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    2021-10-21 17:41:57
    How does Lidar Work?


    Ⅰ Introduction
    Ⅱ Lidar fundamental principle
    Ⅲ Lidar applications 1.Helicopter obstacle avoidance lidar
    2.Chemical warfare agent detection lidar
    3.Airborne Marine Lidar
    4.Imaging lidar can detect underwater objects
    5.Lidar and autonomous driving

    Ⅰ Introduction

    Lidar or LiDAR is an abbreviation for Light Detection and Ranging. Lidar is a radar that uses a laser as a radiation source. Lidar is the product of a combination of laser technology and radar technology. It is composed of a transmitter, antenna, receiver, tracking frame, and information processing. Transmitters are various forms of lasers, such as carbon dioxide lasers, neodymium-doped yttrium aluminum garnet lasers, semiconductor lasers, and wavelength-tunable solid-state lasers, etc... Antennas are optical telescopes. Receivers use various forms of photodetectors, such as photoelectric Multiplier tubes, semiconductor photodiodes, avalanche photodiodes, infrared and visible light multi-detector devices, etc. Lidar adopts two working modes: pulse or continuous wave. The detection methods are divided into direct detection and heterodyne detection.

    Since the first photo was taken by Daguerre and Niepce in 1839, the technique of using the photo to make the plan view (X, Y) of the photo has been in use ever since. In 1901, the Dutch Fourcade invented the stereoscopic observation technology of photogrammetry, which made it possible to obtain ground three-dimensional data (X, Y, Z) from two-dimensional photos. For one hundred years, stereophotogrammetry is still the most accurate and reliable technology for obtaining 3D ground data, and important technology for a country's basic scale topographic map surveying and mapping.

    Ⅱ Lidar fundamental principle

    LIDAR is a system that integrates three technologies: laser, a global positioning system (GPS), and an inertial navigation system (INS) 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 currently maturing terrain LIDAR system for obtaining the ground digital elevation model (DEM) and the hydrological LIDAR system for obtaining the underwater DEM which has been matured. The common feature of these two systems is the use of laser detection and measurement, namely: LIght Detection And Ranging-LIDAR.

    Lidar structure.jpg

    Lidar structure

    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 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 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 is sent. 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, in 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.

    Lidar is a radar system that works in the infrared to ultraviolet spectrum. Its principle and structure are very similar to laser rangefinders. Scientists call a lidar using laser pulses a pulsed lidar, and a lidar using continuous-wave laser beams a continuous wave lidar. The function of lidar is to accurately measure the position (distance and angle), movement state (speed, vibration, and attitude), and shape of the target, and detect, identify, distinguish and track the target. After years of hard work, scientists have developed fire control lidar, detection lidar, missile guidance lidar, range measurement lidar, navigation lidar, etc.

    Ⅲ Lidar applications

    1.Helicopter obstacle avoidance lidar

    At present, lidar has entered the practical stage in low-flying helicopter obstacle avoidance, chemical/biological agent detection, and underwater target detection, and other military application research is also becoming more mature.

    When a helicopter is flying on a low-altitude patrol, it is very easy to collide with hills or buildings on the ground. For this reason, the development of helicopter airborne radar that can evade ground obstacles is a long-cherished wish of people. At present, this kind of radar has been successful in the United States, Germany, and France.

    The U.S.-developed helicopter ultra-low-altitude flight obstacle avoidance system uses a solid-state laser diode transmitter and a rotating holographic scanner to detect wide airspace in front of the helicopter. The ground obstacle information is displayed on the airborne head-up display or helmet display in real-time.

    The Hellas Obstacle Detection Lidar developed by German Daimler Benz Aerospace is even higher. It is a solid 1.54-micron imaging lidar with a field of view of 32 degrees × 32 degrees and can detect wires with a diameter of 1 cm in a distance of 300-500 meters. 

    The pod-mounted CLARA lidar jointly developed by France's Dassault Electronics Company and the British Marconi Company has multiple functions and uses CO2 lasers. Not only can it detect obstacles such as benchmarks and cables, but it also has functions such as terrain tracking, target ranging and indication, and active target indication. It is suitable for airplanes and helicopters.

    2.Chemical warfare agent detection lidar

    Traditional chemical warfare agent detection devices are carried by soldiers, and they move forward while detecting. The detection speed is slow and soldiers are prone to poisoning.

