Lidar

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2D scanning with lidar

Water vapor lidar on the Zugspitze

Lidar (short for English light detection and ranging ), also ladar ( laser detection and ranging ) is the radar related method for optical distance and speed measurement as well as for remote sensing of atmospheric parameters. Instead of radio waves as in radar, laser beams are used.

functionality

Lidar systems for measuring the atmosphere send out laser pulses and detect the light scattered back from the atmosphere . The distance to the point of scattering is calculated from the light transit time of the signals . Cloud and dust particles in the air ( aerosols ) scatter the laser light and enable high-resolution detection and distance measurement of clouds and aerosol layers. More complex systems can be used to determine atmospheric condition parameters and the concentration of atmospheric trace gases . For example, lidar instruments are also used to monitor emission quantities from factory chimneys for compliance with specified limit values .

Depending on the wavelength of the laser light used, lidar systems are more or less sensitive to molecular or particle backscattering. The strength of the backscattering at a wavelength also depends on the respective particle size and concentration. With lidar systems that use several wavelengths, the exact size distribution of the atmospheric particles can therefore be determined.

With sophisticated techniques, lidar can be used to measure a large number of atmospheric parameters: pressure, temperature , humidity, water vapor concentration and the concentration of atmospheric trace gases (ozone, nitrogen oxides , sulfur dioxide , methane and so on). In addition, the optical properties of aerosols and cloud particles can be determined ( extinction coefficient , backscatter coefficient , depolarization ). With a depolarization lidar, the physical state (liquid or solid, i.e. in the case of cloud particles: whether still water or already ice) can be determined (see also polarization ).

Raman lidar systems

Raman lidar systems (see also Raman spectroscopy ) detect signals at other wavelengths in addition to the backscattering of the radiation of a certain wavelength that has just been emitted (elastic backscattering). These signals arise because the molecules that backscatter the light absorb part of the energy of the light particle (the photon ) or add additional energy to it ( inelastic scattering ). The molecules change their vibration or rotation during inelastic scattering (Raman process). The change in energy is only possible in certain “graded” steps (see quantum mechanics ) and these steps are characteristic of the type of molecule. Water molecules, for example, have a lower probability of scattering green light back in red (frequency-doubled Nd: YAG laser light with a wavelength of 532 nm is backscattered at 660 nm). This process is used when determining the water vapor mixing ratio in the atmosphere (water vapor Raman lidar). The intensity of the inelastic Raman scattering is related to the wavelength in the same way as the elastic Rayleigh scattering, i.e. inversely proportional to the fourth power of the wavelength. It therefore makes sense to use lasers in the ultraviolet spectral range, e.g. B. frequency-tripled Nd: YAG lasers (355 nm) or even Xe: Cl excimer lasers with 308 nm. At even shorter wavelengths, however, the absorption by atmospheric ozone dominates, so that the stronger backscattering at greater distances (several kilometers ) no further advantage can be achieved.

Differential absorption lidar

Trace gas concentrations may also - and more accurate for most commodities - with the method of differential absorption lidar (ger .: differential absorption lidar , DIAL ) are measured. With this technology, two laser pulses of different wavelengths are emitted. One of the wavelengths is chosen so that it is absorbed by the substance whose concentration is to be determined (online wavelength); the other wavelength so that it is not absorbed or absorbed as little as possible (off-line wavelength). From the step-by-step comparison of the backscatter signals (each for “on” and “off”), the concentration profile of the substance along the line of propagation of the laser pulses can then be calculated. Absorption coefficients are usually well known from laboratory experiments; DIAL uses the corresponding values ​​for on and off wavelengths to determine the atmospheric trace gas concentration without the need for further calibration of the instrument (the technology is "self- calibrating "). For this, however, the wavelengths of the laser pulses must be set or controlled very precisely. Since the absorption coefficients mostly depend on pressure and temperature, they must be known precisely along the measuring section . This fact plays a major role, especially when it comes to the vertical sounding of the atmosphere. It must also be taken into account that the backscattered light (Rayleigh scattering) experiences a temperature-dependent Doppler broadening. However, this effect does not occur with backscattering from particles (aerosols). Therefore, information about the relationship between Rayleigh scattering and backscattering from particles must also be obtained.

Under aerosol-free conditions and assuming that the spectral distribution of the light is not significantly changed by the trace gas to be measured, the simplified lidar equation applies to the DIAL:

${\displaystyle N(r)={\frac {1}{\Delta \sigma _{\uparrow }(r)+\Delta \sigma _{\downarrow }(r)}}\cdot \left[{\frac {-d}{dr}}\ln {\frac {S_{\mathrm {on} }(r)}{S_{\mathrm {off} }(r)}}\right]}$

Here, the concentration of the measuring trace gas, and the differences of the effective absorption cross-sections on the optical path of the laser beam to the scattering process and on the optical path of the scattering process for lidar receiver and and the backscatter signals of the laser shots on the wavelengths or . In the case of a significantly aerosol-containing atmosphere, the calculation of is generally considerably more complex, since the spectral distribution of the backscattered light is strongly dependent on the distribution of the aerosols. ${\displaystyle N(r)}$ ${\displaystyle \Delta \sigma _{\uparrow }(r)}$ ${\displaystyle \Delta \sigma _{\downarrow }(r)}$ ${\displaystyle (\sigma _{\mathrm {off} }-\sigma _{\mathrm {on} })}$ ${\displaystyle S_{\mathrm {on} }(r)}$ ${\displaystyle S_{\mathrm {off} }(r)}$ ${\displaystyle \lambda _{\mathrm {on} }}$ ${\displaystyle \lambda _{\mathrm {off} }}$ ${\displaystyle N(r)}$

Other uses

Lidar is increasingly replacing radar as a measuring instrument in mobile speed controls . Lidar technology can also be used as an alternative to techniques such as induction loops that are widespread there for stationary speed measurements .

