Aerosols are significant atmospheric constituents that modulate radiation and cloud processes. We evaluated 17-year aerosol profile trends in Barcelona, Spain, from lidar measurements. In summer aerosol reaches 5 km, while in the other seasons it exhibits clear exponential decay. Sahara dust transport affects all seasons, with winter layers above and others penetrating the boundary layer. This study informs the formation of haze and urban preservation strategies in the Mediterranean. The analysis puts in evidence that the averaged net radiative effect is of cooling at both surface level and top of the atmosphere.
The Verification of the Origins of Rotation in Tornadoes Experiment Southeast (VORTEX-SE) provides a wealth of long-duration, high-resolution, vertically pointing observations from active and passive ground-based remote sensing systems enabling characterization of the Atmospheric Boundary Layer (ABL) development over distinct regions that are well known for their relatively high tornado frequency. Application of the Extended Kalman Filter (EKF) to BL height estimation in the convective regime (CBLH) of the diurnal cycle from S-band radar reflectivity observations1 has shown to yield accurate results under simple CBL conditions. In this work, we revisit the radar-EKF technique and investigate its main limitations. For example, during daytime clear-sky conditions such as those prevailing in the BL morning transition, weak turbulence leads to very low reflectivity returns, limiting application of this technique. Additionally, turbulent mixing layers capped with a residual layer, and/or multi-layer scenarios can lead the filter to lose track of the BL signature over time. Doppler Wind Lidar (DWL) observations of the vertical wind velocity variance2 provide complementary CBLH estimates to those of the radar-EKF combination, providing potential to disambiguate more complex convective cases. DWL estimates are, however, strongly influenced by the variance threshold selected. The complementarity of radar and DWL for CBLH estimation is studied in reference to radiosoundings.
Synergistic remote sensing of the atmosphere, combined with adaptive techniques, offers unprecedented opportunities to characterise the evolution of key atmospheric features such as the Atmospheric Boundary Layer (ABL). Using long-duration, high-resolution, profiling observations from active and passive ground-based remote sensing systems during the Verification of the Origins of Rotation in Tornadoes Experiment{Southeast (VORTEX-SE) 2017 field campaign, an attempt is made to characterise ABL development over distinct regions that are well known for their relatively high tornado frequency. In this study, observing systems include an S-band radar, Vaisala CL-31 ceilometer, Doppler Wind lidar (DWL) and radiometric observations from the Collaborative Lower Atmosphere Mobile Profiling System (CLAMPS). In this work, ABL height (ABLH) tracking over the diurnal cycle, and - up to a point - its disambiguation over selected non-precipitating case examples, are attempted. Different observational sets are used, namely, radar reflectivity observations assimilated into a Kalman filter, DWL profiles of the vertical velocity, and virtual potential temperature profiles, as well as radiosoundings and cloudbase reference information collected during Intensive Observation Periods (IOP) carried out in VORTEX-SE, Alabama during 2017. Limitations and advantages of each system are discussed.
KEYWORDS: Space based lasers, Temperature metrology, Atmospheric physics, Radiometry, Performance modeling, Prototyping, Atmospheric sensing, Data modeling, Microwave radiation, Atmospheric modeling
A method for the estimation of Stable Boundary Layer Height (SBLH) using curvature of the potential temperature profiles retrieved by a Microwave Radiometer (MWR) is presented. The vertical resolution of the MWR-derived temperature profile decreases with the height. A spline interpolation is carried-out to obtain a uniformly discretized temperature profile. The curvature parameter is calculated from the first and second order derivatives of the interpolated potential temperature profile. The first minima of the curvature parameter signifies the point where the temperature profile starts changing from the stable to the residual conditions. The performance of the method is analyzed by comparing it against physically idealized models of the stable boundary-layer temperature profile available in the literature. There are five models which include stable-mixed, mixed-linear, linear, polynomial and exponential. For a given temperature profile these five models are fitted using the non-linear least-squares approach. The best fitting model is chosen as the one which fits with the minimum root-mean-square error. Comparison of the SBLH estimates from curvature-based method with the physically idealized models shows that the method works qualitatively and quantitatively well with lower variation. Potential application of this approach is the situation where given temperature profiles are significantly deviant from the idealized models. The method is applied to data from a Humidity-and-Temperature Profiler (HATPRO) MWR collected during the HD(CP)2 Observational Prototype Experiment (HOPE) campaign at Jülich, Germany. Radiosonde data, whenever available, is used as the ground-truth.
