Satellite services (e.g. Internet-over-Satellite or GPS) are getting more and more important in our lives, whereby the operational area is still growing. To guarantee those services, reliable simulation tools are necessary to scale and validate actual and future satellite systems. Therefore, this paper gives an overview about development, functionalities and benefit of a satellite simulation framework which is already available for download, called the Open Source Satellite Simulator (OS3). The use of OMNeT++ in combination with the INET framework allows the release under public license as well as a platform independent implementation. To provide an accurate and comfortable tool, OS3 features a Graphical User Interface, live weather data integration, high resolution altitude data, accurate satellite movements, diff�erent visualization options, channel modeling and much more. In order to ensure the correctness of simulation results, numerous experimental and simulation tests were conducted.