Nowadays, GNSS simulators are complex. The systems can consist of numerous components used to generate or process a signal. The job of a system integrator is to choose the system with the best components, and ultimately build a superior system, while ensuring the most reliable and cost-effective parts are selected.
A typical example of a complex system is a car or flight simulator. Sensors are used to take measurements of an action from a real-life user, e.g., the turning of a steering wheel. Based on these measurements a new simulated position is provided. In this case, it is that the GNSS simulator is capable of generating a matching RF signal. This signal is then processed by a GNSS receiver, and the result is presented back to the user. This tightly coupled system is called hardware-in-the-loop (HIL).