Lidar

LiDAR ( short for. Light detection and ranging), also LADAR (laser detection and ranging ) is a radar (english radio detection and ranging ) very closely related method for optical distance and speed measurement as well as for remote measurement of atmospheric parameters. Instead of radio waves as the radar but laser beams are used.

Distance measurement

The basic function of a LIDAR system is in the distance measurement. The instrument emits laser pulses, and detects the back-scattered light from an object. From the travel time of the signals and the speed of light, the distance is calculated from the object.

Application to atmospheric measurements

Lidar systems for atmospheric measurements emit laser pulses and detect the back-scattered light from the atmosphere. From the time of flight of the signals and the speed of light, the distance is measured to the point of scattering. Clouds and dust particles in the air ( aerosols) scatter the laser light and allow for high-resolution detection and distance measurement of clouds and aerosol layers. With more complex systems can be atmospheric state parameters and the concentration of atmospheric trace gases determine. For example, serve LiDAR instruments and the monitoring of emission quantities of chimneys of factories on compliance with specified limits.

Depending on the wavelength of laser light used LiDAR systems are more or less sensitive to molecular or particle backscattering. The strength of the back scattering depends on the wavelength of the respective particle size and concentration. With LIDAR systems, which emit a plurality of wavelengths, so the exact size distribution of atmospheric particles to be determined.

Using sophisticated techniques can be by means of LiDAR a variety of atmospheric parameters to measure: pressure, temperature, humidity, water vapor concentration, and the concentration of atmospheric trace gases (ozone, nitrogen oxides, sulfur dioxide, methane, etc. ). In addition, the optical properties of aerosols and cloud particles can be determined (extinction coefficient, backscatter coefficient, depolarization ). With a depolarization LiDAR, the aggregate state can (liquid or solid, so when cloud particles: if there are water or even ice) determine (see also polarization).

Raman LiDAR systems

Raman LIDAR systems (see Raman spectroscopy ) to detect, in addition to the backscatter just emitted wavelength ( elastic scattering), signals at other wavelengths. These signals arise from the fact that the light backscattering molecules absorb some of the energy of the light particle ( photon ) or add him extra energy ( inelastic scattering). The molecules change in the inelastic scattering their vibration or rotation ( Raman process). The energy change is only given in " stepped " steps (see quantum mechanics ), and these steps are characteristic of the molecular species. Water molecules scatter example, green light with a small probability red back ( frequency-doubled Nd: YAG laser light with a wavelength of 532 nm is backscattered at 660 nm). This process is used in the determination of the water vapor mixing ratio in the atmosphere ( steam - Raman LIDAR ).

Differential Absorption LiDAR

Trace gas concentrations can also - and more precisely in most materials - using the method of differential absorption lidar (English: differential absorption lidar, DIAL) can be measured. In this technique, two laser pulses of different wavelengths are emitted. One of the wavelengths is chosen so that it is the substance whose concentration is to be determined, absorbed ( on-line wavelength); the other wavelength so that it is not absorbed, or as little as possible ( the off-line wavelength). From the stepwise comparison of the backscatter signals (each for "on" and "off") can then be calculated, the concentration profile of the substance along the emission line of the laser pulses. Absorption coefficients are well known in general from laboratory experiments; DIAL determined by means of the corresponding values ​​for on-and off- wavelength atmospheric trace gas concentration, without any further calibration of the instrument would be required ( the technology is " self-calibrating "). For this, however, the wavelengths of the laser pulses must be very accurately set and checked. Since the absorption coefficients usually depend on pressure and temperature, they must be along the measurement path precisely known. Especially in the vertical sounding of the atmosphere of this fact plays a major role. It must also be considered that the backscattering light ( Rayleigh scattering ) undergoes a temperature-dependent Doppler broadening. However, this effect does not occur ( aerosols ) in the back-scattering of particles. Therefore, information on the relationship between Rayleigh scattering and backscattering must be obtained from particles.

Under aerosol-free conditions and the assumption that the spectral distribution of the light is not significantly altered by the measured trace gas itself, the simplified LiDAR equation for the DIAL applies:

Here, the concentration of the measuring trace gas and the differences in the effective absorption cross-sections in the optical path of the laser beam to the diffusion process or on the optical path of the scattering process for the lidar receiver and and the backscatter signals of the laser shots on the wavelengths or. At a significantly aerosol containing atmosphere is generally much more complex to calculate, as the spectral distribution of the backscattered light is strongly dependent on the distribution of the aerosols.

Other applications

The most visible LiDAR applications in Germany are developed and delivered by the VITRONIC GmbH toll bridges on which detects the approaching vehicles from a LiDAR and tracked to the passage ( tracked ) are. With another LiDAR, which is designed for close range, a 3D contour of the vehicles is determined in order to distinguish from the device data toll trucks from all other vehicles.

LiDAR replaced with the relevant regulatory authorities increasingly the radar as a measurement tool for mobile speed controls.

Also laser rangefinder for handicrafts, construction and surveying work on the LiDAR principle. In principle, all can be applied known principles of measurement for LiDAR and radar ago:

• Transmission of a pulse sequence and measuring the transit time of the reflected signals ( measurement of the distance and the speed are possible)
• Evaluation of the frequency shift of the reflected signals of a frequency-stable lasers for measuring the relative speed (the principle of the Doppler radar, for large distances and speeds )
• Modulating the laser beam with a fixed frequency, for measuring the relative velocity
• Modulation with a variable frequency to measure the distance

In AGVs LiDAR is used for obstacle detection. The insert is also partially standardized in order to avoid accidents with persons who might cross the automatic routes, ( personal protection system ).

Furthermore LiDAR systems of modern passenger aircraft for the detection of turbulence and wind shear in the vicinity are used ( in flight direction).

In the wind energy sector ( SoDAR ) is increasingly addition to acoustic measurement method also used LiDAR to measure horizontal and vertical wind speed and wind direction and example to be transmitted to the control center for the optimal adjustment of the wind turbines. The measurement is typically in the range of 40-200 m, and detects wind velocities between 0 and 70 m / s 0.1 m / s accuracy. The advantage of LiDAR over SoDAR is to lower immunity to noise, thus further spread of technology is foreseeable. Another advantage over SoDAR systems is that commercially available modern LiDAR systems are small and light, and can be 1-2 persons transported and assembled and disassembled. That makes you so interesting for short-term measurements, eg search for a location or for Leistungskennlininenvermessung of wind turbines. Also in the offshore industry is working on the use of LiDAR systems. There are already installed measuring equipment on offshore platforms as well as the first prototypes of buoy -based LIDAR wind measuring instruments. There are also approaches to install the LiDAR directly to the nacelle of wind turbines. The wind LiDAR systems evaluate the caused by the Doppler effect frequency shift between ausgesendetem and received signal, which was previously reflected by aerosols, which were with the wind (and thus in speed and direction equal to the wind) supported. By measuring in at least three different directions, thus the magnitude and direction of the wind vector can be calculated.

In robotics LiDAR systems are used for years for object recognition and environment sensing for use.

They are also used for airborne leak check of gas pipelines (gas remote detection ) with the reliable, laser-based detection of methane in near-surface air layers ( see DVGW sheet G 501 ) was used.

Finally, laser interferometer can also be interpreted as an application example of the Doppler effect for highly accurate range finding, however, does not fall under the term LiDAR.