Glide Path (Arthur C. Clarke Collection)

by Arthur C. Clarke

  “I’ll be bringing the Air Commodore out in thirty minutes, so make sure that the trucks are properly lined up, then get ‘D’ Flight off the ground. By the way, what’s the Met forecast?”

  “Not too bright. There’s a front going through later this morning, and the weather may turn bad about noon, with heavy rain squalls.”

  “Well, as long as it holds off until we can have a few good runs, that’s all that matters. Oh—one other thing. Get the covers off the antennas, so I can show the Air Commodore how they work. It’s always a good idea to blind the opposition with Science.”

  When Alan reached the trucks, which were positioned alongside Runway 320, he found that the setting-up procedure was already far advanced. He waited until the Corporal mechanic had made the final adjustment, then took over. Only a few weeks ago, the mass of switches, controls, and meters in front of him had been utterly meaningless; now he knew the functions of every one of them, and could find them blindfolded.

  Alan studied the screens intently, identifying the pattern of echoes and making a few turns with the tracking wheels. That blob there on the right—it was a radar reflector he had placed himself, three hundred feet from the runway. He cranked the pointer across to it; where did the meter say it was? Two hundred and ninety feet right-only ten feet out. Fair enough.

  “OK,” he said to the Corporal, who had been hovering anxiously in the background. “Let the operators in, and tell ‘D’ Flight to get airborne.”

  As the mechanic cranked the field telephone to Flying Control, the truck was invaded. First came the three WAAF trackers; though they had brought their knitting, this was not necessarily a vote of no confidence in the serviceability of the equipment. Even on the best of days, there were long waiting periods between approaches when there was nothing else for them to do.

  Next entered the Traffic Directors, both of them sergeants with control-tower training, and then the Flight Lieutenant who was to do the actual talking down. The team was complete.

  It was always fascinating to see how swiftly these six individuals, so different in training, rank, and outlook, merged into one entity—the crew. And the crew itself, by some higher symbiosis, then became part of the complex machine it was tending, yet without any loss of human dignity. There was nothing here of the degrading, mindless repetition that Chaplin had satirized in “Modern Times.” For this work was not mechanical; it demanded great skill and understanding. No two approaches were ever the same, and all involved in the operation, from tracker to controller, had to be ready at an instant’s notice to deal with unexpected developments.

  The tense silence of the darkened truck was broken by the roar of C Charlie getting airborne. A minute later the aircraft’s echo emerged from the amorphous blobs of light at the centers of the radar search screens, as it climbed away from the airfield. “I’ve got him,” said the Traffic Director. Then he pressed his transmitter key and articulated carefully: “Sheepdog calling C Charlie. Continue on course three two zero and climb to two thousand feet.” Sheepdog was the unit’s current call sign; it was changed every few days for security reasons, though it seemed to Alan that any bright German signals officer listening to the talk between ground and aircraft could deduce exactly what was happening, and could even estimate the accuracy of the system. But unless Jerry had developed excellent microwave radar himself, he would find it hard to guess how it was done.

  This was now being explained to Air Commodore Burrows, who was standing in front of the transmitter truck, watching the impressive behavior of the antennas. The wood-and-canvas covers that protected them from the weather and from prying eyes had been removed, and they were thrashing back and forth in the clear light of day.

  The Air Commodore had seen hundreds of radar and radio antennas, but these were quite the oddest he had ever met. Long wooden troughs, their curves coated with reflecting tin foil, they looked like overgrown electric fires. The azimuth array was rocking from right to left like a spectator at a tennis match, while its tall, thin companion, the elevation array, was nodding up and down from earth to sky as its invisible beam searched the heavens. Over in the control van the lines of light on the display tubes were moving in exact synchronism with them, reproducing in miniature the strange world they saw—a world that knew neither night nor day, in which cloud and fog had no existence.

