By Stars, Beacons, and Satellites
The lost art-and intimidating science-of aerial navigation.
- By Peter Garrison
- Air & Space magazine, March 2006
(Page 4 of 6)
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.
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.