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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

JPSS J2 Spectralon Performance Prelaunch

Spectralon® is a high reflectance excellent diffuser used to reflect sunlight for use as a calibrator for on-orbit and ground instruments. Radiometric calibration of the reflective bands in the 0.4 to 2.5μm wavelength range is performed by measuring the sunlight reflected from Spectralon®. Reflected sunlight is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon®. On-orbit exposure to sunlight results in solarization due to solar UV and the presence of residual contamination. Spectralon® quality is checked at start of build by measuring the change in reflectance on exposing a witness sample to 100 hours Solar UV as an indication of on orbit performance. For JPSS J2, the witness samples accompanied the sensor till 30 days before launch. Measuring the reflectance change on exposure to Solar UV of the witness samples accompanying the sensor through build and test is a better indication of on orbit performance as this includes any additional contamination during the build and test phase.

spectral reflectance↗
NASA NTRS2024

Assessing Mechanical Properties of Spacecraft Materials under Simulated Low Earth Orbit Atomic Oxygen Conditions

During a space mission, spacecraft surface materials are exposed to various damaging environmental factors including high-energy photons, electrons, atomic oxygen (AO) neutrals and ions, micrometeoroids and orbital debris, vacuum, and large temperature fluctuations. The resulting change in spacecraft material properties can significantly impact the performance and durability of spacecraft systems. Even though all aspects of the space environment can lead to the deterioration of spacecraft components, in low Earth orbit (LEO), the threat posed by AO is especially severe in terms of structural and optical damage, particularly to exterior spacecraft components that are susceptible to oxidation. A comprehensive understanding of material AO-induced weathering is essential for mission planning in the LEO environment. The presented work aims to evaluate the alterations in mechanical properties of selected innovative spacecraft materials and surface electronic system designs, such as Kapton® CR film coated front and back-side with polyimide coating containing AO-resistant filler, Kevlar EXO, and Bendable Electrodynamic Dust Shield (BEDS) architecture, under simulated AO exposure, utilizing the photoelasticity phenomenon in which birefringence is induced in a material when it is subjected to mechanical stress. Changes in stress patterns during the deformation of AO-exposed polymers were assessed using a large-field polariscope under varied deformation types. Patterns of colors were used for qualitative evaluations of residual stress. The stress patterns of the AO-exposed polymers were compared to those of the unexposed ones. Also, these patterns were correlated with data from Bidirectional Reflectance Distribution Function (BRDF) and surface morphology studies.

Yuliya Kuznetsova↗
NASA NTRSDate not supplied

A New BRDF Hotspot Model and its Use in Improving Radiative Transfer Model Efficiency and Application to VLIDORT-based PCRTM Model

The term “hotspot” refers to the sharp increase of reflectance occurring when incident (solar) and reflected (viewing) directions almost coincide in the backscatter direction. The accurate simulation of hotspot directional signatures is important for many remote sensing applications. The RossThick-LiSparse-Reciprocal (RTLSR) Bidirectional Reflectance Distribution Function (BRDF) model is widely used in radiative transfer simulations, and the hotspot model mostly used is from Maignan- Bréon but it typically requires large values of numerical quadrature and Fourier expansion terms in order to represent the hotspot accurately. To improve its use in atmospheric radiative transfer (RT) model simulations, we have developed a modified version based on the Maignan-Bréon’s hotspot BRDF model that converge much faster numerically, making it more practical for use in RT models that require Fourier expansion of BRDF to simulate the top-of-atmosphere (TOA) hotspot signatures. On basis of the vector linearized discrete ordinate radiative transfer model (VLIDORT), we have built a line-by-line based simulation system that can simulate the TOA radiance for atmospheres with 26 gases, different aerosols, dust and clouds, and surface with different BRDF. Using simulated results under diverse atmospheric conditions, we have developed VLIDORT-PCRTM model.

