Literatura científica selecionada sobre o tema "Lidar surface reflectance"

Abstract. This paper presents an innovative retrieval method that translates the CALIOP land surface laser pulse returns into the surface bidirectional reflectance. To better analyze the surface returns, the CALIOP receiver impulse response and the downlinked samples' distribution at 30 m vertical resolution are discussed. The saturated laser pulse magnitudes from snow and ice surfaces are recovered based on information extracted from the tail end of the surface signal. The retrieved snow surface bidirectional reflectance is compared with reflectance from both CALIOP cloud-covered regions and MODIS BRDF–albedo model parameters. In addition to the surface bidirectional reflectance, the column top-of-atmosphere bidirectional reflectances are calculated from the CALIOP lidar background data and compared with the bidirectional reflectances derived from WFC radiance measurements. The retrieved CALIOP surface bidirectional reflectance and column top-of-atmosphere bidirectional reflectance results provide unique information to complement existing MODIS standard data products and are expected to have valuable applications for modelers.

While airborne lidar has confirmed its leading role in delivering high-resolution 3D topographic information during the last decade, its radiometric potential has not yet been fully exploited. However, with the increasing availability of commercial lidar systems which (a) make use of full-waveform information and (b) operate at several wavelengths simultaneously, this potential is increasing as well. Radiometric calibration of the full-waveform information mentioned before allows for the derivation of physical target surface parameters such as the backscatter coefficient and a diffuse reflectance value at bottom of atmosphere (BOA), i.e. the target surface. <br><br> With lidar being an active remote sensing technique, these parameters can be derived from lidar data itself, accompanied by the measurement or estimation of reference data for diffuse reflectance. In contrast to this, such a radiometric calibration for passive hyperspectral imagery (HSI) requires the knowledge and/or estimation of much more unknowns. However, in case of corresponding wavelength(s) radiometrically calibrated lidar datasets can deliver an areawide reference for BOA reflectance. <br><br> This paper presents criteria to check where the assumption of diffuse BOA reflectance behaviour is fulfilled and how these criteria are assessed in lidar data; the assessment is illustrated by an extended lidar dataset. Moreover, for this lidar dataset and an HSI dataset recorded over the same area, the corresponding reflectance values are compared for different surface types.

Abstract The analysis of the sea surface reflectance for different incidence angles based on observations of an airborne Doppler lidar at an ultraviolet wavelength of 355 nm is described. The results were compared to sea surface reflectance models, including the contribution from whitecaps, specular reflection, and the subsurface volume backscattering. The observations show the expected effect of the wind stress on the sea surface reflectance and allow new insights into the significant contribution from subsurface reflectance for large incidence angles. While most of the observations and model results were obtained for isotropic reflectance, first results on anisotropic reflectance are also provided. The results from this study are relevant to future spaceborne wind lidar instruments, for example, the Atmospheric Dynamics Mission (ADM)-Aeolus, which could use the sea surface reflectance for the calibration of intensity and wind.

Abstract. The characteristics of the lidar reflectance of the Earth's surface is an important issue for the IPDA lidar technique (integrated path differential absorption lidar) which is the proposed method for the spaceborne measurement of atmospheric carbon dioxide within the framework of ESA's A-SCOPE project. Both, the absolute reflectance of the ground and its variations have an impact on the measurement sensitivity. The first aspect influences the instrument's signal to noise ratio, the second one can lead to retrieval errors, if the ground reflectance changes are strong on small scales. The investigation of the latter is the main purpose of this study. Airborne measurements of the lidar ground reflectance at 1.57 μm wavelength were performed in Central and Western Europe, including many typical land surface coverages as well as the open sea. The analyses of the data show, that the lidar ground reflectance is highly variable on a wide range of spatial scales. However, by means of the assumption of laser footprints on the order of several tens of meters, as planned for spaceborne systems, and by means of an averaging of the data it was shown, that this specific retrieval error is compatible with the sensitivity requirements of spaceborne CO2 measurements.

