LIDAR data has proved useful in recent years since it gives information such as i tem height and attributes, statistics over large regions, and it all becomes available by capturing the intensity of backscattered pulses in addition to 3D coordinates. “A LiDAR may generate a 2D angle image as well as a 3D picture containing angle/angle/r ange information. A LiDAR can directly measure range in each pixel since it controls when light is emitted, allowing it to calculate range based on the time of flight (ToF) to and from the object in a particular pixel. If there is enough signal, a 3D pictu re can also have grayscale and colour ”. Coherent LiDAR can detect velocity quite precisely. Brief descriptions of airborne laser scanning technology [also known as "light detection and ranging" (LIDAR)] and research findings on its application in forest as sessment and monitoring are included. Many airborne laser scanning missions are being flown with terrain mapping requirements, leading to data sets that are missing important data for vegetation evaluation. As a result, standards and criteria for airborne laser scanning missions are needed to guarantee that they are used for vegetation measurement and monitoring instead of merely landscape mapping (e.g., delivery of all return data with reflection intensity). Keywords: LIDAR, 3D, ToF, Pulsed T ime of Flight, EM, Wavelengths, Electromagnetic pul se. 2. Introduction LiDAR employs electromagnetic (EM) pulses of optical and infrared wavelengths. “It is an active sensor, which means that it emits an EM pulse and receives the signal that bounces back. It is similar to microwave radar, but with a much shorter wavelength. This means it will have far greater angular resolution than radar but will be unable to see through fog or clouds ”. It is similar to passive electro -optical (EO) sensors in wavelengths, except that it creates its own radiation 2 rather than utilizing existing radiation and has several more se nsing modes due to cont rol over the scene illumination (De Raedt et al., 2009 , P.1/10 ). 2.1 “Definition of Key Terms” 2.1.1 LIDAR LIDAR employs lasers to determine the heights of objects such as the ground, woods, and even buildings. “It resembles a radar, which uses radio waves to map things, or a sonar, which uses sound waves to map things, but a LIDAR system utilizes light emitted by a laser to map things. LIDAR, also known as LADAR, is an optical remote sensing technique that detects the characteristics of scattered light to determine the range and/or other information of a distant target structure ” (Olsen, 2016 , P.9/29 ). Figure 1: Overall construction of the LIDAR (Myllylä et al., 1998 , P.3/7 ). 2.1.2 “ToF (Time of Flight) ” Lasers are used in LIDAR to estimate the heights of things like the ground, trees, and even buildings. A LIDAR system is similar to a radar in that it employs radio waves to map things and 3 a sonar in that it uses sound waves to map things, but it uses light generated by a laser to map things. LIDAR, also referred as LADAR, is an optical remote sensing technology for considering a range and/or other data of a distant target structure by detecting th e properties of scattered light. ToF systems are divided into two types: ❖ Indirect (I -ToF): “In I-ToF systems, depth information is computed by detecting the phase shift of the intercepted pulsed or modulated light with reference to the released one ”. ❖ D-ToF: “D-ToF uses avalanche detectors' excellent temporal precision to directly estimate the time of flight of the outgoing light pulse ” (Tontini et al., 2020 , P.1-2/19) 4 “Figure 2: A D-ToF system's block diagram ” (Tontini et al., 2020 , P.2/19 ) 2.2 Background “Scanning LiDAR, stereo vision, and structural light sensors are just a few of the sensors that may be utilized to acquire 3D data today. Scanning LiDAR is the most often used sensor for acquiring 3D data due to its accu racy and wide measurement range ” (Fang et al., 20 18, P.1/15 ). Depending on the power of the transmitter laser, the pulsed time -of-flight method is most commonly employed for measuring distances varying from a few meters to hundreds of meters . Early laser -based distance measuring equipment consisted of manually driven time -of-flight devices intended for multiple -point range measurements. They need precise hand positioning with relation to the thing to be measured, and measurements take time. Scanner -mirror -based devices comprised the second generation of distance measuring tools. These instruments made use of a variety of scanning methods, which resulted in variable sensor performance in terms of scanning speed and picture resolution (Myllylä et al., 1998 , P.2/7 ). 