Multiple-scattering effects can severely influences ground-based lidar measurements when the optical depth is not negligible such as in presence of fog or clouds. This problem can be faced both analytically and by Monte Carlo methods, although as usual the analytical techniques require several simplifications about the microphysical mechanisms whereas Monte Carlo simulation constitutes a more direct approach. In this paper, we discuss an iterative Monte Carlo method to simulate photon multiple scattering in optically dense media. Our results show that it is possible to correct for the multiple scattering influence both extinction and backscattering coefficients obtained by Raman lidar. In particular, for the typical cirrus cloud, the presence of the multiple scattering can lead to an underestimation of the extinction coefficient as large as 100% whereas the backscattering coefficient is almost unaffected by such process. This in turn evidences the strong dependence of the lidar ratio on the multiple-scattering effect.

Laser based altimetry can be very beneficial for planetary exploration, especially in the absence of any significant atmosphere. This technique can provide accurate information on the surface profile (topographic mapping) in a fast and cost effective manner, allowing, within the characteristics of the spacecraft orbit, repeated global coverage of the planet surface. The key characteristics of planetary laser altimetry are therefore an adequate altitude resolution having a range appropriate for full coverage in a compact mission lifetime, an active measurement principle not requiring direct sun illumination, a relatively simple detection chain (as compared to radar based systems), a low resource budget (e.g. mass, power, envelope, data rate) and a relatively simple interface and integration with the spacecraft. A laser altimeter forms a key component for the ESA mission to Mercury, Bepi-Colombo, onboard the Mercury Planetary Orbiter, MPO. The European Space Agency (ESA) is promoting the study of a generic laser altimeter for planetary exploration. This definition study will use Mercury as a reference for the definition of the environmental and operational requirements.

CO.RI.S.T.A. (Consortium of Research on Advanced Remote Sensing Systems) performed a feasibility study funded by Italian Space Agency (ASI) to develop a rangefinder system as payload for microsatellite. The satellite considered for the study is UNISAT, an Italian academic satellite. The studied rangefinder offers the possibility to correct the systematic error of stereoscopic images acquired by MHRRC camera (Miniaturised High-Resolution Reconnaissance Camera) integrated on board the satellite. In order to carry out a compact and reliable altimeter for satellite UNISAT a review and comparison have been made with rangefinder systems both for the microwaves (radar systems) and for the visible infrared wavelength range (laser systems). A pulsed laser altimeter system based on Time Of Flight measurement appears the more suitable for the aforementioned application.

We have estimated the contribution of atmospheric turbulence effects to the satellite laser ranging precision. This work was motivated by the observed discrepancy between the precision of laser ranging to short baseline ground targets and space born targets. The contribution of the atmosphere is expected to be the limiting factor to the satellite laser ranging precision on millimeter level. Two different atmospheric optical models were investigated. The geometry approach showed that at some situations the turbulence-induced random ranging error could reach the millimeter level, as observed in laser ranging experiment. This effect significantly decreases with the station’s altitude above sea level and satellite altitude above horizon. The results depend on the value of the atmospheric outer scale parameter; its value is only approximate due to hardly predictable nature of the turbulence strength height profile. A novel experiment with high repetition rate satellite laser ranging is introduced, which should prove the turbulence contribution to the satellite laser ranging precision.

Hardware development for remote sensing costs a lot of time and money. A virtual instrument based on software modules was developed to optimise a small visibility and cloud base height sensor. Visibility is the parameter describing the turbidity of the atmosphere. This can be done either by a mean value over a path measured by a transmissometer or for each point of the atmosphere like the backscattered intensity of a range resolved lidar measurement. A standard ceilometer detects the altitude of clouds by using the runtime of the laser pulse and the increasing intensity of the back scattered light when hitting the boundary of a cloud. This corresponds to hard target range finding, but with a more sensitive detection. The output of a standard ceilometer is in case of cloud coverage the altitude of one or more layers. Commercial cloud sensors are specified to track cloud altitude at rather large distances (100 m up to 10 km) and are therefore big and expensive. A virtual instrument was used to calculate the system parameters for a small system for heliports at hospitals and landing platforms under visual flight rules (VFR). Helicopter pilots need information about cloud altitude (base not below 500 feet) and/or the visibility conditions (visual range not lower than 600m) at the destinated landing point. Private pilots need this information too when approaching a non-commercial airport. Both values can be measured automatically with the developed small and compact prototype, at the size of a shoebox for a reasonable price.

