What Are the Different Uses of LIDAR Processing?

LiDAR is a system that integrates laser, global positioning system (GPS), and inertial navigation system (INS) technologies to obtain point cloud data and generate accurate digital 3D models. The combination of these three technologies can obtain the surrounding three-dimensional real scene with the same absolute measurement point.

Lidar technology

LiDAR is a system that integrates laser, global positioning system (GPS), and inertial navigation system (INS) technologies to obtain point cloud data and generate accurate digital 3D models. The combination of these three technologies can obtain the surrounding three-dimensional real scene with the same absolute measurement point.

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.

Lidar is short for "Light detection and ranging". Earlier known as light radar, because the light sources used at that time were not lasers. Since the emergence of lasers, lasers are particularly suitable for light radars as high-brightness, low-divergence coherent light. Therefore, current lasers use lasers as light sources, and their names are collectively referred to as laser radars.

Chinese name

Lidar technology

Foreign name

LIght Detection And Ranging-LIDAR

Use

Used to obtain data and generate accurate DEM

Features

Detection and measurement with laser

The most basic working principle of lidar is no different from that of radio radar, that is, a signal is sent by the radar transmitting system, and is collected by the receiving system after the target reflects, and the distance of the target is determined by measuring the running time of the reflected light. As for the radial velocity of the target, it can be determined by the Doppler frequency shift of the reflected light, or two or more distances can be measured and the rate of change can be calculated to obtain the velocity. This is also the basic working principle of the direct detection radar . It can be seen that the basic structure of the direct-detection type lidar is quite similar to that of a laser rangefinder.

Because the speed of light is known, the distance between the laser and the contaminating agent can be calculated based on the time difference between the emitted time of the emitted light and the received time of the backscattered light. This is how lidar can measure distance.

The basic function of laser radar for pollution detection is that a laser as a light source emits a beam of a specific wavelength and hits a polluting agent (or a cloud of poison). After this wavelength of light interacts with the polluting agent, a part of the light is opposite to the emitted beam The direction is reflected back to the receiving device, and the information about the contaminating agent can be obtained after detection.

Lidar detection of atmospheric pollutants is an active method in remote sensing. There are also passive methods. The passive method does not have an artificial light source, and does not emit a beam of light. It only receives thermal radiation from the target cloud itself or radiation from a natural light source (the sun), and then analyzes and determines the conclusion.

Lidar can be classified according to the laser used, detection technology and radar function. Lasers currently used in lidars include carbon dioxide lasers, Er: YAG lasers, Nd: YAG lasers, Raman frequency shifted Nd: YAG lasers, GaAiAs semiconductor lasers, helium-neon lasers, and frequency-doubling Nd: YAG lasers. The erbium-doped YAG laser has a wavelength of about 2 microns, while the GaAiAs laser has a wavelength between 0.8-0.904 microns.

According to different principles of light and pollutant molecules, lidar can be divided into the following types.

Lidar based on absorption principle

Almost all pollution agents (including chemical warfare agents) composed of multiple atoms can absorb light of a specific wavelength, and different compounds absorb light at different wavelengths. To determine which contaminating agent, let the laser emit light of the wavelength that the contaminating agent absorbs most strongly. After a part of the emitted light is absorbed, a small part is always reflected back to the detector, and the concentration of the pollutant can be detected by analyzing the change in the received light intensity. If the wavelength of the light beam emitted by the laser is tuned to overlap with the absorption wavelength of the polluting agent to be measured so that its bandwidth is smaller than the absorption line width of the polluting agent, then the strongest absorption effect will occur. This is called resonance absorption, and the lidar based on measuring resonance absorption is called resonance absorption lidar. In resonance absorption, in addition to thermal energy, fluorescence is also generated. A lidar based on measuring the fluorescence intensity generated during resonance absorption is called a resonance fluorescence lidar.

If the laser is made to emit two beams of light with different wavelengths, the wavelengths of which are respectively located at the absorption peak and absorption valley of the polluting agent to be measured, and the respective intensity of the two beams of light passing through the polluting agent is measured to calculate the concentration of the polluting agent. Differential absorption method, the radar that works with this principle is called differential absorption lidar.

Mie scattering laser radar

In addition to the absorption effect described above, the interaction between light waves and pollutant molecules also has a scattering effect. The so-called scattering refers to a phenomenon in which light propagates in an inhomogeneous medium and deviates from the original direction and spreads in all directions. This phenomenon occurs because the medium is mixed with tiny particles (atoms, molecules, particles) with different refractive indices. If the diameter of the particle is equal to or slightly larger than the wavelength of the beam, the scattering caused is called Mie scattering. In Mie scattering, the wavelength of the incident light is equal to the wavelength of the reflected light. The lidar using the Mie scattering principle is called the Mie scattering lidar.

Raman scattering lidar

If a monochromatic light with a frequency of 0 is used to irradiate the pollution agent, in addition to the component with a frequency of u0 in the scattered light, there are components with a frequency of 0 + and a frequency of 0-, so that the frequency of the incident light occurs. This scattering of displacement is called Raman scattering. Depending on the type of molecule, the frequency shift value of the Raman scattered light is also different, regardless of the frequency 0 of the incident light. It can be seen that by detecting in the backscattered light, it can be inferred which pollutant molecule is present. The radar using this principle is called Raman scattering lidar.

