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NASA NTRS1991

Transport theory for a leaf canopy of finite-dimensional scattering centers

A formalism for photon transport in leaf canopies with finite-dimensional scattering centers that cross shade mutually is developed. Starting from first principles, expressions for the interaction cross sections are derived. The problem of illumination by a monodirectional source is studied in detail using a successive collisions approach. A balance equation is formulated in R3 and the interaction between a leaf canopy and the adjacent atmosphere is discussed. Although the details are those relating to a leaf canopy, the formalism is equally applicable to other media where the constituents cross shade mutually such as planetary surfaces, rings and ridged-ice in polar regions, i.e., media that exhibit opposition brightening.

Myneni, Ranga B.↗

NASA NTRS1991

Relationships of soil, grass, and bedrock over the Kaweah serpentine melange through spectral mixture analysis of AVIRIS data

A linear mixing model is used to model the spectral variability of an AVIRIS scene from the western foothills of the Sierra Nevada and calibrate these radiance data to reflectance. Five spectral endmembers from the AVIRIS data, plus an ideal 'shade' endmember were required to model the continuum reflectance of each pixel in the image. Three of the endmembers were interpreted to model the surface constituents green vegetation, dry grass, and illumination. These are the main transient surface constituents that are expected to change with shifts in land use or climatic influences and viewing conditions ('shade' only). The spectral distinction between the other three endmembers is very small, yet the spatial distributions are coherent and interpretable. These distributions cross anthropogenic and vegetation boundaries and are best interpreted as different soil types. Comparison of the fraction images to the bedrock geology maps indicates that substrate composition must be a factor contributing to the spectral properties of these endmembers. Detailed examination of the reflectance spectra of the three soil endmembers reveals that differences in the amount of ferric and ferrous iron and/or organic constituents in the soils is largely responsible for the differences in spectral properties of these endmembers.

Mustard, John F.↗

NASA NTRS1993

Green vegetation, nonphotosynthetic vegetation, and soils in AVIRIS data

The problem of distinguishing between green vegetation, nonphotosynthetic vegetation (NPV, such as dry grass, leaf litter, and woody material), and soils in imaging-spectrometer data is addressed by analyzing an image taken by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) over the Jasper Ridge Biological Preserve (California) on September 20, 1989, using spectral mixture analysis. Over 98 percent of the spectral variation could be explained by linear mixtures of three endmembers, green vegetation, shade, and soil. NPV, which could not be distinguished from soil when included as an endmember, was discriminated by residual spectra that contained cellulose and lignin absorptions. Distinct communities of green vegetation were distinguished by (1) nonlinear mixing effect caused by transmission and scattering by green leaves, (2) variations in a derived canopy-shade spectrum, and (3) the fraction of NPV.

Roberts, D. A.↗

NASA NTRS1992

Testing the Li-Strahler four-component canopy reflectance model in the HAPEX-Sahel shrub savanna sites using ground reflectance data

The rate at which a light field decays in water is characterized by the diffuse attenuation coefficient k. The Li-Strahler discrete-object canopy reflectance model was tested in two sites, a shrub grass savanna and a degraded shrub savanna on bare soil, in the proposed HAPEX (Hydrologic Atmospheric Pilot Experiment) II/Sahel study area in Niger, West Africa. Average site reflectance was predicted for each site from the reflectances and cover proportions of four components: shrub canopy, background (soil or grass and soil), shaded canopy, and shaded background. Component reflectances were sampled in the SPOT wavebands using a hand-held radiometer. Predicted reflectance was compared to average site reflectance measured using the same radiometer mounted on a backpack with measurements recorded every 5 m along two 1-km transects, also in the SPOT (Systeme Probatoire d'Observation de la Terre) bands. Measurements and predictions were made for each of the three days during the summer growing season, approximately two weeks apart. Red, near infrared reflectance, and the NDVI (normalized difference vegetation index) were all predicted with a high degree of accuracy for the shrub/grass site and with reasonable accuracy for the degraded shrub site.

