We introduce a new unified atmospheric-topographic correction approach that estimates surface geometry directly from the radiance measurement. Surface topography influences the at-sensor radiance measurement, making precise topography modeling critical in applications like vegetation or snow studies in mountainous terrain. Currently, elevation maps are used to derive topographic variables such as the slope and sky-view factor. This process is error-prone since static global digital elevation models do not generally achieve the accuracy required, and even minor mismatches in spatial resolution can introduce significant artifacts in downstream processing. Here we demonstrate that it is possible to estimate topographic parameters directly from spectral data, ensuring perfect physical consistency, temporal coincidence, and spatial alignment. We present experiments estimating topographic slope in two scenes in Southern California, with data from NASA's Next Generation Airborne Visible/Near Infrared Imaging Spectrometer (AVIRIS-NG). We compared our radiance-based estimates against high-resolution lidar datasets. Our initial validation result showed a correlation of R2 = 0.864 (n = 160) over the homogeneous surface of Beckman Auditorium's cone-shaped roof on the Caltech campus in Pasadena, California. We then validate the model over a larger study site near Santa Clarita, California, finding R2 = 0.923 (n = 40, 000) in a 350 x 350 m area. The accuracy of our model estimates, combined with its systematic advantages over the alternative, show the potential of the approach for use in both airborne campaigns and orbital missions.

Models linking surface characteristics within incident solar radiation are inexorably dependent on the topography of the given region. To date, however, most operational surface reflectance retrievals treat this dependence by assuming a flat terrain, leading to significant deviations in the estimated reflectance. Here, we demonstrate that incorporating dynamic topography directly into the joint surface and atmospheric model during retrievals has several advantages. First, it allows for a more complete physical accounting of downwelling illumination, providing more accurate estimates of the absolute magnitude of reflectance. Second, it facilitates a superior resolution of the atmospheric state, most notably due to the confounding influence of atmospheric aerosols and unresolved topographic effects. Our methodology utilizes a practical, high-fidelity, model-driven approach to separate out diffuse and direct irradiation and account for topographic effects during the joint inversion of atmosphere and surface properties. We achieve this by enhancing the atmosphere/surface inversion to account for the radiative transfer effects of surface slope. We further demonstrate how uncertainties in topographic features can be quantified and leveraged within our formulation for a more realistic posterior uncertainty estimates. Our results demonstrate that the inclusion of topographic effects into the retrieval model reduces errors in the reflectance of an only moderately rugged terrain by more than 15%, and that a post hoc accounting of topography cannot achieve these same results.

Equations used to compute Stokes phase matrices for macroscopically isotropic mirror symmetric scat-tering medium from Stokes scattering matrices are reformulated to eliminate numerical instability issues and to eliminate the need to treat positive and negative relative azimuth angles as separate cases (C) 2022 Published by Elsevier Ltd.

MODTRAN7, a polarimetric extension of the MODTRAN6 atmospheric radiative transfer model, is being developed. The vectorized MODTRAN7 will provide band model (BM), correlated-k (Ck), and line- by-line options for computing Stokes vectors. The radiative transfer problem is being solved for Isotropic and Symmetric Media (ISM) using the basic phenomenology described in the classic text by Mishchenko, Travis and Lacis, "Multiple Scattering of Light by Particles" 1. VDISORT, a vectorized version of the DISORT scalar model currently in MODTRAN6, will compute the Stokes vectors for 1-D atmospheres. The MODTRAN method for extracting spherical refractive path contributions from the plane parallel scattering models will be adapted for the polarimetric model. The upgrade is to include new polarimetric optical properties for both the existing aerosol and cloud models within MODTRAN and for recently developed cirrus cloud and dust particulate data. A new algorithm has been developed that enables Generalized Spherical Function (GSF) expansion coefficients to be accurately computed to very high order. MODTRAN has already been restructured to generate Stokes vector data for single scatter solar/lunar applications and validated against the NASA Goddard Space Flight Center model, 6SV.

