LIDAR
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2008-07-15T16:19:01Z
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Reverted edits by [[Special:Contributions/OrgasGirl|OrgasGirl]] ([[User talk:OrgasGirl|talk]]) to last version by Jthiegs
[[Image:Starfield Optical Range - sodium laser.jpg|thumb|right|200px|A [[FASOR (laser physics)|FASOR]] used at the [[Starfire Optical Range]] for [[LIDAR]] and [[laser guide star]] experiments is tuned to the [[Fraunhofer lines|sodium D2a line]] and used to excite sodium atoms [[sodium layer|in the upper atmosphere]].]]
[[Image:Lidar P1270901.jpg|thumb|This [[lidar]] (laser range finder) may be used to scan buildings, rock formations, etc., to produce a 3D model. The lidar can aim its laser beam in a wide range: its head rotates horizontally, a mirror flips vertically. The laser beam is used to measure the distance to the first object on its path.]]
'''LIDAR''' ('''Li'''ght '''D'''etection '''a'''nd '''R'''anging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use [[laser]] pulses. Like the similar [[radar]] technology, which uses radio waves instead of light, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. LIDAR technology has application in [[archaeology]], [[geography]], [[geology]], [[geomorphology]], [[seismology]], [[remote sensing]] and [[atmospheric physics]]<ref name="cracknell">
{{Citation
| last = Cracknell
| first = Arthur P.
| author-link =
| last2 = Hayes
| first2 = Ladson
| author2-link =
| title = Introduction to Remote Sensing
| place=London
| publisher = Taylor and Francis
| year = 2007
| origyear = 1991
| location =
| volume =
| edition = 2
| url =
| doi =
| id =
| oclc = 70765252
| isbn = 0849392551}}
</ref>.Other terms for LIDAR include '''ALSM''' (''Airborne Laser Swath Mapping'') and '''laser altimetry'''. The acronym '''LADAR''' (''Laser Detection and Ranging'') is often used in military contexts. The term '''laser radar''' is also in use but is misleading because it uses laser light and not the radiowaves that are the basis of conventional radar.
== General description ==
The primary difference between lidar and radar is that with lidar, much shorter [[wavelengths]] of the [[electromagnetic spectrum]] are used, typically in the [[ultraviolet]], [[Interferometric visibility|visible]], or [[near infrared]]. In general it is possible to image a feature or object only about the same size as the wavelength, or larger. Thus lidar is highly sensitive to [[Particulate|aerosols]] and [[cloud]] particles and has many applications in atmospheric research and meteorology<ref name="cracknell" />.
An object needs to produce a [[dielectric]] [[discontinuity]] in order to reflect the transmitted wave. At radar ([[microwave]] or [[radio]]) [[frequencies]], a [[metallic]] object produces a significant reflection. However non-metallic objects, such as rain and rocks produce weaker reflections and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies. This is especially true for very small objects (such as single [[molecule]]s and [[aerosols]])<ref name="cracknell" />.
Lasers provide one solution to these problems. The beam densities and coherency are excellent. Moreover the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 [[micrometer (unit)|micrometer]]s to the [[Ultraviolet|UV]] (ca. 250 [[nanometer|nm]]). At such wavelengths, the waves are "reflected" very well from small objects. This type of reflection is called [[backscatter]]ing. Different types of scattering are used for different lidar applications, most common are [[Rayleigh scattering]], [[Mie scattering]] and [[Raman scattering]] as well as [[fluorescence]]. The wavelengths are ideal for making measurements of smoke and other airborne particles (aerosols), clouds, and air molecules<ref name="cracknell" />.
A laser typically has a very narrow beam which allows the mapping of physical features with very high [[Optical resolution|resolution]] compared with radar. In addition, many chemical compounds interact more strongly at visible wavelengths than at microwaves, resulting in a stronger image of these materials. Suitable combinations of lasers can allow for remote mapping of atmospheric contents by looking for wavelength-dependent changes in the intensity of the returned signal.
