"The so-called millimeter wave is a section of radio wave. We call the electromagnetic wave with wavelength of 1 ~ 10mm millimeter wave. It is located in the overlapping wavelength range of microwave and far-infrared wave, so it has the characteristics of two kinds of spectrum. The theory and technology of millimeter wave are the extension of microwave to high frequency and the development of light wave to low frequency.
The so-called millimeter wave is a section of radio wave. We call the electromagnetic wave with wavelength of 1 ~ 10mm millimeter wave. It is located in the overlapping wavelength range of microwave and far-infrared wave, so it has the characteristics of both spectrum. The theory and technology of millimeter wave are the extension of microwave to high frequency and the development of light wave to low frequency.
The so-called millimeter wave radar refers to the radar operating in the millimeter wave band. The ranging principle is the same as that of general radar, that is, send out radio waves (radar waves), then receive echoes, and measure the target position data according to the time difference between sending and receiving. Millimeter wave radar is this radio wave whose frequency is millimeter wave band. Because the wavelength of millimeter wave is between centimeter wave and light wave, millimeter wave has the advantages of microwave guidance and photoelectric guidance. Compared with centimeter waveguide seeker, millimeter waveguide seeker has the characteristics of small volume, light weight and high spatial resolution. Compared with infrared, laser, television and other optical seeker, millimeter waveguide seeker has strong ability to penetrate fog, smoke and dust, and has the characteristics of all-weather (except heavy rain). In addition, the anti-interference and anti stealth ability of millimeter waveguide seeker is also better than other microwave seeker. Millimeter wave radar is a high-precision sensor to measure the relative distance, current velocity and azimuth of the measured object. It was used in the military field in the early stage. With the development and progress of radar technology, millimeter wave radar sensor began to be used in many fields, such as automotive electronics, UAV, intelligent transportation and so on. Characteristics of millimeter wave radar
1. The frequency band is extremely wide. The available bandwidth in the currently used 35g and 94g atmospheric windows is 16g and 23g respectively, which is suitable for various broadband signal processing;
2. Narrow beam can be obtained with small antenna aperture, good directivity, high spatial resolution and high tracking accuracy;
3. It has high Doppler bandwidth, obvious Doppler effect, good Doppler resolution and high velocity measurement accuracy;
4. The influence of ground clutter and multipath effect is small, and the tracking performance is good;
5. Millimeter wave scattering characteristics are sensitive to the details of target shape, so it can improve the ability of multi-target resolution, target recognition and imaging quality;
6. Because millimeter wave radar transmits with narrow beam, it has low interception performance and good anti-jamming performance;
7. Millimeter wave radar has certain anti stealth function.
8. Millimeter wave has the ability to penetrate smoke, dust and fog, and can work all day. Advantages of millimeter wave radar ranging High precision and anti-interference Compared with micro waveguide seeker, millimeter waveguide seeker has the characteristics of small volume, light weight and high spatial resolution. With the same antenna aperture, millimeter wave radar has a narrower beam (generally in the order of milliradians), which can improve the angular resolution and angular measurement accuracy of the radar, and is conducive to anti electronic jamming, clutter jamming and multipath reflection jamming. All weather, all day Compared with infrared, video, laser and other optical seeker, millimeter waveguide seeker has strong ability to penetrate fog, smoke and dust, and has the characteristics of all-weather and all-weather. High resolution multi-target Due to the high operating frequency, large signal bandwidth (such as gigahertz) and Doppler frequency shift may be obtained, which is conducive to improve the measurement accuracy and resolution of range and velocity, and analyze the detailed characteristics of the target. At the same time, millimeter wave radar can distinguish and recognize small targets and multiple targets at the same time, so it has strong spatial resolution and imaging ability. High sensitivity and low false alarm The system has high sensitivity, low false alarm rate and is not easy to be disturbed by external electromagnetic noise. High frequency and low power It has higher transmission frequency and lower transmission power. Speed and distance measurement Using FMCW FM CW, the distance and velocity of multiple targets can be measured at the same time, and the targets can be tracked continuously, even to stationary targets. Long distance and high real-time performance The measurement distance is far, reaching 200 meters in two-way 12 lanes. At the same time, the detection frequency of 38hz and 26ms has strong real-time performance. Working principle of millimeter wave radar Taking the vehicle mounted millimeter wave radar as an example, the radar transmits millimeter wave outward through the antenna, receives the target reflection signal, quickly and accurately obtains the physical environment information around the vehicle body after rear processing (such as the relative distance, relative speed, angle, motion direction, etc. between the vehicle and other objects), and then tracks, identifies and classifies the target according to the detected object information, Then the data fusion is carried out in combination with the dynamic information of the vehicle body, and finally the intelligent processing is carried out through the central processing unit (ECU). After reasonable decision-making, inform or warn the driver in various ways such as sound, light and touch, or make active intervention in time, so as to ensure the safety and comfort of the driving process and reduce the probability of accidents.
