(Seminar Title)

LIDAR TECHNOLOGY

LIDAR TECHNOLOGY

Seminar report submitted for the award of the Degree ofBachelor of Technology inElectronics and Communication Engineeringof Siksha o Anusandhan University

bySweta Mohanty

Department of Electronics & Communication Engineering

DEPARTMENT OF ELECTRONICS & TELECOMMUNICATION ENGINEERINGJADAVPURUNIVERSITYKOLKATA 700 032, INDIAInstitute of Technical Education & ResearchSiksha o Anusandhan University, Bhubaneswar - 751030January, 2014

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DECLARATION

I hereby declare that the report entitled LIDAR TECHNOLOGY submitted to Siksha o Anusandhan University for the award of the degree of Bachelor of Technology in Electronics and Communication Engineering is absolutely based on my own literature review and extensive survey. Wherever contributions of others are involved, every effort has been made to indicate this clearly, with due reference to the literature. I also declare that this report in the present form has not been submitted for award of any degree or diploma or any other academic award anywhere else before.

Sweta MohantyReg. No.: 1011016060, Section: EDepartment of Electronics and Communication Engineering

ABSTRACT

LIDAR is an active remote sensing technology that measures distance with reflected laser light. It measures distance, speed, rotation, chemical composition and concentration. Light detection and ranging (lidar) mapping is an accepted method of generating precise and directly geo- referenced spatial information about the shape and surface characteristics of the Earth. Lidar has become an established method for collecting very dense and accurate elevation data across landscapes, shallow-water areas, and project sites. This active remote sensing technique is similar to radar but uses laser light pulses instead of radio waves. Lidar is typically flown or collected from planes where it can rapidly collect points over large areas. Highly accurate, high-resolution lidar data have particular utility in coastal settings where terrain is generally flat and subtle elevation changes often have significant importance. The potential applications of lidar include forestry, geology, watershed and water quality studies, transportation, safety, cadastral mapping, surveying and archaeology. As lidar is increasingly available for coastal areas, applications that relied on coarse data are being improved with the use of lidar data. It is mainly used in coastal environments.

Keywords: Remote sensing, Light-radar, surveying, elevation, resolution

CONTENTSDECLARATIONiABSTRACTiiCONTENTSiiiLIDAR TECHNOLOGY41.Introduction4 2. Principle of LIDAR.....7 3. LIDAR System Components..8 4. LIDAR Design........9 5. Types of LIDAR...........10 5.1. DIAL..........10 5.2. VIL....//.12 5.3. GALE...//.12 5.4. RAMAN LIDAR.........13 5.5. HSRL..../....146. LIDAR Application..........167. LIDAR Vs RADAR Comparison........198. Advantage and Disadvantage of Lidar.......20 9. Emerging Trends and Technology.22 10. Conclusion.........23 11. Reference....24

LIDAR TECHNOLOGY

1. INTRODUCTION LIDAR is Light detection and ranging, 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 Rader. Lidars are used to precisely measure distances and properties of far away objects.A LIDAR basically consist of transmitter, receiver and detector. The LIDARs transmitter is a laser, while its receiver is an optical telescope. When a short monochromatic light pulse is transmitted into the atmosphere, it will be back scattered by both air molecules and suspended particles. This back-scattered light is collected by a telescope close to the transmitter and detected by a suitable opto electronic detector. The detected signal can then be processed suitably or stored for later processing. Modern LIDAR systems combine the capabilities of radar and optical systems to allow simultaneous measurement of range, velocity, temperature, reflexivity, azimuth and elevation angle. These six dimensions of target information can be utilized in fire control and weapon system applications to allow target acquisition, tracking, classification and imaging. The modulation capabilities of microwave radar systems can be applied to laser transmitters to allow accurate target measurement and time/frequency gating of atmospheric terrain background clutter. The optical resolution associated with laser systems results in a very small angular beam width to allow imaging, aim point assessment precise target tracking and autonomous operation.

What Is Lidar?