LIDAR remote sensing technology has not only applications in geographical, atmospheric or biological sciences but it can also play an important role in the everyday life. Within the last 10 years statistics of European car traffic has shown that about one third of all accidents go back to darkness and poor road conditions. A system collecting information about visibility and distance to following vehicles and setting appropriate rear light intensities could provide a much safer road travel under various environmental conditions. The system that is being developed co-operates with a dirt and brightness sensor to take into account these various external influences on an automobile and applies them to the operation of the rear light. The developed sensors are integrated in an advanced micro-system and capable of providing external environmental data for automatic brightness control within a requested range of light output for constant perceptibility of light signals to the following traffic. This conference gives further information about: (1) construction, optical and laser parameters, (2) application in rear light systems, (3) measurement characteristics, (4) test equipment (LIDAR_Probe), (5) measurement results, test rides, raw data.

Lidar backscatter by hexagonal ice cylinders of cirrus clouds is considered within the framework of both geometric optics and physical optics approaches. Within geometric optics, reasons of great backscattering peak and of large magnitudes of the depolarization ratio are investigated and explained. An approach including diffraction and interference phenomena is discussed.

Multiple-scattering effects sometime bias the ground-based lidar measurements, in particular for density aerosol and cirrus cloud. Both analytical and Montecarlo methods are very useful tools to study this influence. However, for analytical solution, it needs to make some hypotheses and the Montecarlo simulation is only a forward method. In this paper, an itinerative method is introduced based on Montecarlo simulation. Both extinction and backscattering coefficients, obtained by Raman lidar, are corrected for the multiple-scattering influence. For the typical cirrus cloud, the error of the multiple-scattering influence on extinction can be as large as 100%. However, it is negligible of the influence on backscattering coefficient. Therefore, the lidar ratio is also sensitive to the multiple-scattering effect.

Multiwavelength elastic lidar is often used to probe the aerosol profiles of the atmosphere. Normally, the atmosphere is considered homogeneous and an a-priori aerosol ratio is given for each wavelength channel which is then processed independently. However, it is clear that the multiwavelength retrieved backscatter profiles should contain information that can be used to estimate particle size distribution which may provide a new estimate to range dependant aerosol ratio profiles which can be repeated until convergence. In this paper, we illustrate the basic idea of using multiwavelength data using a two wavelength lidar to obtain local information on the lidar ratio which can be used to improve lidar profiling in inhomogeneous atmospheres and show that a key feature of any scheme is the monotonic dependance between the optical data ratio and the distribution parameter. In addition, we extend the approach to a prototypical Nd:YAG three wavelength (355, 532, 1064nm) lidar arrangement and show that while an iterative lidar procedure can be used to extract range dependant profiles, imprecision in the inversion procedure as well as error propagation of the lidar back integration can hamper convergence.

Monitoring aviation safety hazards, such as icing conditions, and retrieving cloud physical properties for climate modeling studies requires cloud thermodynamic phase (water/ice) discrimination. Polarization information from lidar measurement provide such information. Depolarization of lidar backscattering indicates that the scattering cloud particles are non-spherical (i.e., ice clouds). For space based lidar measurements, backscatter from water cloud particles is also depolarized because of multiple scattering. Thus cloud water/ice discrimination is not straight-forward. An alternative method which is less sensitive to multiple scattering is proposed in this study. The new approach is based on the fact that there are big differences in P₄₄ (an element of the scattering phase matrix) at 180° between spherical and non-spherical particles. When the incident beam is left-hand-circularly polarized, backscattering by a nonspherical particle is also left handed. Circular component of backscattering by a spherical particle is right-handed for left-hand-circularly polarized incident beam. Monte Carlo simulations with full Stokes vector indicate that multiple scattering does not affect the sphere/non-sphere determination with this new circular polarization approach.