We present an overview and several important upgrades to the Vector Discrete Ordinate Radiative Transfer (VDISORT) code. VDISORT is a polarized (vector) radiative transfer code that can be applied to a wide range of research problems including the Earth’s atmosphere and ocean system. First, a solution is developed to the complex algebraic eigenvalue problem resulting when the b 2 component of the Stokes scattering matrix is non-zero. This solution is needed to compute the V component of the Stokes vector I=[I∥,I⊥,U,V]T . Second, a significant improvement in computational efficiency is obtained by reducing the dimension of the algebraic eigenvalue by a factor of 2 resulting in a speed increase of about 2 3 = 8. Third, an important upgrade of the VDISORT code is obtained by developing and implementing a method to enable output at arbitrary polar angles by the integration of the source function (ISF) method for partially reflecting Lambertian as well as general non-Lambertian surfaces. Fourth, a pseudo-spherical treatment has been implemented to provide important corrections for Earth curvature effects at near horizontal solar zenith and observation (viewing) polar angles. Fifth, a post-processing single-scattering correction procedure has been developed to enhance the accuracy and speed for strongly forward-peaked scattering. With these significant improvements the results from the upgraded version of the VDISORT code match published benchmark results for Rayleigh scattering, Mie scattering, and scattering by non-spherical cirrus particles. The performance of VDISORT for a polarized incident beam source is equally satisfactory. The VDISORT vector radiative transfer code is made public and freely available for use by the growing polarimetric research community including the space-borne polarimeters on the future NASA PACE and AOS missions.

Efforts have begun on development of MODTRAN7, a polarimetric version of MODTRAN6. A beta-release, modeling single scatter Stokes parameters and linear polarized ground emission is planned for later this year. The basic formalism, single scattering models, and demonstration calculations are presented.

A 3-parameter lineshape function, defined by convolving the Gross and Doppler lineshape functions, was previously proposed as a spectrally universal form. It reduces to the Gross lineshape at microwave and longer wavelengths, and transitions to the Voigt lineshape in the infrared. Unfortunately, the Gross lineshape function is known to not capture the asymmetry of the true microwave lineshape as well as the Weiskopf form. Here, a Center-Shift at Half-Width parameter is introduced to create a modified-Gross lineshape function applicable in the microwave. Its spectral convolution with the Doppler lineshape is demonstrated to provide a 4-parameter lineshape function accurate for the entire ultraviolet to radio wave spectral range.

A new MODTRAN6 output file and post-processing utility has been written to enable generation of single scattering adjacency (aka point spread) functions. The approach relies on Helmholtz reciprocity. A series of MODTRAN single scattering solar radiance calculations are performed to build up a table of adjacency file segment contributions. Changes to the MODTRAN source were minimal, and the postprocessing utility can rapidly define the adjacency function at fine spatial resolution. The basic approach is presented along with sample output.