Abstract. The characteristics of the lidar reflectance of the Earth's surface is an important issue for the IPDA lidar technique (integrated path differential absorption lidar) which is the proposed method for the spaceborne measurement of atmospheric carbon dioxide within the framework of ESA's A-SCOPE project. Both, the absolute reflectance of the ground and its variations have an impact on the measurement sensitivity. The first aspect influences the instrument's signal to noise ratio, the second one can lead to retrieval errors, if the ground reflectance changes are strong on small scales. The investigation of the latter is the main purpose of this study. Airborne measurements of the lidar ground reflectance at 1.57 μm wavelength were performed in Central and Western Europe, including many typical land surface coverages as well as the open sea. The analyses of the data show, that the lidar ground reflectance is highly variable on a wide range of spatial scales. However, by means of the assumption of laser footprints in the order of several tens of meters, as planned for spaceborne systems, and by means of an averaging of the data it was shown, that this specific retrieval error is well below 1 ppm (CO2 column mixing ratio), and so compatible with the sensitivity requirements of spaceborne CO2 measurements. Several approaches for upscaling the data in terms of the consideration of larger laser footprints, compared to the one used here, are shown and discussed. Furthermore, the collected data are compared to MODIS ground reflectance data.

Space missions to study small solar system bodies, such as asteroids and comet cores, are enhanced by lidar that can provide global mapping and serve as navigation sensors for landing and surface sampling. A small swath-mapping lidar using a fiber laser modulated by pseudo-noise (PN) codes is well-suited to small space missions and can provide contiguous measurements of surface topography with <10 cm precision. Here, we report the design and simulation of receiver signal processing of such a lidar using the small all-range lidar (SALi) as a design example. We simulated its performance in measuring the lidar range and surface reflectance by using instrument and target parameters, noise sources, and the receiver correlation processing method under various conditions. In single-beam Reconnaissance mode, the simulation predicted a maximum range of 440 km under sunlit conditions with a range precision as small as 8 cm. In its multi-pixel Mapping mode, the lidar can provide measurements out to 110 km with range precision of 5 cm. The effects of Doppler shift were quantified. From these results, we discuss the need for Doppler compensation via the receiver clock rate. We also describe a novel reflectance measurement method using active laser control, which allows the receiver to use simple comparators for analog-to-digital conversion. This method was simulated with surface reflectance values from 4% to 36% resulting in an RMS precision of 3% and a bias of 1% of the surface reflectance. We also performed an orbital ranging simulation using a shape model of 101955 Bennu for target surface elevation. The range residuals showed a sub-mm bias with a standard deviation of 5 cm. We implemented the receiver processor design on a Xilinx Ultrascale field-programmable gate array (FPGA). It was able to process received signals and retrieve accurate ranges at a single-channel measurement rate of 3050 Hz with a latency of 1.07 ms.

In order to retrieve results comparable under different flight parameters and among different flight campaigns, passive remote sensing data such as hyperspectral imagery need to undergo a radiometric calibration. While this calibration, aiming at the derivation of physically meaningful surface attributes such as a reflectance value, is quite cumbersome for passively sensed data and relies on a number of external parameters, the situation is by far less complicated for active remote sensing techniques such as lidar. This fact motivates the investigation of the suitability of full-waveform lidar as a “single-wavelength reflectometer” to support radiometric calibration of hyperspectral imagery. In this paper, this suitability was investigated by means of an airborne hyperspectral imagery campaign and an airborne lidar campaign recorded over the same area. Criteria are given to assess diffuse reflectance behaviour; the distribution of reflectance derived by the two techniques were found comparable in four test areas where these criteria were met. This is a promising result especially in the context of current developments of multi-spectral lidar systems.