5 2.3 “Concept of the imaging LIDAR ” “Imaging LIDAR is a hybrid device that employs a laser array as the transmitter and a detector array as the receiver. Light f rom a laser diode is guided through an optical fiber , where it travels through a transmitter lens before radiating towards the target surface. Each laser in the array illuminates a portion of the target's overall field of vision. The receiver lens collects a part of the light backscattered from the surface and directs it via an optical fiber to the appropriate detector on the matrix ” (Myllylä et al., 1998 , P.3/7 ). Figure 3: Diagram of the imaging LIDAR (Myllylä et al., 1998 , P.3/7 ). 2.4 3D-laserscanners “Imaging LIDAR is a hybrid technology that uses a laser array as a transmitter and a detector array as a rece iver. Light from a laser diode is steered through an optical fiber , passing via a transmitter lens before propagating towards the target surface. Each laser in the array illuminates a different part of the target's entire field of view. The receiver lens c aptures some of the light 6 backscattered from the surface and sends it to the suitable detector on the matrix through an optical cable ” (Hildebrandt et al., 2008 , P.2/6 ). “Figure 4: LIDAR -type system using the TOF (time of flight) to obtain depth information ” (Hildebrandt et al., 2008 , P.2/6 ). 2.5 Flash LiDAR versus Scanning LiDAR 2.5.1 Flash Lidar “A 3D Flash Lidar emits a pulsed laser beam to the whole solid angle of interest in a single shot. An n x m array of the photosensitive detector (e.g. arrays of photodiodes or CMOS ToF chip) is essential to get a certain resolution of the point cloud ” (Maric & Siedersbeck, 2018 . P.4/22 ). 7 Figure 5: “Basic principle of 3D flash Lidar with simultaneous illumination of the target FoV by t he laser and its conversion to a certain resolution by an n x m array on the receiver side, e.g. a CMOS ToF camera chip ” (Maric & Siedersbeck, 2018 , P.5/22 ). 2.5.2 Scanning LIDAR “A scanning Lidar system is made up of a focused pulsed laser beam that is directed to a tiny solid angle by either a mechanical rotating mi rror or a MEMS (micro electro -mechanical system) mirror. 8 Because the high power pulsed laser beam is only projected within a limited solid angle, the attainable distance with the optical power employed may be significantly grea ter than with 3D Flash system s” (Maric & Siedersbeck, 2018 , P.5/22 ). Figure 6: “Basic principle of a scanning Lidar system with a focused laser beam directed to a small solid angle and a photodiode that receives the signal from this direction ” (Maric & Siedersbeck, 2018 , P.5/22 ). 9 3. Data Analysis 3.1 ToF Range Measurement Principle “ToF technology calculates range by measuring the period it requires for transmitted light to travel to an object and then rebound to the sensor unit. Because of the vacuum speed of light, 𝒄𝟎=𝟐𝟗𝟗 .𝟕𝟗𝟐𝟏𝟎𝟔 𝒎𝒔, is constant (and almost the same as in air), the distance d is proportio nal to half the transit time DT, yielding ”: 𝑑=1 2𝑐0∆𝑇 This demands the sensor and the lighting unit being in the same physical location, which is scientifically impossible. The sensor and the lighting unit are plac ed as near together as possible (Sarbolandi et al., 2018 , P.2/22 ). “ToF allows for the simultaneous recording of the entire image. There is no need for line -by-line scanning or relative motion between the sensor and the objects to be observed. ToF is commonly referred to as LIDAR (light detection and ranging), however this is focused on flash LIDAR rather than scanning LIDAR. ToF may calculate the time of flight of a light pulse in two ways: pulsed operation, which uses charge coupled device (CCD) technology, and continuous wave (CW) ”. “The duration between the emission and reception of a light pulse is determined in pu lsed mode, while the phase shift between the emitted and received modulated light pulses is determined in CW mode Because this technology usually relies on high -intensity light pulses transmitted in very brief bursts during a short integration window, puls ed mode is far more resistant to ambient light and therefore more suitable for exterior applications. CW mode, on the other hand, may be easier to create since the light source does not have to be exceedingly short, with quick rising/falling edges (Brand et al ., 2019, P.1/2 ).When contrasted to the phase shift approach, the time-of-flight approach allows for a straightforward system setup and distance computation, but it has a comparatively big measurement error ”(Brand et al., 2019, P.1/2 ). 10 Figure 7: Simple diagram of time of flight measuremen t (analog.com, 2021) . 