Abstract: MODTRAN is a moderate resolution model and computer code used to predict atmospheric transmittance and background radiance in the microwave, infrared, visible and near ultraviolet spectral regions (0 to 50,000 cm.sup.-1 or 0.2 m to infinity). The code maintains complete compatibility with LOWTRAN 7, specifically, MODTRAN retains all of the capabilities of the LOWTRAN 7 model. Both codes contain the same six built-in model atmospheres; spherical refractive geometry, aerosol models, clouds (water and ice), rain attenuation and options to calculate single scattered solar/lunar radiance, solar/lunar irradiance and multiply scattered thermal and solar radiance. The MODTRAN code improves LOWTRAN'S spectral resolution from 20 to 2 cm.sup.-1 full width/half maximum (fwhm) with an option to vary the resolution between 2 and 50 cm.sup.-1 (fwhm). The band model parameters were formulated from the HITRAN line atlas for twelve atmospheric gases: H.sub.2 O, CO.sub.2, O.sub.3, N.sub.2 O, CO, CH.sub.4, O.sub.2, NO, SO.sub.