Advanced Guidance Approach for Multi-Body/Multi-Actuator Spacecraft Repointing under Attitude Constraints
Creators: name:  A. PONCHE 
name:   A. MARCOS 
name:   T. OTT 
name:   R. GESHNIZJANI 
name:   J. LOEHR 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: FLIGHT DYNAMICS
GNC
AVIONICS
DYNAMICS ANALYSIS
DEGRADED MODES

description: Current and future space observation missions need to perform many (1500 per year) large angle slew maneuvers between the observations that can be planned or are unplanned. For planned observations it is important to maximize the time availability for science observations. For unplanned observations of spontaneous and time definite events it is required to slew within the respective time constraint. Reducing the time spent reorienting the spacecraft is thus an enabler. One of the main levers to achieve this reduction is to focus on the slew maneuvers between observations. In addition, due to the high sensitivity of the scientific instruments, it is essential to orient the spacecraft with safety critical attitude constraints, preventing the sunlight from entering the telescope. This article addresses these needs and presents an advanced guidance strategy for space observation missions under sun avoidance and attitude actuator constraints. Performing autonomous slew maneuvers with attitude constraints requires computational-efficient motion planning algorithms that can be run on-board in definite time. This is challenging as it does not allow implementing optimization algorithms, and no other solution is available in the literature yet. A state-of-practice is to divide the three-axis maneuver in out-of-plane and in-plane single-axis maneuvers of the telescope line of sight. The plane is defined by its sun-pointing normal in this respect. This ensures that the attitude constraints for sun avoidance are explicitly regarded. However, this leads to three consecutive maneuvers. In addition, new space observation missions rely on spacecraft architectures that enable to observe the same target successively with different on-board instruments by having a common movable focusing device, here named primary mirror. This increases scientific return, e.g. due to a larger aperture, but results in multi-body and multi-actuator (MB/MA) dynamics with large masses. The repointing with such a MB/MA configuration involves a mirror steering towards the line of sight of the observing instrument plus a spacecraft slew maneuver. The spacecraft attitude needs thus to be known in real-time after the mirror motion to correctly set the initial conditions for the slew maneuver. This is indispensable when the target attitude is only known shortly before and the observation is time critical, e.g. due to time definite events. The state-of-practice guidance enabling an autonomous on-board implementation is considered as benchmark in this paper to demonstrate a considerable reduction of slew time. The main contribution of this article is an analytical approach that computes systematically a guidance solution in a definite time, requiring only one acceleration and one deceleration phase. This time-efficient coupled-axis guidance relies on the convexity of the permissible attitude zone. In addition, the geometric properties of the angular momentum and torque envelopes of reaction wheel arrays are explicitly taken into account when computing the guidance trajectory. This way, both guidance solutions (the axis-by-axis baseline and the coupled-axis guidance) are guaranteed to satisfy actuator and environmental constraints while being easy to implement on-board and potentially autonomous. Using simulation results, we show that the coupled-axis guidance solution substantially improves the available time for scientific observations for a given spacecraft design and set of observation targets. In summary, the article introduces a novel repointing guidance strategy, applicable to a large variety of MB/MA spacecraft, that increases their agility and scientific observation availability. The results presented in this article are part of a PhD project conducted by the University Carlos III of Madrid, Spain, and Airbus Defence and Space GmbH in Friedrichshafen, Germany.