Lidar, which is commonly spelled LiDAR and also known as LADAR or laser altimetry, is an acronym for light detection and ranging. It refers to a remote sensing technology that emits intense, focused beams of light and measures the time it takes for the reflections to be detected by the sensor. This information is used to compute ranges, or distances, to objects. In this manner, lidar is analogous to radar (radio detecting and ranging), except that it is based on discrete pulses of laser light. The three-dimensional coordinates (e.g., x, y, z or latitude, longitude, and elevation) of the target objects are computed from 1) the time difference between the laser pulse being emitted and returned, 2) the angle at which the pulse was fired, and 3) the absolute location of the sensor on or above the surface of the Earth.

There are two classes of remote sensing technologies that are differentiated by the source of energy used to detect a target: passive systems and active systems. Passive systems detect radiation that is generated by an external source of energy, such as the sun, while active systems generate and direct energy toward a target and subsequently detect the radiation. Lidar systems are active systems because they emit pulses of light (i.e. the laser beams) and detect the reflected light. This characteristic allows lidar data to be collected at night when the air is usually clearer and the sky contains less air traffic than in the daytime. In fact, most lidar data are collected at night. Unlike radar, lidar cannot penetrate clouds, rain, or dense haze and must be flown during fair weather. Lidar instruments can rapidly measure the Earths surface, at sampling rates greater than 150 kilohertz (i.e., 150,000 pulses per second). The resulting product is a densely spaced network of highly accurate georeferenced elevation points (Figure 2-2)often called a point cloudthat can be used to generate three-dimensional representations of the Earths surface and its features. Many lidar systems operate in the near-infrared region of the electromagnetic spectrum, although some sensors also operate in the green band to penetrate water and detect bottom features. These bathymetric lidar systems can be used in areas with relatively clear water to measure seafloor elevations. Typically, lidar-derived elevations have absolute accuracies of about 6 to 12 inches (15 to 30 centimetres) for older data and 4 to 8 inches (10 to 20 centimetres) for more recent data; relative accuracies (e.g., heights of roofs, hills, banks, and dunes) are even better. The description of accuracy is an important aspect of lidar and will be covered in detail in the following sections.

Objectives and Motivation

An objective of this work is to provide evidence to confirm that the measurements recorded by the lidar are at least as accurate as the experimental control. In the case of the lidar data validation experiment, the experimental control is a standard cup anemometer. The motivation behind this project relates to the potential that the lidar holds for reducing the uncertainties associated with wind speed measurement. According to previous work done by Lackner et al., the major sources of cup-anemometer measurement uncertainty includes: 1. Sensor calibration uncertainty 2. Anemometer over-speed effects 3. Vertical flow effects 4. Vertical turbulence 5. Tower shadow effects 6. Sensor boom effects 7. Data reduction accuracy

The largest source of error associated with traditional meteorological (met) tower resource assessment is grouped in what Lackner describes as site assessment uncertainty, or more specifically, shear wind speed extrapolation. Lidar instruments for wind energy resource assessments have the ability to measure up to approximately 150 meters. By eliminating the need to extrapolate wind speed measurements to the hub height of a wind turbine, the lidar is poised to reduce the overall uncertainty involved in wind farm site assessment. The uncertainty analysis presented in section 10 offers further detail with respect to the uncertainty in both lidar and cup anemometer wind speed measurements. A further objective of this work is to more clearly define measurement uncertainty as a function of vertical turbulence intensity for specific sensors. The interest in this research stems from the analysis of the 4th source of cup anemometer measurement uncertainty shown above. A detailed investigation is achieved by an experimental campaign whereby wind speed data from separate sensors are compared. The purpose of this comparison is to demonstrate the degree to which wind measurements are a function of measurement error. The thesis of this analysis is that variability in the vertical flow of air will cause additional measurement uncertainty.

2. BASIC PRINCIPLE OF LIDAR

In RADAR, radio waves are transmitted into the atmosphere, which scatters some of the power back to the Raders receiver. A LIDAR also transmits & receives electro-magnetic radiation; but at a higher frequency, and operate in the ultra violet, visible and infrared region of the electro magnetic spectrum. The LIDAR is also popularly known as LASER RADAR. A LIDAR basically consists of a transmitter, receiver & detector. The LIDARS transmitter is an optical telescope. When a short monochromatic light pulse is transmitted into the atmosphere, it will be back scattered by both air molecules and suspended particles. This back-scattered light is collected by a telescope close to the transmitter and detected by a suitable opto electronic detector. The detected signal can then be processed suitably or stored for later processing.

