Planetary landing: Modelling and control of the propulsion descent

In the propulsion phase, after parachute release, of landing on Mars or the Moon, horizontal motion is obtained by tilting the axial thrust so that it aligns either to the negative velocity vector (gravity turn) or to the requested acceleration vector. The latter strategy is assumed here, as it allows pinpoint landing. As such, tilt angles (pitch and yaw) become proportional to the horizontal acceleration. Instead of designing a hierarchical guidance and control in which horizontal acceleration becomes the attitude control target, a unique control system was designed based on the fourth order dynamics from angular acceleration to position. It is shown that the combined dynamics can be (quasi) input-state linearized except for the nonlinear factor of the tilt angles (the axial thrust imposed by vertical braking). It is also shown that the control design around the reference trajectory (tilt and position) can only exploit a partial input-state linearization, but internal stability can be proved. Stability is al