3.1.1 “Amplitude Modulated Continues -Wave ToF (AMCW -ToF)” For ToF cameras, the AMCW technique is the most commonly used. The fundamental idea behind the operation is to actively brighten t he scene with controlled near infrared (NIR) light. The distance between the sensor and the viewing scene point generates a time shift in the reflected optical signal, which corresponds to a phase shift f [rad] in a periodic signal with a modulation freque ncy fm (Sarbolandi et al., 2018 , P.2/22 ). 3.1.2 “Pulse -Based ToF (PB -ToF)” “In principle, a PB -ToF camera generates a single light pulse th at is reflected in the scene and gathered by a sensor array. One or more optical or electronic shutters detect the incident light pulse in one or two extremely short temporal windows called gates on the sensor. The amount of incident active light is equiva lent to the object distance if the observed object distance is proportionate to the time -of-flight measured in a certain gate ” (Sarboland i et al., 2018 , P.3/22 ). 11 “Figure 8: “The time of flight (ToF) phase -measurement principle ”” (Sarbolandi et al., 2018 , P.2/22 ). 3.2 “How the object yo u are detecting affects your ToF measurements ” The photons will always strike something with its particular characteristics, and all surfaces will reflect light uniquely. When light reaches a surface, it can be absorbed, scattered, or transmitted. The percentage of light reflected by a surface is known as its reflectance. Reflectance, on the other han d, is influenced not only by the surface, but also by the type of light reflected. Be cautious: a surface's reflectance for visible and near -infrared light is not quite the same! When a surface reflects light, it can do so in a variety of ways. Irregular s urfaces can make ToF measurements more difficult by making it more difficult for the sensor to discern which photons came from where. “Photons can bounce off walls and other objects in confined spaces, significantly com plicating the measured distance ” (terabee, 2021) . 12 Figure 9: “The reflection situation ” (terabee, 2021) 3.3 “LIDAR System Options ” 13 3.3.1 “Phase Shift Method for ToF Estimation ” The phase shift method computes the phase difference among emitted and received light by altering the amplitude of the light generated. Aliasing will happen at distances (d) that are a multiple of the speed of light (c) and modulation frequencies (f) 𝑑 = 𝑐∗𝑓/2. To reduce the impact of this phenomena, other frequencies might be used to increase the sensing range. Direct sampling, heterodyne (because the phase of the signal is not changed at intermediate frequency), and under sampling with I/Q demodulation may all be used to measure phase shift (Chevrier & Campanella, 2018 , P.7/25 ). 3.3.2 “Pulse ToF Estimation ” The pulse ToF estimate method uses a direct measurement of the ToF between las er light emission and reception (Chevrier & Campanella, 2018 , P.7/25 ). 14 3.4 The basic principle of LIDAR The basic idea behind this technology is to release a light pulse and then receive the returning one in order to determine the time difference between these two pulses. “The system will be able to provide the three -dimensional coordinates of the target item based on the time difference, the angle at which the light pulse was shot, and the position of the s ystem itself ”. As long as laser pulses move at the speed of light, the distance travelled by the pulse may be determined using the following equation: 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 =𝑡𝑖𝑚𝑒𝑡𝑟𝑎𝑣𝑒𝑙 ∗𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 2 15 And it is multiplied by two since the light goes to the earth and then back to the system (Olsen, 2016 , P.11/29 ). Figure 10: “Distance traveled by the laser pulse ” (Olsen, 2016) . 3.5 Conceptual Discussion of LiDAR A waveform generator generates the laser waveform required to determine range or velocity. A laser is often used to give light, though a LiDAR may operate without one. A few LiDAR systems were created prior to the advent of the laser. In principle, any light source might be used, but since its cr eation, the laser has been the light source of choice for LiDAR. This can be a single laser, a seeded laser, or a master oscillator with one or more laser amplifiers following it. It might also be a laser array. The light must be emitted through a transmit optical aperture. This might be the same or a different aperture than the receive aperture. “If it has the same aperture, it is referred to as a monostatic LiDAR, where mono denotes "one aperture." If the transmit and receive apertures differ, the device is referred to as a bistatic LiDAR, which means it has two apertures. We need a pointing technique and a means to find out where to point the laser at the target we want to observe, thus we need a pointing technique and a means to figure out where to point . To reach a destination, light must travel through a medium, generally the atmosphere. 