THE SCATTERING PROCESS

In a typical LIDAR experiment laser light is transmitted into the atmosphere. The beam can interact with the atmosphere in a number of ways.

Laser light can be scattered elastically (ie; no change in wavelength or colors) from molecules in the atmosphere (Rayleigh scattering) and from particles (Mic scattering). Laser light can be scattered elastically from molecules in the atmosphere. In this instance the wavelength of scattered light is shifted and the change in wavelength depends on the molecule that scattered the light. This is known as RAMAN Scattering. LIDAR in this case is RAMAN LIDAR A specific molecule in the atmosphere can absorb laser light. In small molecules, the molecule at the same or at other wavelength can then radiate this absorbed energy. This process is known as Fluorescence. Under certain circumstances this can be particularly sensitive and specific.

3. LIDAR SYSTEM COMPONENTS

LIDAR BLOCK DIAGRAM

In this configuration the laser is modulated to provide information to the transmitted signal, which is coupled through the interfero meter, optics and scanner to illuminate the scan field of interest. The received signal is coupled, via reciprocity through the interfero meter, to the receiver detector where it is mixed with a sample of the laser signal in the form of a local oscillator.

The receiver output is processed by the signal processor to extract target information and then processed by the data processor where all information is compiled to provide target position, range, velocity and an image. The back scattered Doppler-shifter target signal was then processed in a surface acoustic wave signal processor, recorded on tape and subsequently played back on the ground through a CRT display.The receiver assembly is used to collect back-scattered light and detector, which converts the incoming light stream into electrical impulses. Photodiodes based on silicon do not respond at all in the eye-safe region (1.55m). It is not possible to use InGaAs photodiodes but they generally do not respond to the low levels of light that would be anticipated when the laser beam scatters from a very distant point. We require a photo detector that both responds and has sufficient gain so that the electrical signals are measurable.

InGaAs avalanche photodiodes (APDs) fit this bill. These semi conductors are engineered so that when a 1.55m photon hits them, an electron is efficiently created. In an APD this electron is then accelerated across a barrier by means of an applied voltage where it induces other electrons to follow. One photon therefore, results in a cascade of electrons and hence a measurable current.

One unfortunate drawback of InGaAs avalanche diodes is due to the devices ability to store electrical charge; its high capacitance. This capacitance induces noise in the first stage of electrical amplification. This is a fundamental issue, which affects the ultimate range of the LIDAR system.

We use a 22 diameter InGaAs APD which minimizes the effect of devices capacitance but which requires innovative optical design to effectively couple 10 inches of collected light onto a very small-area detector.

For the MTI radar aspects of the system, Doppler filters are used and the surface acoustic wave delay line processor is programmed to give an MTI cue, in real time, when a Doppler return greater than 5knots is indicated in adjacent pixels. Additionally, it may be noted that several target clutter intensity spots cause false alarm indications, which are distributed through the scene. Altering the target detection algorithms to require an N detection out of M trials detection approach results in significant false alarm reduction.

4.LIDAR DESIGN

The design of basic LIDAR system is illustrated above. To begin a powerful laser transmits a short and intense pulse of light. The pulse is expanded to minimize its divergence, and is directed by a tilted mirror into the atmosphere. As the pulse travels upwards it is scattered by atmospheric constituents and aerosol particles. Light that is back scattered and in the field or view of a telescope receiver is collected and channeled towards the detector by a fibre or other optic. Filters are used to eliminate light away from the lasers wavelength, and a mechanical shutter blocks the intense low level returns. The amount of light received is measured as a function of time (or distance) using sensitive photo detectors, and the signals are digitized for storage on a computer. A timing unit performs co-ordination of the experiment. When each laser pulse exits the atmosphere, another pulse is transmitted and the process is repeated.

As the speed of light c is well known, the time of flight t from the laser to the scattering volume at altitude z and back to the detector is given as t=2z/C. Thus, as the detected lights recorded in sample bins the detector records the time since the laser fired.