Accurate estimation of cloud and aerosol optical depths using backscatter lidar data requires knowledge of the particulate lidar ratio (i.e., the extinction-to-backscatter ratio). In those cases for which a measurement of molecular backscatter can be made on the far side of a layer, knowledge of the lidar ratio can be derived directly from the data. However, obtaining a reliable clear air constraint is a function of layer optical depth, system sensitivity and overall signal-to-noise ratio (SNR). To date, the design constraints imposed on space-based lidars such as LITE and CALIPSO have rendered the use of this retrieval technique virtually impossible for measurements made at 1064 nm. Layers to which the constraint method can be successfully applied are assumed to be homogeneous with respect to particle composition and size distribution, and therefore are characterized by lidar ratios that are range-invariant throughout the layer. By extending this assumption of homogeneity to include the layer backscatter color ratio, this work derives a new technique that simultaneously retrieves both the color ratio and the 1064 nm lidar ratio from two wavelength elastic backscatter lidar measurements of transmissive clouds and/or lofted aerosol layers. Retrieval examples are illustrated using data obtained from LITE. Initial error estimates derived from numerical experiments using simulated data show the retrieval of the backscatter color ratio to be stable, even in the presence of considerable noise in the data.

Lidar (radar laser) systems take advantage of the relatively strong interaction between laser light and aerosol/molecular species in the atmosphere. The inversion of optical atmospheric parameters is of prime concern in the fields of environmental and meteorological modelling and has been (and still is) under research study for the past four decades. Within the framework of EARLINET (European Aerosol LIdar NETwork), independent inversions of the atmospheric optical extinction and backscatter profiles (and thus, of the lidar ratio, as well) have been possible by assimilating elastic-Raman data into Ansmann et al.’s algorithm [the term “elastic-Raman” caters for the combination of one elastic lidar channel (i.e., no wavelength shift in reception) with an inelastic Raman one (i.e., wavelength shifted)]. In this work, an overview of this operative method is presented under noisy scenes along with a novel formulation of the algorithm statistical performance in terms of the retrieved-extinction mean-squared error (MSE). The statistical error due to signal detection (Poisson) is the main error source considered while systematic and operational-induced errors are neglected. In contrast to Montercarlo and error propagation formulae, often used as customary approaches in lidar error inversion assessment, the statistical approach presented here analytically quantifies the range-dependent MSE performance as a function of the estimated signal-to-noise ratio of the Raman channel, thus, becoming a straightforward general formulation of algorithm errorbars.

Optical parametric oscillator (OPO) and sum-frequency mixing (SFM) devices are useful tools for constructing ultraviolet (UV) laser sources for fluorescence spectroscopy. Here, a compact UV-laser sources based on frequency conversion of an actively Q-switched Nd:YAG laser is presented. The second harmonic generation from a Nd:YAG laser was utilized as pump radiation for a periodically poled KTiOPO₄ nanosecond optical parametric oscillator. The OPO-signal and the remaining pump were spatially mode-matched for Type I SFM in a β-barium borate (BBO) crystal and UV radiation at 293 nm could be generated. This corresponds to a conversion efficiency of 2% with respect to the 532 nm harmonic radiation. The wavelength region accessible with this UV source is useful for chemical and biological sensing. Excitation of tryptophan at 293 nm for detection of fluorescence emission in ovalbumin and transthyretin was demonstrated.

