Professional Knowledge

Laser Radar

Lidar (Laser Radar) is a radar system that emits a laser beam to detect the position and speed of a target. Its working principle is to send a detection signal (laser beam) to the target, and then compare the received signal (target echo) reflected from the target with the transmitted signal, and after proper processing, you can obtain relevant information about the target, such as Target distance, azimuth, altitude, speed, attitude, even shape and other parameters, so as to detect, track and identify aircraft, missiles and other targets. It consists of a laser transmitter, an optical receiver, a turntable, and an information processing system. The laser converts electrical pulses into light pulses and emits them. The optical receiver then restores the light pulses reflected from the target to electrical pulses and sends them to the display.
LiDAR is a system that integrates three technologies: laser, global positioning system and inertial navigation system, used to obtain data and generate accurate DEM. The combination of these three technologies can locate the spot of the laser beam hitting the object with high accuracy. It is further divided into the increasingly mature terrain LiDAR system for obtaining ground digital elevation models and the mature hydrological LIDAR system for obtaining underwater DEM. The common feature of these two systems is the use of lasers for detection and measurement. This is also the original English translation of the word LiDAR, namely: LIght Detection And Ranging, abbreviated as LiDAR.
The laser itself has a very precise ranging ability, and its ranging accuracy can reach several centimeters. In addition to the laser itself, the accuracy of the LIDAR system also depends on the internal factors such as the synchronization of the laser, GPS and inertial measurement unit (IMU). . With the development of commercial GPS and IMU, it has become possible and widely used to obtain high-precision data from mobile platforms (such as on airplanes) through LIDAR.
The LIDAR system includes a single-beam narrowband laser and a receiving system. The laser generates and emits a light pulse, hits the object and reflects it back, and is finally received by the receiver. The receiver accurately measures the propagation time of the light pulse from emission to reflection. Because light pulses travel at the speed of light, the receiver always receives the reflected pulse before the next pulse. Given that the speed of light is known, travel time can be converted into a measurement of distance. Combining the height of the laser, the laser scanning angle, the position of the laser obtained from GPS and the direction of laser emission obtained from INS, the coordinates X, Y, Z of each ground spot can be accurately calculated. The frequency of laser beam emission can range from a few pulses per second to tens of thousands of pulses per second. For example, a system with a frequency of 10,000 pulses per second, the receiver will record 600,000 points in one minute. Generally speaking, the ground spot spacing of the LIDAR system ranges from 2-4m. [3]
The working principle of lidar is very similar to that of radar. Using laser as the signal source, the pulsed laser emitted by the laser hits trees, roads, bridges and buildings on the ground, causing scattering, and part of the light waves will be reflected to the receiving of the lidar. On the device, according to the principle of laser ranging, the distance from the laser radar to the target point is obtained. The pulse laser continuously scans the target object to obtain the data of all target points on the target object. After imaging processing with this data, Accurate three-dimensional images can be obtained.
The most basic working principle of lidar is the same as that of radio radar, that is, a signal is sent by the radar transmitting system, which is reflected by the target and collected by the receiving system, and the distance of the target is determined by measuring the running time of the reflected light. As for the radial velocity of the target, it can be determined by the Doppler frequency shift of the reflected light, or it can be measured by measuring two or more distances and calculating the rate of change to obtain the velocity. This is and is also the basic principle of direct detection radars. working principle
Advantages of Lidar
Compared with ordinary microwave radar, because it uses a laser beam, the operating frequency of lidar is much higher than that of microwave, so it brings many advantages, mainly:
(1) High resolution
Lidar can obtain extremely high angle, distance and speed resolution. Usually the angular resolution is not less than 0.1mard, which means that it can distinguish two targets 0.3m apart at a distance of 3km (this is impossible for microwave radar in any case), and can track multiple targets at the same time; the range resolution can be Up to 0.lm; speed resolution can reach within 10m/s. The high resolution of distance and velocity means that distance-Doppler imaging technology can be used to obtain a clear image of the target. High resolution is the most significant advantage of lidar, and most of its applications are based on this.
(2) Good concealment and strong anti-active interference ability
The laser propagates in a straight line, has good directivity, and the beam is very narrow. It can only be received on its propagation path. Therefore, it is very difficult for the enemy to intercept. The laser radar's launch system (transmitting telescope) has a small aperture, and the receivable area is narrow, so it is intentionally launched. The probability that the laser jamming signal enters the receiver is extremely low; in addition, unlike microwave radar, which is susceptible to electromagnetic waves that exist widely in nature, there are not many signal sources that can interfere with the laser radar in nature, so the laser radar is anti-active The interference ability is very strong, suitable for working in the increasingly complex and intense information warfare environment.
(3) Good low-altitude detection performance
Due to the influence of various ground object echoes in microwave radar, there is a certain area of blind area (undetectable area) at low altitude. For lidar, only the illuminated target will reflect, and there is no impact of ground object echo, so it can work at "zero altitude", and the low-altitude detection performance is much stronger than that of microwave radar.
(4) Small size and light weight
Generally, the volume of ordinary microwave radar is huge, the mass of the whole system is recorded in tons, and the diameter of the optical antenna can reach several meters or even tens of meters. The lidar is much lighter and more dexterous. The diameter of the launching telescope is generally only centimeter-level, and the mass of the whole system is only tens of kilograms. It is easy to set up and disassemble. Moreover, the structure of the lidar is relatively simple, the maintenance is convenient, the operation is easy, and the price is low.
Disadvantages of lidar
First of all, the work is greatly affected by the weather and atmosphere. Generally, the attenuation of laser is small in clear weather, and the propagation distance is relatively long. In bad weather such as heavy rain, dense smoke, and fog, the attenuation increases sharply and the propagation distance is greatly affected. For example, the co2 laser with a working wavelength of 10.6μm has the better atmospheric transmission performance among all lasers, and the attenuation in bad weather is 6 times that of sunny days. The range of co2 lidar used on the ground or at low altitude is 10-20km on a sunny day, while it is reduced to less than 1 km on bad weather. Moreover, the atmospheric circulation will also cause the laser beam to be distorted and jittered, which directly affects the measurement accuracy of the lidar.
Secondly, due to the extremely narrow beam of lidar, it is very difficult to search for targets in space, which directly affects the probability of interception and detection efficiency of non-cooperative targets. It can only search and capture targets in a small range. Therefore, lidar is less independent and direct. Used in the battlefield for target detection and search.