The Special Section on Atmospheric Propagation is a forum for the presentation of research on the physics of light propagation, optical remote sensing, and EO/IR effects in the atmosphere, to include distributed volume turbulence, gravity waves, vortex shedding, stably stratified turbulence, persistent eddies, and cloud/aerosol/molecular scattering and absorption, refractive effects such as mirages and over-the-horizon viewing, as well as characterization of these phenomena.Research was solicited on distributed volume turbulence in terms of Kolmogorov and non-Kolmogorov turbulence, optical beam properties, scintillation effects, phase variance, branch points, etc.Also sought were studies on the effects of meteorological phenomena such as refractive layers, boundary layer measurements, stratified turbulence, gravity waves, vortex shedding, large scale eddies, micro-meteorology, and cloud/aerosol extinction.Methods to address these turbulence and extinction effects can be, and have been to some extent, addressed with atmospheric modeling and simulation (M&S) that include multi-phenomena atmospheric characterizations and computationally efficient methods to incorporate physically realistic characterizations into M&S.The M&S techniques exploit numerical weather prediction (NWP) modeling and attempt to enhance it with turbulence and aerosol content characterizations that are not common NWP products.Further improvement of NWP is expected from atmospheric measurement devices that go beyond standard pressure, temperature, humidity, and wind sensors and include the potential implementation of turbulence measuring devices such as sonic anemometers, scintillometers, time-lapse imagers, digital holographic instruments, and aerosol/particle measurement devices such as water and alcohol-based condensation particle counters and particle sizers.Al-Younis et al. developed and studied two approaches for the prediction of optical refraction effects in the lower atmosphere.Low-cost, time-lapse camera systems were deployed to measure image displacements of mountain ridge targets due to atmospheric refraction.Measurements were compared with image displacement predictions provided by (1) a ray-tracing evaluation of NWP data and (2) a machine learning algorithm with measured meteorological values as inputs.The displacement prediction results for both methods were found to be consistent with the field imagery in overall amplitude and phase.Mark Spencer submitted two papers that used wave-optics simulations to look at the Monte Carlo averages associated with turbulence and steady-state thermal blooming (SSTB).The first implemented steady-state simulations; the second considered time-dependent simulations.The goal was to investigate turbulence thermal blooming interaction (TTBI).At wavelengths near 1 μm, TTBI increases the amount of constructive and destructive interference (i.e., scintillation) that results from high-power laser beam propagation through distributed-volume atmospheric aberrations.Abdullah-Al-Mamun and Voelz described the effects of a temperature inversion layer in the lower atmosphere on dispersion and angle of arrival of highly directional beams such as those associated with near-surface free space optical communications systems.For a single wavelength, a linear increase of angle of arrival with initial launch angle was found for the standard atmosphere, but this trend was significantly altered in the presence of an inversion layer.

Remote imaging spectroscopy in the 0.4–2.5-μm visible and shortwave infrared (VSWIR) range captures the majority of solar-reflected energy and enables a wide range of earth surface studies. This spectral range is also influenced by atmospheric effects including absorption from atmospheric gases and aerosols, Rayleigh scattering, and particle scattering. Globally consistent surface measurements must compensate for these atmospheric effects. This article reviews the physical and mathematical foundations of modern VSWIR atmospheric retrieval, focusing on imaging spectrometers. We assess sensitivity of the retrieval to errors in atmospheric state estimation. Finally, we describe some promising avenues of future research to support the next generation of orbital imaging spectrometers.

MODTRAN models the molecular absorption for the entire 0 to 50,000 cm(-1) spectral range. Typically, radiative transfer models define distinct line-shape functions depending on the spectral region. This can produce spectral anomalies at the transitions. A 3-parameter GrossDoppler line-shape function is defined that provides a spectrally-universal model for computing molecular absorption.

The MODTRAN6 radiative transfer model enjoys widespread use throughout the remote sensing community. A multiple line-of-sight option is now available that dramatically increases processing time when spectral transmittances and radiances are required for multiple paths within a scene. The option is demonstrated for three applications: (1) modeling residuals between plane-parallel and spherical earth atmosphere hemispherical fluxes; (2) computing wave boundary layer weighting functions; and (3) generating look-up tables for simulating an airborne visible through shortwave infrared hyperspectral sensor.

A new line-by-line (LBL) algorithm has been developed for use within the MODTRAN (R) 6(1) atmospheric radiative transfer model. The model computes both emitted and scattered line-of-sight radiances utilizing a spherical refractive geometry package and the DISORT discrete ordinate model to solve the 1-D scattering problem. The MODTRAN6 LBL method distinguishes itself from most other monochromatic models in that the radiative transfer problem is solved at arbitrarily fine spectral resolution within disjoint and contiguous 0.1 cm(-1) steps, marching through the user-specified band pass. The advantage of this approach is that the predominantly Lorentzian, temperature and pressure dependent contributions to each 0.1 cm-1 spectral bin from molecular transitions centered more than 0.05 cm-1 from the bin can be summed off-line and fit to a simple analytic form. The line-shape of each molecular transition is explicitly modeled on-the-fly only over a narrow 0.2 cm(-1) sub-region. The challenge of this approach is to ensure that spectral discontinuities do not arise at spectral bin edges, where the method for modeling absorption from individual molecular lines changes abruptly. Interpolations based on the radiative transfer physics of the pre-computed line tail data are introduced to produce a smooth transition across these edges. Spectral validations against LBLRTM verify the fidelity of the approach. The new MODTRAN LBL algorithm is used to quantify the accuracy of the MODTRAN band model and correlated-k statistical approaches under varying conditions. Future upgrades to the MODTRAN band model, correlated-k and LBL methods are also discussed. (C) 2017 Elsevier Ltd. All rights reserved.

