SCIAMACHY is a UV-vis-near IR spectrograph for atmospheric trace gas monitoring, due to be launched on the ESA ENVISAT in 2000. The SCIAMACHY Instrument Simulation Software was used to aid in the design of the instrument, support on- ground performance and calibration measurements, develop the level 1 algorithms and establish in-flight operational scenarios. The instrument simulator is based on a traveling spectrum approach from entrance aperture to detector electronics. Relevant optical properties of the individual components are used to model the spectrum and polarization state of the light at each component in the instrument. Subsequently, noise properties and electronic effects are added. The resulting simulated signal is directly comparable with recently performed measurements on the assembled instrument under flight representative conditions. In this paper the measured SCIAMACHY instrument performance is discussed on the basis of these simulations, in aid of better instrument knowledge required during space operation.