Friend or Foe
Like so many other technologies, RADio Detection and Ranging (RADAR) was a wartime invention. Earliest practical radars date back to the beginning of World War II. Bursts of radio energy reach out through the sky at the speed of light, invisibly illuminating the metal skin of distant aircraft. A sensitive receiver transforms the faint echoes into glowing "blips" -- intensified blobs that linger on a phosphorescent screen, left in behind by a bright, sweep-second hand in overdrive.
The initial intent was belligerent: to detect and destroy hostile aircraft.
A decade later, radars (still called "radar sets") had become a vital instrument of air traffic control. However uncontentious the purpose, the technological principles of civilian radar had not changed: ground-based beams of pulses reflected -- passively -- aloft. It was as if the airplane, once the wary victim of radar's probings at war, was merely a frigid dummy, indifferent to peacetime's admirable motivations.
This disparity, obvious in retrospect, jolted me one Friday in late 1959 following a stormy meeting at NAFEC. I remembered something.
"IFF!" I thought to myself [exclamation point in the original]. Identification Friend or Foe. I shall never forget the moment. A kind of re-discovery. Psychologists call it "bisociation." Had no one else seen the relationship? If not, the explanation is simple. I might have been the only one at NAFEC with a technical background that included IFF.
My first job out of college in 1955 involved me in airborne radar fire control systems. For the U.S., it was still the early days of the guided missile age. ICBM was an unknown acronym. Our project was to equip airplanes to shoot down incoming B47-class bombers. The track-while-scan problem for the military application was different from that faced by air traffic control -- but also kind of the same.
That night, on my way home from NAFEC, I refreshed my memory about how the IFF subsystem had been implemented. I rejoiced in my singular good fortune. I had been at the right place at the right time. But, God! -- all that IFF stuff was classified. Yet, if the FAA had the technology in the works, surely we would have been planning it into the man-machine simulation! Most likely, the Federal righthand simple does not know what the lefthand doeth. I thought of the Grand Canyon. The issue is safety. There were countless innocent lives to be saved in the coming years. In the enterprise of overcoming human fallibility, the promise of technology is great.
Any inventor knows, "the obvious is not always apparent." The concept of IFF was secret. But shit, it was obvious.
Radars are indiscriminate. They reflect their energy off of any metallic object. In the combat situation how do you tell the good guys from the bad guys? -- the friends from the foes? You don't want to go up there and shoot down the wrong airplanes.
The range of our new-fangled guided missiles meant that the interceptor pilot would be unable to identify the target visually -- even if the engagement occured in daytime and in clear weather, conditions we could hardly expect an attacking enemy to select. Our missiles were designed to deliver explosive ordnance by homing in on the same radar echoes used for selecting and tracking the target. Thus did Identification Friend or Foe (IFF) become a hot military project.
An auxiliary transmitter aboard the interceptor put out a separate radio burst relative to the "main bang" (radar jargon for the pulse of radar energy). This "interrogation" signal preceded the radar signal by a fixed time interval of a few microseconds. All U.S. military aircraft were equipped with a receiver tuned to the interrogation signal and a transmitter that issued a reply.
The combination, quite literally contained in a black box, was called a "transponder."
The reply signal was delayed inside the transponder by a pre-set interval based on the "code-of-the-day," which, of course, was confidential. Upon reception on the ground, the reply would be further delayed by a coded interval to produce a "blanking signal." If all the microseconds added up to the correct amount, the radar blip for the friendly aircraft would disappear from the radarscope. So much for the military version.
For air traffic control, the last thing you want is for a blip to be blanked. Instead you want to associate with each radar echo the vital tracking data that radar itself cannot provide. The problem was different in degree, too. Electronically classifying a few combat aircraft into two categories -- friend or foe -- is one thing; identifying a sky full of airliners, one by one, is another.
In the civilian sky, everyone is a friend. And in another sense, everyone is a foe.
That evening in the winter of 1959, I sat down in my rented house in Somers Point, New Jersy, and drew a block diagram in my engineering notebook.