In the field of automotive active safety, automotive millimeter wave radar sensor is one of the core components. Among them, 77GHz millimeter wave radar is an essential key component in intelligent vehicles. It is a sensor that can quickly sense the distance, speed, azimuth and other information of surrounding objects within 0-200m in all-weather scenes. Location The millimeter wave radar sends out directional millimeter waves of corresponding bands through the transmitting antenna. When the millimeter wave encounters an obstacle target, it reflects back, and receives the reflected millimeter wave through the receiving antenna. According to the millimeter wave band, the flight time of the millimeter wave on the way is calculated through the formula, and then combined with the driving speed of the vehicle in front and the driving speed of the vehicle, the relative distance between the millimeter wave radar (the vehicle) and the target and the position of the target can be known. Speed In addition, according to the Doppler effect, the frequency change of the millimeter wave radar and the relative speed of the vehicle and the tracking target are closely related. According to the change of the reflected millimeter wave frequency, we can know the relative motion speed of the obstacle target tracked in front in real time compared with the vehicle. Therefore, when the sensor sends a safe distance alarm, if the vehicle continues to accelerate, or the front monitoring target decelerates, or the front monitoring target is stationary, the frequency of millimeter wave reflected echo will become higher and higher, otherwise, the frequency will become lower and lower. azimuth As for the azimuth measurement of the monitored target, the detection principle of millimeter radar is: after the millimeter wave is transmitted through the transmitting antenna of millimeter wave radar, it meets the monitored object and reflects back. Through the parallel receiving antenna of millimeter wave radar, the azimuth of the monitored target can be calculated by receiving the phase difference of the millimeter wave reflected from the same monitoring target. The schematic diagram is as follows:
azimuth α AZ is the geometric distance d between the receiving antenna rx1 and the receiving antenna rx2 of the millimeter wave radar, and the phase difference b of the reflected echo received by the two millimeter wave radar antennas, and then the azimuth is calculated by the trigonometric function α AZ value, so you can know the azimuth of the monitored target. Position, velocity and azimuth monitoring are the strengths of millimeter wave radar. Combined with its strong anti-jamming ability, millimeter wave radar can work stably all day and all day. Therefore, millimeter wave radar is selected as the core sensing technology of automobile. Millimeter wave radar and lidar With the popularity of automatic driving, lidar has been sought after unprecedentedly, because it has the advantages of high precision, large amount of information and no visible light interference. However, we can note that the current mainstream automatic driving scheme has not completely abandoned millimeter wave radar. What is the reason?
Compared with lidar, the detection distance of millimeter wave radar can easily exceed 200 meters, while lidar is generally less than 150 meters. In the high-speed driving scene, millimeter wave radar is more suitable. Secondly, due to the high requirements for transceiver and assembly process of lidar, it is difficult to reduce the cost. Millimeter wave radar has more advantages in cost because it is a silicon-based chip and has no particularly expensive and complex process. The current price of millimeter wave radar is about 15000, while the price of lidar is still calculated in 10000. Moreover, because the amount of data obtained by lidar is much larger than that of millimeter wave radar, higher performance processors are needed to process data, and higher performance processors also mean higher prices. Therefore, for engineers, millimeter wave radar is still the best choice in simple scenes. However, the shortcomings of millimeter wave radar are also very intuitive. The detection distance is directly restricted by the frequency band loss, unable to perceive pedestrians, and unable to accurately model all the surrounding obstacles. As for the market prospect of millimeter wave radar, a car will be equipped with 3-8 millimeter wave radars. At present, seven millimeter wave radars have been installed in Mercedes Benz's high-end cars. In the next few years, the market scale of vehicle mounted millimeter wave radar will not be underestimated. At present, another very important technology of lidar is solid-state lidar, which actually comes down in one continuous line with traditional radar and millimeter wave radar. Solid-state lidar essentially adjusts the phase of each transmitting and receiving unit. Millimeter wave radar also has the same principle, but millimeter wave radar operates electromagnetic waves, It is much less difficult to realize the device than to change the phase in the optical frequency band. In the future, the combination of solid-state lidar and millimeter wave radar may be a good choice. In short, millimeter wave radar is a sensor that is difficult to be replaced. Although it has shortcomings, the all-weather working state is the biggest advantage. The accuracy of speed measurement and ranging is much higher than that of visual sensor, and the penetration will be better than that of lidar. But on the whole, this is not in conflict, because there will be a trend of integration in the future, especially for autonomous driving. There is no doubt that the three sensors will integrate with each other. Main application classification of millimeter wave radar 1. Guidance radar and fire control radar. At present, this type of radar has some choices in the millimeter wave band. The main reason is to improve the detection ability, reduce the radar volume, reduce the weight and volume, and facilitate integration. 2. Target detection radar. This type of radar mainly detects the target distance, velocity and angle in the observation area through mechanical / electronic beam scanning. Equipped with corresponding data processing unit, it can realize target recognition (scattering characteristics), tracking and prediction (Kalman filter, particle filter, etc.).
