David C. Robertson

MODTRAN Cloud and Multiple Scattering Upgrades with Application to AVIRIS

Abstract Recent upgrades to the MODTRAN atmospheric radiation code improve the accuracy of its radiance predictions, especially in the presence of clouds and thick aerosols, and for multiple scattering in regions of strong molecular line absorption. The current public-released version of MODTRAN (MODTRAN3.7) features a generalized specification of cloud properties, while the current research version of MODTRAN (MODTRAN4) implements a correlated-k (CK) approach for more accurate calculation of multiply scattered radiance. Comparisons to cloud measurements demonstrate the viability of the CK approach. The impact of these upgrades on predictions for AVIRIS viewing scenarios is discussed for both clear and clouded skies; the CK approach provides refined predictions for AVIRIS nadir and near-nadir viewing.

Very narrow band model calculations of atmospheric fluxes and cooling rates

Abstract A new very narrow band model (VNBM) approach has been developed and incorporated into the MODTRAN atmospheric transmittance–radiance code. The VNBM includes a computational spectral resolution of 1 cm−1, a single-line Voigt equivalent width formalism that is based on the Rodgers–Williams approximation and accounts for the finite spectral width of the interval, explicit consideration of line tails, a statistical line overlap correction, a new sublayer integration approach that treats the effect of the sublayer temperature gradient on the path radiance, and the Curtis–Godson (CG) approximation for inhomogeneous paths. A modified procedure for determining the line density parameter 1/d is introduced, which reduces its magnitude. This results in a partial correction of the VNBM tendency to overestimate the interval equivalent widths. The standard two parameter CG approximation is used for H2O and CO2, while the Goody three parameter CG approximation is used for O3. Atmospheric flux and cooling rate p...

MODTRAN2: suitability for remote sensing

MODTRAN2 (1992) is the most recent version of MODTRAN, the Moderate Resolution Atmospheric Radiance and Transmittance Model, first released by the Geophysics Directorate, Phillips Laboratory, in 1990. It encompasses all the capabilities of LOWTRAN 7, the historic 20 cm-1 resolution radiance code, but incorporates a much more sensitive molecular band model with 2 cm-1 resolution. For inversion algorithm applications, MODTRAN2 must prove to be sufficiently accurate when calculating layer- specific perturbations. First steps in establishing this capability have recently been accomplished. DREV (Defence Research Establishment Valcartier, Canada), in conjunction with the Geophysics Directorate, has taken measurements with a surface-based Bomem interferometer (approximately 1 cm-1 resolution), with full supporting sonde profiles (z, T, p, and relative humidity). This suggests that the derivative matrices, typically required for inversion algorithms, may be readily (and rapidly) calculated using MODTRAN whenever its spectral resolution is adequate.

Measured and predicted atmospheric transmission in the 4.0–5.3-μm region, and the contribution of continuum absorption by CO 2 and N 2

High resolution measurements of atmospheric transmission of sunlight from space to altitudes of 12.2 km, 8.53 km, and 5.48 km made over Johnston Island are reported. The spectral region covered is 4.0-5.3 microm. Comparisons of the measured transmission with theoretically synthesized transmission curves are presented. It is shown that the sharp spectral features due to molecular line-by-line absorption can be predicted accurately while the modeling of the continuum absorption features needs further development. A discussion of the current models for CO(2) and N(2) continuum absorption is presented. An alternative mechanism is proposed for continuum absorption, which is based on the spectral properties of atmospheric van der Waals molecular complexes such as CO(2).N(2) and N(2).N(2) dimers.

Band model parameters for the 4.3 μm CO2 band from 200 to 3000°K—II. Prediction, comparison to experiment, and application to plume emission-absorption calculations

Abstract High resolution band-model parameters are presented for the 4.3 μm CO 2 band from 200 to 3000°K. Comparisons are made to experimental data covering this same temperature range. The band model predictions and data are shown to be in good agreement. Application of the band model parameters to plume emission and atmospheric absorption calculations in the CO 2 fundamental band head region (2380–2400 cm -1 ) is discussed. Line-by-line and band-model plume signature predictions are compared for an aircraft and a rocket plume over 0, 0.5, and 5 km atmospheric paths. The line-by-line and band-model predictions for the 0 and 0.5 km paths are in good agreement. For the 5 km path, the band-model overestimates the atmospheric transmission by a factor of two. The reason for this overprediction is discussed and a correction is presented which improves the accuracy of the band-model transmission calculations.

