Surface Bidirectional Reflectance Factor Research Articles

Joint retrieval of surface reflectance and aerosol optical depth from MSG/SEVIRI observations with an optimal estimation approach: 2. Implementation and evaluation

An original method, based on optimal estimation, was presented in a part one of this paper for the joint retrieval of the mean daily total column aerosol optical depth and the surface Bidirectional Reflectance Factor (BRF) from the daily accumulated Meteosat Second Generation–Spinning Enhanced Visible and Infrared Imager (MSG/SEVIRI) observations in the solar channels. The objective of this paper is to evaluate the benefits of the proposed approach and to document the limits of the algorithm assumptions in the context of its implementation in an operational ground segment. A twofold approach is followed. In a first step, by looking at the posterior correlation error matrix the capability of the so‐called Land Daily Aerosol (LDA) algorithm to decouple the surface‐atmosphere signal is analyzed. In particular, the impact of the prior information is investigated in detail. In a second step, the results of the algorithm are compared with independent data sets of aerosol optical depth and surface reflectance. In this phase, the accuracy of the algorithm is evaluated against ground observations from the AERONET network. LDA is shown to be in good agreement with these data, especially when the prior update mechanism is activated. Comparisons with the MODIS surface product showed that the bihemispherical reflectance derived from the LDA products is consistent with the equivalent MODIS white‐sky albedo. Aerosol spatial distributions are comparable in terms of geographical location and intensity, in particular for aerosol episodes with a limited daily variation.

Características espectrais da soja ao longo do ciclo vegetativo com imagens landsat 5/TM em área agrícola no oeste do Paraná

O objetivo deste trabalho foi estudar as mudanças no comportamento espectral da cultura da soja, por meio dos perfis espectrais temporais dos índices de vegetação NDVI e GVI, expressos em diferentes valores físicos: fator de reflectância bidirecional (FRB) aparente, de superfície e normalizado derivados de imagens Landsat 5/TM. Foi monitorada área de cultura de soja localizada próxima ao município de Cascavel - PR, utilizando cinco imagens da safra de 2004/2005, sendo realizados nessas imagens os procedimentos de transformação radiométrica, correção atmosférica e normalização, determinando valores físicos dos fatores de reflectância bidirecional aparente, de superfície e normalizado, respectivamente. Com o intuito de caracterizar a resposta espectral da biomassa da soja, geraram-se imagens referentes aos índices de vegetação NDVI e GVI. Como resultado, a cultura mostrou-se diferente para os tratamentos dos fatores de reflectância bidirecional aparente, de superfície e de normalização. Por meio dos perfis médios espectrais do NDVI e GVI, foi possível acompanhar todo o ciclo da cultura da soja, caracterizando o seu desenvolvimento. Observou-se, ainda, que os dados provenientes do fator de reflectância bidirecional normalizado descaracterizaram a curva espectral da cultura da soja, principalmente em meio à fase de crescimento vegetativo, na data de 9-12-2004.

Polarised Multiangular Reflectance Measurements Using the Finnish Geodetic Institute Field Goniospectrometer

The design, operation, and properties of the Finnish Geodetic Institute Field Goniospectrometer (FIGIFIGO) are presented. FIGIFIGO is a portable instrument for the measurement of surface Bidirectional Reflectance Factor (BRF) for samples with diameters of 10 – 50 cm. A set of polarising optics enable the measurement of linearly polarised BRF over the full solar spectrum (350 – 2,500 nm). FIGIFIGO is designed mainly for field operation using sunlight, but operation in a laboratory environment is also possible. The acquired BRF have an accuracy of 1 – 5% depending on wavelength, sample properties, and measurement conditions. The angles are registered at accuracies better than 2°. During 2004 – 2008, FIGIFIGO has been used in the measurement of over 150 samples, all around northern Europe. The samples concentrate mostly on boreal forest understorey, snow, urban surfaces, and reflectance calibration surfaces.

