A report discusses the design of fast stochastic observers for spacecraft pointing control. In this special context, "observers" signifies mathematical algorithms, implemented on computers aboard spacecraft, through which one processes sensory data (principally, the outputs of star trackers and gyroscopes) to estimate the states (attitudes and angular velocities) of the spacecraft. The development in the report was motivated by the presence of an attitude-dependent bias error in the star-tracker measurement associated with NASA's upcoming SIRTF (Space Infra-Red Telescope Facility) space telescope. This attitude-dependent bias term lies outside of basic linear estimation assumption, and the well-established Kalman theory is no longer optimal. The attitude-dependent bias term forces a step response through the dynamics of the onboard estimator each time the spacecraft is repositioned. If an optimal Kalman filter were used, its sluggish dynamics would create a long undesirable lingering output drift in the pointing response. While this drift error is small (e.g., at the arcsecond level) it cannot be ignored for space telescope applications, and is the main reason that Kalman filters are routinely replaced by simple observers on important missions with stringent pointing requirements such as the Hubble Space Telescope and SIRTF.
In this report, a theoretical analysis of an attitude estimator comprising three decoupled single-axis observers leads to a globally optimal solution for designing a constrained stochastic observer of second-order form. This stochastic observer minimizes the variance of the attitude estimate, subject to a constraint that its poles lie to the left of a specified vertical line in the complex Laplace-transform s-plane. This so-called "fast observer" design allows the step response of the onboard estimator to be sped up with minimal degradation in the variance of the state estimate. Examples are presented to illustrate the optimal tradeoff between observer speed and estimation error.
This work was done by David S. Bayard of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "Fast Observers for Space Telescope Pointing Control with Application to SIRTF," access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Information Sciences category.
NPO-20883
This Brief includes a Technical Support Package (TSP).
Fast Observers for Spacecraft Pointing Control
(reference NPO-20883) is currently available for download from the TSP library.
Don't have an account?
Overview
The document is a technical report detailing the design and implementation of fast stochastic observers for spacecraft pointing control, authored by David S. Bayard at the Jet Propulsion Laboratory (JPL). The primary focus is on improving the efficiency of spacecraft attitude estimation, which is critical for missions requiring precise pointing, such as the Space Infrared Telescope Facility (SIRTF).
Traditional methods, particularly the optimal Kalman filter, often suffer from long settling times, which can extend to several hundred seconds in typical spacecraft applications. This report introduces a novel approach to designing constrained stochastic observers that minimize the variance of attitude estimation errors while ensuring rapid convergence. The design methodology aims to circumvent the limitations of conventional filters by providing a globally optimal solution that adheres to specific constraints regarding the poles of the observer's transfer function.
The report outlines the mathematical framework for these fast observers, emphasizing their ability to significantly reduce settling times without compromising estimation accuracy. For instance, the proposed design can enhance the speed of the observer by an order of magnitude while maintaining less than a 5% degradation in estimation error. This improvement is particularly beneficial in scenarios where the optimal unconstrained solution yields split real roots, which complicate the estimation process.
Additionally, the document discusses the potential applications of these fast observers across a wide range of spacecraft missions, highlighting their operational efficiency. The design principles are illustrated with examples that demonstrate the optimal trade-off between observer speed and performance, showcasing the versatility of the approach in various hardware configurations.
The report also addresses the status of the technology, noting that while the design theory has been developed and is being prepared for journal publication, there has been no commercial application of the technology to date. However, it is confirmed that NASA's SIRTF mission will utilize three fast observers based on this design theory.
In summary, this document presents a significant advancement in spacecraft pointing control technology, offering a robust solution to enhance the performance of attitude estimation systems, which is crucial for the success of future space missions.