5-cm −1 band model option to lowtran5

Modifications to the atmospheric transmission and radiation code LOWTRAN5 are presented which include (1) an increase in the spectral resolution from 20 to 5 cm(-1), (2) the addition of temperature-dependent molecular absorption coefficients, (3) the use of a multiparameter Doppler-Lorentz band model for calculation of molecular transmittance, and (4) the use of the Curtis-Godson approximation for multilayered paths. Comparisons of predictions using the LOWTRAN5 5-cm(-1) band model option to measured transmittance and radiance data are also presented.

Cloud effects in hyperspectral imagery from first-principles scene simulations

Clouds and cloud fields introduce important backscattering, obscuration, shadowing and radiative trapping effects in visible-NIR(near-infrared)-SWIR(short-wavelength infrared) hyperspectral imagery of the ground, especially in off-nadir (slant) viewing geometries where cloud thickness effects reduce the cloud-free line of sight (CFLOS). An investigation of these effects was conducted using monochromatic, multispectral and hyperspectral scene simulations performed with the Spectral Sciences, Inc. MCScene Monte Carlo code. Cloud fields were obtained from the Cloud Scene Simulation Model (CSSM) of Cianciolo and Raffensberger. The simulations took advantage of a data-fusion-based noise-removal method that enabled a dramatic reduction in computation time. Illumination levels at the sunlit ground showed enhancements of up to ~50% due to cloud scattering. Illumination in the cloud shadows was 20% of the full solar illumination or greater, with cloud optical depths of up to 10. Most of this illumination arises from solar scattering off the cloud tops and sides; however, a significant part can be ascribed to radiative trapping between the ground and the clouds, as represented by a local atmospheric spherical albedo. A simulation of a hyperspectral scene with cloud shadows was found to reproduce shadowing effects found in real data. Deeper shadowing is observed with increasing wavelength and in water-band regions, consistent with a previous analysis of cloud shadows in real imagery. The MCScene calculations also predict shadow enhancements of column water vapor retrievals from atmospheric correction/compensation codes, also in accord with field observations. CFLOS fractions were calculated as a function of off-nadir viewing angle and were found to be very accurately represented by a semi-empirical analytical function of both angle and cloud cover.

Infrared Cloud Scene Radiance Model

The development and evaluation of algorithms to detect targets against cloud backgrounds requires a comprehensive understanding of the clutter properties such as the radiance distributions, textures, edge effects, etc. A number of measurement programs are collecting data for this purpose. However they are limited by the vast amounts of data required, and their limited resources for obtaining the data. This paper will describe and show results from a first principles infrared cloud scene radiance model. The work is sponsored by the Naval Surface Warfare Center (NSWC) through a Small Business Innovative Research Program (SBIR) to support IRAMMP (Infrared Analysis Modeling and Measurement Program -- formerly BMAP) as part of the Navys Infrared Search and Track effort. The model is designed to handle arbitrary viewing geometries, atmospheric conditions, and sensor parameters. The output is a two dimensional (n x m pixels) scene radiance map which can be used by system designers, data takers, and analysts.

Aircraft Contrast Signatures In The Infrared Spectral Region

The methodology for calculating aircraft infrared radiation is developed by expressing the signature in its component parts and then modeling each in terms of a few parameters. The basic signature components are: exhaust plume molecular emissions, airframe thermal emissions, various exposed engine hot parts, and scattered ambient radiation (e.g., earth-shine, sunshine and skyshine). When calculating sensor irradiances for systems studies, the close relationship between the aircraft signature and environmental conditions must be considered. This is illustrated with scene contrast signature calculations for several different observer viewing angles and background types.