Conversão de dados radiométricos orbitais por diferentes metodologias de caracterização atmosférica

A conversão de números digitais (NDs) de imagens orbitais em quantidades físicas relacionadas às propriedades espectrais de objetos presentes na superfície terrestre é dependente de fiel caracterização atmosférica, a qual tem sido realizada com medidas obtidas por balões meteorológicos ou por fotômetros solares posicionados em superfície. Recentemente, alguns sensores orbitais vêm produzindo dados que também permitem caracterizar a atmosfera, como os fornecidos pelo sensor MODIS. Neste artigo, valores de Fator de Reflectância Bidirecional de superfície (FRBs) determinados em campo foram comparados com aqueles calculados a partir de imagens TM/Landsat-5 através de modelos de transferência radiativa (MTRs), sendo que os dados de entrada destes modelos referentes à caracterização da atmosfera foram estimados a partir do emprego de um fotômetro solar CIMEL e de produtos atmosféricos MODIS. Os resultados demonstraram que os dados MODIS são aplicáveis na conversão de dados orbitais para valores de FRBs.

Analysis of multi-date MISR measurements for forest and woodland communities, Queensland, Australia

Many theoretical and applied studies have demonstrated that the anisotropic reflectance of the land surface is determined by the structural and optical properties of the land surface. The Multi-angle Imaging SpectroRadiometer (MISR) measures the anisotropic reflectance of the land surface thus has the potential to improve the operational mapping of canopy attributes in Queensland, one of the largest states in Australia. The aim of this study was to assess the relationship between foliage projective cover (FPC) and the shape of the bidirectional reflectance distribution function (BRDF) for a mixed grassland/woodland ecosystem, the Southern Brigalow Belt Biogeographic Region. A practical approach for deriving the surface bidirectional reflectance factor (BRF) from MISR “Local" mode data using existing MISR products is presented. The BRDF typology of the land surface was determined using shape parameters derived from inversion of the linear Ross–Thick Li–Sparse Reciprocal and the non-linear Rahman–Pinty–Verstraete (RPV) models against a time series of MISR “Local" mode surface BRF data. Following an evaluation of the models inversion error, the RPV model was examined for correlation with FPC, a canopy attribute routinely derived from Landsat data in Queensland. Our empirical analyses showed the MISR derived RPV model parameters were qualitatively related to spatial and temporal variations in vegetation structure in Queensland and the k parameter contained information independent of FPC. These results are consistent with published findings from other regions.

Application to MISR land products of an RPV model inversion package using adjoint and Hessian codes

The capability of the non-linear Rahman–Pinty–Verstraete (RPV) model to 1) accurately fit a large variety of Bidirectional Reflectance Factor (BRF) fields and 2) return parameter values of interest for land surface applications motivate the development of a computer efficient inversion package. The present paper describes such a package based on the 3 and 4 parameter versions of the RPV model. This software environment implements the adjoint code, generated using automatic differentiation techniques, of the cost function. This cost function itself balances two main contributions reflecting 1) the a priori knowledge on the model parameter values and, 2) BRF uncertainties together with the requirement to minimize the mismatch between the measurements and the RPV simulations. The individual weights of these contributions is specified notably via covariance matrices of the uncertainties in the a priori knowledge on the model parameters and the observations. This package also reports on the probability density functions of the retrieved model parameter values that thus permit the user to evaluate the a posteriori uncertainties on these retrievals. This is achieved by evaluating the Hessian of the cost function at its minimum. Results from a variety of tests are shown in order to document and analyze software performance against complex synthetic BRF fields simulated by radiation transfer models as well as against actual MISR-derived surface BRF products.

