil
aviation, with no coordination and often no communication at all between the
two. This was, of course, a factor in the 1958 collision. Further, the 1956
collision, while it did not involve the military, did result in part from
communications issues between separate distinct CAA facilities and the
airline’s own control facilities. After 1958, ATC was completely unified into
one organization, the FAA, which assumed the work of the military controllers
of the time and some of the role of the airlines. The military continues to
have its own air controllers to this day, and military aircraft continue to
include privileges such as (practical but not legal) exemption from transponder
requirements, but military flights over the US are still beholden to the same
ATC as civil flights. Some exceptions apply, void where prohibited, etc.

The FAA’s suddenly increased scope only made the practical challenges of ATC
more difficult, and commercial aviation numbers continued to rise. As soon as
the FAA was formed, it was understood that there needed to be major investments
in improving the National Airspace System. While the first couple of years were
dominated by the transition, the FAA’s second director (Najeeb Halaby) prepared
two lengthy reports examining the situation and recommending improvements. One
of these, the Beacon report (also called Project Beacon), specifically
addressed ATC. The Beacon report’s recommendations included massive expansion
of radar-based control (called “positive control” because of the controller’s
access to real-time feedback on aircraft movements) and new control procedures
for airways and airports. Even better, for our purposes, it recommended the
adoption of general-purpose computers and software to automate ATC functions.

Meanwhile, the Cold War was heating up. US air defense, a minor concern in the
few short years after WWII, became a higher priority than ever before. The
Soviet Union had long-range aircraft capable of reaching the United States, and
nuclear weapons meant that only a few such aircraft had to make it to cause
massive destruction. Considering the vast size of the United States (and,
considering the new unified air defense command between the United States and
Canada, all of North America) made this a formidable challenge.

During the 1950s, the newly minted Air Force worked closely with MIT’s Lincoln
Laboratory (an important center of radar research) and IBM to design a
computerized, integrated, networked system for GCI. When the Air Force
committed to purchasing the system, it was christened the Semi-Automated Ground
Environment, or SAGE. SAGE is a critical juncture in the history of the
computer and computer communications, the first system to demonstrate many
parts of modern computer technology and, moreover, perhaps the first
large-scale computer system of any kind.

SAGE is an expansive topic that I will not take on here; I’m sure it will be
the focus of a future article but it’s a pretty well-known and well-covered
topic. I have not so far felt like I had much new to contribute, despite it
being the first item on my “list of topics” for the last five years. But one of
the things I want to tell you about SAGE, that is perhaps not so well known, is
that SAGE was not used for ATC. SAGE was a purely military system. It was
commissioned by the Air Force, and its numerous operating facilities (called
“direction centers”) were located on Air Force bases along with the interceptor
forces they would direct.

However, there was obvious overlap between the functionality of SAGE and the
needs of ATC. SAGE direction centers continuously received tracks from remote
data sites using modems over leased telephone lines, and automatically
correlated multiple radar tracks to a single aircraft. Once an operator entered
information about an aircraft, SAGE stored that information for retrieval by
other radar operators. When an aircraft with associated data passed from the
territory of one direction center to another, the aircraft’s position and
related information were automatically transmitted to the next direction center
by modem.

One of the key demands of air defense is the identification of aircraft—any
unknown track might be routine commercial activity, or it could be an inbound
attack. The air defense command received flight plan data on commercial flights
(and more broadly all flights entering North America) from the FAA and entered
them into SAGE, allowing radar operators to retrieve “flight strip” data on any
aircraft on their scope.

Recognizing this interconnection with ATC, as soon as SAGE direction centers
were being installed the Air Force started work on an upgrade called SAGE Air
Traffic Integration, or SATIN. SATIN would extend SAGE to serve the ATC
use-case as well, providing SAGE consoles directly in ARTCCs and enhancing SAGE
to perform non-military safety functions like conflict warning and forward
projection of flight plans for scheduling. Flight strips would be replaced by
teletype output, and in general made less necessary by the computer’s ability
to filter the radar scope.

Experimental trial installations were made, and the FAA participated readily in
the research efforts. Enhancement of SAGE to meet ATC requirements seemed
likely to meet the Beacon report’s recommendations and radically improve ARTCC
operations, sooner and cheaper than development of an FAA-specific system.

