26-04-2011, 12:52 PM
SUBMITTED BY
VINIT KUMAR MADDHESHIYA
FINAL HARD COPY OF LIDAR ori.docx (Size: 1.19 MB / Downloads: 486)
ASBTRACT
This white paper on LiDAR mapping provides the reader with an introduction to a maturing tech¬nology used to acquire land based Digital Eleva¬tion Models (DEM), and an existing mature tech¬nology used to acquire marine (undersea) DEMs. Common to both the land-based and marine-based systems is the use of lasers, integrated into what are known as LIghtDetectionAndRanging systems, or LiDAR.
The paper begins with an introduction to LiDAR, and follows-through with how the systems are used, a description of land and marine variants, and statements as to what accuracy is attain¬able. Data products driven by applications are outlined, and the ability of these products to be integrated into a GIS is addressed. In turn and integral to the theme of the paper, evidence is presented to support applications developments that will ultimately benefit the public at large.
Where valid, comparisons of LiDAR to other map¬ping technologies are presented to give the reader an understanding of current traditional practices. In many cases the combination of Li¬DAR with other technology creates a result that is unachievable with a single methodology.
Finally, conclusions are presented for discussion purposes that draw on the experiences of LiDAR practitioners and clients, as well as those of Ter¬rapoint.
INTRODUCTION
LiDAR, an acronym for Light Detection and Ranging, is a term that has been around,
discussed, and researched since the early 1950’s. It was not until the development of
accurate positioning systems that LiDAR became to be considered as a viable imaging /
mapping technology.
Looking generally at how LiDAR works, there is typically a LiDAR laser sensor, which
is precision mounted to the underside of an aircraft, and which transmits or pulses a
narrow laser beam towards the earth as the aircraft flies. A receiver additionally affixed
to the aircraft receives the reflection of these pulses as they bounce off the earth below
back to the aircraft. Most LiDAR systems use a scanning mirror to generate a swath of
light pulses. Swath width depends on the mirror's angle of oscillation, and ground-point
density depends on factors such as aircraft speed and mirror oscillation rate. Ranges are
determined by computing the amount of time it takes light to leave an airplane, travel to
the ground and return to the sensor. A sensing unit's precise position and attitude,
instantaneous mirror angle and the collected ranges are used to calculate 3-D positions of
terrain points. As many as 100,000 positions or "mass points" can be captured every
second. The LiDAR sensor essentially records the difference in time between the signal
being emitted and received from a given point, very much like a conventional survey
instrument. The LiDAR data is coupled with additional precise positioning information
gathered by on board Global Positioning Instruments (GPS) and other Inertial Navigation
Systems (INS). Once the total information volume is stored and processed, the resulting
product is an extremely accurate "x.y.z." for every position scanned on the ground
LIDAR uses ultraviolet, visible, or near infrared light to image objects and can be used with a wide range of targets, including non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds and even single molecules. A narrow laser beam can be used to map physical features with very high resolution.
LIDAR has been used extensively for atmospheric research and meteorology. Downward-looking LIDAR instruments fitted to aircraft and satellites are used for surveying and mapping. A recent example being the NASA Experimental Advanced Research Lidar.
Wavelengths in a range from about 10 micrometers to the UV (ca. 250 nm) are used to suit the target. Typically light is reflected via 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. Based on different kinds of backscattering, the LIDAR can be accordingly called Rayleigh LiDAR, Mie LiDAR, Raman LiDAR and Na/Fe/K Fluorescence LIDAR and so on. Suitable combinations of wavelengths can allow for remote mapping of atmospheric contents by looking for wavelength-dependent changes in the intensity of the returned signal.
LIDAR: OVERVIEW
A LiDAR system combines a single narrow-beam laser with a receiver system. The laser produces an optical pulse that is transmitted, reflected off an object, and returned to the receiver. The re¬ceiver accurately measures the travel time of the pulse from its start to its return. With the pulse trav¬elling at the speed of light, the receiver senses the return pulse before the next pulse is sent out. Since the speed of light is known, the travel time can be converted to a range measurement. Combining the laser range, laser scan angle, la¬ser position from GPS, and laser orientation from INS, accurate x, y, z ground coordinates can be calculated for each laser pulse. Laser emission rates can be anywhere from a few pulses per second to tens of thousands of pulses per sec¬ond. Thus, large volumes of points are collected. For example, a laser emitting pulses at 10,000 times per second will record 600,000 points every minute. Typical raw laser point spacing on the ground ranges from 2 to 4 meters
Some LiDAR systems can record “multiple re¬turns” from the same pulse. In such systems the beam may hit leaves at the top of tree canopy, while part of the beam travels further and may hit more leaves or branches. Some of the beam is then likely to hit the ground and be reflected back, ending up with a set of recorded “multiplereturns” each having an x, y, z position. This fea¬ture can be advantageous when the application calls for elevations for not only the ground, but for tree or building heights
As surface types and characteristics vary and change the laser beam’s reflectivity, then theability of the LiDAR to record the return signals changes. For example, a laser used fortopo¬graphic applications will not penetrate water, and in fact records very little data even for the surface of the body of water. Where the appli¬cation calls for a laser to penetrate water to determine x, y, z positions of undersea features, then a slightly different variation of LiDAR technology is used.
DESIGN
In general there are two kinds of lidar detection schema: "incoherent" or direct energy detection (which is principally an amplitude measurement) and Coherent detection (which is best for doppler, or phase sensitive measurements). Coherent systems generally use Optical heterodyne detection which being more sensitive than direct detection allows them to operate a much lower power but at the expense of more complex transceiver requirements .
Fig. 3
In both coherent and incoherent LIDAR, there are two types of pulse models: micropulselidar 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.)[1].
There are several major components to a LIDAR system:
1. 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[1].
2. 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.
3. 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.
4. 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).