| CRTM Gas Absorption Models |
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OPTRAN - Optical Transmittance Model |
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Under clear atmospheric conditions, radiative transfer modeling uses atmospheric absorption coefficients as the key input. The absorption varies with the atmospheric conditions in a complicated way and is often computed through the line-by-line (LBL) models. Although LBL models are accurate, they take considerable time to calculate transmittances for just a few atmospheres. To provide accurate transmittances in a timely fashion, the JCSDA has generated and used fast approximation commonly known as OPTRAN for specific instrument channels. For atmospheric transmittance calculations, the gas absorption coefficients are predicted from the atmospheric parameters at fixed levels of the integrated absorber amount (Kleespies et al., 2004). This approach significantly reduces the coefficients which reside in computer memory and preserves the accuracy. |
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OSS - Optimal Spectral Sampling |
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Recently, a fast and optimal spectral sampling ( OSS) absorption model which is developed by AER Inc. is being tested and integrated as part of CRTM. The OSS is a new approach to radiative transfer modeling which addresses the need for algorithm speed, accuracy, and flexibility. The OSS technique allows for a rapid calculation of radiance for any class of multispectral, hyperspectral, or ultraspectral sensors at any spectral resolution operating in any region from microwave to ultraviolet wavelengths by selecting and appropriately weighting the monochromatic radiances contributed from gaseous absorption and particle scattering over the sensor bandwidth. This allows for the calculation to be performed at a small number of spectral points while retaining the advantages of a monochromatic calculation such as exact treatment of multiple scattering and/or polarization. The OSS method is well suited for remote sensing applications which require extremely fast and accurate radiative transfer calculations: atmospheric compensation, spectral and spatial feature extraction, multi-sensor data fusion, sub-pixel spectral analysis, qualitative and quantitative spectral analysis, sensor design and data assimilation. The OSS was recently awarded a U.S. Patent (#6,584,405) and is currently used as part of the National Polar-Orbiting Operational Environmental Satellite System (NPOESS) CrIS, CMIS, and OMPS-IR environmental parameter retrieval algorithms. Maximum brightness temperature errors from the current look up table calculations are less than 0.05K in infrared and are about 0.01K in microwave wavelengths.
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SARTA - Stand Alone Radiative Transfer for AIRS |
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SARTA is a forward model developed by University Maryland at Baltimore County (UMBC). It is a fast gas absorption model fitted with AIRS observations with the best aacuarcy comparing with all other fast models in the IR wavelengths. Under the CRTM framework, the original SARTA program is re-coded to meet the CRTM standard with tangent-linear and adjoint models have been also completed.
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Zeeman Splitting Effects for Microwave Radiative Transfer |
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When the magnetic fields are present, the atomic energy levels are split into a larger number of levels and the spectral lines of oxygen absorption near 60 GHz are also split. This splitting is called the Zeeman Effect. The absorption lines corresponding to Zeeman splitting also exhibit polarization effects. Polarization has to do with the direction in which the electromagnetic fields are vibrating. This in turn, can have an effect on whether the spectral light can be observed. For example, polarizing sunglasses are often effective in suppressing ambiant glare because light reflected from surfaces has a particular polarization and polarizing sunglasses are designed to not pass that polarization of light. In CRTM, the absorption coefficients including the zeeman effects are calculated from line by line models (Rosenkranz) and are parameterized with various predictors including the earth magnetic field magnitude (B), polarization (left and right circurlarly), temperature and angle between magnetic field and EM propagation direction
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