Daniel R. Wibben

Asteroid Precision Landing via Multiple Sliding Surfaces Guidance Techniques

Autonomous close-proximity operations (hovering, landing) in the low-gravity environment exhibited by asteroids are particularly challenging. A novel nonlinear landing guidance scheme has been developed for spacecraft that are required to execute autonomous closed-loop guidance to a designated point on the asteroid surface. Based on high-order sliding-mode control theory, the proposed multiple sliding surface guidance algorithm has been designed to take advantage of the ability of the system to reach the sliding surface in a finite time. High control activity typical of sliding control design is avoided, resulting in a guidance law that is robust against unmodeled yet bounded perturbations. The proposed multiple sliding surface guidance does not require any off-line trajectory generation, and therefore it is flexible enough to target a large variety of points on the surface without the need of ground-based trajectory analysis. The global stability of the proposed guidance algorithm is proven using a Lyapu...

Mars Atmospheric Entry Guidance via Multiple Sliding Surface Guidance for Reference Trajectory Tracking

To improve the accuracy of Mars landing, future missions will require robust, closedloop guidance schemes for the entry portion of the atmospheric flight. A new nonlinear atmospheric entry guidance law for low lift landers has been developed. The Multiple Sliding Surface Guidance (MSSG) for tracking a reference trajectory is proposed. The guidance method is based on higher order sliding mode control theory adapted to the problem of entry guidance. The higher order sliding mode control has been adapted to account for the specific 2-sliding mode exhibited by the longitudinal motion of the entry vehicle, using bank angle control for following a reference trajectory. The global stability nature of the MSSG law is proven by using a Lyapunov-based approach and the performance is analyzed via a 3-DOF Monte Carlo simulation.

Development of Non-Linear Guidance Algorithms for Asteroids Close-Proximity Operations

In this paper, we discuss non-linear methodologies that can be employed to devise real-time algorithms suitable for guidance and control of spacecrafts during asteroid close-proximity operations. Combination of optimal and sliding control theory provide the theoretical framework for the development of guidance laws that generates thrust commands as function of the estimated spacecraft state. Using a Lyapunov second theorem one can design non-linear guidance laws that are proven to be globally stable against unknown perturbations with known upper bound. Such algorithms can be employed for autonomous targeting of points of the asteroid surface (soft landing , Touch-And-Go (TAG) maneuvers). Here, we theoretically derived and tested the Optimal Sliding Guidance (OSG) for close-proximity operations. The guidance algorithm has its root in the generalized ZEM/ZEV feedback guidance and its mathematical equations are naturally derived by a proper definition of a sliding surface as function of Zero-Effort-Miss and Zero-Effort-Velocity. Thus, the sliding surface allows a natural augmentation of the energy-optimalguidance via a sliding mode that ensures global stability for the proposed algorithm. A set of Monte Carlo simulations in realistic environment are executed to assess the guidance performance in typical operational scenarios found during asteroids close-proximity operations. OSG is shown to satisfy stringent requirements for asteroid pinpoint landing and sampling accuracy.

Integrated guidance and attitude control for asteroid proximity operations using higher order sliding modes

An integrated guidance and attitude control scheme for asteroid proximity operations is presented. The development of this algorithm is motivated by the desire to implement robust and integrated spacecraft GNC schemes for asteroid close proximity operations. Autonomous maneuvering about small bodies is particularly challenging because of the uncertain, low-gravity environment. Based on Higher Order Sliding Mode (HOSM) control theory, the integrated Multiple Sliding Surface Guidance and Control (MSSGC) law has been designed to drive the system to the selected sliding surface in a finite time. The MSSGC scheme integrates the spacecraft’s guidance and attitude control into a common framework that guides the 6-DOF spacecraft to a desired position about the asteroid with the desired orientation, all without the need for a pre-computed reference trajectory. A Lyapunov-based stability analysis shows that the system is globally stable against unmodeled dynamics and perturbations typically expected in small body environments. Results demonstrate that the algorithm is successful in driving the system to the desired target point (either landing on the surface or hovering above a desired location) with zero velocity and with the desired attitude and zero rotational rates.

Switching system model for pinpoint lunar landing guidance using a hybrid control strategy

A novel non-linear spacecraft guidance scheme utilizing a hybrid controller for pinpoint lunar landing is presented. The development of this algorithm is motivated by a) the desire to satisfy more stringent landing accuracies required by future lunar mission architectures, and b) the interest in the ability of a system with multiple controllers to provide robustness and performance that cannot be obtained with a single controller. Based on Hybrid System theory, the proposed Hybrid Guidance algorithm utilizes both a global and local controller to bring the lander safely to the desired target on the lunar surface with zero velocity in a finite time. The hybrid approach is used generally to provide flexibility; many stable controllers can be used for the global and local controllers in the hybrid framework, creating options that allow the algorithm to be tailored to meet mission requirements. The presented case utilizes a global controller that implements an optimal guidance law augmented with a sliding mode to bring the lander from an initial state to a predetermined reference trajectory, at which point the guidance law will switch to that of a LQR-based local controller to bring the lander to the desired point on the lunar surface. The individual controllers are shown to be stable in their respective regions. The behavior and performance of the Hybrid Guidance Law (HGL) is examined in a set of Monte Carlo simulations under realistic conditions. The simulations demonstrate that the HGL is very accurate and results in low residual guidance errors.

Mission Analysis for a Temporary Geocentric Asteroids

This paper presents a mission analysis study for manned missions to a special class of asteroids which have been captured in a temporary geocentric orbit, as demonstrated recently by the discovery of asteroid 2006 RH120. Possible trajectories to 2006 RH120 are surveyed using a preliminary global optimization tool based on differential evolution. The minimum ΔV trajectories found by our methodology show that temporary geocentric manned missions to 2006 RH120 can be as low as 3 km/sec if launched in 2028. Parametric studies show that feasible trajectories may have a total round trip between 150 and 210 days with a stay time between 10 and 20 days. A subset of the best preliminary trajectories are validated using a higher fidelity tool capable of modeling the gravitational influence of the more relevant bodies in the solar system. A brief and preliminary discussion on the options for mission architectures that implement the proposed class of trajectories is finally presented.