6 research outputs found
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
OSIRIS-REx Orbit Determination Performance During the Navigation Campaign
The OSIRIS-REx mission Navigation Campaign consists of three sub-phases: Approach,Preliminary Survey, and Orbital A. Approach was designed for initial characterization ofBennu while matching Bennu's heliocentric velocity. Preliminary Survey provided the firstspacecraft-based estimate of Bennu's mass. This phase consisted of five target flybys witha close approach distance of about 7 km. Orbital A was a two-month phase devoted to theNavigation Team learning the close proximity operations dynamics and environment aroundBennu and transitioning from center-finding optical navigation to landmark feature-basednavigation. This paper provides a detailed summary of the orbit determination performancethroughout the Navigation Campaign
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Development, Analysis, and Testing of Robust Nonlinear Guidance Algorithms for Space Applications
This work focuses on the analysis and application of various nonlinear, autonomous guidance algorithms that utilize sliding mode control to guarantee system stability and robustness. While the basis for the algorithms has previously been proposed, past efforts barely scratched the surface of the theoretical details and implications of these algorithms. Of the three algorithms that are the subject of this research, two are directly derived from optimal control theory and augmented using sliding mode control. Analysis of the derivation of these algorithms has shown that they are two different representations of the same result, one of which uses a simple error state model (Îr/Îv) and the other uses definitions of the zero-effort miss and zero-effort velocity (ZEM/ZEV) values. By investigating the dynamics of the defined sliding surfaces and their impact on the overall system, many implications have been deduced regarding the behavior of these systems which are noted to feature time-varying sliding modes. A formal finite time stability analysis has also been performed to theoretically demonstrate that the algorithms globally stabilize the system in finite time in the presence of perturbations and unmodeled dynamics. The third algorithm that has been subject to analysis is derived from a direct application of higher-order sliding mode control and Lyapunov stability analysis without consideration of optimal control theory and has been named the Multiple Sliding Surface Guidance (MSSG). Via use of reinforcement learning methods an optimal set of gains has been found that make the guidance perform similarly to an open-loop optimal solution. Careful side-by-side inspection of the MSSG and Optimal Sliding Guidance (OSG) algorithms has shown some striking similarities. A detailed comparison of the algorithms has demonstrated that though they are nearly indistinguishable at first glance, there are some key differences between the two algorithms and they are indeed not identical. Finally, this work has a large focus on the application of these various algorithms to a large number of space based applications. These include applications to powered-terminal descent for landing on planetary bodies such as the moon and Mars and to proximity operations (landing, hovering, or maneuvering) about small bodies such as an asteroid or a comet. Further extensions of these algorithms have allowed for adaptation of a hybrid control strategy for planetary landing, and the combined modeling and simultaneous control of both the vehicle's position and orientation implemented within a full six degree-of-freedom spacecraft simulation
OSIRIS-APEX: An OSIRIS-REx Extended Mission to Asteroid Apophis
The Origins, Spectral Interpretation, Resource Identification, and SecurityâRegolith Explorer (OSIRIS-REx) spacecraft mission characterized and collected a sample from asteroid (101955) Bennu. After the OSIRIS-REx Sample Return Capsule released to Earthâs surface in 2023 September, the spacecraft diverted into a new orbit that encounters asteroid (99942) Apophis in 2029, enabling a second mission with the same unique capabilities: OSIRISâApophis Explorer (APEX). On 2029 April 13, the 340 m diameter Apophis will draw within âŒ32,000 km of Earthâs surface, less than 1/10 the lunar distance. Apophis will be the largest object to approach Earth this closely in recorded history. This rare planetary encounter will alter Apophisâs orbit, will subject it to tidal forces that change its spin state, and may seismically disturb its surface. APEX will distantly observe Apophis during the Earth encounter and capture its evolution in real time, revealing the consequences of an asteroid undergoing tidal disturbance by a major planet. Beginning in 2029 July, the spacecraftâs instrument suite will begin providing high-resolution data of this âstonyâ asteroidâadvancing knowledge of these objects and their connection to meteorites. Near the missionâs end, APEX will use its thrusters to excavate regolith, a technique demonstrated at Bennu. Observations before, during, and after excavation will provide insight into the subsurface and material properties of stony asteroids. Furthermore, Apophisâs material and structure have critical implications for planetary defense