Which Way to the Airport?

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.

Moving Chart. No human navigator should be expected to make these comparisons himself, so the Decca system uses a computer that does it for him. A chart of the area being traversed unrolls slowly, its speed controlled by the forward motion of the airplane, as measured by the Decca signals. At the same time a pen, also controlled by the signals, draws a line on the moving chart. The point of the pen represents the airplane's position, and the line shows its course across the terrain represented by the chart. Thus, Decca argues, the pilot can see at a glance where he is and where he has been.

Decca uses rather long radio waves, which, though sometimes subject to interference static, have a longer range than the very high frequency waves of the U.S. system. Therefore, Decca can be used over its full range (more than 200 miles) by helicopters buzzing at treetop level. It is also more fine-grained than VOR/DMET, giving the airplane's position within narrower limits (e.g., it can show which runway a plane is landing on). With the addition of another receiver, the Decca plotter can be hooked into Dectra. a long-distance system that has a range of more than 1,000 miles and is used to guide British airliners across the North Atlantic.

The official British position is that Decca, which covers most of Western Europe and Eastern Canada, should be used in places where traffic is heavy, and busy airports are close together. A demonstration system has been set up in the New York area, but it is not official for U.S. planes.

The U.S. argues that VOR/DMET already handles the heavy U.S. air traffic very well, and that planned improvements will make it even better. VOR stations are in use in most of the non-Soviet world, and more than 60,000 civilian aircraft are equipped to navigate by them--though, as yet, only a handful also have DMET. U.S. experts are urging at Montreal that VOR/DMET be made the recognized worldwide system, even if Decca gets a limited assignment for guiding helicopters and low-flying airplanes.

* The letters mean Very high frequency Omni-Range/Distance Measuring Equipment compatible with Tacan, the U.S. military control system.

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