Caracterização espectral de áreas de gramíneas forrageiras infectadas com a doença "mela-das-sementes da braquiária" por meio de imagens CCD/CBERS-2

Imagens CCD/CBERS-2, nas bandas espectrais CCD2, CCD3 e CCD4, dos anos de 2004 e 2005, de Mirante do Paranapanema - SP, foram transformadas em reflectância de superfície usando o modelo 5S de correção atmosférica e normalizadas radiometricamente. O objetivo principal foi caracterizar espectralmente áreas de pastagens de Brachiaria brizantha em fase de florescimento, isentas e infectadas com a doença "mela-das-sementes da braquiária", possibilitando a sua detecção por meio da comparação entre os valores de reflectância de superfície denominada de Fator de Reflectância Bidirecional de Superfície (FRBS). Teve-se, também, o objetivo de avaliar a eficácia das imagens CCD/CBERS-2 para a obtenção de respostas espectrais de pastagens. Os dosséis sadios e doentes da Brachiaria brizantha foram identificados por meio da análise dos valores de reflectância e dos dados observados no Índice de Estresse Hídrico Acumulativo Relativo da Cultura (ACWSI) obtidos na área de estudo. Os resultados indicaram que as principais diferenças foram a diminuição da reflectância na banda CCD3 e o aumento da reflectância na banda CCD4 nas áreas doentes. A metodologia empregada com o uso de dados do sensor CCD/CBERS-2, associados ao ACWSI, mostrou-se eficaz para discriminar dosséis infectados com a "mela-das-sementes da braquiária".

Local analysis of MISR surface BRF and albedo over GSFC and mongu AERONET sites

We have developed an atmospheric correction algorithm to retrieve the surface bidirectional reflectance factor (BRF) and albedo from Multiangle Imaging SpectroRadiometer (MISR) measurements for small areas around Aerosol Robotic Network (AERONET) sunphotometer sites, using AERONET aerosol and column water vapor information. Our goal is to develop an indirect validation method for MISR surface reflectance products over heterogeneous land. Our algorithm makes independent retrievals with both the Li Sparse-Ross Thick kernel BRF model and the modified Rahman-Pinty-Verstraete BRF model used in the Moderate Resolution Imaging Spectroradiometer and MISR land algorithms, respectively. In this study, we report the first results of processing MISR Collection 4 data for 2003-2004 for two sites, Mongu, Zambia, and Greenbelt, MD. We found that MISR generally provides accurate retrievals of BRF and albedo in both clear and hazy atmospheric conditions, correctly reproducing the parameter time series and spatial distribution. We found that the MISR BRF, on average, is less anisotropic in the visible bands. The difference is greatest in the blue band, but decreases with increasing wavelength such that it is negligible in the near-IR band. This discrepancy originates in part in the MISR aerosol retrieval algorithm over heterogeneous land, which tends to select an aerosol model that favors spectrally invariant shapes of surface BRF. The other part of the discrepancy comes from the surface hemispherical-directional reflectance factor retrieval algorithm where the iteration loop that removes the diffuse atmospheric transmittance is currently turned off. Our initial results suggest that the MISR surface albedo is on average lower than our retrievals by about 0.005 in the green and red bands. In the near-IR, it agreed with our retrievals with the modified Rahman-Pinty-Verstraete model for the Mongu site, but was systematically lower over the Greenbelt site by about 0.016. When significant aerosol absorption is present (Mongu), the albedo discrepancy is additionally biased by the difference between the MISR and AERONET retrievals of aerosol absorption

Parameterized code SHARM-3D for radiative transfer over inhomogeneous surfaces

The code SHARM-3D, developed for fast and accurate simulations of the monochromatic radiance at the top of the atmosphere over spatially variable surfaces with Lambertian or anisotropic reflectance, is described. The atmosphere is assumed to be laterally uniform across the image and to consist of two layers with aerosols contained in the bottom layer. The SHARM-3D code performs simultaneous calculations for all specified incidence-view geometries and multiple wavelengths in one run. The numerical efficiency of the current version of code is close to its potential limit and is achieved by means of two innovations. The first is the development of a comprehensive precomputed lookup table of the three-dimensional atmospheric optical transfer function for various atmospheric conditions. The second is the use of a linear kernel model of the land surface bidirectional reflectance factor (BRF) in our algorithm that has led to a fully parameterized solution in terms of the surface BRF parameters. The code is also able to model inland lakes and rivers. The water pixels are described with the Nakajima-Tanaka BRF model of wind-roughened water surface with a Lambertian offset, which is designed to model approximately the reflectance of suspended matter and of a shallow lake or river bottom.

