By Stars, Beacons, and Satellites
The lost art-and intimidating science-of aerial navigation.
- By Peter Garrison
- Air & Space magazine, March 2006
(Page 3 of 6)
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 In a dusty carton of outdated charts I find only one—a 1969 El Paso sectional—that shows a four-course range.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.