1 Airborne lidar system
1.1 System composition and working principle of lidar
Generally speaking, the airborne lidar system is mainly composed of a laser ranging unit, optical mechanical scanning
Scanning unit, differential GPS and inertial measurement unit
It is used to measure the flight attitude, control unit and digital camera of the aircraft platform. The laser ranging unit mainly includes a laser transmitter and a receiver. The lasers currently used for distance measurement are mainly semiconductor diode lasers and semiconductor pumped ND YAG lasers, which emit laser waves.
The spectrum ranges from 800 to 1600. PIN photodiode Figure 1 Schematic diagram of the airborne LIDAR system
Tubes or avalanche photodiodes (avalanchephotodiodes, APD) are used as detectors. The sensitivity of the detector is very important. Under similar conditions, the range that APD can detect is 4 times that of PIN photodiodes, and some even 10 times. The main physical characteristics of the laser used for distance measurement are that the emitting laser has high power, short pulse, high repetition frequency, etc., and the frequency is adjustable for continuous wave lasers. The laser emits laser light. After shaping, it is emitted from the transmitting system. The signal reflected or scattered by the target is detected by the detector through the receiving system. At the same time, the optical signal is converted into an electrical signal, which is then processed by the computer and processed in an appropriate manner Store it, and measure the distance from the light point to the system by measuring the propagation time of the light signal in space.
Laser ranging is generally divided into two methods: one is pulse ranging, and the other is phase ranging. Phase measurement determines the distance between the laser and the target by measuring the phase difference between the wave emitted by the laser and the wave received by the target.
1.2 Data classification of the airborne lidar system
The data provided by the airborne lidar system is divided into distance image data and echo intensity image data according to the content of the data itself. The distance image data records the distance information from the center of the laser scanner to the ground point, while the echo intensity image data records the intensity of the laser pulse signal returned after the laser pulse emitted from the laser scanner is reflected or scattered by the ground point. . According to the number of echo signals that the laser pulse signal emitted by the laser scanner returns to the laser scanner after being reflected or scattered by the ground point, the lidar data is divided into single-echo data, double-echo data and multi-echo data. In order to facilitate the processing of airborne lidar data, some of the latest airborne lidar systems integrate aerial digital cameras or multispectral imagers, so that they can combine aerial digital images or multispectral images to compare distance image data and echo intensity Image data is processed jointly. These airborne lidar data also include aerial digital image data or multi-spectral image data.
1.3 Data products of airborne lidar
The acquisition, processing and transmission of airborne lidar data are all in digital format, which is easy to generate digital products that can meet various needs. The simplest data format is an ASCII file containing X, Y, and Z coordinate data. These coordinate data reflect the 3D information on the ground collected by the lidar, including the ground, buildings, trees or any reflective objects.
ASCII files with X, Y, and Z can be imported into various software to make rich and valuable products, especially when inputting into GIS software, it can be made into raster DM, and further generate realistic terrain shadow model. With the same DEM, through the combination with traditional aerial photos or digital photography, orthographic maps can also be generated.
Lidar data is also applicable to non-traditional 3D products. Now some software in the PC can make orthographic maps and DE M into simulation clips, which can depict the visual effects of roads, rivers, city centers or jungles. One step further, the computer can also dynamically simulate a 3-dimensional flight perspective.
2 Application of Airborne Lidar System
Compared with the existing measurement methods, the airborne LIDAR technology system can be used as a supplement to photogrammetry on the one hand (Acker-mann, 1991), on the other hand, it is also a competitive technology for traditional measurement techniques. For many measurement applications, airborne LIDAR technology can currently be used in conjunction with a variety of other traditional sensors, including standard aerial cameras, digital cameras, multispectral scanners or thematic imagers. However, in certain applications, such as forestry, coastal engineering or power transmission lines, the unique capabilities provided by airborne LIDAR technology cannot be achieved by any other technology. At present, LIDAR technology is being applied in all countries. For example: Switzerland uses LIDAR technology to monitor the borders of national forests to prevent forest encroachment and implement the plan for returning farmland to forests; Japan uses LIDAR technology to obtain accurate ground DTM data and provide them to all satellite images for geometric precision correction; Malaysia uses this technology to correct The 680-kilometer inclined electrified railway was transformed and expanded into a double track, which greatly saved time and money. According to the relevant information on July 1, 2000, there are about 75 institutions in the world opening about 60 laser sensors for practical applications. Since 1998, the average annual growth rate of the installation of this technical equipment has been 25%. Because of the fast access to 3D digital terrestrial data, the market share of LIDAR is getting bigger and bigger, so that the old American cartography company Shanen Company also introduced this technology; SCOP, EASI and ARC/INFO companies are also actively participating in the post-processing of LIDAR system data the work. www.isurestar.net