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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

The spectral bidirectional reflectance of snow

A radiative transfer model for the bidirectional reflectance-distribution function (BRDF) shows that snow is moderately anisotropic in the near-infrared wavelengths. Although the directional-hemispherical albedo of snow decreases as the grains become larger, the forward scattering also increases, with the result that the illumination and viewing geometry must be considered when interpreting physical properties of the surface layer of the snow pack from remote sensing data. Measurements of the BRDF for a variety of snow conditions were made throughout the winter and spring seasons with a spectroradiometer, for wavelengths from 0.38 to 1.11 microns. Coincident with these the surface grain properties were analyzed by stereological methods. The data show that Mie scattering calculations using an equivalent spherical radius match the directional-hemispherical reflectance, but the BRDF usually shows a small peak in the backscattered direction that would not occur from spherical grains. The sphere with the same surface-to-volume ratio as the ice grains is used as the equivalent sphere.

Dozier, J.↗
NASA NTRS1997

Development and Operation of a Material Identification and Discrimination Imaging Spectroradiometer

Many imaging applications require quantitative determination of a scene's spectral radiance. This paper describes a new system capable of real-time spectroradiometric imagery. Operating at a full-spectrum update rate of 30Hz, this imager is capable of collecting a 30 point spectrum from each of three imaging heads: the first operates from 400 nm to 950 nm, with a 2% bandwidth; the second operates from 1.5 micro-m to 5.5 micro-m with a 1.5% bandwidth; the third operates from 5 micro-m to 12 micro-m, also at a 1.5% bandwidth. Standard image format is 256 x 256, with 512 x 512 possible in the VIS/NIR head. Spectra of up to 256 points are available at proportionately lower frame rates. In order to make such a tremendous amount of data more manageable, internal processing electronics perform four important operations on the spectral imagery data in real-time. First, all data in the spatial/spectral cube of data is spectro-radiometrically calibrated as it is collected. Second, to allow the imager to simulate sensors with arbitrary spectral response, any set of three spectral response functions may be loaded into the imager including delta functions to allow single wavelength viewing; the instrument then evaluates the integral of the product of the scene spectral radiances and the response function. Third, more powerful exploitation of the gathered spectral radiances can be effected by application of various spectral-matched filtering algorithms to identify pixels whose relative spectral radiance distribution matches a sought-after spectral radiance distribution, allowing materials-based identification and discrimination. Fourth, the instrument allows determination of spectral reflectance, surface temperature, and spectral emissivity, also in real-time. The spectral imaging technique used in the instrument allows tailoring of the frame rate and/or the spectral bandwidth to suit the scene radiance levels, i.e., frame rate can be reduced, or bandwidth increased to improve SNR when viewing low radiance scenes. The unique challenges of design and calibration are described. Pixel readout rates of 160 MHz, for full frame readout rates of 1000 Hz (512 x 512 image) present the first challenge; processing rates of nearly 600 million integer operations per second for sensor emulation, or over 2 billion per second for matched filtering, present the second. Spatial and spectral calibration of 66,536 pixels (262,144 for the 512 x 512 version) and up to 1,000 spectral positions mandate novel decoupling methods to keep the required calibration memory to a reasonable size. Large radiometric dynamic range also requires care to maintain precision operation with minimum memory size.

Dombrowski, Mark↗
NASA NTRS1999

Angular and Seasonal Variation of Spectral Surface Reflectance Ratios: Implications for the Remote Sensing of Aerosol over Land

We obtain valuable information on the angular and seasonal variability of surface reflectance using a hand-held spectrometer from a light aircraft. The data is used to test a procedure that allows us to estimate visible surface reflectance from the longer wavelength 2.1 micrometer channel (mid-IR). Estimating or avoiding surface reflectance in the visible is a vital first step in most algorithms that retrieve aerosol optical thickness over land targets. The data indicate that specular reflection found when viewing targets from the forward direction can severely corrupt the relationships between the visible and 2.1 micrometer reflectance that were derived from nadir data. There is a month by month variation in the ratios between the visible and the mid-IR, weakly correlated to the Normalized Difference Vegetation Index (NDVI). If specular reflection is not avoided, the errors resulting from estimating surface reflectance from the mid-IR exceed the acceptable limit of DELTA-rho approximately 0.01 in roughly 40% of the cases, using the current algorithm. This is reduced to 25% of the cases if specular reflection is avoided. An alternative method that uses path radiance rather than explicitly estimating visible surface reflectance results in similar errors. The two methods have different strengths and weaknesses that require further study.