16 LiDAR signals may be transferred over space's vacuum. LiDAR may also be employed in water rather than air or space ” (De Raedt et al., 2009 , P.5/28 ). Figure 11: LiDAR Conceptual diagram (De Raedt et al., 2009 , P.5/28 ). 3.6 LIDAR and Time of Flight LIDAR systems and ToF approaches are critical for self -driving cars to have a perfect view of their surroundings, and they 're also used in a variety of research projects. “The second part of a four-part series on LIDAR systems and ToF methodologies is now available. Light and photons act and manage differently than RF or sound energy, hence LIDAR, like radar and sonar, has a simple principle but a difficult implementation. Nonetheless, LIDAR is successful, and a light - based system can provide both accurate distance and angular resolution because to the small wavelengt h of light, among other factors ” (microcontrollertips, 2019) . The LIDAR system begins with an infrared -emitting laser diode or LED. “Direct ToF calculates the distance to an object by sending out short light pulses and calculating the t ime it takes for each pulse to return to the sensor. Indirect ToF sensors generate a continuous stream of modulated light. A photoreceptor detects any reflections, and the timing and phase of the 17 reflected light are used to calculate the distance to the it em that created the reflection. Direct ToF pulses are typically several nanoseconds long. Shorter pulses offer higher resolution but lower energy, resulting in a reduced sign/noise ratio (SNR) in the acquired reflection; th is is one of numerous tradeoffs ” (microcontrollertips, 2019) . “Figure 12: “The LIDAR system uses the time of flight a reflection of a light beam (pulsed or continuous) to sense the surroundings ”” (microcontrollertips, 2019) . Figure 13: “Example image of using time of flight measurement for people classification and range detection” (analog.com, 2021) . 18 ToF enables the capturing of the entire picture at once. There is no need for line -by-line scanning or relative movement between the sensor and the items being observed. “ToF is sometimes refer red to as LIDAR (light detection and ranging), however this is a flash LIDAR -based approach rather than a scanning LIDAR. ToF may measure the time of flight of a light pulse in two ways: pulsed (using charge coupled device (CCD) technology) and continuous wave (CW) ”. “The time duration between the emission and reception of a light pulse is measured in pulsed mode, whereas the phase shift between the emitted and received modulated light pulses is determined in CW mode. Both modes of working have advantages a nd disadvantages ”. Pulsed mode is more resistant to ambient light and hence more conductive to outdoor applications since this technology typically relies on high intensity light pulses delivered in very brief bursts across a small integration window. CW m ode, on the other hand, may be easier to build since the light source does not need to be as short as possible, with sharp rising and falling edges (Brand et al, 2019, P.1/2 ). However, if the precision of the requirements grows, higher frequency modulation signals will be required, which may be difficult to obtain. High chip resolutions are achievable because to current pixel sizes, allowing not only distance measurements but also object and gesture recognition. The lengths measured range from a few centimeters (10 cm) to several meters (15 m). Unfortunate ly, not everything can be recognized to the same degree. The item's condition, reflectivity, and speed all have an effect on the measurement output. Environmental factors like as fog or bright sunlight may affect the measurement result. The latter is aided by the suppression of ambient ligh t (Brand et al., 2019, P.1 -2/2). 19 Figure 14: Block diagram of a ToF system (Brand et al., 2019, P.2/2 ). 4. Methods and Recommendations 4.1 “Types of LIDAR Systems ” LIDAR sys tems are split into three types: 4.1.1 LIDAR platforms There are two basic approaches for gathering data: ❖ The flown system (Airborne), in which an aero plane is outfitted with a LIDAR system. ❖ The ground -based stationary (Terrestrial LIDAR), in which the sy stem is anchored to the ground (Olsen, 2016 , P.12/29 ). 