Nonlinear Dynamic Inversion-based Guidance and Control for a Pinpoint Mars Entry

A nonlinear control approach is developed for both the attitude and trajectory control and of an Apollo-like capsule with low L/D entering the martian atmosphere, resulting in a very high probability (>99%) of a safe parachute deployment very close to a pre-defined location above Mars surface, enabling a precision l anding (<1km). The attitude control is based on the feedback linearization of the inner (a ttitude dynamics) and outer (attitude kinematics) attitude control loops, in which the de sired dynamics of the linearized loop are tuned such as to ensure both robustness to uncertai nties, moderate demands to the attitude control system and a high tracking performance. The trajectory control also includes nonlinear tracking laws for the drag acceleration a nd range. The trajectory control is based on the modulation of the bank angle, such as to poi nt the lift vector in the direction most suitable to increase or decrease the vehicle’s rate of descent as required. The guidance algorithm associated to the trajectory tracking law attempts to follow a drag acceleration profile, a range profile and a heading angle as the flight progresses, in an attempt to circumvent the known shortcomings of previous entry guidance approaches (namely in terms of range prediction) while avoiding any analy tical predictions, numerical integration of the equations of motion, in-flight trajectory up dates or trajectory optimization during the flight. Monte-Carlo simulations have been carried o ut in high-fidelity 4DOF and 6DOF simulations, introducing significant perturbations to the initial state, the vehicle’s aerodynamics, its’ mass properties and in atmospheric models. Results compare very favorably to existing approaches both in terms of r obustness to uncertainties and overall system performance, while the guidance and control system hereby presented requires a comparably small computational cost.