• Flight Today

By Stars, Beacons, and Satellites

The lost art-and intimidating science-of aerial navigation.

• By Peter Garrison
• Air & Space Magazine, March 01, 2006

One day this will be an airborne life. But by then men will have forgotten how to fly; they will be passengers on machines whose conductors are carefully promoted to a familiarity with labeled buttons, and in whose minds the knowledge of the sky and the wind and the way of the weather will be as extraneous as passing fiction.

—Beryl Markham, West With the Night

To be lost is unpleasant; to be lost at sea or in the sky is particularly so, because the very insubstantiality of the environment infuses it with latent menace. Having been lost, to be found again is inexpressibly exhilarating, almost a rebirth. These experiences, which touch travelers deeply perhaps because they recapitulate the terrors of childhood, may make sailors and airmen shudder in recollection, but at the same time they are an inextricable part of the adventurous life, and so they are in a way precious. All the more now, because the Global Positioning System, the satellite-based means of navigation, is fast making them extinct.

The primitive foundations of navigation are pilotage and dead reckoning. Pilotage is navigation by landmark. The earliest fliers followed rivers and roads from town to town, or, like medieval pilgrims, steered for the nearest steeple. Even the sky itself is a landscape to which a sufficiently sensitive intellect may become attuned: Polynesian “wayfinders” steered their canoes for hundreds of miles with mental sextants and visceral clocks.

Dead reckoning—some say the term comes from “deduced”—means inferring your present position from a knowledge of how long you’ve traveled, at what speed, and in what direction, from your last. This was the method used by ships before the invention of the chronometer made scientific celestial navigation possible. Dead reckoning is subject to the uncertainty of drift—the unknown component of motion owing to wind or ocean currents. The method sometimes works well—Charles Lindbergh hit Ireland right where he intended to after 20 hours over the Atlantic—but it can also fail, as Lindbergh himself later would, by errors of hundreds of miles.

Nor Dark of Night
In 1918 the U.S. Postal Service, with unbureaucratic daring, inaugurated airmail between Washington, D.C., and New York in Army-surplus Curtiss JN-4D Jennies. Because the distance was so short, the airplane’s superior speed gave it only a negligible advantage over ground transportation.

Along the New York-San Francisco route, launched in 1920, there was greater potential for saving time, but darkness was the deal-breaker. When the sun went down, pilots handed their cargo off to trains. President Warren Harding, skeptical of what he perceived to be a costly and inefficient way of delivering mail, threatened to cut off funding.

And so in the dead of the winter of 1921—the worst possible time—the Postal Service staged a bold and very nearly quixotic demonstration of fast transcontinental airmail delivery. Four single-engine, open-cockpit de Havilland biplanes took off, two from New York and two from San Francisco. One eastbound pilot was killed in a takeoff accident in Nevada, and the two westbound airplanes, halted by a snowstorm near Chicago, relinquished their cargo to trains.

Pilot Jack Knight saved airmail. Meeting the surviving eastbound biplane at North Platte, Nebraska, Knight took off in darkness. Guided by bonfires and burning oil drums that had been lit by postal employees and helpful farmers, he flew all night in bitter cold, landing to refuel at Omaha and Iowa City, and reached Chicago in the morning. A relay pilot completed the trip to New York.

Having covered 830 miles in nine hours, Knight proved that the airmail could move even in darkness and bad weather. Lionized in the press as a hero (his name probably helped), he downplayed the difficulties he had faced, though he did concede that “if you ever want to worry your head, just try to find Iowa City on a dark night with a good snow and fog hanging around.” The trail of fire that Knight followed was the beginning of the nation’s first airway system: Funded after all, the Postal Service was soon erecting electric beacons to guide night fliers along its routes.

The Department of Commerce took over responsibility for the system in 1926 and eventually expanded it to 18,000 miles of airways with more than 1,500 beacons. Commerce also produced a series of aeronautical strip charts, the first of which, with topographic, airway, and beacon information for the route from Kansas City, Missouri, to Moline, Illinois, came out in 1927, the year of Lindbergh’s flight from New York to Paris. Only three years later, the first aeronautical sectional chart, depicting the Chicago area on a scale of 1:500,000, appeared. Rudimentary instrument approach charts were printed on the back. Aerial navigation, no longer an awkward improvisation upon sea and surface methods, had come into its own.