Franklin, J.↗

NASA NTRS1993

The IRGen infrared data base modeler

IRGen is a modeling system which creates three-dimensional IR data bases for real-time simulation of thermal IR sensors. Starting from a visual data base, IRGen computes the temperature and radiance of every data base surface with a user-specified thermal environment. The predicted gray shade of each surface is then computed from the user specified sensor characteristics. IRGen is based on first-principles models of heat transport and heat flux sources, and it accurately simulates the variations of IR imagery with time of day and with changing environmental conditions. The starting point for creating an IRGen data base is a visual faceted data base, in which every facet has been labeled with a material code. This code is an index into a material data base which contains surface and bulk thermal properties for the material. IRGen uses the material properties to compute the surface temperature at the specified time of day. IRGen also supports image generator features such as texturing and smooth shading, which greatly enhance image realism.

Bernstein, Uri↗

NASA NTRS1994

UV spectral irradiance measurements in New Zealand: Effects of Pinatubo volcanic aerosol

Since late 1989, regular UV spectral irradiance measurements have been made at Lauder, New Zealand (45 deg S, 170 deg E), whenever weather permits. Here, the instrumentation and measurement strategy are outlined, and early results are discussed. Following the eruption of Mt Pinatubo in June 1991, large amounts of volcanic aerosol were injected into the stratosphere and were subsequently transported to New Zealand's latitudes in the latter half of 1991. This provides an opportunity to investigate the effects of volcanic aerosols on UV irradiances measured at this clean-air site. Although changes in global (sum of diffuse plus direct) irradiances were below the detection threshold, there were significant changes in the partitioning of radiation between the direct beam and diffuse skylight. Decreases by nearly a factor of two in the direct/diffuse ratio were observed at longer wavelengths, and at smaller solar zenith angles (sza's). The aerosol optical depth due to volcanic aerosol over Lauder in December 1991 was 0.15 plus or minus 0.02 at 450 nm, with lower values at shorter wavelengths. Although effects were relatively small in the UVB region, an implication of the changes is that the contrast between shade and direct sun is reduced, so that shaded areas received relatively more radiation in the summer of 1991/92 in New Zealand.

Mckenzie, Richard L.↗

NASA NTRS1993

Temporal changes in endmember abundances, liquid water and water vapor over vegetation at Jasper Ridge

Imaging spectrometry offers a new way of deriving ecological information about vegetation communities from remote sensing. Applications include derivation of canopy chemistry, measurement of column atmospheric water vapor and liquid water, improved detectability of materials, more accurate estimation of green vegetation cover and discrimination of spectrally distinct green leaf, non-photosynthetic vegetation (NPV: litter, wood, bark, etc.) and shade spectra associated with different vegetation communities. Much of our emphasis has been on interpreting Airborne Visible/Infrared Imaging Spectrometry (AVIRIS) data spectral mixtures. Two approaches have been used, simple models, where the data are treated as a mixture of 3 to 4 laboratory/field measured spectra, known as reference endmembers (EM's), applied uniformly to the whole image, to more complex models where both the number of EM's and the types of EM's vary on a per-pixel basis. Where simple models are applied, materials, such as NPV, which are spectrally similar to soils, can be discriminated on the basis of residual spectra. One key aspect is that the data are calibrated to reflectance and modeled as mixtures of reference EM's, permitting temporal comparison of EM fractions, independent of scene location or data type. In previous studies the calibration was performed using a modified-empirical line calibration, assuming a uniform atmosphere across the scene. In this study, a Modtran-based calibration approach was used to map liquid water and atmospheric water vapor and retrieve surface reflectance from three AVIRIS scenes acquired in 1992 over the Jasper Ridge Biological Preserve. The data were acquired on June 2nd, September 4th and October 6th. Reflectance images were analyzed as spectral mixtures of reference EM's using a simple 4 EM model. Atmospheric water vapor derived from Modtran was compared to elevation, and community type. Liquid water was compare to the abundance of NPV, Shade and Green Vegetation (VG) for select sites to determine whether a relationship existed, and under what conditions the relationship broke down. Temporal trends in endmember fractions, liquid water and atmospheric water vapor were investigated also. The combination of spectral mixture analysis and the Modtran based atmospheric/liquid water models was used to develop a unique vegetation community description.