Echoes that are detected soon after the laser fired are from low altitudes, while echoes that are detected later are from higher altitudes. All photons arrive in the range are stored in one range bin, where Dt is the temporal resolution of the measurement. A single profile is recorded each time the laser fires. The single shot profiles are added together at each altitude to build up the signal at each altitude. If a laser fire at a repetition rate of 100 pulses/sec then if we add up to 1000 shots to record a single statistically significant profile the temporal resolution of the measurement Dt is 10sec. 5. TYPES OF LIDAR SYSTEM FOR ATMOSPHERIC MEASURES 5.1.DIAL Differential Absorption LIDAR

Different types of physical processes in the atmosphere are related to different type of light scattering. Choosing different types of scattering processes allows atmospheric composition temperature and wind to be measured. DIAL systems are used for the study of the atmospheric composition. DIAL systems are based on the fact that the absorption of light by the atmosphere is different at different wavelength.

In this measurements are made by two different wavelengths. One wavelength (I on) is chosen in region of high absorption cross section of the gaseous constituent under study, whereas at the second wavelength, (I off) the gaseous absorption should be minimum.

DIAL transmits short pulses of laser light into the atmosphere. The laser beam loses light to scattering and it travels. At each range some of the light is backscattered into the detector because the light takes longer time to return from the more distant ranges, the time delay of the return pulses can be converted into the corresponding distance between the atmospheric scatterer and lidar.

The end result is a profile of atmospheric scattering Vs distance. Analysis of this signal can yield information about the distribution of aerosols in the atmosphere. The amount of backscatter indicates the density of scatterer. This can be used to measure cloud base height or track plumes of pollution.

Other properties of atmosphere can also be deduced from LIDAR return signals. A frequency shift in the light because of Doppler Effect permits the measurement of wind speed. By detecting the depolarization, one can discriminate between liquid droplets and non-spherical ice particles. DIAL is also be used to measures the concentration of atmospheric gases.

Ozone Measurement Using DIAL

In this technique two different laser beams are transmitted vertically into the atmosphere. The laser is tuned between spectral regions of high and low absorption. One has the wavelength 308nm absorbed by ozone and other 351nm, which is not. These two beams scattered elastically by molecules and particles, and a 30 telescope collects the back-scattered light. The 308nm signal falls of much more quickly than the other due to ozone absorption. The difference in absorption of light at different wavelength can be used to determine the amount of ozone. Ozone concentration as a function of altitude can be extracted from a ratio of the two backscattered signals.

5.2.VIL Volume Imaging LIDAR

VIL is an elastic aerosol backscatter LIDAR designed to image the four-dimensional structure of the atmosphere. It can measures formation of clouds, it also gives the aerosol concentration.

The transmitter of a VIL employs a pulsed Nd: YAG laser. The receiver consists of a telescope, interference filter and avalanche photo diode. Scanning is performed using a best computer controlled beam steering unit consist of two flat rotating mirrors mounted at 45 degree on the on the optical axis of the transmitter receiver system.

The VIL transmits a small diameter (0.3cm) beam of light at a wavelength of 1.06 microns out into the atmosphere. As the beam travels through the atmosphere its light is scattered by aerosol particles that fall in the path of the beam. A portion of the beam is scattered back towards the point of origin where a telescope is located that focuses the return light on to a photo diode. The signal is then digitized with a high-speed digitizer and stored along with timing information. The amount of light that comes back to he telescope is proportional to the number of aerosol particles at a particular location in space. The scanning of Volume Imaging LIDAR can provide information about the atmospheres optical properties. It can be used to make 3D maps of aerosol structure, it can measure wind speed and direction and can be used to study atmospheric flows.

5.3.GALE: An Advanced Wind LIDAR

GALE stands for Giant Aperture Lidar Experiment. GALE measures wind and temperature using resonance fluorescence scattering. Resonance fluorescence scattering means that when sodium atoms are illuminated at a precise wavelength (589nm) they become excited and radiate light. The LIDARS receiver measures a fraction of this light.

By adjusting the wavelength of the transmitted signal by a tiny amount, the shift of the spectral time from its central wavelength can be measured. The shift in the central wavelength is called Doppler shift and the wind is determined from the Doppler effect. Wind measurement by GALE shows the dynamic structure of the upper atmosphere due to wave activity.

Temperature is measured by using sodium resonance- fluorescence scattering and by using Rayleigh scattering from air molecule. This Rayleigh scattering is responsible for the blue sky.

The LIDAR equation relates the number of received photo counts to the atmospheric density by equation

P(z) = zN(z)

Where is a constant which depend on individual lidar system, type of scattering and the transmission of atmosphere.

z is the height and N(z) is the number of photo counts at each height. The number of photo counts received by a lidar depends inversely on the square of the altitude.

