To address the 2017 Earth Science Decadal Survey an instrument utilizing two high-throughput F/1.8 Dyson imaging spectrometers is designed to provide 10 nm spectral sampling over the 380-2500 nm range with 180-kilometer swath at 30-meter resolution while delivering high uniformity over the field of view and spectral range.
The Carbon Plume Mapper (CPM) instrument is a high-fidelity imaging spectrometer developed to pinpoint, quantify, and track methane (CH4) and carbon dioxide (CO2) point source emissions. CPM is an optically fast F/1.8 Dyson spectrometer that operates over the spectral range of 400 – 2500 nm with a spectral sampling of 5.0 nm. Three diffraction grating designs were measured in a testbed to provide a reliable prediction of grating performance in a Dyson system to inform CPM grating design. This paper will detail the gratings, testbed design, measurement process, and data used to assess grating efficiency through wavelength (500-1700 nm) of three grating designs, both full aperture and sub-aperture for two field angles, polarized and unpolarized.
The Earth Surface Mineral Dust Source Investigation (EMIT) instrument is a high fidelity imaging spectrometer developed to characterize surface mineralogy of the Earth's dust source regions over the spectral range of 380- 2500 nm and spectral sampling of 7.4 nm. EMIT will close the current knowledge gap in dust source mineral composition by collecting over 1 billion high signal-to-noise ratio spectra in this region of our planet. These new measurements will be used in conjunction with state-of-the-art Earth System Models to understand and reduce the uncertainty in the radiative forcing effect of mineral dust aerosols. EMIT will be deployed on the International Space Station that has an orbit that is well suited for measuring the arid land regions of the Earth. The optical design utilizes a Dyson spectrometer to reduce volume and mass for a fast (F/1.8) and wide swath (1240 samples) optical system. An overview of the EMIT optical design, development, and current status are discussed.
We discuss detailed tolerancing methods developed for imaging spectrometers at NASA Jet Propulsion Laboratory, California Institute of Technology using the Earth Surface Mineral Dust Source Investigation (EMIT) imaging spectrometer as an illustrative example. We tolerance five metrics simultaneously: along-track response function, crosstrack response function, spectral response function, spectral centroid uniformity, and spatial centroid uniformity. A method to calculate tolerancing sensitivities for each metric directly, a method to statistically combine Monte Carlo files from multiple tolerancing runs, and example summary error budgets that communicate the key and driving tolerances for each metric are discussed. These methods facilitate rapid and semi-automated assessment of the predicted performance of imaging spectrometer systems from design through to assembly and launch life cycle, using metrics that are directly relevant to the extraction of accurate spectroscopic data from these instruments.
A sunflower-like starshade positioned between an exoplanet host star and a telescope forms a deep shadow at the telescope enabling the faint exoplanet to be viewed without being overwhelmed by veiling glare from the star. The starshade perimeter has hundreds of meters of sharp edge that are directly exposed to sunlight. The sunlight diffracts and reflects from the edge resulting in a glint pattern that can be brighter than the exoplanet. We have developed models of the edge glint to explain laboratory measurements, to guide the development of edges with minimum glint, and to determine the fundamental glint floor which is set by diffraction. The models include finite difference time domain calculations, Sommerfeld's half-plane diffraction expressions, and a micro-facet scattering model. Models successfully reproduce the features and magnitude of the measured polarization-dependent scatter and show that measured edges are performing near the theoretical limit.
Observations from the Ground-based Multiangle SpectroPolarimetric Imager (GroundMSPI) are used to relate angle of linear polarization (AoLP) measurements to material properties and illumination conditions in sunlit outdoor environments. GroundMSPI is a push-broom spectropolarimetric camera with an uncertainty in degree of linear polarization (DoLP) of ±0.005. This polarimetric accuracy yields useful AoLP images even when the DoLP is less than 0.02. AoLP images are reported with respect to dependency on surface texture, surface orientation, albedo, and illumination conditions. Agreement with well-known principles of polarized light scattering is illustrated, and several special cases are described. Expected observations of AoLP tangential to surface orientation and AoLP perpendicular to the scattering plane are reported. Significant changes in the AoLP are observed from common variations in outdoor illumination conditions. Also, simple variants in material properties change the dominant polarized light scattering process and thus the AoLP. Measurement examples that isolate a 90 deg AoLP flip are shown for a sunny and cloudy day as well as an object of high and low albedo.
