Bidirectional reflectance of three-dimensional diffuse surfaces
Bidirectional reflectance of three dimensional surfaces described by diffuse triangular subareas network for spacecraft thermal design, giving numerical technique
Greene, R. F., Jr.↗
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Advanced Libya-4 radiometric and atmospheric characterization utilizing MODIS and VIIRS full-scan reflective solar band measurements
The NASA Clouds and the Earth's Radiant Energy System (CERES) project provides observed flux and cloud products to the climate science community. The CERES instruments, along with the MODIS and VIIRS imagers, are onboard the Terra, Aqua, NPP, and NOAA20 satellites. In order to produce seamless multi-platform integrated products, long-term sensor stability and inter-calibration are required. Inter-calibration between sensors within the same sun-synchronous orbit relies on Earth invariant targets because simultaneous nadir overpasses are not possible. To facilitate inter-calibration efforts, the CERES Imager and Geostationary Calibration Group (IGCG) has improved the Libya-4 target characterization. Improvements include full scan angle characterization to enable daily observations, clear-sky identification using individual scan angle dynamic spatial homogeneity thresholds, and atmospheric corrections. The water vapor correction was found to be effective across the full scan, whereas the ozone and aerosol corrections were less effective. The atmospheric-corrected normalized radiance temporal fluctuations are similar across scan angles, spectral bands, and between the MODIS and VIIRS imagers, suggesting that the fluctuations are a result of the natural variability of the Libya-4 surface reflectance. The Libya-4 surface variability is more than likely caused by changes in the prevailing winds that alter sand dune orientation and resulting shadows. The full-scan-imager atmosphere and angle corrected reflected solar band radiance trend standard errors are between 0.6% and 1.0%, and for near-nadir observations, are between 0.5% and 0.8%. The advanced characterization suggests that the Libya-4 short-term surface reflectance anomalies may need to be considered for imager stability monitoring and inter-calibration efforts.
Libya-4 Pseudo-invariant Calibration Site↗
Low cost concentrator
The key to concentrator cost effectiveness is the proper design of the reflector surface panels. The low cost concentrator reflective surface design is based on use of a thin, backsilvered mirror glass reflector bonded to a molded structural plastic substrate. This combination of reflective panel material offers excellent optical performance at low cost. The design approach, rationale for the selected configuration, and the development status are described. Reflective panel development and demonstration results are also presented.
Bedard, R. J., Jr.↗
GEONEX: Land Monitoring From a New Generation of Geostationary Satellite Sensors
The latest generation of geostationary satellites carry sensors such as ABI (Advanced Baseline Imager on GOES-16) and the AHI (Advanced Himawari Imager on Himawari) that closely mimic the spatial and spectral characteristics of Earth Observing System flagship MODIS for monitoring land surface conditions. More importantly they provide observations at 5-15 minute intervals. Such high frequency data offer exciting possibilities for producing robust estimates of land surface conditions by overcoming cloud cover, enabling studies of diurnally varying local-to-regional biosphere-atmosphere interactions, and operational decision-making in agriculture, forestry and disaster management. But the data come with challenges that need special attention. For instance, geostationary data feature changing sun angle at constant view for each pixel, which is reciprocal to sun-synchronous observations, and thus require careful adaptation of EOS algorithms. Our goal is to produce a set of land surface products from geostationary sensors by leveraging NASA's investments in EOS algorithms and in the data/compute facility NEX. The land surface variables of interest include atmospherically corrected surface reflectances, snow cover, vegetation indices and leaf area index (LAI)/fraction of photosynthetically absorbed radiation (FPAR), as well as land surface temperature and fires. In order to get ready to produce operational products over the US from GOES-16 starting 2018, we have utilized 18 months of data from Himawari AHI over Australia to test the production pipeline and the performance of various algorithms for our initial tests. The end-to-end processing pipeline consists of a suite of modules to (a) perform calibration and automatic georeference correction of the AHI L1b data, (b) adopt the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm to produce surface spectral reflectances along with compositing schemes and QA, and (c) modify relevant EOS retrieval algorithms (e.g., LAI and FPAR, GPP, etc.) for subsequent science product generation. Initial evaluation of Himawari AHI products against standard MODIS products indicate general agreement, suggesting that data from geostationary sensors can augment low earth orbit (LEO) satellite observations.
geostationary↗
Jet-Surface Interaction Test: Phased Array Noise Source Localization Results
Subsonic jets are relatively simple. The peak noise source location gradually moves upstream toward the nozzle as frequency increases. 2) Supersonic jets are more complicated. The peak noise source location moves downstream as frequency increases through a BBSN hump. 3) In both subsonic and supersonic jets the peak noise source location corresponding to a given frequency of noise moves downstream as jet Mach number increases. 4) The noise generated at a given frequency in a BBSN hump is generated by a small number of shocks, not from all the shocks at the same time. 5) Single microphone spectrum levels decrease when the noise source locations measured with the phased array are blocked by a shielding surface. This consistency validates the phased array data and the stationary monopole source model used to process it. 6) Reflecting surface data illustrate that the law of reflection must be satisfied for noise to reflect off a surface toward an observer. Depending on the relative locations of the jet, the surface and the observer only some of the jet noise sources may satisfy this requirement. 7) The low frequency noise created when a jet flow impinges on a surface comes primarily from the trailing edge regardless of the axial extent impacted by the flow.
