Guidance and control for the propulsion phase of planetary landing

: In the propulsion phase of planetary landing, horizontal motion is obtained by tilting and aligning the axial thrust either to the opposite of the velocity vector or to the requested acceleration vector. The second strategy is assumed here, as it allows free horizontal motion and is preliminary to achieve accurate landing. Instead of designing a hierarchical guidance and control in which horizontal acceleration becomes the attitude reference, a unique control system is designed based on a fourth-order state equation per degree-of-freedom from the angular acceleration to the position coordinate. Following the Embedded Model Control methodology, a unique discrete-time state equation (the embedded model) is derived and employed by guidance, navigation and control. Here only guidance and control are outlined. The whole guidance, navigation and control algorithm has been tested on a high-fidelity descent simulator. The results of Monte Carlo runs for assessing performance versus requirements that are typical of accurate landing are presented and discussed.