The quantum cascade laser is an unipolar semiconductor laser source emitting in the mid-infrared range between 3.5 and 25 μm. During the past ten years after their invention, this technology has reached the level of maturity required for commercialization, and QC lasers have thus become very attractive for a large number of applications, including gas sensing, pollution detection, atmospheric chemistry, detection of compounds, non-invasive medical diagnostics, free-space optical data transmission or even LIDAR. Most common requirements are single-mode operation on thermoelectric cooler, high power and/or continuous-wave. Nowadays several high-power single-mode QC lasers are available at Alpes Lasers in the range from 4.3 to 16.5 μm, with a side-mode suppression ratio larger than 30 dB. We present here a specific high-average power Fabry-Perot quantum cascade laser and a distributed-feedback quantum cascade laser operating near 8 μm.

The GroundWinds direct detection Doppler wind LIDARs located in NH and HI are operational, ground based, multi-order fringe imaging systems capable of detecting Doppler shifts in both Aerosol and Molecular backscatter from 0.25 km to 18 km. The technology developed through these GroundWinds programs will be incorporated and flown on a high altitude (30km) balloon in 2005. The demonstration of GroundWinds Fabry-Perot based incoherent LIDAR technology from a high altitude, downward looking platform to measure winds throughout the entire troposphere and boundary layer will be a significant milestone toward the validation of this technology. Key questions will be answered about the phenomenology of direct detection LIDAR, especially its effectiveness in the optically thick boundary layer. The extensive characterization of the 532nm GroundWinds NH and 355nm GroundWinds HI LIDARs serve as excellent reference points from which performance estimates and technology requirements can be determined to ensure a successful balloon mission. This paper will describe the baseline BalloonWinds instrument specifications; including etalon specifications, system component transmissions, transmit/receive specifications, required laser power, and detector characteristics. This paper will also present performance estimates based on model simulations that employ the baseline system specifications.

The classical radio technique used for FM detection, the frequency discriminator can also be used in optical frequency detection. In this sense, Chanin et al [2] proposed a lidar system that measures atmospheric wind fields by detecting Doppler-shift in the return signal in a differential way by using two Fabry-Perot interferometers or any other high resolution optical filters as frequency discriminators. This technique was also studied and used by Korb et al. They named it "edge-technique." The UPC lidar group is developing a wind lidar based on the "edge-technique." The first prototype of the wind lidar is a continuous-wave system that is able to measure the surface displacement velocity of solid targets. The transmitting laser is the seeder of a Spectra Physics GCR-190 laser, which will be used for the final wind measurements. It includes a Fairy-Perot interferometer, two APD-based optical receivers, and several auxiliary optics, electro-optic and electronic elements. Among them, there is a servo-loop, based on two acousto-optic frequency-shifters and a lock-in amplifier, which is responsible of the proper tuning between the laser and the Fabry-Perot interferometer. To our knowledge, this servo-loop has not been used before for wind lidars based on the edge-technique. The aim of this first prototype is to test the performance of the edge-technique to measure velocities and to assess the role of the servo-loop in the precision of the measurements. The study and design of the prototype, with emphasis in the servo-loop will be presented.

Laser-based remote wind systems work from Doppler-shifted scattering by aerosols in the atmosphere. These devices have been developed over decades, but their integrated opto-components and the associated signal processing are only now reaching an operational state, thanks to new laser and detection systems emerging from the communication industry, where they are becoming stable, eye-safe, cost-effective, and commercial available. A fiber based laser Doppler anemometer (LDA) was developed in the laboratory. Measurements of moving hard targets (wheels, loudspeakers) and soft targets (spray) were carried out and will be reported upon. At the beginning of the development a virtual instrument was used to design the sensor and was subsequently modified after the first flight test using a CO₂ LDA. It was used to design a compact fiber sensor.