Classification based on detection technology

Lidar can be divided into two types: direct detection and coherent detection. Among them, the direct detection laser radar uses pulse amplitude modulation (AM) technology and does not require an interferometer. Coherent detection laser radar can use heterodyne interference, zero-beat interference or offset zero-beat interference. The corresponding tuning techniques are pulse amplitude modulation, pulse frequency modulation (FM) or mixed modulation. According to different functions, lidar can be divided into tracking radar, moving target indication radar, velocity measurement radar, wind shear detection radar, target recognition radar, imaging radar and vibration sensing radar.

Coherent detection-type lidars are also divided into monostable and bistable. In the so-called monostable system, the sending and receiving signals are common. In the so-called monostable system, the sending and receiving signals share one optical aperture. It is isolated by the beginning of transmission / reception (T / R). The T / R switch sends the transmission signal to the output telescope and transmission scanning system for transmission. After the signal is reflected by the target, it enters the optical scanning system and the telescope. At this time, they play the role of optical reception. The T / R switch sends the received radiation to the optical mixer, and the resulting beat signal is focused by the imaging system to a photosensitive detector, which converts the optical signal into an electrical signal, and the low-frequency from the background source is passed by a high-pass filter All components and DC signals induced by the local oscillator are filtered out. The measurement information contained in the final high-frequency component is detected by the signal and data processing system. The difference between bistable systems is that they include two sets of telescopes and optical scanning components. Naturally, T / R switches are no longer needed, and the rest are the same as those of monostable systems.

The US Department of Defense's initial interest in lidar was similar to that of microwave radar, focusing on surveillance, capture, tracking, damage assessment (SATKA), and navigation of targets. However, because microwave radar is sufficient to complete most damage assessment and navigation tasks, military lidar plans have focused on a small number of tasks that the former cannot accomplish well, such as high-precision damage assessment, extremely accurate navigation correction, and high-resolution imaging. An earlier type of lidar, called the "fire pool," was developed by the Lincoln Laboratory of the Massachusetts Institute of Technology in the late 1960s. In the early 1970s, the Lincoln Laboratory demonstrated the ability of a fire pool radar to accurately track satellites and obtain Doppler images. Experiments conducted in the 1980s proved that this CO2 lidar can penetrate certain smoke, detect camouflage, capture air targets and detect chemical warfare agents at long distances. The fire pool lidar developed to the late 1980s uses a highly stable CO2 laser oscillator as a signal source, amplified by a narrowband CO2 laser amplifier, and its frequency is modulated by a single-sideband modulator. Another medium-power argon ion laser working in the blue-green band is combined with the above-mentioned radar beam for angular tracking of the target, and the function of the radar beam is to collect distance-Doppler images, process and display them in real time. Both beams are transmitted and received by a telescope with an aperture of 1.2M. According to reports, researchers from the United States Strategic Defense Agency and the Massachusetts Institute of Technology conducted tracking experiments on a sounding rocket launched from the Atlantic coast of Virginia in March 1990 using the above device. Six minutes after the secondary ignition, the rocket entered the sub-orbit, the climb phase, and threw its payload, an inflatable balloon similar in shape and size to the ballistic missile re-entry vehicle. The balloon has a gas thruster to provide dynamic characteristics consistent with the physical structure of the reentry vehicle and bait. The target is initially tracked by the L-band tracking radar and the X-band imaging radar. The data obtained by these radar sensors were handed over to the fire pool lidar, which successfully obtained images of targets at a distance of about 800 kilometers.

According to Defense Electronics in May 1991, the US Air Force and the Navy were developing "Advanced Technology Lidar Systems (ATLAS)." The system is intended to be mounted on cruise missiles, using CO2 lasers and new infrared radars to guide the cruise missiles to targets. The plan is headed by the Advanced Guidance Department of the Wright Institute at Eglin Air Force Base, Florida. The main contractor, McDonnell Douglas and General Dynamics Conwell, each developed an AGM-130 or cruise missile under a $ 15 million contract. Type weapon. Naval spokesman Captain Lei said at the time that he planned to conduct a flight test of ATLAS with a pod structure in fiscal year 1992; in 1992, the Hughes California-based Optoelectronics and Data Systems Research Group had successfully developed an advanced CO2 lidar. It will be delivered as part of the ATLAS program to the main contractor General Dynamics Conwell. This was reported in "Photonics" in June 1992 and "Defense Electronics" in July. In order to demonstrate the capabilities of the lidar, the Conwell division related it to related signal processing electronics and other components of the guidance system. That is, the processor, navigation sensors and test instruments are loaded into the pod together and hung on the test jet of the Cornwell Division. They are flying at the target at the Eglin Proving Ground. Lidar provides high resolution of the target area. Three-dimensional image. Since then, a variety of air-to-ground weapon navigation, end-pointing, and precision seeking guidance tests have been conducted, which fully show the many unique advantages of this laser radar for missile guidance.

Lidar technology is complex, the development cycle is long, and the equipment is expensive, so to develop it requires not only the relevant high-level specialized personnel, but also a solid economic foundation. It makes it difficult to popularize it. At present it is mainly used in scientific research.

The laser beam emitted by a lidar has a high energy, and it is a difficult problem to protect people.

The development of lidar technology is not sufficient, and some problems have not been completely solved, which limits its application. For example, Raman lidar has theoretically a great advantage in identifying pollutants, but because the current technology is not enough, the few Raman lidars in the world have a small cross section, difficult signal reception, and a long range. It is generally only a few hundred meters and cannot play its due role.

Lidars are usually bulky and heavy, and need to be debugged frequently during use. Solving this problem is not easy.