The Applied Physics Laboratory has developed a physical optics electromagnetic propagation computer code at radar frequencies that features long path length representation of atmospheric waveguides or ducts. Thus, an accurate tropospheric absorption model, emphasizing water vapor and oxygen effects, is required for a realistic code. The HITRAN database now provides a comprehensive characterization of spectral lines from 0 to 10 cm-1. A theoretical local line shape is used with this database, which improves the traditional models. Also, a new semiempirical water vapor continuum model has been developed based on the theoretical work of Birnbaum and the most recent experimental data. The result is an improved model for low-altitude, long path calculations of atmospheric absorption at radar and millimeter wavelengths.