An air traffic control system for the twentieth(!) century

Nearly 40 years ago, the President’s Science Advisory Committee outlined ways to improve the country’s air traffic control system–recommendations that are still salient today.

In recent weeks, news reports have brought attention to a vision of air traffic control that
would increase safety, reduce costs, and increase the capacity of the national and international
air space to facilitate air travel. This same vision was specifically proposed in 1971 by the Air
Traffic Control Panel (ATCP) of the President’s Science Advisory Committee (PSAC), which I led.
(See our final report,
“Improving the Nation’s Air
Traffic Control System” [PDF].)

Neither President George W. Bush’s demand for the Federal Aviation Administration (FAA) to
increase capacity of the commercial aviation sector, nor the disappearance of the private craft
piloted by Steve Fossett will suffice to change a system for the better. But it may help to know
what can be done.

In brief, after a substantial analysis of the problems and opportunities, including a comparison
with the actual FAA plan, the 1971 ATCP proposed an all-satellite system that would replace voice
communication with the cockpit with text communication that would be displayed and stored onboard,
a position-monitoring system that would use time difference of arrival (TDOA) of radio signals to
or from the aircraft to determine their precise positions, and autonomous navigation provided by
TDOA in precisely the fashion that the Global Positioning System (GPS) now operates. This is also a
TDOA system, with the radio receiver and computer in the cockpit, or available by radio relay in
the case of small aircraft that in the 1970s might not have been able to carry computing
capability.

Thus, the three essential functions of air traffic control (communication, monitoring, and
navigation) would all be performed via satellite systems, which might or might not be integrated on
the same satellites.

For the United States, it was clear that a single ground station would suffice to do all of the
computations and control, although the report recommended at least two such facilities to reduce
the vulnerability to natural disaster or attack on the facility.

Both the substance and context of the report are of possible interest. In fact, it is bizarre
that the control of air traffic, which touches the lives of almost all of my readers, has not
attracted more attention from the general public. 1969 was the peak of frustration for flight
delays and disruptions. I flew every few days from LaGuardia Airport in New York City to Washington
National Airport in D.C.; in that era, air traffic controllers would get the aircraft off the
ground on time as much as possible and accommodate landing delays by holding the aircraft in
“stacks” near the terminal area. I recall once taking 11 hours and two stops in flying on the
Washington-New York air shuttle, and 6-hour trips were not that unusual.

ATCP members visited both en-route and terminal air traffic control centers; we had passes that
allowed us to fly in the cockpit of commercial aircraft to gain firsthand experience. As usual, we
met two days a month for deliberations and to gather information from government organizations,
contractors, airlines, airline pilots, and operators of private aviation.

Our report not only analyzed the situation as it existed in 1970, but also the FAA plan for the
future. The FAA plan included greater use of data link to reduce the reliance on radio voice
communication. Our report also investigated altitude-reporting radio transponders that en-route FAA
radar would (and now does) use to display the altitude of the aircraft being viewed by means of the
aircraft-carried beacon responding to the radar pulses. Our approach looked at system performance
and investment and operating costs and took advantage of new computer and communication
technology.

We proposed that satellites should carry out communication, minimizing the communication burden
and the deployment and operating cost by emphasizing data communication instead of voice. We worked
out the number of bits required for standard air traffic control messages and found them well
within the capacity of the era’s economically deployable systems to handle the entire communication
load for 50,000 aircraft simultaneously airborne within the United States. Each of the messages
would be recorded in computer storage at each end and displayed routinely and on demand.

Today, it is easy to describe our proposed “one-way” navigation system, which was indeed GPS.
But in those days, the self-contained GPS computer would have cost several thousand dollars; we
therefore also provided the option of a two-way navigation system that would work with the
aircraft’s position-monitoring system. For such a system, receivers on separate satellites (or
co-located on navigation satellites) retransmit to ground stations a special pulse that each
aircraft would radiate. In principle each aircraft would possess a distinguishable pulse of 511
“chips” that would be broadcast in a sequence of 0.1-microsecond chips, for a total pulse length of
51.1 microseconds, once a second, for each aircraft. But in order not to require 50,000 different
pulses, we categorized them by the pulse repetition rate (PRR), over a range from 1 to 1.1 seconds.
Each aircraft would be assigned a specific PRR, which would serve to distinguish one pulse sequence
from another. The fact that the navigation pulse from aircraft A, for instance, would come at this
precisely fixed interval would allow the ground computers to search for the next pulse from an
aircraft, from a location that for a subsonic aircraft would always be within 1,000 feet of the
previous location. Each pulse would be received on at least four satellites and retransmitted to
the operating ground station and to any number of backup stations.