Remer, L. A.↗
NASA NTRS2002

Organic Solid Matter as a Coloring Agent in Outer Solar System Bodies

Small bodies in the outer Solar System OSS, exhibit a range of color, or slope of the reflectance in the photovisual spectral region, ranging from neutral to very red, sometimes with and sometimes without distinct absorption bands. These objects range in geometric albedo from 0.03 to 1.0, with the higher albedo objects typically showing clear evidence of water ice. Water ice has also been found in a few objects with albedo 0. 1 or less. We explore here the identification of the material or materials that color these icy and non-icy surfaces through scattering models that incorporate minerals, meteoritic material, and organic solids (tholins) produced ID the laboratory by energy deposition in ices and gases. These models must match not only the color in the photovisual region, but the spectral reflectance properties throughout the near-infrared. Among some classes of objects, such as Kuiper Belt objects, the coloring agent may be a single material that is present in greater or lesser abundance, thus accounting for the range in color from neutral to very red. This may also apply to the Centaur objects, the Jovian Trojans, and the outer-main belt asteroids, each taken as a separate class. If so, each class may be colored to varying degrees by a different material, or they all might be colored by a common material that is widespread throughout the OSS, from 3 to 50 AU, and beyond. In this paper, we model the reflectances of "Kuiper Belt objects, Centaurs, Trojans, outer ARAB asteroids, and planetary satellites. Our models show that the reddest surfaces cannot be colored by minerals or meteoritic materials, but can be matched throughout the photovisual and near-infrared by organic solids, specifically certain tholins.

Cruikshank, D. P.↗
NASA NTRS2007

Physical Interpretation of the Correlation Between Multi-Angle Spectral Data and Canopy Height

Recent empirical studies have shown that multi-angle spectral data can be useful for predicting canopy height, but the physical reason for this correlation was not understood. We follow the concept of canopy spectral invariants, specifically escape probability, to gain insight into the observed correlation. Airborne Multi-Angle Imaging Spectrometer (AirMISR) and airborne Laser Vegetation Imaging Sensor (LVIS) data acquired during a NASA Terrestrial Ecology Program aircraft campaign underlie our analysis. Two multivariate linear regression models were developed to estimate LVIS height measures from 28 AirMISR multi-angle spectral reflectances and from the spectrally invariant escape probability at 7 AirMISR view angles. Both models achieved nearly the same accuracy, suggesting that canopy spectral invariant theory can explain the observed correlation. We hypothesize that the escape probability is sensitive to the aspect ratio (crown diameter to crown height). The multi-angle spectral data alone therefore may not provide enough information to retrieve canopy height globally

Schull, M. A.↗
NASA NTRS2012

Characterization of Mineralogy Across Vesta

Dawn VIR spectra are characterized by pyroxene absorptions and no clear evidence for abundant other minerals are observed at the scale of the present measurements. Even though Vesta spectra are dominated by pyroxenes, spectral variation at regional and local scales are evident and distinct color units are identified. Although almost all of the surface materials exhibit spectra like those of howardites, some large units can be interpreted to be material richer in diogenite (based on pyroxenes band depths and band centers) and some others like eucrite-rich howardite units. VIR data strongly indicate that the south polar region (Rheasilvia) has its own spectral characteristics, indicating the presence of Mg-pyroxene-rich terrains (diogenite-like), while the equatorial areas have swallower band depths and average band centers at slightly longer wavelengths, consistent with more eucrite rich materials. Vesta surface shows considerable diversity at smaller scales (tens of km), in terms of spectral reflectance and emission, band depths and slopes. Many bright and dark spots are present on Vesta. Dark spots have low reflectance at visible wavelengths and are spectrally characterized by shallower 1 and 2 micron bands with respect the surrounding terrains. Bright materials have high reflectance and are often spectrally characterized by deep pyroxenes absorption bands. Vesta presents complex geology/topography and the mineral distribution is often correlated with geological and topographical structures. Ejecta from large craters have distinct spectral behaviors, and materials exposed in the craters show distinct spectra on floors and rims. VIR reveals the mineralogical variation of Vesta s crustal stratigraphy on local and global scales. Maps of spectral parameters show surface and subsurface unit compositions in their stratigraphic context. The hypothesis that Vesta is the HED parent body is consistent with, and strengthened by, the geologic and spectral context for pyroxene distribution provided by Dawn.