4.1.1.1 Airborne LiDAR To collect data, airborne LiDAR is mounted to a helicopter or drone. When engaged, Airborne LiDAR sends light towards the ground surface, which returns to the sensor instantly after striking the target, providing a pre cise distance measurement. Topological LiDAR and Bathymetric LiDAR are the two forms of airborne LiDAR (geospatialworld, 2021) 20 “A LIDAR system th at operates from an airborne platform includes several instruments: “the laser device; an inertial navigational measurement unit (IMU) that continuously records the aircraft's attitude vectors (orientation); a high -precision airborne global positioning sys tem (GPS) unit that records the aircraft's three -dimensional position; and a computer interface that manages communication ” (Gatziolis & Andersen, 20 08, P.5/40 ). 4.1.1.2 Terrestrial LiDAR Terrestrial LiDAR systems, as opposed to airborne LiDAR systems, are deployed on moving vehicles or tripods on the earth's surface to acquire precise data points. These are commonly used for monitoring roadways, analyzing infrastructure, and even gathering point clouds from the interior and outside of buildings. There are two types of terrestrial LiDAR systems: mobile LiDAR and static LiDAR (geospatialworld, 2021) . “Mobile LIDAR is the acquisition of LIDAR point cloud data from a rotating platform. A mobile LIDAR system can be made up of any number of LIDAR sensors installed on a moving object. These gadgets might be used in cars, trains, and sometimes even boats. Mobile LIDAR 21 systems, like airborne LIDAR systems, generally include a LIDAR sensor, cameras, GPS (Global Positioning System), and an INS (inertial navigation system). Mobile lidar data may be used to monito r rail network and identify approaching overhead wires, light poles, and road signs near roadways or rail lines ” (desktop.arcgis, 2019) . “Static LIDAR is the collection of LIDAR point clouds from a stationary location. The LIDAR sensor, which is often mounted on a tripod, is a totally portable, laser -based range and imaging technology. The se systems can acquire LIDAR point clouds both inside and outside of buildings. Engineering, mining, surveying, and archaeology are all frequent appl ications for this type of LIDAR ” (desktop.arcgis, 2019) . 4.1.2 The Origin of Energy There are two categories of remote sensing technologies based on the source of energy utilized to identify the target object: passive systems and active systems (Olsen, 2016) . 4.1.2.1 Passive System The passive system identifies items based on the radiations emitted by the object itself, such as near field communication tags. 4.1.2.2 Active S ystem An active system does not rely on an outside source. It creates energy, directs it to the target item, and then detects the radiation induced by the produc ed energy. It appears that relying on a passive system is preferable, as such a system will reduce spent power (Olsen, 2016 , P.13/29 ). 4.1.3 The Object of Interest LIDAR detects whatever the item is; all it takes is for the thing to emit light when it is illuminated. However, in order to detect the seafloor, a LIDAR that can deliver pu lses through water is required (Olsen, 2016 , P.13/29 ). 22 4.1.3.1 Topographic LIDAR Topographic LIDAR is most commonly employed in land surveying, urban planning, and landscape ecology (Olsen, 2016 , P.13/29 ). Figure 15: Topographic LIDAR (desktop.arcgis, 2019) . 4.1.3.2 “Bathymetric LIDAR ” “Bathymetric LIDAR is a type of airborne water -penetrating technology. Most bathymetric LIDAR systems record both elevation and water depth, resulting in an airborne LIDAR survey of the land -water interface. During a bathymetric LIDAR scan, the infrared light (standard laser system) is reflected back to the aircraft from the land and water surface, while the additional green las er enters into the water column ” (Olsen, 2016 , P.13/29 ). Figure 16: Bathymetric LIDAR (desktop.arcgis, 2019) . 23 4.2 How the LIDAR identifies the different objects Qualities that enable LIDAR to provide high -quality information to users The LIDAR system captures the following data throughout the surveying process: I) The angle at which the laser beam was shot. II) The time gap between the emitted and returning pulses. III) The Intensity, which is a highly essential metric, relates to the magnitude of the return signal. Materials with variable spectral reflectance qualities result in varying backscattering laser intensities, in theory. As a result, the LIDAR intensity may be utilized to identify and detect various materials in s cans of natural or urban areas (Olsen, 2016 , P.15/29 ). 4.2.1 Multiple return feature Because the device can catch up to five returns per pulse, the multiple return feature allows LIDAR technology to improve its capacity to look at three -dimensional structures. This is due to the fact that the pulse of light does not merely reflect and return from the top of the tree, but it also penetrates everything (Olsen, 2016 , P.15/29 ). 24 4.3 Time-of-flight (TOF) principle technology comparison Figure 17: Technology C omparison (terabe e, 2021) . 