Sampling the surface bidirectional reflectance distribution function (BRDF): 1. Evaluation of current and future satellite sensors

The characterization and temporal analysis of multispectral and hyperspectral data to extract the biophysical information of the Earth's surface can be significantly improved by understanding its aniosotropic reflectance properties, which are best described by a Bi-directional Reflectance Distribution Function (BRDF). The advancements in the field of remote sensing techniques and instrumentation have made hyperspectral BRDF measurements in the field possible using sophisticated goniometers. However, natural surfaces such as forest canopies impose limitations on both the data collection techniques, as well as, the range of illumination angles that can be collected from the field. These limitations can be mitigated by measuring BRDF in a virtual environment. This paper presents an approach to model the spectral BRDF of a forest canopy using the Digital Image and Remote Sensing Image Generation (DIRSIG) model. A synthetic forest canopy scene is constructed by modeling the 3D geometries of different tree species using OnyxTree software. The field collected spectra from the Harvard forest is used to represent the optical properties of the tree elements. The canopy radiative transfer is estimated using the DIRSIG model for specific view and illumination angles to generate BRDF measurements. A full hemispherical BRDF is generated by fitting the measured BRDF to a semi-empirical BRDF model. The results from fitting the model to the measurement indicates a root mean square error of less than 5% (2 reflectance units) relative to the forest's reflectance in the VIS-NIR-SWIR region. The process can be easily extended to generate a spectral BRDF library for various biomes.

Information about the surface bi-directional reflectance distribution function (BRDF) and albedo is required as a boundary condition for radiative transfer modeling, aerosol retrievals, cloud retrievals, and atmospheric modeling. The typical spatial resolution provided by MODIS and MISR standard surface products (~1km) is insufficient to measure the BRDF of the pure surface types, because most pixels at this scale correspond to mixed classes. We present an approach for the retrieval of the basic surface BRDFs from the observations of MODIS/Terra and MISR using an angular unmixing method. Our analysis is focused on the Atmospheric Radiation Measurement (ARM) Program area in the Southern Great Planes (SGP) region, which is a predominantly agricultural area with a few major crop types. Pure surface classes were identified using high-resolution (30m) Landsat imagery and results of a ground survey. Assuming that the reflectance for each coarse pixel is a linear superposition of reflectances of basic surface types, it is possible to estimate the original BRDF parameters for each landcover type. In our case, three dominant classes were selected: wheat, grass, and baresoil. In the case of wheat and grass, the dispersion of the results is smaller than in the case of soil. This can be explained by the relatively low fractional coverage of the soil class within large pixels and by the significant variability of soil reflectance depending on wetness, soil type (sand, clay, etc.), and other factors. The correlation between the BRDF shape factors and the normalized difference vegetation index (NDVI) has also been analyzed. There is a high degree of correlation between the NDVI and BRDF isotropic factor (r0 in the case of MISR), while the correlation with other BRDF parameters was found to be smaller. In general, the NDVI can be used as a crude proxy for the BRDF shape.

Abstract. Surface albedo is a dominant factor affecting the earth energy balance. Bidirectional reflectance distribution function (BRDF) describes the anisotropic surface reflection of solar radiation. Bidirectional effect is especially noticeable in the forest canopies with three-dimensional canopy structure. Thus, it is challenging to evaluate the canopy structural effects on BRDF using traditional radiative transfer modelling methods, where leaf in canopies is generally assumed to be homogeneous. LESS, a ray-tracing-based three-dimensional (3D) radiative transfer model can effectively simulate bidirectional reflectance factor while fully considering multi-component spectral and structural characteristics of vegetation. In this study, we applied the LESS model to evaluate the impacts of changes in leaf area index (LAI) on canopy reflectance in a Picea crassifolia forest. First, canopy structural parameters obtained from forest inventory were used to construct forest scenes and drive the LESS model. Then, we validated the simulation accuracy of the LESS model against BRDF calculated from MODIS BRDF parameter product using kernel-driven Ross-Li bi-directional reflectance function. Finally, we constructed forest scenes with different LAI to investigate the canopy structural effects on BRDF. We found that the BRDF simulated by the LESS model is in good accordance with the MODIS BRDF data in the red (R2 = 0.97, RMSE = 0.03) and NIR (R2 = 0.94, RMSE = 0.05) bands. The simulated canopy reflectance in the red band displayed ‘dome’ shape and decreased with increasing LAI. In contrast, canopy reflectance in the NIR band displayed ‘bowl’ shape and increased with increasing LAI. Our study highlighted the sensitivity of canopy BRDF to changes in canopy structure and have implications for retrieving LAI from BRDF data.