There are several instruments and methods to retrieve the atmospheric Mixing Layer Height (MLH). However, none of these instruments or methods can measure the development of the MLH under all atmospheric conditions. For example, aerosol signatures measured by backscatter lidars can be used to determine the MLH but this approach is reasonable only when the atmosphere is well-mixed. Microwave Radiometer (MWR) derived profiles have low vertical resolution and cannot resolve fine structures in the boundary layer, especially, at higher altitudes. Here we propose a method which combines data from a ground-based lidar and a MWR, in simulated as well as real measurements scenarios, to overcome these limitations. The method works by fitting an erf-like transition model function to the section of range-corrected lidar backscatter signal. The section of the lidar backscatter signal for fitting the model function is obtained by incorporating the MWR estimates of MLH along with their uncertainties. The fitting is achieved by using an extended Kalman filter (EKF). The proposed approach, by exploiting the synergy between the two instruments, enables to detect MLH with original vertical and temporal resolutions. Test cases combining simulated data for a co-located lidar-ceilometer and a MWR are presented. The simulated data is obtained from the Dutch Atmospheric Large Eddy Simulation (DALES) model for boundary layer studies. Doppler wind lidar along with radiosondes (whenever available) data is used to assess the quality of the synergetic MLH estimates. Data from the HD(CP)2 Observational Prototype Experiment (HOPE) campaign at Jülich, Germany is used to test the proposed method.
In the framework of the project ChArMEx (the Chemistry-Aerosol Mediterranean Experiment, http://charmex.lsce.ipsl.fr/), the variability of aerosol optical, microphysical and radiative properties is examined in three regional background sites on a southwest – northeast (SW–NE) straight line in the middle of the western Mediterranean Basin (WMB). The three sites are on the northward transport pathway of African dust: Ersa, Corsica Island, France (43.00ºN, 9.36ºW, 80 m a.s.l), Palma de Mallorca, Mallorca Island, Spain (39.55ºN, 2.62ºE, 10 m a.s.l) and Alborán, Alboran Island, Spain (35.94ºN, 3.04ºW, 15 m a.s.l). AERONET (AErosol RObotic NETwork) sun-photometer products are mainly used. A preliminary analysis shows that at Ersa and Palma sites the annual aerosol optical depth (AOD) has a similar trend with a peak around 0.2 in July. The winter/spring AOD is lower in Palma than in Ersa, while it is reverse in summer/autumn. The aerosol particle size distribution (and the coarse mode fraction) shows clearly the SW–NE gradient with a decreasing coarse mode peak (and a decreasing coarse mode fraction from 0.5 - 0.35 - 0.2 in July) along the axis Alborán - Palma de Mallorca - Ersa. In addition to the seasonal and annual variability analysis, the analysis of AERONET products is completed with a large variety of ground-based and sounding balloons remote sensing and in situ instruments during the Special Observation Period (SOP) of the ADRIMED campaign in June 2013. The second part of the presentation will focus on the comparison of the observations at Palma de Mallorca and Ersa of the same long-range transported airmasses. The observations include lidar vertical profiles, balloon borne OPC (Optical Particle Counter) and MSG/SEVIRI AOD, among others.
