LIDAR is an acronym for light direction and ranging,sometimes also referred to as Laser Altimetry or Airborne Laser Terrain Mapping (ALTM). The LIDAR system basically consists of integration of three technologies, namely, Inertial Navigation System (INS), LASER, and GPS and is a laser remote sensing technique used in both science and industry. It is the optical equivalent of the microwave radar, and so is often referred to as laser radar.
In the Atmospheric-Optics Laboratory we use lidars for atmospheric research, and obtain measurements of aerosol particulates, clouds, temperatures and water vapour. The measurements are used for studies of transboundary pollution transport and Arctic climate change.LIDAR is much faster than conventional photogrammetric technology and offers distinct advantage over photogrammetry in some application areas.The method has successfully established itself as an important data collection technique, within a few years, and quickly spread into practical applications.
1.1 What’s LIDAR?
LIDAR is similar in principle to Radar. It is a very helpful instrument to observe the Earth’s atmosphere. A lidar transmits light to a target (the atmosphere) , which scatters a small portion of the light back along the line of sight. The lidar is able to infer several characteristics of the target, which subtly alters the incident light. A light may be in the form of short pulses (“pulsed”) or continuously (“continuous-wave”).For the case of pulsed lidar, the range to the target may be measured by timing the flight of the pulse from the laser to the target and back. To construct a lidar system we need a laser to send out powerful light pulses, a telescope to receive the backscattered light, a sensitive detector to measure the intensity, a bunch of electronics, etc. Using lidar we measure where, how and how much light was scattered back.
In radar, radio waves are transmitted into the atmosphere, which scatters some of the power back to radar’s receiver. A lidar also transmits and receives electromagnetic radiation, but at a higher frequency. Lidar operates in the ultraviolet, visible and infrared region of the electromagnetic spectrum.
Different types of physical processes in the atmosphere are related to different types of light scattering. Choosing different types of scattering processes allows atmospheric composition, temperature and wind to be measured.
1.2 How does it work?
LIDAR systems vary by manufacturer, but all use the following instrumentation: a laser source and detector; a scanning mechanism and controller; airborne GPS and IMU equipment; a high-accuracy, high-resolution clock for timing laser emissions, reflections, GPS/IMU, and scan-angle measurements; high performance computers; and high capacity data recorders. With these components, lidar data collection is possible:
o A pulse of laser light is emitted and the precise time is recorded
o The reflection of that pulse from the surface is detected and the precise time is recorded
o Using the constant speed of light, the time difference between the emission and the reflection can be converted into a slant range distance (line-of-sight distance) With the very accurate position and orientation of the sensor provided by the airborne GPS and inertial measurement unit (IMU) data, the XYZ coordinate of the reflective surface can be calculated .
1.3 What are the benefits of LIDAR?
LIDAR offers many advantages over traditional photogrammetric methods for collecting elevation data. These include high vertical accuracy, fast data collection and processing, robust data sets with many possible products, and the ability to collect data in a wide range of conditions.
1.4 Technical Overview
System calibration must occur for each mission. This involves flying a specific pattern of flight lines to highlight systematic offsets to which adjustments are applied during the processing. Ground control also is used to determine any vertical bias that may occur from flight to flight.
LIDAR missions are tailored to meet individual project specifications, based on intended data use. Important collection parameters include:
o Point spacing and point density.
• Also referred to as “average” or “nominal” post spacing, point spacing is a one-dimensional measurement of points along a line. Point density refers to the number of points in a given area. The greater the number of points, the denser the dataset.
o Pulse rate.
• The pulse rate is the speed by which the lidar sensor emits laser pulses. Lidar systems can have fixed or variable pulse rates. Higher pulse rates are associated with denser data sets. The chosen pulse rate for a specific mission will impact the maximum operational altitude (higher pulse rates require lower altitudes).
o Field of view.
• The field of view (FOV) refers to a sensor’s scan angle. The FOV can be fixed or variable and impacts the flying altitude and pulse rate (wider scan angles require higher altitudes and lower pulse rates). For areas containing intense shadows (mountainous regions, for instance) a narrow FOV is preferable to keep the beam nearly perpendicular to the earth’s surface.
o Aircraft altitude.
• The altitude (and speed) of the aircraft during lidar missions depends on the desired point spacing/point density, pulse rate, and field of view. Typically, missions flown at high altitudes and fast speeds produce less dense datasets. Terrain features, aircraft safety, and air traffic control regulations must also be considered when choosing an altitude for a project.
Multiple Return LIDAR
Multiple return lidar yields both range and intensity data from a single pass:
Range data measures the distance from the sensor to the object struck.
Intensity data measures the return signal strength, based on the way the object struck reflects the lidar energy. Intensity data is consistent among similar objects, making it possible to map the information in the form of a matrix, giving the appearance of a gray-scale image.
In multiple return lidar systems, the first pulse measures the range to the first object encountered. The last pulse measures the range to the last object. By acquiring first- and last-pulse data simultaneously, it is possible to measure both tree heights and the topography of the ground.
1.5 Generic Types Of LIDAR
1.5.1 Range finders
Range finder LIDARs are the simplest LIDARs. They are used to measure the distance from the LIDAR instrument to a solid or hard target.
Differential Absorption LIDAR is used to measure chemical concentrations (such as ozone, water vapor and pollutants) in the atmosphere. A DIAL lidar uses two different laser wavelengths which are selected so that one of the wavelengths is absorbed by the molecule of interest while the other wavelength is not. The difference in intensity of the two return signals can be used to deduce the concentration of the molecule being investigated.
1.5.3 Doppler LIDARs
Doppler LIDAR is used to measure the velocity of a target. When the light transmitted from the LIDAR hits a target moving towards or away from the LIDAR, the wavelength of the light reflected / scattered off the target will be changed slightly. This is known as a Doppler shift-hence Doppler LIDAR. If the target is moving away from the LIDAR, the return light will have a longer wavelength, if moving towards the LIDAR the return light will be at a shorter wavelength. The target can be either a hard target or an atmospheric target- the atmosphere contains many microscopic duct and aerosol particles, which are carried by the wind. These are the targets of interest to us as they are small and light enough to move at the true wind velocity and thus enable a remote measurement of the wind velocity.