Roberts, Dar A.↗

NASA NTRS1992

Solar concentrator technology development for space based applications, volume 1

Thermoelectric conversion using a radio-isotope heat source has been used where outer planetary space craft are too far away for absorbing significant solar energy. Solar dynamic power (SDP) conversion is one technology that offers advantages for applications within the inner planet region. Since SDP conversion efficiency can be 2 to 3 times higher than photovoltaic, the collecting surfaces are much reduced in area and therefore lighter. This becomes an advantage in allocating more weight to launched payloads. A second advantage results for low earth orbit applications. The reduced area results in lower drag forces on the spacecraft and requires less reboost propellant to maintain orbit. A third advantage occurs because of the sun-to-shade cycling while in earth orbit. Photovoltaic systems require batteries to store energy for use when in the shade, and battery life for periods of 10 to 15 years is not presently achievable. For these reasons the Solar Dynamics and Thermal Systems Branch at NASA LeRC has funded work in developing SDP systems. The generic SDP system uses a large parabolic solar concentrator to focus solar energy onto a power conversion device. The concentrators are large areas and must therefore be efficient and have low specific weights. Yet these surfaces must be precise and capable of being stowed in a launch vehicle and then deployed and sometimes unfurled in space. There are significant technical challenges in engineering such structures, and considerable investigation has been made to date. This is the first of two volumes reporting on the research done by the Advanced Manufacturing Center at Cleveland State University to assist NASA LeRC in evaluating this technology. The objective of the grant was to restore the solar concentrator development technology of the 1960s while improving it with advances that have occurred since then. This report summarizes the work done from January 1989 through December 1991.

Pintz, A.↗

NASA NTRS1992

Solar concentrator technology development for space based applications, volume 2

Thermoelectric conversion using a radio-isotope heat source has been used where outer planetary space craft are too far away for absorbing significant solar energy. Solar dynamic power (SDP) conversion is one technology that offers advantages for applications within the inner planet region. Since SDP conversion efficiency can be 2 to 3 times higher than photovoltaic, the collecting surfaces are much reduced in area and therefore lighter. This becomes an advantage in allocating more weight to launched payloads. A second advantage results for low earth orbit applications. The reduced area results in lower drag forces on the spacecraft and requires less reboost propellant to maintain orbit. A third advantage occurs because of the sun-to-shade cycling while in earth orbit. Photovoltaic systems require batteries to store energy for use when in the shade, and battery life for periods of 10 to 15 years is not presently achievable. For these reasons the Solar Dynamics and Thermal Systems Branch at NASA LeRC has funded work in developing SDP systems. The generic SDP system uses a large parabolic solar concentrator to focus solar energy onto a power conversion device. The concentrators are large areas and must therefore be efficient and have low specific weights. Yet these surfaces must be precise and capable of being stowed in a launch vehicle and then deployed and sometimes unfurled in space. There are significant technical challenges in engineering such structures, and considerable investigation has been made to date. This is the second of two volumes reporting on the research done by the Advanced Manufacturing Center at Cleveland State University to assist NASA LeRC in evaluating this technology. This volume includes the appendices of selected data sets, drawings, and procedures. The objective of the grant was to restore the solar concentrator development technology of the 1960s while improving it with advances that have occurred since then. This report summarizes the work done from January 1989 through December 1991.

