# How is the FPA (flight-path-angle) computed?

How is FPA computed by the Auto-pilot to follow the desired FPA? One part is clearly VS, is the other part IAS or Ground speed? Where does Ground speed come from?

The flight path angle $$\alpha$$ is calculated by the on-board computers with:
$$\alpha = \text{arctan} \left( \frac{V/S}{\text{GS}} \right)$$