This contribution evaluates an approach using an extended Kalman filter (EKF) to estimate the planetary boundary layer height (PBLH) from lidar measurements obtained in the framework of the European Aerosol Research LIdar NETwork (EARLINET) at 12 UTC ± 30-min. for a 7-year period (2007-2013) under different synoptic flows over the complex geographical area of Barcelona, Spain. PBLH diagnosed with the EKF technique are compared with classic lidar methods and radiosounding estimates. Seven unique synoptic flows are identified using cluster analysis of 5756 HYSPLIT (HYbrid Single Particle Lagrangian Integrated Trajectory) three-day backtrajectories for a 16-year period (1998-2013) arriving at 0.5 km, 1.5 km, and 3 km, to represent the lower PBL, upper PBL, and low free troposphere, respectively. Regional recirculations are dominant with 54% of the annual total at 0.5 km and 57% of the total lidar days at 1.5 km, with a clear preference for summertime (0.5 km: 36% and 1.5 km: 29%). PBLH retrievals using the EKF method range from 0.79 - 1.6 km asl. Highest PBLH are observed in southwest flows (15.2% of total) and regional recirculations from the east (34.8% of total), mainly caused by the stagnant synoptic pattern in summertime over the Iberian Peninsula. Lowest PBLH are associated with north (19.6% of total) and northeast (4.3% of total) synoptic flows, when fresh air masses tend to lower PBLH. The adaptive nature of the EKF technique allows retrieval of reliable PBLH without the need for long time averaging or range smoothing, as typical with classic methods.
The problem of overlap factor (OVF) computation and its near-range sensitivity for medium-size aperture (f/10 , f/11 ) bi-axial tropospheric lidar systems using ray-tracing simulation software is presented. The method revisits both detector and fiber optics coupling alternatives at the telescope focal-plane along with the insertion of a field lens. A sensitivity analysis is carried out as a function of laser divergence, field lens, and detector/fiber positions, detector size, and the fiber’s core diameter and numerical aperture. The ray-tracing approach presented here is straightforward and a comparatively much simpler solution than analytical-based methods. Parametric simulations are carried out to show that both approaches are coincident. Insertion of a field lens proves to be an elegant and low sensitivity solution for OVF enhancement, particularly, in the near-range of the lidar.
This paper presents parameter design methodology and related optomechanical engineering of a 905-nm diode-laser biaxial, eye-safe lidar ceilometer prototype for cloud-height monitoring. Starting with a brief review of the state-of-the-art ceilometer technology, acceptable parameter ranges are identified for the key system parts. Parameter tuning is achieved by imposing goal criteria on the simulated signal-to-noise ratio and laser-telescope overlap factor. The system is based on a low-cost pulsed semiconductor laser, low-cost Fresnel-lens telescope, a low-noise-equivalent power avalanche-photodiode optoelectronic receiver, and collimating/focusing adjustable parts. Finally, preliminary test measurements are presented.
Minimization of the risk associated with spray applications requires a proper understanding of the spray drift
phenomenon. This fact has led to the development of several techniques to measure the deposition on horizontal surfaces
as well as the airborne spray profiles. Assessment of airborne spray drift is particularly difficult because this
phenomenon is subject to variable micrometeorological conditions. However the monitoring of airborne drift has a great
importance since it can be carried over long distances. This paper reviews main sampling techniques currently used to
asses the airborne spray drift, based on passive collectors and tracers. Theoretical principles that determine the efficiency
of passive samplers are studied as well as the performance of different types of tracers. On the other hand, this paper
shows new airborne spray drift assessment techniques based on lidar technology, reviewing its principle of operation as
well as its practical application in several spray drift trials. It is concluded that the lidar technique has significant
advantages over conventional methods, especially in terms of time consumption and monitoring capabilities. However,
the future adoption of lidar technology for airborne spray drift studies will be subjected to the development of lidar
instruments really adapted to this application.