Pintz, A.↗

NASA NTRS1994

Venus cartography

The entire surface of the planet Venus is being mapped at global and regional scales (1:50 million through 1:1.5 million) with synthetic aperture radar (SAR), radar altimeter, and radiometer measurements of physical properties from the Magellan spacecraft. The mapping includes SAR image mosaics, shaded relief maps, and topographic contour overlays made from altimetry data and by radargrammetric methods. Methods used include new techniques of radar image processing that became operational as a result of the Magellan mission. Special cartographic support products prepared by the USGS include: synthetic stereograms, color thematic maps of physical properties, digital shaded relief maps from opposite-look SAR, and topographic maps by radargrammetry. The area being mapped (at a resolution of 75 m/pixel) is roughly equivalent to that of Earth, including seafloors. The mapping is designed to support geologic and geophysical investigations.

Batson, R. M.↗

NASA NTRS1994

Boundary-layer transition detection with infrared imaging emphasizing cryogenic applications

This paper reviews the technique of boundary-layer transition detection using infrared (IR) imaging, emphasizing cryogenic wind-tunnel testing. With the exception of the low-temperature effects on the IR radiation, the discussion is relevant to conventional wind-tunnel and flight testing as well. At low temperatures, IR imaging encounters a reduction in the radiated energy throughout the IR spectrum, combined with a shift to longer wavelengths of the bulk of the radiation. This radiation behavior affects the minimum resolvable temperature difference (MRTD) of the IR imaging system because of its fixed wave band sensitivity. In the absence of commercial long wavelength IR imaging systems, operating at wavelengths longer than 13 micron, some measures can be taken to alleviate the problem caused by the MRTD limitation. The thermal signature of transition can be enhanced by allowing a small and controlled temperature increase of the wind-tunnel flow that induces a transient heat transfer to the model. This action temporarily reveals the model area under the turbulent regime through its higher heating rate compared with the laminar regime. The contrast between the areas exposed to the two regimes can be enhanced by subtraction of thermograms (the equilibrium thermogram from the transient thermogram). Further visual improvement can be obtained through shade stretching or binary shading.

Gartenberg, Ehud↗

NASA NTRS1994

Radiative transfer in shrub savanna sites in Niger: Preliminary results from HAPEX-Sahel. Part 1: Modelling surface reflectance using a geometric-optical approach

To use optical remote sensing to monitor land surface-climate interactions over large areas, algorithms must be developed to relate multispectral measurements to key variables controlling the exchange of matter (water, carbon dioxide) and energy between the land surface and the atmosphere. The proportion of the ground covered by vegetation and the interception of photosynthetically active radiation (PAR) by vegetation are examples of two variables related to evapotranspiration and primary production, respectively. An areal-proportion model of the multispectral reflectance of shrub savanna, composed of scattered shrubs with a grass, forb or soil understory, predicted the reflectance of two 0.5 km(exp 2) sites as the area-weighted average of the shrub and understory or 'background' reflectances. Although the shaded crown and shaded background have darker reflectances, ignoring them in the area-weighted model is not serious when shrub cover is low and solar zenith angle is small. A submodel predicted the reflectance of the shrub crown as a function of the foliage reflectance and amount of plant material within the crown, and the background reflectance scattered or transmitted through canopy gaps (referred to as a soil-plant 'spectral interaction' term). One may be able to combine these two models to estimate both the fraction of vegetation cover and interception of PAR by green vegetation in a shrub savanna.

