What Is LIDAR Scanning?

LiDAR (LightLaser Detection and Ranging) is an abbreviation for laser detection and ranging system.

Lidar system

Lidar

LiDAR (LightLaser Detection and Ranging) is an abbreviation for laser detection and ranging system.

Radar using a laser as a radiation source. Lidar is the combination of laser technology and radar technology. It consists of transmitter, antenna, receiver, tracking frame and information processing. Transmitters are various types of lasers, such as carbon dioxide lasers, neodymium-doped yttrium aluminum garnet lasers, semiconductor lasers, and solid-state lasers with tunable wavelengths; antennas are optical telescopes; receivers use various types of photodetectors, such as photoelectric Multiplier tubes, semiconductor photodiodes, avalanche photodiodes, infrared and visible multiple detection devices, etc. Lidar uses two working modes, pulse or continuous wave. The detection methods include direct detection and heterodyne detection.

Since the first photo was taken by Daguerre and Niepce in 1839, the technology of making photo plan (X, Y) from photos has been used to this day. In 1901, the Dutch Fourcade invented the stereo measurement technology of photogrammetry, which made it possible to obtain three-dimensional ground data (X, Y, Z) from two-dimensional images. For a century, stereophotogrammetry is still the most accurate and reliable technique for obtaining three-dimensional ground data, and it is an important technique for the topographic map of the country's basic scale.

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 the corresponding software and digital stereo photogrammetry workstations have become popular in the production sector. However, the workflow of photogrammetry has basically not changed much, such as the mode of aerial photography-photographic processing-ground surveying (air triangulation) -stereometry-drafting (DLG, DTM, GIS, and others). The cycle of this production mode is too long to meet the needs of the current information society, and it cannot meet the requirements of "digital earth" for mapping.

The development of LIDAR surveying and mapping technology for 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 InertiNavigation System (INS), accurate real-time positioning and attitude have been realized. The University of Stuttgart, Germany, combined laser scanning technology with an instant positioning and orientation system from 1988 to 1993 to form an unloaded laser scanner (Ackermann-19). Since then, the unloaded laser scanners have developed rapidly and have been commercialized since about 1995. At present, more than 10 manufacturers have produced unloaded laser scanners, with more than 30 models to choose from (Baltsavias-1999). The original purpose of developing an unloaded laser scanner was to observe the observations of multiple echoes, and to measure the height model of the ground surface and tree tops. Due to its highly automated and accurate observation results, no-load laser scanners are used as the main DTM production tool.

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

Quickly obtaining high-precision digital elevation data or digital surface data is a prerequisite for the widespread application of airborne LIDAR technology in many fields. Therefore, it is of great theoretical value and practical significance to carry out research on the accuracy of airborne LIDAR data. In this context, scholars at home and abroad have done a lot of research on improving the accuracy of airborne LIDAR data.

Since the flight operation is the first procedure of lidar aerial surveying and mapping, it provides direct starting data for subsequent internal data processing. According to the principle of measurement error and the basic principle of formulating "standards", it is required that the error contained in the results of the previous process should have the least impact on the latter process. Therefore, it is very meaningful to study the airborne lidar operation process and optimize the design of the operation plan to improve the data quality.

LIDAR is a system that integrates three technologies of laser, global positioning system (GPS) and 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 on the object with high accuracy. It is further divided into the currently mature terrain LIDAR system for obtaining ground digital elevation models (DEM) and the mature hydrological LIDAR system for obtaining underwater DEM. The common feature of these two systems is the use of lasers. Detection and measurement, which is exactly the original English translation of the word LIDAR: LIght Detection And Ranging-LIDAR.

The laser itself has a very accurate ranging ability, and its ranging accuracy can reach a few centimeters. The accuracy of the LIDAR system depends on the inherent factors such as laser, GPS, and inertial measurement unit (IMU) synchronization in addition to the laser itself. . 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) via 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 a light pulse from its emission to its reflection back. Because the light pulse travels at the speed of light, the receiver always receives the pulse that was reflected back before the next pulse is sent. Given that the speed of light is known, the time of flight 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, and Z of each ground spot can be accurately calculated. Laser beams can be emitted at frequencies ranging 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 operating in the infrared to ultraviolet spectral range. Its principle and structure are very similar to laser rangefinders. Scientists call laser pulse detection as pulsed lidar, and continuous wave laser detection as continuous wave lidar. The role of lidar is to accurately measure target position (distance and angle), motion state (speed, vibration, and attitude) and shape, and detect, identify, distinguish, and track targets. After years of efforts, scientists have developed fire-control lidars, detection lidars, missile-guided lidars, shooting range measurement lidars, and navigation lidars.

Helicopter obstacle avoidance lidar

At present, lidar has entered the practical stage in obstacle avoidance of low-flying helicopters, detection of chemical / biological warfare agents, and underwater target detection, and other military application research has also matured.

Helicopters are apt to collide with hills or buildings on the ground when conducting low-level patrol flights. For this reason, the development of helicopter airborne radar that can avoid obstacles on the ground is the dream of people. Currently, this radar has been successful in the United States, Germany and France.

Helicopter ultra low altitude flight obstacle avoidance system developed by the United States, using a solid-state laser diode transmitter and a rotating holographic scanner, can 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 for safe flight. Has played a great role in protection.

The Hellas obstacle detection lidar successfully developed by German Daimler Benz Aerospace company is even higher. It is a solid 1.54 micron imaging lidar with a field of view of 32 degrees x 32 degrees and can detect 300-500 A 1-cm thick wire within a meter distance will be installed on the new EC-135 and EC-155 helicopters.

The pod-mounted CLARA lidar jointly developed by Dassault Electronics of France and Marconi of the United Kingdom 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 moving target indication. It is suitable for aircraft and helicopters.

Chemical warfare agent detection lidar

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

Russia successfully developed the KDKhr-1N long-range ground laser gas alarm system, which can detect chemical poisoning attacks at a long distance in real time, determine the slope distance, center thickness, height above the ground, center angle coordinates, and related parameters of the poison aerosol cloud, and Alarm signals can be sent to the army's automatic control system through radio channels or wired lines, which is a big step forward than traditional detection.

The successfully developed VTB-1 telemetry chemical warfare agent sensor technology in 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 measure the chemical warfare agent. Safe and accurate.

Airborne ocean lidar

The traditional underwater target detection device is a sonar. According to the sound wave transmission and reception methods, sonar can be divided into active and passive, which can alert, search, characterize and track targets in the water. But it is very bulky and weighs more than 600 kg, and some even weighs tens of tons. The laser radar uses airborne blue-green lasers to transmit and receive equipment. It emits high-power narrow-pulse lasers to detect and classify targets below the sea surface, which is simple and accurate.

To date, three generations of airborne marine lidar have been developed. The third-generation system successfully developed in the 1990s is based on the second-generation system, adding GPS positioning and altitude setting functions, and the system interfaces with the auto-navigation device to realize automatic control of air routes and altitude.

Imaging lidar detects underwater objects

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

Carman Aerospace has successfully developed an airborne underwater imaging lidar. The biggest feature is that it can image underwater targets. Due to the large coverage area of each laser pulse of imaging laser radar, its search efficiency is much higher than that of non-imaging laser radar. In addition, imaging lidars can display features such as the shape of underwater targets, making it easier to identify targets, which is a major advantage of imaging lidars.