LiDAR

Lidar is a radar system that detects the position, velocity, and other characteristic parameters of a target by emitting a laser beam. In terms of working principle, it is not fundamentally different from microwave radar: it emits detection signals (shock beams) to the target, and then compares the received signal (target echo) reflected back from the target with the transmission signal. After appropriate processing, it can obtain relevant information about the target, such as distance, orientation, altitude, velocity, attitude, and even shape parameters, so as to detect, track, and recognize targets such as airplanes and missiles.



A radar that operates in the infrared and visible light bands and uses laser as the working beam is called a lidar. 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.

Lidar is a radar system that detects the position, velocity, and other characteristic parameters of a target by emitting a laser beam. In terms of working principle, it is not fundamentally different from microwave radar: it emits detection signals (shock beams) to the target, and then compares the received signal (target echo) reflected back from the target with the transmission signal. After appropriate processing, it can obtain relevant information about the target, such as distance, orientation, altitude, velocity, attitude, and even shape parameters, so as to detect, track, and recognize targets such as airplanes and missiles.

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.


Since the first photograph was taken by Daguerre and Niepce in 1839, the technique of using photographs to create photographic plans (X, Y) has been used to this day. In 1901, the Dutch man Fourcade invented the stereo observation technology of photogrammetry, making it possible to obtain three-dimensional ground data (X, Y, Z) from two-dimensional photographs. For a hundred years, stereo photogrammetry has remained the most accurate and reliable technology for obtaining three-dimensional ground data, and is an important technology for national basic scale topographic mapping.

With the development of science and technology and the widespread application of computers and high-tech, digital stereo photogrammetry has gradually developed and matured, and corresponding software and digital stereo photogrammetric workstations have been popularized in production departments. However, the workflow of photogrammetry has not undergone significant changes, such as the modes of aerial photography photography processing ground measurement (aerial triangulation) stereo measurement mapping (DLG, DTM, GIS, and others). The cycle of this production model is too long to meet the needs of the current information society and cannot meet the requirements of the digital earth for surveying and mapping.

The development of LIDAR surveying technology and airborne laser scanning technology originated from the research and development of NASA in 1970. Due to the development of Global Positioning System (GPS) and Inertial Navigation System (INS), precise real-time positioning and attitude determination have become possible. From 1988 to 1993, Stuttgart University in Germany combined laser scanning technology with real-time positioning and orientation systems to form an airborne laser scanner (Ackermann-19). Afterwards, the development of airborne laser scanners was quite rapid, and commercialization began around 1995. Currently, more than 10 manufacturers have produced airborne laser scanners, with over 30 models to choose from (Baltias-1999). The original purpose of developing an airborne laser scanner was to observe multiple echoes and measure the height models of the ground and tree tops. Due to its high degree of automation and precise observation results, the main DTM production tool is the airborne laser scanner.

The laser scanning method is not only the main way to obtain three-dimensional geographic information within the military, but also the data obtained through this method are widely used in resource exploration, urban planning, agricultural development, water conservancy engineering, land use, environmental monitoring, transportation and communication, earthquake prevention and disaster reduction, and national key construction projects. It provides extremely important raw materials for national economy, social development, and scientific research, and has achieved significant economic benefits, demonstrating good application prospects. Compared with traditional measurement methods, the low airborne LIDAR ground 3D data acquisition method has the advantages of low production data field cost and post-processing cost. At present, users urgently need low-cost, high-density, fast speed, and high-precision digital elevation data or digital surface data. Airborne LIDAR technology precisely meets this demand, making it a popular high-tech in various measurement applications.

Rapid acquisition of high-precision digital elevation data or digital surface data is a prerequisite for the widespread application of airborne LIDAR technology in many fields. Therefore, conducting research on the accuracy of airborne LIDAR data has significant theoretical value and practical significance. In this context, scholars at home and abroad have conducted extensive research on improving the accuracy of airborne LIDAR data.

As flight operations are the first step in LiDAR aerial mapping, they provide direct starting data for subsequent office data processing. According to the principle of measurement error and the basic principle of formulating standards, it is required that the errors contained in the results of the previous process should have the minimum impact on the subsequent process. Therefore, it is very meaningful to improve data quality by studying the operation process of airborne LiDAR and optimizing the design of operation plans.

The basic principles of LiDAR

LIDAR is a system that combines laser, global positioning system (GPS), and inertial navigation system (INS) 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 (DEMs) 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 word LIDAR, namely: Light Detection And Ranging 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 2-4m.



Characteristics of LiDAR

Compared with ordinary microwave radar, LiDAR uses a laser beam and operates at a much higher frequency than microwaves, which brings many characteristics, mainly including:

(1) High resolution

Lidar can achieve extremely high resolution in angle, distance, and velocity. Usually, the angular resolution is not less than 0.1mard, which means it can distinguish two targets at a distance of 3km with a distance of 0.3m (which microwave radar cannot achieve in any case), and can track multiple targets simultaneously; The distance resolution can reach 0. lm; The speed resolution can reach up to 10m/s. High resolution of distance and velocity means that clear images of targets can be obtained using distance Doppler imaging technology. High resolution is the most significant advantage of LiDAR, and most of its applications are based on it.

(2) Good concealment and strong resistance to active interference

Laser propagates in a straight line, has good directionality, and the beam is very narrow, which can only be received on its propagation path. Therefore, enemy interception is very difficult, and the aperture of the laser radar transmission system (transmission telescope) is very small, with a narrow reception area. The probability of intentionally emitted laser interference signals entering the receiver is extremely low; In addition, unlike microwave radars, which are susceptible to the widespread influence of electromagnetic waves in nature, there are not many signal sources that can interfere with LiDAR in nature. Therefore, LiDAR has a strong ability to resist active interference and is suitable for working in increasingly complex and intense information warfare environments.

