In the European GALILEO in-orbit validation (IOV) program one essential element in the Galileo Ground Control Segment (GCS) is the Satellite Constellation Simulator (CSIM). The role of CSIM is to provide a complete and reliable environment which will play a key role in the validation of the GCS and in particular in the validation of the Satellite Constellation Control Facility (SCCF). Further to this validation of GCS computer systems it will also be used extensively by the IOV operations staff; first during the operations preparation activities, i.e. to train the Galileo Flight Control Team (FCT) and to validate the Flight Operations Procedures (FOPs); then during the actual in-orbit operations, e.g. to validate on-board software related modifications (e.g. patches validation prior to actual upload in a given spacecraft); finally in the investigation of anomalies. The main technical challenge for CSIM is being able to run multiple spacecraft models and being able to do so in a globally coordinated fashion: CSIM must not only be able to cope with several satellites but it must also be able to represent correctly that those satellites form a constellation delivering a unified service whereby each satellite or ground station model is executed in its own process but controlled by a central simulation environment. CSIM is required to provide a coherent representation of the constellation and the associated ground stations. Last but not least at any given point in time CSIM must be able to cope with “differing” satellites in the sense that they may run different on-board software versions, independent models, activate specific states, be driven by different databases, and so on. This paper will explain with some details the software problems to cope with in order to fulfill the