Stephen Schiller

Results of the 1996 Earth Observing System vicarious calibration joint campaign at Lunar Lake Playa, Nevada (USA)

A joint campaign was held at Lunar Lake Playa, Nevada (USA) in June 1996 to evaluate the accuracy of reflectance-based, vicarious calibrations of Earth Observing Systems (EOS). Four groups participated in the campaign and made independent measurements of surface reflectance and atmospheric transmittance on five different days. Each group predicted top-of-the-atmosphere radiance for several bands in the 400 nm to 2500 nm spectral range. Analysis of the data showed differences of the order of 5% to 10% throughout the spectral region under study. Further study revealed that the major sources of discrepancy are differences in procedures and assumptions in finding the reflectance of field references used to determine the surface reflectance of the test site. Differences caused by varying radiative transfer codes and aerosol assumptions were found to be a relatively small error source, owing to the high reflectance and low turbidity of the test site. Differences in the solar irradiance values used by separate groups were found to be significant, but can be overcome by agreeing on a standard solar irradiance data set. The results from this campaign were used to plan a follow-up campaign in June 1997 that included developing a set of laboratory measurements to characterize the field radiometers which measure surface reflectance, and obtaining a consistent set of reference-panel reflectance factors. The expectation is that disagreement in absolute radiances at the top of the atmosphere generated by these field methods will be reduced to less than 3% if further cooperative work between groups is carried out to develop approaches which will account better for reference panel calibration, the consistent use of atmospheric characterization and radiative transfer codes.

Preflight and Vicarious Calibration of Artemis

Pre-flight and on-orbit calibration of the spectral imagery acquired with the Advanced Responsive Tactically Effective Military Imaging Spectrometer (ARTEMIS) will make use of solar radiation-based methods. Preflight spectral calibration relies on views of a specially-coated to provide a set of known absorption features. Radiometric calibration is determined from views of a spectrally-flat panel illuminated by the sun. At-aperture radiance from the panel is determined through combined measurements by multi-spectral, well-calibrated transfer radiometers and a hyperspectral, field-portable spectrometer. ARTEMIS will rely on vicarious methods for the long-term evaluation of the sensors calibration. The on-orbit radiometric calibration uses the reflectance-based method to provide at-sensor, hyperspectral radiance for the entire spectral range of ARTEMIS. Vicarious calibration of the spectral response will make use atmospheric absorption features and solar Fraunhofer lines across the solar reflective spectrum. The absolute uncertainty of the solar-based radiometric calibrations is less than 3% in spectral regions not affected by strong absorption.

Intercomparison of Approaches to the Empirical Line Method for Vicarious Hyperspectral Reflectance Calibration

Analysis of visible remote sensing data research requires removing atmospheric effects by conversion from radiance to at-surface reflectance. This conversion can be achieved through theoretical radiative transfer models, which yield good results when well constrained by field observations, although these measurements are often lacking. Additionally, radiative transfer models often perform poorly in marine or lacustrine settings or when complex air masses with variable aerosols are present. The empirical line method (ELM) measures reference targets of known reflectance in the scene. ELM methods require minimal environmental observations and are conceptually simple. However, calibration coefficients are unique to the image containing the reflectance reference. Here we compare the conversion of hyperspectral radiance observations obtained with the NASA Glenn Research Center Hyperspectral Imager to at-surface reflectance factor using two reflectance reference targets. The first target employs spherical convex mirrors, deployed on the water surface to reflect ambient direct solar and hemispherical sky irradiance to the sensor. We calculate the mirror gain using near concurrent at-sensor reflectance, integrated mirror radiance, and in situ water reflectance. The second target is the Lambertian-like blacktop surface at Maumee Bay State Park, Oregon, OH, where reflectance was measured concurrently by a downward looking, spectroradiometer on the ground, the aerial hyperspectral imager and an upward looking spectroradiometer on the aircraft. These methods allows us to produce an independently calibrated at-surface water reflectance spectrum, when atmospheric conditions are consistent. We compare the mirror and blacktop-corrected spectra to the in situ water reflectance, and find good agreement between methods. The blacktop method can be applied to all scenes, while the mirror calibration method, based on direct observation of the light illuminating the scene validates the results. The two methods are complementary and a powerful evaluation of the quality of atmospheric correction over extended areas. We decompose the resulting spectra using varimax-rotated, principal component analysis, yielding information about the underlying color producing agents that contribute to the observed reflectance factor scene, identifying several spectrally and spatially distinct mixtures of algae, cyanobacteria, illite, haematite and goethite. These results have implications for future hyperspectral remote sensing missions, such as PACE, HyspIRI, and GeoCAPE.