Franklin, Janet↗

NASA NTRS1999

External Sources of Water for Mercury's Putative Ice Deposits

Radar images have revealed the possible presence of ice deposits in Mercury's polar regions. Although thermal models indicate that water ice can be stable in permanently shaded regions near Mercury's poles, the ultimate source of the water remains unclear. We use stochastic models and other theoretical methods to investigate the role of external sources in supplying Mercury with the requisite amount of water. By extrapolating the current terrestrial influx of interplanetary dust particles to that at Mercury, we find that continual micrometeoritic bombardment of Mercury over the last 3.5 byr could have resulted in the delivery of (3-60) x 10(exp 16) grams of water ice to the permanently shaded regions at Mercury's poles (equivalent to an average ice thickness of 0.8-20 m). Erosion by micrometeoritic impact on exposed ice deposits could reduce the above value by about a half. For comparison, the current ice deposits on Mercury are believed to be somewhere between approx. 2 and 20 m thick. Using a Monte Carlo model to simulate the impact history of Mercury, we find that asteroids and comets can also deliver an amount of water consistent with the observations. Impacts from Jupiter-family comets over the last 3.5 billion years can supply (0.1-200) x 10(exp 16) grams of water to Mercury's polar regions (corresponding to ice deposits 0.05-60 m thick), Halley-type comets can supply (0.2-20) x 10(exp 16) grams of water to the poles (0.07-7 m of ice), and asteroids can provide (0.4-20) x 10(exp 16) grams of water to the poles (0.1-8 m of ice). Although all these external sources are nominally sufficient to explain the estimated amount of ice currently at Mercury's poles, impacts by a few large comets and/or asteroids seem to provide the best explanation for both the amount and cleanliness of the ice deposits on Mercury. Despite their low population estimates in the inner solar system, Jupiter-family comets are particularly promising candidates for delivering water to Mercury because they have a larger volatile content than asteroids and more favorable orbital and impact characteristics than Halley-type comets.

Moses, Julianne I.↗

NASA NTRS2000

BOREAS RSS-19 1994 Seasonal Understory Reflectance Data

The BOREAS RSS-19 team collected airborne remotely sensed images and ground reflectance data for characterizing the radiometric properties of the boreal forest landscape. One objective of BOREAS is to further the understanding of the spectral bidirectional reflectance of typical boreal ecosystem stands in the visible/near-infrared regime. An essential input for any canopy BRDF model is an accurate estimate of the average understory reflectance, both for sunlit and shaded conditions. These variables can be expected to vary seasonally because of species-dependent differences in the phenological cycle of foliar display. In response to these requirements, the average understory reflectance for the flux tower sites of both the NSA (Thompson, Manitoba) and the SSA (Candle Lake, Saskatchewan) was observed throughout the year during five field campaigns. This was done by measuring the nadir reflectance (400 to 850 nm) of sunlit and shaded understory (vegetation and snow cover) along a surveyed LAI transect line (Chen, RSS-07) at each site near solar noon and documenting an average site reflectance. Comparisons between sites reveal differences in the green and infrared regions of the spectra, because of the differing species in the understory for each site. Temporal (seasonal) variation for each site was also observed (06-Feb-1994 to 16-Sep-1994), indicating the changing flora mixtures and changing spectral signatures as the understory matures during the growing season. The data are stored in tabular ASCII files. The data files are available on a CD-ROM (see document number 20010000884) or from the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).

Miller, John R.↗

NASA NTRS2002

Coastal Atmosphere and Sea Time Series (CoASTS)

The Coastal Atmosphere and Sea Time Series (CoASTS) Project aimed at supporting ocean color research and applications, from 1995 up to the time of publication of this document, has ensured the collection of a comprehensive atmospheric and marine data set from an oceanographic tower located in the northern Adriatic Sea. The instruments and the measurement methodologies used to gather quantities relevant for bio-optical modeling and for the calibration and validation of ocean color sensors, are described. Particular emphasis is placed on four items: (1) the evaluation of perturbation effects in radiometric data (i.e., tower-shading, instrument self-shading, and bottom effects); (2) the intercomparison of seawater absorption coefficients from in situ measurements and from laboratory spectrometric analysis on discrete samples; (3) the intercomparison of two filter techniques for in vivo measurement of particulate absorption coefficients; and (4) the analysis of repeatability and reproducibility of the most relevant laboratory measurements carried out on seawater samples (i.e., particulate and yellow substance absorption coefficients, and pigment and total suspended matter concentrations). Sample data are also presented and discussed to illustrate the typical features characterizing the CoASTS measurement site in view of supporting the suitability of the CoASTS data set for bio-optical modeling and ocean color calibration and validation.

