Lidar is a combination of laser and global positioning system

Lidar is a system that combines three technologies: laser, Global Positioning System (GPS), and Inertial Measurement Unit (IMU). Compared to ordinary radar, lidar has advantages such as high resolution, good concealment, and stronger anti-interference ability. With the continuous development of technology, the application of LiDAR is becoming more and more widespread, and its presence can be seen in fields such as robotics, autonomous driving, and unmanned vehicles. There will inevitably be a market if there is demand. With the increasing demand for LiDAR, the types of LiDAR have also become diverse, and they can be divided into different types according to their usage functions, detection methods, and load platforms.


Laser ranging radar

Laser ranging radar determines the distance between the measured object and the test point by emitting a laser beam and receiving the reflected wave of the laser beam, recording the time difference. Traditionally, LiDAR has been used in the field of industrial safety detection, such as the laser wall seen in science fiction films. When someone breaks in, the system will immediately respond and issue a warning. In addition, laser ranging radar is also widely used in the field of space surveying and mapping. But with the rise of the artificial intelligence industry, laser ranging radar has become an indispensable core component in robots. When used in conjunction with SLAM technology, it can help robots achieve real-time positioning and navigation, and achieve autonomous walking. The rplidar series developed by Silan Technology, combined with the Slamware module, is a typical representative of autonomous positioning and navigation for service robots. It can complete laser ranging of tens of thousands of times per second within a 25 meter ranging radius and achieve millimeter level resolution.

Laser velocimetry radar

Laser velocimetry radar is a measurement of the movement speed of an object, which is obtained by performing two laser ranging measurements with specific time intervals on the object being measured. There are two main types of methods for measuring velocity using LiDAR. One is based on the principle of LiDAR ranging, which continuously measures the distance of the target at a certain time interval. The velocity value of the target can be obtained by dividing the difference between two target distances by the time interval, and the direction of velocity can be determined based on the positive or negative difference in distance. This method has a simple system structure and limited measurement accuracy, and can only be used for hard targets with strong laser reflection. Another type of speed measurement method is to use Doppler frequency shift. Doppler frequency shift refers to the frequency difference between the frequency of the received echo signal and the frequency of the transmitted signal when there is a relative velocity between the target and the LiDAR. This frequency difference is called Doppler frequency shift.

Laser imaging radar

Laser imaging radar can be used for detecting and tracking targets, obtaining target orientation and velocity information, etc. It can accomplish tasks that ordinary radar cannot, such as detecting submarines, mines, hidden military targets, and so on. It is widely used in military, aerospace, industrial, and medical fields.

Atmospheric detection lidar

The atmospheric detection lidar is mainly used to detect the density, temperature, wind speed, direction, and concentration of water vapor in molecules and smoke in the atmosphere, in order to monitor the atmospheric environment and predict catastrophic weather such as storms and sandstorms.

Tracking radar

Tracking radar can continuously track a target, measure its coordinates, and provide the target's motion trajectory. Not only used for artillery control, missile guidance, external ballistic measurement, satellite tracking, breakthrough technology research, but also expanding in fields such as meteorology, transportation, and scientific research.

Solid state LiDAR

Solid state LiDAR has a high peak power, an output wavelength range that matches existing optical components and devices, a long output range that matches existing optical components and devices (such as modulators, isolators, and detectors), and atmospheric transmission characteristics. Moreover, it is easy to achieve a main oscillator power amplifier (MOPA) structure. Coupled with conductors such as high efficiency, small size, light weight, high reliability, and good stability, solid state LiDAR is prioritized for application in airborne and space-based systems. In recent years, the focus of LiDAR development has been on diode pumped solid-state LiDAR.