3. Millimeter wave earth observation radar. This type of radar is mainly millimeter wave synthetic aperture radar (SAR). This type of radar mainly realizes earth imaging observation and obtains SAR images of ground areas.
4. Millimeter wave short-range detection radar. This type of radar mainly realizes two-dimensional or three-dimensional imaging detection of targets within 2m. At present, the band of this type of system is 30 ~ 37.5 GHz and 94 ~ 200 GHz or THz. For example, at present, the three-dimensional scanning radar for human security inspection in American airports uses millimeter wave instead of X-ray to detect and image targets inside human clothes and outside human skin, so as to strengthen security; In addition, the non-destructive three-dimensional testing of some special materials is realized by 35GHz band radar or 94GHz and THz band radar.
5. Automotive radar: radar sensors are installed on vehicles to realize vehicle collision avoidance, automatic parking, pedestrian detection, etc. at present, the mainstream automotive radar is 24GHz radar, but due to frequency band control, radio astronomy 5km, it is not allowed to use this band radar, and its large volume (mainly due to large antenna volume), At present, 77GHz automotive radar is gradually being produced and equipped with some high-end vehicles. The main advantages of 77GHz automotive radar are wider allocated frequency band, higher range resolution, smaller volume compared with 24GHz radar and strong target detection ability. However, the production and processing technology of 77GHz radar requires higher requirements. However, at present, this problem is no longer an industry barrier.
The typical military application of millimeter wave radar "long bow" and "no matter after launch" anti tank system is one of the main weapon system development plans of the U.S. Army from 1980s to 1990s. It is installed on the AH-64D attack helicopter manufactured by Boeing company. The purpose is to enable the AH-64D to have high-precision detection and anti tank performance day and night under severe weather and low visibility conditions such as rain, fog, smoke and dust Classification and operational capabilities. The military requires the radar and missile seeker of the system to be light in weight, small in size, high in resolution and work all day, so it can only choose the millimeter wave band. In 1992, the US Army decided to develop a new type of "Hellfire" missile equipped with millimeter wave active radar seeker, later known as "Longbow Hellfire" missile, code named agm-114l, for the "Longbow Apache" weapon system and for attacking ground tanks and armored forces“ The millimeter waveguide seeker of Longbow Hellfire missile has a working frequency of 94 GHz and an operating range of 12 ~ 16 km. For short-range targets or moving targets, the agm-114l millimeter wave seeker can lock the target before launch by using the data sent by apg-78 radar or helicopter target acquisition and calibration sight (TADS); When attacking a long-range fixed target, the missile is launched in the direction of the target, and the inertial navigation system is used to control the missile before the millimeter wave seeker locks the target for terminal guidance (final aiming).
The application of millimeter wave radar in the field of UAV millimeter wave radar has been widely used in military UAV and UAV. Its first application in UAV and the largest in the market at present is the fixed height application of plant protection UAV.
We know that GPs and barometer measure the altitude, and during plant protection, we want UAVs to fly at a fixed altitude above crops, regardless of whether the ground and vegetation fluctuate or not. This is also called simulated ground flight. There are many solutions for this application, such as ultrasound, laser, infrared, binocular and so on. However, because the plant protection environment is mostly poor, there is a lot of dust and water mist, ultrasound and optical based will be greatly disturbed. At present, the altimeter based on millimeter wave radar is the most stable. First, it can penetrate dust and water mist. In addition, it is basically free from interference. Based on beam rather than point reflection, the height just reflects the height of vegetation leaves. The second application of UAV is obstacle avoidance. This is also a battlefield for a variety of sensors. However, we say that millimeter wave radar has the advantages of being unaffected by light, large operating range and reliability, which have been proved in military aircraft, automobiles and UAVs. Of course, the resolution of millimeter wave radar is relatively low. However, due to the advantages of array antenna, this can be greatly improved. Therefore, millimeter wave radar has a lot of room for adjustment, such as beam width, operating range, price, etc. Millimeter wave radar has obvious advantages in UAV height measurement and obstacle avoidance, but it also needs to be supplemented by optics. Application of millimeter wave radar in autopilot function The sensors used for automatic driving mainly include camera, millimeter wave radar, laser, ultrasonic, infrared, etc. Millimeter wave radar has long transmission distance, low atmospheric attenuation and loss in the transmission window and strong penetration, which can meet the requirements of vehicle adaptability to all-weather climate. Moreover, the characteristics of millimeter wave itself determine the millimeter wave radar sensor
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