In principle, a single short pulse of duration 0.1 microseconds at a suitable frequency would
serve. But it would require a peak power of about a megawatt (an average power on the order of 0.1
watts), and in order to use the era’s tiny vacuum tubes or transistors, we selected a so-called
“compressible pulse.”

To minimize the burden on the air traffic control system, each aircraft was proposed to file a
digital flight plan and to follow that plan unless an updated plan was filed. Because all aircraft
could use “area navigation,” they would not bunch (or potentially collide) along the routes defined
by ground navigation aids; we recommended a concept termed “intermittent positive control” that
would require the air traffic controller to intervene only when a potential collision might occur
in the next minute or so, on the assumption that each aircraft followed its flight plan as updated
by the pilot or by the surveillance system if the flight plan was being followed, but perhaps with
some delay.

We gave considerable attention to the ground stations’ physical vulnerability, and some to
intentional interference (jamming) of the links to or from the satellites. In military systems,
operations in the face of hostile acts such as jamming is a primary concern; it would be here as
well, if the world’s air traffic control system depended on unimpaired operation of the system. The
report does consider the benefits of the all-satellite system with a navigation or surveillance
accuracy on the order of 100 feet in providing the basis for blind landings at airfields without
expensive equipment. Airports could, however, have inexpensive pseudolites that would receive GPS
signals and broadcast their own, to provide both a stronger signal locally and one that is
geographically more advantageous than any of those received from satellites. This would
substantially ease the local jamming problem and provide about 1-foot accuracy, entirely adequate
for blind landing and taxi.

Naturally, we planned to distribute the unclassified report widely within the government and to
publish it. Instead, in PSAC’s waning days (President Richard Nixon eventually eliminated it in
February 1973, purportedly “to save money”), the Department of Transportation suppressed the
report–an unprecedented act by a government department. In 2007, I placed a copy in the
“Garwin Archive.”

The TDOA system had historic roots. President Dwight D. Eisenhower proposed the “Open Skies”
regime on July 21, 1955, with the purpose of preventing an accidental war because of a possible
Soviet response to what Moscow might regard as U.S. bombers penetrating its airspace. In response,
I proposed implementing a system in which U.S. bomber aircraft would carry free-running
transmitters (each broadcasting about once every second, the signals from which would be picked up
by special satellites and returned to ground stations on Earth) to provide TDOA locations of the
bombers; the time differences in arrival of radio signals at the ground stations via the various
satellites would be converted to path differences by multiplying by the known speed of radio
waves–the speed of light in vacuum: 300,000 kilometers per second. This would have been used in
exercise mode and presumably turned off in wartime. The purpose was to reassure Moscow that the
bombers remained well outside Soviet territory. (See
“Proposal for an International Air,
Sea, and Space Traffic Control Using Radio Beacons on the Vehicles” [PDF].)

But it was not easy for a new navigation system to gain acceptance. The airlines were fearful
that they would be required to pay for the infrastructure in addition to (rather than as a
replacement for) the existing navigation aids. My military aircraft panel in the 1960s had the same
problem in gaining acceptance for GPS, and I personally visited the Joint Chiefs of Staff in the
Pentagon’s “tank” to brief our proposal in the late 1960s. After the briefing, one of the chiefs
told me, “I don’t have $50 million in my budget this year, and if I did, I would spend it on a
ship.”

In 1980, my respected colleague, Gen. Lew Allen, who had been director of the National Security
Agency before becoming Air Force Chief of Staff, reprogrammed $2 million out of his budget rather
than spending it on GPS. Fortunately, Defense Secretary Harold Brown and Bill Perry, director of
Defense Research and Engineering, put the money back. But it took a long time to deploy GPS,
primarily because of bureaucratic problems and fears from the airlines that they would be forced to
pay an insupportable amount for the system. There were also technical disputes and delays in
perfecting unnecessary technology such as atomic clocks for the satellites. A civilian or military
system could rely on multiple transponders on the ground in geologically stable (and politically
stable) locations for its calibration and for the knowledge of the crystal clock frequency on each
satellite.