Lidar has been used extensively for atmospheric research and [[meteorology]]. With the deployment of the GPS in the 1980's precision positioning of aircraft became possible. GPS based surveying technology has made airborne surveying and mapping applications possible and practical. Many have been developed, using downward-looking lidar instruments mounted in aircraft or satellites. A recent example is the NASA Experimental Advanced Research Lidar.<ref>[http://inst.wff.nasa.gov/eaarl/ 'Experimental Advanced Research Lidar', ''NASA.org'']. Retrieved 8 August 2007.</ref>
== Design ==
[[Image:LIDAR-scanned-SICK-LMS-animation.gif|thumb|A basic LIDAR system involves a laser range finder reflected by a rotating mirror (top). The laser is scanned around the scene being digitised, in one or two dimensions (middle), gathering distance measurements at specified angle intervals (bottom).]]
In general there are two types of lidar systems: '''micropulse lidar''' systems and '''high energy''' systems. Micropulse systems have developed as a result of the ever increasing amount of [[computer]] power available combined with advances in laser technology. They use considerably less energy in the laser, typically on the order of one micro[[joule]], and are often "eye-safe," meaning they can be used without safety precautions. High-power systems are common in atmospheric research, where they are widely used for measuring many atmospheric parameters: the height, layering and densities of clouds, cloud particle properties (extinction coefficient, backscatter coefficient, depolarization), temperature, pressure, wind, humidity, trace gas concentration (ozone, methane, nitrous oxide, etc.)<ref name="cracknell" />.
There are several major components to a lidar system:
#'''Laser''' — 600-1000 [[nanometre|nm]] lasers are most common for non-scientific applications. They are inexpensive but since they can be focused and easily absorbed by the eye the maximum power is limited by the need to make them eye-safe. Eye-safety is often a requirement for most applications. A common alternative 1550 nm lasers are eye-safe at much higher power levels since this wavelength is not focussed by the eye, but the detector technology is less advanced and so these wavelengths are generally used at longer ranges and lower accuracies. They are also used for military applications as 1550 nm is not visible in night vision goggles unlike the shorter 1000 nm infrared laser. Airborne topographic mapping lidars generally use 1064 nm diode pumped YAG lasers, while bathymetric systems generally use 532 nm frequency doubled diode pumped YAG lasers because 532 nm penetrates water with much much less attenuation than does 1064 nm. Laser settings include the laser repetition rate (which controls the data collection speed). Pulse length is generally an attribute of the laser cavity length, the number of passes required through the gain material (YAG, YLF, etc.), and Q-switch speed. Better target resolution is achieved with shorter pulses, provided the Lidar receiver detectors and electronics have sufficient bandwidth<ref name="cracknell" />.
#'''Scanner and [[optics]]''' — How fast images can be developed is also affected by the speed at which it can be scanned into the system. There are several options to scan the azimuth and elevation, including dual oscillating plane mirrors, a combination with a polygon mirror, a dual axis scanner. Optic choices affect the angular resolution and range that can be detected. A hole mirror or a [[beam splitter]] are options to collect a return signal.
#'''Receiver and receiver electronics''' — Receivers are made out of several materials. Two common ones are [[silicon|Si]] and [[InGaAs]]. They are made in either [[PIN diode]] or [[Avalanche photodiode]] configurations. The sensitivity of the receiver is another parameter that has to be balanced in a LIDAR design.
#'''Position and navigation systems''' — Lidar sensors that are mounted on mobile platforms such as airplanes or satellites require instrumentation to determine the absolute position and orientation of the sensor. Such devices generally include a [[Global Positioning System]] receiver and an [[Inertial Measurement Unit]] (IMU).
==Applications==
[[Image:LIDAR equipped mobile robot.jpg|thumb|This LIDAR-equipped [[mobile robot]] uses its LIDAR to construct a map and avoid obstacles.]]
