Global Positioning Systems (GPS) are now in use in many applications, ranging from environmental research, route guidance and automatic fleet management (AVL), automatic pilots, traffic travel-time surveys, air, land, and marine navigation, and many other transportation related systems. In many GPS based system applications the GPS is connected to some form of intelligent electronics which receives the GPS data either from a GPS card or from a hand-held GPS unit, and then performs the required task. When developing and testing GPS based systems, one of the problems is that it is necessary to simulate the output data of a real GPS by generating GPS data. This paper provides the details of a PC based GPS simulator system called GPSIM, which receives the required way-points and routes from the user and simulates a GPS by generating most of the commonly used GPS sentences. The system described is an invaluable tool to GPS system developers. For the covering abstract see IRRD E102946.