Lidar Overview
Introduction
Lidar (or LiDAR or LIDAR) is an acronym for Light Detection And Ranging. The term is also a fusion of the words Radar and Light. Like radar, lidar is an active remote sensing technology but instead of using radio or microwaves it uses light. Because lidar systems provide their own energy they can used in the day or at night. While it can be acquired at night, lidar can not penetrate clouds and rain and therefore can only be acquired in fair conditions. Lidar produces extremely accurate and precise data, often with centimeter level accuracy. Today lidar is used in a variety of application, including forestry, geology, oceanography, mapping and more.
How Lidar Works
Lidar systems send out thousands or even hundred of thousands of laser light pulses every second. The acronym “laser” stands for “light amplification by stimulated emission of radiation.” A laser generates a stream of high energy photons in narrow range of wavelengths. Most terrestrial lidar systems operate in near-infrared portion of the spectrum. These light pulses then strike and reflect off of the surfaces on the earth. The lidar system measures the time it takes for the light pulse to return. These times are recorded and then converted to distances based on the following formula:
The above formula tells us the distance from the sensor to the target, also known as the range. With knowledge of the position (X,Y,Z) of the lidar system sensor and the angle of the laser, the precise 3-dimensional coordinates (X,Y,Z) of the objects on the ground can be calculated. This is done with Direct Georeferencing. The integrated Global Positioning System (GSP), Inertial Measurement Unit (IMU) and onboard computer on the lidar system allow for the direct georeferencing of the points.
Platforms
There are three main types of Lidar platforms: Ground based, Airborne and Satellite.
Ground Based or terrestrial lidar systems are used create highly accurate models and measurements of buildings, archeology sites and rock formations. Ground based systems are usually located on a stationary tripod or they can be located on a moving vehicle. For example the self-driving cars Google is developing and testing have lidar systems mounted on the roofs.
A ground based lidar system is used to analyze and scan the ancient desert city of Petra in the National Geographic program "Time Scanners" Image Credit: National Geographic Airborne lidar platforms include systems mounted on helicopters, airplanes and Unmanned Aircraft Systems (UAS). Fixed-wind aircraft are the most common airborne platforms and are used for topographic lidar mapping. Helicopters are also sometimes used as the can fly lower and slower than an airplane. Lidar systems have become smaller and lighter in recent years and can now be mounted on a variety of Unmanned Aircraft Systems. The use of UAS and lidar will most likely increase substantially in the coming decade. Some of the greatest costs associated with airborne lidar are the high costs of renting a lidar equipped plane and pilot.
Satellite or space-borne lidar platforms are mounted on satellites that orbit the Earth. Satellite lidar platforms tend to cover large areas but with less detail. NASA's Ice Cloud and Land Elevation Satellite (ICESat) carried the Geoscience Laser Altimeter System (GLAS) which collected data on global cloud cover, vegetation canopy and polar ice caps. ICESat-2, the follow-up to the first satellite was launched in 2018. Another space-borne lidar system was installed on the International Space Station in January 2015. The Cloud-Aerosol Transport System (CATS) is designed not measure the Earth's surface but to use lidar to analyze and measure the particles in the Earth's atmosphere.
The Global Ecosystem Dynamics Investigation (GEDI) is a lidar sensor installed on the International Space Station in 2019. The mission of GEDI is to produces high resolution laser ranging observations of the 3D structure of the Earth.
Explorer: Lidar at Mount St. Helens
At Mount St. Helens, lidar based DEMs are used to map pyroclastic and debris flow deposits and analyze changes in the surface elevation of the crater. In 2004 significant deformation of the crater occurred and the growth of a new lava dome was captured with lidar. The new dome grew 110 meters (360 feet) between late September and October 4, 2004.
