The simulation of complex telecommunication systems, such as satellite-based networks, is seldom sufficient to fully understand their real behavior and to evaluate the Quality of Service (QoS) perceived by their users. The Mean Opinion Score (MOS) method, widely used to evaluate the perceived QoS, requires a working system to be used, and this is usually not the case of satellites, when studying new protocols and/or new system architectures. A simulator able to replicate the behavior of the system under exam in real time would be very useful: we call it an emulator. This paper describes an emulator for IP data traffic on a single beam GEO satellite, with TDM/TDMA radio links and on-board circuit switch. The emulator has been implemented in the CNIT Radio Communication Laboratory at Politecnico di Torino.