“Lidar is the product of the combination of laser technology and radar technology. It is composed of 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 optoelectronics 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.
Lidar is the product of the combination of laser technology and radar technology. It is composed of 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 optoelectronics 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 photo plan (X, Y) 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 photographs. For one hundred years, stereo photogrammetry is still the most accurate and reliable technology for obtaining 3D ground data, and an important technology for the country’s basic scale topographic map surveying and 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 the corresponding software and digital stereo photogrammetry workstations have been popularized in the production department. However, the work flow of photogrammetry has basically not changed much, such as aerial photography-photographic processing-ground measurement (aerial triangulation)-stereoscopic measurement-mapping (DLG, DTM, GIS, and others). There is basically no major change. The cycle of this production model is too long to meet the needs of the current information society, nor can it meet the requirements of the “digital earth” for surveying and mapping.
The development of LIDAR surveying and mapping technology airborne laser scanning technology originated from the research and development of NASA in 1970. Due to the development of Global Positioning System (Global Positioning System, GPS) and Inertial Navigation System (InertialInertiNavigation System, INS), accurate real-time positioning and attitude are realized. From 1988 to 1993, Stuttgart University in Germany combined laser scanning technology with real-time positioning and attitude determination system to form an airborne laser scanner (Ackermann-19). After that, the airborne laser scanner developed quite rapidly. Commercialization began in 1995. At present, more than 10 manufacturers have produced airborne laser scanners, with more than 30 models to choose from (Baltsavias-1999). The original purpose of developing an airborne laser scanner was to observe the observations of multiple echoes and to measure the height models of the ground and tree tops. Due to its highly automated and accurate observation results, the airborne laser scanner is the main DTM production tool.
The laser scanning method is not only the main way to obtain 3D geographic information in the military, but also the data obtained through this way is also widely used in resource exploration, urban planning, agricultural development, water conservancy engineering, land use, environmental monitoring, transportation and communication, and earthquake prevention. Disaster reduction and national key construction projects have provided extremely important original data for the national economy, social development and scientific research, and have 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 field cost of production data and post-processing cost. At present, the majority of users urgently need low-cost, high-density, rapid, high-precision digital elevation data or digital surface data. The airborne LIDAR technology just meets this demand, so it has become a popular high-tech in various measurement applications.
The 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, the research on the accuracy of airborne LIDAR data has very important theoretical value and practical significance. In this context, domestic and foreign scholars have done a lot of research on improving the accuracy of airborne LIDAR data.
Since the flight operation is the first process of lidar aerial surveying and mapping, it provides direct calculation 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 errors contained in the results of the previous process should have the least impact on the latter process. Therefore, it is very meaningful to improve the data quality by studying the airborne lidar operation process and optimizing the design operation plan.
LIDAR is a system that integrates three technologies: 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 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 lasers. Detection and measurement, this is the original English translation of the word LIDAR, namely: LIght Detection And Ranging-LIDAR.
The laser itself has a very precise ranging capability, and its ranging accuracy can reach several centimeters. In addition to the laser itself, the accuracy of the LIDAR system also depends on the 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 and widely used 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 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, 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 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 pulse lidar for detection using laser pulses, and continuous wave lidar for detection using continuous wave laser beams. 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.