De Sanctis, M. C.↗
NASA NTRS1992

Extraction of soil and vegetation parameters from high resolution bi-directional reflectance spectra

High spectral resolution reflectance spectra were collected over a semi-desert grassland at both dry and wet season periods. Spectral reflectance measurements were made from several viewing angles at both low and high solar zenith angles. A mixture model was used to separate and extract green vegetation from dry/dead vegetation and soil. The extracted vegetation signal varied greatly with view and sun angle variations such that off-nadir viewing and illuminating angles resulted in the highest vegetation loadings. These variations were normalized with cosine functions for both sun and view angle. These results offer a methodology for standardizing multi-temporal and multi-angular satellite measurements of vegetation activity.

Huete, A. R.↗
NASA NTRS1979

Atlas of soil reflectance properties

A compendium of soil spectral reflectance curves together with soil test results and site information is presented in an abbreviated manner listing those soil properties most important in influencing soil reflectance. Results are presented for 251 soils from 39 states and Brazil. A narrative key describes relationships between soil parameters and reflectance curves. All soils are classified according to the U.S. soil taxonomy and soil series name for ease of identification.

Stoner, E. R.↗
NASA NTRS1982

Reflectance spectroscopy of diogenite meteorite types from Antarctica and their relationship to asteroids

Measurements of the spectral reflectance (0.4-2.5 microns) of three diogenite meteorites found in Antarctica are examined. Comparisons are made with previous measurements of diogenites in order to correlate the mineralogy and petrography with the spectral characteristics that can also be measured on solar system objects of unknown composition. Even though the Antarctic diogenites have the same spectral characteristics as other diogenites, there are measurable differences between all diogenite spectra. Bands caused by spin-forbidden transitions of Fe(2+) ions in pyroxene, which are identified for the first time in reflectance spectra of powdered samples, are revealed in an analysis of the spectra into sums of Gaussian components. It is shown from these laboratory measurements that spectral reflectance measurements of asteroids and other solar system objects made with the visible CVF (circular variable filter) spectrometer used for laboratory measurements can provide useful information on their surface composition.

Mcfadden, L. A.↗
NASA NTRS1992

Estimators of bottom reflectance spectra

Estimators of in situ bottom spectral reflectance are calculated from multi-station optical field data gathered with standard instrumentation from different sites. These spectra are then compared to reflectance spectra measured in the laboratory of the bottom sediments collected in the field for the stations at these different sites. The relative fit of the estimated spectral curves to those measured in the laboratory was measured. The most accurate absolute estimation was provided by the single scattering irradiance model.

Estep, L.↗
NASA NTRS1992

Estimating reflected and emitted components of the radiation balance using remotely-sensed spectral data from KUREX-91

The estimation of incoming and outgoing radiation streams using bidirectional spectral reflectances and bidirectional thermal emittances is described. Good agreement between measured and modeled estimates of the radiation balance is obtained. The data used were collected over selected grassland sites on the Streletskaya Steppe Reserve in Russia in July 1991. Results from this study compare well with results obtained during the first ISLSCP field experiment (FIFE) study in Kansas in 1987-89.

Blad, B. L.↗
NASA NTRS1979

Vacuum ultraviolet reflectance spectra of group H chondrites

The spectral reflectances of seventeen group H chondrites have been measured in the vacuum ultraviolet region, from 5 to 14 eV photon energy (250 to 90 nm, wavelength), as part of a survey of all meteorite types. Olivines, ortho-, and clino- pyroxenes are the three most important silicate minerals in these meteorites, and the spectrum of each mineral contains a distinct peak caused by a transition believed to be excitonic in nature and involving the valence-conduction bandgap. The energy at which this peak occurs varies as a function of mineral composition, and this variation has been determined from the spectra of a number of mineral standards. The observed positions of the olivine and pyroxene peaks in the meteorite spectra are compared to predicted positions based on the standard minerals.