Home on the Range
It was evident that airplanes needed radio, both for communication between pilots and the ground and for defining airways that could be followed in bad weather. The 1930s saw the arrival of non-directional beacons and four-course ranges. NDBs mostly operated in the low-frequency band, between 170 and 600 kilohertz, broadcasting a three-letter Morse code identifier. A loop antenna on the airplane rotated (originally, the pilot or navigator turned a crank; later, rotation was made automatic), and the strength of the signal it received depended on the angle of the loop to the beacon. In the automatic version, the Automatic Direction Finder, a needle on the instrument panel showed the direction to the beacon.

Many airplanes still have these because NDBs, being the cheapest kind of ground navigational aids to install and maintain, are still in widespread use. The ADF operates not only in the low-frequency band but up through AM broadcast frequencies as well, so pilots can fly toward powerful broadcast stations in distant cities and, what they sometimes find equally important, entertain themselves by listening to the radio as well.

The four-course or Adcock ranges were low-frequency beacons with four directional antennas, each transmitting a Morse code signal in a lobe-shaped pattern over roughly a quarter of the compass rose. One antenna repeated the letter A—dot dash—while its neighbors had N—dash dot, and the fourth, opposite the first, had A again. Where neighboring quadrants overlapped, the A and N added up to a continuous tone called a course. If the range was near an airport, as most were, one of the courses led to a runway.

A pilot approaching the station likely heard one letter or the other. If he was unsure of his position, he flew until he crossed a course. He then executed a series of turns designed to determine which of the four courses it was, constantly adjusting the radio volume for greatest sensitivity to tell-tale changes in signal strength. He could now fly to the “cone of silence” over the range, track outbound along the approach course for 10 miles or so, then turn and begin his descent to the airport. A skilled pilot flew the “feather edge” of the course, where the faint clicking of a fragmentary A or N could be heard emerging from the continuous hum like a loose thread from a weave.

Over the years we have moved gradually away from aural indications and toward visual ones, and so the difficulty of steering an airplane by varying tones in a headset, and of judging direction by the swelling or fading of a scratchy signal, seems greater to us today than it apparently did to the DC-3 pilots of the 1930s. Author Ernest K. Gann, who omitted no tribulation of airline flying from his classic autobiography Fate Is the Hunter, passed over bad-weather range approaches almost without comment.

Just after World War II, a new type of four-course range, the VAR, or visual-aural range, appeared, broadcasting on static-resistant frequencies above the AM band. It was “visual” in that, in place of the sounds in the headset, a panel instrument presented the courses as a needle swinging between yellow and blue sectors. But it came too late; the four-course ranges were about to go the way of the open-cockpit biplane.

When I learned to fly in 1961, four-course ranges were still depicted on sectional charts, and I studied them before taking the written test for my instrument rating. Turned out I had wasted my time; the test bypassed the subject completely. In a dusty carton of outdated charts I find only one—a 1969 El Paso sectional—that shows a four-course range. It’s at Chihuahua, Mexico.

The war had given impetus to the development of new navigation systems, as it had driven all kinds of other aeronautical technologies. Bombers above clouds seeking targets below required some web upon which they could crawl to a given intersection. Methods had to be accurate to within a few hundred feet and resistant to jamming.

Several systems used a “master” and two or more “slave” transmitters, which created families of intersecting hyperbolic lines of position. Most of these systems were decommissioned at the end of the war, but one, Consolan (Consolidated Low or Medium Frequency Long-Range Aid to Navigation), was still broadcasting over the north Atlantic in the 1970s, and Loran still serves today (opposite). With these systems, latitude and longitude are determined from the different arrival times of sychronized signals from two or more fixed transmitters.

Long overwater flights still relied heavily on dead reckoning because celestial fixes were not always available. Dead reckoning required a knowledge of drift. Sometimes drift could be observed; optical drift meters enabled navigators to measure the angle at which objects on the ground moved past the airplane. With a layer of clouds below, however, or over
water driven by the wind, drift could not be reliably measured. Navigators turned to the known relationship between isobars—lines of equal barometric pressure—and wind. The wind blows nearly parallel to isobars, and its speed is greater where they are closer together—that is, in areas where pressure is changing rapidly. Navigators could measure changes in barometric pressure along their route by comparing their pressure altimeters with a radar altimeter that gave true height above sea level. From the rate of change of pressure, they could obtain an accurate wind component.

