FIRE-ARMS (Fine InfraRed Explorer for Atmospheric Radiation MeasurementS) software package for simulation of atmospheric radiation spectra, analysis of weighting functions, and solution of the inverse problem of passive IR sensing of the atmosphere has been developed. Some results of IR spectra processing with FIRE-ARMS are presented. The spectra have been obtained with IMG (Interferometric Monitor for Greenhouse gases) Fourier spectrometer operating from onboard an ADEOS (Advanced Earth Observing Satellite) launched in August of 1996 by NASDA (Japan). Using FIRE-ARMS, we have identified some peaks and dips in the IMG spectra, which proved to correspond to 12C18O16O, 13CO2, and HDO isotopic species. The presence of SiO2 aerosol above the Sahara Desert has been revealed from these spectra. The possibility of estimating the cloud top height is also demonstrated.