Gas LiDAR

Gas LiDAR, represented by CO2 LiDAR, operates in the infrared band with low atmospheric transmission attenuation and long detection distance. It has played a significant role in atmospheric wind field and environmental monitoring. However, its large volume and the use of mid infrared HgCdTe detectors must work at a temperature of 77K have limited the development of gas LiDAR. Semiconductor LiDAR can operate continuously at high repetition rates, with the advantages of long lifespan, small size, low cost, and minimal harm to the human eye. It is widely used in Mie scattering measurements with strong backscatter signals, such as detecting cloud base height. The potential application of semiconductor LiDAR is to measure visibility, obtain aerosol extinction profiles in the atmospheric boundary layer, and identify rain and snow, making it easy to make airborne equipment. The CT25K laser cloud detector developed by Vaisala in Finland is a typical representative of semiconductor cloud lidar, with a measurement range of up to 7500m for cloud base height.

Single line LiDAR

Single line LiDAR is mainly used to avoid obstacles, with fast scanning speed, strong resolution, and high reliability. Due to the faster reflection of angular frequency and sensitivity in single line LiDAR compared to multi line and 3D LiDAR, it is more accurate in testing the distance and accuracy of surrounding obstacles. However, single line radar can only scan in a planar manner and cannot measure the height of objects, which has certain limitations. Currently, it is mainly applied to service robots, such as our common sweeping robots.

Multi line LiDAR

Multi line LiDAR is mainly used for radar imaging in automobiles. Compared to single line LiDAR, it has made qualitative changes in dimension enhancement and scene restoration, and can recognize the height information of objects. Multi line LiDAR is typically 2.5D and can achieve 3D. At present, there are mainly 4-wire, 8-wire, 16 wire, 32 wire, and 64 wire launched in the international market. But the price is high, and most car companies will not choose it.

MEMS type LiDAR

MEMS type LiDAR can dynamically adjust its scanning mode to focus on special objects, collect detailed information of farther and smaller objects, and recognize them, which traditional mechanical LiDAR cannot achieve. The entire MEMS system only requires a small reflector to guide a fixed laser beam in different directions. Due to the small size of the reflector, its inertia moment is not significant and can move quickly, reaching a speed that can track 2D scanning mode in less than a second.

Flash type LiDAR

The Flash type LiDAR can quickly record the entire scene, avoiding various troubles caused by the movement of targets or LiDAR during the scanning process. It runs more like a camera. The laser beam will diffuse directly in all directions, so just one quick flash can illuminate the entire scene. Subsequently, the system will use a micro sensor array to collect laser beams reflected back from different directions. Flash LiDAR has its advantages, but there are also certain drawbacks. The larger the pixel, the more signals need to be processed. If a large number of pixels are inserted into a photodetector, it will inevitably bring various interferences, resulting in a decrease in accuracy.

Phased array LiDAR

The phased array LiDAR is equipped with a row of transmitters that can change the direction of laser beam emission by adjusting the relative phase of the signal. At present, most phased array LiDARs are still in the laboratory, and they are still stuck in the era of rotary or MEMS LiDARs.

Mechanical rotating LiDAR

Mechanical rotary LiDAR is an early developed LiDAR with mature technology. However, the structure of mechanical rotary LiDAR systems is very complex, and the prices of core components are also quite expensive, including lasers, scanners, optical components, photodetectors, receiving ICs, as well as position and navigation devices. Due to high hardware costs, mass production is difficult and stability needs to be improved. Currently, solid-state LiDAR has become a development direction for many companies.

Direct detection of LiDAR

The basic structure of direct detection lidar is quite similar to that of laser rangefinder. During work, a signal is sent by the transmitting system, which is reflected by the target and collected by the receiving system. The distance of the target is determined by measuring the time it takes for the laser signal to travel back and forth. As for the radial velocity of the target, it can be determined by the Doppler frequency shift of the reflected light, or by measuring two or more distances and calculating their rate of change to obtain the velocity.

Coherent detection lidar

Coherent detection lidar can be divided into monostable and bistable systems. In the so-called monostable system, the transmitting and receiving signals share a common optical aperture and are isolated by the transmitting receiving switch. The bistable system, on the other hand, includes two optical apertures for transmitting and receiving signals, respectively. The transmitting receiving switch is naturally no longer needed, and the rest is the same as the monostable system.

