LIDAR
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Lidar (light detection and ranging or laser imaging detection and ranging) is a technology that determines distance to an object or surface using laser pulses. Like the similar radar techology, 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.
The acronym LADAR (LAser Detection And Ranging) for elastic backscatter lidar systems is mainly used in military context. The term laser radar is also in use but somewhat misleading as laser light and not radiowaves are used.
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General description
The primary difference between lidar and radar is that much smaller electromagnetic wavelengths are used. 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 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.
Lasers provide one solution to these problem. 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). With these sorts of wavelengths a lidar systems offer much higher resolution than radar. The wavelengths are ideal for making measurements of smoke and other airborne particles (aerosols), clouds, and of air molecules.
A laser typically has a very narrow beam which allows to map atmospheric features with very high resolution compared with radar. Another advantage of lidar is that many chemical substances interact more strongly at visible wavelengths than at microwaves. 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 mostly for atmospheric research and meteorology. More recently a number of surveying and mapping applications have been developed, using downward-looking lidar instruments mounted in aircraft or satellites. Another newer use is to map the eye during LASIK eye surgery, in order to allow the main cutting beam to follow any movements of the eye.
Applications
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.
A word-wide network of observatories use lidars to measure the distance to reflectors placed on the moon, so measuring the moon's position with mm precision and enabling tests of general relativity to be done.
MOLA, the Mars Orbiting Laser Altimeter, used a lidar instrument in a Mars-orbiting satellite to produce a spectacularly accurate global topographic survey of the red planet.
In atmospherics, 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 winds.
One situation where lidar has notable non-scientific application is for vehicle speed measurement. 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. 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.
Military applications are not yet in place, 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.
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 Laser Line Scanner (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 in Sweden and Denmark with 3-D gated viewing imaging at several kilometers range with a range resolution and accuracy less than ten centimeters.
Design
In general there are two types of lidar systems, "high energy" systems and micropulse lidar 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 watt, 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 three major components to a lidar:
- Laser — 600-800 nm lasers are most common for non-scientific applications. They are inexpensive and can be found with sufficient power but they are not eye-safe. Eye-safety is often a requirement for military apps. 1550 nm lasers are eye-safe but not common and are difficult to get with good power output. Laser settings include the laser repetition rate (which controls the data collection speed) and pulse length (which sets the range resolution).
- 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 Si and InGaAs. They are made in either PIN or Avalanche photodiode configurations. The sensitivity of the receiver is another parameter that has to be balanced in a LIDAR design.
See also
External links
- Atmospheric-Optics Laboratory: Lidar Basics (http://aolab.phys.dal.ca/pages/LidarBasics) - at Dalhousie University, Halifax, Canada
- Atmospheric Physics: LIDAR (at University of Wales) (http://users.aber.ac.uk/ozone/lidar.html)
- National Science Digital Library - MPL Quicklook Page (http://www.nsdl.arm.gov/Visualization/mpl/frame.shtml)
- DOE Atmospheric Radiation Measurement MPL Instrument Description (http://www.arm.gov/instruments/static/mpl.stm)
- www.lidar.com
- Puget Sound LIDAR Consortium (http://rocky2.ess.washington.edu/data/raster/lidar/index.htm)de:Lidar