It was possible, merely by knowing the barometric pressures at the starting point and at the destination, to select a single heading to be flown for the entire route, although it was more usual to divvide the route into shorter segments. The airplane might be blown to one side of the course or the other by varying winds along the way, but in the end the cross-track errors would cancel one another and the airplane would arrive on target. Pressure-pattern navigation, together with inertial and celestial, remained an important part of the navigator’s toolkit for flights of more than 350 miles into the 1970s.

For intercontinental airliners and for many military airplanes, inertial—automated dead reckoning—was the principal means of navigation. Gyroscopes and accelerometers measure the motions of the airplane with great precision, constantly integrating the data to determine how far it has traveled and in what direction. The equipment is extremely delicate, precise, and costly, because the allowable margins of error are so small; but new electronic motion-sensing devices, built without moving parts, may yet make inertial navigational gear commonplace.

Steadfast as Thou Art
The most intellectually challenging and aesthetically satisfying form of navigation is celestial—navigating by the stars. To locate yourself by the very framework of the universe—what could be more Godlike? Unfortunately, celestial navigation is a most complicated and cumbersome technique.

The principle, however, is simple. The positions of the sun, moon, and a number of conspicuous stars are tabulated in books called ephemerides (which, by the way, are used by astrologers as well as navigators). The “position” of a heavenly body at a given moment is the point on Earth at which it is exactly overhead—the “substellar point.” Seen from any other point, it is at some angle to the vertical. The observer must be located somewhere on a “circle of position” whose radius is that same angle upon Earth’s circumference, and whose center is the substellar point. Knowing approximately where he is from dead reckoning, the navigator obtains local segments of those circles for three bodies in different areas of the sky, and approximates them on a chart with straight lines. These intersect to form a triangle that represents the observer’s approximate position (see “Celestial Navigation,” How Things Work, Oct./Nov. 2001).

During the 1930s and ’40s, many aircraft that flew transoceanic routes had in their roofs a hemispherical plastic bubble called an astrodome. The navigator would stand with his head in the astrodome to take star sights. Since the horizon was seldom clearly visible from the air, he used a variation of the marine sextant called a bubble octant, which had a bubble level to identify the vertical. A good navigator can take the necessary sights, perform the calculations, and plot the results in 10 minutes, achieving a tolerance of five miles or so.

The astrodome was eventually replaced in pressurized airplanes by a small hole in the cockpit ceiling through which the navigator stuck a periscope; Douglas DC-8s, Boeing 707s, and even early Boeing 747s were still equipped with one. Crews found unexpected uses for the hole, from which air rushed when it was opened. Swissair DC-8 crews called it the “banana hole,” because after eating a banana you could allow it to suck up the peel. U.S. Air Force C-141 crews linked oxygen hoses together, held one end to the hole, and used the other to vacuum the cockpit.

Today, celestial navigation is still the crux of the Federal Aviation Administration navigator’s rating. Only three FAA examiners can administer the test, a two-day ordeal, and applicants are few. The Air Force, however, still trains celestial navigators in large numbers, and refueling aircraft are equipped with electronic star finders for the apocalyptic day that the Global Positioning System satellites fall silent.

“Wink…VOR”
The acronym “VOR” originally stood for visual omnidirectional range, to distinguish such ranges from the old aural four-course variety. As the original significance of “visual” faded from the consciousness of new generations of pilots, the “V” was said to stand for “very high frequency,” and that is the explanation usually given today.

VORs were a major advance. They were much more precise than non-directional beacons had ever been, and immune to most atmospheric static and distortion. But their presentation on the instrument panel was somewhat unintuitive to minds adapted to the automatic direction finder. The instrument was called an OBS, for omni-range bearing selector. Originally, most were colored blue and yellow, in imitation of the VAR four-course range displays. A needle swung from side to side as you turned a knob to rotate a compass rose. When the needle was centered, the position of the compass rose told you your bearing to or from the station. A little triangular pointer or “flag” distinguished between “to” and “from” bearings.

Unlike the NDBs, which pointed a finger, human-like, at the transmitting station (“It’s over there!”), VORs gave you a number, an abstraction that required you to refer to a map to convert the number to a line of position, called a radial. Most aircraft had two VORs, and an exact position could be obtained by the intersection of two radials at a reasonably large angle to each other or, alternatively, by a radial and a distance provided by a radio ranging gadget called distance measuring equipment, or DME.

