A simulation is done where software that is running on a different computer generates data stimulating data that would come from the autopilot sensors. During the simulation, the autopilot is running. It doesn’t “know” it is simulated data, so it responds in the same way it would if it was real sensor data that it was receiving. Physical autopilot hardware that is connected to a simulator is used by hardware in the loop, as opposed to also stimulating the autopilot inside the software. With an open loop simulation, data is fed to the hardware autopilot from the software simulator. In a closed loop simulation, data is fed back into the software simulator as well from the hardware.