The optimization and performance assessment of a reference-beam, continuous-wave, heterodyne low-power laser radar prototype is presented, based on previous homodyne prototypes. It measures both magnitude and sign of the radial component of the displacement velocity. The basic set-up includes a low power (~3 mW) commercial HeNe laser, a beam-splitter, an acousto-optic modulator, and a two-lens system that both focuses the transmitted beam on the target surface and collects the scattered light. Both the reference beam and the radiation collected are focussed onto a Si avalanche photo-detector. The self-aligned configuration of the receiver makes possible, theoretically, to perform optimal mixing between the received scattered radiation and the reference beam. The resulting electrical signal is fed to a transimpedance amplifier and displayed on a spectrum analyser. Laboratory experiments employing as a target the rim of a 50 cm-diameter rotating wheel placed at several distances have been performed. Results concerning detected signal-to-noise ratio, detected-signal spectral width, accuracy of the radial component of the velocity under measurement and system working range will be presented and discussed.

The “lidarist” frequently wishes to process his experimental data to obtain as accurate and clean a representation of water vapor as is consistent with his measurement accuracies. In measurements contaminated by high-frequency noise this usually means smoothing the experimental data by some method (which is equivalent to smoothing with a low-pass filter) to eliminate or greatly reduce the amount of high frequency noise without distorting the desired signal. For data which are continuous in time (analog data) this is commonly accomplished using low-pass RC filters. However, with the increasing use of computer-controlled data acquisition systems which record data in digital form, there has developed a need for techniques which perform the same filtering process on the digitized data. Filtering or smoothing process should be as simple and efficient as is consistent with experimental situation. Poissonian averaging has been using for filtering lidar signal data. In previous work we showed the opportunity of using digital filtering in order to overcome problems created by poissonian averaging. To introduce further improvement in filtering we have used binomial filters; some scientists also use differentiating smoothing in an attempt to compensate for the fact that differentiation reduces the signal-to-noise ratio. This can easily be performed with the binomial filter by convolving either the filter coefficients or the data by sequences [1,0, -1]/2. This may be repeated any number of times to obtain the second-third-, and higher-order derivatives, after which the data are low-pass filtered in usual manner. The advantages of the above adjustable windows compared to the fixed windows are their optimality and flexibility. A wavelet analysis is used to increase signal retrieval. Wavelet analysis represents the next logical step: a windowing technique with variable-sized regions. Wavelet analysis allows the use of long time intervals where we want more precise low-frequency information, and shorter regions where we want high-frequency information.

The solid state photodetectors based on silicon avalanche photodiodes operated in Geiger mode are used for detection of echo signal in laser ranging experiments. The avalanche process nonlinearity enhance the influence of starting conditions to avalanche grow of photodiode output signal. This is the reason why the internal delay of this type of detectors is depended on detected signal intensity, i.e. in case of weak signal it is depended on number of detected photons. The dependence is in the range of 0-200 ps for photon numbers 1-1000 photons. The active quenching and gating circuit with time walk compensation has been constructed to eliminate this effect (Kirchner, 1998). In our experiment, we have used the outputs of the compensation circuit to estimate the photon numbers detected on a shot by shot basis simultaneously with original required time interval estimation. The mutual time difference between the compensated and uncompensated output pulses corresponds to the photon number. Monitoring this time difference by the picosecond event timing device enabled us to monitor the echo signal strength fluctuation on a shot by shot basis in a laser ranging. The experimental results will be presented.

We use measurements and models to develop aerosol models for use in the inversion algorithms for the Cloud Aerosol Lidar and Imager Pathfinder Spaceborne Observations (CALIPSO). Radiance measurements and inversions of the AErosol RObotic NETwork (AERONET) are used to group global atmospheric aerosols using optical and microphysical parameters. This study uses more than 10⁵ records of radiance measurements, aerosol size distributions, and complex refractive indices to generate the optical properties of the aerosol at more 200 sites worldwide. These properties together with the radiance measurements are then classified using classical clustering methods to group the sites according to the type of aerosol with the greatest frequency of occurrence at each site. Six significant clusters are identified: desert dust, biomass burning, urban industrial pollution, rural background, marine, and dirty pollution. Three of these are used in the CALIPSO aerosol models to characterize desert dust, biomass burning, and polluted continental aerosols. The CALIPSO aerosol model also uses the coarse mode of desert dust and the fine mode of biomass burning to build a polluted dust model. For marine aerosol, the CALIPSO aerosol model uses measurements from the SEAS experiment. In addition to categorizing the aerosol types, the cluster analysis provides all the column optical and microphysical properties for each cluster.