    Russia successfully developed the KDKhr-1N long-distance ground laser poison gas alarm system, which can detect chemical poison attacks remotely in real-time, determine the slant distance, center thickness, ground height, center angle coordinates, and poison-related parameters of the poison aerosol cloud, and an alarm signal can be sent to the automatic control system of the army through a radio channel or a wired line, which is a big step forward than traditional detection.

    The VTB-1 remote sensing chemical warfare agent sensor technology successfully developed by Germany is more advanced. It uses two 9-11 micron continuous wave CO2 lasers that can be adjusted at 40 frequencies and uses the principle of differential absorption spectroscopy to remotely detect chemical warfare agents. 

    3.Airborne Marine Lidar

    The traditional underwater target detection device is sonar. According to the way of transmitting and receiving sound waves, sonar can be divided into active and passive sonar, which can alert, search, characterize and track targets in the water. But it is large in size, generally weighing more than 600 kilograms, and some even weighing tens of tons. Lidar uses airborne blue-green laser transmitting and receiving equipment to detect and classify targets under the sea by emitting high-power narrow pulse lasers, which is simple and accurate.

    So far, airborne marine lidar has developed three generations of products. The third-generation system successfully developed in the 1990s is based on the second-generation system, adding GPS positioning and height fixation functions, and the system interfaces with autopilots to realize automatic control of routes and altitudes.

    4.Imaging lidar can detect underwater objects

    The ALARMS airborne mine detection system developed by Northrop Corporation for the US Defense Advanced Research Projects Agency has automatic, real-time detection functions and three-dimensional positioning capabilities, high positioning resolution, and can work for 24 hours. It uses oval scanning to detect suspicious targets in the water.

    The airborne underwater imaging lidar successfully developed by Kaman Aerospace Corporation of the United States is characterized by its ability to image underwater targets. Since each laser pulse of imaging lidar covers a large area, its search efficiency is much higher than that of non-imaging lidar. In addition, imaging lidar can display features such as the shape of underwater targets, making it easier to identify targets, which is already a major advantage of imaging lidar.

    5.Lidar and autonomous driving

    Automotive Lidar.jpg

    Automotive Lidar

    As we all know, the realization path of automatic driving is divided into two schemes: pure vision and lidar. The representative of pure vision is Tesla Autopilot, and the representative of lidar is Waymo. The industry's controversy over lidar has never stopped. In fact, it is because of the lidar camps that are divided into Tesla's pure vision and "other car companies". Musk's theory is that since people don't need lidar and can drive with only "binocular cameras", then artificial intelligence can naturally also.

    The logical problem here is that the human eye itself is not a pair of simple optical lenses. The retina can take care of both day and night, is also responsible for color recognition, and is more effective under relatively bright light. In addition, the human brain does information filtering and processing and makes adaptive decisions based on traffic conditions. The current vehicle-mounted camera technology and artificial intelligence technology cannot reach human learning and decision-making capabilities.

    The most attractive part of the pure visual solution is actually its low cost. The core advantage is that mass production is faster, and cameras are also easier to pass the car regulations for mass production, and it is easier to commercialize the entire vehicle. The disadvantage is that it can only be used in a specific environment, which is very restricted. In fact, it is the problem that the camera cannot recognize objects under strong light. 

    Lidar makes up for the lack of environmental information perception of the camera, and its biggest advantage is that it can use Doppler imaging technology to create a clear 3D image of the target. The distance is determined by measuring the time difference and phase difference of the laser signal. Using the three-dimensional coordinates, reflectivity, and texture information of a large number of dense points on the surface of the target object collected in this process, the three-dimensional model of the measured target and various related data such as lines, surfaces, and bodies can be quickly obtained to achieve environmental perception. 

    The cost of lidar is higher than that of pure vision solutions, but it has a wider range of application scenarios and can compensate for the lack of cameras and radar sensors. The camera is suitable for image recognition, but it is difficult to work in bad weather, nor is it suitable for building a 3D environment; millimeter-wave radar is suitable for use in various climatic conditions and is also suitable for speed and distance judgment, but the resolution is very low, and it is not suitable Identification of item types. Lidar can just make up for the shortcomings of cameras and millimeter-wave radars: it can build a more realistic 3D environment than cameras, and it has more accurate object recognition capabilities than millimeter-wave radars. The Trinity can actually make up for each other's shortcomings.

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