"Sir, if we are standing on the platform, if a train is honking close to the platform, we will feel that the sound of the whistle is gradually sharpening, and the sound of the train gradually lowering when it gradually stands on the platform. Pen @ fun @ pavilion wWw. ｂｉｑUｇE。 The change in the whistle of the info train from sharp to deep is actually a change in the frequency of the sound waves that the sound enters the human ear, and this change is produced because of the movement of the train in close or distant from the ear. We call this behavior the Doppler effect. ”

Ten kilometers west of Baoma, on Pfeifik Lake, an AWACS aircraft converted from a Boeing 314 four-engine seaplane was quietly parked on the shore, and a disc-shaped antenna was installed on the back of the aircraft. Against the backdrop of this aircraft, Norbert Wiener, a Polish-Jew, feared that Lee Kongrong did not understand the new radar, so he explained how it worked at the outset. His extravagant actions made Meng Zhaoying, Ren Zhigong, and others of the Radio Research Institute snicker-The Doppler radar was developed in response to the requirements mentioned in Li Kongrong's proposal, and now Wiener is teaching Li Kongrong the principle of Doppler radar.

Wiener and a Chinese mathematician didn't know what the Chinese were laughing at, but Li Kongrong nodded indifferently, as if he was really a technical layman and needed Wiener to help him popularize the principle of radar.

"Similarly, when a radio detector emits a certain frequency of electromagnetic waves at certain intervals (generally tens of milliseconds) to search for air, if it encounters an active target, under normal circumstances, the target will be close to or far away from the radar, so the frequency of the electromagnetic wave reflected back from the active target and the frequency of the emission will change, and the difference between the two becomes the Doppler frequency, which is proportional to twice the radial velocity of the target and inversely proportional to the radar wavelength. That is, if the target is close to the radar, the Doppler frequency is positive, and vice versa is negative......"

After about fifteen minutes, Professor Vino explained the principles of the new radar, and then he began to describe the problems that existed at hand. "As I said, the Doppler frequency is the difference in the reflection of electromagnetic waves as the target moves. The difference is the comparison of the frequencies of the electromagnetic waves before and after, so in order to compare, we must store the previous ......"

"As far as I know, there is no electromagnetic wave memory in the world." Speaking of the key point, Li Kongrong finally couldn't help but interject.

"......" Wiener thought that Li Kongrong was just a naval officer and didn't understand anything, but he was suddenly a little stunned when he asked like this, he paused for a second before saying: "Yes, there is no electromagnetic wave memory for the time being, but we have thought of a way," Wiener turned his head and glanced at Li Yurong, who smiled. "The main thing is to use the method of delay - the transmission speed of sound waves is only 343 meters per second, while the speed of light propagation is 300,000 kilometers per second. The specific method is to use a nickel wire as the medium, an echo is a radar pulse, according to the amplitude of the pulse, the electromagnetic wave signal is converted into an ultrasonic audio signal, and then the length of the nickel wire is determined according to the interval, and the second echo is compared at a given time. ”

"Amazing invention." Li Kongrong stroked his palms and smiled. He knew the principle of Doppler radar (the result of paying multiple speeding fines), but he didn't know if his vision could be realized in the era when there were no transistors, and he didn't expect the lab to do so simple: just a nickel wire.

"Sir, if the radio detector is placed on the ground, then I think we are close to most of the problems, but the radio detector is mounted on an aircraft several kilometers away. The plane itself is in motion, the echo and the detector have relative velocity, and the target and clutter are no longer zero after subtracting each of the two echoes. We must compensate for this situation.