In order to retrieve results comparable under different flight parameters and among different flight campaigns, passive remote sensing data such as hyperspectral imagery need to undergo a radiometric calibration. While this calibration, aiming at the derivation of physically meaningful surface attributes such as a reflectance value, is quite cumbersome for passively sensed data and relies on a number of external parameters, the situation is by far less complicated for active remote sensing techniques such as lidar. This fact motivates the investigation of the suitability of full-waveform lidar as a “single-wavelength reflectometer” to support radiometric calibration of hyperspectral imagery. In this paper, this suitability was investigated by means of an airborne hyperspectral imagery campaign and an airborne lidar campaign recorded over the same area. Criteria are given to assess diffuse reflectance behaviour; the distribution of reflectance derived by the two techniques were found comparable in four test areas where these criteria were met. This is a promising result especially in the context of current developments of multi-spectral lidar systems.

This paper describes approaches to retrieve important aerosol results from the strong lidar return signals that are received by the space-borne CALIPSO lidar system after reflecting off-ocean surfaces. Relations, from which the theoretically expected values of area under ocean surface returns can be computed, are presented. A detailed description of the lidar system response to the ocean surface returns and the processes of sampling and averaging of lidar return signals are provided. An effective technique that reconstructs the lidar response to surface returnsstarting from down-linked samplesand calculates the area under it, has been developed and described. The calculated area values are validated after comparing them to their theoretically predicted counterpart values. Methods to retrieve aerosol optical depths (AODs) from these calculated areas are described and retrieval results are presented, including retrieval comparison with independent AOD measurements made by an airborne High Spectral Resolution Lidar (HSRL) that yielded quite good agreement. Techniques and results are also presented on using the spectral ratios of the surface response areas to determine spectral ratios of aerosol round-trip transmission and AOD spectral difference, without need of a specific/accurate ocean-surface reflectance model.

