Part 3 HITL
In part 3 a real Pixhawk (AP/PX4) (below in red) is used. The rest of the copter components (frame, motors, IMU, GPS, motor control, etc) are all simulated.
HITL is described for PX4, Ardupilot, and Matlab.
PX4
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Ardupilot
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Matlab
Matlab is used to simulate the world (sensor inputs, flight dynamics, etc).