Wagner, J. K.↗
NASA NTRS2009

Case Studies of Aerosol Remote Sensing in the Vicinity of Clouds

Studying the spatial variability of aerosol properties in the vicinity of clouds is essential to our ability to determine aerosol direct and indirect effects on climate. In this paper, we describe aerosol observations collected near cloud edges by an airborne Sun photometer over dark ocean waters. Focusing on case studies of aerosol measurements near eight cloud edges within a dissipating stratiform cloud deck, we compare the airborne Sun photometer observations to retrievals of aerosol properties using the standard Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol algorithm applied to 500-m-resolution MODIS spectral reflectances. We find a persistent, spectrally neutral increase in the Sun photometer-derived aerosol optical depth (AOD) of up to 10% (0.015) in the 2-km distances closest to the edges of several distinct clouds. At midvisible wavelengths, the MODIS AOD retrievals show similar increases toward cloud edges, although a larger increase in AOD is found in the MODIS along-scan direction. At shortwave infrared (SWIR) wavelengths (1240-2130 nm), the MODIS-derived AOD increases near cloud edges are of the order of 0.03 and as such three times as large as the Sun photometer-derived values. Hence, in contrast to recently discussed bluing of aerosols near cloud edges, i.e., a preferential apparent increase in the visible reflectances of clear-sky pixels due to 3-D radiative transfer effects in the vicinity of clouds, we find a reddening of aerosols in the MODIS 500-m-resolution aerosol retrievals near clouds. This reddening in our study can be traced to larger absolute increases in SWIR reflectances when compared to visible reflectances near clouds, which in turn seem to stem from larger electronic cross talk in the MODIS SWIR bands (5-7). We note that the lack of bluing in our MODIS observations is likely due to the small geometric and optical thicknesses of the clouds considered.

Redemann, Jens↗
NASA NTRS1971

A method of treating the non-grey error in total emittance measurements

In techniques for the rapid determination of total emittance, the sample is generally exposed to surroundings that are at a different temperature than the sample's surface. When the infrared spectral reflectance of the surface is spectrally selective, these techniques introduce an error into the total emittance values. Surfaces of aluminum overcoated with oxides of various thicknesses fall into this class. Because they are often used as temperature control coatings on satellites, their emittances must be accurately known. The magnitude of the error was calculated for Alzak and silicon oxide-coated aluminum and was shown to be dependent on the thickness of the oxide coating. The results demonstrate that, because the magnitude of the error is thickness-dependent, it is generally impossible or impractical to eliminate it by calibrating the measuring device.

Heaney, J. B.↗
NASA NTRS1977

Remotely sensed and laboratory spectral signatures of an ocean-dumped acid waste

An ocean-dumped acid waste plume was studied by using a rapid scanning spectrometer to remotely measure ocean radiance from a helicopter. The results of these studies are presented and compared with results from sea truth samples and laboratory experiments. An ocean spectral reflectance signature and a laboratory spectral transmission signature were established for the iron-acid waste pollutant. The spectrally and chemically significant component of the acid waste pollutant was determined to be ferric iron.

Lewis, B. W.↗
NASA NTRS1987

Spectrometric test of a linear array sensor

A spectroradiometer which measures spectral reflectivities and irradiance in discrete spectral channels was tested to determine the accuracy of its wavelength calibration. This sensor is a primary tool in the remote sensing investigations conducted on biomass at NASA's Goddard Space Flight Center. Measurements have been collected on crop and forest plants both in the laboratory and field with this radiometer to develop crop identification and plant stress remote sensing techniques. Wavelength calibration is essential for use in referencing the study measurements with those of other investigations and satellite remote sensor data sets. This calibration determines a wavelength vs channel address conversion which was found to have an RMS deviation of approximately half a channel, or 1.5 nm in the range from 360 to 1050 nm. A comparison of these results with those of another test showed an average difference of approximately 4 nm, sufficiently accurate for most investigative work.

Brown, Kenneth S.↗