  Deveraux finished his little lecture on the antennas, explaining how they scanned the skies with the narrowest beams yet made by man. Then he led the Air Commodore away from the thrashing antennas and the roaring diesel, into the cloistered calm of the control van.

  The blackout curtains, their hems lined with lead, swung sluggishly aside, then parted to let them through. There was barely room to stand between the wall and the three WAAF trackers crouched over their sloping display panels, but Deveraux was able to crane over the girls’ shoulders and point out the features of the display without interfering with operations.

  They had come at just the right moment, as an aircraft began its approach. There it was—a sharply defined blob of light at the limit of the tube. It was moving even as they watched; with every sweep of the scan it edged a fraction of an inch closer to the glowing maze that marked the airfield. The successive images formed a fading comet tail behind it, drawing the path it was following down the sky.

  Now the creeping echo was only eight miles away. Sometimes it drifted to the left, sometimes to the right, as the pilot changed course according to the Controller’s instructions. And all the while the three WAAF trackers held it transfixed with their shining electronic needles, kept in place by imperceptible movements of their wrists on the handwheels. They seemed quite unaware of the visitors peering over their shoulders; nothing existed in their spheres of consciousness save the echoes crawling toward the end of the runway. Though this was merely a training approach, carried out in broad daylight and good weather conditions by a pilot who could see exactly where he was, the operators could not have been more intent had they been landing a damaged aircraft in thick fog. They took pride in their work, and realized its importance; one day, men’s lives might be in their hands, and when that time came, they would be ready.

  With an abrupt clicking of relays, the radar images suddenly exploded, like pictures painted on the face of an inflating balloon. The final, critical two miles had expanded fivefold, filling the space on the screen originally occupied by ten. Now the last stages of the approach could be watched on a greatly exaggerated scale, so that the slightest deviations from the glide path were clearly visible. On this magnified view, the echoes hopped forward perceptibly between each sweep of the antennas. When the Air Commodore glanced at the elevation display, he could see the echo losing height at what appeared to be a most alarming speed. But the meters indicated that it was still above the glide path, so he must have been misled by the great enlargement of the picture.

  And then, during the last stages of the approach, he noticed a curious phenomenon. At the very bottom of the screen, a ghost echo was matching the movements of the aircraft down the sky. As that descended, so this was creeping upward, apparently from below the level of the ground. The two echoes formed a pair of mirror images, and the Air Commodore realized that he was seeing with his own eyes something that, until now, had been only theoretical knowledge to him. He was watching the earth itself act as a radio mirror.

  It was quite a good one, at this glancing angle. The reflected underground image was almost as bright as the direct one crawling down the glide path, and as they came closer and closer together he saw that this was not merely a pretty proof of the laws of radio propagation. It was also extremely useful, for it showed very precisely the exact position of the ground, as the line bisecting the two images. When these coalesced, the aircraft would have landed.

  But it had no intention of landing on this approach. In the distance the Controller’s voice was saying: “You are now a thousand feet from the end of the runway. Overshoot and change to Channel B.” On the screen, at the same mo
ment, the swiftly moving echo seemed to pull up from the ground; in a matter of seconds it had passed off the edge of the screen, into the blind region behind the precision system’s limited field of view. Simultaneously there was a faintly audible roar beyond the walls of the truck as the plane climbed away from the airfield and prepared to go around again.

  “Well, sir?” said Deveraux, a little too anxiously.

  “Interesting,” was the noncommittal reply. “But can you do it every time?” The Air Commodore waved around the truck, pointing to the banked racks of equipment. “And all this—how serviceable is it? There are far too many operators, and as a pilot I’m not happy about putting myself in the hands of someone sitting safe and snug on the ground.”

  “This is only the prototype,” explained Deveraux patiently. “Professor Schuster’s designed an operational model that will be much simpler and more reliable, and won’t require so large a crew. As for the pilot’s viewpoint—well, they all seem to like it. I suggest you go up presently and try for yourself.”