The well-established and extensively validated atmospheric radiative transfer model MODTRAN® 1 is widely used by the remote sensing community to define the mapping from surface spectral reflectance to solar scatter radiance as a function of atmosphere definition. This mapping is subsequently used in atmospheric correction/compensation models to define the reverse mapping from multi- and hyperspectral images to ground surface spectral reflectances. Since MODTRAN employs a stratified spherical shell atmosphere and accounts for spherical refraction, its applicability extends to low sun and highly off-nadir scenarios. The discrete ordinate module DISORT solves the RTE to compute multiple scattering solar path radiances for MODTRAN. However, there is a problem: DISORT is a plane-parallel atmosphere model. In this paper, a method for modifying DISORT to compute the curved-path segment radiance contributions required by MODTRAN is described.

A validated, polarimetric 3-dimensional simulation capability, P-MCScene, is being developed by generalizing Spectral Sciences' Monte Carlo-based synthetic scene simulation model, MCScene, to include calculation of all 4 Stokes components. P-MCScene polarimetric optical databases will be generated by a new version (MODTRAN7) of the government-standard MODTRAN radiative transfer algorithm. The conversion of MODTRAN6 to a polarimetric model is being accomplished by (1) introducing polarimetric data, by (2) vectorizing the MODTRAN radiation calculations and by (3) integrating the newly revised and validated vector discrete ordinate model VDISORT3. Early results, presented here, demonstrate a clear pathway to the long-term goal of fully validated polarimetric models.

We describe the development of Plume Tracker, an interactive toolkit for the analysis of multispectral thermal infrared observations of volcanic plumes and clouds. Plume Tracker is the successor to MAP_SO2, and together these flexible and comprehensive tools have enabled investigators to map sulfur dioxide (SO2) emissions from a number of volcanoes with TIR data from a variety of airborne and satellite instruments. Our objective for the development of Plume Tracker was to improve the computational performance of the retrieval procedures while retaining the accuracy of the retrievals. We have achieved a 300× improvement in the benchmark performance of the retrieval procedures through the introduction of innovative data binning and signal reconstruction strategies, and improved the accuracy of the retrievals with a new method for evaluating the misfit between model and observed radiance spectra. We evaluated the accuracy of Plume Tracker retrievals with case studies based on MODIS and AIRS data acquired over Sarychev Peak Volcano, and ASTER data acquired over Kilauea and Turrialba Volcanoes. In the Sarychev Peak study, the AIRS-based estimate of total SO2 mass was 40% lower than the MODIS-based estimate. This result was consistent with a 45% reduction in the AIRS-based estimate of plume area relative to the corresponding MODIS-based estimate. In addition, we found that our AIRS-based estimate agreed with an independent estimate, based on a competing retrieval technique, within a margin of ±20%. In the Kilauea study, the ASTER-based concentration estimates from 21 May 2012 were within ±50% of concurrent ground-level concentration measurements. In the Turrialba study, the ASTER-based concentration estimates on 21 January 2012 were in exact agreement with SO2 concentrations measured at plume altitude on 1 February 2012.

