We present our analysis of data taken by SUSIM on-board UARS. We reconstruct the variability of the UV irradiance and compare it to available data. Up to now we model the solar irradiance according to the 3-component model by Unruh et al. (1999) based on LTE synthetic spectra modeled with Kurucz' ATLAS9 code. Our new approach will be that with COSI (COde for Solar Irradiance) we model solar continuum and line formation in spherical symmetry and in non-local thermodynamic equilibrium (non-LTE). We present our first synthetic solar spectra (calculated in LTE) and validate them against spectra computed with Kurucz' ATLAS9 code.