*In geology and seismology a combination of aircraft-based LIDAR and [[GPS]] have evolved into an important tool for detecting [[geological fault|fault]]s and measuring [[uplift]]. The output of the two technologies can produce extremely accurate elevation models for terrain that can even measure ground elevation through trees. This combination was used most famously to find the location of the [[Seattle Fault]] in [[Washington]], [[United States|USA]].<ref>[http://seattlepi.nwsource.com/local/19144_quake18.shtml Tom Paulson. 'LIDAR shows where earthquake risks are highest, ''Seattle Post'' (Wednesday, April 18, 2001)].</ref> This combination is also being used to measure uplift at [[Mt. St. Helens]] by using data from before and after the 2004 uplift.<ref>[http://wagda.lib.washington.edu/data/type/elevation/lidar/st_helens/ 'Mount Saint Helens Lidar Data', ''Washington State Geospatial Data Archive'' (September 13, 2006)]. Retrieved 8 August 2007.</ref>
*Airborne LIDAR systems monitor [[glacier]]s and have the ability to detect subtle amounts of growth or decline. A satellite based system is [[NASA|NASA's]] [[ICESat]] which includes a LIDAR system for this purpose. NASA's Airborne Topographic Mapper<ref>[http://atm.wff.nasa.gov/ 'Airborne Topographic Mapper', ''NASA.gov'']. Retrieved 8 August 2007.</ref> is also used extensively to monitor [[glaciers]] and perform coastal change analysis.
*LIDAR has also found many applications in [[forestry]]. [[canopy (forest)|Canopy]] heights, [[biomass]] measurements, and leaf area can all be studied using airborne LIDAR systems. Similarly, LIDAR is also used by many industries, including Energy and Railroad, and the Department of Transportation as a faster way of surveying. Topographic maps can also be generated readily from LiDAR, including for recreational use such as in the production of [[orienteering]] maps.[http://www.lidarbasemaps.org]
*LIDAR may also be used to measure the speed of atmospheric winds. Doppler LIDAR systems developed by NASA measure atmospheric wind speed along a line. Scanning LIDAR, such as NASA's HALIE LIDAR, have been used to measure atmospheric wind velocity in a large three dimensional cone.<ref>[http://harlie.gsfc.nasa.gov/IHOP2002/Pub&Pats/AMOS%202002%20final.pdf Thomas D. Wilkerson, Geary K. Schwemmer, and Bruce M. Gentry. ''Lidar Profiling of Aerosols, Clouds, and Winds by Doppler and Non-Doppler Methods'', NASA International H2O Project (2002)].</ref> Applications extend to hurricane monitoring. [[ESA|ESA's]] wind mission ADM-Aeolus will be equipped with a doppler LIDAR system in order to provide global measurements of vertical wind profiles.<ref>[http://www.esa.int/esaLP/ESAES62VMOC_LPadmaeolus_0.html 'Earth Explorers: ADM-Aeolus', ''ESA.org'' (European Space Agency, 6 June 2007)]. Retrieved 8 August 2007.</ref> A doppler LIDAR system will be used in the [[2008 Summer Olympics]].<ref>[http://optics.org/cws/article/research/34878 'Doppler lidar gives Olympic sailors the edge', ''Optics.org'' (3 July, 2008)]. Retrieved 8 July 2008.</ref>
*Doppler LIDAR systems are also now beginning to be successfully applied in the renewable energy sector to acquire wind speed, turbulence, wind veer and wind shear data. Both pulsed and [http://www.naturalpower.com/zephir continuous wave systems] are being used. Pulsed systems using signal timing to obtain vertical distance resolution, whereas continuous wave systems rely on detector focussing.
*A world-wide network of observatories uses [[Lunar Laser Ranging Experiment|lidars to measure the distance to reflectors placed on the moon]], allowing the moon's position to be measured with mm precision and [[tests of general relativity]] to be done.
*[[MOLA]], the [[Mars]] Orbiting Laser Altimeter, used a LIDAR instrument in a Mars-orbiting satellite (the NASA [[Mars Global Surveyor]]) to produce a spectacularly precise global topographic survey of the red planet.
