How does lidar work?

Lidar (Optical Laser Detection and Ranging) is the abbreviation of laser detection and ranging system.

A radar that uses a laser as a radiation source. Lidar is a combination of laser technology and radar technology. It consists of transmitter, antenna, receiver, tracking frame and information processing. Emitters are various types of lasers, such as carbon dioxide lasers, Nd-doped yttrium aluminum garnet lasers, semiconductor lasers and solid-state lasers with adjustable wavelengths. The antenna is an optical telescope; The receiver adopts various forms of photodetectors, such as photomultiplier tubes, semiconductor photodiodes, avalanche photodiodes, infrared and visible light multi-detector devices, etc. Lidar works in two modes: pulse or continuous wave. Detection methods are divided into direct detection and heterodyne detection.

[Edit this paragraph] The history of lidar

From the first photo taken by Daguerre and Nipce in 1839, the technology of making photo plane (x, y) by using photos has been used to this day. 190 1 year, the Dutch Fourcade invented the stereo observation technology of photogrammetry, which made it possible to obtain three-dimensional data (x, y, z) of the ground from two-dimensional photos. In the past 100 years, stereo photogrammetry is still the most accurate and reliable technology to obtain three-dimensional data on the ground, and it is also an important technology for national basic scale topographic mapping.

With the development of science and technology and the wide application of computer and high technology, digital stereophotogrammetry has gradually matured, and the corresponding software and digital stereophotogrammetry workstation have been popularized in the production department. However, the workflow of photogrammetry has not changed much, such as aerial photography-photographic processing-ground survey (aerial triangulation)-stereo survey-mapping (DLG, DTM, GIS, etc.). This mode of production has a long cycle, which cannot meet the needs of the current information society, nor can it meet the requirements of "digital earth" for surveying and mapping.

The development of lidar mapping technology and no-load laser scanning technology originated from the research and development of 1970 NASA. With the development of Global Positioning System (GPS) and Inertial Navigation System (INS), accurate real-time positioning and attitude measurement have been realized. The University of Stuttgart, Germany, combined the laser scanning technology with the real-time positioning and attitude determination system of 1988 to 1993, and formed an empty laser scanner (Ackermann- 19). After that, the no-load laser scanner developed rapidly and began to be commercialized around 1995. At present, more than 10 manufacturers have produced no-load laser scanners, and there are more than 30 models available (Baltsavia- 1999). The initial purpose of developing an airborne laser scanner is to observe the observation values of multiple echoes and measure the height model of the surface and the top of the tree. Because of its high automation and accurate observation results, no-load laser scanner is the main DTM production tool.

Laser scanning is not only the main way to obtain three-dimensional geographic information in the military, but also the data obtained in this way are widely used in resource exploration, urban planning, agricultural development, water conservancy projects, land use, environmental monitoring, transportation and communication, earthquake prevention and disaster reduction, national key construction projects and other fields, providing extremely important raw materials for national economy, social development and scientific research, achieving remarkable economic benefits and showing good application prospects. Compared with the traditional measurement methods, the ground 3D data acquisition method of low-altitude airborne lidar has the advantages of low field cost and low post-processing cost. At present, users urgently need low-cost, high-density, high-speed and high-precision digital elevation data or digital surface data, and airborne lidar technology just meets this demand, so it has become a hot high-tech in various measurement applications.

Fast acquisition of high-precision digital elevation data or digital surface data is the premise of wide application of airborne lidar technology in many fields. Therefore, it is of great theoretical and practical significance to study 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.

Because flight operation is the first process of lidar aerial mapping, it provides direct initial data for subsequent indoor data processing. According to the principle of measurement error and the basic principle of formulating "specification", it is required that the error contained in the result of the previous process has the least influence on the next process. Therefore, it is of great significance to study the operation process of airborne lidar and optimize the design of operation scheme for improving data quality.