The radio detector antenna on the plane and the back of the aircraft rotates 360 degrees, and the receiving device can receive reflected waves at different distances, at different altitudes, and from multiple targets, so it is necessary to increase the channel, that is, to increase the number of Doppler filters. On the ground, a filter may be required, one main delay line, two reserve delay lines, and on the aircraft a filter bank must be used, which is likely to require up to 1,000 primary and secondary delay lines, if this is not processed, the frequency that the detector can receive will be very narrow, and the operator will have no way to judge the direction and rate of the target. Delay lines aren't a problem, but dealing with echo signals on those delay lines is a big problem. ”

Saying this, Wiener took a deep breath, and he continued: "This is no longer something I can solve, I think it is better for Professor Johann von Neumann to solve this problem." ”

Many foreign professors and mathematicians joined the Computer Research Laboratory, and the Radio Research Laboratory and the Computer Research Laboratory were jointly responsible for the airborne radio project. In Li Kongrong's mission book, the early warning radar is not a simple pulse-reflecting radar, but a Doppler radar that can detect the speed and direction of the target -- this is extremely important for the early warning aircraft, because the early warning aircraft constantly emits pulse radar waves, its position is completely exposed in the enemy's radio direction finder, and if the other side sends an aircraft to attack, the detection distance of the early warning aircraft is one or two hundred kilometers, that is, it is a matter of more than 10 minutes.

If the direction and speed of the enemy plane cannot be predicted, the early warning aircraft will choose to shut down the aircraft once it detects it, because it is very likely that the enemy plane will attack along the pulse wave. Alternatively, several fighters were sent to escort AWACS aircraft during air patrols. In the warring sea area, the early warning planes will certainly take off to the air for more than 12 hours, and sending fighter planes to escort the aircraft for 12 hours will definitely reduce the aircraft carrier's strike force against the enemy, and it will also consume the fleet's few aviation fuels.

The requirements are simple, but in this day and age they are difficult to achieve. Meng Zhaoying and Ren Zhigong could not complete this task, so they could only find the help of the computer research laboratory, and the professors of the Department of Mechanical and Electrical Engineering of Southwest Associated University were more or less involved, so Li Yurong found Wiener, Wiener found Neumann, and Neumann found a professor and mathematician......

"Sir, I think this is a signal processing problem, and if we want to solve this problem, I think the only way to solve this problem is to put the computer in the laboratory on that plane." Neumann seems to be a bit bald, apparently too smart. For this Jew, who was later called the 'father of computers', Li Kongrong was deeply impressed. "It's just that we have to reduce his size and weight, or we won't get on the plane."

Neumann made a long story short, blinking his eyes as he spoke, as if he was examining what kind of person Li Kongrong was.

"The computer in the lab?" Although the computer in the computer laboratory is not Enia in 1945, it is a behemoth in this era. After the navy had money, the funding of the institute skyrocketed, and Li Kongrong believed that computers could help decipher the secrets of the Japanese army, so he allocated funds to Hua Luogeng and others to make the first electronic computer. Dr. Neumann's ability to come to Switzerland had nothing to do with the computer.

"Yes, Sir. The problem is that it's too big and heavy, so we're going to optimize the formula, mainly using the Fourier transform to reduce the computation, making it a computer that specializes in calculating Doppler frequencies. Neumann said seriously. It takes a little time, and it takes some money. At present, the pilot told us that the plane can carry about 12 tons of cargo, and if necessary, some fuel can be reduced. I don't think the new computer will weigh more than five tons, so it's not impossible to process a signal on a thousand delay lines. ”

The Boeing 314 seaplane was chosen as an AWACS aircraft because of its large load capacity and long endurance (about 20 hours), but Li Kongrong never thought of carrying a five-ton computer and radar on the plane. If this is done, with four or five tons of antennas and other equipment and personnel, the aircraft will definitely have a load of more than ten tons. Ten tons is not impossible to fit, but the tube computer is extremely large, and the air flow on the plane vibrates frequently, can the computer composed of countless tubes operate normally?

"You can try." Li Kongrong thought for a while before saying. "But I was worried about the stability of the computer on the plane, which was very bumpy because of the air currents. Unless the plane is at an altitude of 10,000 meters, but this," Li Kongrong thought of the passenger planes of later generations, and remembered the stratosphere. "This involves several problems, one is that the engine pressurization device must be able to ensure that the aircraft rises to an altitude of 10,000 meters, the second is the radio interference between various electrical appliances, and the third is that it is very cold at an altitude of 10,000 meters, and the radome and wings are likely to freeze, and the computer equipment is ...... if it is to keep warm."

Everyone looked at Li Kongrong and feared that he would veto Neumann's proposal to put the computer on the plane. He promised to give it a try, and a group of mathematicians smiled.