From the given altitude the solid angle is equal to the area of the receiving telescope divided by the square of the altitude. The transmitted beam is not perfectly parallel, so the area illuminated grows with height. Since the number of molecules illuminated by a laser at a given height is both large (due to relatively large size of laser spot, about 2000m2 at 100km altitude) and relatively uniform. The z factor in the conversion from photo counts to density makes the illuminated area constant with height.

5.4. RAMAN LIDAR

Raman LIDAR is based on the process of Raman scattering. This LIDAR is in elastic scattering processes. This means that there is an exchange of energy between scattered photon and the scattering molecules. Raman scattered light is shifted a different amount by different types of molecules. This allows the type of scattering molecules to be identified from the wavelength of the scattered light. This allows us to measure the photons from specific molecule in the atmosphere. Raman scattering is a very weak process and the signal can be two or four orders of magnitude, weaker than elastic backscattered signal. Also the weak scattering cross-section typically limits Raman LIDAR to nighttime measurement at ranges of less than 10Km. To increase the Raman signal and make daytime measurement, high power LIDAR systems have been developed to operate wavelength from 248.5nm to 268.5nm. 5.5. High Spectral Resolution LIDAR (HSRL)

The HSRL measures optical properties of the atmosphere by separating the Doppler broadened molecular backscatter return from the unbroadened aerosol return. The molecular signal is used as a calibration target, which is available at each point in the LIDAR profile. This calibration allows unambiguous measurement of aerosol scattering cross-section, optical depth, and backscatter phase function also measurements of depolarization and multiple scattering can be performed. The HSRL has a significant signal strength advantage over the Raman technique. Another advantage of the HSRL is that it can provide daytime measurements while sky noise background limits the measurement of the weak Raman signal to nighttime.

The fully developed HSRL employs an iodine absorption filter instead of a high-resolution etalon. The spectrum of the electronic transition in molecular iodine has more than 22000 absorption lines in the visible lengths, and 8 of them are easily reached by thermally turning a frequency doubled Nd: YAG laser output. Compared to barium, the advantage of iodine is that instead of requiring a dye laser, a narrow bandwidth, frequency doubled Nd: YAG laser can be used. The received backscatter signal is divided into two channels. One channel detecting the sample from the total backscatter spectrum and the other channel the spectrum filtered by the iodine absorption filter. This signal contains the information about the wings of the molecular spectrum and a small aerosol cross-talk signal.

The measurement shows that, the use of an iodine absorption filter enables accurate measurements of cloud optical parameters. Because of cross talk between channels can be accurately corrected because the 160 rad field of view of HSRL effectively suppresses multiple scattering phase function possible.

6.LIDAR APPLICATIONS

Lidar, as a remote sensing technique, has several advantages. Chief among them are high accuracies, high point density, large coverage areas, and the ability of users to resample areas quickly and efficiently. This creates the ability to map discrete changes at a very high resolution, cover large areas uniformly and very accurately, and produce rapid results. The applications below are examples of some common uses of lidar. Updating and Creating Flood Insurance Rate Maps (Figure 2-6) This application is a major driver in the development and use of lidar data. The application was largely brought about when hurricanes hit North Carolina and the existing mapped flood zones were quickly shown to be inadequate. Forest and Tree Studies A very costly and time-consuming aspect of timber management is the effort spent in the field measuring trees (Figure 2-7). Typically a sample of trees is measured for a number of parameters and the results are statistically extrapolated throughout the harvest area. Trees must be measured to determine how much wood is present, when it is most appropriate to harvest, and how much to harvest. High-resolution, small-footprint lidar has been used to count trees and measure tree height, crown width, and crown depth. From these measurements, the standing volume of timber can be estimated on an individual tree basis, or on a stand level with larger footprint lidar.

Coastal Change Mapping Mapping the coastal zone is an application that highlights the use of lidar data (Figure 2-8) along with GIS layers to increase the utility of both data sets. This highly dynamic region changes on very short timescales (e.g., waves, tides, storms), contains many natural habitats that are highly dependent on elevation, and is densely populated. As a result, the rapid changes can affect significant populations and habitats, both of which are becoming less tolerant to change (i.e., there is less ability to retreat). Lidar data provide the ability to measure specific events as well as longer-term trends. This provides information that can be applied to immediate restoration solutions for critical areas, as well as sustainable planning to minimize future impacts.