The effects of topography on both the bidirectional reflectance distribution function (BRDF) and the hemispherical reflectance (surface albedo) of a forested scene are investigated with the Li-Strahler geometric-optical model. The Li-Strahler geometric-optical model treats a vegetation canopy as an assemblage of partially illuminated tree crowns of spheroidal shape, and through geometric optics and Boolean set theory, models the proportion of sunlit or shadowed canopy and background as functions of view angle, illumination angle, and crown geometry. The model has been modified to accommodate a sloping surface in its computation of bidirectional and hemispherical reflectance. When the BRDF of a flat vegetated surface is compared to the BRDF of a sloping surface that is similarly vegetated, the interaction of the illumination angle and the slope distort the shape of the BRDF. A hemispherical integration of this distorted BRDF provides an albedo for the sloping surface. >

The evaluation of the first available satellite‐based global albedo product at 1‐km resolution is essential for its application in climate studies. We evaluate the accuracy of the Moderate‐Resolution Imaging Spectroradiometer (MODIS) albedo product using available field measurements at Surface Radiation Budget Network (SURFRAD) and Cloud and Radiation Testbed–Southern Great Plains (CART/SGP) stations and examine the consistency between the MODIS surface albedos and the Clouds and Earth's Radiant Energy System (CERES) top‐of‐the‐atmosphere albedos as well as historical global albedos from advanced very high resolution radiometer (AVHRR) and Earth Radiation Budget Experiment (ERBE) observations. A comparison with the field measurements shows that the MODIS surface albedo generally meets an absolute accuracy requirement of 0.02 for our study sites during April–September 2001, with the root mean square errors less than 0.018. Larger differences appear in the winter season probably due to the increased heterogeneity of surface reflectivity in the presence of snow. To examine the effect of spatial heterogeneity on the validation of the MODIS albedos using fine resolution field measurements, we derive an intermediate albedo product from four Landsat Enhanced Thematic Mapper Plus (ETM+) images at 30‐m spatial resolution as a surrogate for the distributed field measurements. The surface albedo is relatively homogeneous over the study stations in growing seasons, and therefore the validation during April–September is supported. A case study over three SURFRAD stations reveals that the MODIS bidirectional reflectance distribution function model is able to capture the solar zenith angle dependence of surface albedo as shown by the field measurements. We also find that the MODIS surface shortwave albedo is consistent with the contemporary and collocated CERES top‐of atmosphere albedos derived directly from broadband observations. The MODIS albedo is also well correlated with historical surface albedos derived from AVHRR and ERBE observations, and a high bias of 0.016 and a low bias of 0.034 compared to those of the latter albedos are reasonable considering the differences in instruments and retrieval algorithms as well as environmental changes.

Assessing the coupling between surface albedo derived from MODIS and the fraction of diffuse skylight over spatially-characterized landscapes

Bi-directional Reflectance Distribution Function (BRDF) measurements that characterise the scattering properties of surfaces are extremely important to the design of optical instruments. Sophisticated codes require the characterisation of the scattering function of optically black and/or differently machined surfaces at several angle of incidence in a wide range of detector angles. These data can be used to develop processes to achieve desired BRDF patterns or to improve stray light suppression techniques. In this paper, we present the characteristics of an automatic, three dimensional, BRDF measurement set up that has been designed and developed at Galileo Avionica by the authors and the most significant experimental results of 3D BRDF data achieved on samples of aluminium alloys, gold and titanium, differently machined and/or black painted. The measurements have been taken at different illumination angles (from the normal incidence up to 45 degrees) at the wavelength of 650 nm. The wide variety of possible applications that can be explored through the proposed equipment, jointed to its flexibility, constitute a reference point for future investigations on the characterisation of the scattering properties of machined materials and paintings.