Modelling and measuring the spectral bidirectional reflectance factor of snow‐covered sea ice: an intercomparison study

AbstractBroadband albedo is a very important geophysical parameter in the Earth surface–atmosphere interaction in either global climate change or hydrological cycle and snowmelt runoff studies. To derive the broadband albedo accurately from satellite optical sensor observation at limited bands and at a single observation angle, the bidirectional reflectance factor (BRF) has to be specified quantitatively. In the present albedo derivation algorithms from the satellite radiance data, the BRF is either modelled or observed. Questions may arise as to how well a BRF model can be in the broadband albedo derivation. To help answer such questions, we studied the performance of a snow‐surface BRF model for two specific cases under large solar zenith angles (65° and 85°). We measured snow‐surface spectral directional reflectance under clear skies. The snow physical properties, such as snow grain size and snow density, at the same sites were also measured. In situ snow physical data are used to simulate the snow‐surface BRF and hemispherical directional reflectance factor (HDRF) through a multilayered azimuth‐ and zenith‐dependent plane‐parallel radiative transfer model. The field measurements and BRF and HDRF simulations all reveal the forward‐scattering nature of snow surface under large solar incidence angles, but the BRF model results depict the strongest forward‐scattering patterns under such solar zenith angles. Because the HDRF is simulated through coupling of the surface BRF with radiative transfer in the atmosphere, the resulting HDRF patterns agree with the field measurements better than the simulated BRF does. The deviation of the simulated HDRF from field‐based clear‐sky directional reflectance (FCDR) is within ±10% for the central (viewing zenith angle <45° ) and lateral sides of the viewing hemisphere. This level of agreement between the simulated HDRF and FCDR also implies that the simulated BRF model can provide remote‐sensing estimates of spectral albedo with an uncertainty of ±10% for the same part of the viewing hemisphere. Further improvement in BRF model performance requires better handling of single scattering properties of snow grains, surface roughness, and atmospheric correction. Also, better procedures and techniques in field measurement are necessary for more accurate assessment of the performance of BRF models. Copyright © 2004 John Wiley & Sons, Ltd.

Using angular and spectral shape similarity constraints to improve MISR aerosol and surface retrievals over land

The Multi-angle Imaging SpectroRadiometer (MISR) instrument on the Terra satellite has demonstrated the capability to retrieve aerosol optical depths, surface bidirectional reflectance factors, and hemispherical reflectances over a wide variety of land surface types. In particular, its multiangular imaging design has enabled the application of algorithms that minimize sensitivity of the aerosol retrievals to the brightness of the underlying surface. The novel aerosol algorithm that was developed prior to launch has had notable quantitative success. Over certain scene types, however, the approach contained obvious spatial artifacts, so a postlaunch refinement to the algorithm was implemented. It constrains the retrieved aerosol models and optical depths such that the implied angular shape of the surface hemispherical–directional reflectance factor (HDRF) is similar among all of the MISR wavelengths. This upgrade has resulted in three tangible benefits: (1) the occurrence of outliers has been dramatically reduced, (2) correlations with AERosol RObotic NETwork (AERONET) aerosol sunphotometer data are quantitatively improved, and (3) the quality of surface products is markedly enhanced. MISR Level 2 aerosol and surface products are archived at the NASA Langley Atmospheric Sciences Data Center. Those having version numbers v0012 and higher incorporate this upgrade in the data processing software.