Xiaozhen Xiong↗
NASA NTRSDate not supplied

Inter-Calibrating the CERES SW Instrument Fluxes Using Deep Convective Clouds(DCC) Within the Same Sun-Synchronous Orbit

The NASA CERES EBAF product provides the scientific community observed TOA fluxes to monitor the Earth’s energy imbalance and to validate climate models. To provide a seamless EBAF 24-year record, the CERES instrument calibration must be stable and consistent across satellite records. Once the Terra and Aqua spacecraft are decommissioned at the end of 2025, the CERES project will rely on CERES instrument observations on the SNPP and NOAA-20 1:30 PM sun-synchronous orbits. The SNPP and NOAA-20 satellite orbit placement will prevent any time matched observations for inter-calibration efforts. The future Libera instrument, which will continue the CERES record and scheduled for launch in 2028 onboard the NOAA-22 satellite, will also need to be inter-calibrated without the aid of time-matched observations. Deep convective clouds (DCC) are the most Lambertian, brightest, tropical Earth invariant targets located at the tropopause making them ideal to radiometrically scale the CERES SW observed radiance to a common reference. The SNPP, NOAA-20 and future NOAA-22 satellites are in the same 16-day repeatable orbits, allowing the DCC targets to be observed with the same angular configuration. The empirically derived SNPP CERES SW channel Bidirectional Reflectance Distribution Function (BRDF) can be easily be applied to the NOAA-20 CERES SW channel radiances since they observe the nearly the same DCC systems. The DCC BRDF corrected radiances are analyzed collectively into radiance probability distribution functions (PDF). By comparing the PDF statistics, the SNPP and NOAA-20 CERES SW channel can be inter-calibrated. The inter-calibration coefficients are validated with the CERES instrument calibration team coefficients to optimize the methodology.

David Doelling↗
NASA NTRS1987

Polarization effects on hard target calibration of lidar systems

The theory of hard target calibration of lidar backscatter data, including laboratory measurements of the pertinent target reflectance parameters, is extended to include the effects of polarization of the transmitted and received laser radiation. The bidirectional reflectance-distribution function model of reflectance is expanded to a 4 x 4 matrix allowing Mueller matrix and Stokes vector calculus to be employed. Target reflectance parameters for calibration of lidar backscatter data are derived for various lidar system polarization configurations from integrating sphere and monostatic reflectometer measurements. It is found that correct modeling of polarization effects is mandatory for accurate calibration of hard target reflectance parameters and, therefore, for accurate calibration of lidar backscatter data.

Kavaya, Michael J.↗
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 NTRS1989

Reflectance and transmittance of snow at high spectral resolution

A radiative-transfer model for the bidirectional reflectance-distribution function (BRDF) shows that snow is moderately anisotropic in the NIR 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 and the transmittance for a variety of snow conditions were made throughout the winter and spring seasons with an SE-590 spectroradiometer, for wavelengths from 0.38 to 1.11 microns. Coincident with these, the surface grain properties were analyzed by stereological methods. A sphere with the same surface-to-volume ratio as the ice grains is used as the equivalent sphere.

Dozier, Jeff↗
NASA NTRS2002

Airborne Spectral Measurements of Ocean Anisotropy during CLAMS

The Cloud Absorption Radiometer (CAR) aboard the University of Washington Convair CV-580 research aircraft obtained bidirectional reflectance-distribution function (BRDF) of Atlantic Ocean and Dismal Swamp between July 10 and August 2, 2001. The BRDF measurements (15 in total, 8 uncontaminated by clouds) obtained under a variety of sun angles and wind conditions, will be used to characterize ocean anisotropy in support of Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) science objectives principally to validate products from NASA's EOS satellites, and to parameterize and validate BRDF models of the ocean. In this paper we present results of BRDF of the Ocean under different sun angles and wind conditions. The CAR is capable of measuring scattered light in fourteen spectral bands. The scan mirror, rotating at 100 rpm, directs the light into a Dall-Kirkham telescope where the beam is split into nine paths. Eight light beams pass through beam splitters, dichroics, and lenses to individual detectors (0.34-1.27 micron), and finally are registered by eight data channels. They are sampled simultaneously and continuously. The ninth beam passes through a spinning filter wheel to an InSb detector cooled by a Stirling cycle cooler. Signals registered by the ninth data channel are selected from among six spectral channels (1.55-2.30 micron). The filter wheel can either cycle through all six spectral bands at a prescribed interval (usually changing filter every fifth scan line), or lock onto any one of the six spectral bands and sample it continuously. To measure the BRF of the surface-atmosphere system, the University of Washington CV-580 had to fly in a circle about 3 km in diameter above the surface for roughly two minutes. Replicated observations (multiple circular orbits) were acquired over selected surfaces so that average BRF smooth out small-scale surface and atmospheric inhomogeneities. At an altitude of 600 m above the targeted surface area and with a 1 degree IFOV, the pixel resolution is about 10 m at nadir and about 270 m at an 80 deg. viewing angle from the CAR.