Practical inversion methods are presented to account for the multiple scattering component of lidar signals. Two variants of the lidar signal inversion for dense smoke plumes, the near-end solution and the optical depth solution, are considered. In both cases, the measured lidar signal, contaminated by multiple scattering, is transformed into a form similar to the single-scattering lidar equation for a single-component atmosphere. To achieve this, a transformation factor related to the range-dependent ratio of the multiple-to-single scattering is determined using an assumed dependence of the ratio on the smoke optical depth.

In this paper we present ultra fast variable optical attenuator to be used for improved performance laser welding systems and for increasing the measurement performance in laser range finders and LIDARs. The improvement in laser range and LIDAR systems is caused due to preventing the reflection of light from the optics of the transmitter that saturates the receiver and thus limits the minimal detection range.

A new laser Doppler velocimeter employing a CO₂ laser has been developed by using its photoacoustic effect. A change of the pressure of a laser discharge tube, induced by mixing of a returned wave with an originally existing wave inside the cavity, is employed to detect the Doppler frequency shift. We found that a Doppler frequency shift as much as 60 kHz was detected, and as well as a good linear relationship between the velocity and the Doppler frequency shift was obtained.

The Advanced Ladar Imaging Simulator (ALIS) is a comprehensive multi-dimensional laser radar system simulator that models complex atmospheric scenes combined with high-resolution solid object scenes. The primary functions of ALIS are to serve as a laser radar sensor design tool, data product generator for exploitation, and a decision aid for implementing system designs. This paper focuses on the software structure of the simulator and the challenges that it presents. The ambient atmospheric scene generation is augmented with built-in approximate plume models or with external large-scale Navier-Stokes computational fluid dynamics plume models. The mixed atmosphere and solid object scene is generated via an adaptively meshed, over-sampled voxel representation predicated jointly on the sensor capabilities and scene complexity. To our knowledge, ALIS goes beyond previous ladar simulators with detailed atmospheric turbulence effects and time-dependent plume dispersion models for direct and coherent detection frequency-agile transceivers. ALIS models a wide range of ladar architectures, treating laser coherence properties, receiver electronics noise/transfer functions, and electronics/photon statistical noise. It provides complex amplitude ladar echo "range cubes” (all range reports along a given line-of-sight) for the composite atmosphere-solid scene. The model complexity and its capability to process large (>10⁹) voxel count scenes is accommodated with a portable, scalable software architecture that supports single processors to fine-grained parallel supercomputers.

BAE SYSTEMS reports on a program to characterize the performance of MEMS corner cube retroreflector arrays under laser illumination. These arrays have significant military and commercial application in the areas of: (1) target identification; (2) target tracking; (3) target location; (4) identification friend-or-foe (IFF); (5) parcel tracking, and; (6) search and rescue assistance. BAE SYSTEMS has theoretically determined the feasibility of these devices to learn if sufficient signal-to-noise performance exists to permit a cooperative laser radar sensor to be considered for device location and interrogation. Results indicate that modest power-apertures are required to achieve SNR performance consistent with high probability of detection and low false alarm rates.

The Lidar In space Technology Experiment (LITE) provided for the first time highly detailed vertical profiles of aerosol and clouds from the Earth’s surface to the middle stratosphere. Validated theoretical results from a Model of Atmospheric Transport and Chemistry (MATCH) can help quantify and qualify the aerosol population as well as identify some consistent patterns of aerosol components for a certain region. The goal of this work is to estimate the degree of confidence on MATCH’s theoretical results, comparing them to the data set retrieved by LITE, in order to improve the lidar aerosol extinction-to-backscatter ratio retrieval algorithm to be applied to the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), NASA’s next mission that will be orbiting a Lidar around the Earth.