"Computers get hot when they're running, and it's best if they freeze." Hua Luogeng was the first to speak, and he didn't have any words.

"The heat dissipation of computer operation is a big problem, and you are not afraid of cold, but you are afraid of heat." Wiener said: "And the higher and higher the plane flies, the weaker the echo on the ground, which is good." At present, airborne radio detectors not only cannot distinguish the speed and direction of the target, but can only find out where the plane is located, and there is also a big error in judging the altitude, and the plane at a distance of 200 kilometers cannot measure the accuracy at all, and the error is thousands of kilometers. ”

"Don't care about the height, it's an acceptable question." Li Kongrong secretly breathed a sigh of relief in his heart, because of the weight of the antenna, wind resistance, and other reasons, it was impossible for the early warning aircraft to use tower radar, but only disk radar. There is certainly no way for the disc radar to measure high accuracy with the ground radar with an antenna as high as 10 meters, and the altitude measurement error of 200 kilometers will of course exceed 1,000 meters.

"Then all that's left is signal processing." Meng Zhaoying finally spoke. "Then there is the stability of the equipment in the sky." He saw that Li Kongrong had been looking at which 314 early warning plane was on the lake, and then said, "Let's get on the plane and take a look." ”

"Good. Let's go up and have a look. Li Kongrong agreed, and came to Pfefik Lake early in the morning to see the early warning aircraft. Soon, Meng Zhaoying and Ren Zhigong took him on board the Boeing 314. The side door to enter is under the wing, and there are two huge propeller engines on the wing, the length of a single propeller is more than two meters, but the fuselage is very high, and the propeller is still more than a meter away from the water. When Li Kongrong saw the side door, he knew that this was not the twelve 314s that Boeing had customized at the request of the Navy, because those planes were all tail-open.

"It's leased from Pan Am." Meng Zhaoying is more aware of the situation of the aircraft. "Pilots are also airline people. When we went to the United States to pick up the plane, we said that our plane had not yet been built, and the airline's plane happened to be idle. ”

"I know these things." The cabin still retains the seats and even the beds of the guests, and the decoration is quite luxurious, and there are some oil paintings hanging in the corridors. Li Kongrong said curiously: "You have installed an antenna on the plane, and the airline has no objection?" ”

"At first, it was opposed, but then Boeing's engineers promised to restore the original state after the dismantling, and the airlines were okay. The main reason is that airlines have ordered 12 planes at once, and although the number of passengers has increased, the planes are still idle. Ren Zhigong explained.

He walked in front of Meng Zhaoying, and at this time the corridor seemed to have come to an end, and a room of more than twenty square meters (judging from the layout on the wall, this was obviously a dining room) was full of instruments. He walked over to two half-human-tall locker-like instruments and said, "This is the radio detector." This is the transmitter," he said, pointing to the smaller, circular display, "This is the receiver." ”

Li Kongrong looked at the display of the receiving device, obviously, unlike the display on the submarine sonar that only displayed two points, this was a new flat plane display. "It's already on?" Only then did Li Kongrong realize that the radar was on.

"It's open." Meng Zhaoying said, he pointed to the northwest position of the display screen and said: "This is not the sea, but you can also see the planes taking off and landing at Zurich Airport." ”

"Can you see the planes in Zurich?" Li Kongrong was interested, but he didn't see anything.

There was calm on the "......" monitor, Meng Zhaoying was a little embarrassed, he hurriedly looked at his watch, and finally breathed a sigh of relief: "It's too early and there is no plane, the earliest flight is around a quarter past eight, and there are times when the weather is bad." ”

"It's okay." Li Kongrong was unimpressed. He was satisfied to see that the radar was a flat display rather than a single-line display like on a submarine. "Let's introduce this probe first."

"This," Meng Zhaoying groaned slightly. Although the radar used in this test was rudimentary, anything contained in it could be said to last for three days and three nights, but in the end, he omitted too much technical content, and only said: "According to the design, when searching the sea surface, it can detect aircraft within 150 kilometers and flying more than 150 meters; On the surface of the sea, large ships can be detected within 300 kilometers. This is related to the sea state, if the sea condition is not good, clutter increases, the detection range will be reduced. ”