*In atmospheric physics, lidar is used as a remote detection instrument to measure densities of certain constituents of the middle and upper atmosphere, such as [[potassium]], [[sodium]], or molecular [[nitrogen]] and [[oxygen]]. These measurements can be used to calculate temperatures. Lidar can also be used to measure wind speed and to provide information about vertical distribution of the [[aerosol]] particles.
*In oceanography, lidars are used for estimation of [[phytoplankton]] [[fluorescence]] and generally biomass in the surface layers of the ocean. Another application is airborne lidar [[bathymetry]] of sea areas too shallow for [[hydrographic]] vessels.
*One situation where LIDAR has notable non-scientific application is in traffic speed [[police|law enforcement]], for vehicle speed measurement, as a technology alternative to [[radar gun]]s. The technology for this application is small enough to be mounted in a hand held camera "gun" and permits a particular vehicle's speed to be determined from a stream of traffic. Unlike RADAR which relies on [[doppler effect|doppler shifts]] to directly ''measure'' speed, police lidar relies on the principle of time-of-flight to calculate speed. The equivalent radar based systems are often not able to isolate particular vehicles from the traffic stream and are generally too large to be hand held. While there are distinct advantages to being able to pick out one vehicle in a pack, LIDAR has very serious problems associated with "sweep" error. Sweep error is almost always present because automobiles are typically targeted at distances ranging from several hundred feet to over one thousand feet. If the targets were flat surfaces moving forward, such as a large semi-tractor trailer truck, LIDAR can be more accurate. But, when the target is a jelly-bean-shaped automobile or SUV, sweep error is inevitable. Most traffic LIDAR systems send out a stream of approximately 100 pulses over the span of three-tenths of a second. A "black box," proprietary statistical algorithm picks and chooses which progressively shorter reflections to retain from the pulses over the short fraction of a second.
*Military applications are not yet known to be in place and are possibly classified, but a considerable amount of research is underway in their use for imaging. Their higher resolution makes them particularly good for collecting enough detail to identify targets, such as tanks. Here the name LADAR is more common.
*Five lidar units produced by the German company [[Sick AG]] were used for short range detection on [[Stanley (vehicle)|Stanley]], the [[driverless car|autonomous car]] that won the 2005 [[DARPA Grand Challenge]].
*Lidar has been used to create [[Adaptive Cruise Control]] (ACC) systems for automobiles. Systems such as those by Siemens and Hella use a lidar device mounted in the front of the vehicle to monitor the distance between the vehicle and any vehicle in front of it. In the event the vehicle in front slows down or is too close, the ACC applies the brakes to slow the vehicle. When the road ahead is clear, the ACC allows the vehicle to speed up to speed preset by the driver.
*Laser imaging systems can be divided into scanning systems and non-scanning systems. The scanning system can again be divided into sub-groups by the way the laser beam is scanned across the object. Beam-scanners scan a narrow beam, typically in lines on top of each other, therefore this type of system is called a '''L'''aser '''L'''ine '''S'''canner (LLS). Fan-beam scanners scan a fan-shape beam across the object.
*3-D imaging is done with both scanning and non-scanning systems. "3-D gated viewing laser radar" is a non-scanning laser radar system that applies the so-called gated viewing technique. The gated viewing technique applies a pulsed laser and a fast gated camera. There are ongoing military research programmes in [[Sweden]], [[Denmark]], the USA and the [[United Kingdom|UK]] with 3-D gated viewing imaging at several kilometers range with a range resolution and accuracy less than ten centimeters.
