A brief history of development
The basic concept of radar was formed in the early 20th century. But it was not until around the Second World War that radar developed rapidly. As early as the beginning of the 20th century, some scientists in Europe and America knew that electromagnetic waves were reflected by objects. 1922, Italian g Marconi published a paper about the possible detection of objects by radio waves. The US Naval Laboratory found that bistatic CW radar can detect ships passing through it. 1925, the United States began to develop a pulse modulation radar that can measure the distance, and used it to measure the height of the ionosphere for the first time. In the early 1930s, some countries in Europe and America began to develop pulse modulation radar for detecting aircraft. 1936, the United States developed a pulse radar for detecting aircraft with a range of 40 kilometers and a resolution of 457 meters. 1938, Britain set up an early warning radar chain on the continental coastline near France to observe enemy planes.
Early warning radar chain
During the Second World War, radar technology developed rapidly due to operational needs. As far as frequency bands are concerned, the devices and technologies before the war can only reach tens of megahertz. At the beginning of the war, Germany first developed high-power triode and quadrupole, which increased the frequency to more than 500 MHz. This not only improves the accuracy of radar search and aircraft guidance, but also improves the performance of anti-aircraft gun control radar, so that anti-aircraft gun has a higher hit rate. 1939, Britain invented a 3000 MHz power magnetron, and the ground and aircraft were equipped with microwave radars using this magnetron, which gave the Allies an advantage in air combat and air-sea battle. In the late World War II, the United States further increased the frequency of the magnetron to 10 GHz, realizing the miniaturization of airborne radar and improving the measurement accuracy. In the aspect of anti-aircraft gun fire control, the accurate automatic tracking radar developed by the United States
SCR-584 let anti-aircraft guns shoot down a plane, and the hit rate ranged from thousands of shells at the beginning of the war to dozens of shells hitting a plane.
In the late 1940s, moving target display technology appeared, which was beneficial to find targets in the background of ground clutter and cloud rain. High performance moving target display radar must emit coherent signals, so power traveling wave tube, klystron, forward wave tube and other devices have been developed. High-speed jet aircraft appeared in 1950s, and low-altitude penetration aircraft, medium and long-range missiles and military satellites appeared in 1960s, which promoted the rapid improvement of radar performance. In the 1960s and 1970s, computers, microprocessors, microwave integrated circuits and large-scale digital integrated circuits were applied to radar, which greatly improved the radar performance, reduced its size and weight, and improved its reliability. In the new radar system and technology, moving target display, monopulse angle tracking and pulse compression technology have been widely used in the 1950s. Phased array radar appeared in the 1960s. In 1970s, solid-state phased array radar and pulse Doppler radar came out.
In China, radar technology began in the early 1950s. The radar developed by China has been equipped with troops. China has developed two-coordinate and three-coordinate warning guidance radars for air defense, ground-to-air missile guidance radar, long-range missile initial ranging radar and reentry ranging recovery radar. The large radar developed by China is also used to observe satellites launched by China and other countries. In civil use, navigation and collision avoidance radars for ocean-going ships, navigation and control radars for airports and weather radars have been produced and applied. The airborne synthetic aperture radar developed by China has been able to obtain a large area of clear mapping. A new generation of radar developed in China uses computers or microprocessors and digital information processing technology of medium and large scale integrated circuits, and its frequency has been extended to millimeter wave band.
principle of operation
The radar antenna shoots the electromagnetic energy provided by the transmitter to a certain direction in space, and the objects in this direction reflect the electromagnetic waves encountered. These reflected waves carry the information of objects, which are received by radar antenna and sent to radar receiving equipment for processing, thus extracting useful information needed by people and filtering out useless information.
Radar can be divided into two categories: continuous wave radar and pulse radar. Single-frequency continuous wave radar is the simplest form of radar, and it is easy to obtain the change rate (that is, radial velocity) of the distance between the moving target and the radar. Its main disadvantages are: ① it is impossible to measure the target distance directly, and if the target distance is to be measured, frequency modulation must be carried out, but the target distance measured by frequency modulation continuous wave is far less accurate than that of pulse radar; ② It is easy to confuse targets in multi-target environment; ③ The receiving antenna and transmitting antenna of most continuous wave radars must be separated, which requires a certain degree of isolation.
pulse radar
It is easy to achieve accurate ranging, and the received echo is in the rest period of the transmitted pulse, so there is no problem of isolation between the receiving antenna and the transmitting antenna, so most pulse radars have the same receiving antenna and transmitting antenna. Because of these advantages, pulse radar (Figure 1) occupies a major position in all kinds of radars. The pulse signal emitted by this radar can be a rectangular pulse with a single carrier frequency, as in the case of ordinary pulse radar; It can also be a pulse modulation signal in the form of coding or frequency modulation. Through matched filtering in the receiver, the signal bandwidth is increased and very narrow pulses are output, thus improving the ranging accuracy and range resolution of radar. This is a pulse compression radar. In addition, the phases between adjacent pulses transmitted by radar can be incoherent (random) or coherent signals with certain regularity. The spectrum purity of coherent signals is high, and good display performance of moving targets can be obtained.
target location
Locating ground and sea targets means measuring their distance and direction relative to radar. The positioning of air targets needs to measure the distance, azimuth and altitude at the same time. This kind of radar is called three-coordinate radar. Measuring the distance is actually measuring the time difference between the transmitted pulse and the echo pulse, because electromagnetic waves travel at the speed of light and can be converted into the accurate distance of the target. The azimuth of the target is measured by the sharp azimuth beam of the antenna. Under the same narrow beam condition, the single pulse method can obtain higher measurement accuracy than the single beam (see tracking radar). Narrow elevation beam measurement. According to the elevation angle and distance of the target, the target height can be calculated, and the accurate elevation angle can also be obtained by monopulse method.