Generalizing the definition of the bi-directional reflectance distribution function

EOS-AM1 is the first component of NASA's Earth Observing System (EOS). As centerpiece to Mission to Planet Earth, EOS will provide satellite observations to determine the extent, causes, and regional consequences of global climate change. EOS-AM1 is intended to obtain information about the physical and radiative properties of clouds; air-land and air-sea exchanges of energy, carbon, and water; measurements of important trace gases in the atmosphere; and volcanology. It carries five advanced instruments: advanced spaceborne thermal emission and reflection radiometer (ASTER) provided by the Ministry of International Trade and Industry of Japan, Clouds and Earth's Radiant Energy System provided by NASA's Langley Research Center, Multi-angle Imaging SpectroRadiometer provided by the Jet Propulsion Laboratory, Moderate Resolution Imaging SpectroRadiometer provided by NASA's Goddard Space Flight Center, and Measurements of Pollution in the Troposphere provided by the Canadian Space Agency. The project is currently in its D Phase and is maintaining schedule for a June 1998 launch. Fabrication of Flight Model hardware is being completed and integration and subsystems testing is underway. During the next six months, all instruments will be delivered to the spacecraft contractor for integration with the spacecraft bus. System- level compatibility, performance, and environmental testing will follow. The ambitious science objectives, associated data quality and instrument/spacecraft technology considerations, and the current development status will be discussed. The EOS-AM1 project is managed by Goddard Space Flight Center.

Surface bi-directional reflectance distribution function (BRDF) and albedo properties are retrieved over the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) area. A landcover-based fitting approach is employed by using a newly developed landcover classification map and the MODIS 10-day surface reflectance product (MOD09). The surface albedo derived by this method is validated against other satellite systems (e.g. Landsat-7 and MISR) and ground measurements made by an ASD spectroradiometer. Our results show good agreements between the datasets in general. The advantages of this method include the ability to capture rapid changes in surface properties and an improved performance over other methods under a frequent presence of clouds. Results indicate that the developed landcover-based fitting methodology is valuable for generating spatially and temporally complete surface albedo and BRDF maps using MODIS observations.

Land surface broadband albedo is a critical variable for many scientific applications. Due to the scarcity of spectral albedo measurements of the Earth's surface environments, it is useful to construct broadband albedo from spectral albedo data obtained by multi‐angle satellite observations. The Multi‐angle Imaging SpectroRadiometer (MISR) onboard NASA's Earth Observing System (EOS) Terra satellite provides land surface albedo products from multi‐angular observations; however, the products have not been comprehensively validated. We convert MISR spectral albedos to total shortwave albedos and validate them using ground measurements at different validation sites. For most surface types, a published narrowband to broadband conversion formula was used, but a new conversion formula for snow and ice covered sites is developed in this study where the spectral range of the instrument is different. Several comparisons are made: (1) between MISR directional‐hemispherical reflectance (DHR) or albedo and MODIS (Moderate Resolution Imaging Spectroradiometer) DHR; and (2) between MISR spectral DHR and bi‐hemispherical reflectance (BHR). The results show that: (1) both the value and the temporal trends of the MISR shortwave albedo and the ground measured shortwave albedo are in good agreement, with the exception of the snow and ice sites; (2) the MISR DHR conforms well to MODIS DHR; and (3) the values of MISR DHR and BHR are nearly identical.

G-function and HOt SpoT (GHOST) reflectance model: application to multi-scale airborne POLDER measurements

User needs and assessing the impact of low latency NASA Earth observation data availability on societal benefit

Ground processing of data from the Multi-angle Imaging SpectroRadiometer (MISR) instrument, part of NASA's Earth Observing System (EOS), exploits new and unique science algorithms not previously used operationally. A range of data products from Level 1 through Level 3 is being produced. Because of MISR's unprecedented design, extensive prototyping was required from a relatively early stage. The data throughput is large, necessitating an innovative software design approach that maximizes performance. The systematic science processing software was developed at the Jet Propulsion Laboratory, with data processing occurring at the NASA Langley Research Center using the EOS Core System, a collaborative arrangement that works well. With the availability of actual mission data following launch on the Terra spacecraft in December 1999, MISR's computational needs have become better known, and many improvements have been made to both the science software and the production system to achieve a successful overall data processing capability. This paper provides information about MISR data for the science user, and describes the nature and scope of implementation and operations activities.

While NASA's Earth Observing System has an ambitious, even visionary quality, its data system—EOSDIS—seems more often than not cloaked in an air of unreality. “Does anybody really know what the system is?” asked one scientist close to the project. Undoubtedly, a major reason for this impression has been the drawn‐out, 2‐year deliberation over a contractor for the $3 billion data system—a decision that has just been put off again.