Podboy, Gary↗
Telescope With Reflecting Baffle
Telescope baffle made from combination of reflecting surfaces. In contrast with previous ellipsoidal reflecting baffles, new baffle reflects skew rays more effectively and easier to construct. For infrared telescopes, reflecting baffles better than absorbing baffles because heat load reduced, and not necessary to contend with insufficiency of infrared absorption exhibited by black coatings.
Linlor, W. I.↗
Real-Time Keratometer
Optical/electronic keratometer system produces contour map of corneal surface illuminated by infrared light pulses. Prevents operator error and eliminates need to apply fluorescent liquids to cornea. Keratometer provides both video display and numerical recording for corneal-transplant and radial-keratotomy surgery and for contact-lens fitting. Optical system of keratometer, circular grating pattern transmitted to corneal surface reflected onto identical circular grating on surface of fiber plate, producing Moire pattern. Data-analysis-and-display mode, laser light pulsed, and Moire pattern converted to digital information in frame grabber. Digital picture information sent to computer for processing and display.
Frazer, Robert E.↗
Amazon Forests Maintain Consistent Canopy Structure and Greenness During the Dry Season
The seasonality of sunlight and rainfall regulates net primary production in tropical forests. Previous studies have suggested that light is more limiting than water for tropical forest productivity, consistent with greening of Amazon forests during the dry season in satellite data.We evaluated four potential mechanisms for the seasonal green-up phenomenon, including increases in leaf area or leaf reflectance, using a sophisticated radiative transfer model and independent satellite observations from lidar and optical sensors. Here we show that the apparent green up of Amazon forests in optical remote sensing data resulted from seasonal changes in near-infrared reflectance, an artefact of variations in sun-sensor geometry. Correcting this bidirectional reflectance effect eliminated seasonal changes in surface reflectance, consistent with independent lidar observations and model simulations with unchanging canopy properties. The stability of Amazon forest structure and reflectance over seasonal timescales challenges the paradigm of light-limited net primary production in Amazon forests and enhanced forest growth during drought conditions. Correcting optical remote sensing data for artefacts of sun-sensor geometry is essential to isolate the response of global vegetation to seasonal and interannual climate variability.
dry season↗
Coastal and Inland Aquatic Data Products for the Hyperspectral Infrared Imager (HyspIRI)
The HyspIRI Aquatic Studies Group (HASG) has developed a conceptual list of data products for the HyspIRI mission to support aquatic remote sensing of coastal and inland waters. These data products were based on mission capabilities, characteristics, and expected performance. The topic of coastal and inland water remote sensing is very broad. Thus, this report focuses on aquatic data products to keep the scope of this document manageable. The HyspIRI mission requirements already include the global production of surface reflectance and temperature. Atmospheric correction and surface temperature algorithms, which are critical to aquatic remote sensing, are covered in other mission documents. Hence, these algorithms and their products were not evaluated in this report. In addition, terrestrial products (e.g., land use land cover, dune vegetation, and beach replenishment) were not considered. It is recognized that coastal studies are inherently interdisciplinary across aquatic and terrestrial disciplines. However, products supporting the latter are expected to already be evaluated by other components of the mission. The coastal and inland water data products that were identified by the HASG, covered six major environmental and ecological areas for scientific research and applications: wetlands, shoreline processes, the water surface, the water column, bathymetry and benthic cover types. Accordingly, each candidate product was evaluated for feasibility based on the HyspIRI mission characteristics and whether it was unique and relevant to the HyspIRI science objectives.
Infrared↗
A physical model for predicting bidirectional reflectances over bare soil
While most previous attempts to retrieve soil surface optical property characteristics have proceeded through a fitting of empirical functions to the data, an optimization technique is presently applied to a physically-based surface reflectance model developed for the study of planetary surfaces. This inversion procedure is shown to allow the direct estimation of the single-scattering coefficient, two parameters describing the 'hot spot' phenomenon, and two parameters describing the scattering phase function. A comparison of inversion technique results with both synthetic data and actual observations shows the model to be capable of predicting the observed bidirectional reflectances as well as directional-hemispherical reflectances; it can also build the complete radiance field over the upward hemisphere.
