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.
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.