(telegraph) transmitter
It can be a magnetron oscillator. This is the way of early microwave radar transmitter, and simple radar is still in use. Modern high-performance radar requires signal coherence and high frequency stability. Therefore, it is necessary to use the crystal oscillator as a stable frequency source and obtain the required coherence, stability and power through the frequency doubling power amplification chain. Power traveling wave tube or klystron is the most commonly used power amplifier in the last stage of amplification chain. When the frequency is lower than 600 MHz, microwave triode or microwave quadrupole can be used.
Pulse modulator
It generates modulation pulses for the transmitter switch. It must have the pulse width needed to emit high-frequency pulses and provide the modulation energy needed to switch the transmitting tube. Using vacuum tube or transistor as discharge switch is called hard tube modulation; Hydrogen thyratron, as a discharge switch for artificial line energy storage, is called hose modulation. In addition, electromagnetic elements can also be used for pulse switch modulation. The general requirements for modulation pulse are steep starting and falling edges and flat pulse top.
Transmit-receive switch
It cuts off the receiving branch when transmitting pulse, so as to minimize the energy of transmitting pulse leaking to the receiving branch. When the transmitting pulse ends, the transmitting branch is disconnected, and all the echo signals received by the antenna enter the receiving branch through the transceiver switch. The transceiver switch usually consists of a special gas-filled tube. During launch, the gas-filled tube is ionized and ignited to form a short-circuit state, and the open-circuit state is restored after the launch pulse passes. In order not to block the echo of short-range targets, the time from ionization short-circuit state to ionization elimination open-circuit state is extremely short, usually on the order of microseconds, and some radar systems are on the order of nanoseconds.
aerial
Radar must have high target orientation accuracy, which requires the antenna to have a narrow beam. When searching for a target, the antenna beam scans a certain airspace. Scanning can adopt mechanical rotation method or electronic scanning method. Most antennas have only one beam, but some antennas have several beams at the same time. The energy distributed in the antenna sidelobe should be as small as possible, and the antenna with low sidelobe is needed for anti-jamming.
receiving machine
Generally, superheterodyne type is adopted. The front end of the receiver has a low noise and high frequency amplifier stage. The amplified carrier frequency signal and the local oscillator signal are mixed into an intermediate frequency signal. Analog signal processing (such as pulse compression and moving target display). ) in the intermediate frequency amplification stage, and then the target signal is detected and transmitted to the display. When using digital signal processing, in order to reduce the processing speed, the signal should be mixed to zero intermediate frequency; In order to keep the phase information, the zero intermediate frequency signal is decomposed into two orthogonal signals, which enter two different branches respectively, and then the two branches are digitally processed, and then the processing results are combined.
Radar emits electromagnetic energy into space in a directional way. By receiving radio waves reflected by objects in space, the direction, height and speed of objects can be calculated, and the shape of objects can be detected. A radar aimed at the ground can detect the exact shape of the ground.
1922, Taylor and Yang suggested that two warships should be equipped with high-frequency transmitters and receivers to search for enemy ships. 1924, appleton and barnett measured the height of the stage by radio waves reflected from the ionosphere. Blair and Duff in the United States use pulse waves to measure the Haveser layer. 193 1 year, the United States naval research laboratory developed a radar based on the beat frequency principle and began to let the transmitter emit continuous waves. Three years later, we switched to pulse wave 1935, and developed a "learning cellar" with magnetron wavelength 16 cm. Other ships can be found in foggy days or at night. This is the beginning of the peaceful use of radar. 1936 65438+ 10 in October, W. Watt of Britain established the first radar station in Britain on the coast of Sofk. The British Air Force added five more aircraft, which played an important role in the Second World War.
1937 the radar XAF test of the first us warship was successful.
194 1 year, the Soviet union equipped early warning radar on the plane for the first time. 1943, MIT developed the plane position indicator of airborne radar, which can shoot moving aircraft. He invented the microwave early warning radar which can distinguish dozens of targets at the same time. 1947, American Bell Telephone Laboratory developed LFM pulse radar. In the mid-1950s, the United States was equipped with a long-range early warning radar system to detect supersonic aircraft. Soon, pulse Doppler radar was developed.
1959, General Electric Company of the United States developed the ballistic missile early-warning radar system, which can launch and track missiles 3000 miles away and 600 miles high, with an early warning time of 20 minutes.
1964, the United States installed the first space orbit surveillance radar to monitor artificial earth satellites or spacecraft. 197 1 year, Iyuca, Canada and other three people invented the holographic matrix radar. At the same time, digital radar technology appeared in the United States.
Radar can be divided into military radar and civil radar according to its use. Military radar includes warning radar, guidance radar, friend or foe identification, etc. Civil radar includes navigation radar, weather radar, speed measuring radar, etc.
Military radar
Civil radar
Weather radar is a clairvoyance and perspective instrument for detecting meteorological changes in the atmosphere. Meteorological radar intermittently emits electromagnetic waves (pulses) into the air, and then receives electromagnetic waves (echoes) scattered by meteorological targets to detect the spatial position and characteristics of meteorological targets within a radius of more than 400 kilometers, which plays an important role in monitoring and early warning of disastrous weather, especially sudden and medium-sized disastrous weather.
Weather radar
The word radar comes from English radar, a radio wave detection device. It is called "clairvoyance". Seeing the word "thunder" immediately reminds people of the lightning in the horizon, highlighting a fast word. Naturally, the "clairvoyance" function of radar is even more impressive.