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CONTENTS 1) INTRODUCTION 2) PRINCIPLE OF LIDAR 3) LIDAR INSTRUMENTATION 4) LIDAR DESIGN 5) TYPES OF LIDAR a) DIAL b) VIL c) GALE d) RAMAN LIDAR e) HSRL 6) LIDAR APPLICATIONS 7) LIDAR/RADAR COMPARISONS 8) CONCLUSION 9) REFERENCES ABSTRACT ‘LIDAR’ is the acronym for Light Detection & Ranging. Since the instrument detects and radiates we can simply locate it to RADAR ie, radio detection and ranging. Both have the same working principles, but the only difference between these two is that in the medium used for ranging and detecting. In LIDAR laser is used in the same way as radio waves in RADAR. LIDAR popularly known as the Laser-Radar uses lasers as the transmitting materials. The laser is back scattered by the target object and the received signal gives not only the range of the object but also gives us an idea about the composition of the object, which is got from its light scattering properties. The LIDAR technique for detecting and ranging was widely recognized only after the discovery of Ruby Laser by Miaman in 1960. Rapid developments since then in the laser technology, opto electronics and fast signal processing devices have resulted in establishing the potentials of LIDAR’S for a wide variety of atmosphere studies. Different types of LIDARS are being used depending on the type of scattering processes and for type of signal. 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 reffered 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 LIDAR’s 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. BASIC PRINCIPLE OF LIDAR In RADAR, radio waves are transmitted into the atmosphere, which scatters some of the power back to the Rader’s 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 LIDAR’S 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 colours) from molecules in the atmosphere (Rayteigh 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 depend 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. LIDAR INSTRUMENTATION The main components of the LIDAR are, A solid state ‘LASER’ which produce brief pulses of high intensity light. An ‘Optical Telescope’ which directs the laser emission into the sky, and collect the light back scattered by atmosphere. A ‘Detector’, which analyses the back, scattered light. LASER Laser is the abbreviated form for ‘Light Amplification by Stimulated Emission of Radiation’. As the name indicates process of stimulated emission is used for the amplification of light. Different kinds of lasers are used depending on the power and wavelength required. The laser may be either continuous wave or pulsed. Some important lasers are given below. Solidstate laser : Ruby laser, Nd: YAG Gas laser : He-Ne laser Liquid laser : Dyes laser Semi conductor laser : Diode laser High voltage electricity causes the quartz flash tube to emit an intense burst of light, exiting some of the atoms in the Ruby crystal to high level. At a specific energy level, some atoms emit particles of light called Photons. At first the photons are emitted in all directions. Photon from one atom stimulates emission of photon from other atom and the light intensity is rapidly amplified. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification. The photons leave through the partially silvered mirror. This is laser light. For some LIDAR applications more than one laser is used. The final output of both channels of the transmitter is pulsed with repetition rate of 20 times/second and the pulse width is about 7ns. OPTICAL TELESCOPE Usually the LIDAR uses the same optical telescope as transmitter and receiver. It uses an optical switch, to switch the LIDAR between transmit and receive mode, by a high speed (2400 rpm) mechanical shutter. The rotation of the shutter is phase locked with the firing of laser so that the out going light passes through one of five symmetrically placed apertures. While light backscattered from ranges in excess of 10 km is directed into the direction system by the reflective skyward surface of the shutter when it rotates into the received beam. To prevent the saturation of the detection system by the strong signal returned from the lower atmosphere, the shutter apertures are shaped to allow the portion of the received light reaching the detection system to increase with the range over the interval of 10 km to 18 km. The receiving system records the scattered light received by the receiver at fixed time intervals. LIDARS typically use extremely sensitive detectors called photo multiplier tubes to detect back scattered light. Photo multiplier tubes convert the individual quanta of light, photons, first into electric currents and then into digital photo counts that can be stored and processed on a computer. DETECTOR : SPECTROMETER The detectors used are usually spectrometers, which also function as optical filters. They rejecting unwanted optical signal so that only the required wave length of light is analyzed. A typically used spectrometer is F.P.S [FABRY PEROT SPECTROMETER]. The F P S consists of two etalons 0.3 nm band pass interference filter and a cooled photo multiplier detector. Each etalon consists of pair of extremely flat glass plate placed parallel to each other. The facing surface is coated with a partially reflecting layer. Each etalon act as a filter with a pass-band that is periodic in wavelength. In order to maintain the stability of etalons a high level of thermal and vibration isolation has been incorporated in the design. The fixed and scanning wavelength modes of the F.P.S will allow the measurement of the total power of the backscattered signal. The response function of the F.P.S will be obtained by the injection of light from the out going laser pulses to the receiver. Nd : YAG LASER SYSTEM It stands for “ Neodymium: Yttrium Aluminium Garnet ”. It is a solid-state laser in which energy levels of Neodymium ion take part in laser emission. Four Level System It is a four level laser system. Level 1[E1] is the ground level while E2, E3, E4 are excited levels of system. Atoms