Improving MODIS surface BRDF/Albedo retrieval with MISR multiangle observations

We explore a synergistic approach to use the complementary angular samplings from the Multi-angle Imaging SpectroRadiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) to improve MODIS surface bidirectional reflectance distribution function (BRDF) and albedo retrieval. Preliminary case studies show that MODIS and MISR surface bidirectional reflectance factors (BRFs) are generally comparable in the green, red, and near infrared. An information index is introduced to characterize the information content of directional samplings, and it is found that MISR angular observations can bring additional information to the MODIS retrieval, especially when the MISR observations are close to the principal plane. We use the BRDF parameters derived from the MISR surface BRFs as a priori information and derive a posteriori estimates of surface BRDF parameters with the MODIS observations. Results show that adding MISR angular samplings can reduce the relative BRF prediction error by up to 10% in the red and green, compared to the retrievals from MODIS-only observations which are close to the cross-principal plane.

Ground measurements of surface BRF and HDRF using PARABOLA III

The ground‐based Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA), version 3, provides multiangle measurements of sky and ground radiances on a spherical grid of 5° in the zenith‐to‐nadir and azimuthal planes in eight spectral channels. The hemispherical directional reflectance factor (HDRF) can be measured directly by comparing the radiance reflected by the surface in given direction to that reflected from a reference surface simultaneously observed by the PARABOLA 3. The surface bidirectional reflectance factor (BRF) cannot be measured directly, however, because of the presence of the sky diffuse illumination. The contribution of the diffuse sky radiance to the radiance reflected by the natural target surface is computed, and removed, using an iterative technique. Two approaches are employed: the first requires knowledge of the atmospheric optical depth, and the second requires the simultaneous measurements of the radiance reflected by a standard surface panel under the same atmospheric and illumination conditions. Ground measurements of the BRF and HDRF for dry lake surfaces were obtained from the PARABOLA 3 observations with better than ±10% accuracy. The results described in this work are used primarily for the vicarious calibration of the Multiangle Imaging Spectroradiometer (MISR) onboard the Earth Observing System (EOS) Terra platform and for validation of MISR BRF retrievals of selected Earth surface targets.

PARABOLA III: A sphere‐scanning radiometer for field determination of surface anisotropic reflectance functions

The Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere III (PARABOLA III) is a sphere‐scanning radiometer. The original PARABOLA was built to study the relationship between surface morphology and reflected radiation properties. Follow‐on work led to the design of an improved radiometer, the PARABOLA III. This in‐situ sensor will be used to validate surface reflectances at angles measured by the Multi‐angle Imaging SpectroRadiometer (MISR), a global imager flown on the Earth Observing System (EOS)‐Terra orbital spacecraft. Derived PARABOLA III data products include the surface bidirectional reflectance factor, and sky and surface radiances for the upward and downward viewing hemispheres. This paper describes the design, calibration, and operation of the JPL PARABOLA III.

Validation of a DDV‐based aerosol optical depth retrieval algorithm using multialtitude spectral imagery

One of the classical weaknesses of remote sensing algorithms is their statistically limited validation data sets. Validation procedures are generally carried out over sparse ground‐based data sets which are orders of magnitude smaller in number than the image products they are meant to validate and often of a physical scale that is inconsistent with the footprint scale of a single pixel. The multialtitude regression methodology seeks to address this problem by exploiting the flexibility and programmability of an airborne sensor whose potential for acquiring algorithm test data is much more commensurate with the validation needs of high altitude or satellite sensors. The multialtitude regression procedure was employed to validate a single‐altitude aerosol optical depth (AOD) inversion algorithm which uses an atmospherically resistant vegetation index criterion to select dense dark vegetation (DDV) pixels in a boreal forest image acquired by the CASI (Compact Airborne Spectrographic Imager) imaging spectrometer. The multialtitude regression procedure permits the extraction of AOD images that are reasonably independent of the surface bidirectional reflectance factor (BRF) required as input to the DDV‐based AOD inversion algorithm. This independence of surface BRF is a fundamental requirement for the validation of DDV‐based algorithm since the basic weakness of this algorithm is its sensitivity to surface BRF variations. The results indicate that the multialtitude regression procedure is an effective tool for validating DDV inversion algorithms.