(3) Good low altitude detection performance

Due to the influence of various ground echoes, microwave radar has blind spots (undetectable areas) in certain areas at low altitudes. For LiDAR, only the illuminated target will produce reflection, and there is no influence of ground echoes. Therefore, it can work at zero altitude, and its low altitude detection performance is much stronger than that of microwave radar.

(4) Small size and light weight

Usually, ordinary microwave radars have a large volume, with the entire system weighing in tons, and the aperture of the optical antenna can reach several meters or even tens of meters. Lidar, on the other hand, is much lighter and more agile. The aperture of the transmitting telescope is generally only in centimeters, and the minimum weight of the entire system is only a few tens of kilograms, making it easy to install and dismantle. Moreover, the structure of LiDAR is relatively simple, easy to maintain, easy to operate, and the price is also relatively low.



Disadvantages of LiDAR

Firstly, work is greatly affected by weather and atmospheric conditions. Laser generally attenuates less in clear weather and propagates over longer distances. In bad weather such as heavy rain, thick smoke, and fog, the attenuation increases sharply and the transmission distance is greatly affected. A CO2 laser with a working wavelength of 10.6 μ m has the best atmospheric transmission performance among all lasers, and its attenuation in bad weather is six times that in sunny weather. The operating range of CO2 LiDAR used on the ground or at low altitude is 10-20km on sunny days, while it is reduced to within 1km on bad weather. Moreover, atmospheric circulation can cause distortion and jitter of laser beams, directly affecting the measurement accuracy of LiDAR.

Secondly, due to the extremely narrow beam of LiDAR, it is very difficult to search for targets in space, which directly affects the interception probability and detection efficiency of non cooperative targets. It can only search and capture targets within a small range, so LiDAR is less commonly used directly on the battlefield for target detection and search.

Wonderful use

Lidar is a radar system that operates in the spectral range from infrared to ultraviolet, and its principle and structure are very similar to those of a laser rangefinder. Scientists refer to the use of laser pulses for detection as pulse LiDAR, and the use of continuous wave laser beams for detection as continuous wave LiDAR. The function of LiDAR is to accurately measure the position (distance and angle), motion state (velocity, vibration, and attitude), and shape of the target, detect, recognize, distinguish, and track the target. After years of effort, scientists have developed fire control LiDAR, detection LiDAR, missile guided LiDAR, range measurement LiDAR, navigation LiDAR, and more.

Helicopter obstacle avoidance LiDAR

At present, LiDAR has entered the practical stage in low altitude helicopter obstacle avoidance, chemical/biological warfare agent detection, and underwater target detection, and other military application research is also becoming increasingly mature.
Helicopters are prone to colliding with small hills or buildings on the ground during low altitude patrol flights. Therefore, developing a helicopter mounted radar that can avoid ground obstacles is a dream for people. At present, this type of radar has been successful in the United States, Germany, and France.

The helicopter ultra-low altitude obstacle avoidance system developed in the United States uses a solid laser diode transmitter and a rotating holographic scanner to detect a wide airspace in front of the helicopter. Ground obstacle information is displayed in real-time on the airborne head up display or helmet display, providing great assurance for safe flight.

Daimler, Germany. The successful development of the Hel by Mercedes Benz Aerospace Company?? LAS obstacle detection lidar is even better. It is a solid-state 1.54 micron imaging lidar with a field of view of 32 degrees x 32 degrees, capable of detecting wires with a diameter of 1 centimeter within a distance of 300-500 meters. It will be installed on the new EC-135 and EC-155 helicopters.
The CLARA lidar, jointly developed by French company Dassault Electronics and British company Marconi, has multiple functions and uses a CO2 laser. 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, suitable for airplanes and helicopters.

Chemical warfare agent detection lidar

The traditional chemical warfare agent detection device is carried by soldiers, who move forward while detecting, and the detection speed is slow, and soldiers are prone to poisoning.

The KDKhr-1N long-distance ground laser gas alarm system successfully developed by Russia can detect chemical agent attacks in real time and distance, determine the slant distance, center thickness, ground clearance height, center angle coordinates, and agent related parameters of the aerosol cloud, and send alarm signals to the automatic control system of the troops through wireless channels or wired lines, which is a big step forward from traditional detection.

The VTB-1 type telemetry chemical warfare agent sensor technology 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 differential absorption spectroscopy principles to telemetry chemical warfare agents, which is both safe and accurate.

Folding airborne ocean lidar

The traditional underwater target detection device is sonar. According to the emission and reception methods of sound waves, sonar can be divided into active and passive types, which can alert, search, classify, and track underwater targets. But it has a large volume, usually weighing over 600 kilograms, and some even weigh tens of tons. Lidar, on the other hand, utilizes airborne blue-green laser emission and reception equipment to detect and classify underwater targets by emitting high-power narrow pulse lasers, which is both simple and accurate.


So far, airborne ocean lidar has developed three generations of products. The third-generation system, developed successfully in the 1990s, is based on the second-generation system and adds GPS positioning and altitude determination functions. The system interfaces with automatic navigation devices to achieve automatic control of flight routes and altitudes.

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 and real-time detection functions and three-dimensional positioning capabilities. It has high positioning resolution and can work 24 hours. It uses egg shaped scanning to detect suspicious underwater targets.

The airborne underwater imaging LiDAR developed by the American company Kaman Aerospace is characterized by its ability to image underwater targets. Due to the large coverage area of each laser pulse in imaging LiDAR, its search efficiency is much higher than that of non imaging LiDAR. In addition, imaging LiDAR can display the shape and other characteristics of underwater targets, making it easier to identify targets, which has become a major advantage of imaging LiDAR.