*At the [[Joint European Torus|JET]] [[nuclear fusion]] research facility, in the UK near [[Abingdon, Oxfordshire]], LIDAR [[Thomson Scattering]] is used to determine [[Electron]] Density and Temperature profiles of the [[Plasma (physics)|plasma]].<ref>[http://www.jet.efda.org/pages/focus/001lidar/index.html CW Gowers. ' Focus On : Lidar-Thomson Scattering Diagnostic on JET' ''JET.EFDA.org'' (undated)]. Retrieved [8 August] 2007.</ref>
*LIDAR has been used in the recording of a music video without cameras. The video for the song "House of Cards" by [[Radiohead]] is believed to be the first use of real-time 3D laser scanning to record a music video.<ref>[http://creativity-online.com/?action=news:article&newsId=129514§ionId=behind_the_work Nick Parish, ''From OK Computer to Roll computer: Radiohead and director James Frost make a video without cameras'', CREATIVITY (July 13, 2008)].</ref>
==See also==
* [[Laser rangefinder]]
* [[Atomic line filter]]
* [[Time domain reflectometry]]
* [[Optical time domain reflectometer]]
* [[Satellite laser ranging]]
* [[Sonar]]
* [[Radar]]
* [[LIDAR detector]]
* [[Optech Incorporated]], a company focusing on lidars
==References==
{{reflist|2}}
== External links ==
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* [http://home.iitk.ac.in/~blohani/LAS_Converter.htm LAS Converter]. LAS is LiDAR data Exchange Format Standard specified by ASPRS. Many users need LAS data, as supplied by operator, to be converted into ASCII format or from LAS1.0 to LAS1.1 format. This facility is not available in several common software. This free utility can help in data conversion.
* [http://home.iitk.ac.in/~blohani/LiDAR_Tutorial/Airborne_AltimetricLidar_Tutorial.htm An Airborne Altimetric LiDAR tutorial]. A tutorial on altimetric LiDAR.
* [http://inst.wff.nasa.gov/eaarl/ NASA Experimental Advanced Airborne Research Lidar]. NASA's EAARL is an airborne Lidar designed to map complex coastal environments above and below the water, within vegetated areas. It is also being used to map the bottom topography is shallow braided rivers and streams.
* [http://www-calipso.larc.nasa.gov/ CALIPSO]: The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite -- space-based laser remote sensing of clouds and aerosols for a better understanding of climate change issues
* [http://www.eol.ucar.edu/lidar NCAR REAL]: NCAR's Earth Observing Laboratory (EOL) created the Raman-shifted Eyesafe Aerosol Lidar. This eyesafe '''high energy''' lidar with scanning capability expands the applications to include mapping urban atmospheric pollutants and studies of dispersion very near the surface of the earth.
* [http://www.etl.noaa.gov/technology/instruments/floe/ NOAA Oceanographic (Fish) Lidar]
* [http://shoals.sam.usace.army.mil/ Joint Airborne Lidar Bathymetry Technical Center of Expertise (JALBTCX)]
* [http://eosl.gtri.gatech.edu/index.jsp EOSL] - The Electro-Optical Systems Laboratory at [[GTRI]] has a nationally known program in lidar research and development.
* [http://ramanlidar.gsfc.nasa.gov The NASA Goddard Space Flight Center's Raman Lidar Laboratory] - This laboratory has a ground and an upcoming airborne Raman lidar measuring water vapor, aerosols and other atmospheric species
* [http://lidar.cr.usgs.gov/ The USGS Center for LIDAR Information Coordination and Knowledge (CLICK)] - A website intended to "facilitate data access, user coordination and education of lidar remote sensing for scientific needs."
*[http://www.ikg.uni-hannover.de/publikationen/publikationen/2006/brenner_tutorial_part1.pdf] Tutorial slides on LIDAR (aerial laser scanning): Principles, errors, strip adjustment, filtering.
*[http://www.ikg.uni-hannover.de/publikationen/publikationen/2006/brenner_tutorial_part2.pdf] Tutorial slides on LIDAR (aerial laser scanning): Extraction and modelling.
* Weier, John. 2004. [http://www.conbio.org/cip/article53C3D.cfm Conservation in 3D]. ''Conservation in Practice'' 5(3):39-41. On the conservation applications of LIDAR.
{{Meteorological equipment}}
[[Category:Meteorological instrumentation and equipment]]
[[Category:Atmospheric and ocean optics]]
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