: The MODTRAN (MODerate resolution atmospheric TRANsmission) computer code is widely used throughout the Department of Defense (DoD), in many other Federal Government departments and agencies, and worldwide by research scientists in many fields for the prediction and analysis of optical measurements through the atmosphere. The code is embedded in many DoD operational and research sensor and data processing systems, particularly those involving the removal of atmospheric effects, commonly referred to as atmospheric correction, in remotely sensed multi- and hyperspectral imagery. These MODTRAN calculations are the most time-consuming part of the atmospheric correction process. Given the ever increasing capabilities of spectral sensors to quickly generate enormous quantities of data, combined with the need for image analysts to provide actionable information to the Warfighter in a timely manner, significantly speed up of MODTRAN processing was desired. Furthermore, MODTRAN was written in an outdated programming style and language (Fortran77), making it time-consuming and costly to upgrade and maintain, and difficult to integrate with other software. To address these limitations, SSI has developed the NextGen MODTRAN code (MODTRAN6), with a focus on current Air Force needs related to remote sensing applications used for Wide Area Surveillance.

Polarimetric measurements can substantially enhance the ability of both spectrally resolved and single band imagery to detect the proliferation of weapons of mass destruction, providing data for locating and identifying facilities, materials, and processes of undeclared and proliferant nuclear weapons programs worldwide. Unfortunately, models do not exist that efficiently and accurately predict spectral polarized signatures for the materials of interest embedded in complex 3D environments. Having such a model would enable one to test hypotheses and optimize both the enhancement of scene contrast and the signal processing for spectral signature extraction. The Phase I set the groundwork for development of fully validated polarimetric spectral signature and scene simulation models. This has been accomplished 1. by (a) identifying and downloading state-of-the-art surface and atmospheric polarimetric data sources, (b) implementing tools for generating custom polarimetric data, and (c) identifying and requesting US Government funded field measurement data for use in validation; 2. by formulating an approach for upgrading the radiometric spectral signature model MODTRAN to generate polarimetric intensities through (a) ingestion of the polarimetric data, (b) polarimetric vectorization of existing MODTRAN modules, and (c) integration of a newly developed algorithm for computing polarimetric multiple scattering contributions; 3. by generating an initial polarimetric model that demonstrates calculation of polarimetric solar and lunar single scatter intensities arising from the interaction of incoming irradiances with molecules and aerosols; 4. by developing a design and implementation plan to (a) automate polarimetric scene construction and (b) efficiently sample polarimetric scattering and reflection events, for use in a to be developed polarimetric version of the existing first-principles synthetic scene simulation model, MCScene; and 5. by planning a validation field measurement program in collaboration with the Remote Sensing and Exploitation group at Sandia National Laboratories (SNL) in which data from their ongoing polarimetric field and laboratory measurement program will be shared and, to the extent allowed, tailored for model validation in exchange for model predictions under conditions and for geometries outside of their measurement domain.

A Line-By-Line (LBL) option is being developed for MODTRAN6. The motivation for this development is two-fold. Firstly, when MODTRAN is validated against an independent LBL model, it is difficult to isolate the source of discrepancies. One must verify consistency between pressure, temperature and density profiles, between column density calculations, between continuum and particulate data, between spectral convolution methods, and more. Introducing a LBL option directly within MODTRAN will insure common elements for all calculations other than those used to compute molecular transmittances. The second motivation for the LBL upgrade is that it will enable users to compute high spectral resolution transmittances and radiances for the full range of current MODTRAN applications. In particular, introducing the LBL feature into MODTRAN will enable first-principle calculations of scattered radiances, an option that is often not readily available with LBL models. MODTRAN will compute LBL transmittances within one 0.1 cm-1 spectral bin at a time, marching through the full requested band pass. The LBL algorithm will use the highly accurate, pressure- and temperature-dependent MODTRAN Padé approximant fits of the contribution from line tails to define the absorption from all molecular transitions centered more than 0.05 cm-1 from each 0.1 cm-1 spectral bin. The beauty of this approach is that the on-the-fly computations for each 0.1 cm-1 bin will only require explicit LBL summing of transitions centered within a 0.2 cm-1 spectral region. That is, the contribution from the more distant lines will be pre-computed via the Padé approximants. The status of the LBL effort will be presented. This will include initial thermal and solar radiance calculations, validation calculations, and self-validations of the MODTRAN band model against its own LBL calculations.