An operational satellite simulator is a digital representation of the satellite. It is organized into a set of virtual models that imitate both the subsystems that make up the satellite, such as the power supply, transmitters and receivers, among others, as well as the space environment and the ground stations that interact with the satellite. A significant function of the satellite simulator is to support engineers in defining operational procedures (command sequences) necessary when physical changes occur to the satellite after its launch. These changes can be anticipated, such as natural wear and tear on equipment over time, or unforeseen, such as unexpected failures or unplanned use of payload instruments. Thus, the simulator helps test new procedures considering various constraints and find solutions without exposing the satellite to risks, which can maximize the use of the satellite's resources. Although primarily developed in software, it can include hardware models. Its minimum function is to interpret commands received from the Mission Operations System, execute them considering the context, and produce responses exactly like the telemetry from the real satellite. Depending on the degree of fidelity required in representing real systems, the operational simulator can be the most complex software in a space mission. Due to its multidisciplinary nature, it requires developers to interact with various areas of knowledge and domains related to the satellite, stations, communication protocols, the space environment, and more. The Simulator for CBERS 3&4 satellites (SimCBERS) was developed between 2007 and 2017 by the National Institute for Space Research (INPE) to meet Brazilian operational requirements for this mission, such as facilitating the analysis of unforeseen situations that could occur during the satellite's life. One of the main challenges in the SimCBERS simulator project was to create a standardized specification for the models of the satellite's 15 subsystems, as well as to find a language that faithfully represents the behavior of the subsystems and that can be understood by engineers from the various subsystem disciplines, such as electrical, mechanical, mathematical, computing, optics, etc. The development of the SimCBERS Simulator was supported by subsystems engineers and by mission managers verifying the specifications and validating the simulations results. SimCBERS' solutions and challenges leveraged research topics such as the use of Artificial Intelligence techniques in models, interaction and visualization with virtual reality, and more. The development of this product also enabled the rapid development of the Simulator for the AMAZONIA-1 Satellite, which served to train operators and evaluate and find solutions for unexpected failure situations. (Responsável: Ana Maria Ambrosio)