Shoreline Mapping Lidar coverage is expanding rapidly along the U.S. shoreline, and as a result lidar data are increasingly being used for shoreline mapping, including defining shoreline positions and quantifying rates of shoreline change. Shorelineand its many tidal datum variationsis commonly referenced as a boundary component in legal descriptions, as the point of origin for jurisdictional boundaries, and as the boundary between public and private ownership (Figure 61). These various definitions of shorelines are based on water levels that are influenced by tides and currents. Inundation Mapping Inundation, whether from sea-level rise or storm surge, is a common coastal application using elevation data. Lidar data provide the accuracy to both model and delineate the potential extent of flooding from different forms of inundation due to the high accuracy of the data and the ability to resolve small features that influence flow paths. In sea-level rise scenarios, the lidar data can be used to model topographic changeor morphologic changethat would be generated from a nearly uniform rise in water level and also to identify areas that are potentially susceptible to flooding. Coastal flooding from storms represents a rise in water level that is not uniform. Lidar data can be input in surface water models as well as map flooding extent.Storm surge is typically modelled using either the Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model or the Advanced Circulation (ADCIRC) model. There are advantages and disadvantages to both models, and lidar data can be, and have been, incorporated into both models. The DEM grid that the ADCIRC model uses generally has a higher resolution than the SLOSH model, but ADCIRC cannot yet be run operationally for oncoming storms. Of course, neither SLOSH nor ADCIRC run at or near the resolution of even the coarsest lidar data. The output of the SLOSH model is an envelope of water that includes water depth (Figure 6-6). When these outputs are combined with a lidar-derived topographic surface (Figure 6-7), flood areas, extents, and depths are more precisely defined than those generated using the best available regional data that, in this case, has a lower spatial resolution and also poorer accuracy. Wetland Habitat Delineation Elevation is an extremely important variable in defining coastal wetland habitats (Figure 6-9), and in the face of changing sea levels, evolution in marsh micro-habitats (i.e., low marsh vs. high marsh) will depend on the sea-level change rates and ability of the marsh to accrete vertically (i.e., increase elevation through deposition of sediment and detritus). If the system lacks sediment or biogenic (e.g., plant material, shells) deposition, then the marsh habitats will migrate to higher elevations, if possible, as sea-level rises. In a system where sediment or biogenic deposition can match rises in sea-level, the marshes and their micro-habitats will continue to maintain equilibrium. In either case, new marsh will be created, if environmental variables permit, where uplands once existed. Wetlands pose a particular challenge when working with lidar data because the upright vegetation is dense and near to the ground, and the marsh surface is partially composed of it. The following example, using lidar data from Charleston County, South Carolina, examines the use of cell size to determine the bare-earth surface in a marsh area. Even if the data are classified, the traditional routines used typically do not completely remove marsh vegetation from the data.

OTHER LIDAR APPLICATIONS

LIDAR GUN

LIDAR GUN is also known as police lidar. This is used to point out the vehicles and detect their speed. It is a great use to traffic regulation. It is used for checking the speedy vehicles from distances. The basic principle is that the laser is used to aim the moving object and the reflected light is used to calculate the speed.

AIR BORNE LIDAR

LIDARS have been used extensively in air borne systems, mainly for getting depth of a water body and for terrain mapping. Air borne LIDAR is an aircraft mounted laser system designed to measure the 3D co-ordinates of a passive target. The LIDAR operating principle uses pulsed light to measure the variations in surface features. The distance from the aircraft to the ground features is determined by measuring the elapsed time between the generation and return of each laser pulse. These laser pulses are stored on a hard drive for a post mission processing. From the processed data a 3D model of the earthSurface is created.

EXAMPLE OF LIDAR TECHNOLOGY

Estimating the tree height:

Some pulses reach the forest floor, others reflect from the understory and canopy. Here the above graph directly depicts the height of the forest canopy, i.e. the graph is plotting the return signals along x- axis and height of the tree is along y-axis. The light which has received by receiver in shorter period will represent the highest point of tree and the light signal which has return at last represents the forest floor.