Hooker, Stanford B.↗

NASA NTRS2003

Performance of the Star Tracker Lightshades on the Earth Observing Satellite (EOS) Aqua

The TRW built EOS Aqua spacecraft uses two Ball Aerospace CT-602 star trackers to provide attitude updates to the 3-axis, zero momentum, controller. Two months prior to the scheduled launch of Aqua, Ball reported an error in the design of the star tracker lightshades. The lightshades, which had been designed specifically for the EOS Common spacecraft, were not expected to meet the stray light rejection requirements of the mission, thus impacting the overall spacecraft pointing performance. What ensued was an effort to characterize the actual performance of the existing shade design, determine what could be done within the physical envelope available, and modify the hardware to meet requirements. Changes were made based on this review activity and Aqua was launched on May 4, 2002. To date the spacecraft is meeting all of its science pointing requirements. Reported here are the lightshade design predictions, test results, and the measured on orbit performance of these shades.

Kenny, Thomas↗

NASA NTRS2003

Performance of the Star Tracker Lightshades on the Earth Observing Satellite (EOS) Aqua

The TRW built EOS Aqua spacecraft uses two Ball Aerospace CT-602 star trackers to provide attitude updates to the 3-axis, zero momentum, controller. Two months prior to the scheduled launch of Aqua, Ball reported an error in the design of the star tracker lightshades. The lightshades, which had been designed specifically for the EOS Common spacecraft, were not expected to meet the stray light rejection requirements of the mission and thus impact the overall spacecraft pointing performance. What ensued was an effort to characterize the actual performance of the existing shade design, determine what could be done within the physical envelope available, and modify the hardware to meet requirements. Changes were made based on this review activity and Aqua was launched on May 4, 2002. To date the spacecraft is meeting all of its science pointing requirements. Reported here are the lightshade design predictions, test results, and the measured on orbit performance of these shades.

Kenney, Thomas↗

NASA NTRS2004

High Spectral Resolution MODIS Algorithms for Ocean Chlorophyll in Case II Waters

The Case 2 chlorophyll a algorithm is based on a semi-analytical, bio-optical model of remote sensing reflectance, R(sub rs)(lambda), where R(sub rs)(lambda) is defined as the water-leaving radiance, L(sub w)(lambda), divided by the downwelling irradiance just above the sea surface, E(sub d)(lambda,0(+)). The R(sub rs)(lambda) model (Section 3) has two free variables, the absorption coefficient due to phytoplankton at 675 nm, a(sub phi)(675), and the absorption coefficient due to colored dissolved organic matter (CDOM) or gelbstoff at 400 nm, a(sub g)(400). The R(rs) model has several parameters that are fixed or can be specified based on the region and season of the MODIS scene. These control the spectral shapes of the optical constituents of the model. R(sub rs)(lambda(sub i)) values from the MODIS data processing system are placed into the model, the model is inverted, and a(sub phi)(675), a(sub g)(400) (MOD24), and chlorophyll a (MOD21, Chlor_a_3) are computed. Algorithm development is initially focused on tropical, subtropical, and summer temperate environments, and the model is parameterized in Section 4 for three different bio-optical domains: (1) high ratios of photoprotective pigments to chlorophyll and low self-shading, which for brevity, we designate as 'unpackaged'; (2) low ratios and high self-shading, which we designate as 'packaged'; and (3) a transitional or global-average type. These domains can be identified from space by comparing sea-surface temperature to nitrogen-depletion temperatures for each domain (Section 5). Algorithm errors of more than 45% are reduced to errors of less than 30% with this approach, with the greatest effect occurring at the eastern and polar boundaries of the basins. Section 6 provides an expansion of bio-optical domains into high-latitude waters. The 'fully packaged' pigment domain is introduced in this section along with a revised strategy for implementing these variable packaging domains. Chlor_a_3 values derived semi-analytically and Chlor_a_2 values derived empirically using the O Reilly et al. OC3M algorithm from MODIS Terra radiances are compared to field chlorophyll-a concentrations in Sections 7 and 8.

Carder, Kendall L.↗