Figure 13. Schematic view of radar targets on successive sweeps of the antenna.
Radar is not a TV camera or a photographic lens. It does not, at least at present, produce a picture of the physical appearance, shape, size, or color of the thing it detects. The scope shows only tiny spots of light on the flat surface of a screen. A pointer something like a clock hand continually sweeps around the dial at a given speed. A complete rotation may take from two to fifteen seconds, depending on the type of the set. This sweep hand keeps pace with the rotation of the radar antenna as it scans the sky by sending out radio pulses. When they encounter a solid object, they bounce off and return to the set as echoes which show as “blips,” or spots of light, on the radarscope. The operator must interpret these spots and try to identify them as planes, helicopters, balloons, ships, mountains, clouds, birds, storms, hurricanes, or phantom echoes of various kinds. Safe commercial flying depends on the accuracy of these identifications, as does the security of the country in periods of international tension.
Radar only reports. It does not interpret. If the sweep hand on successive rotations shows a spot of light apparently moving from position A to position B, to C, to D, the operator generally concludes that the blips represent a single object that is moving at a certain speed in a certain direction (see [Figure 13]). If successive sweeps show a spot of light that remains at position A, he usually concludes that it represents a stationary object. If the blip moves a very great distance in the interval between two sweeps or seems to jump erratically from one position to another, an amateur might interpret it as a spacecraft flying at incredible velocity—a flying saucer. But an expert would probably conclude, especially under certain weather conditions, that the scope was picking up echoes from two or more separate objects, one reflecting briefly at position A, another at position B, and so on.
The Principle of Radar
Radar is an electronic assembly far too complex for detailed description here, but its basic principle is simple. It is merely an echo machine that reflects radio waves instead of sound waves. To illustrate by a rough analogy, let us imagine that a man is standing in the middle of an open field on a very dark night. He wants to find out something of the contours of the surrounding country but his only tools are a compass, a watch with luminous dial and hands, and a large megaphone. He raises the megaphone to his lips, points it directly north, and gives a sharp and piercing call: “Hi!” He now cups his hand to his ear and listens for an echo. Hearing no reply, he deduces that in the north there are no hills, tall buildings, or other obstructions that might have produced an echo.
Changing his position, he turns to the east and tries the experiment again. After an interval his call returns as a faint echo: “Hi!” The time elapsed between call and echo, according to his watch, is ten seconds. His call has taken five seconds to reach the object and five seconds more to return. Since he knows that sound travels at the rate of about 1000 feet a second, he deduces that an obstruction lies in the east, about 5000 feet away. Slowly changing position, he repeats his call at various points around the compass. Some echoes take longer to return than others, indicating more distant objects. Other echoes come back in a fraction of a second, showing an object very close. Thus he gradually constructs a mental map of the surrounding terrain.
Radar detects and locates objects in a similar way, by reflecting sharp pulses of radio waves. But spurious echoes, which sometimes deceive the operator, can also appear on the scope. These “anomalous” or abnormal returns may have one of several causes, including the nature of the radar mechanism itself. To help explain this, let us go back to our analogy of the man in the open field. Let us suppose that the man has mechanized his device. To ease the strain on his vocal cords, he has built a megaphone with a record-playing device. The megaphone rotates automatically and sends out a recorded “Hi!” once every twenty seconds, as regular as clockwork. To increase the sensitivity of his hearing, he wears ear trumpets that point in the same direction as the megaphone. This procedure is more effective than cupping his ears and eliminates some of the extraneous noise that might come in from the rear and the sides.
With this improved equipment the man now repeats his experiment. As before, he gets no signal from the north. When he turns to the east he gets an echo after ten seconds, just as he did during his first experiment. As he continues to turn slowly, like a minute hand on a clock dial, he mentally maps the positions of the echoes as distances along the hand from the center of the dial, and compares this new map with the crude one he constructed earlier. Basically the two agree.
But wait! From the southwest he hears a new echo that did not occur in his earlier experiment. It returns after two seconds and thus apparently comes from an obstruction 1000 feet away. Puzzled, the man decides to walk toward the object and check his observation. After he has covered half the distance he stops, sends out a call, and listens for the echo. The indicated distance to the echo-producing object is now 500 feet, just as he calculated. And so he goes on, checking at intervals. When he has covered 990 feet he knows that he should reach the obstruction at any moment and to avoid colliding with it in the darkness he proceeds with extreme caution—995, 996, 997, 998, 999 feet. He puts out his hand, expecting to touch a building or a stone wall, and warily takes the last step. But he finds no structure of any kind, merely level ground. And at the same moment he finds to his astonishment that he can no longer detect the echoes he had been following. What has happened? Has his equipment been malfunctioning? Or was the unknown structure perhaps a vehicle from outer space that waited until he was practically touching it and then rose silently in an enormous burst of speed and vanished?
The man checks and finds that his equipment is functioning perfectly, since he can still pick up echoes from the terrain he had mapped earlier. He then walks back ten feet and listens once more for an echo from the phantom structure. Again he gets a signal, apparently from an obstruction just ten feet ahead. Has the mysterious object suddenly returned? But how could it have done so without disturbing the atmosphere or making a noise? By this time our man is frightened as well as puzzled, but he boldly decides to make one more experiment. He walks again to the point where the obstruction should be. Signaling again to the southwest, he now gets a faint echo apparently from a distance of 10,000 feet. Tired as he is, he starts walking toward this new obstruction and eventually reaches his goal. He now finds the true source of the returns—a high hill that rises abruptly from the plain. The hill is 10,000 feet away from the position indicated by the original series of echoes, and 11,000 feet away from the place he stood when he first sent out the signals.