Pinty, Bernard↗
Temperature-dependent reflectivity of silicon carbide
The spectral reflectivity of a commercial silicon carbide (SiC) ceramic surface was measured at wavelengths from 2.5 to 14.5 microns and at temperatures ranging from 358 to 520 K using a NASA-developed multiwavelength pyrometer. The SiC surface reflectivity was low at the short wavelengths, decreasing to almost zero at 10 microns, then increasing rapidly to a maximum at approximately 12.5 microns, and decreasing gradually thereafter. The reflectivity maximum increased in magnitude with increasing surface temperature. The wavelength and temperature dependence can be explained in terms of the classical dispersion theory of crystals and the Lorentz electron theory. Electronic transitions between the donor state and the conduction band states were responsible for the dispersion. The concentration of the donor state in SiC was determined to be approximately 4 x 10 exp 18 and its ionization energy was determined to be approximately 71 meV.
Ng, Daniel↗
APC: A New Code for Atmospheric Polarization Computations
A new polarized radiative transfer code Atmospheric Polarization Computations (APC) is described. The code is based on separation of the diffuse light field into anisotropic and smooth (regular) parts. The anisotropic part is computed analytically. The smooth regular part is computed numerically using the discrete ordinates method. Vertical stratification of the atmosphere, common types of bidirectional surface reflection and scattering by spherical particles or spheroids are included. A particular consideration is given to computation of the bidirectional polarization distribution function (BPDF) of the waved ocean surface.
surface reflection↗
Prairie grassland bidirectional reflectances measured by different instruments at the FIFE site
Land surface reflectance measurements were obtained during the First ISLSCP Field Experiment (FIFE) field campaigns utilizing a variety of airborne and ground-based spectral radiometers. To study the validity of the assumption that the values obtained by the several different teams and instruments were interchangeable, the surface radiation measurement teams converged on a common site for one day during the fifth intensive field campaign in 1989. The bidirectional reflectances from the various instruments were basically found to be comparable.
Deering, D. W.↗
Measurement of bidirectional reflectance using a photographic technique
Monochromatic radiation angular distribution surface reflectance measurement using photographic reflectometer
Loehrlein, J. E.↗
Laser Pulse Bidirectional Reflectance from CALIPSO Mission
In this Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) study, we present a simple way of determining laser pulse bidirectional reflectance over snow/ice surface using the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) 532 nanometer polarization channels' measurements. The saturated laser pulse returns from snow and ice surfaces are recovered based on surface tail information. The method overview and initial assessment of the method performance will be presented. The retrieved snow surface bidirectional reflectance is compared with reflectance from both CALIOP cloud cover regions and Moderate Resolution Imaging Spectroradiometer (Earth Observing System (EOS)) (MODIS) Bi-directional Reflectance Distribution Function (BRDF) / Albedo model parameters. The comparisons show that the snow surface bidirectional reflectance over Antarctica for saturation region are generally reliable with a mean value of about 0.90 plus or minus 0.10, while the mean surface reflectance from cloud cover region is about 0.84 plus or minus 0.13 and the calculated MODIS reflectance at 555 nanometers from the BRDF / Albedo model with near nadir illumination and viewing angles is about 0.96 plus or minus 0.04. The comparisons here demonstrate that the snow surface reflectance underneath the cloud with cloud optical depth of about 1 is significantly lower than that for a clear sky condition.
Lu, Xiaomei↗
Vega landing sites - Venera 15/16 unit analogs from Pioneer Venus reflectivity and RMS slope data
Pioneer Venus radar data on surface properties have been used to compare the Vega spacecraft landing sites with the northern 1/4 of Venus mapped by the orbiters Venera 15 and 16. The regions surrounding both landing sites possess surface reflectivity and small-scale roughness properties most similar to those of mapped volcanoes and volcanic plains regions and different surface properties than those of mapped tectonic units. Regions analogous to the Vega 1 site are relatively rare, covering 2.8 percent of the mapped surface. Vega 2 analogs are much more common and cover 22.6 percent of the surface. Neither landing site is representative of the nearby highlands of Aphrodite, but the Vega 2 landing site is similar to much of the northern plains of Venus.
Bindschadler, Duane L.↗
Mars: Spectral signatures seen and unseen
Intrepretation of Mars surface reflectance spectra and geochemical modeling of the surface material are different approaches to the same problem (determining the compositional nature and evolution of the surface of Mars). The degree to which these two approaches agree is studied. This study involves: current understanding of the reflectance of Mars, and reflectance spectra of proposed Mars constituents.
Blaney, Diana L.↗
Solar total energy-large scale experiment, Shenandoah, Georgia
The design and development of a 7 meter diameter parabolic dish solar collector are discussed. Each of the four main subsystems of the collector: (1) reflector, (2) mount and drives, (3) receiver and (4) the controls, is discussed briefly with the major emphasis on the receiver design. To minimize development risks and production costs, a dish design based on use of stamped aluminum petals (sectors) was chosen. This design is similar to the design of a communication antenna already commercially produced. The reflective surface of the petals has a total reflectance of .86 and a specularity (dispersion) of 8 mrd. This performance is obtained by mechanical polishing and chemical brightening of the petal surface, followed by application of a clear RTV silicone protective coating. Selection of the material and weather proofing coated are discussed. Results from performance tests on an engineering development dish collector are presented and compared with pretest predictions.
Hensley, W. R.↗