A 6-channel dichroic-based polychromator is presented as the spectrally selective unit for the U.P.C. elastic/Raman
lidar. Light emission is made at 355-nm (ultraviolet, UV), 532-nm (visible, VIS) and 1064-nm (near infrared, NIR)
wavelengths. In reception, the polychromator is the spectral separation unit that separates the laser backscattered
composite return into 3 elastic (355, 532, 1064-nm wavelengths) and 3 Raman channels (386.7, 607.4 and 407.5-nm
(water-vapor) wavelengths). The polychromator houses photo-multiplier tubes (PMT) for all the channels except for the
NIR one, which is avalanche photodiode (APD) based.
The optomechanical design uses 1-inch optics and Eurorack standards. The APD-based receiver uses a XY-axis
translation/elevation micro-positioning stage due to its comparatively small active area and motorised neutral density
filters are used in all PMT-based channels to avoid detector saturation. The design has been specially optimized to
provide homogeneous spatial light distribution onto the photodetectors and good mechanical repeatability. All channels
are acquired in mixed analog and photon-counting mode using Licel® transient recorders, which are controlled by
means of a user friendly LabVIEWTM interface.
The paper focuses on the main polychromator optical design parameters, that is, light collimation trade-offs, end-to-end
transmissivity, net channel responsivity, light distribution and spot size onto the photodetectors.
The polychromator along with the rest of the U.P.C. lidar system has successfully been tested during a recent lidar
system intercomparison campaign carried out in Madrid (Spain) during Oct. 2010.
A solution based on a Kalman filter to trace the evolution of the atmospheric boundary layer (ABL) sensed by an elastic
backscatter lidar is presented. An erf-like profile is used to model the mixing layer top and the entrainment zone
thickness. The extended Kalman filter (EKF) enables to retrieve and track the ABL parameters based on simplified
statistics of the ABL dynamics and of the observation noise present in the lidar signal. This adaptive feature permits to
analyze atmospheric scenes with low signal-to-noise ratios without need to resort to long time averages or rangesmoothing
techniques, as well as to pave the way for an automated detection method. First EKF results based on
synthetic lidar profiles are presented and compared with a typical least-squares inversion for different SNR scenarios.
The Roque de los Muchachos Observatory, located on the island of La Palma in the Canary Islands, is home of many
astronomical facilities. In the context of the Extremely Large Telescope Design Study, an intensive lidar campaign was
performed in the ORM near the Jacobus Kapteyn Telescope (17°52'41.2" W, 28°45'40.1" N, 2395 m asl) between 26th
May and 14th June 2008. The goal of the campaign was to characterize the atmosphere in terms of planetary boundary
layer height and aerosol stratification vs. synoptic conditions. As a by-product an estimate of the aerosol optical
thickness was also obtained and compared to the total atmospheric extinction coefficient measured by the Carlsberg
Meridian Telescope.
KEYWORDS: Signal to noise ratio, LIDAR, Interference (communication), Raman spectroscopy, Linear filtering, Receivers, Fermium, Frequency modulation, Analog electronics, Photon counting
In this paper we estimate the signal-to-noise ratio (SNR) at the opto-electronic receiver output of both elastic and Raman lidar channels by means of parametric estimation of the total noise variance affecting the lidar system.
In the most general case, the total noise variance conveys contributions from photo-induced signal-shot, dark-shot and thermal noise components. While photo-inducted signal-shot variance is proportional to the received optical signal (lidar return signal plus background component), dark-shot and thermal noise variance components are not. This is the basis for parametric estimation, in which the equivalent noise variance in any receiving channel is characterized by means of a two-component vector modeling equivalent noise parameters.
The algorithm is based on simultaneous low-pass and high-pass filtering of the observable lidar returns and on weighted constrained optimization of the proposed variance noise model when fitting an estimate of the observation noise.
A noise simulator is used to compare different noisy lidar channels (i.e. with different pre-defined noise vectors or dominant noise regimes) with the two-component noise vectors estimate retrieved. Both shot-dominant and thermal-dominant noise regimes, as well as a hybrid case are studied. Finally, the algorithm is used to estimate the SNR from lidar returns from tropospheric elastic and Raman channels with satisfactory results.
