What is LiDAR ?
Light Detection and Ranging (LiDAR) is a popular remote sensing technology that uses laser light to detect and measure distances of objects or targets. A laser beam is emitted towards the object and scattered beam is collected by sensors located within the LiDAR module. Differences in laser return times and wavelength carry information about the object to make a digital 3D representation of the target.
The LiDAR has improved the precision of sensing compared to the traditional remote sensing technologies like Radar (radio waves) and Sonar (sound waves). The laser precision of it can map features of an object or an environment as a point cloud that appears as solid physical objects on display. It uses different wavelengths starting from infrared (10 micrometers) to ultraviolet (250 nanometers) light to image objects. Different kinds of scattering such as Rayleigh scattering, Raman scattering, Mie scattering, and others are used in different applications. Identifying wavelength-dependent changes of the scattered signal allows remote mapping of atmospheric contents and object properties.
Types of LiDAR
These systems can be classified into various types based on its orientation, platform, and other features. Few different types are listed below
- Ground-based: Terrestrial static LiDAR systems are installed on the earth surface for accurate collection of data points. These are applicable for observing highways, scanning buildings, analyzing infrastructures, and mapping of geology. Mobile systems consist of two or more scanners installed on a moving vehicle to map data along a path.
- Airborne: In this system, a laser scanner is installed on a helicopter or drone to create a 3D point cloud of the landscape. These kinds of systems are mainly used in military and surveying applications. Space-borne systems are used in satellite mapping and other applications.
- Orientation: These are classified based on the orientation of the LiDAR. For example, collision avoidance systems are side-looking, atmospheric LiDAR looks up, and altimeters look down.
LiDAR System Components
Various components from different technology background are used in a system and few main components are listed as follows
- Lasers: The use of lasers depends on different types of LiDAR and its applications. A wavelength around 1000nm is used in airborne types. Wavelengths in the infrared range are often used in topography mapping and obstacle avoidance. The eye-safe laser source must operate in pulsed mode to use in autonomous vehicle systems.
- Optics and Scanners: Speed of a scanner decides the speed at which images can be developed. The choice of optics affects the angular resolution and range that can be detected. Dual oscillating plane mirrors in combination with a polygon mirror and a dual-axis scanner are needed to scan the azimuth and elevation. Scattered signals are usually collected using a beam splitter or a mirror.
- Photo-detectors: It is a device that detects and records the back-scattered signals. Avalanche photodiodes and photomultipliers are two common photo-detectors used in systems.
- Navigation and Positioning systems: In mobile LiDAR types, it becomes necessary to determine the absolute position and the orientation of the sensor. The accurate geographical information regarding the sensor position and the accurate orientation of the sensor is obtained from Global Positioning Systems (GPS) and an Inertial Measurement Unit (IMU) respectively.
In simple terms, LiDAR emits laser light at an object and calculates the time it takes to return to the receiver. The laser source fires rapid pulses of light at a surface and a sensor measures the amount of time it takes for each pulse to scatter back. This process is repeated in quick intervals to build a complex map of the object. The distance between the static system and the target object can be calculated at high accuracy using the formula given below
The distance of the target object = (Time of Flight x Speed of Light) / 2
In mobile LiDAR, as the sensor is moving, the data from GPS and IMU such as height, location, and orientation of the instrument is critical. The data are downloaded and processed using application-specific computer software. Geographically registered longitude (X), latitude (Y) and elevation (Z) for every data point is calculated to create detailed topographic maps.
The use of LiDAR has penetrated almost all major sectors and few applications are listed below
- Autonomous vehicles: This industry has benefited more from LiDAR technology than any other industry. Self-driving cars, robots, or drones navigate in autonomous mode using LiDAR. It is used for obstacle detection and avoidance. Adaptive Cruise Control (ACC) which has LiDAR technology to control the speed of the vehicles based on traffic density.
- Physics and Astronomy: NASA’s Mars Global Surveyor uses LiDAR to produce a precise global topographic survey of the red planet. In atmospheric physics, LiDAR is used to measure densities of certain constituents of the middle and upper atmosphere. LiDAR is used to gather information about the distribution of the aerosol particles and also to calculate wind speeds.
- Archaeology: LiDAR is used for mapping features under forest canopy and planning of field campaigns. Its data can be easily integrated into the Geographic Information System (GIS) for analysis and interpretation. Digital elevation models (DEMs) of archaeological sites can reveal micro-topography of hidden vegetation using LiDAR.
- Agriculture: it is used to create a topographical map of fields. Further, determine where to apply costly fertilizers based on the recorded data of slopes and sun exposures to the farmland. Furthermore, it also used to categorize the farm yield into zones of low, medium, and high. It is employed for monitoring insects in the field. LiDAR with machine learning is used for plant species identification.
- Biology and conservation: LiDAR is applied to estimate and assess the biodiversity of organisms. It is also applied in many applications of forestry.
- Military and Law enforcement: Enforcement agencies use LiDAR for recording vehicle speed measurements. Laser-Induced Fluorescence (LIF) based LiDAR is used for detecting the presence of bio-threats in aerosol form. Also, high-resolution systems can identify targets such as tanks and other enemy targets.