  “I mean to,” said the Air Commodore grimly. “I’ve seen far too many systems that worked beautifully with pilots that were used to them, but were no good with anyone else. But I’ll watch a few more approaches first.”

  He spent another hour inside the control van, and the dozen approaches he saw were uniformly successful. One by one, B Baker, C Charlie, and S Sugar bisected the runway with monotonous precision.

  The thirteenth approach was just starting when the telephone to Flying Control rang like the voice of doom. Deveraux answered it, and put the receiver back with an expression of annoyance. Through long experience, everyone in the truck knew what had happened.

  “Runway change,” he announced. “The wind’s swung round—we’ve got to move to 270.” He turned to the Air Commodore.

  “As a matter of fact, sir, this is rather handy. If you like, we’ll land one of our aircraft and drive you round to ‘D’ Flight. By the time you’re airborne, we’ll be operating on the new runway.” He gave a slightly deprecatory, think-nothing-of-it kind of cough. “And it will give us a chance,” he added, “to show you just how quickly we can move to a new site when we have to.”

  He did not guess how much the unit’s luck had changed with the changing wind.

  13

  Runway 270 was a particularly easy one to operate from; Alan had lined up the equipment on it dozens of times, and knew its radar pattern by heart. There were the great amoeba-like blobs of Numbers Three and Four hangars; there were the smaller blips of the Liberators parked in the dispersal sites—and there were the radar reflectors he had carefully set up to identify the line of the runway.

  It had taken them only twenty minutes to move to the new position and to get the equipment lined up again. S for Sugar, now piloted by Air Commodore Burrows, was already airborne and was being vectored around the sky by the Traffic Director; in another five minutes, the Air Commodore would be doing his first GCD approach. Everything looked perfect; the signals were coming in nicely, and Flight Lieutenant Arnold Evans, who was handling the talk-down, was the best controller in the business next to Dr. Wendt. As long as the Mark I behaved itself, and the Air Commodore did exactly what the Controller told him, they would have nothing to worry about.

  “He’s on the cross-wind leg,” the Traffic Director called to the girls in the back room. “You should be seeing him in a couple of minutes.”

  “I’ve got him!” cried the elevation tracker. “Nine miles out!”

  Alan peered over the girl’s shoulder. Yes, there was a nice fat echo, at just the right altitude. A moment later it appeared on the azimuth display as well; even at this distance, it was already well lined up with the runway. This would be a piece of cake.

  “I’ll take him now,” said the Controller, when the echo had approached to within six miles. He switched on his transmitter and began the familiar routine: “Sheepdog calling S Sugar. Continue on course two seven zero. Are you receiving me? Over.”

  “Receiving you loud and clear,” said the Air Commodore’s voice. Did it sound a little baffled? Alan could not be sure; but nothing could go wrong now—it was a beautiful line-up.

  The Controller had an absurdly easy job; though the aircraft was a little slow at responding to orders, for the last three miles it needed no corrections at all. To Alan, watching both displays at once—he was quite good at this by now—the echo seemed almost glued to the glide path.

  “You are now a thousand feet from the end of the runway,” said the Controller, giving his final instruction. “Go ahead and overshoot.”

  But the echo did nothing of the sort. Instead of climbing away into the sky once more, it got closer and closer to the ground. Well, that was the pilot’s own affair, for he was flying visually now and could see exactly what he was doing. But it was a little surprising, and Alan passed the news on to the now-relaxed Controller.

  “I think he’s landing on this approach. Was he cleared with Flying Control?”

  F/Lt. Evans sounded annoyed and surprised.

  “No—he wasn’t. He should have known better than that. Maybe he’s had engine trouble.”

  “Well, he’s on the deck now. Quite a smooth touchdown, by the look of it.”

  The descending blip had merged into the ground echoes and was trundling swiftly along the runway; in a few seconds it would come abreast of the trucks and would disappear from the radar displays as it entered the blind zone behind the antennas.