Gatebe, C. K.↗
NASA NTRS2016

Airborne Spectral BRDF of Various Surface Types (Ocean, Vegetation, Snow, Desert, Wetlands, Cloud Decks, Smoke Layers) for Remote Sensing Applications

In this paper we describe measurements of the bidirectional reflectance-distribution function (BRDF) acquired over a 30-year period (1984-2014) by the National Aeronautics and Space Administration's (NASA's) Cloud Absorption Radiometer (CAR). Our BRDF database encompasses various natural surfaces that are representative of many land cover or ecosystem types found throughout the world. CAR's unique measurement geometry allows a comparison of measurements acquired from different satellite instruments with various geometrical configurations, none of which are capable of obtaining such a complete and nearly instantaneous BRDF. This database is therefore of great value in validating many satellite sensors and assessing corrections of reflectances for angular effects. These data can also be used to evaluate the ability of analytical models to reproduce the observed directional signatures, to develop BRDF models that are suitable for sub-kilometer-scale satellite observations over both homogeneous and heterogeneous landscape types, and to test future spaceborne sensors. All of these BRDF data are publicly available and accessible in hierarchical data format (http:car.gsfc.nasa.gov/).

albedo↗
NASA NTRS2017

Multispectral Snapshot Imagers Onboard Small Satellite Formations for Multi-Angular Remote Sensing

Multispectral snapshot imagers are capable of producing 2D spatial images with a single exposure at selected, numerous wavelengths using the same camera, therefore operate differently from push broom or whiskbroom imagers. They are payloads of choice in multi-angular, multi-spectral imaging missions that use small satellites flying in controlled formation, to retrieve Earth science measurements dependent on the targets Bidirectional Reflectance-Distribution Function (BRDF). Narrow fields of view are needed to capture images with moderate spatial resolution. This paper quantifies the dependencies of the imagers optical system, spectral elements and camera on the requirements of the formation mission and their impact on performance metrics such as spectral range, swath and signal to noise ratio (SNR). All variables and metrics have been generated from a comprehensive, payload design tool. The baseline optical parameters selected (diameter 7 cm, focal length 10.5 cm, pixel size 20 micron, field of view 1.15 deg) and snapshot imaging technologies are available. The spectral components shortlisted were waveguide spectrometers, acousto-optic tunable filters (AOTF), electronically actuated Fabry-Perot interferometers, and integral field spectrographs. Qualitative evaluation favored AOTFs because of their low weight, small size, and flight heritage. Quantitative analysis showed that waveguide spectrometers perform better in terms of achievable swath (10-90 km) and SNR (greater than 20) for 86 wavebands, but the data volume generated will need very high bandwidth communication to downlink. AOTFs meet the external data volume caps well as the minimum spectral (wavebands) and radiometric (SNR) requirements, therefore are found to be currently feasible in spite of lower swath and SNR.