Les connaissances sur la distribution et les propriétés physico-chimiques des particules aérosols dans la troposphère ont été identifiées par le Groupe d’experts Intergouvernemental sur l’Évolution du Climat (GIEC) comme la principale source d’incertitude dans l’étude de l’évolution du climat. Une caractérisation des types, des propriétés optiques et de la distribution verticale des aérosols à l’échelle régionale est nécessaire pour réduire cette source d’incertitude et certaines zones comme la Sibérie sont encore mal documentées. Les concentrations en aérosol de la Sibérie dépendent de sources naturelles, comme les feux de forêt saisonniers ou le transport vers le nord des poussières désertiques, mais également des sources anthropiques comme celles des zones exploitations d’hydrocarbures ou le transport à longue distance des émissions du Nord de la Chine. Afin de contribuer à l’amélioration de cette caractérisation des sources d’aérosol en Sibérie, nous avons dans un premier temps analysé les mesures de deux campagnes aéroportés réalisées sur des distances de plusieurs milliers de km en juillet 2013 et juin 2017. L’avion était équipé d’un lidar à rétrodiffusion à 532 nm ainsi de mesures in-situ de monoxyde de carbone (CO), de carbone suie (BC) et des distributions en taille des aérosols. Ces observations ont été étudiées en synergie avec celle du lidar spatial CALIOP et des missions MODIS et IASI. La gamme d'altitude des couches d'aérosols et le rôle de l’âge sur les propriétés optiques (épaisseur optique (AOD532), dépolarisation, rapport de couleur) sont discutés pour chaque type d'aérosol. Les résultats d’un vol au-dessus des régions d’extraction du gaz correspond au plus fortes AOD532, et des concentrations en BC supérieure à celle des émissions des zones urbaines et a permis une estimation du rapport lidar de ces panaches d'aérosols mal documentés dans la littérature. La deuxième partie du travail a consisté à proposer une alternative à la restitution indirecte de l’AOD532 par l’instrument CALIOP à partir de l’inversion du signal lidar de rétrodiffusion atténué. Cette méthode utilise la réflectance du signal lidar de CALIOP par la surface et a déjà été utilisée au-dessus des océans ou des nuages d’eau liquide optiquement opaques pour calculer une valeur AOD. Dans ce travail, nous avons ainsi développé et évalué une restitution des AOD à partir de la réflectance CALIOP de surface pour les zones continentales. Deux méthodologies ont été utilisées afin de déterminer la réflectance lidar de surface non atténuée par les aérosols: (i) sélection des observations CALIOP en condition de ciel clair sur 7 ans d’observation (ii) extrapolation de la relation de linéarité entre la réflectance lidar de surface atténuée et la transmission atmosphérique. Si ces deux méthodes donnent de bons résultats dans les zones de faible réflectance lidar de surface (< 0.75 sr-1) la première méthode n’est pas utilisable sur les zones désertiques. L’utilisation de ces AOD lidar mesurées directement au-dessus des surfaces continentales permet d’améliorer le biais (|ME| < 0.034) et la dispersion (< 0.145) en comparaison aux observations MODIS. Ceci améliore beaucoup les résultats des comparaisons CALIOP-MODIS obtenus avec la restitution indirecte des AOD une analyse des profils verticaux de rétrodiffusion lidar atténuée avec un biais < 0.174 et une dispersion < 0.234
Knowledge of the distribution and physico-chemical properties of aerosol particles in the troposphere has been identified by the Intergovernmental Panel on Climate Change (IPCC) as the main source of uncertainty in the study of climate change. Characterization of the types, optical properties and vertical distribution of aerosols at the regional scale is needed to reduce this source of uncertainty and some areas such as Siberia are still poorly documented. Aerosol concentrations in Siberia depend on natural sources, such as seasonal forest fires or northward transport of desert dust, but also on anthropogenic sources such as those from hydrocarbon mining areas or long-range transport of emissions from northern China. In order to contribute to the improvement of this characterization of aerosol sources in Siberia, we first analyzed the measurements of two airborne campaigns carried out over distances of several thousand km in July 2013 and June 2017. The aircraft was equipped with a back-scattering lidar at 532 nm, as well as in-situ measurements of carbon monoxide (CO), black carbon (BC) and aerosol size distributions. These observations were studied in synergy with those of the CALIOP spaceborne lidar and the MODIS and IASI missions. The altitude range of the aerosol layers and the role of age on the optical properties (optical thickness (AOD532), depolarization, color ratio) are discussed for each type of aerosol. The results of a flight over the gas extraction regions corresponded to the highest AOD532 and higher BC concentrations than the emissions from urban areas and allowed an estimation of the lidar ratio of these aerosol plumes poorly documented in the literature. The second part of the work consisted in proposing an alternative to the indirect restitution of the AOD532 by the CALIOP instrument from the inversion of the attenuated back-scattering lidar signal. This method uses the surface reflectance of the CALIOP lidar signal and has already been used over oceans or optically opaque liquid water clouds to calculate an AOD value. In this work, we have thus developed and evaluated an AOD restitution from the CALIOP surface reflectance for continental areas. Two methodologies were used to determine the surface lidar reflectance not attenuated by aerosols: (i) selection of CALIOP observations under clear sky conditions over 7 years of observation (ii) extrapolation of the linearity relationship between attenuated surface lidar reflectance and atmospheric transmission. If these two methods give good results in areas of low surface lidar reflectance (< 0.75sr-1), the first method is not usable in desert areas. The use of these LIDAR AOD measured directly over continental surfaces improves the bias (|ME| < 0.034) and dispersion (< 0.145) compared to MODIS observations. This greatly improves the results of the CALIOP-MODIS comparisons obtained with the indirect restitution of the AODs an analysis of the vertical profiles of attenuated lidar backscatter with a bias < 0.174 and dispersion < 0.234

The spectral reflectance, namely the fraction of the incident radiation flux reflected at a given wavelength, is an intrinsic characteristic of each natural or artificial surface. It seemed therefore interesting to investigate the potential of multiwavelength lidars in target identification by means of a spectral reflectance analysis; because of its wavelength agility the CO2 laser is a very attractive one for this kind of analysis. The aim of this paper is to present the results of spectral reflectance measurements of relevant target surface materials at the CO2 laser wavelengths in order to forecast the performances of a differential reflectance lidar.