As we were refining our proposal during our ATCP deliberations, we attempted to learn the
judgment of some of those experienced in military navigation systems and contacted a group at the
Institute for Defense Analyses (IDA). As technical people, they were quite interested, but they
differed with our analysis. We had worked with David D. Otten at TRW Corporation to make a
bottom-up assessment of the cost of the installed equipment and ended up with replication costs of
$1,800 for a private aircraft and $4,100 for a commercial aircraft for a system that would provide
a “fix” every second with an accuracy of 100 feet. Rather than validating or contesting these
conclusions, the IDA group took a different approach–a regression analysis (for existing systems)
of navigation costs versus accuracy and fix time. Their conclusion was that our system would cost
about $200,000 per copy.

Naturally, we met with the analysts following receipt of their report, and I complained that
they had done nothing to verify or challenge our analysis. They said that this was intentional: If
our point-design system approach failed in development, then we would be forced into a conventional
mode of operation, and we would have the same costs as everybody else. I protested that the program
should be canceled if it did not achieve these goals. Apparently, that’s the problem: Military
systems in general are committed to achieve a goal, and if it cannot be achieved with the planned
schedule and budget, the goal is pursued anyhow at whatever costs or delays. In the commercial
world, developments are allowed to fail; it is inconceivable that the world would rely at any time
on a single design of a digital camera. A similar approach is highly desirable for the design and
elements of a worldwide air traffic control system.

Technically, the ATCP was an enjoyable activity, both from the point of view of learning and
inventing the technology and for the interpersonal relations among panel members. Bureaucratically,
it was disappointing to have our recommendations ignored and suppressed. Economically, it is tragic
to have denied the public for two decades or more the benefits of a modern air traffic control
system. The report considered potential system designs, but recognized that no such design could be
selected with such a small amount of preliminary work. It therefore advocated a specific approach
to doing the further analyses and developments required to field such a system.

When George A. Keyworth first took office as President Ronald Reagan’s science adviser, I made a
special trip to Washington to give him my views of existing problems and opportunities in the
fields of national security and technology. I specifically presented the all-satellite air traffic
control system as something that would have an enormous impact on both U.S. commercial and military
capabilities and provide systems, products, and services for the United States to sell to the rest
of the world. This was particularly true also for the introduction of a new system of
vertical-landing passenger aircraft that the new ATC system would facilitate, providing a major
market for the U.S. aerospace industry and an option for greatly increasing the capacity of the air
transport system by physically increasing the takeoff and landing capacity of busy airports by
reducing the dependence on long runways.

As is well-known, GPS was deployed by the military and has been a wild success since it proved
its utility during the first Gulf War. In the work of the ATCP, we suggested that some major
airline such as United might want to adopt and deploy such a system for corporate use, which would
reduce the difficulties involved in the decision to go ahead.

It would be good to have a high-level technical committee similar to the ATCP convened
today–with a substantial budget for technical support work–to bring the proposal up-to-date and
to provide a conservative implementation plan. The scope for updating is clear from the definition
of “high-performance computer” in the 1971 report as about 65 million operations per second,
compared with a desktop computer costing $500 in 2007 and routinely capable of 1,000 million
operations per second.

But the important goal is to deploy a reliable system rapidly, of a type that could be upgraded
continuously and that would realize benefits in savings and safety even during early
deployment.

It is clear that the United States now has the capability to design such a system, but it is
unclear that it would be able to decide on such a system. The air force secretary recently informed
the acting undersecretary of defense for acquisition that the first three space-based infrared
satellites (SBIRS-high) won’t be launched until 2009–seven years after their original launch
date–and that the cost is projected to exceed $11 billion, considerably more than the 1996
projection of $2.5 billion. The SBIRS-high system would replace the constellation of “Defense
Support Program” satellites that have operated since the 1970s to provide warning within less than
a minute of any significant ballistic missile launch anywhere on the Earth’s surface.

Perhaps a country with a less evolved bureaucracy might successfully design and deploy such a
system to provide worldwide air traffic control services. With a 10-year time to full operation, if
we start now, it might be operational in 2017–at least a third of a century after it was
technically feasible.