A 905-nm 5-kHz rep. rate diode-laser biaxial lidar ceilometer (cloud-height monitoring) prototype is presented. The
prototype uses a low-cost Fresnel lens and a low-NEP avalanche photodiode (APD) opto-electronic receiver. The article
presents the opto-mechanical engineering of both the system and main subsystems involved as well as the system energy
link-budget.
The reception subsystem is based on a low-cost Fresnel-lens telescope and collimating and focusing adjustable parts,
which include a rectangular slit diaphragm to minimise background radiance. Equivalent focal length, background
radiance rejection gain, confusion circle and imaged spot characteristics onto the photodiode surface are also formulated
and discussed by means of a geometrical optics approach.
The emission subsystem uses a beam expander to ensure eye-safety (maximum exposure levels) and ad-hoc mechanics
to provide enough degrees of freedom for emission-reception overlap factor (OVF) adjustment. At this point, an
overview of future alternative mechanical solutions for enhanced pointing accuracy and trade-offs among different laser
diode-based solutions is presented. This part is complemented with OVF simulations of the prototype designed.
Finally, preliminary test measurements at our premises in North Campus (UPC) are introduced as raw and rangecorrected
processed signals.
Implementation of the pure-vibrational Raman spectra lidar method for simultaneous measurements of atmospheric water-vapour, aerosol extinction and backscatter coefficients is reported. A Q-switched Nd:YAG laser provides the three elastic wavelengths of 1064, 532 and 355 nm while the return signal is collected by a 40-cm aperture telescope. A spot-to-spot fiber bundle conveys the light from the telescope focal plane to a specific polychromator especially simulated and designed with care on minimizing optical losses and physical dimensions. The reception field of view, which is limited by the fiber bundle characteristics, is the same for all wavelengths. By means of four customised dichroic filters and beam splitters, light is separated into the three elastic wavelengths (355, 532, 1064 nm) as well as the 386.7- and 607.4-nm N2-Raman-shifted wavelengths, and the 407.5-nm H2O-Raman-shifted wavelength. Signal detection is achieved by using avalanche photodiodes at 1064 and 532 nm and analog acquisition while photomultiplier tubes and fast photon counting acquisition at the rest of the wavelengths. A specific design of the optoelectronics of the receiving channels is controlled by a distributed CPU thanks to a user-friendly LabViewTM interface. User-configurable scanning tools are built-in, but can also be customized. In this work an overview of the system though particularly geared to the polychromator unit is presented as well as a power link-budget assessment, which is to include simulation of end-to-end transmissivities, will be discussed for the main channels involved. The first measurements have already been made at 1064, 532, and 607.4 nm.
Information about atmospheric variables, such as presence and characteristics of supended particulates (aerosols), concentration of chemical species, humidity, temperature, etc., can be obtained remotely in an energetic-efficient manner from radiation produced by optical sources and backscattered by the atmosphere, drawing on the relatively strong interaction between electromagnetic radiation at optical wavelengths and particulates and molecules in the atmosphere. Atmospheric probing systems based on laser sources can provide information as a function of range and, being their operating principle essentially the same as that of radar (except for the fact that in atmospheric probing the target is by definiti8on the atmosphere zone illuminated by the laser, instead of a hard target), they are usually called laser radars or lidars (from Light Detection And Ranging). In addition to their ability to perform remote measurements- a characteristic that they share with other systems-lidars feature the availability of quasi-real-time data, high spatial resolution and a relative convenience to perform three-dimensional scans of the atmospheric volume under study. Basic lidar principles and capabilities are discussed in this work.
