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.

Remote sensing instruments, both aircraft and on-orbit platforms, undergo extensive laboratory calibrations to determine their geometric, spectral, and radiometric responses. Additional in-flight radiometric calibrations can be performed using well-characterized earth targets. The Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign provided such an opportunity when the ER-2 aircraft overflew Railroad Valley on August 13 and 15, 2019. Surface reflectances were available from the August 4, 2019 field team and from the Radiometric Calibration Network (RadCalNet) portal, and spectral aerosol optical depths from an on-site AERosol RObotic NETwork (AERONET) sunphotometer. The Enhanced MODIS Airborne Simulator (eMAS), the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI), and the “Classic” Airborne Visible/Infrared Imaging Spectrometer (AVIRIS-C) sensors individually performed a vicarious calibration using their respective methodologies and selection of input parameters. A comparison of the at-sensor radiances predicted from these independent analyses highlights some of the uncertainties in the inputs, including choice of solar irradiance model. Although good agreement, within 5%, is found at visible wavelengths, difference can be as large as 15% in the shortwave infrared (SWIR). This highlights the need for the remote sensing community to agree upon a standard solar model, to remove sensor-to-sensor biases derived from in-flight calibrations.

Daytime skylight has long been known to be impacted by the scattering of particles of various origins (e.g., organic matter, dust, soot, urban pollutants, biomass burning, water vapor, sea salt) in the visible and near infrared (roughly 0.4 to 2.5 μm). Previous work described at the AMOS Conference has shown that skylight radiance is, in turn, the limiting factor for observations of satellites during the daytime. However, actual observations of daytime skylight radiance climatologies at sites are rare in the literature. To fill the gap, MODerate resolution atmospheric TRANsmission (MODTRAN®) modeling using default parameters have been used to estimate performance. In this paper, in situ data is used to constrain MODTRAN modeling at two sites with the results showing the default parameters most likely do not reflect the actual skylight radiances at these sites.

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.

Methods to radiometrically calibrate a non-imaging airborne visible-to-shortwave infrared (VSWIR) spectrometer to measure the Greenland ice sheet surface are presented. Airborne VSWIR measurement performance for bright Greenland ice and dark bare rock/soil targets is compared against the MODerate resolution atmospheric TRANsmission (MODTRAN®) radiative transfer code (version 6.0), and a coincident Landsat 8 Operational Land Imager (OLI) acquisition on 29 July 2015 during an in-flight radiometric calibration experiment. Airborne remote sensing flights were carried out in northwestern Greenland in preparation for the Ice, Cloud, and land Elevation Satellite 2 (ICESat-2) laser altimeter mission. A total of nine science flights were conducted over the Greenland ice sheet, sea ice, and open-ocean water. The campaign's primary purpose was to correlate green laser pulse penetration into snow and ice with spectroscopic-derived surface properties. An experimental airborne instrument configuration that included a nadir-viewing (looking downward at the surface) non-imaging Analytical Spectral Devices (ASD) Inc. spectrometer that measured upwelling VSWIR (0.35 to 2.5 µm) spectral radiance (Wm-2sr-1µm-1) in the two-color Slope Imaging Multi-polarization Photon-Counting Lidar's (SIMPL) ground instantaneous field of view, and a zenith-viewing (looking upward at the sky) ASD spectrometer that measured VSWIR spectral irradiance (W m−2 nm−1) was flown. National Institute of Standards and Technology (NIST) traceable radiometric calibration procedures for laboratory, in-flight, and field environments are described in detail to achieve a targeted VSWIR measurement requirement of within 5 % to support calibration/validation efforts and remote sensing algorithm development. Our MODTRAN predictions for the 29 July flight line over dark and bright targets indicate that the airborne nadir-viewing spectrometer spectral radiance measurement uncertainty was between 0.6 % and 4.7 % for VSWIR wavelengths (0.4 to 2.0 µm) with atmospheric transmittance greater than 80 %. MODTRAN predictions for Landsat 8 OLI relative spectral response functions suggest that OLI is measuring 6 % to 16 % more top-of-atmosphere (TOA) spectral radiance from the Greenland ice sheet surface than was predicted using apparent reflectance spectra from the nadir-viewing spectrometer. While more investigation is required to convert airborne VSWIR spectral radiance into atmospherically corrected airborne surface reflectance, it is expected that airborne science flight data products will contribute to spectroscopic determination of Greenland ice sheet surface optical properties to improve understanding of their potential influence on ICESat-2 measurements.

: 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.

The Air Force Weather Agency (AFWA) has a long history of providing global cloud analyses and forecasts. Until recently, their focus has been on determining the cloud amount and cloud type. Satellite-based World-Wide Merged Cloud Analysis (WWMCA) data provided by the AFWA are analyzed to understand and assess their capability to characterize cloud single scattering parameters at optical wavelengths. WWMCA represents the most refined version of AFWA's cloud depiction and forecast system and includes up to four cloud layers and 38 cloud parameters per file at each hemispheric grid point. Findings on WWMCA's determination of cloud optical depth (COD), consistency with synoptic-scale cloud fields, and its ability to support radiative transfer calculations are as follows: (1) the WWMCA optical depth is strongly correlated with the theoretical optical depth at 550 nm computed using WWMCA's cloud microphysical parameters. (2) WWMCA captures the large-scale spatial variation of COD as represented by the Mei-yu/Baiu onset and progression of synoptic cloud fields as well as the time-dependent character of mesoscale features. (3) WWMCA does not provide single scattering albedo and the cloud phase function which are needed to solve the radiative transfer equation. Because WWMCA is based on passive sensor processing, obscured cloud layers are not accounted for in the calculation of COD, which may lead to an underestimation of total COD. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

The MODTRAN6 radiative transfer (RT) code is a major advancement over earlier versions of the MODTRAN atmospheric transmittance and radiance model. This version of the code incorporates modern software architecture including an application programming interface, enhanced physics features including a line-by-line algorithm, a supplementary physics toolkit, and new documentation. The application programming interface has been developed for ease of integration into user applications. The MODTRAN code has been restructured towards a modular, object-oriented architecture to simplify upgrades as well as facilitate integration with other developers' codes. MODTRAN now includes a line-by-line algorithm for high resolution RT calculations as well as coupling to optical scattering codes for easy implementation of custom aerosols and clouds.