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
This paper presents a validation case study of Multi-angle Imaging SpectroRadiometer (MISR) surface products where its bidirectional reflectance (BRF) measurements during the Southern Africa Regional ...Science Initiative (SAFARI 2000) campaign are compared with those coincidently evaluated on the ground and from the air, using the Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA) and Cloud Absorption Radiometer observations, respectively. The presence of haze and smoke during the campaign provided a case study to evaluate the effect of atmospheric correction on MISR surface products. Two surface types were considered in the analyses: the bright desert-like surface of the Pan and the dark grassland that surrounds it. The results show that for the dark surface the BRF values retrieved from MISR are in good agreement, within 5%, with those obtained from field data. For the bright desert-like pan surface, better agreement, within ~10%, was found in all channels on the clear day but only in the forward scattering on the hazy day. A comparison of MISR aerosol retrievals to those obtained from three independent ground measurements suggests that, in the presence of a highly reflective surface, small uncertainties in the MISR aerosol retrievals become magnified at larger optical depths, causing errors in the surface BRF retrievals
A recent paper, J.V. Martonchik et al., IEEE Trans. Geosci. Remote Sensing, vol.36, p.1212-27 (1998), compared approximate radiative transfer results for top-of-atmosphere (TOA) radiance, using ...various algorithms published in the literature. We show that one of the algorithms was used incorrectly, resulting in its poor performance as stated in that paper. Correct usage produces results with errors typically less than 3%, which compares favorably to the other tested algorithms.
The first generic and climatological aerosol retrievals using AirMISR data are presented. Multiangle observations at 672 and 867 nm, in a cloud‐free region over dark water in Monterey Bay on June 29, ...1999, yield complementary generic and climatological results. The generic retrieval produces cross‐section‐weighted, column‐mean aerosol properties: midvisible aerosol optical depth between 0.05 and 0.10, with a preference for values on the low side of the range, particle number‐mean characteristic radius between 0.25 and 0.45 μm, and imaginary index of refraction <0.004, with 0.0 as the most likely value. These properties correspond to a “medium‐to‐large, spherical” column‐average particle. The climatological retrieval identifies a maritime air mass, having a total aerosol optical depth about 0.1, and mixing ratio for sea‐salt particles (large, spherical) of 50%, based on optical depth in MISR Band 2, and 40% for the sulfate plus carbonaceous (medium, spherical) components, to an accuracy of about ±15%. These results are in good agreement with the limited nearby surface‐based and aircraft observations available. The analysis also shows that over dark water, pixel‐to‐pixel scene variability can contribute more to the retrieval uncertainty than camera calibration and that high spatial variance of the reflectance, in addition to geometric considerations, is a better indicator of Sun glint contamination than geometry alone. This work represents an early step toward the goal of using MISR multiangle data to add spatial detail and information about temporal variability to the global aerosol climatology.
The multi-angle imaging spectroradiometer (MISR) instrument is designed to provide global imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. The MISR standard ...products include vegetation canopy green leaf area index (LAI) and fraction of photosynthetically active radiation absorbed by vegetation (FPAR). These products are produced using a peer-reviewed algorithm documented in the EOS-AM1 (Terra) special issue of the Journal of Geophysical Research. This paper presents results on spatial distributions of LAI and FPAR of vegetated land surfaces derived from the MISR LAI/FPAR algorithm with bidirectional reflectance data from the polarization and directionality of the Earth's reflectance (POLDER) instrument over Africa. The results indicate that the proposed algorithm reflects the physical relationships between surface reflectances and biophysical parameters and demonstrates the advantages of using multiangle data instead of single-angle data. A new method for evaluating bihemispherical reflectance (BHR) from multi-angle measurements of hemispherical directional reflectance factor (HDRF) was developed to prototype the algorithm with POLDER data. The accuracy of BHR evaluation and LAI/FPAR estimation is also presented. To authors demonstrate the advantages of using multi-angle data over single-angle data of surface reflectance.