A method based on the advantage of a bi-wavelength lidar system has been developed and tested. The method departs from the fact that in nearly transparent atmospheres (weak extinctions), the ratio of the lidar signals at both wavelengths gives the Ångstrom exponent in backscatter with very low uncertainty. The method consists in (i) assuming a linear relationship between the Ångstrom exponent in backscatter and the Ångstrom exponent in extinction (usually called Ångstrom exponent), (ii) approximating by a first order development the ratio of the transmittances, (iii) and forcing the resulting Ångstrom exponent. Profiles of the Ångstrom exponent, and the aerosol backscatter coefficient at 1064 and 532 nm were retrieved. The aerosol backscatter coefficient profiles were compared to Klett inversion results in the lowest part of the atmosphere containing aerosols, [0 - 3 km], in a rural environment in summertime: the agreement is better than 12 % at 1064 nm, and better than 35 % at 532 nm, indicating a greater dependency to the initial assumptions in the visible range.
Regular aerosol backscatter measurements using an elastic-backscatter lidar were performed between May 2000 and November 2002 in Barcelona (Spain), in the frame of EARLINET (European Aerosol Research Lidar Network). The mixing layer height, required to understand the chemical and physical processes taking place in the low troposphere, was one of the major parameters to be retrieved. Three analytic definitions of the ML height have been tested using the range squared-corrected lidar signal: (1) the minimum of its first derivative, (2) the minimum of its second derivative, and (3) the minimum of the first derivative of its logarithm. The strong coastal and orographic influences and the climatological settling of Barcelona determine the complexity of its atmospheric boundary layer dynamics and the high heterogeneity of the lidar signals. Therefore, single lidar analyses do not allow an unambiguous determination of the mixing layer height in many cases and complementary data are needed, such as synoptic maps, backtrajectories, radiosoundings and solar irradiance profiles. The resulting mixing layer heights were compared to radiosoundings, and the second method was found to give statistically the best results. This definition was used to process the whole dataset. A number of 162 days and 660 profiles were examined. The mixing layer height was inferred in cases such as low clouds, Saharan dust events and sea breeze and mountain induced recirculation. Variations between 300 and 1450 m were observed over the three years.
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.
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.
The newly developed Nd:YAG portable 3-D-scanning lidar from the Universitat Politecnica de Catalunya (UPC, Technical University of Catalonia, Barcelona, Spain) was used to improve our knowledge of the aerosols properties in the Barcelona area where an important number of pollution and saharan dust events can be observed all year round in the atmosphere. The system simultaneously operated at the 1064-nm and 532-nm elastic wavelengths, and was used in its scanning mode from 15 degrees to 70 degrees from zenith with 5 degree steps. A variational method was used to invert the multi-angular profiles and to retrieve the aerosols optical thickness and backscatter coefficient at each wavelength without making any assumption on the aerosol type. At the same time, the ratio of the backscatter profiles was used to retrieve the profile of the Angstrom coefficient in backscatter. The backscatter-to-extinction ratio could not be calculated directly but various values of this parameter were used in Klett method until backscatter coefficient profiles could match the one retrieved with the variational method (at least in some altitude regions). Very good agreement (differences less than 20%) was observed in the 0.3 - 2.5 km region with a value of 0.030 sr-1, whereas no agreement could be achieved above where supposedly mixed aerosols were initially observed. The lidar profiles closer to the zenith at both wavelengths allowed to calculate a new Angstrom coefficient in backscatter that is compared to the one retrieved by the variational method. The comparison showed good agreement in the lower layers and thus validated the backscatter-to-extinction ratio profiles used in the Klett method. However, the difficulties encountered to invert the lidar signal above an altitude of 1.6 km show that non negligible inhomogeneities of the atmosphere were present in each line of sight, proving the dense and fast-moving aerosol load over the Barcelona basin.
A novel configuration for a reference-beam, continuous-wave, heterodyne low-power radar prototype is presented. It measures both magnitude and sign of the radial component of the displacement velocity. The basic set-up includes a low power (~10 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 focused 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 analyzer. 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, system working range, and system tolerance in focus-adjustment distance will be presented and discussed. Compared to a previous homodyne prototype presented by the authors, the present system shows a shorter working range (~12 m compared to nearly ~16 m in the homodyne prototype). We attribute this smaller range to the additional losses in the acousto-optic modulator.
