GSIM is a simulator of infrared missile seekers. It simulates a missile flight scenario in a 3D environment from launch to arrival at the target. The radiation from objects and background is calculated using weather parameters, e. g. solar radiation and air temperature. After adding atmospheric transmission and sensor properties, the resulting seeker image is created. The primary output from GSIM is a sequence of images, generated by the seeker during the flight. In April 2005 helicopter trials were performed, where missile-like image sequences were recorded. The helicopter followed a slant path towards a number of ground vehicles, standing in a field. The recorded data has been used for evaluation of GSIM. At the evaluation, modelled temperature on background and targets was compared with radiometrically measured temperature. Simulations in GSIM were made of a selection of the recorded sequences. The difference between modelled and measured temperature is about 1 - 2 K, for both targets and background. When comparing the simulated and the original sequence, the visual impression is that there is an apparent similarity between the images. Long distance images show the strongest resemblance to reality, due to the limited level of detail in the background model.