7.LIDAR/RADAR COMPARISONS

LIDAR system synthesis, performance evaluation and analysis established that optical radar techniques had many wavelength related advantages and some disadvantages. Large information bandwidth and extremely high angular resolution possibilities are some of the advantages of LIDARs over Radars, while low efficiencies and atmospheric propagation limitations are some of the disadvantages.

8. Advantages of LIDAR Technology:The other methods of topographic data collection are land surveying, GPS, inteferrometry, and photogrammetry. LiDAR technology has some advantages in comparison to these methods, which are being listed below:

o Higher accuracy Vertical accuracy 5-15 cm (1s) Horizontal accuracy 30-50 cm o Fast acquisition and processing Acquisition of 1000 km2 in 12 hours. ADEM generation of 1000 km2 in 24 hours. o Minimum human dependence As most of the processes are automatic unlike photogrammetry, GPS or land surveying. o Weather/Light independence Data collection independent of sun inclination and at night and slightly bad weather. o Canopy penetration

LiDAR pulses can reach beneath the canopy thus generating measurements of points there unlike photogrammetry.

o Higher data density

Up to 167,000 pulses per second. More than 24 points per m2 can be measured. Multiple returns to collect data in 3D.o GCP independence Only a few GCPs are needed to keep reference receiver for the purpose of DGPS. There is not need of GCPs otherwise. This makes LiDAR ideal for mapping inaccessible and featureless areas.o Additional data LiDAR also observes the amplitude of back scatter energy thus recording a reflectance value for each data point. This data, though poor spectrally, can be used for classification, as at the wavelength used some features may be discriminated accurately.

Cost:It has been found by comparative studies that LiDAR data is cheaper in many applications. This is particularly considering the speed, accuracy and density of data.

DISADVANTAGES:

LIDAR sensors can only collect during cloud coverage if the clouds are above the height of the airborne platform.

LIDAR sensors can only collect data in reasonably good weather and cannot collect data in rain, fog, mist, smoke, or snowstorms.

In areas of dense vegetation coverage, the LIDAR pulses, in most cases, will not be able to penetrate through the foliage to the ground unless ample openings in the vegetation exist and the spot size of the pulse is small and densely spaced.

Imagery data (digital photos or satellite imagery) are needed to perform proper vegetation classification and removal when producing bare earth models from multiple return LIDAR data.

9. EMERGING TRENDS AND TECHNOLOGY

LASER technology is a vast field involving optical communication and remote sensing, behind the drive to put more lidar in space lies the continuing development of solid state lasers. With the advancement in laser system, improved and effective lasers have been developed which improvise on the exciting system. New semi conductor solid state lasers have led to improved performance of Lidar systems employed in remote sensing.

The MARS POLAR LANDER has a lidar based system using, which it will be possible to measure and completely analyses the Mars atmosphere for the first time. This is an important landmark in space exploration.

FUTURE SCOPE OF LIDAR

Earth science: Long Duration orbiting Instruments providing global monitoring of the atmosphere and land. Planetary science: Land based scientific instruments providing geological and atmospheric data of solar system bodies. Landing Aid: Sensors provide hazard avoidance, guidance and navigation data.Rendezvous and Docking Aid: sensors providing spacecraft bearing, distance , and approaching velocity.

10. CONCLUSION

It has been shown that LIDAR can be used for atmospheric research in quite different application. Mainly this is related to detailed process studies, where advantage is taken of the capability to observe the vertical distribution of key constituents with rather high resolution. Although most measurements are quite demanding, the technique has been developed to a stage where high accuracy can be achieved. In the case of ozone measurements this can be done in a kind of routine operation, already in the case of water vapour or temperature. Some further development is required to optimize the performance and to achieve the routine operation.

The use of LIDAR has not been limited to atmospheric studies but with many space applications being planned to put into use. This will help us in exploring the unexplored and unexplained phenomenon in our solar system.

11.REFERENCES

[1] Martin D. Adams, Lidar Design, Use, and Calibration Concepts for Correct Environmental Detection, IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, Vol. 16, No. 6,Pp.753-761, December 2000.

[2] Fei Wang , LIDAR data acquisition methods in emergency management applications. Eng. Phys. Dept., Tsinghua Univ., Beijing, China, Geoinformatics, 2011 19th International Conference on 24-26 June 2011.