An approximation to improve accuracy in the derivation of surface reflectances from multi‐look satellite radiometers

Many applications of Earth‐observing satellite radiometer measurements in the visible and near infra‐red require surface reflectances, necessitating the use of atmospheric correction algorithms. Performing these corrections usually requires the assumption of Lambertian surfaces, a limitation that becomes serious with the new generation of multi‐look satellite radiometers. In this letter, we present and test theoretically a multiple‐channel approximation that does not assume a Lambertian surface, but instead assumes that the wavelength dependence of the shape of the surface bidirectional reflectance factor distribution function is negligible in comparison with the wavelength dependence of the surface and atmospheric scattering properties. The approximation is characterised by a mean ratio, k, that may be retrieved by a suitable atmospheric correction process along with surface reflectances and the important atmospheric parameters, without assuming a canopy or soil model or using “dark” pixels to obtain the atmospheric optical thickness. We test the approximation with a canopy model that includes the hot‐spot effect, and, when using ATSR‐2 configurations, it yields at best an order‐of‐magnitude reduction in the error of the surface reflectance (at the surface) over the Lambertian assumption, and in more stringent conditions, a factor of three improvement. We indicate how the approximation might be implemented in an atmospheric correction algorithm.

Retrieval of surface directional reflectance properties using ground level multiangle measurements

Knowledge of the directional reflectance properties of natural surfaces such as soils and vegetation canopies is essential for classification studies and canopy model inversion. Atmospheric correction schemes, using various levels of approximation, are described to retrieve surface bidirectional reflectance factors (BRFs) and directionalhemispherical reflectances (albedos) from multiangle radiance measurements taken at ground level. The retrieval schemes are tested on simulated data incorporating realistic surface BRFs and atmospheric models containing aerosols. Sensitivity of the atmospherically corrected BRFs and associated directional-hemispherical reflectances to various aerosol properties and the sun-view geometry is illustrated. A measurement strategy for obtaining highly accurate surface reflectance properties also is examined in the context of instrument radiometric calibration, knowledge of the atmospheric properties, and sun-view angular coverage.

Atmospheric effects in the remote sensing of surface albedo and radiation absorption by vegetation canopies

Abstract A one‐dimensional turbid medium model of a vegetation canopy that includes specular reflection and the hot spot effect is used to calculate surface bidirectional reflectance factors (BRFs), albedo, absorbed solar and photosynthetically active radiation and, canopy photosynthetic efficiency. Simulated surface BRF distribution is then used as the lower boundary condition in the atmospheric radiative transfer problem. A horizontally homogeneous cloudless midlatitude continental atmosphere with both molecular and aerosol loading is assumed throughout. The dynamics of surface and planetary albedo are examined for varying surface and atmospheric parameters. The relationship between planetary and surface albedo and its sensitivity to problem parameters is investigated. An algorithm for the estimation of surface albedo is outlined and the magnitude of errors incurred is discussed. An effort is made to relate canopy radiation absorption to top‐of‐the‐atmosphere spectral vegetation indices and the sensitiv...

Influence of atmospheric aerosols and desert reflectance properties on satellite radiance measurements

The influence of surface bidirectional reflectance factors, including shadowing, and of atmospheric aerosol variability are modeled for their effects on the remote sensing of desert targets from space in the 0.7-micron region at high spatial resolution. The white sand reflectance data of Salomonson (1968) are used as the basis for the simulation. The effects of the surface bi-directional reflectance and atmospheric aerosol on the nadir-normalized reflectance measured at the satellite are discussed individually and jointly. The net influence of these two factors is shown to depend on the magnitude of other parameters, such as the surface reflectance and solar zenith angle.