This article is aimed at describing the technology, system architecture and specifications of a new 3D Nd:YAG scanning lidar. Main features of the system are interspersed low-range and far-range exploration, open user-configuration scanning tools and a specific architectural design based on parallel CPU control, a LabView user interface and a digitally controlled optoelectronic receiver. The latter provides key advantages to the whole system architecture such as calibration of lidar returns in terms of absolute power and repeatability. Issues concerning system responsivity calibration, receiver gain self-calibration, automatic gain control and synchronization offset-drift zeroing and the like, all of which are of prime importance for the lidarist, are presented. As far as we know, these contributions are new to the state-of-the-art of the community of optical and electronic lidar system designers.
Results presented in this contribution correspond to the first measurements made with the transportable lidar system developed by the Universitat Politecnica de Catalunya, which were collected during the eyra 2000 in the city of Barcelona. The system uses a Nd:YAG laser at 1064 nm wavelength and 0.35 J pulse energy at a 20 Hz pulse repetition rate, has a 20 cm diameter telescope and a scanning range of 120 degrees in elevation and 300 degrees in azimuth. In this study only vertical profiles of atmospheric extinction will be shown. Data were acquired under several meteorological situations, showing the influence that this factor has in the arrangement of aerosols in the vertical dimension, and the distributions of backscatter extinction coefficients obtained from the lidar. Data have also been compared to radiosonde profiles acquired in Barcelona at a near time and have shown how the vertical arrangement of aerosols is correlated with changes in atmospheric stability condition, water content and wind direction.
In 1992 an experimental campaign took place in Barcelona whose main goal was the study of the circulatory patterns of air pollutants in the region. An elastic-backscatter lidar operating at a wavelength of 1.064 micrometers , being mainly sensitive to changes in the distribution of aerosols was used. The study of temporal series of lidar vertical scans revealed a multilayer arrangement of the aerosols above the city. A few years after that experiment, the UPC started to develop its own lidar system. The first stadium of the project was the construction of an elastic-backscatter lidar. In 1997, in the frame of a meteorological situation very similar to the one present during the first campaign, the UPC lidar acquired vertical profiles of elastic- backscatter in the Barcelona's atmosphere. The study of the temporal series also showed the development of elevated layers of aerosols at similar altitudes to the ones observed during the first campaign. The second experiment seems to confirm that the formation of elevated layers in Barcelona is a typical behavior for the region during the summer period and not just a particular situation.
Up to now, retrieval of the atmospheric extinction and backscatter has mainly relied on standard straightforward non-memory procedures such as slope-method, exponential- curve fitting and Klett's method. Yet, their performance becomes ultimately limited by the inherent lack of adaptability as they only work with present returns and neither past estimations, nor the statistics of the signals or a prior uncertainties are taken into account. In this work, a first inversion of the backscatter and extinction- to-backscatter ratio from pulsed elastic-backscatter lidar returns is tackled by means of an extended Kalman filter (EKF), which overcomes these limitations. Thus, as long as different return signals income,the filter updates itself weighted by the unbalance between the a priori estimates of the optical parameters and the new ones based on a minimum variance criterion. Calibration errors or initialization uncertainties can be assimilated also. The study begins with the formulation of the inversion problem and an appropriate stochastic model. Based on extensive simulation and realistic conditions, it is shown that the EKF approach enables to retrieve the sought-after optical parameters as time-range-dependent functions and hence, to track the atmospheric evolution, its performance being only limited by the quality and availability of the 'a priori' information and the accuracy of the atmospheric model assumed. The study ends with an encouraging practical inversion of a live-scene measured with the Nd:YAG elastic-backscatter lidar station at our premises in Barcelona.
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