Continuous LiDAR

From the principle of laser, continuous laser means that there is always light coming out, just like turning on the switch of a flashlight, and its light will always be on (except in special circumstances). Continuous laser is a method of collecting data at a certain height by continuously shining light to the desired height. Due to the working characteristics of continuous laser, only one point of data can be collected at a certain moment. Due to the uncertain nature of wind data, using a point to represent wind conditions at a certain altitude is obviously somewhat one-sided. Therefore, some manufacturers compromise by rotating 360 degrees and collecting multiple points on this circular edge for average evaluation. Obviously, this is a concept of multi-point statistical data in a virtual plane.

Pulse type LiDAR

The laser output from pulsed laser is discontinuous, but rather flickering. The principle of pulsed laser is to emit tens of thousands of laser particles. According to the internationally recognized Doppler principle, the reflection of these tens of thousands of laser particles is used to comprehensively evaluate the wind conditions at a certain altitude. This is a three-dimensional concept, which is why there is a theory of detection length. From the characteristics of lasers, pulsed lasers measure several tens of times more points than continuous lasers, and can more accurately reflect a certain altitude wind condition.

Airborne LiDAR

Airborne LiDAR is a technology that tightly integrates laser ranging equipment, GNSS equipment, and INS equipment, using a flying platform as a carrier. By scanning the ground, it records the attitude, position, and reflection intensity of the target, obtains three-dimensional information of the ground, and deeply processes it to obtain the required spatial information. There is broad potential and prospects in both military and civilian fields. Airborne LiDAR has a close detection range. When the laser is transmitted in the atmosphere, its energy is attenuated due to the influence of the atmosphere. The operating range of LiDAR is within 20 kilometers, especially in harsh weather conditions such as dense fog, heavy rain, smoke, and dust. The operating range will be greatly shortened, making it difficult to work effectively. Atmospheric turbulence can also reduce the measurement accuracy of LiDAR to varying degrees.

Vehicle mounted LiDAR

Car mounted LiDAR, also known as car mounted 3D laser scanner, is a mobile 3D laser scanning system that can emit and receive laser beams, analyze the turning time of the laser when it encounters the target object, calculate the relative distance between the target object and the vehicle, and use the collected 3D coordinates, reflectivity, and other information of a large number of dense points on the surface of the target object to quickly reconstruct the target's 3D model and various map data, establish a 3D point cloud map, and draw an environmental map to achieve the goal of environmental perception. The role of on-board laser radar in the auto driving "car building" tide is becoming more and more important. Enterprises such as Google, Baidu, BMW, Bosch, Delphi, etc. have all used laser radar in their auto drive system, driving the rapid expansion of the on-board laser radar industry.

Ground based LiDAR

Ground based LiDAR can obtain 3D point cloud information in forest areas, using point cloud information to extract the position and height of individual trees. It not only saves manpower and material resources, but also improves the accuracy of extraction, and has advantages that other remote sensing methods cannot match. Through the analysis of the forestry application of this technology both domestically and internationally, and the verification of the results of later research on this invention, it is expected that this technology will be used to extract various forest parameters in larger research areas in the future.

Spaceborne LiDAR

The spaceborne radar adopts a satellite platform, with a high orbit and wide observation field, and can reach every corner of the world. This provides a new approach for obtaining three-dimensional control points and digital ground models in overseas areas, which is of great significance for both national defense and scientific research. Spaceborne LiDAR also has the ability to observe the entire celestial body. The exploration programs of the Moon and Mars carried out by the United States include spaceborne LiDAR, and the data provided by it can be used to create comprehensive three-dimensional topographic maps of celestial bodies. In addition, satellite borne LiDAR can also play an important role in measuring the vertical distribution of vegetation, sea surface height, cloud and aerosol vertical distribution, and monitoring special climate phenomena. Through the above introduction of the characteristics, principles, and application fields of LiDAR, I believe everyone can also roughly understand the different attributes of various types of LiDAR. Currently, in the increasingly competitive field of LiDAR, creating low-cost, mass-produced LiDAR is a dream that many startups want to achieve. But developing and mass producing LiDAR is not easy. Rich industry experience and reliable technology are necessary to ensure its dominant position in this wave of trends.