Portable Ground-Based Atmospheric Monitoring System (PGAMS) for the calibration and validation of atmospheric correction algorithms applied to satellite images

Detecting changes in the Earths environment using satellite images of ocean and land surfaces must take into account atmospheric effects. As a result, major programs are underway to develop algorithms for image retrieval of atmospheric aerosol properties and atmospheric correction. However, because of the temporal and spatial variability of atmospheric transmittance, it is very difficult to model atmospheric effects and implement models in an operational mode. For this reason, simultaneous in situ ground measurements of atmospheric optical properties are vital to the development of accurate atmospheric correction techniques. Presented in this paper is a spectroradiometer system that provides an optimized set of surface measurements for the calibration and validation of atmospheric correction algorithms. The portable ground-based atmospheric monitoring system (PGAMS) obtains a comprehensive series of in situ irradiance, radiance, and reflectance measurements for the calibration of atmospheric correction algorithms applied to multispectral and hypserspectral images. The observations include: total downwelling irradiance, diffuse sky irradiance, direct solar irradiance, path radiance in the direction of the north celestial poles, path radiance in the direction of the overflying satellite, almucantar scans of path radiance, full sky radiance maps, and surface reflectance. Each of these parameters are recorded over a wavelength range from 350 to 1050 nm in 512 channels. The system is fast, with the potential to acquire the complete set of observations in only 8 to 10 minutes depending on the selected spatial resolution of the sky path radiance measurements.

Experimental and model-based derivation of atmospheric point spread functions

Radiometric error exists for most remote sensing applications in the visible and near-infrared regions due to atmospheric effects which are often characterized by absorption and scattering mechanisms that modify the direct radiance and path radiance components of the total radiance observed by the sensor. However, another source of error exists due to scattering, and often turbulence, that causes loss of contrast between bright and dark targets. The phenomenon, sometimes called the adjacency effect, occurs when light from a bright surface is scattered into the field of view of a darker target, thus causing the brighter target to appear darker while the darker target appears brighter. At a given point in time and space, this effect can be modeled as an atmospheric point spread function (PSF). Much of the work to characterize atmospheric PSF has been done in coastal regions where there is a high aerosol concentration and a well-defined edge (shoreline). The work described attempts to extend these observations to an inland, mid-continent atmosphere. A unique spectral/spatial target was developed and deployed in a grassy area near the Brookings, South Dakota, airport. It consisted of a large tarp, 30 by 60 meters in size, against a uniform background. The tarp was made of nylon and dyed so that it was very bright in the blue region of the spectrum with a reflectance peak of 0.6 at approximately 510 nm. Over the rest of the visible spectrum the reflectance was approximately 0.05, and in the near infrared it again rose to roughly 0.5. The uniform background was recently mowed grass and, therefore, exhibited a typical vegetative spectrum that was quite dark in the blue. Since aerosol scattering is typically greatest in the blue portion of the visible spectrum, this target with its well defined spatial edge and unique spectral signature was assembled in an attempt to optimize detection of atmospheric PSF near 500 nm wavelengths.

Direct measurement of the multiple-scattering of solar radiation

Describes the results of a unique field experiment to evaluate the atmospheric scattering of solar radiation reflected off of a bright ground target using ground-based instruments. The effort is part of an experimental and theoretical program to fully evaluate the atmospheric point spread function. During the boreal summer of 1997, a unique spectral/spatial target was deployed in the middle of a large uniform grassy area near Brookings, South Dakota, airport. The target consisted of a large vinyl covered nylon tarp 30 by 60 metres in size. The vinyl was dyed to produce a surface that was very bright only in the blue region of the visible spectrum.

Evaluation of the aerosol scattering phase function from PGAMS observation of sky path radiance

A knowledge of the spectral and geometrical distribution of sky path radiance at the time a remote sensing image of the Earths surface is acquired can greatly improve the application of atmospheric correction and vicarious calibration techniques. The focus of this investigation is to evaluate the sensitivity of the radiance exiting the bottom and top of the atmosphere to the representation of the aerosol single-scattering phase function for these applications. Hyperspectral almucantar sky path radiance measurements obtained with the Portable Ground-based Atmospheric Monitoring System (PGAMS) are compared to synthetic spectra generated by MODTRAN3.

Calibration of MODTRAN3 with PGAMS observational data for atmospheric correction applications

The portable ground-based atmospheric monitoring system (PGAMS) is a spectroradiometer system that provides a set of in situ solar and hemispherical sky irradiance, path radiance, and surface reflectance measurements. The observations provide input parameters for the calibration of atmospheric algorithms applied to multispectral and hyperspectral images in the visible and near infrared spectrum. Presented in this paper are the results of comparing hyperspectral surface radiances calculated using MODTRAN3 with PGAMS field measurements for a blue tarp and grass surface targets. Good agreement was obtained by constraining MODTRAN3 to only a rural atmospheric model with a calibrated visibility and surface reflectance from PGAMS observations. This was accomplished even though the sky conditions were unsteady as indicated by a varying aerosol extinction. Average absolute differences of 11.3 and 7.4 percent over the wavelength range from 400 to 1000 nm were obtained for the grass and blue tarp surfaces respectively. However, transformation to at-sensor radiances require additional constraints on the single-scattering albedo and scattering phase function so that they exhibit the specific real-time aerosol properties rather than a seasonal average model.