LIDAR

Light Detection and Ranging LIDAR is an optical remote sensing technology used to collect a wide range of topographic data.

Introduction to LIDAR:
LIDAR is similar to radar technology, which uses radio waves, a form of electromagnetic radiation that is not in the visible spectrum. 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 applications in Archaeology, Geography, Geology, Geomorphology, Seismology, remote sensing and many more areas.

Light detection and ranging (LIDAR), also known as airborne laser scanning (ALS), is an emerging remote sensing technology with promising potential to assisting mapping, monitoring, and assessment of forest resources. Compared to traditional analog or digital passive optical remote sensing, LIDAR offers tangible advantages, including nearly perfect registration of spatially distributed data and the ability to penetrate the vertical profile of a forest canopy and quantify its structure.

LIDAR Systems
A LIDAR system operating from an airborne platform comprises a set of instruments: the laser device; an inertial navigational measurement unit (IMU), which continuously records the aircraft’s attitude vectors (orientation); a high-precision airborne global positioning system (GPS) unit, which records the three-dimensional position of the aircraft; and a computer interface that manages communication among devices and data storage. The system also requires that a GPS base station installed at a known location on the ground and in the vicinity (within 50 km) of the aircraft, operate simultaneously in order to differentially correct, and thus improve the precision of, the airborne GPS data.

The laser device emits pulses (or beams) of light to determine the range to a distant target. The distance to the target is determined by precisely measuring the time delay between the emission of the pulse and the detection of the reflected (backscattered) signal. There are two types of LIDAR acquisition differentiated by how backscattered laser energy is quantified and recorded by the system’s receiver. With waveform LIDAR, the energy reflected back to the sensor is recorded as a (nearly) Continuous signal. With discrete-return, small-footprint LIDAR, reflected energy is quantized at amplitude intervals and is recorded at precisely referenced points in time and space. Popular alternatives to the term “point” include “return” and “echo.” The energy amplitude pertaining to each return is known as intensity.

System Specifications
LIDAR systems have been evolving for more than a decade, and will likely continue to evolve even faster in the years to come. Hence, when planning data acquisition, it is essential to obtain specifications of currently available systems. Such specifications will determine both data acquisition costs and, quite likely, the feasibility of projects the acquired data are expected to support. The major operational specifications of a LIDAR system are outlined below:

• Scanning frequency is the number of pulses or beams emitted by the laser instrument in 1 second. Older instruments emitted a few thousand pulses per second. Modern systems support frequencies of up to 167 kHz (167,000 pulses per second). A high-frequency system can generate desired return densities by operating on an aircraft that flies higher and faster than an aircraft carrying a lower frequency system, thereby reducing flying time and acquisition costs.

• Scanning pattern is the spatial arrangement of pulse returns that would be expected from a flat surface and depends on the mechanism used to direct pulses across the flight line. Of the four scanning patterns supported by instruments used in acquiring laser data, the seesaw pattern and its stabilized equivalent are the most common. In these two patterns, the pulse is directed across the scanning swath by an oscillating mirror, and returns are continuously generated in both directions of the scan. In the parallel line pattern, a rotating polygonal mirror directs pulses along parallel lines across the swath, and data are generated in one direction of the scan only. The elliptical pattern is generated via a rotating mirror that revolves about an axis perpendicular to the rotation plane.

• Beam divergence Unlike a true laser system, the trajectories of photons in a beam emitted from a LIDAR instrument deviate slightly from the beam propagation line (axis) and form a narrow cone rather than the thin cylinder typical of true laser systems. The term “beam divergence” refers to the increase in beam diameter that occurs as the distance between the laser instrument and a plane that intersects the beam axis increases. Typical beam divergence settings range from 0.1 to 1.0 millirad. At 0.3 millirad, the diameter of the beam at a distance of 1000 m from the instrument is approximately 30 cm. Because the total amount of pulse energy remains constant regardless of the beam divergence, at a larger beam divergence, the pulse energy is spread over a larger area, leading to a lower signal-to-noise ratio.

• Scanning angle is the angle the beam axis is directed away from the “focal” plane of the LIDAR instrument The maximum angle supported by most systems does not exceed 15 degrees. The angle is recorded as positive toward the starboard and negative toward the port side of the aircraft. The combination of scanning angle and aboveground flight height determines the scanning swath.

• Footprint diameter is the diameter of a beam intercepted by a plane positioned perpendicularly to the beam axis at a distance from the instrument equal to the nominal flight height. It is thus a function of both beam divergence and the above-target flight height. The distribution of pulse energy is not uniform over the extent of the footprint. It decreases radially from the center and can be approximated by a two-dimensional Gaussian distribution.

• Pulse length is the duration of the pulse, in nanoseconds (ns). Along with discretization settings, it determines the range resolution of the pulse in multiple return systems, or the minimum distance between consecutive returns from a pulse.