Representative examples from 3 years of measurements from JPL’s ground-based multiangle spectropolarimetric imager (GroundMSPI) are compared to a Mueller matrix bidirectional reflectance distribution function (mmBRDF). This mmBRDF is used to model polarized light scattering from solar illuminated surfaces. The camera uses a photoelastic-modulator-based polarimetric imaging technique to measure linear Stokes parameters in three wavebands (470, 660, and 865 nm) with a ±0.005 uncertainty in degree of linear polarization. GroundMSPI measurements are made over a range of scattering angles determined from a fixed viewing geometry and varying sun positions over time. This microfacet mmBRDF model predicts an angle of the linear polarization that is consistently perpendicular to the scattering plane and therefore is only appropriate for rough surface types. The model is comprised of a volumetric reflection term plus a specular reflection term of Fresnel-reflecting microfacets. The following modifications to this mmBRDF model are evaluated: an apodizing shadowing function, a Bréon or Gaussian microfacet scattering density function, and treating the surface orientation as an additional model parameter in the specular reflection term. The root-mean-square error (RMSE) between the GroundMSPI measurements and these various forms of the microfacet mmBRDF model is reported. Four example scenes for which a shadowed-Bréon microfacet mmBRDF model yields realistic estimates of surface orientation, and the lowest RMSE among other model options are shown.
Starshades, combined with future space telescopes, provide the ability to detect Earth-like exoplanets in the habitable zone by producing high contrast ratios at small inner working angles. The primary function of a starshade is to suppress light from a target star such that its orbiting planets are revealed. In order to do so, the optical edges of the starshade must maintain their precise in-plane profile to produce the necessary apodization function. However, an equally important consideration is the interaction of these edges with light emanating from our own Sun as scattered and/or diffracted sunlight can significantly degrade the achievable contrast. This paper describes the technical efforts performed to obtain precision, low-scatter optical edges for future starshades. Trades between edge radius (i.e. sharpness) and surface reflectivity have been made and small-scale coupons have been produced using scalable manufacturing processes. A custom scattered light testbed has been developed to quantify the magnitude of scattered light over all sun angles. Models have also been developed to make predictions on the level of reflected and/or diffracted light for various edge architectures. The results of these studies have established a current baseline approach which implements photochemical etching techniques on thin metal foils.
Remotely sensing plant canopy water status remains a long term goal of remote sensing research.
Established approaches to estimating canopy water status — the Crop Water Stress Index, the Water
Deficit Index and the Equivalent Water Thickness — involve measurements in the thermal or reflective
infrared. Here we report plant water status estimates based upon analysis of polarized visible imagery of a
cotton canopy measured by ground Multi-Spectral Polarization Imager (MSPI). Such estimators potentially
provide access to the plant hydrological photochemistry that manifests scattering and absorption effects in
the visible spectral region.
Many models used to represent the boundary condition for the separation of atmospheric scattering from the surface reflectance in polarized remote sensing measurements assume that the polarized surface reflectance is spectrally neutral. The Spectral Invariance Hypothesis asserts that the magnitude and shape of the polarized bidirectional reflectance factor (pBRF) is equal for all wavelengths. In order to test this hypothesis, JPL's Ground-based Multiangle SpectroPolarimetric Imager (GroundMSPI) is used to measure polarization information of different outdoor surface types. GroundMSPI measures the linear polarization Stokes parameters (I, Q, U), at three wavelengths, 470 nm, 660 nm, and 865 nm. The camera is mounted on a two-axis gimbal to accurately select the view azimuth and elevation directions. On clear sky days we acquired day-long scans of scenes that contain various surface types such as grass, dirt, cement, brick, and asphalt and placed a Spectralon panel in the camera field of view to provide a reflectance reference. Over the course of each day, changing solar position in the sky provides a large range of scattering angles for this study. The polarized bidirectional reflectance factor (pBRF) is measured for the three wavelengths and the best fit slope of the spectral correlation is reported. This work reports the range of best fit slopes measured for five region types.
Representative examples from three-years of measurements from JPL's Ground-based Multiangle SpectroPolarimetric Imager (Ground-MSPI)[1] are compared to a model for the surface polarized bidirectional reflectance distribution matrix (BRDM). Ground-MSPI is an eight-band spectropolarimetric camera mounted on a rotating gimbal to acquire push-broom imagery of outdoor landscapes. The camera uses a photoelastic-modulator-based polarimetric imaging technique to measure linear Stokes parameters in three wavebands (470, 660, and 865 nm) with a ±0.005 uncertainty in degree of linear polarization (DoLP). Comparisons between MSPI measurements, BRDM models, and common modifications to the model are made over a range of scattering angles determined from a fixed viewing geometry and varying sun positions over time. The BRDM model is comprised of a volumetric reflection term plus a specular reflection term of Fresnel-reflecting micro-facets. We consider modifications to this model using a shadowing function and two different micro-facet scattering density functions. We report the root-mean-square error (RMSE) between the Ground-MSPI measurements and BRDM model. The BRDM model predicts an angle of the linear polarization (AoLP) that is perpendicular to the scattering plane. This is usually, but not always, observed in Ground-MSPI measurements and in this work we offer explanations for some of the deviations from the model.
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