Nag, Sreeja↗
NASA NTRS2021

Characterization of Bidirectional Transmissive and Reflective Properties of Black Silicon

This paper describes the initial work of characterizing the transmissive and reflective properties of black silicon diffusers. The diffusers were fabricated from a 100 mm diameter black silicon sample at NASA’s Goddard Space Flight Center (GSFC). The directional hemispherical reflectance from 250 nm to 2500 nm and BRDF/BTDF measurements at 632.8 nm, 1064 nm, and 1550 nm were measured using the GSFC Diffuser Calibration Laboratory’s (DCL) spectrophotometer and optical scatterometer. The diffusers exhibit a low level of specular reflection up to ~1100 nm with no evidence of retroscatter. The measurements are traceable to those made at the National Institute of Standards and Technology (NIST).

Bidirectional Reflectance Distribution Function↗
NASA NTRS2023

Assessment of the Performance of the Atmospheric Correction Algorithm MAJA for Sentinel-2 Surface Reflectance Estimates

The correction of atmospheric effects on optical remote sensing products is an essential component of Analysis Ready Data (ARD) production lines. The MAJA processor aims at providing accurate time series of surface reflectances over land for satellite missions, such as Sentinel-2, Venμs, and Landsat 8. The Centre d’Études Spatiales de la Biosphère (CESBIO) and the Centre National d’Études Spatiales (CNES) share a common effort to maintain, validate, and improve the MAJA processor, using state-of-the-art ground measurement sites, and participating in processor inter-comparisons, such as the Atmospheric Correction Intercomparison Exercise (ACIX). While contributing to the second ACIX-II Land validation exercise, it was found that the candidate MAJA dataset could not adequately be compared to the main reference dataset. MAJA reflectances were corrected for adjacency and topography effects while the reference dataset was not, excluding MAJA from a part of the performance metrics of the exercise. The first part of the following study aims at providing complementary performance assessment to ACIX-II by reprocessing MAJA surface reflectances without adjacency nor topographic correction, allowing for an un-biased full resolution comparison with the reference Sentinel-2 dataset. The second part of the study consists of validating MAJA against surface reflectance measurements time series of up to five years acquired at three automated stations. Both approaches provide extensive insights on the quality of MAJA Sentinel-2 Level 2 products.

ROSAS↗
NASA NTRS1987

Computer controlled facility for measurement of the bidirectional scattering distribution function

A computer controlled facility for measurement of the bidirectional scattering distribution function is described, with application to the evaluation of spacecraft surface contamination in optical and thermal control systems. An automatic detector/sampleholder allows a 360-deg detector scan around the sample for a complete bidirectional reflectance and transmittance distribution function characterization. The system demonstrates reduced data acquisition and reduction cycle time, and improved accuracy.

Carosso, Paolo A.↗
NASA NTRS2012

Coherent Detector for Near-Angle Scattering and Polarization Characterization of Telescope Mirror Coatings

A report discusses the difficulty of measuring scattering properties of coated mirrors extremely close to the specular reflection peak. A prototype Optical Hetero dyne Near-angle Scatterometer (OHNS) was developed. Light from a long-coherence-length (>150 m) 532-nm laser is split into two arms. Acousto-optic modulators frequency shift the sample and reference beams, establishing a fixed beat frequency between the beams. The sample beam is directed at very high f/# onto a mirror sample, and the point spread function (PSF) formed after the mirror sample is scanned with a pinhole. This light is recombined by a non-polarizing beam splitter and measured through heterodyne detection with a spectrum analyzer. Polarizers control the illuminated and analyzed polarization states, allowing the polarization dependent scatter to be measured. The bidirectional reflective or scattering distribution function is normally measured through use of a scattering goniometer instrument. The instrumental beam width (collection angle span) over which the scatterometer responds is typically many degrees. The OHNS enables measurement at angles as small as the first Airy disk diameter.

Macenka, Steven A.↗
NASA NTRS1988

Standards For Bidirectional Reflectance And Transmittance

Sample-to-sample variations reduced. Improved reference or standard samples for measurement of bidirectional reflectance or transmittance distribution functions made by depositing single layers of spheres of same size on flat surfaces. New reference samples replicated easily and consistently, durable with moderate care. Used to reconcile different measurement facilities.

Witherow, William K.↗