4.4 Advantages and Disadvantages Using the rotating micrometer technique, the geometric details of the objects are efficiently retrieved from the measurements provided by the 3 -D occupancy grid. When radar data is combined with LIDAR measurement s, it provides information about the dynamic properties of the obstacle, such as velocity and location of the obstacle for the sensor location, which helps the 25 vehicle or driver decide what action to take to ensure safety. The sole worry is the computation al cost of implementing this data processing methodology. 26 4.5 Applications The primary concept is to measure the time difference between the emitted and returned pulses, record the laser angle, and then find the target. The primary purpose of this technology at the outset was to detect the ground of the target region and determine its elevation. The initial application of LIDAR with pulsed ToF data was to create attractive high resolution elevation maps to find holes, picks, hills, and even streams and roads in previously unknown locations. Following are the applications of LIDAR with P -ToF, ❖ Architectural Surveying Tools ❖ LIDAR for Automotive Scanning ❖ Drones ❖ Gas Detection ❖ Laser Range Finder (LIDAR) ❖ Laser Scanners for Safety ❖ Imaging of the retina ❖ Autom ation and robotics (Texas Instruments, 2017 , P.1/29 ) Also For a long time, trees were thought to be a source of noise in the survey process, and people worked hard to eliminate as much noise as possible in order to get high resolution elevation data. Later, they found that the noise they were attempting to eliminate included highly vital information that may be highly valuable! Since then, LIDAR has been used to determine the height of trees and structures, to determine the best time for harvesting, and to identify natural and urban cover surfaces. Furthermore, it is utilized as a complement to data detecting natural environments: ❖ Differentiation between rock a nd soil layers . ❖ Detection of changes in snow cover. ❖ Wetland hydrology and flood modelling ❖ The lava flows are ageing. ❖ Mapping of coastal land cover. 27 ❖ Classification of trees, identification of snags (Olsen, 2016 , P.22/29) 4.5.1 Oceanography “LiDAR technology is used by authorities to determine the precise depth of the ocean's surface in order to find any object in the event of a maritime mishap or for research reasons. Aside from finding items, LiDAR is also used to calc ulate phytoplankton fluorescence and biomass at the ocean's surface, which is otherwise difficult ” (geospatialworld, 2021) . 4.5.2 Digital Elevation or T errain Model “Terrain elevations are extremely important in the construction of roadways, huge structures, and bridges. Because LiDAR technology contains x, y, and z coordinates, it is relatively simple to create 3D representations of elevations so that in terested parties may draw appropriate conclusions more simply ” (geospatialworld, 2021) . 4.5.3 Agriculture & Archaeology “Typical agricultural applications of LiDAR technology include production rate analysis, crop reconnaissance, and seed dispersions. It is also utilized for campaign planning, mapping under the forest canopy, and other purposes ” (geospatialworld, 2021) . 5. Conclusion With the advancement of LIDAR systems, they are becoming increasingly helpful in a variety of applications, and the entire world will rely on various LIDAR sys tems to discover, gather, and update data from various areas. For many IoT -enabled applications, ToF is a diverse and precise distance measurement solution. New sensor generations expand a product's capability, improve dependability, and give a competitive advantage. Sensors that were initially designed for a certain market are increasingly finding homes in unexpected places. Accuracy, resolution, and linearity are all important factors to consider when measuring 3D using LIDAR . One of the most significant aspects of LIDAR design is the optical system. According to the experimental results, appropriate optical systems were examined in this study based on the required detection range and distance resolution. Also b ecause of its great precision and 28 resolution , LIDAR outperforms pre -existing RADAR and SONAR technology. 3D mapping is extremely useful in industries such as rescue, agriculture, and wind farm optimization. 3D mapping with LIDAR is incredibly exact, reliable, and cost -effective. The LIDAR is connect ed to a pan tilt servo motor kit, which creates a 3D map of the front hemisphere. As a result, this study discusses various 3D optical measurements utilizing LIDAR with pulsed time of flight, as well as the use of LIDAR in many sectors. 29 6.