By the mid-1950s the country was thickly dotted with VOR and DME transmitters—low circular structures, each with a slender truncated cone rising from the center. Dead reckoning sank into disuse, surviving only in questions on the private pilot written test, as pilots came to depend on VORs as stepping stones from one place to another.

VORs made it unnecessary to monitor the wind. With non-directional beacons, if the pilot did not include drift in his heading, a crosswind would push the airplane off track. The automatic direction finder needle would swing progressively to the side as the pilot, continually adjusting his heading, flew an unintentional curved line toward the station. VOR radials are fixed tracks in space; a pilot automatically compensates for wind drift if he merely keeps the OBS needle centered.

One of the unintended side effects of the VOR network was to superimpose upon the familiar map of the United States one that accorded to little-known places like Wink, Texas, or Hector, California, the same familiarity as St. Louis and Indianapolis. It drilled their names into the consciousness of pilots who, droning along, often at night, over a featureless landscape, fastened their attention upon, for lack of anything better, a Morse code identifier and a faint, scratchy tenor intoning, over and over, “Wink...VOR...”

I Once Was Lost, But Now Am Found
Each advance in navigational technique brought an improvement in accuracy, reliability, or ease of use. Each was in one way or another a simplification, but it also required new learning and new insights from its users. And then came GPS, the satellite-based system, originally intended for military tracking and targeting, that identified the location of anything on or near Earth within a few feet.

GPS changed everything. It was GPS—or rather the boundless varieties of digital processing of raw GPS data—that brought navigation to maturity, and the great historical traditions of navigation to their knees. A drug irresistible to even the fiercest Luddite, GPS at one infantalizes and deifies us. With GPS there are no landmarks, no beacons, no airways, except as relics of earlier times. There is only the surface of the planet. GPS makes skill, intuition, and judgment unnecessary. Navigation, that great and noble art, whose traditions reach back into the darkness of prehistory, has degenerated into a computer game. Orientation, sense of direction, dead reckoning, line of position, pilotage, weather sense, drift, heading, track, course, VOR, NDB, precession, magnetic variation, estimation, latitude, longitude, azimuth, elevation, lost, found—relics all.

Sidebar: Where the Beacons Beckon

We were barely off the runway at Helena, Montana, when I caught sight of the first beacon winking from the ridge ahead. The sun had set an hour ago, and the mountains stood in inky silhouette against the pale western sky.

We crept westward against a smooth, steady headwind that cut our groundspeed to 100 mph. The skyshine paled and vanished. The darkness was complete now: Black tree-cloaked mountains below us, black star-flecked sky above. From our cruising altitude, 8,500 feet, just above the highest mountaintops, we could see two or three beacons at a time, stretching out ahead of us and curving gradually to the right. The first was MacDonald Pass, then Avon; there was a gap at the sparsely lighted town of Drummond, then the chain picked up again. Five more beacons would wink into life ahead of us, spell out their identifiers in Morse code with a fainter red light, and slowly pass beneath before we emerged from the mountains at Mullan Pass, just east of Coeur d’Alene, Idaho.

The Montana airway beacons are the last survivors of a great system of more than 1,500 that once dotted the country. Initiated by the Postal Service, which realized that airmail could offer no speed advantage if airplanes were idle during the night, the beacon system grew from a 1919 experiment with strings of bonfires to guide airmail pilots across the Great Plains into an 18,000-mile network of federal airways managed, after 1926, by the Department of Commerce. It survived into the 1970s, though by then few pilots were aware of it. When the Federal Aviation Administration decommissioned the beacons, Montana, which had 39 of them marking routes through the mountains, took over those within its borders. Ultimately, it kept 17 operating in its mountainous western half, linking Coeur d’Alene, Missoula, Helena, Great Falls, and Butte.

The French novelist Marcel Proust, writing about a midnight stroll through the streets of Paris when it was being bombarded by the Germans in World War I, describes the reassuring feeling of being watched over by a benevolent power that the defensive searchlights crisscrossing the sky gave him. I felt the same way about the beacons. The guidance of radio signals is cold and abstract, but a light on a distant mountain, winking rhythmically, emits a personal, human warmth. “This way!” it seems to say. “I am here.”

—Peter Garrison