As it happened, well, it didn’t happen. SATIN became interconnected with
another planned SAGE upgrade to the Super Combat Centers (SCC), deep
underground combat command centers with greatly enhanced SAGE computer
equipment. SATIN and SCC planners were so confident that the last three Air
Defense Sectors scheduled for SAGE installation, including my own Albuquerque,
were delayed under the assumption that the improved SATIN/SCC equipment should
be installed instead of the soon-obsolete original system. SCC cost estimates
ballooned, and the program’s ambitions were reduced month by month until it was
canceled entirely in 1960. Albuquerque never got a SAGE installation, and the
Albuquerque air defense sector was eliminated by reorganization later in 1960
anyway.

Flight Service Stations

Remember those Flight Service Stations, the ones that were originally built by
the Post Office? One of the oddities of ATC is that they never went away. FSS
were transferred to the CAB, to the CAA, and then to the FAA. During the 1930s
and 1940s many more were built, expanding coverage across much of the country.

Throughout the development of ATC, the FSS remained responsible for non-control
functions like weather briefing and flight plan management. Because aircraft
operating under instrument flight rules must closely comply with ATC, the
involvement of FSS in IFR flights is very limited, and FSS mostly serve VFR
traffic.

As ATC became common, the FSS gained a new and somewhat odd role: playing
go-between for ATC. FSS were more numerous and often located in sparser areas
between cities (while ATC facilities tended to be in cities), so especially in
the mid-century, pilots were more likely to be able to reach an FSS than ATC.
It was, for a time, routine for FSS to relay instructions between pilots and
controllers. This is still done today, although improved communications have
made the need much less common.

As weather dissemination improved (another topic for a future post), FSS gained
access to extensive weather conditions and forecasting information from the
Weather Service. This connectivity is bidirectional; during the midcentury FSS
not only received weather forecasts by teletype but transmitted pilot reports
of weather conditions back to the Weather Service. Today these communications
have, of course, been computerized, although the legacy teletype format doggedly
persists.

There has always been an odd schism between the FSS and ATC: they are operated
by different departments, out of different facilities, with different functions
and operating practices. In 2005, the FAA cut costs by privatizing the FSS
function entirely. Flight service is now operated by Leidos, one of the largest
government contractors. All FSS operations have been centralized to one
facility that communicates via remote radio sites.

While flight service is still available, increasing automation has made the
stations far less important, and the general perception is that flight service
is in its last years. Last I looked, Leidos was not hiring for flight service
and the expectation was that they would never hire again, retiring the service
along with its staff.

Flight service does maintain one of my favorite internet phenomenon, the phone
number domain name: 1800wxbrief.com. One of the odd manifestations of the
FSS/ATC schism and the FAA’s very partial privatization is that Leidos
maintains an online aviation weather portal that is separate from, and competes
with, the Weather Service’s aviationweather.gov. Since Flight Service
traditionally has the responsibility for weather briefings, it is honestly
unclear to what extend Leidos vs. the National Weather Service should be
investing in aviation weather information services. For its part, the FAA seems
to consider aviationweather.gov the official source, while it pays for
1800wxbrief.com. There’s also weathercams.faa.gov, which duplicates a very
large portion (maybe all?) of the weather information on Leidos’s portal and
some of the NWS’s. It’s just one of those things. Or three of those things,
rather. Speaking of duplication due to poor planning…

The National Airspace System

Left in the lurch by the Air Force, the FAA launched its own program for ATC
automation. While the Air Force was deploying SAGE, the FAA had mostly been
waiting, and various ARTCCs had adopted a hodgepodge of methods ranging from
one-off computer systems to completely paper-based tracking. By 1960 radar was
ubiquitous, but different radar systems were used at different facilities, and
correlation between radar contacts and flight plans was completely manual. The
FAA needed something better, and with growing congressional support for ATC
modernization, they had the money to fund what they called National Airspace
System En Route Stage A.

Further bolstering historical confusion between SAGE and ATC, the FAA decided
on a practical, if ironic, solution: buy their own SAGE.

In an upcoming article, we’ll learn about the FAA’s first fully integrated
computerized air traffic control system. While the failed detour through SATIN
delayed the development of this system, the nearly decade-long delay between
the design of SAGE and the FAA’s contract allowed significant technical
improvements. This “New SAGE,” while directly based on SAGE at a functional
level, used later off-the-shelf computer equipment including the IBM
System/360, giving it far more resemblance to our modern world of computing
than SAGE with its enormous, bespoke AN/FSQ-7.

And we’re still dealing with the consequences today!

[1] It also laid the groundwork for the consolidation of the industry, with a
1930 decision that took air mail contracts away from most of the smaller
companies and awarded them instead to the precursors of United, TWA, and
American Airlines.