  The Controller reached toward his mike and pressed the transmit key. Unless the aircraft had developed a fault, there was no reason why the pilot, whatever his rank, should disobey standard operating procedures. F/Lt. Evans had never reprimanded an Air Commodore, and rather looked forward to the experience.

  “Sheepdog calling S Sugar,” he said. “Are you receiving me? Why have you landed? Over.”

  There was a considerable delay before the reply sounded in his earphones. When it came, it was surprisingly faint; and even over the low-fidelity RT circuit, it crackled with indignation.

  “I don’t know what you think you’re doing,” spluttered the distant and furious voice of Air Commodore Burrows, “but at the moment I am about five miles out to sea, somewhere over the Channel. And if I’d kept the rate of descent you gave me, by this time I’d be in the drink. I was only a hundred feet from the water when I broke off. I am returning to the airfield and will request permission to land visually. Over and OUT.”

  That last explosive syllable was still echoing in the Controller’s ears when the control van reverberated to the roar of great engines. As the heavy bomber that had just landed—without the slightest assistance from GCD—went taxiing past them, one needed little knowledge of aircraft to appreciate that no modest twin-engined Anson created that amount of noise.

  In the silence that followed, the elevation tracker made what was widely acclaimed as the most superfluous remark of the day.

  “You know, sir,” she said to the still-stricken Alan, “I did think it was rather a big echo for S Sugar.”

  ***

  The post-mortem took place in Professor Schuster’s little office; though the atmosphere was one of extreme gloom, there were no recriminations. With heroic self-restraint, none of the Americans said: “If only we had been doing the controlling…!” But that had been against the ground rules; a major purpose of the demonstration had been to prove that GCD did not need a team of Ph.D.s to run it.

  It was perfectly obvious what had happened, and a quick check with Flying Control had confirmed the instant diagnosis of everyone who had been working in the truck. Somewhere on its circuit around the sky, S Sugar had passed within a mile or so of a Liberator coming home after a mission, and the two echoes had been confused. Thinking they were following S Sugar, the operators had started tracking the wrong aircraft. As the Liberator had been making a normal landing in good visibility, it was scarcely surprising that the radar had reported an excellent approach. And so poor S Sugar, left to his own d
evices in quite another part of the sky, was being solemnly assured that he was nicely on the runway, only half a mile from touchdown—while all the while the indignant and astonished pilot stared at the open sea…

  Wearing the wool-lined flying jacket that was his all-purpose protection against British weather and British heating systems, Professor Schuster sat quietly at his desk while Deveraux poured the sad story into his ears. He showed no signs of worry, disappointment, or anger; indeed, he might have been listening to one of his students explaining how a physics experiment had gone wrong. Which, in a sense, was not far from the truth.

  “If there’s a weakness in the system,” he said, “it’s just as well that we’ve discovered it—though I wish we could have done it some other time. What we’re up against is a simple technical problem; therefore, it must have a simple technical solution.”

  Schuster pulled across a large writing pad and began to cover it with neat sketches.

  “The weak point is here, just before the final approach starts. The Traffic Director has an aircraft on his display and is bringing it into line with the runway. The girls in the back room are looking at their screens, and they see an echo approaching. How can they be sure it’s the same one? Ninety-nine times out of a hundred it will be; today, unfortunately, it wasn’t.”

  “You know,” said Hatton, “I thought of this a long time ago, but decided that the risk was too small to bother about. Well, I was wrong; the statistics have caught up with us.”

  “If you don’t like the odds in a game,” said Benny, “there’s one way of improving them. You can mark the cards.”

  “That’s the answer,” exclaimed Dr. Wendt. “We have to put a label on the echo we’re interested in. Anything will do—a pointing arrow, a little circle, X marks the spot. Just as long as it distinguishes our aircraft from any others that are stooging around.”

  “Don’t we already have enough electronic markers cluttering up the picture?” grumbled Deveraux.