The ATC version of the transponder would have to shift its reply over a range of discrete steps. They would not be secret, of course. The intention is to provide the ground equipment with the ability to measure the interval automatically in order to complete each identification.
Alas, the round-trip speed of radio waves is 164 yards per microsecond. It did not take a genius to decide that "pulse-separation modulation," was not up to the task. The computer's "binary" system was far more efficient. With just 12 discrete bits, you can make up 4,096 different codes, more than enough to identify all the airliners aloft at that time. This was a significant distinction, possibly patentable, from the IFF -- all the more reason to write everything down, so as not to catch hell from the company attorneys about "interference claims" later on.
The transponder-assisted system for track-while-scan took shape in a matter of hours. There were times when I could not draw and write fast enough. I trembled with excitement and the utter rightness of my private effort. Forgoing sleep, forsaking family, I worked at the kitchen table through the weekend, cerebral fires burning. By Monday, the basic specifications were ready to discuss. But with whom?
That point marked the beginning of the hard part.
Telemetry even in 1960 was old art. Sputnik, don't forget. Onboard data of any kind could in theory be sent from airliners to ground computers for processing -- fuel remaining, cockpit temperature, number of passengers on board, galley inventory. For flight safety, of course, the data with highest priority would be altitude.
The details of building an electronically encoding altimeter was beyond my own craft skills, but when I began to disclose the transponder to my colleagues the next week, Al Jackson, our group leader, and two others suddenly and independently invented altitude telemetry several charts ahead of my presentation -- the sure sign of a meritorious idea.
My notes were scant for the UHF radio sections of the transponder, too. Designing the logic of the airborne equipment, though, would be -- as we would say decades later -- a "slam-dunk."
Ah, but the transponder would be expensive, and that worried me. With the electronic technology of the fifties, the box required one 12AT7 (a vacuum tube about the size of your thumb) per bit of register storage. I expected the FAA would encounter resistance to any regulation that mandated the installation of a costly accessory -- especially in private aircraft.
Unlike other instruments on the control panel, the transponder would not show the pilot anything. Oh, how badly did I underestimate the significance of this one factor! For example, a "competitor for panel space," so to speak, was soon to come along. Distance measuring equipment -- DME, it is called. DME gives the pilot a direct read-out of distance to an upcoming checkpoint -- plus ground-speed and estimated time-enroute. Hot shit. The transponder only has a panel light, which flickers uselessly with each interrogation. Ho-hum.
Enlightened self-interest is undeniably a weaker impulse than self-interest.
With transponders aloft, however, the ground-based automation of track-while-scan would become altogether straightforward. ATC's computers had everything they needed: azimuth and range -- and now altitude along with unambiguous identification of each blip. Integrated with their respective time-of-day clocks, the machines in the radar room could automatically discern directions of flight and compute aircraft speeds. Most important, we now had the vital data to project and resolve conflicts.
In a fanciful moment, I even jotted some notes about storing within the ATC computer memory a "field of terrain elevations" (the term "database" was not then in use), by means of which an aircraft's potential conflict with undulations in the surface of the earth could be detected.
All these processing tasks demanded much of the computer, but I did not worry about that. However massive the hardware might need to be back in the sixties, it was firmly on the ground. As for cost -- what is the worth of 128 lives!
The biggest technological problem was the controller interface. My notebook was filled with alternative designs for keyboards and displays. First there were separate consoles, which would sit adjacent to the radarscopes, much like the trays of flight progress strips. That was far from ideal. They required the controllers to look away from their screens constantly.
To make altitude-reporting transponders practical, the displays needed a technology we did not yet have: video character generators. I drafted a proposal for the requisite development program and another for improvements in the "scan converters." NAFEC's man-machine simulator we were building already applied the latter to transform the radar sweep into television-like (raster) format. Perfecting these essential subsystems would give the FAA the ability to superimpose the transponder data on the radar screen alongside the blips.
A long road lay ahead. My first objective was to gain the support of my own company.
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