Laser Radar is a radar system that detects the position, velocity, and other characteristics of a target by emitting a laser beam. Its working principle is to emit detection signals (shock beams) to the target, and then compare the received signal (target echo) reflected from the target with the transmission signal. After appropriate processing, relevant information about the target can be obtained, such as distance, orientation, altitude, velocity, attitude, and even shape parameters, so as to detect, track, and recognize targets such as aircraft and missiles. 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 out. The light receiver then restores the light pulses reflected back from the target into electrical pulses and sends them to the display.

Radar function

LiDAR (Light Detection and Ranging) is the abbreviation for laser detection and ranging systems, also known as Laser Radar or LADAR (Laser Detection and Ranging).
An active remote sensing device that uses a laser as the emitting light source and employs photoelectric detection technology. Lidar is an advanced detection method that combines laser technology with modern optoelectronic detection technology. It consists of transmission system, reception system, information processing, and other components. The emission system consists of various forms of lasers, such as carbon dioxide lasers, neodymium doped yttrium aluminum garnet lasers, semiconductor lasers, wavelength tunable solid-state lasers, and optical beam expansion units; The receiving system adopts a telescope and various forms of photodetectors, such as photomultiplier tubes, semiconductor photodiodes, avalanche photodiodes, infrared and visible light multi detector devices, etc. Lidar operates in two modes: pulse or continuous wave. The detection methods can be divided into different types based on the detection principle, such as Mie scattering, Rayleigh scattering, Raman scattering, Brillouin scattering, fluorescence, Doppler, etc.

Composition principle

LiDAR is a system that combines laser, global positioning system, and inertial navigation system technologies to obtain data and generate accurate DEMs. The combination of these three technologies can accurately locate the spot of the laser beam hitting an object. It is further divided into the increasingly mature terrain LiDAR system used to obtain ground digital elevation models and the mature hydrological LIDAR system used to obtain underwater DEMs. The common feature of these two systems is the use of lasers for detection and measurement, which is also the English translation of the term LiDAR, namely: Light Detection And Ranging, abbreviated as LiDAR.

Laser itself has a very precise ranging ability, with a ranging accuracy of several centimeters, and the accuracy of LIDAR system depends not only on the laser itself, but also on internal factors such as synchronization of laser, GPS, and inertial measurement unit (IMU). With the development of commercial GPS and IMU, obtaining high-precision data from mobile platforms (such as on airplanes) through LIDAR has become possible and widely used.

The LIDAR system includes a single narrowband laser and a receiving system. The laser generates and emits a beam of light pulse, which hits an object and reflects back, ultimately being received by the receiver. The receiver accurately measures the propagation time of light pulses from emission to reflection back. Because light pulses propagate at the speed of light, the receiver always receives the previous reflected pulse before the next one is emitted. Given that the speed of light is known, propagation time can be converted into a measurement of distance. By combining the height of the laser, the scanning angle of the laser, the position of the laser obtained from GPS, and the direction of laser emission obtained from INS, the coordinates X, Y, and 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 10000 pulses per second will record 600000 points in one minute by the receiver. Generally speaking, the ground spot spacing of LIDAR systems varies from 2 to 4m. 



The working principle of LiDAR is very similar to radar. Laser is used as the signal source, and the pulse laser emitted by the laser hits trees, roads, bridges, and buildings on the ground, causing scattering. A part of the light wave will be reflected onto the LiDAR receiver. According to the principle of laser ranging, the distance from LiDAR to the target point can be calculated. The pulse laser continuously scans the target object to obtain data of all target points on the target object. After imaging processing with this data, accurate three-dimensional images can be obtained.

The basic working principle of LiDAR is no different from that of radio radar, which is to send a signal by the radar transmission system, which is reflected by the target and collected by the receiving system. 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 by measuring two or more distances and calculating their rate of change to obtain the velocity.