Guidance refers to the questions ”where am I going to?”, ”How can the vehicle follow a trajectory?”. The trajectory itself is prepared by the Flight Planning System (FPS) (or Mission Planning System, MPS). During the flight, the Flight Management System (FMS) knows the trajectory and gives the current portion to the Guidance System (GS) . The Guidance System is in charge of converting the high level parameters (trajectory, waypoint positions) into a set of lower level orders that can be understood by the control, typically altitude, heading or directly a load factor, that is an acceleration. In some cases the guidance can have the role to compute a trajectory between 2 points.
It is also in charge of maintaining the trajectory and the other high level parameters. That means the guidance is itself a control loop. Typically the guidance is the Position Control System (PCS).
This is where things become tricky. Piloting and Guidance are essentially similar. The difference to keep in mind is that the pilot is a low level control loop whereas the guidance is a high level control loop. A nice world would be a world where guidance and pilot loops are two independent loops. Unfortunately both are coupled, and designing one, you need to keep the other one in mind.
Next chapter will be about GNC and human pilot.
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If economy was an airplane, that airplane would go across a severe zone of turbulences. Then, a passenger would ask the air hostess “can I talk to the pilot, please? Because this pilot does not seem very good”. The hostess would answer...