Schematic representation of the pointing direction of the satellite’s long axis θ and thus of the orientation of the satellite-fixed frame, including the contribution of the longitudinal physical libration γ. The satellite is represented at periapsis (M = 0), at apoapsis (M = π), and at a arbitrarily chosen location along its orbit. In this work, we define the satellite-fixed reference frame such that the satellite’s long axis points towards the planet at apo- and periapsis.