• Number of returns (per beam/pulse) is the maximum number of individual returns that can be extracted from a single beam. Certain systems can identify either the first or the first and last returns. Most modern systems can identify multiple returns (e.g., up to five) from a single beam.

• Footprint spacing is the nominal distance between the centers of consecutive beams along and between the scanning lines, which, along with the beam divergence, determines the spatial resolution of LIDAR data. The footprint spacing is a function of scanning frequency, the aboveground flight height, and the velocity of the aircraft.

Applications of LIDAR
Archaeology - LIDAR has many applications in the field of archaeology including aiding in the planning of field campaigns, mapping features beneath forest canopy, and providing an overview of broad, continuous features that may be indistinguishable on the ground. LIDAR can also provide archaeologists with the ability to create high-resolution digital elevation models (DEMs) of archaeological sites that can reveal micro-topography that are otherwise hidden by vegetation. LIDAR-derived products can be easily integrated into a Geographic Information System (GIS) for analysis and interpretation. Beyond efficiency, its ability to penetrate forest canopy has led to the discovery of features that were not distinguishable through traditional geo-spatial methods and are difficult to reach through field surveys. An April 2009 flyover of the Maya city of Caracol used LIDAR equipment to help scientists construct a 3-D map of the settlement in western Belize. The survey revealed previously unknown buildings, roads, and other features in just four days, scientists announced at the International Symposium on Archaeometry in Tampa, Florida.

Atmospheric Studies - NASA plans to test a laser-based sensor (Fall, 2010) in space that will help scientists better understand global climate and how it might be changing. The instrument, called LITE (LIDAR In-Space Technology Experiment), will orbit the Earth while positioned inside the payload bay of Space Shuttle Discovery. During this nine-day mission, LITE will measure the Earth's cloud cover and track various kind of particles in the atmosphere. Designed and built at the NASA Langley Research Center, LITE is the first use of a LIDAR system for atmospheric studies from space. LIDAR is similar to the radar commonly used to track everything from airplanes in flight to thunderstorms. But instead of bouncing radio waves off its target, LIDAR uses short pulses of laser light. Some of that light reflects off of tiny particles in the atmosphere and back to a telescope aligned with the laser. By precisely timing the LIDAR "echo," and by measuring how much laser light is received by the telescope, scientists can accurately determine the location, distribution and nature of the particles. The result is a revolutionary new tool for studying constituents in the atmosphere, from cloud droplets to industrial pollutants, that are difficult to detect by other means.

Bathymetry - Although it is still beyond our reach to acquire precise and high-resolution seafloor depths from space, LIDAR bathymeters on fixed wing and rotary aircraft can penetrate the water column to collect seafloor data at depths of up to approximately 230 feet (70 meters). Bathymetric LIDAR uses a high powered laser to transmit electromagnetic energy, specifically in near-infrared and green wavelengths, from the aircraft platform through the water column and make a time difference measurement to calculate the seafloor depth. Most modern units today use a frequency between 200 and 4,000 Hz, which can result in upwards of 14 million measurements per hour and a horizontal spacing of approximately 0.5 to 6 meters on the seabed. While airborne, a laser altimeter mounted in the aircraft pulses both of these wavelengths to the surface of the water and measures the time it takes for the energy to return. The infrared light is reflected back to the aircraft from the water surface while the green light travels through the water column. Energy from the green light reflects off the seafloor and is captured by the airborne sensor. The water depth is obtained by determining the time difference between the infrared and green laser reflections using a simple calculation that incorporates the properties of the water column along with system and environmental factors.

Contour Mapping - LIDAR Contour Mapping is a rapid, cost-effective source of high-accuracy, high-density elevation data for many traditional topographic mapping applications. The technology allows large area topographic surveys to be completed significantly faster and  at a reduced cost compared to traditional survey methods.

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. 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. Raman LIDAR is also used for measuring the concentration of atmospheric gases, but can also be used to retrieve aerosol parameters as well. 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. Synthetic Array LIDAR allows imaging LIDAR without the need for an array detector.

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. Airborne LIDAR systems monitor glaciers and have the ability to detect subtle amounts of growth or decline.

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.

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. 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. 3D mapping Airborne LIDAR sensors are used by companies in the Remote Sensing area to create point clouds of the earth ground for further processing (e.g. used in forestry). A common format for saving these points (with parameters like x, y, return, intensity, elevation) is the LAS file format.

Law Enforcement - LIDAR devices have been developed that can pinpoint oncoming cars in traffic and determine their rate of speed with a great degree of accuracy.

Oceanic Studies - LIDAR imaging has been used to analyze oil contamination in the Gulf of Mexico resulting from the 2010 BP oil rig disaster.