Features and Benefits
Increased Spectral Resolution
Comparison of MODTRAN®4 spectral resolution (top left) to that of MODTRAN®5 (bottom right) for the 9.6 μ O3 band.
MODTRAN®5 introduces a state-of-the-art band model formalism, developed to support increased spectral resolution. Updated band model spectral bin data have been generated on grids of 15.0, 5.0, 1.0 and now 0.1 cm-1. The MODTRAN®5 band model features an exact expansion for the finite bin Voigt equivalent width, a Padé approximant treatment of line tails, and a bimodal model of line strength distributions. Accurate transmittances, radiances and fluxes are generated at a spectral resolution as fine as 0.2 cm-1, sufficient for modeling current thermal region ultra-spectral imaging sensors.
Atmospheric Correction Data Generation
MODTRAN®5 atmospheric correction data (top) and a mapping of radiance to reflectance obtained from the data (bottom).
MODTRAN®5 includes a new option to generate atmospheric correction data. This spectral data provides the information required to map radiances from down-looking solar-region hyperspectral imagers into surface reflectances. A refined version of the DISORT multiple scattering module within MODTRAN®5 computes the spherical albedo from the ground and both sun-to-ground and sensor-to-ground diffuse transmittances. This is combined with MODTRAN®5 atmospheric backscatter and both sun-to-ground and sun-to-ground-to-sensor direct transmittances to complete the suite of atmospheric correction data.
Auxiliary Species Options
MODTRAN®5 0.2 cm-1 spectral resolutions contrast signature produced by including acetylene (C2H2) absorption in a nadir view simulation from space. Ambient vertical column density may be closer to 0.01 Atm-cm than the value of 0.06 Atm-cm used here.
MODTRAN®5 now allows users to add molecular species not included in the MODTRAN®5 terrestrial atmosphere default list. Constituent concentrations are defined via vertical density profiles, and optical data via either spectral cross-sections or MODTRAN®5 band model data. Default profiles are included for a number trace molecular species, and band model data is provided for all the HITRAN molecules.
Additional MODTRAN®5 band model data for many species are available from SSI (modtran@spectral.com).
Brightness Temperature
Sample of MODTRAN®5 Brightness Temperature output for the AIRS sensor (bottom left) and a simulated broad band spectrum (top right). The MODTRAN®5 analytic expansion (black curve) avoids errors introduced by inverting the Planck function based on the band center (red).
MODTRAN®5 spectral grid and channel output files have been expanded to include brightness temperatures. The radiance to brightness temperature conversion is straightforward for narrow spectral channels, but not so for broader bands such as those commonly encountered with multi-spectral imagers. A newly derived algorithm defines the radiance to brightness temperature mapping using channel spectral moments, which are precomputed from simple analytic formulas for common filters (square, triangular Gaussian, sinc, Hamming) or from user-specified tabulated response functions. The conversion is typically accurate to better than 1 mK.
Additional Features
- Angstrom Law aerosol optical property input options
- Binary output option with ASCII-to-binary conversion tools
- Surface liquid water options
- Greatly expanded suite of data in spectral channel output file
- A temperature vertical profile perturbation option
- Option to set H2O amount based on model atmosphere relative humidity profiles
- Added surface directional emissivity and top-of-atmosphere solar irradiance output
- Eliminated DISORT solar angle dependent problems and hard stops
- Removed restrictions on maximum number of DISORT streams
- Updated to HITRAN2008
- Input specifying the wavelength to be used for computing refraction
- Independent scaling of density profiles for each molecular species
- O2 - O2 visible spectral region absorbance continuum
- Increased maximum band model temperature from 305 to 330K
