A report proposes an alternative method of control for precision landing on a remote planet. In the traditional method, the attitude of a spacecraft is required to track a commanded translational acceleration vector, which is generated at each time step by solving a two-point boundary value problem. No requirement of continuity is imposed on the acceleration. The translational acceleration does not necessarily vary smoothly. Tracking of a nonsmooth acceleration causes the vehicle attitude to exhibit undesirable transients and poor pointing stability behavior. In the alternative method, the two-point boundary value problem is not solved at each time step. A smooth reference position profile is computed. The profile is recomputed only when the control errors get sufficiently large. The nominal attitude is still required to track the smooth reference acceleration command. A steering logic is proposed that controls the position and velocity errors about the reference profile by perturbing the attitude slightly about the nominal attitude. The overall pointing behavior is therefore smooth, greatly reducing the degree of pointing instability.
This work was done by Gurkirpal Singh of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Mechanics category.
NPO-40585
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Alternative Attitude Commanding and Control for Precise Spacecraft Landing
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Overview
The document titled "Alternative Attitude Commanding and Control for Precise Spacecraft Landing" from NASA's Jet Propulsion Laboratory outlines advanced methodologies for achieving precision landings on extraterrestrial bodies, particularly focusing on Mars. It emphasizes the necessity for six-degree-of-freedom control to ensure that landers can touch down at specific locations with the desired orientation relative to the surface.
The primary challenge addressed is the highly coupled nature of commanding and control in precision landing scenarios, which differs significantly from traditional spacecraft operations. In typical applications, the vehicle's attitude must track a desired translation acceleration, which can lead to undesirable transients due to the non-smooth nature of acceleration commands. To mitigate this, the document proposes an innovative approach involving a "Steering Logic" that slightly perturbs the attitude command around a nominal pointing. This method allows for smoother acceleration commands, enhancing the stability of terrain sensors that are crucial for detecting landing hazards and providing reliable surface-relative state updates.
The document details three key design elements for the control system:
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Tracking Commander Function: This function is designed to improve the tracking of desired inertial positions, velocities, and accelerations, and is based on algorithms that have been successfully implemented in missions like Cassini.
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Steering Logic: This novel approach significantly enhances pointing stability compared to existing techniques by ensuring that both the attitude and acceleration commands are smooth, thereby reducing the computational load on the flight processor.
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Controller Design: Rooted in traditional rigid body control methods, this controller is adapted for the unique configurations of vehicles used in precision landing applications, ensuring that the control signals are effectively realized within a zero control error tolerance.
The document concludes that the proposed architecture and algorithms offer substantial improvements over existing methods, particularly in terms of computational efficiency and reliability in sensing terrain and detecting hazards. The insights provided are applicable not only to Mars landings but can also be adapted for other extraterrestrial bodies, making this research significant for future space exploration missions.