in level1 are excited to level4 from which they make a non-radiative transition to the level 3. Level 3 corresponds to upper laser level and is a metastable state having longer lifetimes. The transition from level3 to level 2 forms the laser transmission. The level 2 has very little lifetime, so that the atoms in level 2 are quickly moved to level1. This maintains population inversion between level3 and level 2. The main pump lines for the excitation of the ion have 8100A and 7500A wavelength and the pumping is done using krypton arc pumps. The laser has the wavelength of 10600A. The difference between lower laser level and ground laser level is 0.26 eV, the ratio of its population to that of ground state at room temperature is very small. So the lower laser level is unpopulated. It finds applications in range finders, resistor trimming micro matching operation, welding and hole drilling. TYPICAL 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 Dopplershifter 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 draw back of InGaAs avalanche diodes is due to the device’s 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. 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 laser’s 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. Echos that are detected soon after the laser fired are from low altitudes, while echos 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 upto 1000 shots to record a single statistically significant profile the temporal resolution of the measurement ‘Dt’ is 10sec. The power incident on a LIDAR receiver as a result of scattering from an atmosphere containing particular matter is given by: Where the optical depth, is defined as T(r) = Where, e (r )dr Eo C Ar r a m e - Laser pulse energy in joules Speed of light in m/sec. Collecting area of the Receiver in m² Range of the scattering volume in mts. Aerosol scattering cross section /unit volume in m Scattering cross section/unit volume for air molecule - The extinction cross-section per unit volume = Back scatter phase function in str LIDAR SYSTEM FOR ATMOSPHERIC MEASURES 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 crossection of the gaseous constituent under study, where as 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. 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. Schematic Diagram of VIL 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 atmosphere’s 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. GALE: An Advanced Wind LIDAR GALE stands for Giant Aperture Lidar Experiment. GALE measures wind and temperature using resonance flourescence scattering. Resonance fluorescence scattering means that when sodium atoms are illuminated at a precise wavelength (589nm) they become excited and radiate light. The LIDAR’S 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 Reyleigh scattering from air molecule. This Reyleigh 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) = 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 z² factor in the conversion from photo counts to density makes the illuminated area constant with height. The temperature can be found by using the two methods. 1) g = Acceleration due to gravity A = Area of transmitting beam Z = Height 2) Using the ideal gas law This law is the empirical relation between Ie ; P = RT Where ‘R’ is the universal gas constant. Ie; 287 J/Kg for air Combining the above equations we get the relation to the temperature measurement. 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 crosssection 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. 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 suppress multiple scattering phase function possible. LIDAR APPLICATIONS Weather Forecasting Clouds Clouds are formed by the ascent of moist air in the lower atmosphere. Different types of clouds form at different altitude and are an indicator of atmosphere stability. The arrival of different types of clouds helps us to protect the next day’s weather. Stratospheric Aerosols The atmosphere contains a small proportion by volume of microscopic suspended particles generally refered to as aerosols. These particles play a major role in atmospheric chemistry. They help in the cooling of lower atmosphere and assist the destruction of ozone. A portion of the stratospheric aerosols is natural in origin, coming from sources such as meteor ablation and volcanic events. The remainder of the anthropogenic origin, coming from sources such as CFC emission and from high altitude aircraft exhausts. There are many applications of LIDAR technology that are used in both science and industry. Atmospheric science Dynamic measurements: temperature, winds and waves. Climate measurements: clouds, aerosols & water vapour. Ozone measurements: depletions and polar stratospheric clouds. High altitude trace metal measurements: sodium and potassium. Astronomy Sodium layer guide stars for adaptic optics. Planetary surface relief mapping. Topographic mapping Erossion monitoring. Bathymetry (underwater mapping) Harbour profiling for marine safety. Forest ground and canopy measurement Used to access forest growth and health Building and factory construction Measurement allow for precise prefabrication, improving efficiency and reducing cost. Mine shaft mapping Allows cavern monitoring for worker safety. Aircraft docking For safe aircraft maneuvering near airport terminals. Automobile speed monitoring A replacement for hand held radar guns. 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 earth surface is created. 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. 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 Mar’s atmosphere for the first time. This is an important landmark in space exploration. 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. REFERENCES 1) “Laser Radar Systems and Techniques” by Christian G. Bachman. 2) “ Laser Radar Systems” by Albert V. Jalalian. 3) “ LIDAR- Principle and Operation”. 4) “ Exploring the atmosphere with LIDAR” by R. Sica. 5) http:// www.noaa.com 6) http:// www.lidar.com