Remote sensing, surveying, and monitoring have, in the last few years, reached new heights with the power of the LiDAR technology. The proliferation of LiDAR, which stands for Light Detection and Ranging, is now seen in multiple industries including forestry, disaster management, weather prediction, construction, archaeology, autonomous vehicles, and many more.
It is important to note, however, that there are many types of LiDAR, and choosing the right one is crucial for its successful usage. Here are outlined the different types of LiDAR, first based on functionality and then based on application.
Types of LiDAR by Functionality
Let us take a brief look at the different types of LiDAR classified by the way they are set up and the way they function.
As the name suggests, terrestrial LiDAR is a system that works on the ground. It can be either mounted on a moving vehicle or implanted at a static location. Either way, terrestrial LiDAR data is beneficial for applications that require a detailed survey of the ground or “a closer look” at objects.
Some applications of terrestrial LiDAR include construction, self-driving vehicles, road surveys, city surveys, and so on. Terrestrial LiDAR can be further classified into Mobile and Static versions. Here is how these two differ.
A mobile LiDAR setup typically comprises a sensor, a global positioning system (GPS), an inertial navigation system (INS), and a few cameras. It is mobile because the unit is placed on top of a moving vehicle, such as a car or a train.
From this moving vehicle, the LiDAR unit continues to send out laser pulses in all directions and read the reflections. These valuable point clouds (data points) and then processed to understand the conditions of roads and railway tracks, identify unwanted obstacles on the road, and so on.
In self-driven cars, an advanced rotating LiDAR sensor is mounted on top of the car that detects the presence of pedestrians/other vehicles on the road.
In some applications, it is advantageous to have the LiDAR unit fixed at one point rather than have it move around. Such applications use static LiDAR.
In this setup, the LiDAR unit is mounted on a static object, which is usually a tripod. If needed, the entire unit can be moved to another location along with the tripod. In essence, even though this unit is not mobile, it is fully portable.
A static LiDAR unit continues to send laser pulses to the surrounding area from a fixed point. The data is then used to understand the characteristics of the surrounding. This functionality is highly useful in applications such as building construction, mining, engineering, etc.
When the LiDAR unit is airborne, it means that the system is placed either in an aircraft or a helicopter that continues to hover above the surface of the earth, sending laser pulses downward as it moves.
Airborne LiDAR can scan vast areas in a shorter time as compared to terrestrial LiDAR. This makes airborne LiDAR systems suitable for those applications that require a bird’s eye view of an area spanning multiple acres.
Based on what kind of area the LiDAR unit scans, airborne systems can be further classified into topographic LiDAR and bathymetric LiDAR. Read on for more information on these types.
Topographic LiDAR is used to scan any kind of land, wherein the laser pulses sent down to the surface of the earth provide an estimate of the various characteristics of the area. The rise and the fall of the surface are mapped out based on the altitude of the structures that reflect the laser beams.
In short, topographic LiDAR is used to chalk out the topographic map of a particular piece of land. Applications of topographic LiDAR include forestry, urban planning, ecology, infrastructure mapping, geomorphology, and so on.
While topographic LiDAR can remotely sense any kind of land, it does not work very well when water bodies have to be scanned. To accomplish this task, another type of airborne LiDAR system called the bathymetric LiDAR is used.
A bathymetric LiDAR sensor consists of all the components of a topographic LiDAR plus an extra characteristic that allow the unit to send green laser pulses. These pulses can penetrate the water surface and return to the airborne vehicle.
Data collected in this manner gives an estimation of the depth of the water bodies. When used in conjunction with the topographic sensors, these units can identify shorelines and elevations more distinctly. Coastal engineering and marine sciences typically benefit from using such LiDAR systems.
LiDAR units can also be set up in satellites that revolve around the earth. With satellite LiDAR systems, it is possible to scan greater portions of not just the earth but also the atmosphere above the earth.
Multiple such space-borne LiDAR systems have been used by NASA to understand cloud positioning above the earth, vegetation, the state of the ice on the two poles, and so on. More advanced satellite LiDAR units are being developed that can read particles in the atmosphere as well.
Types of LiDAR by Application
Although most LiDAR systems are similar in the basic working principle, certain applications require specific types of LiDAR units. Here we classify the LiDAR systems based on these specifications.
Differential Absorption LiDAR (DIAL)
This technology is used to measure the gas concentrations in the atmosphere, more specifically, to monitor the levels of ozone. DIAL systems can be both terrestrial and airborne and can also measure pollution levels.
Such a system uses a tunable laser to produce two wavelengths of laser pulses that record data from the peak of a gas absorption line and another in a low-absorption region. Hence, the name, differential absorption.
Measuring the movement of wind can be tricky using any form of remote sensing equipment, primarily because the wind changes direction rapidly. Wind LiDAR has been designed for this very purpose.
Advanced LiDAR systems that provide 360-degree monitoring of wind continuously can be extremely beneficial in understanding turbulences, wind speed, and wind direction using multiple data points.
Raman LiDAR is a terrestrial LiDAR system that is specifically used to detect and measure levels of water vapor and aerosols in the environment. While a conventional LiDAR unit perceives its data from the backscattering of reflected laser pulses, Raman LiDAR can go further and detect the signals present in the backscattering at different wavelengths.
This process is called inelastic scattering. These signals are essentially produced by the presence of molecules. Using this technique, the LiDAR can evaluate the presence of aerosols with optimum accuracy.
HSRL, which stands for High Spectral Resolution LiDAR, is another way to determine the presence of aerosols in the atmosphere. However, in contrast to a Raman LiDAR, HSRL is an airborne system.
The unit works with the spectral distribution of the reflected LiDAR signals to differentiate between pulses reflected by molecules from those sent back by aerosols.
As LiDAR continues to be successfully used in various applications, the knowledge of different types of LiDAR gives the users an edge when deciding on the right system. Based on various functionalities, technology, setup, and applications, LiDAR units can be classified into many different types.
From being airborne to terrestrial, from mobile to static, LiDAR units can be flexible to be used as the need arises. With advances in technologies such as differential absorption and inelastic scattering, LiDAR has the potential to be used across more applications in the future.
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