Science: Which Way to the Airport?

• Share

That deepening problem of modern times—giant airliners swooping in on airports thick with ever-increasing traffic—sat like a brooding presence last week at the meeting of the International Civil Aviation Organization at Montreal. The conference's purpose: to select a common system of worldwide air navigation.

The problem should be purely technical. But commercial competition and nationalistic pride confuse the issue. The British press has been loud in defense of the British Decca navigation system. Cried the Daily Express: "The Americans are pushing their own system, acknowledged to be less effective, but with big dollar investments behind it." Though the U.S. press has paid little attention, U.S., Government and commercial agencies are propagandizing vigorously in favor of the U.S. navigation system VOR/DMET.

Bearing & Distance. VOR/DMET and Decca are radically different. VOR/DMET* gives the distance and direction from each of many individual stations. The navigator can tune to the frequency of a VOR/DMET station and see his compass bearing from that station appear in degrees on a dial. Then he sends a signal to the station, which replies by telling him his distance from it in nautical miles. By plotting the bearing and measuring off this distance on his chart, he can pinpoint his airplane's position and set his course accordingly.

So far, comparatively few airplanes or stations have the full distance-measuring equipment. But a navigator or pilot can get a fix by tuning in two stations and getting his bearing from each. His position is the point where the two bearing lines cross on the chart. VOR/DMET uses very high frequency radio waves, which are seldom bothered by static from thunderstorms. Disadvantage is that high frequency waves are line-of-sight (like those used for TV), and therefore stop at the horizon. Airplanes flying above 20,000 ft. can detect them 200 miles away. But for low-flying airplanes and helicopters, their range may be only a few miles, hence the need for many stations in a VOR/DMET system.

Master & Slaves. Decca, developed mostly in Britain (but invented by William O'Brien, a U.S. engineer), is a "hyperbolic" system that uses groups of four stations. One of them is a master, the others "slaves" arranged around it 60-100 miles apart. The Decca receiver is set to receive the waves of all three slave stations, and measure the microseconds of time they take to reach it. When the waves from two stations arrive at the same time, the airplane must be at the same distance from both stations.

A line can be drawn on a chart to show all such positions, other lines to show places at unequal distances (half as far, 0.8 as far, etc.). The receiver simultaneously measures the distance from the third slave station, and this information generates other theoretical lines on the chart. If the airplane is on two lines of different sets, it must be at their point of intersection.