Science: Errors in the Air

The Air Force radar operator at Goose Bay, Labrador, last week was watching bright blips drifting across his radarscope as he traced the track of airplanes approaching from Europe. The planes seemed to be scattered all over the sky, sometimes as far as 150 miles off course. Only when the planes came within reach of land radio guidance did the blips slant crabwise back to their proper courses.

The dance of the drifting blips was impressive proof that the problem of long-range aircraft navigation has yet to be licked. The most spectacular new guidance systems still strain to keep up with the swiftest new planes.

Clinging Twilight. The basic navigation tool is still the time-honored sextant, with which a navigator shoots the stars (or planets, sun or moon) to fix his plane’s position above the surface of the spinning earth. Sextants have been vastly improved since the days of sailing ships, and a competent navigator can make a fix that is accurate to within ten miles. If weather permits, he takes about five fixes during a transatlantic crossing.

Weather does not always favor the celestial navigator. Far up where modern jets fly (up to 40,000 ft.), heavy clouds are rare, and the brighter celestial bodies generally shine through thin, high cirrus clouds. But at twilight, when the sun drops just under the horizon, there are anxious stretches when a navigator can spot no stars against a bright sky lit from below. If he is heading eastward, he soon flies into darkness, and his guiding stars reappear. But fast jets almost keep pace with the sun, and on westward flights the baffling, starless twilight may last for several hours.

The familiar magnetic compass is another source of trouble; it is unreliable in the northern latitudes near the magnetic pole, and most North Atlantic flights are close to the top of the world. Gyrocompasses have a different affliction: they drift slowly from their true reading and require continual resetting. An error of 3° is not uncommon. Uncorrected, it can carry a 550-m.p.h. jetliner 28 miles off course in a single hour, slanting the course dangerously close to the track of other planes.

The cockpits of modern jet aircraft now bristle with a host of complicated electronic gadgets that have been designed to give the harried navigator a hand. The most important:

> VOR (Very High-Frequency Omni-Range), which requires land-based transmitters, can guide a transatlantic liner for only 150 miles over the ocean. VOR is useful chiefly just after take-off and just before landing.

> LOR AN (Long-Range Navigation) can give a navigator an accurate, medium-range fix. But despite the promise of its name, LORAN does not reach dependably to the middle of the Atlantic, and strong “atmospherics” (static) can put it out of action.

Radar Wrinkles. The most promising newcomer among aircraft navigation instruments is Doppler radar. The name honors the 19th century Physicist Christian Doppler, who discovered that sound waves transmitted from a moving object change in frequency. This Doppler effect applies equally to radio waves.

Navigation Dopplers shoot three or four slim radar beams downward from an airplane. Reflected from land or sea, the radar pulses from each beam are changed slightly in frequency by the airplane’s motion, and complicated computers translate the change into a continuing record of the swift ship’s forward progress and sideward drift. Doppler navigation works best over land, where any wrinkle makes a fine radar reflector. It also works well over the sea when the water is roiled by waves. Day or night, it can scan the earth’s surface through the densest cloud deck. Military aircraft use Doppler radar extensively since they would not have ground radio guidance near enemy territory. Next summer the U.S. Federal Aviation Agency will station two radar ships in the Atlantic to watch every passing TWA airliner to compare Doppler-guided test track with true course.

But Doppler calculations are subject to cumulative errors that grow larger the longer they go unchecked. During a long series of tests last spring, one-quarter of the aircraft observed flew more than 40 miles away from the positions recorded by their Dopplers. One of them went 70 miles astray. Sometimes the Dopplers failed entirely; often the dual computers of a single Doppler installation gave individual readings many miles apart.

In the vast air room over the North Atlantic navigation errors balance out, and the airlines’ safety record has been excellent. But as the transocean lanes fill up with more and faster aircraft, both the human navigators and their nonhuman helpers will have to learn how to stay more tightly on their assigned courses.