Also laser rangefinder for handicrafts, construction and surveying work on the lidar principle. In principle, all of the measurement principles known from radar can be used for lidar.

Lidar systems are also used in the field of driver assistance systems for automobiles and "automated driving". In driverless transport vehicles z. B. Lidar is used for obstacle detection. The use here is also partially standardized in order to avoid accidents with people who could cross the automatic routes ( personal protection system ). The systems used here are i. d. Usually designed as compact sensor modules. In a typical design, the laser beam is deflected horizontally over a wide angular range (up to 360 °), but vertically only a few angles are implemented channel by channel (e.g. 16 channels with 2 ° spacing each). This is typically completely sufficient for obstacle detection.

Furthermore, wind lidar systems are used by modern passenger aircraft to detect turbulence and shear winds in the close range (in the direction of flight).

In the wind energy industry , in addition to acoustic measurement methods ( sodar ), lidar is also increasingly being used to measure horizontal and vertical wind speed and wind direction . B. to the control center for the optimal setting of the wind turbines. The measurement is typically carried out in the range of 40-200 m and records wind speeds between 0 and 70 m / s with an accuracy of 0.1 m / s. The advantage of lidar over sodar is that it is less susceptible to noise, which means that the technology will become more widespread. Another advantage over Sodar systems is that modern, commercially available lidar systems are small and light and can be transported or set up and dismantled by one or two people. This also makes them interesting for short-term measurements, e.g. B. when searching for a location or for measuring the performance characteristics of wind turbines . The use of lidar systems is also being worked on in the offshore industry. There are already installed measuring devices on offshore platforms as well as the first prototypes of buoy-supported lidar wind measuring devices. There are also approaches to install the lidar directly on the nacelle of wind turbines.

The wind lidar systems evaluate the frequency shift between the transmitted and received signal caused by the Doppler effect , which was previously reflected by aerosols that were carried with the wind (and thus the same as the wind in speed and direction). By measuring in at least three different directions, the amount and direction of the wind vector can be calculated.

In the robotics lidar systems are used for years for object recognition and environment detection used.

They are also used for air-based leak testing of natural gas pipelines ( remote gas detection ) through the reliable, laser-based detection of methane in layers of air close to the ground (see DVGW data sheet G 501).

See also

• Airborne laser scanning
• Iterative Closest Point Algorithm

literature

• Claus Weitkamp: Lidar - range-resolved optical remote sensing of the atmosphere . Springer, New York 2005, ISBN 0-387-40075-3 .
• Takashi Fujii: Laser remote sensing . CRC, Taylor & Francis, Boca Raton 2005, ISBN 0-8247-4256-7 .
• Albert Ansmann: Advances in atmospheric remote sensing with lidar . Springer, Berlin 1997, ISBN 3-540-61887-2 .

Web links

Commons : Lidar  - collection of images, videos and audio files

• http://lidar.ssec.wisc.edu/
• LiDAR in atmospheric research ( memento from January 26, 2016 in the Internet Archive ) on the website of the Institute for Meteorology and Climate Research (IMK-IFU) of the Karlsruhe Research Center (as of January 2014)
• Claus Brenner: Aerial Laser Scanning . International Summer School “Digital Recording and 3D Modeling” . Aghios Nikolaos, Crete, Greece, April 24-29 April 2006, accessed July 24, 2010 (tutorial on airborne lidar; PDF file; 16.88 MB)
• Spiegel-Online: “ Flying laser reveals buried buildings ” . Article and picture series from July 26, 2012 about LiDAR in aerial photography archeology
• Jörg Römer: Archeology in the bushes at Spiegel-Online from September 10, 2018

Individual evidence

1. ↑ POLISCAN SPEED STATIONÄR - VITRONIC - the machine vision people. In: www.vitronic.de. Retrieved July 5, 2016 . 
2. ↑ H. Härter, LIDAR technology, sensor technology as a key technology for autonomous driving, electronics practice, July 17, 2017 [1]
3. ↑ SICK (Ed.): LMS prospectus . ( PDF [accessed on March 3, 2008]).
4. ↑ Jörg Böttcher: Online Compendium Measurement Technology and Sensor Technology. Retrieved October 30, 2019 . 
5. ↑ Meteotest: About us. June 5, 2020, accessed June 5, 2020 . 
6. ↑ Archived copy ( memento of the original from January 5, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
7. ↑ Archived copy ( memento of the original from January 8, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. 
8. ↑ 3E. Retrieved June 5, 2020 (American English). 
9. ↑ Archived copy ( memento of the original from April 10, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
10. ↑ Doppler Lidar Systems - MITSUBISHI ELECTRIC. Retrieved June 5, 2020 . 
11. ↑ http://www.iftas.de/Main/Solutions IfTAS