|
|
|
|
LIDAR
|
| |
|
| |
LIDAR (Light Detection and Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target.
Discussion
Ask a question about 'LIDAR' Start a new discussion about 'LIDAR' Answer questions from other users |
Recent Posts
Encyclopedia
LIDAR (Light Detection and Ranging) 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.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, 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 aerosols and cloud particles and has many applications in atmospheric research and meteorology.
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 molecules and aerosols).
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 micrometers to the UV (ca. 250 nm). At such wavelengths, the waves are "reflected" very well from small objects. This type of reflection is called backscattering. 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.
A laser typically has a very narrow beam which allows the mapping of physical features with very high 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.
Design
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 microjoule, 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.).
There are several major components to a lidar system:
- Laser — 600-1000 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 focused 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 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.
- 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.
- Photodetector and receiver electronics — Two main photodetector technologies are used in lidars: solid state photodetectors, such as silicon avalanche photodiodes, or photomultipliers. 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
Other than those applications mentioned above, there are a wide variety of applications of LIDAR.
Meteorology
The first LIDARs were used for studies of atmospheric composition, structure, clouds, and aerosols. Initially based on ruby lasers, LIDARs for meteorological applications were constructed shortly after the invention of the laser and represent one of the first applications of laser technology.
Elastic backscatter LIDAR is the simplest type of lidar and is typically used for studies of aerosols and clouds. The backscattered wavelength is identical to the transmitted wavelength, and the magnitude of the received signal at a given range depends on the backscatter coefficient of scatterers at that range and the extinction coefficients of the scatterers along the path to that range. The extinction coefficient is typically the quantity of interest.
Differential Absorption LIDAR (DIAL) is used for range-resolved measurements of a particular gas in the atmosphere, such as ozone, carbon dioxide, or water vapor. The LIDAR transmits two wavelengths: an "on-line" wavelength that is absorbed by the gas of interest and an off-line wavelength that is not absorbed. The differential absorption between the two wavelengths is a measure of the concentration of the gas as a function of range. DIAL LIDARs are essentially dual-wavelength elastic backscatter LIDARS.
Raman LIDAR is also used for measuring the concentration of atmospheric gases, but can also be used to retrieve aerosol parameters as well. Raman LIDAR exploits inelastic scattering to single out the gas of interest from all other atmospheric constituents. A small portion of the energy of the transmitted light is deposited in the gas during the scattering process, which shifts the scattered light to a longer wavelength by an amount that is unique to the species of interest. The higher the concentration of the gas, the stronger the magnitude of the backscattered signal.
Doppler LIDAR is used to measure wind speed along the beam by measuring the frequency shift of the backscattered light. Scanning LIDARs, such as NASA's HARLIE LIDAR, have been used to measure atmospheric wind velocity in a large three dimensional cone. ESA's wind mission ADM-Aeolus will be equipped with a Doppler LIDAR system in order to provide global measurements of vertical wind profiles. A doppler LIDAR system was used in the 2008 Summer Olympics to measure wind fields during the yacht competition.
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 are being used. Pulsed systems using signal timing to obtain vertical distance resolution, whereas continuous wave systems rely on detector focusing.
Geology
In geology and seismology a combination of aircraft-based LIDAR and GPS have evolved into an important tool for detecting faults 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, USA. This combination is also being used to measure uplift at Mt. St. Helens by using data from before and after the 2004 uplift. Airborne LIDAR systems monitor glaciers and have the ability to detect subtle amounts of growth or decline. A satellite based system is NASA's ICESat which includes a LIDAR system for this purpose. NASA's Airborne Topographic Mapper is also used extensively to monitor glaciers and perform coastal change analysis.
Physics and Astronomy
A world-wide network of observatories uses 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 September, 2008, NASA's Phoenix Lander used LIDAR to detect snow in the atmosphere of Mars.
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.
At the 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.
Biology and conservation
LIDAR has also found many applications in forestry. 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.
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. Much of this research has been conducted at the Center for Coastal and Ocean Mapping Joint Hydrographic Center by Dr. Shachak Pe'eri and Captain Armstrong.
Military and law enforcement
One situation where LIDAR has notable non-scientific application is in traffic speed law enforcement, for vehicle speed measurement, as a technology alternative to radar guns. 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 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. LIDAR has the distinct advantage of being able to pick out one vehicle in a cluttered traffic situation as long as the operator is aware of the limitations imposed by the range and beam divergence.
Contrary to popular belief LIDAR does not suffer from “sweep” error when the operator uses the equipment correctly and when the LIDAR unit is equipped with algorithms that are able to detect when this has occurred. A combination of signal strength monitoring, receive gate timing, target position prediction and pre-filtering of the received signal wavelength prevents this from occurring. Should the beam illuminate sections of the vehicle with different reflectivity or the aspect of the vehicle changes during measurement that causes the received signal strength to be changed then the LIDAR unit will reject the measurement thereby producing speed readings of high integrity. For LIDAR units to be used in law enforcement applications a rigorous approval procedure is usually completed before deployment. Jelly-bean shaped vehicles are usually equipped with a vertical registration plate that, when illuminated causes a high integrity reflection to be returned to the LIDAR, many reflections and an averaging technique in the speed measurement process increase the integrity of the speed reading. In locations that do not require that a front or rear registration plate is fitted headlamps and rear-reflectors provide an almost ideal retro-reflective surface overcoming the reflections from uneven or non-compliant reflective surfaces thereby eliminating “sweep” error. It is these mechanisms which cause concern that LIDAR is somehow unreliable.
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, the autonomous car that won the 2005 DARPA Grand Challenge.
Vehicles
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. Often, the lasers are placed onto the bumper. 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.
Imaging
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 UK with 3-D gated viewing imaging at several kilometers range with a range resolution and accuracy better than ten centimeters.
Coherent Imaging Lidar is possible using Synthetic Array Heterodyne Detection which is a form of Optical heterodyne detection that enables a staring single element receiver to act as though it were an imaging array.
Imaging LIDAR can also be performed using arrays of high speed detectors and modulation
sensitive detectors arrays typically built on single chips using CMOS and hybrid CMOS / CCD
fabrication techniques. In these devices each pixel performs some local processing such as
demodulation or gating at high speed down converting the signals to video rate so that
the array may be read like a camera. Using this technique many thousands of pixels / channels may be acquired simultaneously. In practical systems the limitation is light
budget rather than parallel acquisition.
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.
See also
External links
- . A tutorial on altimetric 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.
- : 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
- : 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.
-
-
- - The Electro-Optical Systems Laboratory at GTRI has a nationally known program in lidar research and development.
- - This laboratory has a ground and an upcoming airborne Raman lidar measuring water vapor, aerosols and other atmospheric species
- - A website intended to "facilitate data access, user coordination and education of lidar remote sensing for scientific needs."
- Weier, John. 2004. . Conservation in Practice 5(3):39-41. On the conservation applications of LIDAR.
|
| |
|
|
